Hydrocarbon Thermal Isomerizations, Second Edition

Hydrocarbon Thermal Isomerizations by Joseph J. Gajewski • • • ISBN: 0122733517 Pub. Date: April 2004 Publisher: Else...

Author: Joseph J. Gajewski

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Hydrocarbon Thermal Isomerizations by Joseph J. Gajewski

• • •

ISBN: 0122733517 Pub. Date: April 2004 Publisher: Elsevier Science & Technology Books

Preface to the Second Edition It is rare to be able to visit an old book on fundamental chemical processes and to be able to modify and amplify the conclusions with the insight from recent experimental and theoretical results. It was a privilege to follow this pursuit even though it was embarrassing at times when typographical errors, incorrect citations of fact, and fundamental mistakes in the previous version became obvious. Hopefully more errors were corrected than were introduced in the wealth of new information. An important aspect of this edition is the inclusion of calculational results that provide insight, often more than was anticipated, into these relatively simple reactions. I thank particularly Ernest Davidson, Ken Houk, and Wes Borden for their willingness to collaborate and for their many contributions to this science. Thanks are also due to John Baldwin for employing new experimental techniques to provide important details on a number of the pathways for some of the simplest hydrocarbons. Acknowledgement is also due to the graduate students and post docs who continued to pursue some of this work in my laboratory, namely Jangir Emrani, Jose´ Jimenez, Mike Squicciarrini, Andrea Gortva, Jeff Scheibel, Chris Hawkins, Charles Benner, Jahagir Emrani, Jurjus Jurayj, Kyle Gee, Ken Tupper, Thomas Kreek, Leif Olson, Nancy Brichford, Nathan Harris, Jo Ann Currey, and Ilya Kuchuk. Gitendra Paul, a long time post doc pursued a number of problems in the area. Also, visitors Chris Samuel, Adrian Brook, and Richard Holder were especially helpful. To these and others who worked on different problems I thank you for your interest and your contributions both experimentally and intellectually. You enriched my understanding of chemistry. I hope that the undergrads who participated in this and other research projects, namely Bob Gajewski, Ruth Kinder, John Gajewski and Paiboon Ngernmeesri learned as much as I did in the course of their efforts. Thanks are also due to the U.S. Department of Energy for their support during the writing of this edition. Finally, I thank Jack Crandall and Mike Montgomery for their insights, criticisms, and friendship over the years. Joseph J. Gajewski

1 Introduction

CONTENTS 1 Introduction 2 Activation in Unimolecular Reactions 3 Transition State Theory 4 Non-interacting Biradical Hypothesis 5 Woodward –Hoffmann Rules 6 Energetics and Stereochemistry of Concerted Reactions 7 Calculations 8 Scope of Coverage of this Book References

1

1 1 3 3 4 7 8 9 9

INTRODUCTION

Unimolecular isomerizations of hydrocarbons have played a central role in developing the understanding of chemical transformations. Since these apparently simple processes are not catalyzed, are usually independent of solvent, and require activation only by heat, their temperature and pressure dependence has provided impetus for and calibration of statistical mechanical and quantum mechanical models of reaction kinetics and energy surfaces. Because this volume will emphasize the structural changes and energetics of a large number of unimolecular hydrocarbon isomerizations, only a brief overview of kinetic theories is possible, and this is mostly for the purposes of defining terms. 2

ACTIVATION IN UNIMOLECULAR REACTIONS1

A molecule cannot undergo a unimolecular reaction unless it is provided with sufficient energy to traverse whatever barrier separates it from product. In purely

2

Hydrocarbon Thermal Isomerization

thermal reactions, energy is provided by collision with other molecules, so firstorder behavior is not obvious. However, the rate-determining step in thermolysis reactions is bond cleavage or electron reorganization, and not activation when the reactant is of reasonable size and the pressures are moderately high. First-order kinetics result because the activated molecule undergoes a bimolecular loss of energy to bath gas molecules. Recognition and understanding of this behavior is due to Lindemann and Hinshelwood (LH) and quantification is due to Rice, Ramsperger, and Kassel (RRK). According to the LH hypothesis for activation, collision of a reactant molecule, A, with one or more hot bath molecules, Mp, gives an activated reactant, Ap, which either reacts unimolecularly to give product, B, or collides with a cool bath molecule, M, and loses its energy (Eq. (1.1)). k1

k2

A þ Mp Y A p þ M ! B

ð1:1Þ

k21

The rate expression for formation of B, assuming low steady-state concentrations of Ap, is given in Eq. (1.2). dB=dt ¼ k1 k2 AMp =ðk21 M þ k2 Þ

ð1:2Þ

At high pressures k21 M . k2 , so Eq. (1.3) results. dB=dt ¼ k1 k2 AMp =ðk21 MÞ

ð1:3Þ

Further, at high pressure both Mp and M are constant and can be included in a constant related to the equilibrium between hot and cold M, given Eq. (1.4) where Kk2 is the observed high pressure first-order rate constant. dB=dt ¼ Kk2 AM

ð1:4Þ

Note that k2 is characteristic of only A, but K is characteristic of both A and M. If the pressure is so low that k21 M , k2 then Eq. (1.2) reduces to a second-order expression, Eq. (1.5), where the rate-determining step in the reaction is collisional activation of A by Mp, and this is what is observed with small molecules at low pressures. dB=dt ¼ k1 AMp

ð1:5Þ

For isomerization of small hydrocarbons (fewer than five carbons), the observed rate constants are generally independent of total pressure at pressure greater than about 1 atm. However, at lower pressures the first-order rate constants “fall off” and are dependent on pressure. Importantly, the smaller the molecule the higher the pressure at which the fall off behavior occurs. The fall off behavior as a function of molecular size is explained by arguing that the energy in the activated molecule is randomly distributed among a number of vibrational modes, and for a reaction to occur, i.e. k2 ; the energy must be localized in the appropriate vibration. Therefore, k2 is proportional to ½ðE 2 Eact Þ=Es21 ; where s is the number of vibrations in the molecule, and the energy factor is the fractional

Introduction

3

excess energy over that necessary to overcome the barrier for step k2 : Therefore, larger molecules have smaller values of k2 which is often less than k21 M leading to independence of the rate on pressure. Finally, the proportionality constant in an expression for k2 is generally agreed to be the rate of a molecular vibration, roughly 5 £ 1012 s21 at room temperature. Whether or not energy is randomly distributed and localized in a statistical fashion has been questioned and demonstrated not to be strictly true; just how much deviation from statistical behavior occurs is not substantial where tested (see Chapter 7, Section 4), but the stereospecificity of 1,3-shifts which appear to occur via biradicals has been argued to result from such behavior, vide infra. 3

TRANSITION STATE THEORY

Erying absolute rate theory or transition state theory allows a convenient dissection of the Arrhenius expression, k ¼ A expð2Eact =RTÞ; into thermodynamic quantities since the theory assumes a near equilibrium between the reactant and transition state. In transition state theory, the first-order rate constant (at the high pressure limit) is given by Eq. (1.6). k ¼ kkT=h 2 expðDS‡ =RÞexpð2DH ‡ =RTÞ

ð1:6Þ

where k is the transmission coefficient, usually assumed to be unity, k is the Boltzmann constant, and h is Planck’s constant. Identification of Eq. (1.6) with the Arrhenius expression after recognition that DH ‡ ¼ Eact 2 nRT, where n is 1 for firstorder reactions, gives DS‡ ¼ R½ln A 2 lnðkT=hÞ þ 1: Thus to convert Arrhenius parameters to activation parameters, a temperature must be specified. As a practical matter, the center of the temperature range utilized in measuring the kinetics should be used. However, often the temperature range is not stated, so in this book the reported parameters will not be converted to a common basis. But for all intents and purposes, activation energies and enthalpies are roughly equivalent, and deviations of log A from 12.5 represent the direction of the activation entropy: higher – more positive; lower – more negative with every log unit of deviation representing 4.6 e.u. 4

NON-INTERACTING BIRADICAL HYPOTHESIS

In unimolecular reactions, it is useful to compare the experimentally determined activation energy with the bond dissociation energy (BDE) of the bond presumed to be broken in the rate-determining step. If Eact is lower than the BDE, either the assumption about which bond is being broken is incorrect or the reaction is concerted, i.e. bond breaking is assisted by bond making. Indeed, concert may be quantified in these terms although as a practical matter, activation energies can have

4


some error, more if a small temperature range is chosen, and BDEs are especially difficult to determine particularly when the starting material is strained. Furthermore, BDEs are particularly difficult to measure even in simple cases.2 Finally, when the radical fragments are within the same molecule it is tempting to assume that these do not interact; this is, however, an assumption. Therefore, some leeway is necessary in assessing whether or not a reaction is concerted, let alone the extent of or lack of concert. Often the required BDE is obtained by removing two hydrogens from an appropriate hydrocarbon whose heat of formation is known3 along with the C –H BDEs and the BDE of dihydrogen to give the heat of formation of the noninteracting biradical (Scheme 1.1).

Scheme 1.1

This then can be compared to the heat of formation of reactant. Invariably this leads to low values for the BDE (by roughly 10 kcal/mol) compared with the experimentally determined activation energies necessary to break a C – C bond in non-concerted reactions like the isomerizations of cyclopropane and cyclobutane. These seem to lead to biradicals, which react almost as fast as they are formed and as fast as rotation around a bond with a 6-fold barrier that should be small. 5

WOODWARD –HOFFMANN RULES

Few theories of stereospecificity have touched the analytic sensitivities and moved the creative abilities of chemists as that contained in the classic papers of Woodward and Hoffmann in 1965.4 Their initial ideas had developed into a cogent set of Rules described in a later publication,5 which will be summarized below. The Rules for the stereospecificity apply only to pericyclic reactions which are concerted. The Rules apply neither to non-concerted reactions nor to those that are not pericyclic. Pericyclic reactions are those which involve a monocyclic transition state having a conjugated array of interacting orbitals, one per atom. Three types of pericyclic processes have been recognized: electrocyclic reactions, cycloadditions, and sigmatropic shifts. In electrocyclic reactions, a linear conjugated polyenyl system with n p orbitals is in equilibrium with a cyclic system with n 2 2 p orbitals and one new s bond (Scheme 1.2).

Introduction

5

Scheme 1.2

The course of this reaction is conrotatory, i.e. the p orbitals at the termini of the polyene rotate in the same direction to make (or break) the s bond in a concerted reaction, if 4n electrons are involved. If 4n þ 2 electrons are involved, the concerted reaction occurs in a disrotatory manner, i.e. the p orbitals at the termini rotate in opposite directions. Cycloadditions (and their retrograde reactions) can occur between two conjugated p electron systems and are designated as m þ n where m and n are the number of p orbitals in each component. However, more important is the number of p electrons in each component. If there are 4n þ 2 electrons in the transition state, the concerted reaction will occur in a face to face, i.e. a suprafacial, suprafacial, manner (or less likely in an edge to edge or antarafacial, antarafacial manner). If there are 4n electrons involved, the concerted reaction must occur in a face to edge or suprafacial, antarafacial manner (Scheme 1.3).

Scheme 1.3

Sigmatropic shifts of the order i; j are those transformations involving migration of a s bond flanked by one or two conjugated p systems to a new position whose termini are i and j atoms removed from the original bond. The concerted reaction with 4n þ 2 electrons total will occur in a suprafacial, suprafacial (or antarafacial, antarafacial) manner while those with 4n electrons will occur suprafacially on one component and antarafacially on the other component (Scheme 1.4).

6


Scheme 1.4

A generalization allows easy application of the Rules to any concerted pericyclic reaction involving two-electron bonds: for the “allowed” reaction there must always be an odd number of suprafacial (as opposed to antarafacial) uses of bonds. Here, suprafacial is defined as utilizing both atoms of a s bond with retention of configuration or both with inversion; with a p bond, the p orbitals must be used from the same face of the bond. An antarafacial use of a s bond would give retention at one atom and inversion at the other; with a p bond, the p orbitals must be used from opposite faces of the bond. In order to apply the generalization, it is convenient to use arrows to indicate the use of atoms in s and p bonds. The allowed pathway is the one which allows the arrow heads to join, e.g. Scheme 1.5.

Scheme 1.5

Introduction

7

Two different selections of bond usages are given for each reaction in Scheme 1.5. The use of bonds is arbitrary so long as there are an odd number of s uses for the “allowed” stereopathway. If more than one stereopathway can be envisioned, other factors such as steric effects or even electronic effects of substituents are important in determining the preferences. It should be noted here that a hydrogen atom could only be used with retention when it is involved in a sigmatropic shift. Finally, to extend the generalization to even electron charged systems, an empty p orbital (a cationic site) must be used in a suprafacial manner although it does not contribute to the count of s uses, and an anionic site can be used in either an antarafacial or suprafacial manner, and it does contribute to the number of s uses in the latter case. The generalized Rules might seem arbitrary, but they are based on fundamental considerations of cyclic delocalization in transition states.6 If there are 4n þ 2 electrons involved, the number of negative overlaps in basis set orbitals must be even for the delocalization to occur, i.e. stabilization of the transition state. If there are 4n electrons involved, the number of negative overlaps in basis set orbitals must be odd for delocalization to occur. The former is a so-called Hu¨ckel p overlap system while the latter is a Mo¨bius p overlap system. Finally, Woodward and Hoffmann have emphasized the application of the Rules only to monocyclic orbital interactions. They have pointed out that the generalizations when applied to the prismane to benzene thermal isomerization lead to the prediction that it can occur in an allowed fashion with minimal steric difficulty. However, prismane is a very stable compound despite being 90 kcal/mol less stable than benzene. The application of the generalization here ignores the polycyclic nature of the orbital interaction, which upon examination by a correlation diagram reveals that prismane would generate a doubly excited state of benzene were it to open in the apparently allowed manner upon heating.5 6 ENERGETICS AND STEREOCHEMISTRY OF CONCERTED REACTIONS The activation parameters for a unimolecular reaction provide evidence for whether or not a reaction is concerted. If the activation energy is substantially less than that expected for complete homolysis of the appropriate bond, then it is likely that bond making is assisting bond breaking in the transition state. Often activation energies in concerted reactions are dramatically lower than the BDE of the bond being broken as in the 1,5-hydrogen shifts of cis-1,3-pentadienyl systems. Thus a concerted reaction is one in which there is a continuous change in bonding from reactant to product via a single potential energy maximum along the reaction coordinate. If a reaction is concerted by the enthalpic criterion described earlier, then the Woodward –Hoffmann Rules will apply and will invariably predict the reaction stereopathway (“Violations…There are none…Nor can violations be expected…” – Woodward, 1970). However, observation of allowed stereochemistry does not prove

8


that a reaction is concerted nor does observation of “forbidden” stereochemistry reveal an exception to the Rules. Steric effects or dynamical effects can produce an allowed stereochemical path despite the lack of concert, and other effects may intercede or overwhelm the electronic preferences predicted by the Rules. As an example of the latter, the hydroboration reaction is often written as a 2 þ 2 cycloaddition of a B – H bond to an olefin, yet the boron binds to the olefin with an empty p orbital and transfers the hydride from another orbital. This is not a pericyclic reaction when one atom utilizes two orthogonal orbitals in the process.

7

CALCULATIONS7

The combination of developments in Quantum Mechanical Theory, especially approximations to all all-valence electron calculations, and fast computers has resulted in a symbiotic relation between experiment and theory in the area of hydrocarbon isomerizations. At a minimum, a reliable calculation of the relative energies and geometries of isomeric compounds is desirable, and, in the best of all possible worlds, the relative energies and geometries of all transition states within 10 kcal/mol of the lowest energy transition state as well as that of all accessible intermediates should be obtained, i.e. the potential energy surface – hopefully with corrections for zero point energies as a result of a calculation of the harmonic frequencies of the stationary states. This goal has been achieved in only a few cases, and those were obtained only after many less-than-successful approximations to the Schro¨dinger equation. It is required to utilize large basis sets (at least 6-31Gp in the Gaussian Frame of Reference) for the Self-Consistent Field calculations, to include multiconfigurational wave functions because of the weak bonding in the transition states (MCSCF), and to include configuration interaction with all electrons to adequately reproduce electron correlation, i.e. the microscopic avoidance of electron – electron repulsion (Dynamic Correlation). Further, the dynamics of molecules traversing the potential surface appear to play a role in determining the relative utilization of reaction paths of similar energies. In addition, substantial efforts need be made to ensure that the geometry optimizations give rise to true minima or maxima as stationary points on the potential energy surface, i.e. those with zero, or one and only one imaginary vibrational frequency, respectively. Success in calculations is defined as reproducing the experimental activation energies, entropies, kinetic isotope effects, and stereospecificity, more or less. It is fair to argue that the energy surfaces for most isomerizations with fewer than eight carbons have been or can be treated at a reasonably high level of theory, and these are discussed in the context of the experiments later. Finally, Density Function Theory, which includes correlation in a semi-empirical way and utilizing large basis sets has also been successful, particularly with concerted pericyclic reactions such as the Cope rearrangement.8

Introduction

8

9

SCOPE OF COVERAGE OF THIS BOOK

The major focus of this book are those hydrocarbon isomerizations which have been at or near the center of experimental and theoretical efforts by physical organic chemists and were emphasized in the first edition. While some attempt was made to include all hydrocarbon isomerizations, invariably some will have been overlooked (not intentionally), and those involving very large carbon systems were excluded for reasons of space and generalizability. A comprehensive coverage of all derivatives of a particular hydrocarbon would be impossible, as would inclusion of all references in a particular study. As much as possible, the reactions of parent hydrocarbons are the major focus, and substituents are examined in light of their perturbations on the parent compound(s). Occasionally the parent material had not been studied, in which case substituted materials will be discussed. The references given are hopefully the latest and will include references to earlier studies.

REFERENCES 1.

2. 3. 4. 5. 6. 7. 8.

For detailed expositions see J.W. Moore and R.G. Pearson, Kinetics and Mechanism, 3rd edn, Wiley, New York (1981); S.W. Benson, The Foundations of Chemical Kinetics, McGraw-Hill, New York (1960); H.S. Johnston, Gas Phase Reaction Rate Theory, Ronald Press, New York (1966); P.J. Robinson and K.A. Holbrook, Unimolecular Reactions, Wiley/Interscience, New York (1972). S.J. Blanksby and G.B. Ellison, Acc. Chem. Res., 36, 255 (2003) and references contained therein. See also S.W. Benson, Thermochemical Kinetics, 2nd edn, Wiley, New York (1976). J.B. Pedley, R.D. Naylor, and S.P. Kirby, Thermochemical Data of Organic Compounds, 2nd edn, Chapman and Hall, London (1986). R.B. Woodward and R. Hoffmann, J. Am. Chem. Soc., 87, 395 (1965). See also p. 2046, p. 2511, p. 4388, p. 4389. R.B. Woodward and R. Hoffmann, Angew. Chem. Int. Ed. Engl., 8, 781 (1969); R.B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry, Academic Press, New York (1970). H.E. Zimmerman, J. Am. Chem. Soc., 88, 1564– 1566 (1966). For a summary see W.T. Borden and E.R. Davidson, Acc. Chem. Res., 29, 67 (1996). K.N. Houk, J. Gonzalez, and Y. Li, Acc. Chem. Res., 28, 81 (1995).

2 CH4

CONTENTS 1 Methane: Tetrahedral Inversion, Planar Arrangements References

11 12

1 ETHANE METHANE: TETRAHEDRAL INVERSION, PLANAR ARRANGEMENTS Since methane is tetrahedral, a potentially detectable isomerization is inversion of configuration. However, high level calculations1 have placed the transition state for this reaction 5 –20 kcal/mol above the transition state for cleavage of methane to a methyl radical and a hydrogen atom ðlog k ¼ 14:88 2 103 000=2:3RTÞ; so the reaction is of only academic concern.2 In this spirit, B3LYP using a 6-311Gpp basis set found a pyramidal structure ð, H – C – H ¼ 77:88Þ for the transition state (Yoshizawa) reflecting an earlier assessment that planar methane would have a HOMO with a non-bonding pair of electrons3 so the pyramidal distortion is not unexpected. Planar carbon has long been a subject of experimental and theoretical efforts. Preparation of fenestranes was thought to be one approach but even the latest wellcharacterized structure, cis,cis,cis,trans-[5.5.5.6]tribenzofenestrane has central bond angles distorted only to 120.8 and 115.58 (Scheme 2.1).4

12


Scheme 2.1

Perhaps the most interesting (and complex) all carbon structure to be calculated at the MP2/6-31G(d) level with energies at the MP2/6-311 þ G(2d,p) level that has a planar carbon is dimethanospiro[2.2]alkaplanane (Scheme 2.2).5

Scheme 2.2

However, at the B3LYP/6-311G(3d,2p) level it is a transition state so it was suggested (and calculated) that real planarity could be achieved if the central carbon be oxidized to a dication using electron accepting groups on the periphery of the molecule.6

REFERENCES 1. 2. 3. 4. 5. 6.

Reference 4 of D.R. Rasmussen and L. Radom, Pure Appl. Chem., 70, 1977 (1998); K. Yoshizawa and A. Suzuki, Chem. Phys., 271, 41 (2001). H.B. Palmer and T.J. Hirt, J. Phys. Chem., 67, 709 (1963). R. Hoffmann, R.W. Alder, and C.F. Wilcox, Jr., J. Am. Chem. Soc., 92, 4992 (1970). B. Bredenko¨tter, U. Flo¨rke, and D. Kuck, Chem. Eur. J., 7, 3387 (2001). D.R. Rassmussen and L. Radom, Angew. Chem. Intl Ed. Engl., 38, 2876 (1999), and references contained therein. Z.-X. Wang and P. von R. Schleyer, J. Am. Chem. Soc., 124, 11979 (2002). see also H. Hopf, Classics in Hydrocarbon Chemistry, Wiley-VCH, Weinheim, Federal Republic of Germany (2000), Chapter 6.

3 C2H2 – C2H6

CONTENTS 1 C2H2 1.1 Acetylene Dyotropic Reactions 2 C2H4 2.1 Ethylene Geometric Isomerization and Substituent Effects 3 C2H6 3.1 Ethane Dyotropic Reactions 3.2 Ethane Cleavage Reaction References

1 1.1

13 13 15 15 16 16 17 18

C2H2 Acetylene Dyotropic Reactions

The only obvious thermal isomerization of acetylene is transposition of the atoms at either end of the triple bond. This type of “dyotropic” process1 was observed in the parent molecule using deuterium and carbon labeling.2 Thus, flash vacuum pyrolysis of [1-13C,1-2H]acetylene at temperatures above 7008C resulted in protium appearing at C1 of the recovered acetylene (Scheme 3.1).

Scheme . 3.1

14


The reaction was clearly intramolecular since no scrambling of hydrogen and deuterium between molecules was observed below 10508C. Previous to this study, the transposition of groups in phenyl acetylene was observed between 500 and 7008C. Here, it was suggested3 that either a phenyl or a hydrogen shift occurred to give phenylvinylidene, which rearranges as is well known (Scheme 3.2).4

Scheme . 3.2

Evidence which supports this hypothesis was the formation of phenanthrene and 1,2-benzazulene from pyrolysis of biphenyl-2-ylacetylene. Both products were envisioned to result from intramolecular insertion and addition, respectively, of a vinylidene (Scheme 3.3).

Scheme . 3.3

The possibility that both groups on the acetylene migrate simultaneously was not excluded, but it was viewed as less likely from these types of experiments.5 Early high level calculations indicated that vinylidene is 40 kcal/mol less stable than acetylene and that vinylidene has an 8.6 kcal/mol barrier to rearrange to acetylene.6 These are not inconsistent with the energetics suggested by

C2H2 – C2H6

15

the experiments. Subsequently, the singlet – triplet gap in vinylidene was found to be 2.06 eV by PE spectroscopy favoring the singlet state.7 Calculations at the CCSD level using the TZ þ 2P basis set found a classical barrier for hydrogen shift in the singlet of 3 kcal/mol, which could become as low as 1.4 kcal/mol with zero point energy corrections.8 Further, the overall barrier for isomerization of acetylene was found to be 43 kcal/mol. Thus, it was argued that vinylidene was a true intermediate. However, dynamics calculations on the hydrogen shifts in other carbenes revealed the importance of tunneling, which has the effect of lowering both the calculated enthalpy and entropy of activation.9 Even more remarkable is the fact that the calculated deuterium kinetic isotope effect for the hydrogen shift in simple carbenes is small because, it was argued, deuterium tunnels more than hydrogen in these cases. Thus singlet vinylidene is, at best, a shallow minimum on the reaction pathway.

2 2.1

C2H4 Ethylene Geometric Isomerization and Substituent Effects

Geometric isomerization about olefins, presumably by p bond rotation, has been addressed in a variety of systems including the parent compound where log A is approximately 13 and the activation energy is roughly 65 kcal/mol.10 Shock-tube pyrolysis of dideuterioethylene gave log k ¼ 14:0 2 65 000=2:3RT:11 Calculations at the CI-SD/6-31Gpp level are consistent with an activation energy of 65 kcal/ mol.12 Furthermore, the point was made that the first p bond energy of acetylene is 9 kcal/mol higher than that of ethylene despite the fact that acetylene has higher heat of hydrogenation for the first p bond. In 1-deuteriopropene, log k ¼ 12:16 2 61 300=2:3RT:13 In the conversion of cisto trans-2-butene, log k ¼ 14:28 2 64 900=2:3RT and in the Z to E conversion of 2,3-dimethyl-3-hexene, log k ¼ 14:34 2 61 400=2:3RT:14 Conjugated systems have been examined extensively. Among many, the anti to syn isomerization of 3-ethylidenecyclohexene has log k ¼ 13:8 2 54 000=2:3RT and the reverse reaction has log k ¼ 13:79 2 53 600=2:3RT:15 In all these cases, the activation energy reflects the strength of the p bond and extent of conjugation in the starting material and in the transition state. For instance, careful consideration of the extent of conjugation of double bonds led to the conclusion that the resonance energy of allyl radical species in the transition state for double bond geometric isomerization is 13.5 kcal/mol.15 Rotation around carbon – carbon double bonds has a curiously large deuterium kinetic isotope effect. This was found to be reasonably reproduced by MCSCF calculations at the 6-311Gpp level.16 In effect, the vibrations of sp2 radical-like carbon are lower in frequency than those of sp2 carbons of double bonds.

16 3 3.1


C2H6 Ethane Dyotropic Reactions

Ethane can, in principle, undergo interchange of hydrogens between C1 and C2 (Scheme 3.4).

Scheme . 3.4

While never observed, this reaction is the simplest possible in the category of type I dyotropic reactions.17 These are defined as reactions involving migration of m-p or potentially p atoms from one site to another with concomitant migration of n atoms from the migration terminus to the reaction origin (Scheme 3.5).

Scheme . 3.5

The reaction could proceed through an “anti-like” transition state as depicted for the ethane double hydrogen shift, which would be a dyotropic reaction of order 1,1. In this case, the reaction is “forbidden” if the interaction of the orbital on the migration origin with the orbital on the migration terminus is ignored so that a monocyclic array of interacting orbitals would be involved. The reaction could also proceed through a “syn-like” transition state as depicted in Scheme 3.5, but the electronic preferences would be the same. If chains of atoms migrate with retention of configuration (or suprafacially) then, only when the total number of electrons involved is 4n þ 2; giving a Hu¨ckel aromatic transition state, can the reaction be concerted. If 4n electrons are involved then one group must be used in an antarafacial manner to give a Mo¨bius aromatic transition state in order for the reaction to be concerted. So, for instance, a 3,3 dyotropic reaction involving transposition of allyl groups is a forbidden reaction if both allyls are used suprafacially since a cyclic array of eight electrons is involved. However, a 5,3 transposition involving a pentadienyl group and an allyl group, both used in a suprafacial manner, should enjoy some transition state stabilization (Scheme 3.6).

C2H2 – C2H6

17

Scheme . 3.6

The examples of such reactions in all carbon cases are few and far between,18 but they have been observed in heteroatomic systems.19 Type II dyotropic reactions were defined in a companion paper20 to that of the type I reaction. Here, two sets of atoms are transferred simultaneously from two atoms to two other atoms, resulting in transposition of p bond(s). The simplest example is that of transfer of a hydrogen atom from each carbon of ethane to the carbons of ethylene (Scheme 3.7).

Scheme . 3.7

Here, a six-electron, monocyclic array of interacting orbitals is involved and could proceed through an aromatic transition state. There are few of these reactions reported, and all involve intramolecular transfer of dihydrogen through six-21 and ten-22 electron Hu¨ckel aromatic transition states. 3.2

Ethane Cleavage Reaction

Homolytic cleavage of ethane to two methyl radicals is characterized by log k ¼ 16:75 2 89 500=2:3RT:23 The BDE has been determined to be 90.1 kcal/mol.24 Fundamental to considerations of energetics of thermal isomerizations are the BDEs of the bonds being broken in the rate-determining step. Thus it is important to note that substitution of hydrogen in ethane by simple alkyl groups lowers the BDE by roughly 1 kcal/mol per group but steric and delocalization effects are important. Thus the BDE of the central C –C bond in tetramethylbutane is only 78.6 kcal/mol and that of the allylic C –C in 1-pentene is 75.4 kcal/mol. The latter is important because it reflects the resonance energy of the allyl radical, which is 12.5 kcal/mol by comparison to the BDE of the central C– C bond in butane, which is 87.9 kcal/mol.24

18


REFERENCES 1. M.T. Reetz, Tetrahedron, 29, 2189 (1973). 2. R.F.C. Brown, F.W. Eastwood, and G.P. Jackman, Aust. J. Chem., 31, 579 (1978). 3. R.F.C. Brown, K.J. Harrington, and G.L. McMullen, J. Chem. Soc., Chem. Commun., 123 (1974). 4. H.D. Hartzler, Carbene (R.A. Moss and M. Jones, Jr., eds), Wiley/Interscience, New York, Vol. 2, 43 (1975). 5. R.F.C. Brown, Eur. J. Org. Chem., 3211 (1999), for a brief summary of experimental work. 6. C.E. Dykstra and H.F. Schaefer, III, J. Am. Chem. Soc., 100, 1378 (1978). See also C.E. Dykstra, C.A. Parsons, and C.L. Oates, J. Am. Chem. Soc., 101, 1962 (1979) for calculations on higher cumulene carbenes. 7. K.M. Ervin, J. Ho, and W.C. Lineberger, J. Chem. Phys., 91, 5974 (1989). 8. M.M. Gallo, T.P. Hamilton, and H.F. Schaefer, III, J. Am. Chem. Soc., 112, 8714 (1990). 9. J.W. Storer and K.N. Houk, J. Am. Chem. Soc., 115, 10426 (1993). 10. J.E. Douglas, B.S. Rabinovitch, and F.S. Looney, J. Chem. Phys., 23, 315 (1955). 11. D.K. Lewis, B. Brandt, L. Crockford, D.A. Glenar, G. Rauscher, J. Rodriguez, and J.E. Baldwin, J. Am. Chem. Soc., 115, 11728 (1993). 12. A. Nicolaides and W.T. Borden, J. Am. Chem. Soc., 113, 6750 (1991), and references contained therein. 13. M.C. Flowers and N. Jonathan, J. Chem. Phys., 50, 2805 (1969). 14. W. von E. Doering, W.R. Roth, R. Bauer, R. Breuckmann, T. Ebbrecht, M. Herbold, R. Schmidt, H.-W. Lennartz, D. Lenior, and R. Boese, Chem. Ber., 122, 1263 (1989), and references contained therein. 15. W. von E. Doering, W.R. Roth, R. Bauer, H. Boenhe, R. Breuckmann, J. Ruhkamp, and O. Wortmann, Chem. Ber., 124, 1461 (1991). 16. L.P. Olson, S. Niwayama, H.-Y. Yoo, K.N. Houk, N.J. Harris, and J.J. Gajewski, J. Am. Chem. Soc., 118, 886 (1996). 17. M.T. Reetz, Angew. Chem. Int. Ed. Engl., 11, 129 (1972). 18. See J.B. Lambert and D. Stec, Croat. Chem. Acta, 65, 737 (1992). 19. X.Y. Zhang, K.N. Houk, S. Lin, and S.J. Danishefsky, J. Am. Chem. Soc., 125, 5111 (2003), for example and references contained therein. 20. M.T. Reetz, Angew. Chem. Int. Ed. Engl., 11, 130 (1972). 21. T.J. Chow and M.F. Ding, Angew. Chem. Int. Ed. Engl., 25, 1121 (1986); K.N.Houk, Y. Li, M.A. McAllister, G. O’Doherty, L.A. Paquette, W. Siebrand, and Z.K. Smedarchina, J. Am. Chem. Soc., 116, 10895 (1994); L.A.Paquette, G.A. O’Doherty, and R.D. Rogers, J. Am. Chem. Soc., 113, 7761 (1991). 22. H. Geich, W. Grimme, and K. Proske, J. Am. Chem. Soc., 114, 1492 (1992); See also W. Grimme, K. Pohl, J. Wortmann, and D. Frowein, Justus Liebigs Ann. Chem., 1905 (1996). 23. For a critical analysis of many reports see S.W. Benson and H.E. O’Neal, Kinetic Data on Gas Phase Unimolecular Reactions, NSRDS-NBS 21, US Government Printing Office, Washington, DC (February 1970), 383– 384. 24. S.J. Blanksby and G.B. Ellison, Acc. Chem. Res., 36, 255 (2003), for a summary of many important BDEs. See also. J. Berkowitz, G.B. Ellison, and D. Gutman, J. Phys. Chem., 98, 2744 (1994).

4 C3H4 – C3H6

CONTENTS 1 C3H4 1.1 Allene Racemization 1.2 Cyclopropene to Propyne and Allene 1.3 Allene to Propyne 1.4 Energy Surface for C3H4 2 C3H6 2.1 Cyclopropane – Geometric Isomerization 2.2 Cyclopropane Structural Isomerization 2.3 1,3-Hydrogen Shifts References

1 1.1

19 19 20 24 24 25 25 29 30 31

C3H4 Allene Racemization

1,3-Disubstituted allenes are chiral, and when resolved, can be racemized thermally in a first-order reaction. Roth determined log kenant ¼ 13:61 2 46 170=2:3RT for the racemization of 2,3-pentadiene at 1– 2 in a pressure-independent reaction.1 The pathway most likely involves a 908 rotation around one p-bond to give an allylic biradical where the electrons are in mutually perpendicular orbitals (Scheme 4.1).

Scheme 4.1

20


Consideration of bond energies gave a value of 14 kcal/mol for the resonance energy of the allylic species. An alternative hypothesis for the racemization involving reversible nonstereospecific closure of the allene to singlet cyclopropylidene would appear to be ruled out by theory. The cyclopropylidene was found to be 62 kcal/mol less stable, by an SCF calculation at the 4-31G þ DZP level, than the allene, and methyl groups should not dramatically affect that energy difference.2 1.2

Cyclopropene to Propyne and Allene

Cyclopropene undergoes a thermal isomerization to propyne 3 with log k ¼ 13:25 2 37 400=2:3RT and to allene with log k ¼ 13:25243 400=2:3RT:4 The simplest reaction pathway would appear to be C1 – C3 cleavage to a biradical, which is also vinylcarbene, followed by a vicinal hydrogen shift from C2 to either C3 or C1, respectively (Scheme 4.2).

Scheme 4.2

However, the energy surface for formation of the acetylene is much more complex. As a result of MRCI –MCSCF calculations with a 4-31G þ DZP basis set, two hydrogens migrate: the first is a 1,3-shift to generate a propenylidene and the second is a vicinal hydrogen shift to establish the acetylene as is well known for vinylidenes (Scheme 4.3).5 Further, the pathway of Scheme 4.2 was found to be nearly 20 kcal/mol higher than the favored one!

Scheme 4.3

C3H4 – C3H6

21

Evidence for this pathway came first from the observation of a decreased rate of reaction with alkyl substitution at C1 rather than a rate increase due to increased substitution.6 This rate retardation results from preventing the 1,3-hydrogen shift to make the vinylidene, resulting in slower formation of acetylene via the pathway described in Scheme 4.2. Further, pyrolysis of carbon-labeled 1,3,3-trimethylcyclopropene gave one acetylene, which could be derived from either pathway (Scheme 4.2 or 4.3), and nearly equal amounts of another acetylene, which could only be derived by Scheme 4.3 (Scheme 4.4).7

Scheme 4.4

Finally, a small amount of degenerate isomerization of 1-isopropyl-3,3dimethylcyclopropene was observed, which could only reasonably arise via the vinylidene derived from cleavage of the less substituted bond (Scheme 4.5).7

Scheme 4.5

Subsequent work found that a small amount of 1-ethylcyclopropene was formed reversibly in the pyrolysis of 1,3-dimethylcyclopropene which, again, could best be derived via the vinylidene derived from cleavage of the less substituted bond (Scheme 4.6).8

22


Scheme 4.6

However, a major product from 3-substituted cyclopropenes is a conjugated diene resulting apparently from a 1,4-hydrogen shift in the originally formed biradical/carbene. In the case of 1,3-dimethylcyclopropene, 1,3-pentadienes are formed which is consistent with cleavage of the more substituted bond followed by the 1,4-hydrogen shift. Furthermore, some acetylene product could also result via this biradical although some is probably formed via the vinylidene, given the other evidence. This work also determined the activation parameters for the overall loss of 1,3dimethylcyclopropene as log k ¼ 13:36 2 39 300=2:3RT and that for the formation of 1-ethylcyclopropene as log k ¼ 12:29 2 39 800=2:3RT: The activation parameters for loss of the latter material were found to be very similar to that of the former cyclopropene. Still later work using 2-deuterium-labeled material not only demonstrated that the ethyl group migrated in preference to the methyl group in the vinylidene by a factor of roughly 3, but also found a deuterium isotope effect of 1.32 at 509 K.9 This isotope effect could result from a pathway like that depicted in Scheme 4.2, but the authors suggested that it arises from the 1,3-hydrogen shift to form the vinylidene. According to the same calculations, the formation of allene follows the course described in Scheme 4.2, but ring opening to a biradical/carbene is reversible, having a transition state enthalpy of 58.5 kcal/mol relative to propyne and a barrier to allene of 66.1 kcal/mol relative to propyne. Surprisingly, this same biradical was not along the path to propyne although it is obviously accessible at the energies required for that conversion. This biradical would appear to be responsible for the observation of racemization of optically active 1,3-diethylcyclopropene which occurs roughly 10 times faster than the structural isomerization ðlog krac ¼ 11:8 2 32 600=2:3RT and log kstruct ¼ 10:4 2 32 300=2:3RTÞ (Scheme 4.7).10

C3H4 – C3H6

23

Scheme 4.7

Here, it is most likely that the more substituted bond undergoes reversible cleavage to give racemized starting material. It must be responsible for the 1,4hydrogen shift to the conjugated dienes given the substitution pattern observed. However, structural isomerization to the acetylene can occur by either a vicinal hydrogen shift in the biradical or by slower formation of the less substituted biradical followed by the 1,3-hydrogen shift to a vinylidene followed by its rearrangement. The originally formed 1,3-biradical is an interesting species in its own right because it is also vinylcarbene. Such species were generated independently from diazo compounds and as lithium carbenoids and were shown to give cyclopropenes (Scheme 4.8).11

Scheme 4.8

Of theoretical interest is the fact that generalized valence bond calculations in 1977 on vinylcarbene indicated that the planar singlet is 12– 14 kcal/mol less stable than the planar triplet ground state.12 Subsequent calculations at much higher levels gave values between 11.7 and 13.3 kcal/mol.13 Furthermore, the heat of formation of the triplet species was found to be 93 ^ 3 kcal/mol.

24


1.3

Allene to Propyne

The kinetics of the reversible conversion of allene to propyne were determined in shock tube experiments at 1030 – 1220 K giving log k ¼ 13:17 2 60 400=2:3RT 14 or log k ¼ 14:48 2 69 000=2:3RT 15 or DGact ¼ 65 kcal=mol:16 The reaction is intramolecular, and it was noted that cyclopropene could be an intermediate because its estimated heat of formation is only 20 kcal/mol higher than that of allene.4 Further, since cyclopropene gives propyne faster than allene, propyne could scramble hydrogens via cyclopropene faster than it could give allene, and indeed, at 1000 K in a flow system, 1-deuteriopropyne gives 3-deuteriopropyne roughly four times faster than allene.17 The pathway for conversion of the propyne to cyclopropene presumably involves the vinylidene of Scheme 4.3. 1.4

Energy Surface for C3H4

The relative heats of formation for all relevant species were calculated in ref. 2, and Scheme 4.3 reveals some of that information. There is reasonable correspondence between the calculated relative enthalpies and the experimental heats of formation. Further, the experimental heat of formation of singlet vinylcarbene is around 104 kcal/mol, which, if compared with that of cyclopropene from experiment (66 kcal/mol), allows it to be accessible in the pyrolysis, but the calculated transition state heat of formation to this species is 102 kcal/mol. Given the error in the experimental determination of the triplet energy of vinylcarbene (3 kcal/mol) and the variation in the calculated singlet – triplet energy splitting (2 kcal/mol), the values can be reconciled. An energy surface is presented below, which combines the experimental and calculated heats of formation using the lowest possible value for vinylcarbene (Scheme 4.9).

Scheme 4.9

C3H4 – C3H6

2 2.1

25

C3H6 Cyclopropane – Geometric Isomerization

Cyclopropane has long been known to undergo a thermally induced structural rearrangement to propylene. The first-order rate constant at high pressures is log k ¼ 15:1 2 65 200=2:3RT:18 The trimethylene biradical was invoked as an intermediate in the reaction. Subsequent observations that cis- and trans-1,2dideuteriocyclopropane interconvert roughly 20 times faster than the structural isomerization with log k ¼ 16:41 2 65 100=2:3RT 19 can be understood in terms of reversible formation of the trimethylene species (Scheme 4.10).

Scheme 4.10

Thermochemical estimates of the heat of formation of trimethylene with no interaction between the radical sites revealed that it is 8– 10 kcal/mol more stable than the transition state for the geometric isomerization.20 It therefore would appear that the reactions involve non-concerted ring opening and closure as well as a vicinal hydrogen shift to give propylene. However, cyclopropane geometric isomerization became the subject of intense scrutiny after Hoffmann, using Extended Hu¨ckel Theory, suggested that the ring opening was more favorable via conrotatory motions rather than disrotatory motions. His argument was that the antisymmetric p-like biradical is not destabilized by filled orbital interaction with the perpendicular p-AO at C2 in contrast to the symmetric p-like biradical. This has the consequence of suggesting double inversion at C1 and C3 via conrotatory ring opening and closure. Thus in 1,2-dideuteriocyclopropane, geometric isomerization occurs by C1 –C3 and C2 – C3 bond fission while C1 – C2 bond fission gives back the starting material in the case of the cis compound but could result in racemization of optically active trans material (Scheme 4.11).

Scheme 4.11

26


Berson first pursued the question with optically active trans-1,2-dideuteriocyclopropane and found that kgeo is 1.07 ^ 0.04 times that for loss of optical activity of the sample, ka :21 This result is consistent with predominant double inversion at all bonds (a factor of . 13) relative to single inversion reactions provided the kinetic isotope effects are small, which is not unreasonable at the temperatures of the reaction. If a random biradical reaction at all bonds were involved then kgeo =ka would equal 1.50, and if only single inversion at each site were involved the ratio would be 2 (Scheme 4.12). In this scheme, only the forward rate constants are described with subscripts indicating whether a single or double inversion path is utilized and which atoms are involved. A “2” multiplies each single inversion rate constant because two different bonds can break to result in each single inversion reaction. The calculation of kgeo recognizes that every conversion of trans to cis gives twice as much of an equilibrium mixture. The calculation of ka recognizes that formation of enantiomeric trans material gives twice as much racemic material as the formation of a cis compound. The calculation of the ratio in each case assumes no isotope effect on the bond being broken. The fact of this high specificity is remarkable considering the estimate of the heat of formation of trimethylene.

Scheme 4.12

Berson’s results were later confirmed by Baldwin.22 However, Baldwin subsequently examined the isomerizations of quadruply labeled cyclopropane with results more or less consistent with a random biradical.23 Thus, the appropriate 1,2,3-trideuteriocyclopropane with a carbon-13 label was prepared optically active and pyrolyzed at 407.08C (see Scheme 4.13). It was found that kgeo =ka ¼ 1:48; which is consistent with the random biradical pathway. The difference between the

C3H4 – C3H6

27

dideuterio- and trideuteriocyclopropane results has been attributed to an isotope effect which, in the case of the dideuterio compound, must substantially increase cleavage of the C1 –C2 bond if a biradical path were involved. This is inconsistent with expectations based on alpha secondary deuterium isotope effects; so some other unknown effect must be involved.

Scheme 4.13

Subsequent to Hoffmann’s theoretical work, Salem located a transition state for one center inversion at the ROHF þ (3 £ 3)-CI level using a minimal basis set.24 This is the (0,90) trimethylene species. Also found was a transition state for double inversion, but subsequent theoretical work by Schaeffer at the TCSCF/DZ level25 found that this species, the p-cyclopropane or the (0,0) trimethylene species has two imaginary frequencies, so it could not be a transition state. Later calculations at the GVB/6-31Gp level of theory with one pair of electrons correlated located the transition states for single and double inversion; the preference for conrotatory double inversion over single inversion was found to be roughly 1 kcal/mol, which corresponds to a factor of only 2 at the temperatures of the reaction.26 Further the deuterium isotope effect for the dideuterio compound was calculated to be 1.13, i.e. normal, as expected. A depiction of the potential energy surface for thermal geometric isomerization of cyclopropane is given in Scheme 4.14.

28


Scheme 4.14

More recent approaches addressing dynamical effects on the reaction reveal a greater propensity for double inversion via mostly conrotatory motions over single inversion in the 1,2-dideuterio case by factors of 2.3– 3.5 and 4.7, which are significantly less than that determined by Berson and Baldwin (refs. 21 and 22, respectively).27 Another significant milestone in the trimethylene story is the observation of the singlet species derived from photolysis of cyclobutanone and a determination of its lifetime before formation of cyclopropane at 122 fs.28 For the 2,2-dideuterio species the lifetime is 129 fs and that for the 1,1,3,3-tetradeuterio species is 183 fs. These effects were reasonably reproduced by DZP-TCSCF wave functions, which indicate that double inversion occurs four times more often by conrotatory over disrotatory paths.29

C3H4 – C3H6

29

Concern over the energy surface for the geometric isomerization spawned numerous experiments with 1,2-disubstituted and 1,2,3-trisubstituted derivatives, almost all of which revealed a preference for random biradical formation tempered by steric effects. Notable exceptions to this generalization include Berson’s work with phenylcyclopropane21 and 1,2-dimethyl-1,2-bis(trideuteriomethyl)cyclopropane.30 The former case, which reveals a propensity for double inversion, stands in contrast to Willcott’s work with vinylcyclopropane. Further, an extensive study by Baldwin using carbon-13 as an additional stereolabel revealed that double inversion was only , 60% as fast as single inversion at the phenyl-bearing carbon and , 20% as fast as single inversion at the methylene carbons.31 The origin of the differences is not clear. The tetramethylcyclopropane case reveals a strong preference for single inversion if the assumption is made that only the C1 – C2 bond breaks. Reasonable steric effects would appear responsible for the behavior of this derivative. In the case of the other 1,2-disubstituted derivatives, it is reasonable that the most substituted bond cleaves to a greater extent than the less substituted bonds. 2.2

Cyclopropane Structural Isomerization

The structural isomerization of cyclopropane bears scrutiny since it involves a vicinal hydrogen atom shift to a radical site, a reaction having little precedence in free radical chemistry. These reactions are invariably slower than the geometric isomerization of cyclopropanes described in Section 2.1. Further, radical stabilizing groups on the cyclopropane increase the rate of geometric isomerization but have a smaller effect on structural isomerization. So, while the geometric isomerization of 1,2-dimethylcyclopropane is ca. 7 times faster than that of the parent, structural isomerization is only 2 times faster, and the product distribution reflects a substantial amount of cleavage at the less substituted bond (Scheme 4.15).32

Scheme 4.15

Steric effects are also important in the hydrogen shift with perhaps the most extreme case being that of 1,1,2,2-tetramethylcyclopropane where the hydrogen shift must necessarily involve cleavage of the most substituted bond as well as

30


forcing the methyl groups into a near planar geometry for the shift to occur. So here the geometric isomerization, as determined from the 1,2-bis(trideuteriomethyl)-1,2dimethyl material and involving just single inversion, is nearly 200 times faster than the hydrogen shift.33 The hydrogen shift is also retarded in the isomerization of bicyclo[2.1.0]pentane to cyclopentene relative to geometric isomerization via bridgehead double inversion. The origin of this effect is unknown, and the results stand in contrast to the relative rates of the two processes with bicyclo[3.1.0]hexane (see Chapter 7, Section 3). Finally, it is of theoretical interest that geometry optimizations at the CASSCF(4,4).T2ZP level with energies calculated at still higher levels along with zero point energy corrections revealed that the activation energy for the vicinal hydrogen shift in trimethylene was only 2.2 kcal/mol lower in energy than a structural isomerization pathway via a 1,3-hydrogen shift to ethyl carbene followed by a vicinal hydrogen shift to propylene.34

2.3

1,3-Hydrogen Shifts

The Conservation of Orbital Symmetry Rules argue that the 1,3-shift of hydrogen in propenyl systems is electronically allowed only if the allylic moiety is used in an antarafacial manner (Scheme 4.16).

Scheme 4.16

However, this is difficult to achieve sterically, so very high temperatures necessary to cause complete dissociation would appear to be required to observe this isomerization as a pure thermal, uncatalyzed process. More than a few calculations have verified this when X ¼ H or R.35 Nonetheless, in the gas phase, butanone enolate undergoes a reasonably rapid 1,3hydrogen shift.36 Subsequent calculations at a variety of levels indicate that this and other hydrogen shifts in charged systems, X ¼ O2 and COþ, undergo the antarafacial reaction with energies less than that required for complete dissociation, typically 30 kcal/mol, which still makes these very high energy processes.37

C3H4 – C3H6

31

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

28.

W.R. Roth, G. Ruf, and P.W. Ford, Chem. Ber., 107, 48 (1974). M. Yoshimine, J. Pacansky, and N. Honjou, J. Am. Chem. Soc., 111, 4198 (1989). K.B. Wiberg and W.J. Bartley, J. Am. Chem. Soc., 82, 6375 (1960). I.M. Bailey and R. Walsh, J. Chem. Soc., Faraday Trans. I, 74, 1146 (1978). See also R. Walsh, J. Chem. Soc., Faraday Trans. I, 72, 2137 (1976). R.F.C. Brown, Recl. Trav. Chim. Pays. Bas., 107, 655 (1988). R. Walsh, C. Wolf, S. Untiedt, and A. de Meijere, J. Chem. Soc., Chem. Commun., 421 (1992). I.R. Likhotvorik, D.W. Brown, and M. Jones, Jr., J. Am. Chem. Soc., 116, 6175 (1994). H. Hopf, W.G. von der Schulenburg, and R. Walsh, Angew. Chem. Int. Ed. Engl., 36, 381 (1997). W.G. von der Schulenburg, H. Hopf, and R. Walsh, Angew. Chem. Int. Ed. Engl., 38, 1128 (1999). E.J. York, W. Dittmar, J.R. Stevenson, and R.G. Bergman, J. Am. Chem. Soc., 94, 2882 (1972). E.J. York, W. Dittmar, J.R. Stevenson, and R.G. Bergman, J. Am. Chem. Soc., 95, 5680 (1973). G. Closs and L.E. Closs, J. Am. Chem. Soc., 85, 99 (1963). G. Closs, L.E. Closs, and W.A. Bo¨ll, J. Am. Chem. Soc., 85, 3796 (1963). J.H. Davis, W.A. Goddard, III, and R.G. Bergman, J. Am. Chem. Soc., 99, 2427 (1977). J.C. Poutsma, J.J. Nash, J.A. Paulino, and R.R. Squires, J. Am. Chem. Soc., 119, 4686 (1997). A. Lifshitz, M. Frenklach, and A. Burcat, J. Phys. Chem., 79, 1148 (1975). J.N. Bradley and K.O. West, J. Chem. Soc., Faraday Trans. I, 71, 967 (1975). J.M. Simmie and D. Melvin, J. Chem. Soc., Faraday Trans. I, 74, 1337 (1978). H. Hopf, H. Priebe, and R. Walsh, J. Am. Chem. Soc., 102, 1210 (1980). T.S. Chambers and G.B. Kistiakowsky, J. Am. Chem. Soc., 56, 399 (1934). B.S. Rabinovitch, E.W. Schlag, and K.B. Wiberg, J. Chem. Phys., 28, 504 (1958); E.W.Schlag, and B.S. Rabinovitch, J. Am. Chem. Soc., 82, 5996 (1960). S.W. Benson, J. Chem. Phys., 34, 521 (1961); H.E.O’Neal, and S.W. Benson, J. Chem. Phys., 72, 1866 (1968). J.A. Berson, L.A. Pedersen, and B.K. Carpenter, J. Am. Chem. Soc., 98, 122 (1976); J.T.Wood, J.S. Arney, D. Corte`s, and J.A. Berson, J. Am. Chem. Soc., 100, 3855 (1978). S.J. Cianciosi, N. Ragunathan, T.B. Freedman, L.A. Nafie, and J.E. Baldwin, J. Am. Chem. Soc., 112, 8204 (1990). S.J. Cianciosi, N. Ragunathan, T.B. Freedman, L.A. Nafie, L.A. Lewis, D.A. Glenar, and J.E. Baldwin, J. Am. Chem. Soc., 113, 1864 (1991). Y. Jean, L. Salem, J.S. Wright, J.A. Horsley, C. Moser, and R.M. Stevens, Pure Appl. Chem. Suppl. (23rd Congr.), 1, 197 (1971); J.A.Horsley, Y. Jean, C. Moser, L. Salem, R.M. Stevens, and J.S. Wright, J. Am. Chem. Soc., 94, 279 (1972). Y. Yamaguchi, Y. Osamura, and H.F. Schaefer, J. Am. Chem. Soc., 105, 7506 (1983). S.J. Getty, E.R. Davidson, and W.T. Borden, J. Am. Chem. Soc., 114, 2085 (1992). See references in this paper for an overview of all the previous theoretical efforts addressing the cyclopropane energy surface. C. Doubleday, Jr., K. Bolton, and W.L. Hase, J. Am. Chem. Soc., 119, 5251 (1997); C. Doubleday, Jr., K. Bolton, and W.L. Hase, J. Phys. Chem. A, 102, 3648 (1998); D.A.Hrovat, S. Fang, W.T. Borden, and B.K. Carpenter, J. Am. Chem. Soc., 119, 5253 (1997); D.A. Hrovat, S. Fang, W.T. Borden, and B.K. Carpenter, J. Am. Chem. Soc., 120, 5603 (1998) (correction). S. Pedersen, J.L. Jerek, and A.H. Zewail, Science, 266, 1359 (1994).

32 29. 30. 31. 32. 33. 34. 35. 36. 37.

Hydrocarbon Thermal Isomerization J.E. Baldwin, T.B. Freedman, Y. Yamaguchi, and H.F. Schaefer, III, J. Am. Chem. Soc., 118, 10934 (1994). J.A. Berson and J.M. Balquist, J. Am. Chem. Soc., 90, 7343 (1968). J.E. Baldwin and T.C. Barden, J. Am. Chem. Soc., 106, 5312 (1984). M.C. Flowers and H.M. Frey, Proc. R. Soc. (Lond.), Ser. A, 257, 122 (1960); M.C. Flowers and H.M. Frey, Proc. R. Soc. (Lond.), Ser. A, 260, 424 (1961). H.M. Frey and D.C. Marshall, J. Chem. Soc., 3052 (1962); J.A. Berson and J.M. Balquist, J. Am. Chem. Soc., 90, 7343 (1968). H.F. Bettinger, J.C. Rienstra-Kiracofe, B.C. Hoffman, H.F. Schaefer, III, J.E. Baldwin, and P. von R. Schleyer, J. Chem. Soc., Chem. Commun., 1515 (1999). K.N. Houk, Y. Li, and J.D. Evanseck, Angew. Chem. Int. Ed. Engl., 31, 682 (1992) and references contained therein. K. Sannes and J.I. Brauman, J. Am. Chem. Soc., 117, 10088 (1995). C.E. Hudson and D.J. McAdoo, J. Org. Chem., 68, 2735 (2003).

5 C4H4 – C4H8 CONTENTS 1 C4H4 1.1 Cyclobutadiene and Tetrahedrane 2 C4H6 2.1 Bicyclo[1.1.0]butane to Butadiene 2.2 Cyclobutene to 1,3-Butadiene 2.3 Methylenecyclopropane Degenerate Rearrangement 2.3.1 More Complex MCPs 3 C4H8 3.1 Cyclobutane Geometric Isomerization and Cleavage 3.2 1,2-Dimethylcyclobutanes 3.3 1,1,2,2-Tetramethylenecyclobutane References

1 1.1

33 33 35 35 38 41 45 46 46 48 50 50

C4H4 Cyclobutadiene and Tetrahedrane

The (CH)4 energy surface is of great concern. Molecules, which inhabit this surface, include cyclobutadiene and tetrahedrane. Many cyclobutadienes have been prepared including the parent compound.1 The infrared spectrum of the parent has been interpreted in terms of a D2h structure consistent with a pseudo Jahn – Teller distorted rectangular, singlet species (Scheme 5.1).2 The two possible isomers of vicinally substituted dideuteriocyclobutadiene have been trapped and the difference in activation parameters for isomerization and trapping (from ln ki/ktrap) have been determined to be DDH ‡ ¼ 1:5 kcal=mol and DDS‡ ¼ 8 e:u:3 It was suggested that heavy atom tunneling could be responsible for the isomerization.4

34


Scheme 5.1

Calculations on the energy surface for cyclobutadiene with STO-3G, double Zeta, and 6-31Gp basis sets and full p and s CI indicate that the activation energy for interconversion of the rectangular singlets is roughly 10 kcal/mol ðC – C ˚ ) with the D4h singlet ðC – C distance ¼ 1:45 A Þ being distances , 1:54 and 1.37 A 5 the transition state. Further, the square triplet is ca. 10 kcal/mol less stable than the square singlet. Cyclobutadiene undergoes rapid dimerization in a formal 2 þ 2 fashion but more likely the reaction is a 4 þ 2, Diels – Alder-like process.6 Most interesting is the fact that dimerization cannot occur when cyclobutadiene is generated within the cavity of a hemicarcerand.7 Heavily substituted cyclobutadienes are more amenable to study because dimerization is a slower process than with the parent. Examination of numerous substituted cyclobutadienes provides evidence for the presence of two isomers, but perhaps the most revealing is the study of 1,2-di-tert-butyl-3-(nonadeuterio-tertbutyl)cyclobutadiene whose carbon-13 spectrum indicated splitting of the formally equivalent ring carbons (Scheme 5.2).8

Scheme 5.2

This is consistent with equilibration of the two rectangular isomers which must interconvert with an activation energy less than 2.5 kcal/mol since decoalescence of the carbon signals does not occur even at 2 1858C. The crystal structure of the tetra˚ tert-butylcompound at 2 1508C indicates bond distances of 1.53 and 1.44 A

C4H4 – C4H8

35

suggesting that the substituent interaction distorts the rectangle toward the square.9 B3LYP calculation with a large basis set indicates that the distances should be 1.354 ˚ .10 and 1.608 A Tetra-tert-butyltetrahedrane was prepared by Maier by the photolysis of tetra-tertbutylcyclopentadienone.11 X-ray crystallographic analysis established that the bond ˚ , but the arc of electron density between carbons is roughly lengths are 1.497 A 12 ˚ 1.7 A. Further, it rearranges to tetra-tert-butylcyclobutadiene upon melting at 1328C in what must be a symmetry “forbidden” reaction. Its heat of formation suggests a strain energy of 130 kcal/mol which is probably due mostly to the rings and not to the substituents. However, roughly only half of the strain is relieved upon ring opening leading to a modest activation energy for the conversion to the cyclobutadiene. Calculations at the G2 level indicate that tetrahedrane itself is roughly 26 kcal/mol less stable that cyclobutadiene.13 Somewhat disconcerting is the fact that while DFT calculations10 provide a similar energy difference they also indicate that tetra tert-butyltetrahedrane is ca. 3 kcal/mol more stable than the cyclobutadiene. Tri-tert-butyl trimethylsilyl tetrahedrane14 and tetra-trimethylsilyltetrahedrane15 have also been prepared. The former gives the cyclobutadiene at 1608C, and the latter is stable up to 3008C. Interestingly, B3LYP calculations indicate that the latter tetrahedrane is 9 kcal/mol more stable than the corresponding cyclobutadiene. These same calculations provide an energy difference of 24.3 kcal/mol in the parent case favoring the cyclobutadiene which is close to the G2 value so the value with the tetra TMS derivative is probably believable. 2 2.1

C4H6 Bicyclo[1.1.0]butane to Butadiene

Bicyclo[1.1.0]butane was first prepared by Lemal who also found that it rearranged to butadiene upon heating. 16 Subsequently, it was determined that log k ¼ 14:52 2 41 400=2:3RT:17 Doering found that the 1,3-dimethyl-substituted material gave only 2,3-dimethylbutadiene indicating that the central bond was not broken leaving cross-ring external bond fissions to be the likely pathway for what appears to be a retro 2 þ 2 cycloaddition (Scheme 5.3).18

Scheme 5.3

36


The activation energy is less than a reasonable estimate of the bond dissociation energy of one of the external bonds; so it is likely that the reaction is concerted. Indeed, Closs showed that the cleavage of the two bonds occurs predominantly, but not exclusively in an s þ a sense. Thus exo,exo-2,4-dimethyl material gives a 93.2:3.9 mixture of cis,trans- and trans,trans-2,4-hexadiene while exo,endomaterial gives a 1.2:95 mixture of the two dienes, respectively.19 Scheme 5.4 shows not only the reaction stereochemistry but the pathway expected if the reaction were concerted.

Scheme 5.4

Small amounts of trans-2-methylvinylcyclopropane are formed, possibly from a 1,4-hydrogen shift in a biradical species. Woodward and Hoffmann pointed out that in the opening of bicyclobutane, a 2s þ 2a pathway to cyclobutene is also possible, and this species, upon conrotatory ring opening (see Section 2.2), will give diene with stereochemistry equivalent to the minor product in Scheme 5.4. Thus the reaction may be more stereospecific than what is reflected by the product distributions. On the other hand, MINDO/3 calculations, which often favor biradical formation, suggest that the reaction occurs via a biradical.20 But subsequent calculations at various ab initio levels revealed a concerted asynchronous reaction pathway.21,22 In the latest effort using large basis sets and 10/10 MCSCF approaches,22 the pathway of Scheme 5.4 was found to be of lowest energy by 14 kcal/mol. The transition state ˚ while the cross-ring bond is stretched by only has one bond stretched to 2.25 A ˚ from starting material, and the methylene group is twisted by approximately 0.05 A

C4H4 – C4H8

37

258C from normal. The reaction pathway proceeds with the stretching and twisting motions about the cross-ring bond as in the scheme. The next higher energy pathway reverses the twisting of the first bond but not the second. The low energy pathways is referred to as a “conrotatory” one and the latter as a “disrotatory” pathway, but these descriptors are at best “poor” and at worse “misleading”. Remarkably, the thermal isomerization of tricyclo[3.1.0.02,6]hexane gives all-cis-1,3-cyclohexadiene (Scheme 5.5).23 Interestingly, the activation energy ðlog k ¼ 13:6 2 41 700=2:3RTÞ is similar to that of the parent molecule.24

Scheme 5.5

Here, the reaction would appear to be required to follow the forbidden pathway. However, recent studies of the potential energy surface for this reaction with multiconfiguration methods25 find that the “allowed” pathway is utilized to produce a trans,cis-1,3-cyclohexadiene which has only a 3 kcal/mol barrier to double bond isomerization to the all-cis-1,3-cyclohexadiene (Scheme 5.6).

Scheme 5.6

Precedence for this pathway comes from Wiberg’s study of the thermal isomerization of tricyclo[4.1.0.02,7]heptane which gives bicyclo[3.2.0]hept-6ene, which most reasonably is formed from cis,trans-1,3-cycloheptadiene (Scheme 5.7).26

Scheme 5.7

38


Further evidence for the pathways described above comes from labeling studies in which deuterium replaced both hydrogens at the bicyclobutane bridgehead positions, leading to the cyclic dienes with deuteriums at carbons 2 and 3.24 This study also included an unsaturated derivative of the tricyclo heptane above as well as measurements of activation parameters for all of the bridged bicyclobutane systems known at that point of time. 2.2

Cyclobutene to 1,3-Butadiene

Cyclobutene has long been known to isomerize to 1,3-butadiene. The kinetics of isomerization of the parent cyclobutene were determined to be log k ¼ 13:4 2 32 900=2:3RT (Scheme 5.8).27 The activation energy is more than 30 kcal/ mol lower than that for cleavage of cyclobutane (see Section 3).

Scheme 5.8

Vogel was the first to recognize that these reactions were stereospecific.28 He found that the dimethyl ester of cis-cyclobutene-3,4-dicarboxylate gives only the dimethyl ester of cis,trans-muconic acid at 1208C (Scheme 5.9). Similar stereochemical observations were made by Criegee who found that cis-1,2,3,4tetramethylcyclobutene gives on cis,trans-3,4-dimethyl-2,4-hexadiene while the trans compound gives the trans,trans-diene at 2008C (Scheme 5.9).29

Scheme 5.9

C4H4 – C4H8

39

More early examples of the stereospecificity were provided by Adam.30 Further, the rate data for variously substituted cyclobutenes are contained in early reviews,31 and all have very low activation energies if conrotatory ring opening can occur. The stereospecificity of the cyclobutene isomerization was rationalized in the first of the Woodward– Hoffmann papers in 1965 which outlined the principle of Conservation of Orbital Symmetry in Concerted Reactions. This isomerization was defined as a conrotatory electrocyclic process. Note should be made of the possibility that the conrotatory pathway could also result in a cis,cis-2,4-hexadiene from a trans-3,4-disubstitued cyclobutene, but unfavorable steric interactions apparently intervene when a sterically large group rotates inward on the molecular system. Thus trans-1,2,3,4-tetramethylcyclobutene undergoes rearrangement with log k ¼ 13:85 2 33 600=2:3RT while the cis isomer reacts with log k ¼ 14:1 2 37 000=2:3RT so at 1778C the trans material reacts 49 times faster than the cis isomer.32 Subsequent investigations have attempted to quantify the extent of concert in the rearrangement and the difficulty in opening cyclobutenes in a disrotatory sense. Stephenson and Brauman compared the rate of isomerization of cis,cis- to trans,trans1,4-dideuterio1,3-butadiene to the rate of formation of the cis,trans-isomer in an FVP apparatus at 3 Torr and 910 K (Scheme 5.10).33 The rate constant for tt to cc isomerization was 50 times greater than that for tt to tc isomerization. At this temperature, the rate factor corresponds to a free energy difference of 7.1 kcal/mol.

Scheme 5.10

In a related work, Brauman and Golden examined the energetics of ring opening of cyclobutenes which were cis-fused to other rings that are constrained to open in a disrotatory manner. After correcting for strain energy differences, they suggested that the conrotatory ring opening mode is 15 kcal/mol lower in free energy than the disrotatory model in constrained systems.34 In a careful gas chromatographic study of the pyrolysis of cis-3,4-dimethylcyclobutene, Brauman found a 10.7 kcal/mol preference for conrotatory over random ring opening. However, after correcting for the steric difficulty of conrotatory ring opening, a 15 kcal/mol preference for conrotatory opening was suggested.35

40


In a more highly substituted system, Z,E-2,3,4,5-tetraphenyl-2,4-hexadiene was pyrolyzed for extensive periods to find “mistakes” resulting in the formation of E,Ediene.36 Here the kinetics of equilibration of the two geometric isomers of the Z,E1,1,1-trideuterio compound were known, and the reaction was assumed to proceed via the cis-3,4-dimethylcyclobutene through conrotatory closure and opening. Thus E,E-diene could only result from non-concerted or disrotatory opening. However, even after a 51-day pyrolysis at 1248C no mistakes were found indicating at least a 7.3 kcal/mol preference for conrotatory opening (Scheme 5.11).

Scheme 5.11

The preferred direction of the two possible conrotatory ring openings of cyclobutenes appears not to be due to steric effects since molecules like 3-isopropyl3-methylcyclobutene give roughly twice as much Z as E diene. This indicates a preferential “inward” rotation of the larger group.37 Further, inward rotation of a CF3 group is preferred over inward rotation of a fluorine atom.38 In general, the relative p donor ability of the substituent increases the tendency for it to rotate “outward” while the opposite is true for p acceptor substituents. Calculations reveal that the origin of this “torquoselectivity” is the stabilizing interaction of the p acceptor LUMO with the HOMO of the transition state which is the two-electron bond being broken.39 Indeed the calculated energy difference between inward and outward rotation of a substituent correlates with Taft’s soR parameters. Finally, secondary deuterium kinetic isotope effects in the ring opening of cis- and trans-dideuterio isomers are very similar, kH =kD2 ¼ 1:22 and 1.21, respectively, at 139.58C.40 However, pyrolysis of 3-deuteriocyclobutene results in formation of more Z- than E-1-deuteriobutadiene, in a ratio of 1.10: 1 and kH =kDinner ¼ 1:15 and kH =kDouter ¼ 1:04.40 SCF-MP2/6-31Gp calculations on the conrotatory transition state structure roughly reproduced the isotope effects and suggested differential extents of rehybridization at the inner and outer C –H bonds – more at the outer carbon than the inner carbon. This would appear to contradict the usual notion that more sp3 to sp2

C4H4 – C4H8

41

rehybridization would result in a larger secondary deuterium isotope effect. Alternatively, differential effect may be the result of a steric effect: C –D bonds are shorter than the C –H bonds by virtue of the anharmonicity of the C – H vibration and the lower zero-point energy of the C – D bond. 2.3

Methylenecyclopropane Degenerate Rearrangement

Methylenecyclopropane (MCP) is remarkably stable upon heating. However, at temperatures above 1508C, substituted materials reveal a degenerate isomerization which interchanges ring and exomethylene carbons, a reaction which would appear to involve the trimethylenemethane (TMM) biradical (Scheme 5.12). The TMM species, however, is unique in many ways. There is the question of spin multiplicity, singlet or triplet, and the relative energies, geometries, and reactivities of either species. It is, however, likely that the thermal reaction of MCP involves TMM singlet species.

Scheme 5.12

In studying the least perturbed MCP to date, Chesick found that logðkf þ kb Þ ¼ 14:26 2 40 400=2:3RT for the interconversion of 2-methyl-MCP and ethylidenecyclopropane.41 The activation energy is roughly 8 kcal/mol higher than what might be expected for homolytic cleavage of the highly strained cyclopropane bond (strain energy , 41 kcal=molÞ to give a biradical having resonance energy roughly comparable to that of an allyl radical recognizing that the TMM species is crossconjugated (Scheme 5.13).

Scheme 5.13

To provide information on the TMM intermediate geometry, the thermolyses of the optically active trans- and of cis-2,3-dimethyl MCP, T and C, respectively, were

42


pursued.42 The salient features of the isomerizations are as follows: (a) geometric isomerizations of the starting materials were competitive with structural isomerizations to the methylethylidenecyclopropanes ðkT!C =kT!MEC < 0:5 and kC!T =kC!MEC < 1:5 with KT=C < 3Þ; (b) the products were formed with partial inversion at the remaining chiral center from the trans-dimethyl MCP; and (c) quantification of the racemization required that at 5 –10% of the racemization occured via processes that did not involve reactions of the cis-dimethyl MCP. The data suggested that a common orthogonal DMTMM species could be responsible for both geometric and structural isomerization with inversion, and the excess racemization could be the result of some incursion of a planar, achiral TMM species. Scheme 5.14 depicts the interconversions with partitioning ratios from the orthogonal DMTMM species. Interestingly, both T and C give MEC with virtually the same rate constant.

Scheme 5.14

Subsequent work with deuterated exomethylene materials established that the products were formed with a substantial normal secondary deuterium isotope effect ðkH =kD2 < 1:30Þ; but the geometric isomerization of the starting material had virtually no isotope effect.43 The origin of the former effect would appear to be the increased inertia involved in the necessary 908 rotation of the exomethylene carbon in making the methylethylidenecyclopropanes.44 However, the lack of an isotope effect on geometric isomerization is of concern since it should have been inverse, particularly in the case of the pyrolysis of Cd2 if an intermediate TMM species were formed after the rate-determining step, namely, cleavage of the cis material. That is, more of the geometrically isomerized material should be formed because the biradical, once formed, gives the structural isomers more slowly due to the isotope effect. Nonetheless, it is likely that there is also a normal isotope on the ring opening due to the loosening of frequencies about the exomethylene;45 thus, the product of all

C4H4 – C4H8

43

isotope effects could result in the observed effects. This would then be consistent with an energy surface involving opening of both T and C to a common, orthogonal biradical which partitions to all products, consistent with Scheme 5.14. Further, the intervention of a TMM biradical (presumably singlet) and not a concerted 1,3sigmatropic shift is consistent with Chesick’s high activation energy. There is, however, one experiment which indicates that the planar TMM species is not involved directly in the formation of the 1,3-shift product. Baldwin found that optically ring deuterium labeled 2-methoxymethylmethylenecyclopropane gives geometric isomers and the 1,3-shift product; the latter being formed without any direct racemization (Scheme 5.15).46

Scheme 5.15

Why this system undergoes both one- and two-center epimerization at C2 and C3 without giving some 1,3-shift product with epimerization at C2 is unclear, especially in light of the strong computational support for accessibility of the planar TMM singlet, vide infra. Further, pyrolysis of methyl-labeled 6-methylenebicyclo[3.1.0]hexanes revealed the likelihood of at least partial intervention of the planar TMM species.47 Studies of the TMM biradical itself began in the late sixties when Dowd observed an ESR spectrum interpretable as that of the TMM triplet species from a sample of 4-methylenepyrazoline that was photolyzed in a glassy matrix at 2 1968C. Further, all six hydrogens were equivalent from the splitting of the half-field resonance line ðDms ¼ 2 transitions) thus indicating that the TMM triplet had D3h symmetry.48 Similar observations were made upon photolysis of 3-methylenecyclobutanone. Both reactions were claimed to produce MCP as the sole product. However, photolysis of the ketone at 2 778C in butadiene gives not only MCP but

44


1,4-dimethylenecyclohexane (DMCH), the dimer of TMM, as well as the adducts of TMM with butadiene. Yeshurun photolyzed 4-methylenepyrazoline in the gas phase and found MCP to be the product, but benzene-sensitized photolysis of the pyrazoline in the gas phase gave both MCP and DMCH.49 Further, addition of triplet oxygen to the photolysis suppressed the formation of DMCH (Scheme 5.16).

Scheme 5.16

All of this data suggest that cyclization is the fate of singlet TMM and triplet TMM dimerizes; further, the singlet to triplet interconversion has no temperature dependence and is irreversible. The latter suggestion is reinforced by photoelectron spectroscopy which indicates that the planar triplet is 16.1 kcal/mol more stable than the planar singlet,50 and by numerous calculations over the years, the latest of which at the 6-31Gp CASPT2N level reveals that the planar triplet is ca. 13 kcal/mol more stable than the orthogonal singlet, and the planar singlet is 1.6 kcal/mol less stable than the orthogonal singlet.51 The latter calculation is not inconsistent with the observation of excess racemization with optically active T (Scheme 5.14). The reactivity hypothesis above is on much firmer ground as a result of the work by Berson on a TMM species constrained to a five-membered ring. When 7isopropylidene-2,3-diazabicyclo[2.2.1]hept-2-ene was photolyzed at 2 1968C, a triplet ESR spectrum is observed, and the only dimeric product is formed upon warming to 2 808C.52 However, photolysis at 2 788C gives 5-isopropylidenebicyclo[2.1.0]pentane from the NMR, and this species dimerizes in a first-order reaction with log k ¼ 9:2 2 13 200=2:3RT:53 The latter reaction would appear to involve rate-determining ring opening followed by intersystem crossing (Scheme 5.17).

C4H4 – C4H8

45

Scheme 5.17

2.3.1

More Complex MCPs

The first suggestion of the degenerate rearrangment of MCPs occurred in 1932 when Feist’s ester, trans-2,3-dicarbethoxymethylenecyclopropane,54 was melted and gave two new structural isomers.55 However, the structures of all compounds were assigned only in 1952 by Ettlinger56 who also prepared the cis isomer of Feist’s acid.57 The structural assignments were confirmed by X-ray crystallographic analysis58 and by hydrogenation studies.59 The rearrangement, therefore, would appear to occur via cyclopropane bond homolysis to a TMM biradical, which recloses to give the thermodynamically more stable products as in Scheme 5.14. Subsequently, Ullman showed that the thermal rearrangement of optically active Feist’s ester gave optically active isomerization products, thereby ruling out sole intervention of a planar TMM species.60 Later, Doering and Roth reinvestigated the Feist’s ester pyrolysis and found mostly inversion of configuration at the remaining chiral center in the a,b-unsaturated product.61 Further, only very slow racemization of starting material was observed, and little, if any, cis isomer was found. Knowledge of this result misled Woodward and Hoffmann to cite the MCP rearrrangement as an example of a concerted 1,3-sigmatropic shift (Scheme 5.18).

Scheme 5.18

46


Still later, Doering examined the thermal interconversions of 3-methyl-2-cyanoethylidenecyclopropane and found that all four isomers with a cyano group on the ring were interconverted at similar rates, a result not consistent with a concerted reaction, which would predict formation of only two of these isomers, not three from the other, see for example (Scheme 5.19).

Scheme 5.19

Finally, for examples of the MCP rearrangement in ring systems, see the works by Billups,62 Dolbier,63 and Roth,64 and the review by Brandi.65 For a highly detailed study of the 2-phenylmethylenecyclopropane rearrangement, see the paper by Roth.66

3 3.1

C4H8 Cyclobutane Geometric Isomerization and Cleavage

Cyclobutane undergoes a thermally induced cleavage to ethylene with log k ¼ 15:6 2 62 500=2:3RT (Scheme 5.20).67

Scheme 5.20

At very low pressures propylene and 1-butene are also formed, but these appear to be the result of surface catalysis.68 The retro 2 þ 2 cycloaddition was thought to proceed via a biradical since the activation energy is at least equal to or higher than

C4H4 – C4H8

47

the BDE of one bond in cyclobutane. Further, studies with alkyl-substituted materials revealed that geometric isomerizations accompany cleavage; so biradical intermediates seemed to be involved, vide infra. Dervan69 examined the decomposition of 3,4-dideuterio-cis-tetrahydropyridazine and found that kcleavage =kcyclization of a presumed biradical intermediate of the geometry of starting material was 2.2. It was further found that krotation =kcyclization in this biradical was 12 ^ 3 (Scheme 5.21). Thus, it appears that the unsubstituted tetramethylene biradical undergoes rotation ca. 5.5 times faster than cleavage and 12 times faster than cyclization.

Scheme 5.21

Subsequently, Chickos pyrolyzed optically active trans-1,2-dideuteriocyclobutane and found that kisomerization =kloss opt: act: ¼ 1:5 ^ 0:4 (Scheme 5.22).70 This is consistent with a nearly, but not necessarily totally, random biradical in the reaction. Importantly, the ratio of the rates of geometric isomerization and cleavage (2.2) is very similar to cyclization/cleavage ratio observed by Dervan for the diazene described above. And, remarkably, the ratio of trans- to cis-dideuterioethylene produced at very short reaction times is not very different from that found for the cisto trans-dideuterioethylene (55:45) in Dervan’s reaction at similar temperatures. This correspondence is suggestive of similar behavior of similar biradical intermediates in the two reactions.

Scheme 5.22

48


However, Goldstein examined the product distribution from pyrolysis of all cis1,2,3,4-tetradeuteriocyclobutane and concluded that the ratio of krotation to kcleavage to kcyclization was 120 ^ 50 : 1:5 ^ 1:1 : 1:71 Whether or not these values are within the experimental error of those reported by Dervan and by Chickos is of concern since Doubleday had tried to reconcile the differences by a more complicated mechanistic scheme in which “stereorandomness is built into the formation and decay of the biradical”.72 Doubleday’s calculations involved geometry optimization at the 4 £ 4 CAS MCSCF level with a 6-31Gp basis set with energies recalculated using multireference CI (SDCI) with frequency calculations to generate the enthalpy surface. Two biradical minima were obtained: the gauche and the anti-species with both being responsible for cleavage, and with the anti-species being 2 kcal/mol more stable than the gauche biradical. The initial calculations indicated small energy differences between the gauche biradical and transition states leading from it, but subsequent calculations at the MRCI-CASSCF level with multiply polarized basis set functions provided a broad, flat potential surface for the gauche biradical and transition states with the gauche biradical being “a mere dimple” on the surface.73 The 1-ps lifetime of this species as determined by Zewail74 could only be attributed to entropy considerations assuming that the biradical, indeed, was an intermediate. 3.2

1,2-Dimethylcyclobutanes

The biradical pathway for the cyclobutane cleavage and geometric isomerization was first pursued by Walters with cis- and trans-1,2-dimethylcyclobutane which interconvert roughly one-fifth as fast as they undergo cleavage to propylene at 4008C (Scheme 5.23).75 There is also 5– 10% ethylene and 2-butene formed in the reaction, and, further, there is some preservation of stereochemistry at C1 and C2. The ratio of cis- to trans-2-butene formed from the trans compound is 0.125 while that from the cis compound is 2 at low conversions.76

Scheme 5.23

C4H4 – C4H8

49

Significantly, Dervan observed the same ratios of trans- to cis-2-butene from the trans- and cis-3,4-dimethyl-3,4,5,6-tetrahydropyridazines at 4158C suggesting that similar biradical species are being generated in both reactions. From the ratio of products, the ratio of rate constants for reaction of the two isomeric forms of, presumably, the gauche biradical could be determined.77 Thus from the transstarting material, kcleav =krot ¼ 2:8 and kcyc =krot ¼ 0:5: From the cis compound, kcleav =krot ¼ 1:3 and kcyc =krot ¼ 1:4: These data are also suggestive of a relatively flat potential energy surface, but one in which the biradical derived from the trans compounds does not easily convert to products from the biradical derived from the cis compound. Subsequently, Chickos examined the pyrolysis of cis-anti- and trans-1,2dimethyl-cis-3,4-dideuteriocyclobutane at 5108C in a flow system. The cis-anticis isomer undergoes fragmentation to cis- and trans-propene-d1 in a 1.5:1 ratio, while the ratio of the trans- to cis-propenes from the trans-cis isomer is 1:1 at conversions of 26 and 56 %, respectively (Scheme 5.24).78 The Principle of Conservation of Orbital Symmetry predicts that the retro 2 þ 2 reaction should proceed in a s þ a fashion if it were a concerted process. If this were the case, then the propylene formed from the cis-anti-cis isomer should have been exactly a 1:1 mixture of deuterium isomers. The fact that it is not a 1:1 mixture rules out concert as the sole reaction pathway initiated by cleavage of the more substituted bond. Significantly, the ratios of the 2-butenes formed by cleavage of the less substituted bond was similar to that observed by Walters as described above. Further, the ethylene formed was entirely scrambled just as was observed much earlier by Srinivasan with the cis-anti-1,2-dimethyl-cis-3,4-dideuteriocyclobutane isomer upon heating in a static system at 4258C.79

Scheme 5.24

In the pyrolysis of the cis compound, the recovered dimethyl cyclobutane consisted of a nearly 5:1 ratio of cis- to trans-1,2-dimethyl materials. The recovered cis material contained approximately 40% of the cis-syn-cis isomer relative to the trans material. Thus, the stereochemistry of the starting material was not highly compromised in this case even though the reaction proceeded through more than one

50


half-life. In the case of the trans compound, there was little cis-1,2-dimethylcyclobutane (2%) formed at 25% reaction. It is significant that the double rotation product from the cis isomer is formed only to the extent of roughly 40% of the single rotation product. There are, of course, two other bonds that can be broken to result in single rotation besides the C1 –C2 bond, but this would occur only to the extent of roughly 10% of the more substituted bond cleavage; so double rotation would appear to occur roughly at the same rate as either single rotation process in this case. 3.3

1,1,2,2-Tetramethylenecyclobutane

In the case of 1,1,2,2-tetramethylenecyclobutane made optically active by virtue of labeling trans-methyls with deuterium, pyrolytic retro 2 þ 2 cycloaddition is much faster than either double or single rotation.80 This must represent the difficulty of reclosure of any biradical to generate two contiguous quaternary centers relative to cleavage.

REFERENCES 1. For reviews see G. Maier, Angew. Chem. Int. Ed. Engl., 27, 309 (1988). G. Maier, H.P. Reisenauer, T. Preiss, H. Paci, D. Ju¨rgen, R. Tross and S. Senger, Pure Appl. Chem., 69, 113 (1997). T. Bally and S. Masamune, Tetrahedron, 36, 343 (1980). 2. G. Maier, H.Y.-G. Hartan and T. Sayrac, Angew. Chem. Int. Ed. Engl., 15, 226 (1976). S. Masamune, Sugihara, K. Morio and J.E. Bertie, Can. J. Chem., 54, 2679 (1976). 3. D.W. Whiteman and B.K. Carpenter, J. Am. Chem. Soc., 102, 4272 (1980). D.W. Whiteman and B.K. Carpenter, J. Am. Chem. Soc., 104, 6473 (1982). 4. B.K. Carpenter, J. Am. Chem. Soc., 105, 1700 (1983). 5. H. Kollmar and V. Staemmler, J. Am. Chem. Soc., 99, 3583 (1977). W.T. Borden and E.R. Davidson, J. Am. Chem. Soc., 100, 388 (1979). J.A. Jafri and M.D. Newton, J. Am. Chem. Soc., 100, 5012 (1978). 6. Y. Li and K.N. Houk, J. Am. Chem. Soc., 118, 880 (1996). 7. D.J. Cram, M.E. Tanner and R. Thomas, Angew. Chem. Int. Ed. Engl., 30, 1024 (1991). D.J. Cram, Nature, 356, 29 (1992). 8. G. Maier, H.-O. Kalinowski and K. Euler, Angew. Chem. Int. Ed. Engl., 21, 693 (1982). 9. H. Irngartinger and M. Nixdorf, Angew. Chem. Int. Ed. Engl., 22, 403 (1983). 10. M. Balci, M.L. McKee, P. von R. Schleyer, J. Phys. Chem. A, 104, 1246 (2000). 11. G. Maier, S. Pfriem, U. Scha¨fer and R. Matusch, Angew. Chem. Intl. Ed. Engl., 17, 520 (1978). 12. H. Irngarteinger, R. John, G. Maier and R. Emerick, Angew. Chem. Int. Ed. Engl., 26, 356 (1987). 13. M.N. Glukortsev, S. Laiter and A. Pross, J. Phys. Chem., 99, 6828 (1995). See B.S. Jursic, J. Mol. Struct. Theochem., 507, 185 (2000), for a summary of calculations on the parent system. 14. G. Maier, D. Born, I. Bauer, R. Wolf, R. Boese and D. Cremer, Chem. Ber., 127, 173 (1994). 15. G. Maier, J. Neudert, O. Wolf, D. Pappusch, A. Sekiguchi, M. Tanaka and T. Matsuo, J. Am. Chem. Soc., 124, 13819 (2002). 16. D.M. Lemal, F. Menger and G.W. Clark, J. Am. Chem. Soc., 85, 2529 (1963).

C4H4 – C4H8 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52.

51

R. Srinivasan, A.A. Levi and I. Haller, J. Phys. Chem., 69, 1775 (1965). H.M. Frey and I.D.R. Stevens, Trans. Faraday Soc., 61, 90 (1965). W. von E. Doering and J.F. Coburn, Jr., Tetrahedron Lett., 991 (1965). G.C. Closs and P.E. Pfeffer, J. Am. Chem. Soc., 90, 2452 (1968). M.J.S. Dewar and S. Kirschner, J. Am. Chem. Soc., 97, 2931 (1975). P.B. Shevlin and M.L. Mckee, J. Am. Chem. Soc., 110, 1666 (1988). K.A. Nguyen and M.S. Gordon, J. Am. Chem. Soc., 117, 3835 (1995). M. Christl and G. Bruntrup, Chem. Ber., 107, 3908 (1974). M. Christl, U. Heineman and W. Kristof, J. Am. Chem. Soc., 97, 2299 (1975). S.R. Davis, K.A. Nguyen, K. Lammertsma, D.L. Mattern and J.E. Walker, J. Phys. Chem. A, 107, 198 (2003). K.B. Wiberg and G. Szeimies, Tetrahedron. Lett., 1235 (1968). W. Cooper and W.D. Walters, J. Am. Chem. Soc., 80, 4220 (1958). R.W. Carr, Jr. and W.D. Walters, J. Phys. Chem., 69, 1073 (1965). E. Vogel, Angew. Chem., 66, 640 (1954). E. Vogel, Angew. Chem., 68, 189 (1956). E. Vogel, Justus Liebigs Ann. Chem., 615, 14 (1958). R. Criegee and K. Noll, Justus Liebigs Ann. Chem., 627, 1 (1959). W. Adam, Chem. Ber., 97, 1811 (1965). R. Criegee, D. Seebach, R.E. Winter, B. Bo¨rretzen and H.-A. Brune, Chem. Ber., 98, 2339 (1965). R. Criegee, Angew. Chem. Int. Ed. Engl., 7, 559 (1968). G.R. Branton, H.M. Frey and R.F. Skinner, Trans. Faraday Soc., 62, 1546 (1966). L.M. Stephenson, R.V. Gemmer and J.I. Brauman, J. Am. Chem. Soc., 94, 8620 (1972). J.I. Brauman and D.M. Golden, J. Am. Chem. Soc., 90, 1920 (1968). J.I. Brauman and W.C. Archie, Jr., J. Am. Chem. Soc., 94, 4262 (1972). G.A. Doorakin and H.H. Freedman, J. Am. Chem. Soc., 90, 5310 (1968), see also p. 6896. M.J. Curry and I.D.R. Stevens, J. Chem. Soc. Perkin Trans., 2, 1391 (1980). W.R. Dolbier, Jr., H. Koroniak, D.J. Burton, A.R. Bailey, G.S. Shaw and S.W. Hansen, J. Am. Chem. Soc., 106, 1871 (1984). W. Kirmse, N.G. Rondan and K.N. Houk, J. Am. Chem. Soc., 106, 7989 (1984). For a review see W.R. Dolbier, Jr., H. Koroniak, K.N. Houk and C. Sheu, Acc. Chem. Res., 29, 471 (1996). J.E. Baldwin, V.P. Reddy, L.J. Schaad and B.A. Hess, Jr., J. Am. Chem. Soc., 110, 8555 (1988), and ref. 9 of this paper. J.P. Chesick, J. Am. Chem. Soc., 85, 2720 (1962). J.J. Gajewski, J. Am. Chem. Soc., 90, 7178 (1968). J.J. Gajewski, J. Am. Chem. Soc., 93, 4450 (1971). J.J. Gajewski, C.W. Benner, B.N. Stahly, R.F. Hall and R.I. Sato, Tetrahedron, 38, 853 (1982). L.P. Olson, S. Niwayama, H.-Y. Yoo, K.N. Houk, N.J. Harris and J.J. Gajewski, J. Am. Chem. Soc., 118, 886 (1996). J.J. Gajewski, L.P. Olson and K.J. Tupper, J. Am. Chem. Soc., 115, 4548 (1993). J.E. Baldwin and G.E.C. Chang, Tetrahedron, 38, 825 (1982). W.R. Roth and G. Wegener, Angew. Chem. Int. Ed. Engl., 14, 758 (1975). see also J.J. Gajewski and S.K. Chou, J. Am. Chem. Soc., 99, 5696 (1977). P. Dowd, A. Gold and K. Sachdev, J. Am. Chem. Soc., 90, 2715 (1968). P. Dowd, Acc. Chem. Res., 5, 242 (1972), and refs citied therein. J.J. Gajewski, A. Yeshurun and E.J. Bair, J. Am. Chem. Soc., 94, 2138 (1972). P.G. Wenthold, J. Hu, R.R. Squires and W.C. Lineberger, J. Am. Chem. Soc., 118, 475 (1996). S.B. Lewis, D.A. Hrovat, S.J. Getty and W.T. Borden, J. Chem. Soc., PII, 2339 (1999). J.A. Berson, R.J. Bushby, J.M. McBride and M. Tremelling, J. Am. Chem. Soc., 93, 1544 (1971).

52 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80.

Hydrocarbon Thermal Isomerization M. Rule, M.G. Lazzara and J.A. Berson, J. Am. Chem. Soc., 101, 7091 (1979). F. Feist, Ber. Dtsch. Chem. Ges., 26, 747 (1893). G.A.R. Kon and H.R. Naji, J. Chem. Soc., 2557 (1932). M.G. Ettlinger, J. Am. Chem. Soc., 74, 5805 (1952). M.G. Ettlinger and F. Kennedy, Chem. Ind. (Lond.), 166 (1956). D. Lloyd, T.C. Downie and J.C. Speakman, Chem. Ind. (Lond.), 492 (1954). D.R. Petersen, Chem. Ind. (Lond.), 904 (1956). E.F. Ulman, J. Am. Chem. Soc., 81, 5386 (1959). E.F. Ulman, J. Am. Chem. Soc., 82, 505 (1960). W. von E. Doering and H.-D. Roth, Tetrahedron, 26, 2825 (1970). W.E. Billups, B.A. Baker, K.H. Leavell and E.S. Lewis, J. Org. Chem., 40, 1702 (1975). W.R. Dolbier, Jr., K. Akiba, J.M. Riemann, C.A. Harmon, M. Bertrand, A. Bezaquet and M. Santelli, J. Am. Chem. Soc., 93, 3933 (1971). W.R. Roth, H. Wildt and A. Schlemenat, Eur. J. Org. Chem., 4081 (2001). A. Brandi and A. Goti, Chem. Rev., 98, 589 (1998). W.R. Roth, M. Winzer, H.-W. Lennartz and R. Boese, Chem. Ber., 126, 2717 (1993). C.T. Genaux, F. Kern and W.D. Walters, J. Am. Chem. Soc., 75, 6196 (1953). J.N. Butler and R.B. Ogawa, J. Am. Chem. Soc., 85, 3346 (1963). R.W. Vreeland and D.F. Swinehart, J. Am. Chem. Soc., 85, 3349 (1963). P.C. Beadle, D.M. Golden, K.D. King and S.W. Benson, J. Am. Chem. Soc., 94, 2943 (1972). D.S. Santilli and P.B. Dervan, J. Am. Chem. Soc., 101, 3663 (1979). P.B. Dervan and D.S. Santilli, J. Am. Chem. Soc., 102, 3863 (1980). J.S. Chickos, A. Annamalai and T.A. Keiderling, J. Am. Chem. Soc., 108, 4398 (1986). The analysis of mechanism is similar to that for optically active trans-1,2dideuteriocyclopropane – see Chapter 4, Section 2.1. M.J. Goldstein, M.J. Cannarsa, T. Kinoshita and R.F. Koniz, Stud. Org. Chem. (Amsterdam), 31, 121 (1987). C. Doubleday, Jr., J. Am. Chem. Soc., 115, 11968 (1993). C. Doubleday, Jr., J. Phys. Chem., 100, 15083 (1996). S. Pedersen, J.L. Herek and H.A. Zewail, Science, 266, 1359 (1994). H.R. Gerberich and W.D. Walters, J. Am. Chem. Soc., 83, 3935 (1961), see also p. 4884. R.W. Carr, Jr. and W.D. Walters, J. Am. Chem. Soc., 88, 884 (1966). P.B. Dervan, T. Uyehara and D.S. Santilli, J. Am. Chem. Soc., 101, 2069 (1979). Y.-S. Wang and J.S. Chickos, J. Org. Chem., 52, 4776 (1987). R. Srinivasan and J.N.C. Hsu, J. Chem. Soc. Chem. Commun., 1213 (1972). J.A. Berson, D.C. Tompkins and G. Jones, II, J. Am. Chem. Soc., 92, 5799 (1970).

6 C5H6 –C5H10

CONTENTS 1 C5H6 1.1 1,3-Cyclopentadiene 1,5-Hydrogen and Alkyl Shifts 1.2 Bicyclo[2.1.0]pentene to Cyclopentadiene 1.3 Vinylidenecyclopropane to 1,2-Dimethylenecyclopropane 1.4 [1.1.1]Propellane to 1,2-Dimethylenecyclopropane 2 C5H8 2.1 Vinylcyclopropane to Cyclopentene 2.1.1 Substituted Vinylcyclopropanes 2.2 Bicyclo[2.1.0]pentane 2.2.1 Cyclopentane-1,3-diyl Formation from 2,3-Diazabicyclo[2.2.1]hept-2-ene 2.3 Bicyclo[1.1.1]pentane 2.4 Methylenecyclobutane Degenerate Rearrangement and Retro 2 þ 2 Cleavage 2.5 Spiropentane 2.6 Allene – Ethylene Cycloadditions 2.7 cis-1,3-Pentadiene Hydrogen Shift 3 C5H10 3.1 1-Pentene Retro Ene Reaction References

1 1.1

53 53 56 57 58 59 59 63 66 67 68 68 72 77 79 80 80 81

C5H6 1,3-Cyclopentadiene 1,5-Hydrogen and Alkyl Shifts

Cyclopentadiene (CPD) undergoes an intramolecular vicinal (1,5-) hydrogen shift. The reaction was first recognized when a mixture of double bond isomers was

54


obtained when specific alkyl-substituted CPDs were heated above room temperature.1 Roth studied the kinetics of rearrangement of 5-protio-1,2,3,4,5-pentadeuteriocyclopentadiene and found log k ¼ 12:11 2 24 300=2:3RT for conversion of starting material to the 1-protio material, and this material rearranges to the 2-protio isomer. The sequence of conversions is consistent with 1,5-deuterium shifts (Scheme 6.1).2

Scheme 6.1

The possibility that the reaction could proceed by acid dissociation was dismissed by the observation that the reaction occurred only 1.4 times faster in acetone than in the gas phase.2 Thus, the reaction would appear to be an “allowed”, concerted reaction, particularly since the BDE of the doubly allylic C –H is roughly 75 or 50 kcal/mol higher than the observed activation energy. An insight into the nature of the transition state was provided by deuterium kinetic isotope effect studies in which kH =kD for conversion of 5-methyl-CPD to 1-methyl-CPD using the perdeuterio compound was used to determine kD : Here, log kH =kD ¼ 21:0 2 2450=2:3RT; which indicates hydrogen, and not deuterium, tunneling although the large temperature dependence was also attributed to a non-linear transition state with nearly complete loss of one C – H bending mode (Scheme 6.2).3

Scheme 6.2

Recognition should be made of the fact that kD includes both a primary and a secondary kinetic isotope effect, but the latter is usually much smaller than the former and should not contribute greatly to the large difference in activation energies (for “normal” primary hydrogen kinetic isotope effects, the difference in activation energies is usually around 1 kcal/mol).

C5H6 – C5H10

55

Alkyl shifts in CPD have been observed. For instance, 1,5,5-trimethyl-CPD gives all possible double bond isomers of 1,2,3-trimethyl-CPD above 2008C with DH ‡ ¼ 40:3 kcal=mol and DS‡ ¼ 24 e:u: or DH ‡ ¼ 43:8 kcal=mol and DS‡ ¼ 0:7 e:u: (Scheme 6.3).4

Scheme 6.3

However, Willcott found evidence for a radical chain process in these reactions.5 Thus,1,5-dimethyl-5-deuteriomethyl-CPD gives 23% d0 and 15% d2 product in addition to the expected d1 material. Nonetheless, spiro[4.4]nona-1,3-diene undergoes ring expansion to bicyclo[4.3.0]nonadienes with “allowed” retention of stereochemistry at the migrating carbon, so the reaction appears to be concerted in these cases (see Chapter 10, Section 3). Interestingly, 5-trimethylsilyl-CPD exhibits a temperature-dependent 1H NMR spectrum above room temperature allowing estimation of a rate constant of 1000 s21 at 808C.6 A 1,5-silyl shift would account for the absence of other isomers. Subsequently, Ashe directly observed the other isomers resulting from hydrogen shift and showed that 1,5-silyl migration is 106 times faster than the 1,5-hydrogen shift (Scheme 6.4).7

Scheme 6.4

Finally, migrations of other groups around CPD, particularly of metals, have been well-established,8 but circumambulatory migration of a carbocation around the ring with symmetry-allowed inversion of this group at each step is particularly intriguing (Scheme 6.5).9 In addition, the migration of ester groups has been studied by Hoffmann.10

56


Scheme 6.5

1.2

Bicyclo[2.1.0]pentene to Cyclopentadiene

Brauman and Golden reported the isomerization of bicyclo[2.1.0]pentene (BCP) (which is formed by photolysis of CPD), to CPD with log k ¼ 14:21 2 26 990=2:3RT (Scheme 6.6).11 The authors assumed, quite reasonably, that the reaction involved bridgehead bond cleavage in a disrotatory sense. This was confirmed by Baldwin who found that vicinal bis-carbon-13-labeled BCP gave only vicinally labeled CPD, which removes from consideration the carbon skeletal rearrangements.12

Scheme 6.6

The activation energy is remarkably high considering the greater than 50 kcal/mol strain relief which if superimposed on the cyclobutene to butadiene reaction energetics would suggest a negative activation energy were it not for the fact that the latter is an allowed conrotatory process while the former would appear to be a “forbidden” process. The ring opening reaction is accompanied by an extraordinarily large deuterium kinetic isotope effect at the bridgehead positions. With 1,2,4-trideuterio-5,5dimethylbicyclo[2.1.0]pentene, kH =kD3 is 1.85 at 24.68C.13 Since the 2-deuterio derivative has a very small isotope effect, it must be the bridgehead hydrogens that are responding to deuterium substitution. Magnetic and intersystem crossing isotope effects were ruled out, and changes in hybridization are almost certainly insufficient to cause such an effect. Perhaps it is simply the superposition of a primary isotope effect resulting from the motion of hydrogens from reactant to transition state. Some hint of this follows from the temperature dependence, which while not remarkable, reveals a ratio of frequency factors, AH =AD ¼ 0:54; which is typically more associated with tunneling (when tunneling is not involved, the value approaches ˚ in unity). Calculations at the HF/3-21G level reveal that the C1 – C4 bond is 1.82 A the transition state, but the system has not extensively depyramidalized,14 which is not inconsistent with an early transition state for this very exothermic reaction. Another possible reaction, namely circumambulatory 1,3-shifts of the C5 carbon in BCP have not been observed except with carbomethoxy and cyano substitution.

C5H6 – C5H10

57

Thus, endo-5-carbomethoxy-1,5-dimethylbicyclo[2.1.0]pentene gives the 2,5dimethyl isomer at only 08C with exclusive inversion at C5 (Scheme 6.7).15

Scheme 6.7

The rate of ring walk is roughly 13 times that for ring opening with a free energy of activation for the ring walk of 21.7 kcal/mol. However, the rate of ring walk for the exo-5-carbomethoxy isomer was only one-thirtieth of that for ring opening. It would appear that radical stabilizing groups sufficiently weaken the cyclopropane ring bonds to C5 to allow the ring walk reaction. Jensen calculated the energy surface for the walk reaction in the parent case at various levels of theory and found the inversion transition state to be 8 kcal/mol lower than a retention transition state with CASSCF/3-21Gp.16 Subsequent calculations at the CASPT2N/CASSCF(8/8)/c-31G(d) level revealed 11.1 kcal/mol difference in the inversion/retention transition states.17 1.3

Vinylidenecyclopropane to 1,2-Dimethylenecyclopropane

Crandall first reported formation of 1,2-dimethylenecyclopropanes in the pyrolysis of vinylidenecyclopropanes in the per-methyl series.18 The rearrangement would appear to involve cleavage of the C2 – C3 bond to give a trimethylenemethane biradical species which reforms the cyclopropane ring between different carbons (Scheme 6.8).

Scheme 6.8

Subsequent work by Roth revealed that reversible 1,2- and 1,3-bond fission occurs rapidly prior to rearrangement in these systems probably because of stabilization of radicals by the remote double bond.19 Conia reported the conversion of vinylidenecyclopropane itself to some 1,2dimethylenecyclopropane upon heating at 3208C for 3 min in a static system.20 Subsequently, Aue reported an estimate of the free energy of activation at 1508C of

58


41 kcal/mol,21 a value that appears too high considering the observations by Szeimies with [1.1.1]propellane (Scheme 6.9).

Scheme 6.9

Other derivatives have been studied.22 1.4

[1.1.1]Propellane to 1,2-Dimethylenecyclopropane

Wiberg first synthesized and pyrolyzed [1.1.1]propellane in solution to give methylenecyclobutene at only 1148C.23 However, Szeimies subsequently heated the propellane in a flow system at 4308C and found 1,2-dimethylenecyclopropane to be the major product.24 More recently, the reaction was found to give an 87:13 mixture of the dimethylenecyclopropane and vinylidenecyclopropane, which is invariant with the extent of reaction and appears to be an equilibrium mixture of the two (Scheme 6.9). Further, the activation parameters for loss of the propellane were determined to be log k ¼ 14:02 2 39 660=2:3RT:25 The reaction appears to resemble the retro 2 þ 2 cycloaddition of bicyclo[1.1.0]butanes. 3-Methylenecyclobutene was also formed as a result of heterogeneous catalysis. Calculations at various levels of theory, DFT and CAS (6,6)/6-311Gp, indicated that one bond cleavage is involved to give an intermediate resembling a non-planar carbene complex of a double bond which gives a higher energy transition state to form 1,2-dimethylenecyclopropane.25 This non-planar carbene species, however, was not a minimum at the CCSD/6-311Gp level, and the reaction appears concerted at this level (Scheme 6.10).

Scheme 6.10

C5H6 – C5H10

59

It is also interesting that the calculated energy of a planar 3-methylenecyclobutylidene, which could be responsible for formation of methylenecyclobutene by a vicinal hydride shift and for inversion of the propellane, is similar to those of the transition states forming the cyclopropane products. Remarkable about the propellane is the substantial kinetic and thermodynamic stability of the material23 and the fact that what might appear to be the weakest bond, namely the bridgehead – bridgehead bond, is not cleaved in the high-temperature process in the absence of other molecules. Of interest is the fact that the parent material does polymerize via this bond generating remarkable materials.26 Related reactions involving di- and trimethylene bridged propellanes, which give interesting and understandable products in this context, were also studied by Szeimies (Scheme 6.11).24

Scheme 6.11

2 2.1

C5H8 Vinylcyclopropane to Cyclopentene

Overberger discovered that vinylcyclopropane rearranged to cyclopentene on heating.27 Minor amounts of hydrogen-shifted, ring-opened isomers were also observed. The Arrhenius parameters for all processes were determined,28 in particular, that for formation of cyclopentene is log k ¼ 13:6 2 49 600=2:3RT: The lower activation energy for this reaction relative to those for reaction of cyclopropane is consistent with ring opening to an allylically stabilized biradical followed by reclosure with allylic rearrangement or by slower vicinal hydrogen shifts (Scheme 6.12).

60


Scheme 6.12

That a biradical is a likely intermediate follows from the fact that the activation energy for the rearrangement is at least equal to, if not higher, than a thermochemical estimate of the enthalpy difference between the starting material and a biradical having no interaction between the radical sites. Evidence for formation of a biradical intermediate in the parent system was found when a mixture of trans- and cis-2-deuteriovinylcyclopropane with the trans isomer in excess gave a nearly 1:1 mixture of the two isomers with log k ¼ 14:02 2 47 100=2:3RT; which is at least five times faster than the rearrangement.29 It must be recognized as well that the rearrangement reaction requires an s-cis arrangement of the substituted allyl moiety while geometric isomerization can proceed either through an s-cis or s-trans allylic radical (Scheme 6.13).

Scheme 6.13

Moreover, it is possible that the geometric and structural isomerizations proceed via separate pathways, one biradical, the other concerted. It is even possible that both reactions are concerted and not involve any biradicals at all! These were the questions that occupied attention for more than 40 years. Subsequently, evidence was provided for a randomly rotating biradical in the geometric isomerization of vinylcyclopropane.30 Pyrolysis of cis,anti-2,3-dideuterio-1-vinylcyclopropane to only 20% conversion to cyclopentene allowed recovery of labeled vinylcyclopropane. In the recovered material, there was hydrogen at the anti-methylene position at 0.7 ppm in the NMR signal. The deuterium-decoupled spectrum revealed a six-line pattern, a doublet of doublets due to the trans isomer and a doublet due to the cis,syn isomer. Integration indicated a 2:1 ratio, respectively. Assuming only C1, C2 bond fission, which is warranted by the low activation energy for reaction in comparison with cyclopropane itself, a 2:1 ratio of isomers under kinetic control is consistent with intermediacy of a randomly

C5H6 – C5H10

61

rotating biradical or with a collection of biradical conformations that give rise to the statistical isomer distribution (Scheme 6.14).

Scheme 6.14

Concern for the stereochemistry of the 1,3-shift portion of the reaction, i.e. the pathway for formation of cyclopentene, was addressed by examination of numerous substituted materials, but ultimate resolution of the problem was provided by pyrolysis31 of syn-E-2,3,20 -trideuteriovinylcyclopropane at 3008C to roughly only 1% conversion, which resulted in a 16:60:24 ratio of Z,Z-, E,Z-, and E,E-3,4,5trideuteriocyclopentene, respectively. Pyrolysis of syn-Z-2,3,20 -trideuteriovinylcyclopropane to obtain roughly 1% of cyclopentene product resulted in a 23:41:36 ratio of Z,Z-, E,Z-, and E,E-3,4,5-trideuteriocyclopentene, respectively (Scheme 6.15). The Z,Z- and E,E- products from syn-E are formed by the ar and ai pathways, respectively, and these same products from syn-Z are formed by the sr and si pathways, respectively, thus providing data on the relative utilization of the four pathways.

Scheme 6.15

A minor (because of the very short reaction times) correction for the equilibration of the starting materials to the various stereoisomers30 allowed calculation of the relative rates of the si, sr, ar, and ai pathways to be 40:23:13:24. Thus there is little

62


evidence for dominant Orbital Symmetry Control in this 1,3-shift, but the slightly less than random distribution of the stereoisomers has been of concern. Theory has been applied to this rearrangement at a number of levels. Density Functional Theory was used to obtain an energy surface that predicted essentially a random product distribution.32 Calculations using four-electron MCSCF theory with a 6-31Gp basis set found no intermediates on the potential energy surface for the reaction.33 Starting with the cisoid conformation of the vinyl group, two pathways to a transition state, TSG, which is responsible for geometric isomerization were found depending on the direction of the rotation around the C2,C3 bond. Further, each of these transition states leads to a still higher energy transition state, TSSI, which is responsible for the 1,3-shifts. From one, the si and ai pathways are followed while from the other, the sr and ar pathways are followed although the pathway to the antarafacial products is slightly higher in energy, ca. 0.3 kcal/mol. Scheme 6.16 provides the details.

Scheme 6.16

C5H6 – C5H10

63

Subsequently, the semi-empirical MO program, AM1, was parameterized to give the MCSCF energy surface so that dynamics calculations could be performed.34 Different starting energies distributed in different normal modes provided trajectories to the rearrangement products. With the lowest energy and all vibrations at their ZPE level, the si trajectory was utilized most often (71%) with all the other pathways utilized to roughly equal extents. Only with higher starting energies was a more random product distribution obtained, and at 9.82 kcal/mol with 7.8 kcal/mol in the IRC, an si:sr:ar:ai ratio equal to 40:24:8:27, respectively, was obtained. This is remarkably similar to that observed. The authors further point out that the suprafacial products arise via rotation of the vinyl group toward C2, but the antarafacial products come from the biradical resulting from rotation of the vinyl group away from C2, that is via species TS-ca of Scheme 6.16. This latter species must revert back to the other species to the right of the scheme in addition to giving geometrically isomerized starting material. 2.1.1

Substituted Vinylcyclopropanes

Normal secondary deuterium KIEs at C2 and C20 of vinylcyclopropane in its conversion to cyclopentene have been observed.35 The latter was subsequently confirmed as being 1.21 for two deuteriums at 3008C.36 The origin of this normal effect would appear to be inclusion of a rotational mode of C20 along the reaction coordinate in the formation of cyclopentene from TSSI; thus, the difference in ZPEs of that mode with and without deuterium at C20 in the reactants is lost in the transition state for the 1,3-shift. Methyl substitution was utilized early on to probe the energy surface of the vinylcyclopropane rearrangement. The possibility that formation of an intermediate biradical which could also reclose back to vinylcyclopropane was explored separately by Frey and by Roth in an examination of cis- and trans-2methylvinylcyclopropane.37 The cis material undergoes a relatively fast homo1,5-hydrogen shift to Z-1,4-hexadiene with log k ¼ 11:03 2 31 200=2:3RT while the trans compound gives both the Z-diene and what was claimed to be 3methylcyclopentene with log k ¼ 14:78 2 48 640=2:3RT and log k ¼ 13:67 2 48 640=2:3RT; respectively. These observations are not inconsistent with the results of the parent system with regard to reversible ring opening except that the formation of diene intercedes when it is stereoelectronically possible, i.e. in the cis isomer (Scheme 6.17). (Note that the vinyl group must lie over the cyclopropane ring for the hydrogen shift to proceed.)

Scheme 6.17

64


However, the formation of 3-methylcyclopentene requires cleavage of the less substituted bond which was acknowledged to be less likely by the authors. Subsequently, the 1,3-shift product was identified as 4-methylcyclopentene, and it was formed with log k ¼ 13:67 2 48 640=2:3RT:38 The first attempt to determine the stereochemistry of vinylcyclopropane rearrangement utilized methyl groups at both C2 and C20 . Thus, optically active trans-2-methyl-1-(trans-propenyl)cyclopropane was pyrolyzed, and after correction for the racemization of starting material, it was concluded that the cyclopentenes were formed in a 65:22:8:5 ratio via the si; sr; ar; ai pathways, respectively.39 Thus, 73% of the reaction occurs by the allowed pathway. However, this result could have been prejudiced by the fact that the si and ar products are more stable than the forbidden products. Unfortunately, cis isomer could not be subjected to the pyrolytic rearrangement because another reaction, namely the homo-1,5-hydrogen shift to substituted dienes occurred (Scheme 6.18).

Scheme 6.18

In an effort to determine the stereochemistry of the 1,3-shift of what at the time was thought to be a minimally perturbed vinylcyclopropane system, which also would not suffer from extensive geometric isomerization via an s-trans alkylallylic species, Olson prepared and pyrolyzed cis-2,3-dideuterio-trans-1-(Z-2-deuterio-1-tert-butylethenyl)cyclopropane.40 The 1H NMR of the epoxide of the major cyclopentene product from short-term pyrolysis revealed greater than 90% suprafacial inversion stereochemistry in the rearrangement despite the fact that the stereochemistry of the starting material was also compromised in a faster reaction, apparently via reversible formation of an s-cis alkylallylic species (Scheme 6.19).

Scheme 6.19

C5H6 – C5H10

65

Given the minimal perturbation at the migration terminus and the migrating carbon, the result would appear contrary to that observed in the parent system. Subsequently it was discovered using DFT approaches that the tert-butyl group sufficiently distorted the allylic radical so as to promote what appears to be an “allowed” reaction.41 Earlier, the pyrolyses of optically active trans- and cis-2-isopropenyl-1cyanocyclopropane were examined with the finding that optical and geometric isomerizations of starting materials occurred 20 – 30 times faster than structural rearrangement to 4-cyano-1-methylcyclopentene.42 The optical isomerization of the trans material is more than twice as fast as geometric isomerization, but with the cis material the rates of both reactions were within 10% of one another. Moreover, the cyano-bearing carbon rotates roughly twice as fast as the isopropenyl-bearing carbon in the geometric interconversion as judged by the relative rates of enantiomers produced in each case. In addition, the stereochemical fate of the migrating carbon in the 1,3-shift was examined at very short reaction times where the stereointegrity of the starting material had not been severely compromised. The major enantiomer of product from both the (2 )-trans and (þ )-cis (epimeric at C1) materials is the same, so the trans material undergoes dominant (2.4:1) inversion at the migrating carbon, C1, while the cis isomer reveals a 1.5-fold preference for retention at C1 in the rearrangement (Scheme 6.20).

Scheme 6.20

Recognizing that cyano rotation is preferred over isopropenyl rotation and suggesting that the direction of rotation about C1 might be so as to rotate the cyano group outward, the same orthogonal biradical was proposed to account for the major 1,3-shift product in each case. Closure of this intermediate by a least motion

66


pathway would give the major product. The other stereopathway could be due to inward rotation of the cyano group or possibly competitive rotation about the isopropenyl-bearing carbon. For a recent review of the vinylcyclopropane rearrangement, see Baldwin in Chemical Reviews.43 2.2

Bicyclo[2.1.0]pentane

Bicyclo[2.1.0]pentane gives cyclopentene and 1,4-pentadiene upon heating, with log k ¼ 14:1 2 45 600=2:3RT and 14:4 2 52 300=2:3RT; respectively (Scheme 6.21).44

Scheme 6.21

Chesick reported similar activation parameters.45 Baldwin reported activation parameters for interconversion of the two cis-2,3-dideuteriobicyclo[2.1.0]pentanes finding log k ¼ 13:9 2 37 800=2:3RT; which provides evidence for pre-equilibrium bridgehead double inversion.46 Work on substituted materials was also reported, but earlier Chesick reported similar activation parameters for the 2-methyl derivative.47 It seems reasonable that the geometric isomerization would occur by a 1,3cyclopentane biradical, since the estimated strength of the bridgehead bond is roughly 30 kcal/mol due to relief of strain (Scheme 6.22). Further, this species could undergo both vicinal hydrogen shift and bond cleavage to the structurally isomerized products.

Scheme 6.22

Remarkably, Berson found that the cleavage reaction occurs with substantial stereospecificity (10 – 30-fold) in the sense of a conrotatory opening, judging by the results of pyrolysis of the trans- and the cis,syn- and cis,anti-2,3-dimethylbicyclo[2.1.0]pentanes (see, e.g. Scheme 6.23).48 The result suggests that the biradical is a symmetric species formed by disrotatory motions in the ring opening, so cleavage must occur in a conrotatory sense not unlike that of cyclobutene.

C5H6 – C5H10

67

Scheme 6.23

It is of interest that the triplet biradical has been characterized by ESR and that the signals disappear above 1.3 K to give bicyclo[2.1.0]pentane.49 However, there is no temperature dependence for the reaction below 20 K suggesting that intersystem crossing to the singlet is the slow step, and ring closure is very fast with a barrier less than 2 kcal/mol perhaps allowing tunneling or more complicated phenomena.50 This implies a very small singlet –triplet gap, which was confirmed by calculation.51 The calculation also indicates that the nearly planar biradical has but a small barrier to reclosure despite the thermochemical estimates of the central bond BDE. The thermolyses of substituted bicyclo[2.10]pentanes have been studied,52 but the most intriguing observation is that by Dougherty who found that 1,4-diphenylbicyclo[2.1.0]pentane undergoes bridgehead double inversion with DH ‡ ¼ 12:2 kcal=mol and DS‡ ¼ 216:4 e:u:53 The low enthalpy of activation is a tribute to the stability of the radical sites imparted by the phenyl groups. Further, this biradical is remarkably persistent in its triplet state at room temperature.54 2.2.1 Cyclopentane-1,3-diyl Formation from 2,3-Diazabicyclo[2.2.1]hept-2-ene Interest in the cyclopentane1,3-biradical stems not only from its ease of formation from bicyclo[2.1.0]pentane, but also its formation from 2,3-diazabicyclo[2.2.1]hept2-ene by either thermolysis or photolysis. Remarkably, the gas phase thermolysis of exo-deuterated material gave mostly inverted bicyclo[2.1.0]pentane (Scheme 6.24).55

Scheme 6.24

68


Various explanations have been given such as stepwise cleavage of the two C –N bonds with concerted backside displacement of the nitrogen molecule by the carbon radical and, intriguingly, conservation of momentum upon loss of dinitrogen that results in the double inversion product.56 This latter explanation has been pursued by Carpenter who has championed non-statistical dynamics as an explanation for many stereochemical pathways in thermal reactions.57 Experimentally, Carpenter found that the specificity in the denitrogenation reaction, i.e. double inversion over retention, is a factor of five at low pressures but becomes smaller with increasing pressures in supercritical fluids so that at 200 bar, it is around 2.5. This is argued to be consistent with collisional deactivation of a dynamically controlled intermediate to one, which is nearly thermally equilibrated which closes in a statistical fashion. CASPT2 calculations reveal that the transition state for nitrogen loss involves simultaneous cleavage of both C –N bonds, and this is 7.3 kcal/mol more favorable than cleavage of only one C –N bond. Further, the cyclopentane-1,3-singlet species has C1 symmetry and is the only minimum on the surface with a 1.2 kcal/mol barrier to closure. On the other hand, Adam has examined the photochemical denitrogenation in more substituted derivatives and found that bridgehead substitution reduces the extent of inversion to the point where a bis-bridgehead diphenyl material undergoes exclusive double retention.58 He further found that 12 e-/10 O CASSCF geometry optimization with CASPT2 energies revealed that the one C –N bond-broken species was the low-energy intermediate in the parent case. 2.3

Bicyclo[1.1.1]pentane

One of the more interesting rearrangements which has received little attention is the retro 2 þ 2 reaction of bicyclo[1.1.1]pentane to 1,4-pentadiene.59 The reaction has log k ¼ 15:24 2 49 000=2:4RT (Scheme 6.25).60 Pyrolysis of the 1,3-dimethyl derivative gives 2,4-dimethyl-1,4-pentadiene with log k ¼ 16:2 2 53 000=2:3RT:

Scheme 6.25

2.4 Methylenecyclobutane Degenerate Rearrangement and Retro 2 1 2 Cleavage Chesick found that methylenecyclobutane undergoes thermal cleavage to ethylene and allene with log k ¼ 15:68 2 63 300=2:3RT:61 The activation is comparable to that for cleavage of cyclobutane itself suggesting that the double bond does not assist in the overall reaction. However, Doering and Gilbert found that 2,2-dideuterio-

C5H6 – C5H10

69

methylenecyclobutane and dideuteriomethylenecyclobutane interconvert at a much lower temperature with logðkf þ kb Þ ¼ 14:77 2 49 500=2:3RT:62 These authors made the reasonable suggestion that the degenerate rearrangement is assisted by formation of an allylic radical, but cleavage requires twisting of the allyl radical with concomitant loss of resonance energy (Scheme 6.26). It is difficult to argue for concert in this 1,3-shift since the activation energy is equal to, if not higher than, the estimated BDE of the C2 – C3 bond.

Scheme 6.26

Subsequent theoretical work explored the energy surface for the degenerate rearrangement using substantial levels of configuration interaction (CASSCF and MP4(SDQ)/6-31Gp) for energies with HF/3-21G geometries. This suggested that the allyl – ethyl biradical prefers an orthogonal conformation with the migrating carbon plane perpendicular to the plane of the allyl.63 Rotation around the migrating carbon requires only 1 kcal/mol. The transition state for ring opening to the orthogonal species occurs in stages, which effectively result in conrotatory opening of the cyclobutane ring with rotation around the opposite bond in the ring occurring to maximize overlap during the reaction (Scheme 6.27).

Scheme 6.27

70


This mode of ring opening was suggested earlier64 to explain the results of elegant experiments conducted by Baldwin on Z-2-methylethylidenecyclobutane. This material undergoes both the degenerate 1,3-shift as well as epimerization at C2 faster than all other geometric and structural isomerizations by a factor of roughly 10,65 so attention focused on this material. Subsequent experimental work focused on the interconversions, as revealed by NMR, of the four deuteriumlabeled, racemic Z diastereomers, which provided the rate constants reported in Scheme 6.28.

Scheme 6.28

It is important to note that only three phenomenological rate constants characterize the interconversions, but seven different mechanistic processes could be involved. These are the two allowed 1,3-shift pathways (suprafacial inversion and antarafacial retention), the two forbidden 1,3-shift pathways (suprafacial retention and antarafacial inversion), C2-epimerization, C3-epimerization, and C2 – C3 double epimerization. Scheme 6.28 reveals the combinations responsible for the three rate constants. Fortunately, there is one other rate constant that must be included, namely, that for racemization (k ¼ 4:12 £ 1025 s21 at 3228C which could arise by 2 £ (sr þ si þ C2ep þ C2 – C3ep). While the mechanistic rate constants remain indeterminant, limits on their range are provided by the experimental rate constants, and it was argued that the predominant stereomode for utilization of the allylic framework must be antarafacial, with both retention and inversion at C3. The reason for this unusual stereochemistry arises from the fact that kc is twice as large as kb ; and since kc could only arise by epimerization at C2 or C3, there could be little contribution of

C5H6 – C5H10

71

suprafacial pathways to the racemization. However, given the experimental error in the rate constants due to overlapping NMR signals, it is not unlikely that kb ¼ kc and that both are roughly 40% of ka : If this were the case, then the stereopathways of Scheme 6.29 involving conrotatory opening of the ring to an orthogonal biradical, which closes roughly one-third as fast as epimerization at C3, can rationalize the data65 and is consistent with the calculations63 on the mode of ring opening.

Scheme 6.29

The only remarkable aspect of the reaction is then the preferred outward rotation of the C2 methyl which may be the result of minimizing repulsion between the methyl and C3 as the C2 – C3 bond is being broken. It is interesting to compare the dimethyl case to a dicarbomethoxy derivative. Here pyrolysis of 2R; 3R-cis-2,3-dicarbomethoxy-1-dideuteriomethylenecyclobutane gave 1,3-shift product with a carbomethoxy group Z to the deuterated carbon, but with an 87:13 preference for 3R over 3S 3-carbomethoxy-2,2-dideuterio-1carbomethoxymethylenecyclobutane.66 This data is consistent again with preferred “outward” rotation of the substituent at C2, but the preference for inversion at C3 is less than half of that in the dimethyl case (Scheme 6.30). Further, there was no epimerization at C3 presumably because of the thermodynamic driving force to make the conjugated ester.

72


Scheme 6.30

2.5

Spiropentane

Spiropentane gives methylenecyclobutane upon heating at high pressures with log k ¼ 15:86 2 57 570=2:3RT:67 The activation energy is higher than the estimated BDE of the C1 – C2 bond which is probably broken initially. So a biradical intermediate would appear to be involved, and this undergoes rearrangement by cyclopropyl methylene migration (Scheme 6.31). At lower pressures, a retro 2 þ 2 reaction to allene and ethylene occurs, no doubt the result of incomplete collisional deactivation of vibrationally hot methylenecyclobutane product.68

Scheme 6.31

The question of geometric isomerization of spiropentane itself was partly addressed by Doering and Gilbert who ruled out a potential degenerate rearrangement of spiropentane, which might result from cleavage of both peripheral bonds, by labeling both C1 and C2 with deuterium (Scheme 6.32).62 Subsequently, Gilbert found that the interconversion of cis- and trans-1,2-dideuteriospiropentane occurred 10 times faster than the structural isomerization having log kgeo ¼ 14:5 2 51 500=2:3RT (Scheme 6.32), suggesting that a biradical is formed reversibly,69 most likely by peripheral and not radial bond fission (see Scheme 6.31).

C5H6 – C5H10

73

Scheme 6.32

Evidence for reversible peripheral bond fission in simple alkyl-substituted spiropentanes was provided by Burka who found that both proximal- and distal-1,4dimethylspiropentane gave the medial isomer at least 20 times faster than the proximal – distal isomer interconversion (Scheme 6.33). The rate constant for the interconversion of distal- and medial-1,4-dimethylspiropentane was log k ¼ 14:7 2 50 000=2:3RT:70

Scheme 6.33

Because of the interest in stereochemistry of 1,3-biradicals, Chang pyrolyzed optically active trans-1,2-dimethylspiropentane and syn-4,4-dideuterio-cis-1,2dimethylspiropentane and determined the rate constants for the thermally induced interconversions shown in Scheme 6.34.71 With the cis isomer, double inversion (cis to cis interconversion) is preferred by a factor of 1.8 over the two single inversion (cis to trans conversion) pathways, but there is little preference for double inversion over single inversion with the trans isomer. Similar observations were made with the 1,2,4-trimethylspiropentanes.72

74


Scheme 6.34

These results were originally interpreted as suggesting a preference for disrotatory ring opening of spiropentane to a biradical which therefore closed in a disrotatory fashion since outward rotation of both methyl groups in the cis isomers would be sterically unimpeded, while disrotatory opening of the trans isomers would necessarily rotate one methyl “inside”. If the reaction were a conrotatory process then the trans isomers would have the greater preference for double inversion since the methyls could both rotate outward. These conclusions were not supported by calculations at the 2/2 CASSCF and CASPT2N level using a 6-31Gp basis set.73 A roughly 1 kcal/mol preference for conrotatory ring opening in spiropentane and the dimethyl derivatives was calculated. In addition, there are two equivalent pathways for conrotatory ring opening of cis-1,2-disubstituted materials. However, there is only one favored pathway for conrotatory ring opening of the trans isomers, i.e. outward, since “inward” rotation of both methyls should result in severe steric destabilization between the methyls. The stereochemistry of the structural isomerization of spiropentane to methylenecyclobutane was studied by Burka using methyl-substituted carbethoxy spiropentanes.74 In these cases, the carbomethoxymethylidenecyclobutanes are the kinetic products, and geometric isomerization is slower than the structural rearrangement. Retention at the migrating carbon was favored by a factor of 2– 9 as determined from pyrolysis of the cis- and trans-4,5-dimethyl-1-carbethoxyspiropentanes (Scheme 6.35). The stereochemistry of the carbethoxy group in the trans starting materials was highly dependent on the initial stereochemistry which appears to be the result of a steric repulsion between a methyl syn and the ester preventing outward rotation. No such prohibition appears in the lone cis case, and indeed, the ester group rotated equally well in either direction. Further, there appeared to be no polar character to the transition state since the reaction was not accelerated in acetonitrile solvent relative to benzene solvent.

C5H6 – C5H10

75

Scheme 6.35

The stereochemistry at the migration terminus and the direction of rotation of the carbethoxy group was examined with racemic 2,4-dimethyl-1-carbethoxyspiropentanes. Here, only the compounds with methyl trans to carbethoxy could be examined since the four cis-methyl-carbethoxy isomers undergo rapid double epimerization at C1 and C2 via a reversible homo-1,5-hydrogen shift before structural isomerization, thus scrambling the stereolabeling. Three of the four methyl trans to carbethoxy compounds rearranged with 63, 71, and 82% retention, respectively, at both C2 and C4 (or inversion at both – no distinction is possible with racemic materials) and rotation of the ester-bearing carbon was consistent with a 2s þ 2a allowed reaction (Scheme 6.35). In the case of the least stereospecific reaction, the other major

76


pathway was also a 2s þ 2a pathway with inversion at C2 and retention at C4 (or vice versa) giving a total of 82% allowed stereochemistry.75 The fourth trans isomer gave an isomer which could only result from C5 (methylene) migration possibly because steric effects in the allowed pathway involving C4 migration are sufficiently destabilizing to prevent it. Examination of the two allowed rearrangement reactions with retention at both C2 and C4 which are too high in energy to occur, namely that of one of the trans-4,5dimethyl compounds and one of the 2,4-dimethylcompounds, reveals in each a proximal relationship between two groups 1,4 with respect to one another. As the allowed migration occurs, these groups must approach one another to an even greater extent. All this is consistent with concert in the rearrangement. However, it is not clear why other concerted pathways are not involved to a significant extent, particularly one involving retention at the migrating carbon, C4, and inversion at the migration origin, C2. Perhaps calculations might provide some insight. The stereochemistry of the migrating carbon in a spiropentane fused to a cyclobutane ring was examined76 and was found to occur with only a small preference for retention (a factor of three from the trans compound of Scheme 6.36 and no preference with the cis isomer). This system, however, will not permit an allowed reaction with retention at both the migrating carbon and the migration terminus without generation of a trans double bond in a five-membered ring, so a biradical pathway is likely. In the parent system, the kinetics of rearrangement yield log k ¼ 12:94 2 33 750=2:3RT; and with appropriately deuterated material, bridgehead double inversion was found to occur much faster with log k ¼ 14:08 2 29 000=2:3RT:

Scheme 6.36

Finally, Burka found that there was complete retention at the migrating carbon of trans-4,5-dimethyl-1,1-dicyanospiropentane.77 However, the cis-anti isomer gave

C5H6 – C5H10

77

little of the expected rearrangement product and gave instead 1,1-dicyano-2-methylanti-3-ethylidenecyclobutane (Scheme 6.37). This product must result from initial radial bond fission most likely to produce a zwitterionic species involving a malonitrile anion and a cyclopropyl cation in which the ring opens in a disrotatory fashion to an allyl cation. Anion – cation combination then leads to the major product. The reaction rate is at least 10 times faster in acetonitrile than in benzene, which is consistent with the zwitterion hypothesis. Further, the change in mechanism from the trans isomer is not inconsistent with the increased rate of solvolysis of cis-2,3-dimethyl-anti-1-chlorocyclopropane relative to its other isomers.78

Scheme 6.37

2.6

Allene –Ethylene Cycloadditions

An important reaction that is on the spiropentane/methylenecyclobutane energy surface is the 2 þ 2 cycloaddition of allene and appropriately substituted ethylenes to give methylenecyclobutane. Studies on the intermolecular cycloaddition indicate that a biradical intermediate is involved although the reaction often occurs with high stereospecificity about the ethylenic moiety. Thus dimethyl maleate gives substantial stereospecificity in the formation of the two possible regio-isomers (Scheme 6.38).79

Scheme 6.38

78


Similar results were obtained in ketene – ethylene cycloadditions.80 Evidence for biradicals comes from kinetic and product-determining secondary deuterium isotope effects in the addition of acrylonitrile to perdeuterio and 1,1-dideuterioallene, respectively, where the kinetic effect is only 4% but there is a 21% preference for formation of the product with deuterium on the exo methylene group (Scheme 6.39).81

Scheme 6.39

These observations require a product-forming intermediate which responds to isotopic substitution formed after the rate-determining step, which does not respond to isotopic substitution. Further evidence on the stereospecificity of the reaction was provided in a study of the cycloaddition of R-2,3-pentadiene with acrylonitrile in which all four 2-methyl3-ethylidenecyanocyclobutane products were formed with an excess of the R configuration at C2 (Scheme 6.40).82

Scheme 6.40

C5H6 – C5H10

79

The observation can be rationalized by generation of a biradical from the least hindered approach of the acrylonitrile to the allene with rotation about the allene to generate a nearly statistical ratio of the dimethylallyl radicals with closure occurring by least motion faster than rotation about the newly formed s bond (Scheme 6.40). 2.7

cis-1,3-Pentadiene Hydrogen Shift

The 1,5-hydrogen shift in cis-1,3-pentadienes is a facile thermal process first described by Wolinsky (Scheme 6.41).83

Scheme 6.41

Roth84 and Frey85 examined the kinetics of a number of these reactions and found low pre-exponential terms, , 1011 s21, and low activation energies, 36 – 38 kcal/ mol. The low pre-exponential term is consistent with a transition state with fewer degrees of freedom than the starting material, and the activation energies observed are lower than the BDE of the C5 –H bond by roughly 40 kcal/mol. Moreover, Roth found a very large primary deuterium kinetic isotope effect in the reaction of 5,5,5trideuterio-cis-1,3-pentadiene, 12.2 at 258C, which requires a hydrogen shift in the rate-determining step. All this is consistent with a concerted reaction. If the reaction is concerted then it should proceed with suprafacial use of the pentadienyl moiety, and this is the case with 5S-2-deuterio-6-methyl-2-trans-4-cisoctadiene.86 Upon pyrolysis at 2508C, this material gave a mixture of 7R-transand 7S-cis-dienes of the same optical purity and with high stereospecificity (Scheme 6.42).

Scheme 6.42

80


At higher temperatures, other 1,5-hydrogen shifts intervene, but the suprafacial reaction was found to be at least 8 kcal/mol lower in energy than any antarafacial reaction. Finally, it might be mentioned that the large hydrogen isotope effect might be consistent with a tunneling process since the KIE is larger than what is normally observed for primary deuterium effects. Tunneling is characterized by a larger difference in H/D activation energies than what might be expected for the loss of zero point energy for one stretching vibration. However, here ED 2 EH ¼ 1:56 kcal/mol and AH =AD ¼ 0:932;87 so the larger KIE is probably the result of changes in the ZPE of more than one vibration. Calculations at the MP2/6-31Gp level using an RHF/ 3-21G optimized Cs geometry (Scheme 6.43), reproduced the activation energy for the reaction.88 DFT calculations on numerous substituted pentadienes at the B3LYP/ 6-31Gp level have been reported.89

Scheme 6.43

3 3.1

C5H10 1-Pentene Retro Ene Reaction

While not an isomerization, the retro ene reaction of 1-pentene is the prototype for many intramolecular variants and should be discussed here. Upon heating, 1-pentene gives ethylene and propylene with log k ¼ 11:2 2 49 900=2:3RT (Scheme 6.44).90

Scheme 6.44

The activation parameters strongly suggest a concerted hydrogen transfer with C – C bond cleavage; in particular, the C –C bond energy is greater than 70 kcal/mol mole and the C –H bond energy is greater than 80 kcal/mol. Further, for a cleavage reaction, the entropy of activation is negative by about 5 e.u. The most favorable geometry for a concerted process is depicted in Scheme 6.44.

C5H6 – C5H10

81

The scope of the ene and retro ene reaction was the subject of an excellent early review.91

REFERENCES 1. 2. 3. 4.

V.A. Mironov, E.V. Sobeloev and A.N. Elizarova, Tetrahedron, 19, 1939 (1963). W.R. Roth, Tetrahedron Lett., 1009 (1964). S. McLean, C.J. Webster and R.J.D. Rutherford, Can. J. Chem., 47, 1555 (1969). J.W. de Haan and H. Kloosterziel, Recl. Trav. Chim. Pays. Bas., 84, 1594 (1965); W.C. Herndon and J.M. Manion, J. Org. Chem., 33, 4504 (1968). 5. M.R. Willcott, III and I.M. Rathburn, III, J. Am. Chem. Soc., 96, 938 (1974). 6. H.P. Fritz and C.G. Kreiter, J. Organomet. Chem., 4, 313 (1965). 7. A.J. Ashe, III, J. Am. Chem. Soc., 92, 1233 (1970). 8. F.A. Cotton, Acc. Chem. Res., 1, 257 (1968); G. Bo¨che, F. Heidenhain and B. Staudigl, Angew. Chem. Int. Ed. Engl., 18, 218 (1979). 9. H.E. Zimmerman and D.S. Crumrine, J. Am. Chem. Soc., 90, 5612 (1968); T.M. Brennan and R.K. Hill, J. Am. Chem. Soc., 90, 5614 (1968); R.F. Childs and S. Winstein, J. Am. Chem. Soc., 90, 7146 (1968); H. Hart, T.R. Rogers and J. Griffiths, J. Am. Chem. Soc., 91, 754 (1969); J.A. Berson and N.M. Hasty, Jr., J. Am. Chem. Soc., 93, 1549 (1971); P. Vogel, M. Saunders, N.M. Hasty, Jr. and J.A. Berson, J. Am. Chem. Soc., 93, 1551 (1971). 10. R.W. Hoffmann, P. Schmidt and J. Backes, Chem. Ber., 109, 1918 (1976); R.W. Hoffmann and J. Backes, Chem. Ber., 109, 1928 (1976). 11. J.I. Brauman and D.M. Golden, J. Am. Chem. Soc., 90, 1920 (1968); D.M. Golden and J.I. Brauman, Trans. Faraday Soc., 65, 464 (1969); For the synthesis of BCP see E.E. van Tamelen, J.I. Brauman and L.E. Ellis, J. Am. Chem. Soc., 93, 6145 (1971). 12. G.D. Andrews and J.E. Baldwin, J. Am. Chem. Soc., 99, 4853 (1977). 13. J.E. Baldwin and N.D. Ghatlia, J. Am. Chem. Soc., 111, 3319 (1989). 14. P.N. Skancke, K. Yamashita, and K. Morokuma, J. Am. Chem. Soc., 109, 4157 (1987). 15. F.-G. Kla¨rner and F. Adamsky, Angew. Chem. Int. Ed. Engl., 18, 674 (1979); F.-G. Kla¨rner, E.L. Eliel, S.H. Wilen, and N.L. Allinger (eds), Topics in Stereochemistry, Wiley, New York, 15, 1 (1984). 16. F. Jensen, J. Am. Chem. Soc., 111, 4643 (1989). 17. M.B. Reyes, E.B. Lobkovsky, and B.K. Carpenter, J. Am. Chem. Soc., 124, 641 (2002). 18. J.K. Crandall and D.R. Paulson, J. Am. Chem. Soc., 88, 4302 (1966). See also D.R. Paulson, J.K. Crandall, and C.A. Bunnell, J. Org. Chem., 35, 3708 (1970). 19. Unpublished work communicated privately to JJG. 20. R. Bloch, P. Le Perchec, and J.-M. Conia, Angew. Chem. Int. Ed. Engl., 9, 798 (1970). 21. D.H. Aue and M.J. Meshishnek, J. Am. Chem. Soc., 99, 223 (1977). 22. For reviews A. Brandi and A. Goti, Chem. Rev., 98, 589 (1998). 23. K.B. Wiberg and F.H. Walker, J. Am. Chem. Soc., 104, 5239 (1982). 24. J. Belzner and G. Szeimies, Tetrahedron Lett., 27, 5839 (1986). 25. O. Jarosch, R. Walsh, and G. Szeimies, J. Am. Chem. Soc., 122, 8490 (2000). 26. M.D. Levin, P. Kaszynski, and J. Michl, Chem. Rev., 100, 169 (2000). 27. C.G. Overberger and A.E. Borchert, J. Am. Chem. Soc., 82, 1007 (1960). 28. M.C. Flowers and H.M. Frey, J. Chem. Soc., 3547 (1961). 29. M.R. Willcott, III and V.H. Cargle, J. Am. Chem. Soc., 89, 723 (1967). 30. M.R. Willcott, III and V.H. Cargle, J. Am. Chem. Soc., 91, 4310 (1969). 31. J.E. Baldwin, K.A. Villarica, D.I. Freedberg, and F.A.L. Anet, J. Am. Chem. Soc., 116, 10845 (1994); J.E. Baldwin, Comput. Chem., 19, 222 (1998).

82 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52.

53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68.

Hydrocarbon Thermal Isomerization K.N. Houk, M. Nendel, O. Wiest, and J.W. Storer, J. Am. Chem. Soc., 119, 10545 (1997). E.R. Davidson and J.J. Gajewski, J. Am. Chem. Soc., 119, 10543 (1997). C. Doubleday, M. Nendel, K.N. Houk, D. Thweatt, and M. Page, J. Am. Chem. Soc., 121, 4720 (1999). J. Chickos, Abstracts of papers, 187th Meeting of the American Chemical Society, St Louis, MO, ORGN 228 (1984) J.E. Baldwin and K.A. Viliarica, Tetrahedron Lett., 7905 (1994). R.J. Ellis and H.M. Frey, J. Chem. Soc., 5578 (1964). W.R. Roth and J. Ko¨nig, Justus Liebigs Ann. Chem., 688, 28 (1965). G.D. Andrews and J.E. Baldwin, J. Am. Chem. Soc., 98, 6705 (1976). J.J. Gajewski and L.P. Olson, J. Am. Chem. Soc., 113, 7432 (1991); J.J. Gajewski, L.P. Olson, and M.R. Willcott, III, J. Am. Chem. Soc., 118, 299 (1996). M. Nendel, D. Sperling, O. Wiest, and K.N. Houk, J. Org. Chem., 65, 3259 (2000), and references contained therein. W. von E. Doering and K. Sachdev, J. Am. Chem. Soc., 96, 1168 (1974); W. von E. Doering and K. Sachdev, J. Am. Chem. Soc., 97, 5512 (1975). J.E. Baldwin, Chem. Rev., 103, 1197 (2003). C. Steel, R. Zand, P. Hurwitz, and S.G. Cohen, J. Am. Chem. Soc., 86, 679 (1964). M.L. Halberstadt and J.P. Chesick, J. Am. Chem. Soc., 84, 2688 (1962). J.E. Baldwin and J. Ollerenshaw, J. Org. Chem., 46, 2116 (1981). J.P. Chesick, J. Am. Chem. Soc., 84, 3250 (1962). J.A. Berson, W. Bauer, and M.M. Campbell, J. Am. Chem. Soc., 92, 7515 (1970). S.L. Buchwalter and G.L. Closs, J. Am. Chem. Soc., 97, 3857 (1975). J. Wang, C. Doubleday, Jr., and N.J. Turro, J. Am. Chem. Soc., 111, 3962 (1989). M.P. Conrad, R.M. Pitzer, and H.F. Schaefer, III, J. Am. Chem. Soc., 101, 2245 (1979); C.D. Sherrill, E.T. Seidl, and H.F. Schaefer, III, J. Phys. Chem., 96, 3712 (1992). J.J. Tufariello, J.H. Chang, and A.C. Bayer, J. Am. Chem. Soc., 101, 3315 (1979); M.J. Jorgenson and A.F. Thacher, Chem. Commun., 1030 (1969); J.-P. Goselaude, H.-U. Gonzenbach, J.-C. Perlberger, and K. Schaffner, Helv. Chim. Acta, 59, 2919 (1976); A.J. Ashe, III, J. Am. Chem. Soc., 95, 818 (1973). F.D. Coms and D.A. Dougherty, J. Am. Chem. Soc., 111, 6894 (1989). F.D. Coms and D.A. Dougherty, Tetrahedron Lett., 29, 3753 (1988). W.R. Roth and M. Martin, Liebigs Ann. Chem., 702, 1 (1967); W.R. Roth and M. Martin, Tetrahedron Lett., 47, 4695 (1967). E.L. Allred and R.L. Smith, J. Am. Chem. Soc., 91, 6766 (1969). M.B. Reyes and B.K. Carpenter, J. Am. Chem. Soc., 120, 1641 (1998); M.B. Reyes and B.K. Carpenter, J. Am. Chem. Soc., 122, 10163 (2000). W. Adam, H. Garcıá, M. Diedering, V. Martı´, M. Olivucci and E. Palomares, J. Am. Chem. Soc., 124, 12192 (2002). K.B. Wiberg and D.S. Connor, J. Am. Chem. Soc., 88, 4437 (1966). R. Srinivasan, J. Am. Chem. Soc., 90, 2752 (1968). J.P. Chesick, J. Phys. Chem., 65, 2170 (1961); see also R.L. Brandaur, B. Short and S.M.E. Keller, J. Phys. Chem., 65, 2269 (1961). W. von E. Doering and J.C. Gilbert, Tetrahedron, Suppl., 7, 397 (1966). P.N. Skancke, N. Koga, and K. Morokuma, J. Am. Chem. Soc., 111, 1559 (1989). J.J. Gajewski, J. Am. Chem. Soc., 98, 5254 (1976). J.E. Baldwin and R.H. Fleming, J. Am. Chem. Soc., 95, 5249 (1973). W. von E. Doering, Lecture at 21st Natl. Org. Chem. Symp., (1969); E.T. Fossel, Diss. Abstr., 826B (1971). M.C. Flowers and H.M. Frey, J. Chem. Soc., 5550 (1961). P.J. Burkardt, Ph D Dissertation, University of Oregon, Eugene, (1962); P.J. Burkardt, Diss. Abstr., 23, 1524 (1962) and the previous reference.

C5H6 – C5H10 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91.

83

J.C. Gilbert, Tetrahedron, 25, 1459 (1969). J.J. Gajewski and L.T. Burka, J. Am. Chem. Soc., 94, 8857 (1972). J.J. Gajewski and M.J. Chang, J. Am. Chem. Soc., 102, 7542 (1980). J.J. Gajewski, R.J. Weber, and M.J. Chang, J. Am. Chem. Soc., 101, 2100 (1979). W.T.G. Johnson, D.A. Hrovat, and W.T. Borden, J. Am. Chem. Soc., 121, 7766 (1999). J.J. Gajewski and L.T. Burka, J. Am. Chem. Soc., 94, 8865 (1972). Scheme 5.50 of the first edition inappropriately interchanged the results from two of the isomers. W.R. Roth and K. Enderer, Justus Liebigs Ann. Chem., 733, 44 (1970). J.J. Gajewski and L.T. Burka, J. Am. Chem. Soc., 93, 4952 (1971). P. von R. Schlyer, G.W. Van Dine, U. Scho¨llkopf, and J. Paust, J. Am. Chem. Soc., 88, 2868 (1966). See also, S.J. Cristol, R.M. Sequeira, and C.H. DePuy, J. Am. Chem. Soc., 87, 4007 (1965). E.F. Kiefer and M.Y. Okamura, J. Am. Chem. Soc., 90, 4187 (1968). R.W. Holder, J. Chem. Educ., 53, 81 (1976). S.-H. Dai and W.R. Dolbier, Jr., J. Am. Chem. Soc., 94, 3946 (1972). J.E. Baldwin and U.V. Roy, Chem. Commun., 1225 (1969). J. Wolinsky, B. Chollar, and M.D. Baird, J. Am. Chem. Soc., 84, 2775 (1962). W.R. Roth and J. Ko¨nig, Justus Liebigs Ann. Chem., 699, 24 (1966). H.M. Frey and R.J. Ellis, J. Chem. Soc., 4770 (1965); H.M. Frey and B.M. Pope, J. Chem. Soc. A, 1701 (1966). W.R. Roth, J. Ko¨nig, and K. Stein, Chem. Ber., 103, 426 (1970). Recalculated values by linear regression analysis of the rate constants reported in Roth (1966); see G.-Y. Shen, R.Tapia, W.H. Okamura, J. Am. Chem. Soc. 109, 7499 (1987). F. Jensen and K.N. Houk, J. Am. Chem. Soc., 109, 3139 (1987). N.J. Saettel and O. Wiest, J. Org. Chem., 65, 2331 (2000). R. Walsh, cited by K.W. Egger and P. Vitins, Int. J. Chem. Kinet., 6, 429 (1974). H.M.R. Hoffmann, Angew. Chem. Int. Ed. Engl., 8, 556 (1969). See also B.B. Snider and B.M. Trost, Comprehensive Organic Synthesis, Pergamon Press, Oxford, 5, 1 –27 (1991).

7 C6H4 –C6H10

CONTENTS 1 C6H4 1.1 cis-1,2-Diethynylethene to p-Benzyne 2 C6H6 2.1 (CH)6 Isomer Interconversion 2.2 Benzene Topomerization (Degenerate Rearrangement) 2.3 3,30 -Biscyclopropenyl Degenerate Rearrangement 2.4 1,5-Hexadiyne to Bisallene to 3,4-Dimethylenecyclobutene, Benzene, and Fulvene 2.5 1,2-Hexadien-5-yne Degenerate Rearrangement and Conversion to 2-Ethynyl-1,3-butadiene 2.6 cis- and trans-1,3-Hexadien-5-yne to Benzene 3 C6H8 3.1 Bicyclo[2.1.1]hexene to Bicyclo[3.1.0]hexene 3.2 Bicyclo[3.1.0]hexene Degenerate Rearrangement 3.3 Bicyclo[3.1.0]hexene to 1,4- and 1,3-Cyclohexadiene 3.4 trans-1,3,5-Hexatriene Geometric Isomerization and 3-Vinylcyclobutene to cis-1,3,5-Hexatriene and to Cyclohexadiene 3.5 cis-1,3,5-Hexatriene to 1,3-Cyclohexadiene 3.6 1,5-Shifts in 1,3-Cyclohexadienes 3.7 1,4- and 1,3-Cyclohexadiene to Benzene and Dihydrogen 3.8 anti-Tricyclo[3.1.0.02,4]hexane to 1,4-Cyclohexadiene 3.9 Bicyclo[2.2.0]hex-2-ene to 1,3-Cyclohexadiene 3.10 2-Methylenebicyclo[2.1.0]pentane to 3- and 4-Methylenecyclopentene 3.11 1-Ethynyl-2-methylcyclopropane and 1-Hexen-5-yne to 1,2,5-Hexatriene 3.12 Cyclopropylallene to 3-Methylenecyclopentene 3.13 cis-1,2,4-Hexatriene to cis-1,3,5-Hexatriene 3.14 1,2-Dimethylenecyclobutane Degenerate Rearrangement, Tetramethyleneethane, and the Allene Dimerization

86 86 88 88 92 94 95 97 98 101 101 103 106 106 108 110 112 113 114 114 115 117 118 118

86


3.14.1 Allene Dimerization 3.15 Biscyclopropylidene to Methylenespiropentane and to 1,2-Dimethylenecyclobutane 3.16 Vinylmethylenecyclopropane, Allylidenecyclopropane, and 3-Methylenecyclopentene 3.17 3-Allylcyclopropene Degenerate 3,3-Shift 3.18 [2.1.1]Propellane 4 C6H10 4.1 1,5-Hexadiene 3,3-Shift – the Cope Rearrangement 4.1.1 Substituent Effects on the 1,5-Hexadiene 3,3-Shift 4.2 The Claisen Rearrangement of Allylvinyl Ethers 4.3 Bicyclo[2.2.0]hexane to 1,5-Hexadiene 4.3.1 Substituted Bicyclo[2.2.0]hexanes 4.4 Bicyclo[2.1.1]hexane to 1,5-Hexadiene 4.5 Vinylcyclobutane to Cyclohexene 4.6 Cyclohexene Retro Diels –Alder Reaction 4.7 Biscyclopropyl 4.8 Bicyclo[3.1.0]hexane to Cyclohexene and 1-Methylcyclopentene 4.9 cis-2-Methylvinylcyclopropane to cis-1,4-Pentadiene References

1 1.1

121 124 127 132 133 134 134 140 146 152 153 154 154 158 160 161 162 163

C6H4 cis-1,2-Diethynylethene to p-Benzyne

In an important extension of 3,3-shifts and hexatriene rearrangements, Bergman demonstrated the pairwise interchange of C1 and C3 with C4 and C6 of cis-1,2diethynylethene at 2008C in the gas phase (Scheme 7.1).1

Scheme 7.1

C6H4 – C6H10

87

The half-life at this temperature was roughly 30 s which indicates a free energy of activation of 32 kcal/mol. The benzene-1,4-biradical was suggested as an intermediate since radical-like products are generated in solution, particularly benzene when 1,4-cyclohexadiene is present. Subsequent work revealed intramolecular trapping of the biradical2 and polarized emission signals in the NMR are consistent with the reactive state being the singlet state.3 The closed form, butalene, appears to have been generated by Breslow in a lithium dimethylamide-induced elimination of 1-chlorobicyclo[2.2.0]hexa2,5-diene at 08C.4 N,N-dimethylaniline was the major product which was deuterated to the extent of 75% at the para position when N-deuteriodimethylamine was the solvent. Both the meta and ortho positions were also deuterated (Scheme 7.2).

Scheme 7.2

When the elimination was conducted at 2 358C in the additional presence of tetramethylenediamine and diphenylisobenzofuran, an adduct of 1-dimethylaminobicyclo[2.2.0]hexa-2,5-diene derived by addition of the deuterated solvent to the butalene was formed. In this reaction all the deuterium was found at the bridgehead position at the level of NMR detection (Scheme 7.3).

Scheme 7.3

88


The Bergman and Breslow observations complement one another in that both ring-opened and ring-closed forms of p-benzyne can be generated. DFT calculations suggest that the closed form is 37 kcal/mol less stable than the open form and that the barrier to ring opening of the closed form is less than 6 kcal/mol5 which is not very different than the earlier MINDO 3 calculations.6 Bergman estimated from thermochemical considerations that the diyl is 14 kcal/mol less stable than 1,2-diethynylethene and resides in an 18 kcal/mol potential energy well. NO and oxygen trapping experiments indicate that the biradical has a 19.8 kcal/mol barrier to reclosure with the biradical being 8.5 kcal/mol less stable than the enediyne.7 A high level calculation by Morokuma which appears to include dynamic correlation places the biradical 22 kcal/mol above the enediyne with a transition state energy of 37.6 kcal/mol.8 The transition state ˚ with only slight bond length for the forming bond in the electrocyclization is 1.95 A distortion of the other bond lengths from that of the starting material. The heat of formation of the p-benzyne biradical was determined to be 138 kcal/mol, and was confirmed by a study of the photoelectron spectrum of the radical anion.9 Further, the singlet – triplet splitting was found to be only 3.8 kcal/mol in favor of the singlet. Substantial interest in the Bergman rearrangement resulted when naturally occurring enediyne antibiotics such as esperamicin and calicheamicin were discovered.10 These molecules are stable at ambient temperatures, but could be activated chemically. The activation process appears to force the ends of the diynes into closer proximity resulting in a dramatic lowering of the temperature necessary for cyclization to the biradical which then abstracts hydrogen from nearby organic residues, most often sugar residues in nuclei acids. Extensive work has revealed that the rates of the cyclization are proportional to the difference in steric energy of reactant and biradical as opposed to diminished distances.11 Finally, the hydrogen abstraction reaction has been examined theoretically using MCSCF methods. The barrier does not appear to be affected by the singlet – triplet gap in the biradical.12

2 2.1

C6H6 (CH)6 Isomer Interconversion

The question of structure, stability, and properties of benzene has held a central position in organic chemistry. The Kekule’s hypothesis for the stability of cyclohexatriene prompted development of theory whose extensions to transition states has had as much impact as on ground states. However, there are five known isomers of (CH)6: benzene, Dewar benzene, benzvalene, prismane, and biscyclopropenyl (Scheme 7.4). The highly strained quadricyclo[2.2.0.02,5,03,6] hexane isomer is also of interest.

C6H4 – C6H10

89

Scheme 7.4

Dewar benzene was prepared in 1963 by lead tetraacetate decarboxylation of the diacid from the photolysis of 1,2-dihydrophthalate esters.13 Benzvalene was isolated from photolysis of benzene in 196714 and prepared in quantity from lithium cyclopentadienide, methylithium, and methylene chloride.15 Prismane was prepared from benzvalene in 1972.16 Bicyclopropenyl itself was prepared in 198917 although alkyl derivatives were characterized earlier and shown to undergo 3,3-sigmatropic shifts, vide infra. Substituted derivations of all the (CH)6 hydrocarbons, particularly the permethyl materials.18 It is a well-documented yet remarkable tribute to the Conservation of Orbital Symmetry Hypothesis that benzene is not the immediate result of attempts to synthesize any of the other (CH)6 isomers since their strain energies and the substantial resonance energy of benzene confer enormous exothermicity on the conversion of any of these other isomers to benzene. Indeed, there is no obvious direct “symmetry allowed” pathway on the thermal energy surface that connects any of the other isomers, except perhaps benzvalene, to benzene. Indeed, heating above 1008C is required to affect the isomerizations. In solution, Dewar benzene gives benzene with DH ‡ ¼ 23 kcal=mol and DS‡ ¼ 25 e:u:19 not unexpectedly by central bond fission.20 Most interesting is the fact that this isomerization is accompanied by emission of light.21 Benzvalene, although explosively unstable in concentrated solution, gives benzene with log k ¼ 13:7 2 26 700=2:3RT; further, the reaction is exothermic by 67.5 kcal/ mol.22 A biradical reaction might be involved although the activation energy would appear to be lower than the anticipated BDE of an external bicyclobutane bond (see Chapter 5, Section 2). Subsequent calculations (see below) suggest that the reaction is concerted, although weakly so. Light is not emitted in this reaction nor in the isomerization of prismane to benzene which has a half-life of 11 h at 908C (Katz, 1973). How this occurs will be addressed below in the discussion of the permethyl derivatives. Finally, 3,30 -dimethylbiscyclopropenyl gives the xylenes with an activation energy of roughly 37 kcal/mol; moreover, if the pyrolysis is conducted at 2008C for short times followed by rapid cooling, luminescence is observed as deduced by light emission from added 9,10-dibromoanthracene.23 Since only Dewar benzene gives off light in the isomerization to benzene and the reaction has a lower activation energy, it was proposed as an intermediate in the isomerization of the biscyclopropenyl (Scheme 7.5).

90


Scheme 7.5

The formation of Dewar benzene from 3,30 -bicyclopropenyl most reasonably occurs via opening of the cyclopropene single bond to a biradical (see Chapter 4, Section 1) which undergoes an expansion of the second ring to give a biradical which can close to the product (Scheme 7.6).24

Scheme 7.6

In the most thoroughly studied series, it was found that hexamethylprismane gives hexamethyl Dewar benzene with log k ¼ 14:50 2 33 800=2:3RT and hexamethylbenzene with log k ¼ 13:94 2 33 200=2:3RT (Oth, 1968). Hexamethylbenzvalene was detected as a steady-state intermediate in the NMR spectrum of the reaction suggesting that the portion of the reaction leading from the prismane to the benzene involves rate-determining formation of the benzvalene. If the observations with the parent system are extrapolatable from to this, hexamethylbenzvalene must give hexamethylbenzene directly. From the heats of reaction derived from calorimetry and the activation enthalpies, an enthalpy surface for the interconversion can be constructed (Scheme 7.7).

C6H4 – C6H10

91

Scheme 7.7

Of interest is the fact that the relative heats of formation of the permethyl isomers are almost identical to those calculated by MINDO 3 for the parent molecules.25 More recent, higher level (G2) calculations utilizing isodesmic reactions have provided very different relative heats of formation. The absolute values are: benzene 19.6 kcal/mol; Dewar benzene 95 kcal/mol; prismane 131 kcal/mol; benzvalene 90.5 kcal/mol; and 3,30 -biscyclopropenyl 138 kcal/mol.26 The experimental value for the heat of formation of benzene is 19.8 kcal/mol, and the experimental difference in enthalpy between benzvalene and benzene is 67.5 kcal/mol. The latter value is reproduced by the calculation to within 4 kcal/mol. These calculations along with the experimental activation data from the above provides the enthalpy surface of Scheme 7.8.

Scheme 7.8

92


The formation of the permethyl Dewar benzene (and presumably the parent compound) from the prismane appears to be a “symmetry forbidden” retro 2 þ 2 cyclization perhaps proceeding via a biradical which can also give the benzvalene via a 1,2-shift of the cyclopropane (Scheme 7.9).

Scheme 7.9

The pathway for conversion of benzvalene to benzene has been studied theoretically with perhaps the best calculations – CISD(T)/DZP//B3LYP/DZP þ ZPVE – revealing an unsymmetrical, but concerted transition state nearly resembling a biradical having opposite bicyclobutane bond lengths of 2.32 and ˚ .27 Further, the heat of formation of this transition state relative to benzene is 1.704 A 100.9 kcal/mol which is not inconsistent with the experimental activation energy. Before concluding this section on benzene and its isomers, it should be noted that the aromaticity associated with benzene which is usually attributed solely to delocalization of the p system has been questioned.28 Calculations suggest that relief of bond compression within a localized double bond more than make up for compression of the sigma bond between two double bonds. This may contribute substantially to the “aromaticity” in benzene. Indeed, a simple Hook’s law ˚) calculation assuming a reasonable force constant for the sp2 – sp2 s bond (5 md/A ˚ , suggests roughly a 7 kcal/mol stabilization in and an equilibrium distance of 1.48 A ˚ relative to cyclohexatriene the sigma system in benzene with six bonds of 1.40 A ˚ and three bonds of 1.48 A ˚. with three bonds of 1.34 A 2.2

Benzene Topomerization (Degenerate Rearrangement)

In a remarkable set of experiments, Scott subjected numerous aromatic hydrocarbons to high temperature – . 10008C – pyrolysis in a flow system and discovered carbon scrambling. With benzene-1,2-bis-13C there was isomerization to the 1,3- and 1,4-isomers in a 6:1 ratio, respectively, when 72% of the 1,2-isomer was recovered (Scheme 7.10).29

C6H4 – C6H10

93

Scheme 7.10

The data is consistent with a reversible sequential reaction converting the 1,2- to the 1,3-, then to the 1,4- isomer. Subsequent theoretical work focused on the mechanism of this transformation. At the 6-31Gp MP3 level, formation of a carbene followed by a hydrogen shift was the preferred pathway (Scheme 7.11).30

Scheme 7.11

Diotropic rearrangement to fulvene and bicyclo[3.1.0]hex-1,3-diene are not accessible from benzene, and reversible formation of benzvalene was ca. 20 kcal/mol higher in enthalpy at this level of theory. Subsequent calculations at the CISD(T)/DZP//B3LYP/DZP þ ZPVE level, however, indicated that in addition to the carbene pathway above, reversible formation of benzvalene could account for the isomerization reaction.31 Benzvalene is not formed via a biradical, but if it were, then the 1,4-isomer could be formed directly from the first benzvalene, a pathway not in accord with the experimental observations (Scheme 7.12).

Scheme 7.12

94


2.3

3,30 -Biscyclopropenyl Degenerate Rearrangement

3,30 -Biscyclopropenyls undergo a facile 3,3-shift as observed with the 3,30 -dimethyl materials (Scheme 7.13).32 The 10:1 ratio of threo to meso product indicates that the chair transition state is favored over a boat transition state by roughly 2 kcal/mol in this case.

Scheme 7.13

In a careful study of the n,n 0 -dimethyl-3,30 -biscyclopropenyl interconversions and the xylene isomer distribution, Bergman concluded that dimethylprismane could not be involved which reinforces the conclusion based on observation of luminescence in the pyrolysis of 3,30 -biscyclopropenyls and not in the pyrolysis of prismane (see Section 2.1). The relative rate constants and distributions at 165.58C are given in Scheme 7.14.33

Scheme 7.14

Bergman suggested that the data is consistent with conversion to Dewar benzenes via a ring expansion of the usual biradical generated upon pyrolysis of

C6H4 – C6H10

95

cyclopropenes (see Scheme 7.9). The interconversion of the meso and threo isomers prior to formation of the xylenes supports this hypothesis. 2.4 1,5-Hexadiyne to Bisallene to 3,4-Dimethylenecyclobutene, Benzene, and Fulvene Huntsman first observed that 1,5-hexadiyne gave 3,4-dimethylenecyclobutene upon heating at 2508C with an activation enthalpy of 34.4 kcal/mol and an activation entropy of 2 9.4 e.u.34 A reasonable reaction pathway would invoke a 3,3-shift to bisallene followed by electrocyclization of the destabilized conjugated diene moiety (Scheme 7.15).

Scheme 7.15

Indeed, Hopf observed that bisallene gave 3,4-dimethylenecyclobutene rapidly at 2008C.35 That the reaction is a 3,3-shift followed by a cyclization also follows from deuterium labeling studies which indicate conversion of 1,6-dideuterio-1,5hexadiene to 1,2-dideuterio-3,3-dimethylenecyclobutene.36 The stereochemistry of the reaction was studied by pyrolysis of meso-3,4dimethyl-1,5-hexadiyne in a flow system at 3508C. The syn,anti-isomer of 3,4diethylidenecyclobutene was formed. Pyrolysis of the threo isomer gave mostly the anti,anti-isomer (Scheme 7.16).

Scheme 7.16

96


However, in an important unpublished work, Huntsman and Onderak showed that the major product from the threo isomer at 2208C in a static system was nearly a 1:1 mixture of both the anti,anti- and syn,syn- diethylidenecyclobutene isomer.37 At higher temperatures, a 92:8 mixture of the two isomers, respectively, are formed, presumably by equilibration via the threo dimethylbisallene, Scheme 7.16. The observations are consistent with the mechanism proposed above with a ratedetermining, stereospecific 3,3-shift followed by a rapid, reversible conrotatory formation of the dimethylenecyclobutenes. The 3,3-shift must proceed in a fashion that transfers asymmetry from C3 and C4 to the bisallene, presumably via an orbital symmetry controlled reaction. Calculations at various levels have been performed to locate the transition state for the 3,3-shift portion of the reaction. At the RHF/6-21Gp level, both breaking ˚ , but at the CASSCF/6-31Gp level the breaking and making bonds are roughly 1.94 A ˚ and the making bond is 1.82 A ˚ . Finally at the B3LYP/6-31Gp level, the bond is 1.70 A distances are 1.77 and 1.93, respectively, and the transition state is nearly planar.38 At still higher temperatures, 3,4-dimethylenecyclobutene gave both fulvene and benzene in a 2:1 ratio, respectively, in a flow system.36 Further, when the 1,2dideuterio material was pyrolyzed, the fulvene appears to have deuterium on the 2 and 3 positions while the benzene is 93% ortho labeled with only 5% meta labeled and 2% para labeled.36 The proposed mechanism for these conversions invoked reversion to the bisallene follwed by recyclization in two different modes to produce biradicals whose resonance structures are carbenes which undergo insertion(s) into adjacent CH bonds (Scheme 7.17).

Scheme 7.17

More recent work involving pyrolysis of dimethylenecyclobutene in deuterium gas revealed that the high-temperature products were formed initially without incorporation of deuterium.39 However, at higher temperatures, deuterium was incorporated suggesting that a radical chain pathway is involved. Interestingly, a small amount of 1,3-hexadien-5-yne was also observed as a product, and it is known to give benzene at these temperatures (see Section 2.6). A likely pathway for the formation of dieneyne is a 1,5-hydrogen shift (Scheme 7.18).

C6H4 – C6H10

97

Scheme 7.18

Substituted derivatives of dimethylenecyclobutenes are described in a review.40 2.5 1,2-Hexadien-5-yne Degenerate Rearrangement and Conversion to 2-Ethynyl-1,3-butadiene Hopf discovered a rapid, reversible, degenerate 3,3-sigmatropic rearrangement of 1,2-hexadien-5-yne as a result of deuterium scrambling between the terminal acetylene position and an allenyl position (Scheme 7.19).41

Scheme 7.19

Work on kinetics with the 4-methyl derivative allowed determination of log k ¼ 10:84 2 30 800=2:3RT; Scheme 7.19.42 The pre-exponential factor is comparable to that for the 3,3-shift of 1,5-hexadiene, but the activation energy is 4 kcal/mol less, possibly because of the weaker cumulative p system. The reaction is interesting in that the only possible symmetry along the reaction coordinate is that resulting from a center of inversion. Indeed, the calculated distance for the bonds ˚ .43 being broken and made in the transition state at the B3LYP/6-31Gp level is 1.76 A

98


However, at the CASSCF/6-31Gp level, the transition states are unsymmetrical with distances of 1.65 and 1.86 reflecting the formation of an intermediate C2 symmetric 1,4-cyclohexadiene-1,4-diyl. The apparent pathological predisposition for biradicals of multiconfiguration methods without inclusion of dynamic correlation is probably responsible for the latter result. Most interesting is the subsequent rearrangement of the dienyne to 2-ethynyl-1,3butadiene at 5008C in a flow system.44 The suggested mechanism involves the strained bisolefin, 1,4-bicyclo[2.2.0]hexadiene, formed by a 2 þ 2 process, possibly by the diyl described above, which undergoes a six-electron shift preserving the central bond. This pathway is supported by the deuterium distribution from the 3deuterio derivative (Scheme 7.20).

Scheme 7.20

2.6

cis- and trans-1,3-Hexadien-5-yne to Benzene

It was reported that both cis- and trans-1,3-hexadiene-5-yne gave benzene at 2758C.44 The cis isomer gives benzene four times faster than it isomerizes to the trans isomer. The authors indicated that the most reasonable precursor to benzene is the cis isomer; and a hydrogen shift is required either during or subsequent to a cyclization (Scheme 7.21).

Scheme 7.21

C6H4 – C6H10

99

Using the oxygen trapping technique, Roth determined log kðcis to cyclic alleneÞ ¼ 9:54 2 31 300=2:3RT:45 Further, the cis compound is ca. 65 kcal/mol less stable than benzene, and log kðcyclic allene to benzeneÞ ¼ 12:13 2 21 100= 2:3RT; and log kðcyclic allene to cisÞ ¼ 11:5 2 10 900=2:3RT: The latter reaction involves a hydrogen shift that appears to be intramolecular; indeed, it may be a 1,5hydrogen shift in a cyclic diene system. The data places the trappable intermediate cyclic allene intermediate roughly 85 kcal/mol above benzene. However, while calculations at the mr-ci þ Q/cc-pVDZ/DFT level reproduces these values the stability of the intermediate is not consistent with the experimental conclusion.46 The authors found that the cyclic allene is only 75 kcal/mol above benzene and the singlet biradical is 8.9 kcal/mol higher in energy with the triplet another 2.4 kcal/ mol higher in energy. The authors suggest that oxygen is trapping the singlet biradical and not the cyclic allene (Scheme 7.22).

Scheme 7.22

The intermediacy of the cyclic allene was also demonstrated by a different trapping experiment. Pyrolysis of the dienyne in the presence of styrene gave, among many other products, the two 2 þ 2 cycloaddition product of the cyclic allene.47 The conversion of 1,3-hexadien-5-yne to benzene can occur by another mechanism, namely, an acetylene to vinylidene rearrangement followed by insertion – cyclization into a vinyl C – H bond (Scheme 7.23).

100


Scheme 7.23

Support for this pathway comes from the observation of two deuterio isomers of biphenyl when 1-phenyl-6-deuterio-1,3-hexadien-5-yne is pyrolyzed.48 The ratio of the two suggests that the cyclic allene pathway is utilized only to the extent of 33% relative to the vinylidene pathway (Scheme 7.24).

Scheme 7.24

At still higher temperatures, apparently radical chain reactions occur. Zimmermann has provided an excellent review of these reaction types.49 One of the most intriguing applications of this reaction occurs in Scott’s pyrolytic synthesis of corannulene, which is one-third of C60, from 7,10-diethynylfluoranthene (Scheme 7.25).50

Scheme 7.25

Whether this reaction proceeds via the cyclic allene or the vinylidene is unclear.

C6H4 – C6H10

3 3.1

101

C6H8 Bicyclo[2.1.1]hexene to Bicyclo[3.1.0]hexene

Bond first prepared bicyclo[2.1.1]hexene and showed that it rearranges to bicyclo[3.1.0]hex-2-ene above 1508C (Scheme 7.26).51 The reaction has log k ¼ 13:95 2 35 170=2:3RT in the gas phase.52

Scheme 7.26

The stereochemistry of the reaction was first investigated by Roth who found that the anti- and syn-6-methylbicyclo[2.1.1]hexenes gave different proportions of the exo- and endo-6-methylbicyclo[3.1.0]hexenes in the sense of dominant inversion at the migrating carbon by factors of 197 and 2, respectively. Small amounts of the 4-methylbicyclo[3.1.0]hexenes were also formed (Scheme 7.27).53

Scheme 7.27

The results were interpreted in terms of an “allowed” thermal 1,3-sigmatropic shift with suprafacial-inversion stereochemistry, and the lower specificity in and higher temperature necessary for the rearrangement of syn material was argued to be due to steric inhibition of the inversion pathway.

102


Subsequently, this rearrangement in the syn (endo)-5-deuterio parent compound was examined by Carpenter who found that at the lowest temperature, 135.28C, the ratio of inversion to retention was 98:2 in isooctane solution.54 At 1978C, the ratio was 93:7 (Scheme 7.28).

Scheme 7.28

However, the percent inversion with the 1- and 2-phenyl-5-deuterio derivatives was 91 and 88%, respectively, and was independent of temperature over a 608C range. It was suggested that the lack of temperature dependence indicated nonstatistical, i.e. dynamical control of the reaction as opposed to two pathways with identical activation enthalpies but differing in their entropy requirements. Indeed, it was suggested that “In the case of the [1,n ] sigmatropic shifts in rigid bicyclic molecules, all of which seem to show a preference for inversion of configuration at the migrating carbon when there is an energetically accessible biradical mechanism, one can argue that conservation of momentum will favor inversion because it is the ‘continuation’ of the motion that transformed the originally pyramidal tetracoordinate carbon of the reactant into the planar trigonal carbon of the biradical.” The hypothesis of dynamical control has been pursued in biradical reactions, vide infra, but the notion that temperature independence of reaction pathways is a criterion for such behavior seems inappropriate since the parent bicyclo[2.1.1]hexene does not show such behavior despite the fact that it, having many fewer vibrational modes, should be most likely to do so. The question of whether or not the stereochemistry of the rearrangement of the parent compound is under orbital symmetry control requires that the reaction be concerted. However, the heat of formation of the parent material and the heat of formation of the methyl cyclopentene diyl suggests a BDE of less than roughly 22 kcal/mol for the nonconcerted cleavage of the C1 – C5 bond55 which is 12 kcal/mol less than that observed. However, these types of BDE estimates are invariably 8– 10 kcal/mol less. So, it is unclear whether or not the reaction in the parent case is perhaps partially concerted.

C6H4 – C6H10

3.2

103

Bicyclo[3.1.0]hexene Degenerate Rearrangement

In an unpublished work, Doering and Roth examined the thermal equilibration of 6,6-d2- and 4,4-d2-bicyclo[3.1.0]hexene (Scheme 7.29).56 This reaction is a vinylcyclopropane rearrangement involving the bridgehead bond. Since the reaction is constrained to a six-membered ring it cannot proceed via an allowed si or ar pathway.

Scheme 7.29

Cooke prepared and pyrolyzed optically active 2,4,4-d3 material at 3188C in a flow reactor finding that equilibration with the 2,6,6-d3 material was 0.75 times as fast as racemization of the sample.57 This suggests that double epimerization of the bridgehead carbons is 33% faster than formation of a Cs biradical species (Scheme 7.30).

Scheme 7.30

Further, the 2,4-exo-dideuterio material gave an unequal distribution of three possible isomeric products at low conversions, 2,6-exo-dideuterio, 2,4-endodideuterio, and 2,6-endo-dideuterio material in a 51:38:11 ratio, respectively. These products are the result of the sr, the double epimerization, and the ai stereopathways, respectively (Scheme 7.31) and they are consistent with the results from the pyrolysis of the trideuterio material. It would seem reasonable that the sr product and the latter two products arise from the two conformers of Cs symmetric biradicals of (Scheme 7.31). However, the unequal distribution of the latter two

104


products argues against this unless there is another pathway for excess double epimerization of the bridgehead carbons. More recently, Baldwin confirmed Cooke’s observations with the dideuterio-labeled material and determined the activation parameters for the three reactions as log Aðs21 Þ ¼ 14:1; 14.2, and 14.1, respectively, and Eact ¼ 43:8; 44:3; and 44.8 kcal/mol, respectively.58 Thus, the difference in the reaction pathways is attributable to difference in enthalpies not entropies.

Scheme 7.31

The ab initio calculated potential energy surface for the reactions, (4,4)CASPT2/6-31Gp, found a transition state for ring opening of the bridgehead bond to give a Cs symmetric biradical species in a very shallow well.59 This can give the sr product. Also found was a C2v transition state converting one Cs species into another which can give the double epimerization and the ai product. The potential energy surface would suggest that these two products should be formed in equal amounts. The greater than 3 to 1 ratio determined experimentally was attributed to dynamical effects. However, just why the enthalpic differences observed by Baldwin were not found by calculation is unclear. Further, these differences would appear to be inconsistent with an interpretation based on dynamical effects. The lower activation energy for the double epimerization product may result from an unsymmetrical transition state which has thus far evaded computational discovery. Earlier, it was reported that the rates of racemization and deuterium scrambling of a-thujene at 2008C were nearly identical and consistent with an intermediate of Cs

C6H4 – C6H10

105

symmetry.60 Subsequent work found that log krac for 3,4,4-trideuterio-a-thujene was , 14:33 2 43 400=2:3RT:61 Further from the re-resolved pyrolsate from (2 ) material after ca. 45% conversion it was determined that the sr 1,3-shift product was the major product in both solution and the gas phase, 19.4 and 22.6%, respectively, while the percentage of the double epimerization product was 7.0 and 14.8%, respectively, and the percentage of the ai 1,3-shift product was 8.1 and 8.8% , respectively (Scheme 7.32). The gas phase results are comparable to those of Cooke with the parent compound. However, the solution results might be interpreted in terms of the potential energy surface calculated by Houk for the parent system in a reaction that is not dominated by dynamical effects due to increased collisional deactivation.

Scheme 7.32

In a related example, Swenton found evidence for only the sr 1,3-shift product with optically active endo-5,6-diphenylbicyclo[3.1.0]hexene at 1308C. Here log k ¼ 12:6 2 32 400=2:3RT (Scheme 7.33).62 The lower activation energy is consistent with increased stabilization of a non-planar biradical by a phenyl group and the lack of other products can be reasonably attributed to a higher activation energy for formation of a planar species due to the presence of vicinal phenyl groups which must interact sterically in the transition state for conformational interconversion.

106


Scheme 7.33

3.3

Bicyclo[3.1.0]hexene to 1,4- and 1,3-Cyclohexadiene

At higher temperatures than those for the degenerate rearrangement (see Section 3.2), bicyclo[3.1.0]hexane undergoes hydrogen shifts to give 1,4- and 1,3cyclohexadiene with log k ¼ 14:1 2 50 200=2:3RT and log k ¼ 14:3 2 50 200=2:3RT; respectively.63 The reaction would appear to involve central bond fission to a biradical followed by vicinal hydrogen shifts in either of two directions (Scheme 7.34).

Scheme 7.34

3.4 trans-1,3,5-Hexatriene Geometric Isomerization and 3-Vinylcyclobutene to cis-1,3,5-Hexatriene and to Cyclohexadiene Upon heating trans-1,3,5-hexatriene apparently undergoes central bond geometric isomerization to its cis isomer since 1,3-cyclohexadiene results. The reaction has log k ¼ 12:91 2 44 300=2:3RT (Scheme 7.35).64,65

Scheme 7.35

C6H4 – C6H10

107

Since the cis isomer cyclizes to the cyclohexadiene with an activation energy roughly 12 kcal/mol lower (see Section 3.5), it is reasonable to assume that the activation parameters determined are those for the geometric isomerization recognizing that only the cis isomer can cyclize. The activation volume for the cyclization of the cis material is 2 9.8 cc/mol.66 In a geometrically constrained case, Doering found that geometric isomerization of the central bond in a conjugated triene system had log k cis to trans ¼ 12:03 2 41 700=2:3RT (Scheme 7.36).1

Scheme 7.36

Comparisons of the activation enthalpy for this reaction to that for geometric isomerization of ethylene and 2-butene led to an estimate of 13 ^ 2 kcal=mol for allyl radical resonance energy once correction for hexatriene resonance energy was made. The pathway by which the geometric isomerization occurs in the acyclic case could be either direct central p bond rotation via orthogonal allyl radicals or by closure to 3-vinylcyclobutene followed by opening to the cis isomer. The latter pathway is not stereoelectronically permitted in the geometrically constrained case of Scheme 7.36. However, the former possibility was addressed by Pinski who found almost exclusive conversion of 3-vinylcyclobutene to trans-1,3,5-hexatriene with an activation energy of 26.8 kcal/mol (Scheme 7.37).67

Scheme 7.37

The formation of the cis compound required substantially higher temperatures.68 In a geometrically constrained case that could permit ring expansion, Paul found that both the E-triene and the cyclohexadiene were formed in a 3:1 ratio, respectively, kinetically (Scheme 7.38).69

108


Scheme 7.38

The formation of the hexalin was envisioned to occur via the Z-triene although a direct ring expansion could not be ruled out. The E-triene also rearranged to the cyclohexadiene most likely via formation of a vinylcyclobutene although direct geometric isomerization was not ruled out. Activation parameters for the conversions were determined by estimation of rate constants by numerical integration of the product data as a function of temperature to give: log kðVCB – EtrieneÞ ¼ 13:6 2 33 000=2:3RT; log kðVCB – HexalinÞ ¼ 11:8 2 30 800=2:3RT; and log kðEtriene – HexalinÞ ¼ 9:2 2 26 200=2:3RT: The errors in the latter parameters are relatively high because this is a relatively slow reaction, and the errors in the rate constants for reversion of the Etriene to the vinylcyclobutene are very high because this is an even slower reaction. Not ruled out is the possibility that the vinylcyclobutene undergoes a 3,3-shift to the related vinylcyclobutene which could open to the Z-triene and give the hexalin product. Finally, there is a great history of geometric isomerization about the external double bonds in terminal alkyl substituted 1,3,5-hexatrienes. These occur mostly by 1,7-hydrogen shifts (See Chapter 6), but there is also a direct pathway (see Section 3.5).

3.5

cis-1,3,5-Hexatriene to 1,3-Cyclohexadiene

Thermal isomerization of cis-1,3-5-hexatrienes to 1,3-cyclohexadienes was first recognized in the pyrolytic conversion of previtamin D2 and vitamin D2, which interconvert by 1,7-hydrogen shifts, to pyro- and isopyrocalciferol at higher temperatures (Scheme 7.39).70

C6H4 – C6H10

109

Scheme 7.39

This electrocyclization proceeds in an allowed disrotatory stereomode.71 The kinetics of the cyclization in the parent molecule were determined to be log k ¼ 11:85 2 29 900=2:3RT (Scheme 7.35).72 Further, the reaction was found to be exothermic by 14.5 kcal/mol. Evidence for disrotatory cyclization in a simple case was provided by Marvell73 and by Vogel74 who found that trans, cis, trans-2,4,6-octatriene gives at least 99.9% pure cis-5,6-dimethyl-1,3-cyclohexadiene with an activation energy of 29.4 kcal/ mol and an entropy of activation of 2 7 e.u. (Scheme 7.40).

Scheme 7.40

110


Further, the cis,cis,trans isomer interconverts with the all-cis isomer at 1008C presumably by reversible 1,7-hydrogen shifts, vide infra. At higher temperatures, this mixture gives three products: the cis- and the trans-5,6-dimethyl-1,3cyclohexadienes as well as 1,6-dimethyl-1,3-cyclohexadiene, the latter presumably by a 1,5-hydrogen shift in the initial cyclohexadiene products. Marvel and Spangler have provided numerous examples of these cyclizations.75 It is of historical significance that Havinga, in 1961, suggested that electronic factors predicts a “cis ring closure” would be preferred in these thermal cyclizations.76 Secondary deuterium kinetic isotope effects in the cyclization are very different, kH =kD ¼ 0:88 and 1.05, for the Z,Z- and E,E-1,6-deuterio derivatives, respectively.77 SCF/6-31Gp calculations for the disrotatory transition state structure roughly reproduced the isotope effects and suggested some steric crowding between the hydrogens on the inside which is relieved with deuterium present. However, the system is much more complicated: there should be an inverse effect at both sites because sp2 carbon is being converted to sp3 carbon, and opposing this should be a normal isotope effect due to loss of zero-point energy associated loss of bending vibrations being converted to reaction coordinate motion. However, these factors should affect both Z and E hydrogens equally but to an unknown extent, and the differential effect must be superimposed on these considerations. Interestingly, the interconversions of the terminal dialkylhexatrienes of Scheme 7.40 have been quantified and the 1,7-hydrogen shift isomer which can be responsible for the cis,cis- cis,trans- geometric isomerization has been found to be an intermediate.78 Further, the kinetics of formation and loss of the 1,7-shift isomer require that there be an additional pathway responsible for the geometric isomerization. Even more remarkable is the fact that upon extended heating of these isomers, the trans, trans isomer is also formed. Since these reactions occur with activation free energies (31 kcal/mol) roughly 13 kcal/mol lower than the cyclization reactions, there must be a still different pathway for the double bond geometric isomerizations. It should be noted that the deuterium kinetic isotope effect in a 1,7-shift, which must occur antarafacially (See Chapter 6) is 7.0 at 658C but shows strong a strong temperature dependence, Eact ðDÞ 2 Eact ðHÞ ¼ 2:0 indicative of tunneling.79 Finally, the preferred direction of disrotatory ring closure has been addressed just as in the electrocyclic ring opening of cyclobutene. In the six-electron case, p acceptor substituents control the direction of rotation in the same sense as in cyclobutene but the “torquoselectivity” effect is attenuated.80 Further, steric effects tend to play a bigger role, and both effects were attributed to the disrotatory nature of the transition state. 3.6

1,5-Shifts in 1,3-Cyclohexadienes

Like cyclopentadiene, 1,3-cyclohexadiene undergoes a 1,5-hydrogen shift which has been observed in methyl-substituted materials (see Section 3.5). In 5-methyl-

C6H4 – C6H10

111

1,3-cyclohexadiene, the reaction give 1-methyl-1,3-cyclohexadiene with log k ¼ 11:2 2 35 200=2:3RT (Scheme 7.41), and similar activation parameters were determined for the dimethylcyclohexadienes (see Section 3.5).81

Scheme 7.41

The activation energy is higher than that in cyclopentadiene probably reflecting the greater distance the hydrogen must traverse. Further, while the activation energies are similar to those for acyclic dienes (see Chapter 5), the pre-exponential term is lower probably reflecting less loss of entropy in going to the transition state because of the relatively rigidity of the ring. Calculations using the B3LYP functional with the 6-31Gp basis set located a Cs symmetric transition state for the hydrogen shift having an activation energy of 41.9 kcal/mol.82 The breaking and making C – H distances in the transition state ˚ longer than that calculated for the 1,5-hydrogen were calculated to be 1.49, 0.175 A ˚ longer than that in cis-1,3-pentadiene. shift in cyclopentadiene and nearly 0.06 A Schiess examined the 1,5-shift of carbonyl groups in 1,3-cyclohexadiene and found that the rate of rearrangement for the formyl material is roughly 125 times faster than that for the hydrogen shift while that for the acetyl derivative is only 4 times faster.83 Further, the carbomethoxy derivative is substantially slower (by a factor of 70). Here it is likely that there is substantial bond formation to the carbonyl group recognizing that it is carbocation-like so the interaction is not unlike that in transannular p participation; however, it is unclear as to the extent of ionic character in this transition state, so (Scheme 7.42) is merely meant to be provocative.

Scheme 7.42

Finally, alkyl shifts may occur in these systems although they are not the major pathways. For instance, in a flow system at 4608C, 5,5-diethyl-1,3-cyclohexadiene gives mostly 2,3-dimethylethylbenzene upon heating followed by dehydrogenation

112


of the cyclohexadiene to a phenyl group. This was viewed to involve a six-electron ring opening followed by a 1,7-hydrogen shift followed by a six-electron cyclization (Scheme 7.43).

Scheme 7.43

Only small amounts of 1,3-diethyl benzene, the alkyl group migration product, were found (4%); however, 24% of ethyl benzene was also found suggesting that radical cleavage processes were involved. Some 1,4-diethylbenzene (4%) was also formed which was argued to result from subsequent electrocyclic and 1,7-hydrogen shift processes.84

3.7

1,4- and 1,3-Cyclohexadiene to Benzene and Dihydrogen

The title reaction is not an isomerization but an interesting dehydrogenation reaction. 1,4-Cyclohexadiene undergoes dihydrogen loss with log k ¼ 12:02 2 42 700=2:3RT:85 The reaction would appear to be a syn elimination by a retro Diels –Alder-like pathway since trans-3,6-dideuterio-1,4-cyclohexadiene gives benzene containing 94% of only one deuterium (Scheme 7.44).86

Scheme 7.44

C6H4 – C6H10

113

On the other hand, though 1,3-cyclohexadiene gives benzene, cyclohexene is also produced, and its yield is diminished by addition of propylene suggesting a radical chain process,85 a hypothesis confirmed by Benson87 and by Thrush.88 Only at very low compound pressures (, 0.04 Torr) does 1,3-cyclohexadiene undergo a firstorder conversion to benzene. Here log k ¼ 13:67 2 61 600=2:3RT: Thrush suggested that the unimolecular loss of hydrogen could occur by reversion to bicyclo[3.1.0]hexane followed by reopening to 1,4-cyclohexadiene (see Section 3.3) (Scheme 7.45).

Scheme 7.45

3.8

anti-Tricyclo[3.1.0.02,4]hexane to 1,4-Cyclohexadiene

When anti-tricyclo[3.1.0.02,4]hexane is heated at 1658C, 1,4-cyclohexadiene results; the half-life at this temperature is 3 h (Scheme 7.46).89

Scheme 7.46

The kinetics reveal log k ¼ 13:70 2 36 750=2:3RT:90 The activation energy is similar to that for ring opening of bicyclo[2.1.0]pentane suggesting that the ratedetermining step is ring opening followed by fast cleavage of the relatively weak cyclopropane bond. Interestingly, at lower pressures, benzene was found as a product as well, and the ratio of the two products was dependent on pressure. This suggests that the diene is formed vibrationally hot and loses hydrogen in competition with collisional deactivation.

114 3.9


Bicyclo[2.2.0]hex-2-ene to 1,3-Cyclohexadiene

Upon heating bicyclo[2.2.0]hex-2-ene rearranges to 1,3-cyclohexadiene with log k ¼ 13:6 2 31 700=2:3RT 91 or log k ¼ 13:87 2 33 000=2:3RT:92 Goldstein found that exo,cis-5,6-dideuteriobicyclo[2.2.0]hex-2-ene gives exclusively cis-5,6dideuterio-1,3-cyclohexadiene without any formation of the endo,cis geometric isomer (Scheme 7.47).

Scheme 7.47

This finding is consistent with a “forbidden” disrotatory cleavage of the central bond followed by electron reorganization as opposed to generation of some planar species which could reclose to geometrically isomerized starting material. Also consistent with lack of concert in this reaction is the fact that the activation energy is comparable to that for central bond rupture in the saturated analog, namely, bicyclo[2.2.0]hexane (see Section 4) so little energetic benefit is provided by the double bond. 3.10 2-Methylenebicyclo[2.1.0]pentane to 3- and 4-Methylenecyclopentene Baldwin found that flow system pyrolysis of 2-methylenebicyclo[2.1.0]pentane at 2358C gives 1,2,5-hexatriene reversibly and 3- and 4-methylenecyclopentene in a 1.4:1 ratio (Scheme 7.48).93

Scheme 7.48

Roth determined the activation parameters for the three reactions as log k ¼ 13.51 2 36 500/2.3RT, 13.3 2 35 800/2.3RT, and 13.11 2 35 800/2.3RT, respectively.94 Roth also studied the interconversion of the exo- and endo-5-methyl derivatives which probably results from bridgehead double inversion (Scheme 7.49).

C6H4 – C6H10

115

Scheme 7.49

The rate constants are log k(exo – endo) ¼ 13.45 2 24 500/2.3RT and log k(endo – exo) ¼ 13.22 2 23 600/2.3RT. The very low activation energies for the geometric isomerization relative to bicyclo[2.1.0]pentane itself is not a function of 5-methyl substitution since the 5,5-dimethyl derivative ring opens with the same activation parameters as the parent so the increased rate probably reflects the stabilization of the biradical by the exo-methylene group.

3.11 1-Ethynyl-2-methylcyclopropane and 1-Hexen-5-yne to 1,2,5-Hexatriene Hopf found that cis- and trans-1-ethynyl-2-methylcyclopropane apparently equilibrate rapidly at 3408C in a flow system with 35 s residence times, but the cis isomer undergoes a homo-1,5-hydrogen shift to 1,2,5-hexatriene which itself undergoes a reversible 3,3-sigmatropic shift to 1-hexene-5-yne (Scheme 7.50).95 The triene also apparently cyclizes to 2-methylenebicyclo[2.1.0]pentane which opens, then undergoes a vicinal hydrogen shift to both 3- and 4-methylenecyclopentene. The ratio of the latter two isomers is 1.3:1 which is similar to that obtained from 2-methylenebicyclo[2.1.0]pentane itself (see Section 3.9). In addition, the triene also undergoes a 1,3-hydrogen shift to 1,3,5-hexatriene, which was not isolated, but its electrocyclization product, 1,3-cyclohexadiene is formed to a significant extent. At higher temperatures the cyclized products dominate the reaction mixture.

Scheme 7.50

116


The interconversion of 1-hexen-5-yne and 1,2,5-hexatriene was studied by Huntsman who found that the methylenecyclopentenes were formed at the expense of the triene.96 Furthermore, the activation energy is 32.7 kcal/mol and the activation entropy is 2 11.6 e.u.97 The nature of the transition state in the 3,3-shift was examined by Berson who found that (SE)-4-methyl-2-hepten-6-yne gave recovered starting material and both (RZ)- and (SE)-4-methyl-1,2,5-heptatriene and the latter two gave (RZ)-4-methyl-2hepten-6-yne all reversibly with high stereospecificity prior to formation of cyclized and dimeric products (Scheme 7.51).98 These results are consistent with an allowed stereopathway via a chair-like transition state as opposed to a forbidden one or the one in which an effectively planar cyclohexene-1,4-diyl is involved.

Scheme 7.51

Subsequent calculations have confirmed the “allowed, aromatic” nature of the transition state using various DFT methods.99 The breaking bond in the eneyne is on ˚ and the making bond is 1.88 A ˚ . Unfortunately, like the Cope an average 1.83 A rearrangement CASSCF calculations on this system indicate favorable formation of ˚ , respectively100) a diyl intermediate (bond distances of 1.87 and 1.67 A which reveals the need for inclusion of Dynamic Correlation. This and the related reactions have been reviewed by Hopf.101 Of related interest is the fact that cis-2-allyl-1-ethynylcyclopropane rearranges to 1,2,5,7-octatetraene with log k ¼ 8:2 2 25 100=2:3RT (Scheme 7.52).102 Apparently the allyl group sufficiently weakens the bond to the hydrogen that must be transferred to lower the energetic demands so that the subsequent isomerizations do not occur at the lower temperatures needed to accomplish the hydrogen shift in this case.

Scheme 7.52

C6H4 – C6H10

3.12

117

Cyclopropylallene to 3-Methylenecyclopentene

Roth examined the kinetics of the isomerization of cyclopropylallene to 3-methylenecyclopentene and found log k ¼ 14:08 2 50 200=2:3RT:103 Furthermore, cis-2-deuterio-1-allenylcyclopropane undergoes geometric isomerization 4.5 times faster than rearrangement (Scheme 7.53).

Scheme 7.53

The activation parameters are not dissimilar to those for the vinylcyclopropane rearrangement to cyclopentene (see Chapter 5). An interesting aspect of the rearrangement was provided by Olson who found a 15% rate reduction with two deuteriums replacing the hydrogens on the exocyclic methylene group (Scheme 7.54).104

Scheme 7.54

This result was attributed to the mass effect of deuterium slowing the rotation of the exomethylene in the formation of product. Subsequently, Olson collaborated with Houk to characterize rotational isotope effects both experimentally and theoretically.105

118


3.13

cis-1,2,4-Hexatriene to cis-1,3,5-Hexatriene

5-Methyl-cis-1,2,4-hexatriene undergoes a facile 1,5-hydrogen shift to give cis-2methyl-1,3,5-hexatriene (Scheme 7.55).106

Scheme 7.55

The activation free energy for the reaction was determined to be 24.6 kcal/mol at 113.48C. The relatively low activation free energies probably result from weaker p bonds in the allene moiety compared with that involved in the 1,5-hydrogen shift of cispentadiene systems (see Chapter 6, Section 2). 1,5-Hydrogen shifts have been examined in other vinylallene systems.107 Okamura determined the primary deuterium kinetic isotope effect in a vitamin Dlike model system as a function of temperature to obtain ED 2 EH ¼ 1:62 kcal=mol and AH =AD ¼ 0:835 (Scheme 7.56).108 These results suggest a normal primary KIE without tunneling, not unlike those determined by Roth for the 1,5-hydrogen shift in cis-1,3-pentadiene (see Chapter 6, Section 2).

Scheme 7.56

3.14 1,2-Dimethylenecyclobutane Degenerate Rearrangement, Tetramethyleneethane, and the Allene Dimerization 1,2-Dimethylenecyclobutane (DMC) undergoes a degenerate carbon scrambling reaction at temperatures above 2508C at low pressures in the gas phase with log k ¼ 14:45 2 46 800:109,110 Further, the isotopomers are formed kinetically as a 2:1 mixture of 1,3- and 3,3-shift products suggesting that an orthogonal or rapidly rotating 2,20 -bisallyl biradical is involved (Scheme 7.57).109

C6H4 – C6H10

119

Scheme 7.57

The stereochemistry of the ring opening was examined by Shih who found that trans-3,4-dimethyl-1,2-dimethylenecyclobutane, T, and the corresponding cis isomer, C, gave product distributions suggestive of a dominant, but not exclusive conrotatory processs.111 Further, the activation parameters were comparable to those determined by Doering and Dolbier for the parent system albeit lowered due to methyl substitution. The fact that the reaction of T is only a factor of two faster than that of C despite the conrotatory nature of the reaction suggests that substantial twisting about the central bond occurs in the transition state to relieve the steric interactions. Indeed, if twisting about the central bond occurs in the sense of meshed bevel gears relative to the conrotatory motions about the other bonds occurs then the facts can be rationalized (Scheme 7.58).

Scheme 7.58

Indeed, these motions represent the lowest energy pathway for ring closure of a singlet bisallyl biradical according to 6-31Gp MCSCF calculations by Gilbert.112 The bisallyl biradical, however, has been demonstrated to undergo rapid rotation around the central bond. Thus, pyrolysis of optically active T gives optically inactive 1,3-shift product at only 9% conversion; further, starting T racemized to the extent of 17.3%.113 This indicates that ring opening of T is reversible and that reclosure to T is at least 2.2 times faster than rearrangement, and that central bond rotation is fast compared with all other events.

120


Remarkably, the activation energy for this rearrangement is substantially higher (ca. 14 kcal/mol) than anticipated for formation of a bisallyl biradical; that is, the bisallyl biradical is about 32 kcal/mol less stable than non-conjugated DMC. It is hard to imagine that delocalization of the cisoid diene moiety in the reactant occurs to this extent. Perhaps complete delocalization of the allylic moieties is not achieved in the transition state for reasons that are not clear. The suggestion113 that the biradical might be anti-aromatic has been criticized by experts in theory.114 There are related concerns about the bisallyl biradical, namely the structure and stability of both singlet and triplet states. In 1970, Dowd observed a triplet ESR spectrum attributed to the bisallyl species [or tetramethyleneethane (TME)] by photolysis of 3,4-dimethylenecyclopentanone and also by photolysis of DMC itself,115 and subsequent work found a linear Curie plot suggesting that the triplet is either the ground state or very close to it (Scheme 7.59).116

Scheme 7.59

After much debate MCSCF(6,6)/3-21G-CI calculations finally revealed that the singlet is the ground state for both planar and perpendicular geometries, but the triplet is lower in energy than the singlet by roughly 1 kcal/mol when the dihedral angle is 508C.117 However, there is no difference in energy between the 508C triplet and the perpendicular singlet according to the calculations, and the perpendicular singlet is roughly 2 kcal/mol more stable than the planar singlet. All of this is consistent with the optical studies above, and it is consistent with the Borden – Davidson conjecture that biradicals with disjoint degenerate HOMOs (i.e. HOMOs with orbital densities on separate atoms) will not have deep triplet ground states and those with non-disjoint HOMOs like trimethylenemethane will definitely have a triplet ground state.118. Further, Iwamura made magnetic measurements on a nearly planar TME biradical (that from Scheme 7.63) and concluded that the singlet and triplet states are nearly equal in energy.119 All of this data, however, are inconsistent with studies of the pyrolysis reactions of 7-methylene-bicyclo[3.2.0]hept-2-ene.120 This material undergoes a degenerate rearrangement via a potentially freely rotating TME species, but using added oxygen and SF6 the authors deduced that the triplet was 7.4 kcal/mol more stable than the singlet. The origin of the discrepancy may rest in the rate constants assumed for trapping both singlet and triplet TME by oxygen. Finally, the observation that triplet TME could be produced in the photolysis of DMC at low temperatures in a matrix contrasts with the observation of degenerate

C6H4 – C6H10

121

structural isomerization upon photolysis at 254 nm in the vapor phase. Under these conditions, a 2:1 ratio of bis dideuteriomethylene isomers is produced from 1,2bis(dideuteriomethylene)cyclobutane as in the thermal process – see Scheme 7.57. Further, photolysis of T and C gave rearrangement products via the same stereochemical mode as the thermolysis, but with lower specificity – see Scheme 7.58. Lastly, the quantum yield for the structural photoisomerization of the parent compound is inversely proportional to pressure so that in solution, no isomerization is found. This is consistent with the isomerization occurring during the internal conversion process off the singlet excited surface, that is, the isomerization is a thermal process with a competing collisional deactivation.121 Interestingly, ring closure to a 1(4)-bicyclo[2.2.0]hexene122 does not seem to occur because the dimers123 usually formed from it were not observed. 3.14.1

Allene Dimerization

Related to the degenerate rearrangement of DMC is the allene 2 þ 2 dimerization which gives mostly DMC at low conversions.124 Deuterium isotope effect studies by Dolbier strongly suggest a reaction pathway involving rate-determining formation of the 2,20 -bisallyl biradical. Thus, dimerization of a mixture of allene and perdeuterio allene gave a statistical product distribution indicating no ratedetermining isotope effect. However, dimerization of 1,1-dideuterioallene revealed an isotope effect on the product distribution since a 1:2.3:1.3 ratio of 3,3,4,4-, 1dideuteriomethylene-3,3-dideuterio- and 1,2-bisdideuteriomethylene-DMCs, respectively, were formed (Scheme 7.60).125 The original analysis in the latter experiment was by NMR and was confirmed by Shih using mass spectrometry after degradation of the product mixture in the same manner as described previously.109 The origin of the product determining isotope effect would appear to be slower rotation of the CD2 group over the CH2 group in the allyllic moieties in the formation of the cyclobutane ring.

Scheme 7.60

122


The same isotope effects were observed in the dimerization of 1,2-cyclononadiene indicating the same type of energy surface.126 The importance of this latter observation is the earlier finding by Moore and Bach that the dimerization of optically active 1,2-cyclononadiene produced a dimer mixture consisting mostly of one compound, namely the cis,anti,anti material while dimerization of racemic allene gave mostly a 2:1 mixture of trans,anti,anti and cis,anti,anti material, respectively.127 These observations are consistent with an allowed 2s þ 2a cycloaddition which can be envisioned to occur from the sterically least hindered orientation of the optically active allene to give the meso isomer (Scheme 7.61). The same orientation with one of the allenes being the enantiomer will give the trans,anti,anti product.

Scheme 7.61

Other orientations of 2s þ 2a cycloaddition will give different, less stable, dimers having one or two syn-substituted exo-methylenes, but no other orientation will give the product distribution observed. It is also possible to imagine using the other double bonds of the allenes in a 2 þ 2 þ 2 or 2 þ 2 þ 2 þ 2 fashion, but only the allowed pathways will give the observed stereochemistry. The question of whether the bisallyl biradical is involved in the allene dimerization has been answered in the affirmative.128 Dimerization of 1,1dimethylallene at 1508C and thermolysis of 4,5-dimethylene-3,3,6,6-tetramethyl3,4,5,6-tetrahydropyridazine at the same temperature gave the three isomeric allenes in the same ratio. Further, direct photolysis of the azo compound just above room temperature gave the same product distribution when the thermal data was extrapolated to that temperature. Finally, the relative energies of all species on this energy surface need to be placed. Unfortunately, the activation energy for the allene dimerization is unknown,

C6H4 – C6H10

123

but that of an intramolecular variant, namely, 1,2,8,9-decatetraene to cis-7,8dimethylenebicyclo[4.2.0]octane occurs with log k ¼ 9:53 2 31 200=2:3RT:129 Since the heat of formation of allene is 47 kcal/mol130 and that of 1,3dimethylenecyclobutane is 53.5,131 a crude enthalpy surface can be constructed assuming that 1,2-dimethylenecyclobutane is roughly 5 kcal/mol more stable than its isomer by virtue of conjugation, albeit in an s-cis form. The uncertainty in this value is almost matched by that of the bisallyl species itself. As indicated above, it is roughly 32 kcal/mol less stable than unconjugated DMC or roughly 37 kcal/mol less stable than DMC stabilized by 5 kcal/mol. This enthalpy surface is depicted in Scheme 7.62.

Scheme 7.62

Finally, the thermolysis of 2,3-dimethylenebicyclo[2.2.0]hexane has been examined in an effort to determine the behavior of a 2,20 -bisallyl biradical forced to be nearly planar. The hydrocarbon dimerizes in a first-order reaction in modestly concentrated solution, and the presumed biradical can be trapped with 2,4-hexadiene with loss of stereochemistry of the diene.132 However, in the vapor phase at 1108C, first-order isomerization to a 2:1 mixture bicyclo[4.2.0]octa-1,5diene (BOD), and 3,4-dimethylene-1,5-hexadiene (DMHD), occurs.133 Re-examination of the solution behavior led to different dimers than reported previously,132 and these were formed with CIDNP emission signals suggesting reaction out of the triplet state. It was also found that the bis allene, 1,2,6,7octatetraene gave almost exclusively DMCH at low pressures at 1008C, but at high pressures and in solution a 3:2 ratio of the same products from the dimethylenebicyclo 2.2.0 hydrocarbon were formed. Further, heating BOD at higher temperatures gave DMHD (Scheme 7.63).

124


Scheme 7.63

This suggests the BOD is formed vibrationally hot from the allene. Some 1,2divinylcyclobutene is formed at moderate pressures, but it too gives DMHD at higher temperatures. All of these interconversions have been quantified;134 however, DMHD appears to be the thermodynamic sink at higher temperatures due to its higher vibrational (bond rotational) entropy. Finally, flash photolysis of an azo precursor to the bisallyl species involved in these interconversions gave a 2:1 ratio BOD and DMHD confirming the hypothesis that the TME biradical, presumably as the singlet, is involved.135 3.15 Biscyclopropylidene to Methylenespiropentane and to 1,2-Dimethylenecyclobutane Pyrolysis of biscyclopropylidene at , 2008C gives methylenespiropentane136 which on heating at 3008C for 10 h gives both 1,2- and 1,3-dimethylenecyclobutane in a 7:1 ratio (Scheme 7.64).137

Scheme 7.64

The isotope effects on the conversion of 2,2-dideuterio-biscyclopropylidene to methylenespiropentane was examined. NMR integration allowed determination of the isotope effect on bond breaking (1.24) and the isotope effect on closure when the bond to the CD2 group was broken (1.14) (Scheme 7.65).

C6H4 – C6H10

125

Scheme 7.65

A reasonable interpretation of the data would have reversible ring opening to an orthogonal TMM-like biradical follwed by rate-determining closure to product. If a rotation isotope effect (k H/k D2) of 1.5 is involved in all processes, the experimental results can be accommodated provided the orthogonal species returns to starting material roughly five times as fast as it rearranges although, Dolbier provided other possible explanations. The subsequent formation of 1,2-dimethylenecyclobutanes can arise from C1,C3 ring cleavage in the methylenespiropentane followed by ring opening of the cyclopropyl radical which gives the 2,20 -bisallyl biradical. Alternatively, both dimethylenecyclobutane isomers could result from cleavage of the C4,C5 bond followed by migration of either C1 or C2 to give the 1,2- and 1,3-dimethylenecyclobutanes, respectively (Scheme 7.66).

Scheme 7.66

Both reaction paths give the same distribution of carbons, so a labeling experiment could not distinguish between them. However, opening the methylenecyclopropane moiety would appear to be more favorable by relief of ring strain due to the trigonal carbon in the cyclopropane ring and formation of an allyl radical. On the other hand, the former pathway generates a less stable cyclopropyl radical, which also has a modest barrier to ring opening. Nonetheless, the former pathway would appear to be favored.

126


In an interesting case where cleavage of the less substituted ring above is favored by incorporation into a bicyclo[2.1.0] system, Berson found a 7:1 ratio of 1,2- and 1,3-dimethylenecyclobutanes (Scheme 7.67).138

Scheme 7.67

A related set of rearrangements was observed by Roth with a dimethylene bridged methylenespiropentane which undergoes a reversible methlenecyclopropane rearrangement at 1208C, the product of which undergoes a reversible retro Diels– Alder reaction at 1508C. Ultimately, this set of isomers gives bicyclo [4.2.0]octa1,5-diene and 3,4-dimethylene-1,5-hexadiene (Scheme 7.68).139

Scheme 7.68

Formation of the latter materials would appear to proceed from cyclopropyl radical fission in the trimethylenemethane species responsible for the interconversion of the two methylenespiropentanes. Historically, it was Crandall who first discovered the interconversions of the biscyclopylidenes and methylenespiropentane using hexamethyl-substituted materials (Scheme 7.69).140

Scheme 7.69

C6H4 – C6H10

127

Subsequently, Dolbier determined the activation parameters for the rearrangements of the tetramethyl compounds of Scheme 7.70 where log k1 ¼ 14:94 2 39 200=2:3RT and log k3 ¼ 14:77 2 38 800=2:3RT: The less-stable olefin is formed in each case, and it rearranges to the more stable olefin at higher temperatures with log ðk2 þ k22 Þ ¼ 15:72 2 46 100=2:4RT and with Keq ¼ 3:141

Scheme 7.70

Finally, the heats of formation of biscyclopropylidene and methylenespiropentane can be reasonably estimated to be 80 and 75 kcal/mol, respectively, which is consistent with the observations. Further, they are substantially less stable than the dimethylenecyclobutanes of Scheme 7.64. 3.16 Vinylmethylenecyclopropane, Allylidenecyclopropane, and 3-Methylenecyclopentene 2-Vinylmethylenecyclopropane was first prepared by Shields and Billups who also reported its thermal rearrangement to 3-methylenecyclopentene (Scheme 7.71).142 Subsequently, the kinetics of the rearrangement was determined in a stirred flow vapor phase reactor to give log k ¼ 11:48 2 25 800=2:3RT:143 The reaction would appear to involve the trimethylenemethane biradical which undergoes an allylic rearrangement.

Scheme 7.71

128


Gilbert also examined this rearrangement using deuterium-labeled material of Scheme 7.72.144 He observed interconversion with ring-labeled material with log k ¼ 11.8 2 24 900/2.3RT, which is roughly 10 times faster than formation of 3methylenecyclopentane. However, a 50:50 mixture of exo-methylene and ringlabeled 3-methylenecyclopentane is formed kinetically before complete equilibration of the label in the starting material. This is consistent with sole intermediacy of the two geometric isomers of the orthogonal vinyltrimethylenemethane biradical. Of the two, only the cisoid form can give ring-expanded product; however, it can also be responsible for the equilibrated methylenecyclopropanes, and the transoid form can only give the methylenecyclopropanes (Scheme 7.72). Rapidly equilibrating planar biradicals could also be involved.

Scheme 7.72

Davidson addressed both the singlet and triplet energy surface for these and the related allylidenecyclopropane rearrangement with 6-31Gp 6e2/6 orbital CASSCF calculations (Scheme 7.73).145 Not only were the experimental observations above reproduced, but conrotatory closure of the cisoid orthogonal singlet to the fivemembered ring was favored by 2.8 kcal/mol over disrotatory closure. Further, it was only the transoid singlet which could achieve a planar geometry which is roughly 3 kcal/mol higher in energy than the orthogonal singlet, and, of course, the planar triplet is the most stable biradical species, but it apparently is not involved in these thermal reactions presumably because intersystem crossing is slow compared with the other reactions of the singlet biradicals.

C6H4 – C6H10

129

Scheme 7.73

Apparently methyl substitution perturbs the energy surface dramatically. cis- and trans-3-methyl-2-vinylmethylenecyclopropane both equilibrate and undergo the methylenecyclopropane rearrangement slowly relative to forming 4-methyl-3methylenecyclopentene (Scheme 7.74).143 Further, there appears to be a bias for the allylic rearrangement occurring with almost exclusive use of C3 as the migrating group in what nominally is a 1,3-sigmatropic shift. This would rule out a common orthogonal biradical as an intermediate in these rearrangements.

Scheme 7.74

In a related reaction, 2,2-dimethylallylidenencyclopropane rearranges to 3-isopropylidenecyclopentene at 1908C.142 Subsequent work by Kende revealed that both

130


the anti and syn isomers rearrange to the same 2:1 mixture of 3-isopropylidenecyclopentene and 4,4-dimethyl-3-methylenecyclopentene. Further, 1-isopropylidene-2-vinylcyclopropane was found to be an intermediate in the process, Scheme 7.75146 and this material was in equilibrium with a small amount of 3,3-dimethyl-2vinylmethylenecyclopropane (Keq ¼ 123).

Scheme 7.75

It was proposed that both allylidenecyclopropanes gives the 2-vinylmethylenecyclopropanes via a trimethylenemethane biradical, and this species can also be responsible for formation of the cyclopentenes. In an effort to examine the stereochemistry of the overall rearrangement, Roth pyrolyzed the cis- and trans-2,3-dimethyl-trans-1-butenylidenecyclopropanes and found nearly the same distribution of 4,5-dimethyl-3-ethylidenecyclopentenes under conditions where starting materials did not interconvert and products did not equilibrate (Scheme 7.76).147

Scheme 7.76

C6H4 – C6H10

131

The data makes it clear that there is no stereospecificity, only stereoselectivity, in the rearrangement, thus ruling out any element of concert. It is, however, interesting that the rearrangement of allylidenecyclopropanes fused to five- and six-membered rings do show stereospecificity in a thermal double ring expansion. Thus, Cohen showed that the trans- and cis-7-butenylidenebicyclo[4.1.0]heptanes give different stereoisomers of 8-methylbicyclo[5.3.0]deca1(2),9-diene (Scheme 7.77).148

Scheme 7.77

Further, when optically active trans material was heated, there was no racemization of starting material, but there was complete racemization of product even at short reaction times.145 The data indicates that planar or rapidly equilibrating biradicals are involved, but rearrangement, presumably via a cisoid orthogonal biradical, is faster than ring closure or rotation around transoid-biradicals to produce the cisoid orthogonal biradical. Examination of the structures makes it clear that a conrotatory closure of the cisoid orthogonal species is necessary to give the observed stereochemistry, just as calculated in the parent case (Scheme 7.78).145 If there are any doubts about the likelihood that the reaction is not concerted, it should be recognized that the overall stereochemistry of the transformation represents a thermally forbidden pathway by application of the Principle of Conservation of Orbital Symmetry.

132


Scheme 7.78

3.17

3-Allylcyclopropene Degenerate 3,3-Shift

3-Allylcyclopropenes undergo a degenerate 3,3-sigmatropic shift and an intramolecular 2 þ 2 cycloaddition. Padwa showed that 3-allyl-3-methyl-1,2-diphenylcyclopropene equilibrates with 3-allyl-2-methyl-1,3-diphenyl.cyclopropene and 6-methyl-1,2-diphenyltricyclo[3.1.1.02,5]hexane at 1508C (Scheme 7.79).149

Scheme 7.79

A temperature study of the conversion of the 2-methylderivative to the 3-methyl material revealed log k ¼ 12:1 2 31 900ð^2500Þ=2:3RT: As a result of work with more substituted derivatives, it was suggested that a chair-like 1,4-biradical arising from only bond making could account for all the observations. The biradical could cleave to the rearrangement product or after ring flip from the chair to the boat conformation, it could close to the tricyclohexane (Scheme 7.80).

C6H4 – C6H10

133

Scheme 7.80

Alternatively, the two processes could be independent; however, it is not unreasonable that bond formation is favorable to relieve the strain of one trigonal carbon in the three-membered ring and to avoid the generation of a third trigonal center as well as the potential for anti-aromatic destabilization were allylic bond cleavage to occur – compare to Chapter 7, Section 4.1, hexadiene 3,3-shift. 3.18

[2.1.1]Propellane

[2.1.1] Propellane has not been isolated150 nor has its isomerization products been characterized. Nonetheless, the potential energy surface has been calculated at a high level of theory, CAS(12,12)PT2/6-31G(d) among others, and isomerization to 3-methylenecyclopentylidene which should give hydrogen shift products was found to be the low energy pathway with an activation energy around 30 kcal/mol (Scheme 7.81).151

Scheme 7.81

Ring-opening pathways to 1,2- and to 1,3-dimethylenecyclobutane were found to be higher in energy by 25 and 10 kcal/mol, respectively.

134 4 4.1


C6H10 1,5-Hexadiene 3,3-Shift – the Cope Rearrangement

The 3,3-sigmatropic shift in allyltoluenes was pursued by Hurd152 in 1934 who was searching for an all-carbon Claisen rearrangement.153 Cope provided strong evidence for the rearrangement in substituted systems so that the rearrangement is now known as the Cope rearrangement.154 Humski, et al. demonstrated that the reaction in the parent case was exclusively a 3,3-shift without any 1,3-shift component in 1970155 although work with substituted materials earlier indicated that this was the case. Thus allyl radicals are not involved on the low energy path in simple systems, rather a delocalized or concerted transition state is involved (Scheme 7.82).

Scheme 7.82

Indeed, Golden and Benson studied the dissociation of 1,5-hexadiene to two allyl radicals by flash vacuum pyrolysis and found a very high activation energy for this process, log k ¼ 15:6 2 62 200=2:3RT:156 A separate determination gave log k ¼ 14:79 2 57 700=2:3RT:157 (The origin of the difference is not obvious, but the activation free energies at 500 K are not very different.) For the rearrangement of the parent compound with deuterium labels assuming no kinetic or equilibrium deuterium isotope effect in the reaction log k ¼ 10:36 2 34 300=2:3RT:158 Doering and Roth provided stereochemical evidence that is consistent with a chair-like transition state.159 Thus, upon heating threo-3,4-dimethyl-1,5-hexadiene gave a 9:1 mixture of E,E- and Z,Z-2,6-octadiene, respectively, with less than 1% of the E,Z-diene being formed kinetically, presumably via a boat-like transition state. To confirm the hypothesis, pyrolysis of erythro-3,4-dimethyl-1,5-hexadiene gave 99.7% E,Z-2,6-octadiene as is expected from a chair-like transition state and only

C6H4 – C6H10

135

0.3% of the E,E isomer (Scheme 7.83). From the data it was estimated that the chair transition states were favored over the boat transition state by at least 5.7 kcal/mol.

Scheme 7.83

The interpretation of the Doering– Roth experiment lay unquestioned for nearly a decade, a period of time, which saw recognition of the role of Conservation of Orbital Symmetry and increased awareness of the need for stereochemical labeling at every site in a reaction to deduce its three-dimensional course. A complete analysis of the geometric possibilities for the 3,3-shift by Goldstein recognized the possibility of four Orbital Symmetry allowed, that is, concerted pathway via chair, boat, twist (or helix), and plane transition states (Scheme 7.84).160

Scheme 7.84

136


In the chair and boat processes, both allylic fragments are used in a suprafacial manner although secondary electronic and steric considerations apparently favor the former.161 The two other transition states, twist and plane, utilize the allylic fragments each in an antarafacial manner. Importantly, the twist transition state is also consistent with the results of the Doering – Roth experiment. A threedimensional graph of the stereochemical possibilities is given in Scheme 7.85 utilizing methyls and labeled methyls as stereomarkers. The graph consists of two mutually orthogonal hexagons representing two stereochemical families. Interconversion of stereoisomers within each family can occur only by chair and boat transition states. However, interconversion of the two families can occur via twist and plane transition states. Only some of the connections are shown in the graph. Careful inspection and comparison to Goldstein’s schemes will reveal an error in one of Goldstein’s predictions.

Scheme 7.85

C6H4 – C6H10

137

While the original Doering –Roth experiment could not distinguish between the two pathways, chair or twist because there were no additional stereochemical labels on C-1 or C-6, Hill did, in fact, provide the decisive evidence in favor of the chair form although the twist pathway was not considered in 1967.162 Hill found that 3R3-phenyl-3-methyl-E-1,5-heptadiene gave 83% of 3S-3-methyl-6-phenyl-E-1,5heptadiene along with 4% of its 3R isomer. In addition, 12% of the 3R Z isomer and 1% of the 3S Z isomer were found (Scheme 7.86). The two major products, 3SE and 3RZ, can only come from either a chair or a plane transition state, but the latter is unlikely because of very poor sigma overlap between the reaction centers. Only the minor products can come from a twist transition state, but a boat transition state is also possible and probably likely in view of subsequent experiments specifically designed to provide information on the twist and plane transition states, all of which is negative, vide infra.

Scheme 7.86

Benner and Hawkins prepared and pyrolyzed R,R-4,5-dimethyl-Z,Z-1,1,1,8,8,8hexadeuterio-2,6-octadiene (trans,cis,cis) and found log k ¼ 11:36 2 36 000=2:3RT for formation of the ttt diastereomer and log k ¼ 10:73 2 37 400=2:3RT for conversion of ttt back to the tcc isomer. Furthermore, label scrambling in the ttt isomer had a free energy of activation of 41.5 kcal/mol at 2408C, which is virtually identical to that for the 3,3-shift in 1,5-hexadiene itself. Then in an effort to uncover evidence for a twist transition state optically active R,R4,5-dimethyl-Z,Z-1,1,1,8,8,8-hexadeuterio-2,6-octadiene was pyrolyzed, see Scheme 7.85. Because of the labeling pattern, the 3,3-shift could occur many times via chair and boat transition states and still stay in the same stereochemical

138


family. However, a twist or plane transition state would result in formation of the other stereochemical family. Oxidation of the reaction mixture to the 2,3dimethylsuccinic acids followed by proton NMR in an optically active solvent would reveal the antara – antara reaction path from integration of the methyl hydrogens in the trans acid. However, with increasing pyrolysis times the only product observed in addition to those from chair or boat transition states were 1,3shift products most likely resulting from cleavage to 1,3-dimethylallyl radicals followed by recombination. The boat and cleavage reactions were roughly 6 kcal/ mol higher in energy than the chair-like 3,3-shifts in this system.163 Doering, utilizing phenyl labels instead of methyl labels, made similar observations except that the activation parameters for the chair-like 3,3-shift (racemization of the threo, trans, trans isomer) were DH ‡ ¼ 21:3 kcal=mol; DS‡ ¼ 213:2 e:u: and DV ‡ ¼ 27:4 cc=mol and those for cleavage (i.e. formation of the meso compound) were DH ‡ ¼ 30:7 kcal=mol; DS‡ ¼ 2:1 e:u: and DV ‡ ¼ 13:5 cc=mol:164 The stereochemical observations in all systems, therefore, require that the chairlike species is the lowest energy transition state for the 3,3-shift of 1,5-hexadiene. The boat-like transition state is 5.8 kcal/mol higher in free energy than the chair transition state from Goldstein’s determination of the activation parameters for the conversion of meso,E,Z-1,3,4,6-tetradeuterio-1,5-hexadiene to the threo isomers (Scheme 7.87), namely DH ‡ ¼ 44:7 ^ 2:0 kcal=mol and DS‡ ¼ 23:0 ^ 3:6 e:u:165 and comparison with Doering’s activation parameters for the rearrangement. The similarity of this energy difference with that determined by Doering and Roth with the 3,4-dimethyl-1,5-hexadienes is reassuring.

Scheme 7.87

However, the Goldstein experiment could not distinguish between the boat and the plane transition state, but the 3,3-shift of 1,4-dimethylenecyclobutane studied by Hoffmann,166 which has an activation free energy of only 6 kcal/mol higher than that for the parent compound, indicates that the boat transition state is not energetically inaccessible in the Goldstein experiment (Scheme 7.88).

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139

Scheme 7.88

In another example of using molecular constraints to force chair and boat transition states on a 3,3-shift, Shea determined the activation parameters for the irreversible rearrangements of the threo- and meso-bis-2-methylenecyclopentyls to 1,2-bis(1cyclopentenyl)ethane (Scheme 7.89).167 The threo isomer must rearrange via a chair (or much less likely, a twist) transition state; its activation parameters are DH ‡ ¼ 28 kcal=mol and DS‡ ¼ 211:4 e:u: The meso isomer must rearrange via a boat (or much less likely, a plane) transition state; its activation parameters are DH ‡ ¼ 41:8 kcal=mol and DS‡ ¼ 20:4 e:u: The difference in free energy between the two reactions is 8 kcal/mol, again consistent with other observations. However, another important factor emerges from the kinetics determined by Doering, Goldstein, and Shea: the activation entropy for the boat transition state is roughly 10 e.u. higher than that for the chair transition state. In other words, the boat transition state is “looser” than the chair transition state providing a free energy benefit of roughly 5 kcal/mol for the boat transition state. This partially compensates for a much larger difference in activation enthalpies favoring the chair transition state.

Scheme 7.89

140


Finally, while the twist transition state appears to be very high in energy in acyclic systems, at least as high as the cleavage reaction, such a transition state was thought to be involved in a skeletal rearrangement of a bicyclo[3.2.0]hepta-3,6-dien-2-one studied by Mukai, Scheme 7.90.168 However, subsequent pyrolysis of a deuteriumlabeled bicyclo[3.3.0]octa-2,6-diene that has a similarly positioned diene system resulted in no rearrangment even at 4508C.169 This lead to the reasonable suggestion that Mukai’s rearrangement is not a 3,3-shift but a cyclobutene ring opening to a Z,E,Z-cyclic triene which recloses to a different cyclobutene (Scheme 7.90) (see Chapter 8, Section 2).

Scheme 7.90

4.1.1

Substituent Effects on the 1,5-Hexadiene 3,3-Shift

In the paper describing the kinetics of the 3,3-shift of 1,1-dideuterio-1,5hexadiene,158 the specter of a non-concerted reaction path via a 1,4-cyclohexane diyl was raised. This species could result from exclusively bond making between C1 and C6 without concomitant cleavage of the C3,C4 bond. The estimated heat of formation of this species as a non-interacting biradical was 38 kcal/mol above 1,5hexadiene by the usual technique of addition twice the secondary C – H bond dissociation energy minus the BDE of dihydrogen to the heat of formation of cyclohexane and comparing this the heat of formation of 1,5-hexadiene.

C6H4 – C6H10

141

Subsequently, the bond dissociation energy of a secondary C – H was determined to be 98 kcal/mol,170 which places the heat of formation of the diyl species roughly 42 kcal/mol above 1,5-hexadiene and 8 kcal/mol above the heat of formation of the transition state for the 3,3-shift. This would appear to be a mininum estimate given that a similar calculation of butane 1,4-diyl is only 61 kcal/mol above cyclobutane while the transition state for formation and cleavage of this species is 4 kcal/mol higher in energy (see Chapter 5, Section 3). Thus, the case for a non-concerted 3,3-shift via a cyclohexane-1,4-diyl is weak. Nonetheless, substituent effects on the rate of the 3,3-shift were intially interpreted in terms of the diyl species. In particular, Dewar found the 2-phenyl and 2,5,diphenyl-1,5-hexadiene rearrange 40 and 1600 times, respectively, more rapidly than that of the parent diene.171 Further, semi-empirical MINDO/3 calculations supported the proposition that even the parent species proceeded via the chair-like cyclohexane-1,4-diyl.172 These observations and calculations provided stimulus for a substantial effort in the subsequent years to address the question of transition state structure in and the energy surface for the 3,3-sigmatropic shift of 1,5hexadiene. However, one obvious measure of the inadequacy of the diyl species as a transition state or intermediate is the relatively small effect of the C2-phenyl group on the rate of reaction. The phenyl would stabilize the double bond by 2.6 kcal/mol,173 but would increase the stability of the diyl by ca. 13 kcal/mol, the benzyl radical resonance energy. A 10 kcal/mol effect on the rate at roughly 450 K should have increased the rate by a factor of 100 000 per phenyl not only 40fold. Another important observation was provided155 even before the Dewar papers, namely that the secondary deuterium kinetic isotope effect (SDKIE), at C4 of a 3,3-dicyano substituted-1,5-hexadiene was a larger normal effect in an absolute sense than the inverse SDKIE at C6. This must represent more bond breaking at C4, i.e. rehybridization from sp3 to sp2, than bond formation at C6 involving rehybrization from sp2 to sp3, a result inconsistent with a diyl-like transition state whose SDKIEs would be opposite in magnitude. Subsequent SDKIE determination by Conrad on 3-alkyl substituted -1,5-hexadiene and 2-phenyl-1,5hexadiene and on carbon-13 labeled 2,5-diphenyl-1,5-hexadiene revealed a change in transition state structure from a species with more bond making than breaking to one with a nearly a fully formed 1,4-cyclohexane diyl in that order, respectively.174 The interpretation of all the data focused on a variable transition state whose structure was dictated by the nature and placement of substituents. As was customary even in the 1970s, depiction of reactions involving two or more bonds, in particular the E2 elimination, utilized a three-dimensional potential energy diagram first proposed by Rory More O’Ferrall and Bill Jencks which served to illustrate the variation of transition states between extremes. Scheme 7.91 represents this energy surface applied to the 3,3-shift. The MOF-J diagram depicts two structural changes: C3,C4 bond breaking to generate two allyl radicals and C1,C6 bond making to generate the 1,4-cyclohexane

142


Scheme 7.91

diyl at the extremes. The combination of these two leads to product. The potential energy coordinate increases out of the plane to the viewer. Thus for the 3,3-shift of the parent diene, reactant and product are equally low points in energy while the two extreme transition state possibilities are high points in energy with the actual transition state occuring at an energy saddle point somewhere within the diagram. Such a scheme was first proposed for the 3,3-shift by Wehrli et al. without any data to locate transition structures.175 However, Conrad’s SDKIEs provided a metric. In a minimally substituted system, the transition state is roughly one-third the way along the bond breaking coordinate and two-thirds along the bond making coordinate. Progress along each coordinate is measured by the natural logarithm of the SDKIE at the appropriate carbon ratio against the maximum value expected for that SDKIE in the structure at the extreme of behavior being measured. The latter values were originally assumed to be the equilibrium isotope effect (EIE) for fractionation of deuterium between reactant and product at the appropriate carbon. Subsequently, due to the theoretical considerations by Houk176 and experimental work by Olson,177 the EIE for conversion of reactant to two allyl radicals was revised upward because C – H bending frequencies for allyl radicals are lower than those for olefinic carbon. Determinations of EIEs are not direct, and the values are temperaturedependent just as the SDKIEs. Nonetheless, the broad range of the ratios reveals a substantial variation in transition state structure when utilized in the linear free energy relationship described. Humski’s SDKIEs for the 3,3-dicyano substituted

C6H4 – C6H10

143

diene make it clear that the transition state more closely resembles two allyl radicals while Conrad’s SDKIEs for the 2,5-diphenyl substituted diene suggests a transition state which is very close to the diyl. The transition state structure variation found from the SDKIEs indicates that the relative energies of the two extreme structures are the major factors controlling transition state structure and stabiity. Substitution, which stabilizes one over the other draws the transition state in the direction of the stabilization, and, of course, results in stabilization of the transition state, but not necessarily by the full effect expected on the extreme transition state structure. This variation has been characterized as the “perpendicular” effect because the transition state perturbation occurs in vibrations orthogonal to that associated with the reaction coordinate motion. This stands in contrast to the “parallel” effect in which substituents perturb the relative energies of reactant and product. This latter effect is also known as Hammond’s Postulate. However, the parallel effect is but a minor consideration in the systems described thus far because the energy difference between reactant and product is very small or zero. The power of this type of analysis is illustrated by the activation entropies determined for reactions proceeding by boat-like transition states. The boat-1,4cyclohexane-diyl should be highly destabilized by steric and perhaps electronic effects relative to two allyl radicals. Therefore, the transition state is not only higher in energy but more resembles two allyl radicals; thus the transition state should be looser as is observed. Moreover, those 3,3-shifts that must proceed via boat geometries in cyclic systems are very high-temperature processes if they are not assisted by strain relief when the doubly allylic CC bond is broken (see Chapter 9, Section 4, for example). In the cases of the boat-like 3,3-shifts of cis1,2-divinylcyclopropanes and -cyclobutanes as well as those of bullvalene (CH)10, semibullvalene (CH)8 and related compounds including a cyclopentadiene dimer (a bicyclo[2.2.1]hept-2-ene system) double allylic bond fission is a much lower energy alternative due to strain relief. Therefore, these rearrangements should have transition state strongly resembling the two allyl radical alternative, and when this hypothesis was tested in the case of Barbaralane, C9H10, it was not found wanting. One important aspect of all the observations, stereochemistry and kinetic isotope effects, is the stimulus provided to theoretical chemistry. The inadequacy of the semi-empirical MINDO approach led to efforts at a more fundamental level. However, initially, the size of even the parent structure made difficult application of large basis sets and adequate levels of correlation. Further, the partial open shell nature of the transition state, multiconfiguration SCF approaches are required. Remarkably, Morokuma, Borden, and Hrovat were able to locate both a chair and a boat transition state at the CASSCF 3-21Gp level having bond making and breaking ˚ , respectively.178 However, with the advancement in distances of 2.09 and 2.32 A computer power, MCSCF with a 6-31Gp basis set with configuration interaction (CI) ˚ bond distances and an aromatic transition state found evidence for a diyl with 1.64 A ˚ bond distances179 thus seemingly reinforcing the original semiwith 2.19 A

144


empirical calculations by Dewar but not be capable of reproducing the SDKIEs! Remarkably, density functional theory (DFT), using B3LYP functionals with a 631Gp basis set provided results180 which closely matched Conrad’s SDKIEs. The ˚ . It was only with the breaking and making bond distance was calculated to be 2.04 A addition of dynamic correlation to CASSCF and using a 6-311G(2d,2p) basis set could the SKDIEs be reproduced by multiconfiguration methods.181 It should be noted that not only has the chair-like transition state been characterized, but the boatlike transition state was characterized by calculations as well. As expected from the more favorable entropy of activation for reactions proceeding via the latter species, ˚ while the bond breaking and making bond lengths for the latter species are 2.20 A ˚ the bond length in the chair-like transition state is 1.885 A. Even more recently, concerns about substituent effects in the 3,3-shift were raised by Doering.182 Two extreme situations were considered. In one, the “Centauric” transition state, there is an independent action of substituents making conflicting, but full, electronic demands on the two halves of the transition region, the diyl and two allyl radicals. In the second, the “Chameleonic” model, the substituent groups in position of opposing demands are no more effective than the stronger of them. If one where to quantify the two models proposed, Eqs. (7.1) and (7.2), respectively, would apply where the activation free energy observed is the dependent variable with the activation free energies to form the diyl and the two allyl radicals are the independent variables both of which might be obtained from external studies. In fact, Doering cast the analysis in terms of enthalpy units alone in order to avoid temperature effects, but clearly rates are a function of both enthalpy and entropy and temperature. Centauric Chameleonic

DG‡ ð3; 3-shiftÞ

/ DG‡ ðdiylÞ þ DG‡ ðtwo allyl radicalsÞ

ð7:1Þ

DG‡ ð3; 3-shiftÞ / aDG‡ ðdiylÞ þ bDG‡ ðtwo allyl radicalsÞ

ð7:2Þ

where a ¼ 1 or 0 and b ¼ 0 or 1, respectively. Examination of the activation energies for rearrangement of 2-phenyl-, 2,5diphenyl-, 1,3-diphenyl-, 1,6-diphenyl-, 1,3,5-triphenyl-, and 1,3,4,6-tetraphenyl1,5-hexadiene, all thermoneutral reactions to avoid thermodynamic considerations led Doering to conclude that neither model was appropriate. In the analyses, great care was taken to correct for the effect of phenyl on the stability of the starting material in each case. Almost two decades previous to the Doering papers a reasonable model for substituent rate effects was proposed that was based on a geometric model for the MOF-J energy surface for the 3,3-shift.183 Thus, a hyperbolic paraboloid surface equation could be differentiated to obtain coordinates and the activation free energy for the saddle point (the transition state) cast in terms of the relative free energies for formation of the diyl and the two allyl radicals, the same independent variables of Eqs. (7.1) and (7.2). Equation 7.3, which relates the independent variables by the harmonic mean is based on the simplest hyperbolic paraboloid surface, that is, one with linear edge potentials. Slightly more realistic models were also explored,

C6H4 – C6H10

145

but Eq. (7.3) suffices to model the data once it is recognized that equation provides no option to vary the dependence of the activation free energy on the coupling of the two perpendicular extremes. Thus a proportionality constant could multiply the harmonic mean of the perpendicular extremes, and if this constant could be determined by linear regression of all the observations, then some confidence can be placed in the model which now is represented by Eq. (7.4). DG‡ ð3; 3-shiftÞ ¼ DG‡ ðdiylÞ £ DG‡ ðtwo allyl radicalsÞ=ðDG‡ ðdiylÞ þ DG‡ ðtwo allyl radicalsÞ 2 DG‡ ðrxnÞ

ð7:3Þ

DG‡ ð3; 3-shiftÞ ¼ c £ DG‡ ðdiylÞ £ DG‡ ðtwo allyl radicalsÞ=ðDG‡ ðdiylÞ þ DG‡ ðtwo allyl radicalsÞ 2 DG‡ ðrxnÞ

ð7:4Þ

Indeed, it proved possible to correlate the data but an almost exact fit could be obtained provided that the experimental activation free energy for 2-phenyl-1.5hexadiene was increased by 1.7 kcal/mol.184 The reported error limits171 were sufficiently high to justify this assumption. Further the correspondence of the activation entropy to those of other 3,3-shifts was sufficiently poor that coupled with Conrad’s observation that similar materials reacted slower and a report185 that the rate constant for rearrangement of the 2-phenyl derivative at 1648C is only one-twelfth as large as that determined from the originally reported activation parameters reinforced the assumption. Finally, it should be recognized that the harmonic mean equation, Eq. (7.4), is a mathematical statement of the notion that in the 3,3-shift, substituents that increase the stability of the more stable alternative, diyl or two allyl radicals, to a greater extent than substituents that increase the stability of the less stable alternative. In effect, increasing the stability of the less stable alternative results more in drawing the transition state toward the previously described less stable structure than in stabilizing the transition state. A visual model might be generated by attaching the opposite corners of a square sheet of rubber over two vertical stakes of uneven length and tying the remaining corners to the ground. If a rod could be balanced on the diagonal between the stakes, it would rest near the shorter one. If now the shorter stake were made shorter by a given increment, the rod, if it were still balanced, would rest lower than if the longer stake were shortened the equivalent amount. There should be recognition of the theoretical pursuits of Houk and Borden a which have been successful in reproducing the experimental activation energies for not only phenyl substituted 1,5-dienes, but cyano-substituted ones as well.186 While substituents like alkyl, halogens, and hydroxyl groups at C2 through C5 of 1,5-hexadiene have little effect on the rate of the Cope rearrangement, an anionic substituent at C3 has a dramatic effect. Evan’s first demonstrated an enormous rate acceleration (1010 –1017 fold!) upon conversion of 3-hydroxy-1,5dienes to their conjugate bases when the counterion is potassium ion and the solvent is HMPA.187 Further, addition of 18-crown-6 to a THF solution of the

146


anion resulted in a dramatic rate acceleration. The origin of the rate effect appears to be weaking of the C3 –C4 bond by the anionic substituent.188 Gee showed that the rearrangement of the potassium salt of 3-hydroxy-3-methyl-1,5-hexadiene is first order in both THF and DMSO solvent with the rate being roughly 1000 times faster in the latter solvent compared to the former.189 Since small tertiary alkoxides are tetrameric and dimeric in the two solvents, respectively, the reaction must proceed within the aggregates. However, in DMSO, the rate was inversely proportional to added potassium ion suggesting that dissociation to a triple ion (two alkoxides sharing a single potassium ion) is involved, and large SDKIEs at C4 implicate that the dissociation is a pre-equilibrium process. Further, smaller inverse SDKIEs at C6 along with the large effects at C4 indicate a dissociative-like transition state, although evidence for a concerted reaction via a chair transition state.190 Gee also discovered that one equivalent of 18-crown-6 was necessary for complete reaction in THF and that the SDKIEs also indicated a dissociative-like transition state. Remarkably, the oxyanionic Cope rearrangement also occurs in the gas phase.191 Houk found a delocalized transition state for the oxyanionic Cope rearrangement at the B3LYP/6-31 þ Gp level; however, it has a ˚ C1 – C6 bond distance and a 2.33 A ˚ C3 –C4 bond distance suggesting a 3.28 A 192 dissociative-like transition state. Attempts to persuade the conjugate base of 3-amino-substituted 1,5-dienes to undergo the Cope rearrangement met with no success; only cleavage products were isolated,192 and calculations at the B3LYP/6-31 þ Gp level indicated that, if anything, the amide anion bearing derivative should dissociate heterolytically because acrolein imine has an unfavorable electron affinity for homolytic cleavage which should promote the rearrangement. In yet another interesting system, Dolbier examined the 3,3-shift of 1,1,6,6tetrafluoro-1,5-hexadiene to the 3,3,4,4-tetrafluoro isomer which has activation parameters somewhat lower than those of the parent system: DH ‡ ¼ 29:9 kcal=mol and DS‡ ¼ 218:5 e:u:193 However, while the terminal geminal difluoro substitution on the threo- Shea compound (Scheme 7.89) lead to similar activation parameters as the hydrocarbon, the activation parameters for the meso isomer were extraordinary in the sense of a very high activation enthalpy and a positive entropy of activation, 49.5 kcal/mol and 8.1 e.u., respectively. The inhibition of the boatlike transition state by terminal fluorine substitution was argued to be a steric effect preventing the formation of a bond with eclipsed C – F interactions. Finally, there has been both an experimental and theoretical study of 3,3-shifts which proceed via two-allyl radicals as a result of the 1,5-diene system being constrained to ring systems which would not easily permit close approach of the diene termini.194 4.2

The Claisen Rearrangement of Allylvinyl Ethers

While not a hydrocarbon rearrangement, the 3,3-shift of allylvinyl ethers to g,dunsaturated carbonyl compounds, the Claisen rearrangement,195 is so closely related

C6H4 – C6H10

147

to the Cope Rearrangement and is so important in mechanistic and synthetic chemistry that it bears scrutiny in the current context. In the parent system the rearrangement occurs at 160 –2008C in the gas phase with DH ‡ ¼ 29:7 kcal=mol and DS‡ ¼ 27:7 e:u:196 The reaction proceeds predominantly via a chair-like transition state judging by the product distribution from E,E-propenyl-2-butenyl ether, namely a 95:5 ratio of threo to erythro product; the latter product is the expected result of a boat-like transition state (Scheme 7.92).197

Scheme 7.92

In an effort to assess the relative extent of bond making and breaking in the transition state, Conrad determined the normal SDKIE at C4 of allylvinyl ether to be roughly 45% that of the equilibrium isotope effect (EIE) at that carbon estimated from the equilibration of 1,1-dideuterio- and 3,3-dideuterioallyl acetate at 1608C catalyzed by mercuric acetate.198 Conrad also found that the inverse SDKIE at C6 of allylvinyl ether was roughly one-sixth that of the EIE at that carbon estimated from an interpolation of the EIE for the all-carbon 3,3-shift at various temperatures. Thus, the transition state involves more bond breaking than making and is “early” with respect to reactant and product as expected for a highly exothermic reaction (ca. 2 17 kcal/mol).199 This data was supported by carbon-13 and oxygen-18 kinetic isotope effects determined by Shine and Saunders200 and CASSCF calculations by Houk.201 DFT calculations with the BLYP functional also provide a transition state which closely reproduces the kinetic isotope effects (Scheme 7.93); also included in the scheme are the results of an MP4(SDQ) calculation by Singleton.202 Concerns about the structure of the transition state, polar or non-polar, were raised when Carpenter found that the reaction occurred with a faster rate in polar solvents and particularly in aqueous solvents.203 Brandes provided quantitative

148


Scheme 7.93

rate data in pure solvents for an allylvinyl ether with a six-carbon chain attached to C4 which terminated in either a sodium carboxylate or a methylcarboxylate.204 The former provided water, trifluoroethanol (TFE), and DMSO solubility while the latter provided solubility in less polar solvents. Since the rates of the two substrates were comparable in those solvents in which both were soluble, it was possible to deduce that the effect of water relative to cyclohexane was a factor of roughly 200 in rate or a factor of almost 4 kcal/mol at 808C. The rate data correlated with that for the solvolysis of tert-butyl chloride in those solvents which actually promote the solvolysis, but the slope was sufficiently small so as to rule out ionization as a mechanistic alternative. Nonetheless, the effect of water on the 3,3-shift, like that on the solvolysis reaction, is dramatic and is in need of explanation. As a result of Monte Carlo calculations of the effect of many water molecules on the transition state as determined by an ab initio MO calculation, Jorgenson argued that the water donated two hydrogen bonds to the developing carbonyl oxygen in the Claisen rearrangement thus stabilizing the transition state.205 On the basis of free energies of transfer of chloride ion between water and various solvents, in particular, TFE, Gajewski argued that if hydrogen bond donation were so important, the rearrangement would be faster in TFE than in water.206 Further, as a result of attempting to correlate the solvent rate data with various parameters including the Kirkwood –Onsager function of dielectric constant, ð1 2 1Þ=ð21 þ 1Þ; the free energies of transfer of chloride ion, and the Hildebrand cohesive energy density, ðDHvap 2 RTÞ=Vmolar it was concluded that in water hydrogen bond donation

C6H4 – C6H10

149

accounted for roughly half the rate acceleration and the cohesive energy density accounted for the other half. The dielectric function was not a significant factor suggesting that there was little change in dipole moment from ground state to transition state. The cohesive energy density appears to be important in water because there is a negative volume of activation for the Claisen rearrangement.207 Thus, in water there is less disturbance of the hydrogen bonding network of water in the transition state than in the ground state. This hydrophobic-like explanation also applies to the solvolysis reaction of tert-butyl chloride.208 On the other hand, in TFE solvent, hydrogen bond donation to the transition state is the dominant factor in both the Claisen rearrangment and the solvolysis reaction. Supportive of the suggestion that ionization is not a major pathway in the Claisen rearrangement of the parent compound is the fact that the SDKIEs in aqueous solution are comparable to those in the gas phase and in m-xylene.209 Furthermore, attempts to solvolyze 1,1-dideuterioallyl mesylate in aqueous methanol resulted in no ionization to an allyl cation; instead, the direct displacement product was formed exclusively. Finally, determinations of a solvent kinetic isotope effect in deuterium oxide resulted in values around unity ^ 10%.210 In the solvolysis reaction of tertbutyl chloride the value is 40% at room temperature.211 It is possible to cause allylvinyl ethers to ionize by providing cation stabilizing substituents and Lewis acids or Lewis acidic solvents.212 Substituents have a dramatic effect on the rate of the Claisen rearrangement. Perhaps the greatest acceleration results from p donor groups at C2. Ireland explored the effect of oxyanion and silylether substitution at C2 and also showed that the chair transition state is favored over the boat by roughly 3 kcal/mol.213 Johnson and Eschenmoser found dramatic rate accelerations with alkoxy and nitrogen donors, respectively, and even fluorine has a large effect.214 The origin of this effect was provided by Emrani who determined the SDKIEs at C4 and C6 of purified allyl (1-trimethylsilylvinyl) ether.215 The normal effect at C4 is large and nearly comparable to that expected for formation of an allyl radical while that at C6 is small and inverse which is also consistent with allyl radical generation, Scheme 7.94. Further, the reaction rate in acetonitrile was only three times that in carbon tetrachloride thus ruling out transition states more or less polar than reactants. The promotion of a more dissociative transition state by donors at C2 can be rationalized by the formation of radical character alpha to the developing carbonyl group which in turn is dramatically stabilized by oxygen and amine donors in the form of ester and amide resonance energies, respectively, whose values are greater than 10 kcal/ mol and nearly 20 kcal/mol, respectively. This and the subsequent isotope effect determinations on allylvinyl ether 3,3-shifts can be summarized by a More O’Ferrall – Jencks diagram similar to that used for the 1,5-hexadiene 3,3-shift (Scheme 7.94). Besides the question of the relative extents of bond making and breaking, the overall structure of the transition state in the Ireland– Claisen rearrangement is a chair-like structure. Included in Scheme 7.94 is the aromatic Claisen transition state whose SDKIEs were first determined by McMichael.216 It is significant that this

150


Scheme 7.94

transition state is not only highly dissociative but “late”. This is consistent with the fact that the 3,3-shift is endothermic as a result of destroying aromatic resonance energy, but still resembling a relatively stabilized phenoxy radical. This interpretation is not inconsistent with more recent heavy atom and deuterium kinetic isotope effects determined by Singleton217 and with calculations at the Becke3LYP/6-31Gp level whose O – C(alpha) and C(ortho) – C(gamma) bond ˚ , respectively.218 lengths in the transition state are 2.12 and 2.19 A Oxygen donors at C4 and C6 provide a modest rate acceleration and, in addition, make the rate more susceptible to polar solvents.219 However, no SDKIEs have been determined to address the question of transition state structure. Sigma electron withdrawing groups like trifluoromethyl have no effect at C4 which is consistent with partial radical and not polar character at that site.220 Trifluoromethyl at C2 does provide some rate acceleration. However, the origin of this effect is not clear since the SDKIEs are similar to those of the parent molecule,

C6H4 – C6H10

151

but this may result from ground state destabilization, Scheme 7.94. The placement of radical stabilizing groups like cyano at C4 of allylvinyl ether has a substantial effect in increasing the rate of the Claisen rearrangement.221 This is not inconsistent with the development of some radical character at C4. The effect of the radical stabilizing group, carboalkoxy, at C2 forms an important chapter in the Claisen rearrangement. The only pericyclic reaction found in all of biology involves the Claisen rearrangement of chorismic acid to prephenic acid in the shikimic acid pathway for formation of precursors to phenylalanine in plants.222 Thus, it is important to understand the reaction not only in vitro but in vivo. The enol pyruvate moiety of chorismate, could be studied in the simplest case, particularly with respect to isotope effects. The SDKIEs with 2-carbomethoxy allylvinyl ether revealed a transition state with more bond making character than the parent (Scheme 7.94).203 This is the likely result of the radical stabilizing group stabilizing the 3-oxacyclohexane-1,4-diyl species, a not unexpected result. But this left open the question of why chorismate undergoes the Claisen rearrangement rapidly at only 558C in aqueous solution. Knowles determined the secondary tritium kinetic isotope effect at the bond breaking and bond making sites of chorismate and concluded that the transition state was more dissociative not associative as in the parent case with radical stabilizing groups at C2.203 As to why the chorismate rearrangement occurs so rapidly and with a dissociative transition state was addressed by Jurayj who prepared a series chorismate esters successively stripped of substituents.203 In the absence of both the hydroxyl and the carboalkoxy group on the ring, the rearrangement in aqueous methanol occurred rapidly at 2 158C (Scheme 7.95).

Scheme 7.95

Similar relative rates were observed in organic solvents. It is important to recognize that addition of the hydroxyl slowed the reaction, and addition of the carboalkoxy group further slowed the reaction to that of the parent chorismic acid at pH 7. The conclusion from the experiments is that the chorismate rearrangement is dissociative because the potential cyclohexadienyl radical which could be generated is highly stabilized in the hyperconjugated form of benzene and a hydrogen atom.223 This is consistent with the fact that the cyclohexenyl derivative is relatively sluggish in undergoing the Claisen rearrangement.

152


Thus chorismate is unique among all Claisen systems by virtue of the cyclohexadienylic moiety, which is attached to the enolpyruvate. Consistent with this is unpublished work by Jurayj who found that the equivalent cycloheptadienyl enolpyruvate was much more sluggish in its rearrangement.224 The effect of hydroxyl in retarding the rate of rearrangement may be due to the usual effect of allylic hydroxyl namely stabilization of the double bond by hyperconjugative stabilization to give hydroxide ion and an allylic cation. The rate retarding effect of the ring carboalkoxy group is probably due to the usual steric retardation of bond formation by substitution at the bond making site. Finally, the rate of rearrangement of chorismate to prephenate is roughly a million times faster in the presence of the enzyme, chorismate mutase. The origin of a nearly 9 kcal/mol effect has attracted the attention of many. The enzyme can be an entropy trap by pre-organizing the two freely rotatable bonds into the chair-like transition state structure. However, the entropy contribution due to bond rotation is roughly 5 e.u. per bond which would lead to at most a 3 kcal/mol effect at room temperature. Clearly, more factors must come into play. One of these is the observation that a number of proton donating (at pH 7) amino acids are near the pyruvate oxygen in crystal structures of various chorismate mutases with a transition state analogue.225 Thus, hydrogen bond donation may provide some, if not all, of the extra driving force. There is also evidence for an aspartate or a glutamate near the hydroxyl on the ring, which can accept a hydrogen bond from the hydroxyl hydrogen. This would have the effect of lessening the stabilization of the cyclohexadienyl unit by the hydroxyl and contribute to the rate acceleration induced by the enzyme. It has also been argued that the enzyme reduces the electrostatic repulsion between the carboxylates to allow the 3,3-shift to occur.226 4.3

Bicyclo[2.2.0]hexane to 1,5-Hexadiene

The thermal cleavage of bicyclo[2.2.0]hexane (BCH), to 1,5-hexadiene was first reported by Srinivasan.227 Steel determined that log k ¼ 13:4 2 36 000=2:3RT:228 Goldstein found substantial stereospecificity in the reaction corresponding to an allowed retro 2 þ 2 cycloaddition. Thus exo-2,3,5,6-tetradeuteriobicyclo[2.2.0]hexane cleaves almost exclusively (98%) to meso, cis, trans-1,3,4,6-tetradeuterio-1,5hexadiene with DH ‡ ¼ 36:0 kcal=mol and DS‡ ¼ 1:5 e:u:229 Further, Goldstein showed that the exo starting material interconverted with its endo isomer roughly three times faster than cleavage with DH ‡ ¼ 35:0 kcal=mol and DS‡ ¼ 1:4 e:u: The stereochemistry of the reactions is consistent with ring opening to a boat diyl which undergoes conformation interconversion with the chair diyl which can result in geometric isomerization as well as cleave in a least motion fashion (Scheme 7.96). Goldstein also pointed out that BCH has a heat of formation 9 kcal/mol higher than that of 1,5-hexadiene at 2508C. Thus the transition state for the geometric and structural isomerizations are 43 and 44 kcal/mol, respectively, above 1,5-hexadiene. The estimated heat of formation of the likely intermediate, a non-interacting chair cyclohexane diyl is 42 kcal/mol above 1,5-hexadiene; so it can be an intermediate in

C6H4 – C6H10

153

Scheme 7.96

both reactions. Subsequent calculations at the CASSCF/CASPT2 level reproduced these relative enthalpies except that the chair-diyl is not a stationary point; however, a twist-boat biradical was found to be a common intermediate.230 Further, a halfchair diyl was found to be the transition state for the cleavage to 1,5-hexadiene. Consideration of the relative entropic contributions to the potential energy surface at the temperatures of the reactions suggest the addition of approximately 9 kcal/mol to the energy of all species described above except 1,5-hexadiene. This then leads to the free energy surface for the reactions including the Cope rearrangement of 1,5hexadiene described in Scheme 7.97.

Scheme 7.97

4.3.1

Substituted Bicyclo[2.2.0]hexanes

In the first published study of the stereochemistry of the BCH cleavage reaction, Paquette found that pyrolysis of exo,cis-2,3-dicarbomethoxybicyclo[2.2.0]hexane

154


gives an 1:1:8 mixture of meso-3,4-dicarbomethoxy-1,5-hexadiene, dimethyl trans,trans- and cis,trans-2,6-octadienedioates, respectively.231 This was interpreted in terms of cleavage of a chair-like cyclohexanediyl intermediate. A similar interpretation was advanced earlier by Roth to rationalize the results of thermolysis of 5,6-dimethyl-2,3-diazobicyclo[2.2.2]oct-2-enes.232 Roth also found that pyrolysis of the dimethyl derivative corresponding to Paquette’s compound gave a 3:37:55 mixture of the same types of cleavage products.233 4.4

Bicyclo[2.1.1]hexane to 1,5-Hexadiene

Bicyclo[2.1.1]hexane gives 1,5-hexadiene upon heating with log k ¼ 15:17 2 55 000=2:3RT (Scheme 7.98).234

Scheme 7.98

Perhaps the most significant aspect of this retro 2 þ 2 cycloaddition is the fact that the activation energy is a little less than 10 kcal/mol lower than that for cleavage of the parent cyclobutane. 4.5

Vinylcyclobutane to Cyclohexene

Vinylcyclobutane undergoes both structural isomerization via a 1,3-shift to cyclohexene with log k ¼ 13:86 2 48 600=2:3RT and cleavage to 1,3-butadiene and ethylene with log k ¼ 14:87 2 50 700=2:3RT (Scheme 7.99).235 The two reaction pathways are utilized roughly equally, and a reasonable hypothesis for them would focus on generation of both cisoid- and transoidbiradicals, the former of which could give products of both pathways while the latter

Scheme 7.99

C6H4 – C6H10

155

could only be responsible for cleavage. In principle, both biradicals could reclose to geometrically isomerized vinylcyclobutane. Similar reactions with substituted derivatives were found in the 1960, for instance, that of isopropenylcyclobutane.236 One of the first observations of geometrically isomerized starting material in a relatively simple vinylcyclobutane system was by Jordan.237 Thus cis- and trans-2-ethylvinylcyclobutane equilibrated with rates similar to the other reactions of each isomer. Significantly, pyrolysis of the optically active trans isomer led to the inverted and retained 4-ethylcyclohexene products in a 59:41 ratio indicating a slight preference for what might be the allowed suprafacial-inversion reaction although no evidence bears on the stereochemistry of the allylic unit, supra- or the forbidden antarafacial use. Further work by Jordan analyzed the interconversions of trans-, and cis-2-methyl1-(trans- and cis-1-propenyl)cyclobutane. A 1.9:1 ratio of trans- and cis-3,4dimethylcyclohexenes were formed from both trans-propenyl compounds and 7:1 and 9:1 ratio of the trans- to cis-3,4-dimethylcyclohexenes was formed from the cispropenyl trans- and cis-2-methylcyclobutanes, Scheme 7.100. Again, there could be

Scheme 7.100

no quadrisection of the data to reveal the relative utilization of all four possible 1,3sigmatropic shift pathways, but the near identical ratios of the trans- to cis-3,4dimethylcyclohexenes from the two trans-propenyl compounds and from the two

156


cis-propenyl compounds suggests formation of a nearly random biradical which has a small preference for formation of trans-dimethylcyclohexene. Subsequent work by Baldwin allowed the quadrisection of stereopathways by using non-racemic trans-propenyl compounds.238 The results are almost, but not quite consistent with a common biradical with some stereochemical bias suggesting that complete randomization about all bonds has not been achieved prior to ring closure. Given in Scheme 7.101 are just the 1,3-shift products with their stereopathway from each isomer.

Scheme 7.101

Nearly equal amounts of the major product enantiomer (of the trans product) are formed, and in both cases, there is a preference for suprafacial use of the allylic moiety. However, there is roughly twice as much product resulting from antarafacial use of the allylic moiety in from the starting cis isomer as from the trans isomer which comes at the expense of the cis-dimethylcyclohexene which is the result of a suprafacial use of the allylic moiety. A factor of two, however, represents only 0.8 kcal/mol at this relatively high temperature. Of significance in this connection is early work by Dervan with non-racemic trans-1,2-trans,trans- and trans,cis-bispropenylcyclobutanes.239 In both cases, the major 1,3-shift products, which in fact resulted from migration over the transpropenyl moiety almost exclusively, were the result of the si and the sr pathways used in roughly equal amounts with less than 10% of the antarafacial products being formed. Whether this represents two concerted pathways, one allowed and the other forbidden or the generation of a biradical which lives insufficiently long to allow much formation of antarafacial product is unknown. Finally, in the case of trans- and cis-1-cyano-2-trans-propenylcyclobutane, which has remote deuterium labels to provide diastereomers that allows quadrisection of the pathways, the product distributions obtained are reminiscent of those from the 2-methyl-1-propenylcyclobutanes (Scheme 7.102).240

C6H4 – C6H10

157

Scheme 7.102

The conclusion would appear to be that the vinylcyclobutane rearrangement involves biradicals, possibly formed by conrotatory-level motions (see Chapter 7, Section 3.14), Scheme 7.103, which are incompletely equilibrated before cyclization to cyclohexene because there is always a preference for utilization of the allylic moiety in a suprafacial way.

Scheme 7.103

The vinylcyclobutane 1,3-sigmatropic shift is at the core of many mechanistic studies which are described elsewhere in this book and in a review by Leber and Baldwin.241

158 4.6


Cyclohexene Retro Diels – Alder Reaction

While not strictly an isomerization reaction, the retro 4 þ 2 reaction of cyclohexene, Scheme 7.104, bears scrutiny in the context of this chapter.

Scheme 7.104

The reaction occurs at low pressures in the gas phase with log k ¼ 15:18 2 66 200=2:3RT 242 or with log k ¼ 14:93 2 65 200=2:3RT:243 The reaction occurs only because of the low pressure since cyclohexene is more stable than the addends despite an unfavorable entropy below 1000 K. The question of whether the Diels –Alder reaction is concerted or stepwise via a biradical is an old but important one. The stereochemical preservation of the dienophilie added suggests concert, but the biradical pathway has had proponents. A careful analysis of the enthalpies of the various species on the energy surface244,235 showed that the biradical intermediate, generated from vinylcyclobutane, while nearly as stable as the Diels– Alder transition state in the parent case, is kinetically inaccessible under the reaction conditions. An extension of that analysis to include the contribution of entropy changes at 473 K is given in Scheme 7.105. Using data from the previous section, vinylcyclobutane undergoes cleavage with partial rate-determining homolysis of the transoid allylic biradical moiety.

Scheme 7.105

C6H4 – C6H10

159

Presumably, the cisoid allylic biradical can do the same with the same activation free energy so the transition state necessary to obtain the required cisoid biradical from the addends is roughly10 kcal/mol higher in free energy than the observed transition state energy for the Diels– Alder/retro Diels– Alder reaction. Unknown is the depth of the energy wells of the biradicals and the activation free energy for their cleavages and combinations, but that is irrelevant to the argument. Interestingly, shock tube pyrolysis of the cis-1,2,3,4,5,6-hexadeuteriocyclohexenes resulted in partial conversion to trans-1,2-dideuterioethylene with 33% of the ethylene so labeled at 1244 K. More remarkable was the observation that 3,3,6,6tetradeuteriocyclohexene gave small amounts (5%) of 1,1-dideuterioethylene suggesting that ring contraction to vinylcyclobutane had occurred although formation of bicyclo[3.1.0]hexene might also be involved (Scheme 7.106).245

Scheme 7.106

Kinetic modeling based on the scheme revealed that vinylcyclobutane was formed with log k ¼ 15:4 2 72 300=2:3RT which places this pathway, which presumably involves a cisoid biradical, 6.3 kcal/mol higher in enthalpy (and free energy) than the retro 4 þ 2 cycloaddition. This difference is 5 kcal/mol than the thermochemical estimates of Scheme 7.105, which is based on a temperature roughly 5008C lower. Bartlett provided evidence for the accessibility of the biradical path by finding 0.029% vinylcyclobutane in the 4 þ 2 cycloaddition of butadiene and ethylene.246 Given the temperature of the reaction, this represents roughly a 10 kcal/mol

160


preference for the Diels– Alder transition state over a stepwise one to give, presumably, a transoid biradical. The stereochemistry of the 4 þ 2 addition was examined with deuterium substitution. Thus, when cis- or trans-1,2-dideuterioethylene was reacted with 1,1,4,4-tetradeuterio-1,3-butadiene at 1858C and 1800 psi, the cyclohexene isomers were formed with retention of stereochemistry in the ethylene derived portioned to the extent of 100:1.247 MINDO calculations, which pathologically favor biradical as opposed to concerted transition states, predict an unsymmetrical transition state for the 4 þ 2 cycloaddition;248 however, an early ab initio calculation revealed the presence of a symmetrical transition state with a biradical path roughly 2 kcal/mol higher in energy.249 The neglect of overlap in the MINDO framework may be a source of difficulty as pointed out by Houk.250 While symmetrical transition states are the rule with 4 þ 2 reactions with symmetrical addends, with unsymmetrical addends, the transition states are unsymmetrical by both experiment using secondary DKIEs251 and calculation.252 4.7

Biscyclopropyl

Pyrolysis of biscyclopropyl with log k ¼ 15:36 2 60 710=2:3RT leads to a myriad of products, most of which are olefins as in the pyrolysis of cyclopropane itself.253 However, a small amount (ca. 2%) of cyclohexene is also formed as a primary product (Scheme 7.107). The details of this transformation have yet to be established, although it is likely that it results from cleavage in the initially formed biradical. Also formed is 1-methylcyclopentene presumably via isopropenylcyclopropane which is formed by external cyclopropane bond cleavage.

Scheme 7.107

C6H4 – C6H10

161

A related observation is that by Doering who added methylene to vinylcyclopropane at low pressures in the gas phase and found a 12:1 mixture of biscyclopropyl and cyclohexene; apparently no other products are formed (Scheme 7.108).254

Scheme 7.108

An important not unrelated observation was made by Rabinovitch who added methylene-d2 to the hexafluorovinylcyclopropane of (Scheme 7.109).255

Scheme 7.109

Extrusion of difluorocarbene ensued, but the distribution of identical products, except for deuterium, depended on pressure indicating that energy does not rapidly randomize among all the vibrational modes in the molecule. So that at higher pressures, deactivation occurs to give more loss from the ring remote from that formed in the reaction. This cannot be an isotope effect because addition of CH2 to the dideuterio vinylcyclopropane gives the same result, namely loss of CF2 from the deuterated cyclopropane ring. 4.8

Bicyclo[3.1.0]hexane to Cyclohexene and 1-Methylcyclopentene

Bicyclo[3.1.0]hexane gives cyclohexene and 1-methylcyclopentene with log k ¼ 13:29 2 57 400=2:3RT and log k ¼ 13:89 2 61 170=2:3RT; respectively (Scheme 7.110).256

162


Scheme 7.110

Both products appear to be a cyclopropane to propylene type of thermal isomerization. The lower activation energy for formation of cyclohexene is probably the result of strain in the five-membered ring which is released upon bridgehead bond homolysis.

4.9

cis-2-Methylvinylcyclopropane to cis-1,4-Pentadiene

cis-2-Methylvinylcyclopropane gives cis-1,4-pentadiene with log k ¼ 11:03 2 31 200=2:3RT (Scheme 7.111).257

Scheme 7.111

The activation parameters indicate a concerted process, particularly the very low activation energy for a C – H bond cleavage. The cis geometry of the starting material is demanded by steric considerations, and the geometry of the product requires that the reaction occurs out of a conformation with the vinyl group over the ring. Direct proof of this hypothesis was provided in a study of homodienyl hydrogen shift with cis-2-(2-propyl)-1(E)-propenylcyclopropane and its optically active, deuterium-labeled derivative which gave the hydrogen shift

C6H4 – C6H10

163

product with high stereospecificity at each of the three sites of stereogenicity (Scheme 7.112).258

Scheme 7.112

trans-2-Methylvinylcyclopropane requires much higher temperatures to react, and it apparently does so by isomerizing to the cis material with log k ¼ 14:7 2 48 640=2:3RT; and this product undergoes the hydrogen shift, Scheme 7.111.257 Also formed in the reaction is less than 10% of 4-methylcyclopentene, the vinylcyclopropane rearrangement product.

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170 224. 225. 226. 227. 228. 229. 230. 231. 232. 233. 234. 235. 236. 237. 238. 239. 240. 241. 242. 243. 244. 245. 246. 247. 248. 249. 250. 251. 252. 253. 254. 255. 256. 257.

258.

Hydrocarbon Thermal Isomerization There may be other stereoelectronic factors that affect the relative rates which were cited by negatively responding referees. A.Y. Lee, P.A. Karplus, B. Ganem, and J. Clardy, J. Am. Chem. Soc., 117, 3627 (1995), and references contained therein. S. Hur and T.C. Bruice, J. Am. Chem. Soc., 125, 5964 (2003). S. Cremer and R. Srinivasan, Tetrahedron Lett., 21, 24 (1960). C. Steel, R. Zand, P. Hurwitz, and S.G. Cohen, J. Am. Chem. Soc., 86, 679 (1964). M.J. Goldstein and M.S. Benzon, J. Am. Chem. Soc., 94, 5119 (1972). D. Hrovat and W.T. Borden, J. Am. Chem. Soc., 123, 4069 (2001). L.A. Paquette and J.A. Schwartz, J. Am. Chem. Soc., 92, 3215 (1970). W.R. Roth and M. Martin, Tetrahedron Lett., 3865 (1967). W.R. Roth, private communication to R.G. Bergman, cited in R.G. Bergman, Free Radicals, (J.K. Kochi ed), Wiley, New York (1973). R. Srinivasan and A.A. Levi, J. Am. Chem. Soc., 85, 3363 (1963). H.M. Frey and R. Pottinger, J. Chem. Soc. Faraday Trans., 1, 1827 (1978). R.J. Ellis and H.M. Frey, Trans. Faraday Soc., 59, 2076 (1963). L.M. Jordan, PhD. Thesis, Yale University, New Haven, Conn (1974) with J.A. Berson, Diss. Abstr. Int. B., 35, 5332 (1975) J.E. Baldwin and R.C. Burrell, J. Am. Chem. Soc., 123, 6718 (2001). J.A. Berson and P.B. Dervan, J. Am. Chem. Soc., 95, 269 (1973). X. Cheng, PhD Dissertation, Harvard University (1989) with W. von, E. Doering, Diss. Abstr. Int. B., 50, 3472 (1990) cited from the next reference. P.A. Leber and J.E. Baldwin, Acc. Chem. Res., 35, 279 (2002). M. Uchiyama, T. Tomioka, and A. Amano, J. Phys. Chem., 68, 1878 (1964). D.C. Tardy, R. Ireton, and A.S. Gordon, J. Am. Chem. Soc., 101, 1508 (1979). W. von E. Doering, M. Franck-Neumann, D. Hasselmann and R.L. Kaye, J. Am. Chem. Soc., 94, 3833 (1972). D.K. Lewis, B. Brandt, L. Crockford, D.A. Glenar, G. Rauscher, J. Rodriguez, and J.E. Baldwin, J. Am. Chem. Soc., 115, 11728 (1993). P.D. Bartlett and K.E. Schueller, J. Am. Chem. Soc., 90, 6071 (1968). K.N. Houk, Y.-T. Lin, and F.K. Brown, J Am, Chem. Soc., 108, 554 (1986). M.J.S. Dewar, A.C. Griffin, and S. Kirschner, J. Am. Chem. Soc., 96, 6225 (1974). R.E. Townshend, G. Ramunni, G. Segal, W.J. Hehre and L. Salem, J. Am. Chem. Soc., 98, 2190 (1976). P. Caramella, K.N. Houk, and L.N. Domelsmith, J. Am. Chem. Soc., 99, 4511 (1977). J.J. Gajewski, K.B. Peterson, J.R. Kagel and Y.C.J. Huang, J. Am. Chem. Soc., 111, 9078 (1989). B.R. Beno, K.N. Houk, and D.A. Singleton, J. Am. Chem. Soc., 118, 9984 (1996). M.C. Flowers and H.M. Frey, J. Chem. Soc., 1689 (1962). W. von E. Doering, J.C. Gilbert, and P.A. Leermakers, Tetrahedron, 24, 6863 (1968). J.D. Rynbrandt and B.S. Rabinovitch, J. Chem. Phys., 54, 2275 (1971). H.M. Frey and R.C. Smith, Trans. Faraday Soc., 58, 697 (1962). R.J. Ellis and H.M. Frey, Proc. Chem. Soc. (Lond.), 221 (1964), and J. Chem. Soc., 5578 (1964); See also W.R. Roth and J. Ko¨nig, Justus Liebigs Ann. Chem., 688, 28 (1965); For many examples see M.J. Jorgenson and A.F. Thacher, Tetrahedron Lett., 4651 (1969). P.A. Parziale and J.A. Berson, J. Am. Chem. Soc., 113, 4595 (1991).

8 C7H6 –C7H12

CONTENTS 1 C7H6 1.1 1,2-Diethynylcyclopropane to 1,4,6-Bicyclo[3.2.0]heptatriene and Fulvenallene 1.2 Benzocyclopropene to Fulvenallene 1.3 Spiro 2,4 hepta-1,4,6-triene to Bicyclo[3.2.0]hepta-1,3,6-triene 1.4 1,2,4-Heptatriene-6-yne to meta-Toluene Diyl 1.5 Phenyl Carbene to 1,2,4,6-Cycloheptatetraene 2 C7H8 2.1 Tropilidene, Norcaradiene, and Toluene –Tropilidiene “Skin” and “Bones” Degenerate Rearrangements 2.2 Bicyclo[3.2.0]hepta-2,6-diene Degenerate Rearrangement and Isomerization to 1,3,5-Cycloheptatriene; Tricyclo[4.1.0.02,7]heptene 2.3 Norbornadiene, Quadricyclane and Tricyclo[3.2.0.02,4]hept-6-ene 2.4 2-Ethynyl-1-vinylcyclopropane 2.5 Spiro[2.4]heptadiene Geometric Isomerization and Rearrangement 2.6 1-(cis-2-Methylcyclopropyl)-1,2-propadiene to 1,3,6-Heptatriene 2.7 Norpinene Degenerate Rearrangement 2.8 Benzene Oxide 2.9 o-, p-Isotoluene, Homofulvene, Methylenebicyclo[2.2.0]hex-2-ene, and Toluene 2.10 6-Methylenebicyclo[3.1.0]hex-2-ene to Homofulvene 3 C7H10 3.1 cis-, trans-1,2-Divinylcyclopropane to 1,4-Cycloheptadiene 3.1.1 Heterologs 3.2 1,1-Divinylcyclopropane to 1-Vinylcyclopentene 3.3 4-Vinylcyclopentene Degenerate Rearrangement 3.4 1,3-Dimethylcyclopentane Degenerate Rearrangement 3.5 Bicyclo[3.2.0]hept-2-ene to Bicyclo[2.2.1]hept-2-ene 3.6 Bicyclo[2.1.0]pentane-5-spirocyclopropane, Bicyclo[3.2.0]hept-1-ene, 5-Methylenebicyclo[2.1.1]hexane,

172 172 173 174 174 175 176 176 180 182 184 185 187 188 188 188 189 191 191 193 193 194 195 196

172


1,2,6-Heptatriene, 3-Methylene-1,5-hexadiene and Tricyclo[4.1.0.01,3]heptane 3.7 Bicyclo[3.2.0]hept-6-ene to 1,3-Cycloheptadiene 3.8 Bicyclo[4.1.0]hept-1(2)-ene to 6-Methylenebicyclo[3.1.0]hexane 3.9 3-Vinylmethylenecyclobutane to 4-Methylenecyclohexene 3.10 Tricyclo[4.1.0.02,4]heptane to Bicyclo[4.1.0]hept-3-ene 3.11 cis,cis-1,3,5-Heptatriene 1,7-Hydrogen Shift 4 C7H12 4.1 Bicyclo[4.1.0]heptane 4.2 cis-2-Methylvinylcyclobutane Homodienyl Hydrogen Shift References

1

198 202 202 203 204 204 204 204 205 206

C7H6

1.1 1,2-Diethynylcyclopropane to 1,4,6-Bicyclo[3.2.0]heptatriene and Fulvenallene In an interesting variant on the 1,5-hexadiyne 3,3-shift, Bergman showed that cis1,2-diethynylcyclopropane gives bicyclo[3.2.0]hepta-1,4,6-triene with a half-life of 3.5 h at 968C. The reaction presumably involves generation of meso-1,2,3,4cycloheptatetraene which undergoes cyclization in a “forbidden” disrotatory fashion. On the other hand, the trans material requires heating to 2008C to affect the rearrangement with Eact ¼ 42:8 kcal/mol and an entropy of activation of 8.1 e.u. The data suggest that the trans material isomerizes to the cis isomer which then rearranges to the bisallene and on to the bicycloheptatriene (Scheme 8.1).

Scheme 8.1

There are interesting pressure effects on the product distribution from the trans isomer. At 2478C and 100 Torr, only the bicyclic triene is formed, but at 0.1 Torr, 20% of fulvenallene and 9.6% of ethynylcyclopentadiene are formed in addition to the bicyclic triene.1 Since the latter two materials are the products from high temperature pyrolysis of the bicyclic triene, the suggestion was made that

C7H6 – C7H12

173

the bicyclic triene is formed vibrationally hot in the pyrolysis and that it rearranges prior to collisional deactivation in the low pressure pyrolysis. Deuterium labeling studies indicated that the course of the pyrolysis of the bicyclic triene involved cleavage of the C1,C7 bond followed by a vicinal hydrogen (or deuterium) shift that gave a biradical in which 1,5-hydrogen shifts could give an appropriate cyclopentadienyl radical which is the fulvenallene (Scheme 8.2).2

Scheme 8.2

Subsequent work by Bergman and Vollhardt examined the rearrangements of the corresponding diethynyl epoxide and episulfide which, in the cis cases, gave the appropriate heterobicyclic triene; however, the trans-episulfide gave the desulfurized trans-diethynylethene.3 1.2

Benzocyclopropene to Fulvenallene

Benzocyclopropene is converted to fulvenallene and ethynylcyclopentadiene at 8008C and low pressures.4 Wentrup showed that the methylene group is converted to the exo-methylene groups of fulvenallene to the extent of 85%; therefore, there was 15% carbon scrambling, apparently in fulvenallene. The dominant label distribution was rationalized by cleavage of the cyclopropene moiety to a carbene which underwent ring contraction (Scheme 8.3).

Scheme 8.3

Wentrup also showed that 3-ethylfulvenallene is converted to styrene and benzocyclobutene at 10008C suggesting carbene – carbene rearrangement similar to that observed by W.M. Jones and M. Jones, Jr. (see Chapter 4, Section 1).

174 1.3


Spiro[2,4]hepta-1,4,6-triene to Bicyclo[3.2.0]hepta-1,3,6-triene

Spiro[2,4]hepta-1,4,6-triene undergoes a 1,5-carbon shift to bicyclo[3.2.0]hepta1,3,6-triene with log k ¼ 12 2 24 700=2:3RT in chloroform solution.5 The reaction is not irreversible, but rapid dimerization of the bicyclic triene to 2 þ 2 and 4 þ 4 cycloaddition products apparently drives the reaction (Scheme 8.4).

Scheme 8.4

Calculations at various levels suggest that the relative energy difference between the two isomers is small. Also calculated was the relative energy of the 1,3-carbon shift product from the spirotriene, namely norbornatriene, but it was 50 kcal/mol higher in energy than the spirotriene The bicyclic triene has also been subjected to theoretical scrutiny and has a low-lying triplet state6 which may be responsible for the dimerization. Finally, calculations6,7 revealed that a possible reaction intermediate, namely cycloheptatetraene, which might be formed by a necessarily disrotatory cyclobutene ring opening in the bicyclic diene, would be higher in energy than the observed activation energy. 1.4

1,2,4-Heptatriene-6-yne to meta-Toluene Diyl

1,2,4-Heptatriene-6-yne undergoes an electrocyclization to what appears to be an a,3-toluene diyl which rapidly abstracts hydrogen from the environment to give toluene (Scheme 8.5).8 In methanol, two products are isolated one of which appears to be radical-derived and the other zwitterion-derived.

Scheme 8.5

This reaction is at the core of the action of the antibiotic neocarzinostatin, which removes hydrogens from DNA, and as such has received much attention.8

C7H6 – C7H12

175

Questions have surrounded the nature of the intermediate(s), diyl, zwitterion, or cyclic allene. The most recent experiments suggest that two pathways are involved in methanol solvent since there is a dependence of the product ratio on methanol concentration, but calculations even with high solvent dielectric constant do not allow the potential energy of the zwitterion to be within 20 kcal/mol of the biradical.9 Further, the non-planar and therefore enantiomeric cyclic allene was calculated to be approximately only 7 kcal/mol less stable than the singlet biradical at the CASPT2N 6-31Gp//CASSCF(2,2)/6-31Gp level. Thus the diyl and the allene were suggested to be responsible for the radical and “zwitterionic” products, respectively. 1.5

Phenyl Carbene to 1,2,4,6-Cycloheptatetraene

The gas phase rearrangements of phenyl carbene, particularly the conversion to cycloheptatrienylidene, which is actually 1,2,4,6-cycloheptatetraene, and probably involving bicyclo[4.1.0]hepta-2,4-6(7)-triene, are particularly fascinating (Scheme 8.6).10

Scheme 8.6

Even more remarkable is the observation that photochemical decomposition of the azo precursor to p-tolylcarbene in hemicarceplexes resulted in an NMR spectrum (in the perdeuterio hemicarceplex) which consisted of two diastereomers in a 1:1.8 ratio at room temperature. Above 988C the two methyl singlets (which are at d ¼ 21:47 and 2 1.57 ppm, respectively) broaden suggesting interconversion with activation energies between 19.6 and 19.1 kcal/mol and log A of 12.2 and 12.3, respectively (Scheme 8.7).

Scheme 8.7

176


Calculations at various levels, CASPT2N11 and DFT,12 indicate that the singlet cycloheptatrienylidene is the transition state for the interconversion of the enantiomeric cyclic allenes, and it is roughly 20 kcal/mol less stable than the cyclic allenes. 2

C7H8

2.1 Tropilidene, Norcaradiene, and Toluene –Tropilidiene “Skin” and “Bones” Degenerate Rearrangements Tropilidene (1,3,5-cycloheptatriene) is a non-planar, tub-like molecule with only a 6.3 kcal/mol barrier to ring inversion.13 It is converted to toluene upon heating14 with log k ¼ 13:54 2 51 100=2:3RT 15 or log k ¼ 13:9 2 52 200=2:3RT:16 The most reasonable course of this rearrangement is disrotatory electrocyclization to norcaradiene followed by homolytic cleavage of an external cyclopropane bond to a 1,3-biradical and subsequent vicinal hydrogen shift (Scheme 8.8).

Scheme 8.8

Norcaradiene is accessible from tropilidene as evidenced by the formation of Diels– Alder adducts of it when the monocycle is treated with various dienophiles.17 Further, cyclohepatrienes substituted with p electron accepting groups at C7 are in observable equilibrium with, if not less stable than, the norcaradienes.18 These electron withdrawing groups apparently interact with and remove electron density from the antibonding p-like Walsh orbital that makes up part of the fusion bond in norcaradiene thus making the bicycle more energetically accessible.19 The difference in energy between cycloheptatriene and norcaradiene has been estimated at 4 –5 kcal/mol from solvolytic data on 7-tropylcarbinyl and 7-caradienylcarbinyl derivatives.20 Of importance is the preparation of norcaradiene itself and the observation that it gives cycloheptatriene with log k ¼ 11:7 2 6500=2:3RT (108C range at 100 K); so the free energy necessary to give norcaradiene from cycloheptatriene is roughly 11 kcal/mol.21 Heating cycloheptatriene at roughly 1008C results in 1,5-hydrogen shifts; this is the so-called “skin” rearrangement to distinguish it from the carbon skeletal or “bones” rearrangement.22 With 7-deuteriocycloheptatriene log k ¼ 11:2 2 31 500=2:3RT for formation of 3-deuterio material which gives the other deuterium isomers (Scheme 8.9).23

C7H6 – C7H12

177

Scheme 8.9

In order to answer the question posed in Scheme 8.8 as to whether or not external cyclopropane bond homolysis was reversible in norcaradiene, Willcott and Berson examined the pyrolysis of 3,7,7-trimethyl-cycloheptatriene.24 Remarkably, the 2,7,7- and 1,7,7-isomers were formed in a 10:1 ratio, respectively, along with small amounts of m- and p-cymene. Importantly, substantial quantities of 1-methyl-3isopropenyl-1,4-cyclohexadiene and 2-methyl-3-isopropenyl-1,4-cyclohexadiene were also found, and these reverted to the cycloheptatrienes under the reaction conditions. Therefore, the interconversion could be the result of biradical formation or reversible homodienyl-1,5-hydrogen shifts (Scheme 8.10).

Scheme 8.10

178


This mechanistic ambiguity does not occur in the pyrolysis of the methyl-7,7dicyanonorcaradienes which cannot undergo the hydrogen shifts, yet interconvert at only 558C.25 Fascination with the bones rearrangement stems from the circumambulatory nature of the transformation and the question of orbital symmetry control. According to the Rules, the interconversion of the norcaradienes derived from the tropilidenes should proceed with retention of the migrating carbon since the pentadienyl unit must be utilized in a suprafacial manner. Elegant experiments have revealed that the migration of C7 occurs mostly with inversion in substituted cases. Thus, Kla¨rner found that optically active 2,7dimethyl-7-carbomethoxy- and 2,7-dimethyl-7-cyanocycloheptatriene found that the systems undergo racemization with increasing pyrolysis times; moreover, inversion at C7 occurred upon each rearrangement.26 Further, kinetic analysis led to the conclusion that 1,5-hydrogen shifts could not be involved (Scheme 8.11).

Scheme 8.11

Similar results were obtained by Baldwin using optically active, deuteriumlabeled 3,7-dimethyl-7-methoxymethylcycloheptatriene.27 Here, the forbidden suprafacial inversion pathway is roughly 30 times faster than the “allowed” suprafacial retention pathway (Scheme 8.12).

C7H6 – C7H12

179

Scheme 8.12

The circumambulatory isomerization is probably not concerted, judging by the high temperatures necessary to affect it in the absence of radical stabilizing groups at C7. Therefore, the observed forbidden stereochemistry must have resulted from other factors. Calculations on the singlet energy surface for ring opening of the parent norcaradiene using the 6-31Gp basis set in multiconfiguration SCF with dynamic correlation (CASPT2N) revealed only a 0.5 kcal/mol difference in the allowed and forbidden transition states favoring the retention pathway, but UB3LYP þ ZPE favored the inversion pathway by 0.9 kcal/mol.28 There was no significant bonding ˚. from C7 to ring carbons in either transition state since the distance is roughly 2.5 A UB3LYP/6-31Gp calculations on the transition states derived from 7-cyano-7methylcycloheptatriene revealed a 2.5 kcal/mol preference for the inversion over the retention pathway, a result attributed to steric effects. Similar results were obtained using the 6-31Gp basis set in multiconfiguration SCF with dynamic correlation (MROPT2) and zero point energy corrections. These revealed a small preference, 1.8 kcal/mol, for retention over inversion in transition states linking norcaradienes.29 At the B3LYP (UHF) the two transition states were of equal energy, and at the CASSCF (8/8) level, the inversion transition state was favored by 1.2 kcal/mol. The calculations, therefore, make no clear prediction about stereochemistry. Also of interest is the finding that the hydrogen shift to give toluene occurs only from the transition state that involves retention at all levels of theory. Further, calculations on the cycloheptatriene-to-norcaradiene equilibrium at the B3LYP(RHF) level come closest to experiment, namely 6.5 kcal/mol for

180


the enthalpy difference and 10 kcal/mol for the activation energy. The values from MROPT2 are 8.9 and 7.32 kcal/mol, respectively, and those from CASSCF are 21.6 and 11.8 kcal/mol, respectively. Finally, the activation energies (without zero point corrections) calculated for the 1,5-hydrogen shift in cycloheptatriene by MROPT2, CASSCF, and B3LYP are 38.7, 60.2, and 40.6 kcal/mol, respectively, and a zero point correction would lower these energies by about 4 kcal/mol. Clearly, the CASSCF method without dynamic correlation is suspect just as in the Cope rearrangement (see Chapter 7, Section 4.1). 2.2 Bicyclo[3.2.0]hepta-2,6-diene Degenerate Rearrangement and Isomerization to 1,3,5-Cycloheptatriene; Tricyclo[4.1.0.02,7]heptene The conversion of bicyclo[3.2.0]hepta-2,6-diene to cycloheptatriene proceeds with log k ¼ 14:0 2 39 500=2:3RT:30 The reaction would appear to be a forbidden disrotatory cyclobutene ring opening which occurs with reasonable facility due to the stabilization of developing radicals by an adjacent double bond (Scheme 8.13).

Scheme 8.13

Alternatively, ring opening in a conrotatory manner to a cis,trans,cis- or a trans,cis,cis-1,3,5-cycloheptatriene followed by a 1,5-hydrogen shift could account for the observations, but 1,4,4-trimethylbicyclo[3.2.0]hepta-2,6-diene gave 3,7,7trimethylcycloheptariene at virtually the same rate as the parent; therefore, the hydrogen shift pathway is unlikely.31 However, also formed in the latter pyrolysis was roughly 20% of 4,4,7-trimethylbicyclo[3.2.0]hepta-2,6-diene which is the formal 3,3-shift product. If it is a 3,3-shift, then it must occur by an antarafacial, antarafacial pathway. Alternatively, reversible ring opening to a cis,trans,cis-triene can also provide this isomer (Scheme 8.13). Reversible conrotatory ring opening of these dienes was postulated31 as the explanation for the remarkable 3,3-shifts in these systems which are required to proceed in an antarafacial, antarafacial manner.32 Thus, lumitropolone methyl ether (1-methoxybicyclo[3.2.0]hepta-3,6-dienone) rearranges to the 3-methoxy isomer (Scheme 8.14).

C7H6 – C7H12

181

Scheme 8.14

Antarafacial, antarafacial 3,3-shifts are unknown in the parent 1,5-diene system (see Chapter 7, Section 4.1), and a somewhat geometrically related diene, bicyclo[3.3.0]octa-2,6-diene, does not undergo a 3,3-shift at temperatures approaching 3008C.31 Of course, this diene does not have a four-membered ring whose strain would be relieved somewhat in the transition state. Subsequent work by Otterbacher discovered the formal 3,3-shift product (ca. 5%) from the parent 3.2.0 diene using deuterium as a label at C7.33 Substitution of a methyl group at C1 in various derivatives led to a small rate enhancement in the formation of cycloheptatrienes but slowed the rate of the 3,3-shift seeming to suggest that different mechanistic pathways were involved in the two reactions: a disrotatory ring opening provides the cyclic c,c,c-cyclic triene, but a conrotatory opening to the c,t,c-cyclic triene would be destabilized by a methyl group rotating inward (Scheme 8.15).

Scheme 8.15

The 4,4-dimethyl-2-deuterio case represents two interesting exceptions in that the 3,3-shift product constitutes roughly 45% of the reaction (Scheme 8.15), and substitution of a C1 methyl reduces the 3,3-shift only to 20% of the total

182


(Scheme 8.13). The geminal dimethyl substitution may force the carbon termini involved in the formal 3,3-shift closer thus promoting this pathway, and the distortion may remove some of the offending steric interactions resulting from C1 methyl substitution. In any event, these reactions do not appear to be concerted since the estimated BDE for the central bond is probably around 15 kcal/mol lower than the activation energy for the reactions. Indeed, it is likely that two different configurations of two allyl radicals are involved in the two reactions, ring opening and 3,3-shift (Scheme 8.16).

Scheme 8.16

Finally, tricyclo[4.1.0.02,7]hept-3-ene gives almost exclusively bicyclo[3.2.0]hepta-2,6-diene upon heating at 1508C with a half-life of 1.5 h (Scheme 8.17).34 It was suggested that this rearrangement involves the symmetry-allowed opening of the bicyclobutane to a 1,3,5-cycloheptatriene with one trans double bond followed by conrotatory closure to the bicycloheptadiene.

Scheme 8.17

2.3

Norbornadiene, Quadricyclane and Tricyclo[3.2.0.02,4]hept-6-ene

Bicyclo[2.2.1]heptadiene (norbornadiene) gives cycloheptatriene upon heating with log k ¼ 14:68 2 50 610=2:3RT:35 Also formed in the reaction is cyclopentadiene and acetylene, the retro Diels –Alder products with log k ¼ 14:68 2 51 900=2:3RT and toluene with log k ¼ 14:23 2 53 140=2:3RT: Most likely, the initial reaction proceeds via cleavage of the C1 – C7 bond to give a biradical which can form norcaradiene and then cycloheptatriene or undergo a hydrogen shift to toluene, but the retro 4 þ 2 reaction must result from C1 – C2 (and C3 – C4) bond fission (Scheme 8.18).

C7H6 – C7H12

183

Scheme 8.18

The intervention of a biradical in the isomerization reactions is supported by the likelihood that the C1 – C7 bond dissociation energy is 12– 15 kcal/mol lower than the activation energies. An interesting sidelight on this reaction is the fact that 7-tert-butoxynorbornadiene undergoes isomerization much faster with log k ¼ 13:6 2 35 500=2:3RT:36 Alkoxy substitution on the migrating carbon in 1,3-shifts seems to promote these reactions as well, and the reactions are independent of solvent polarity, ruling out charged intermediates. Norbornadiene is the product of pyrolysis of quadricyclane. The reaction is exothermic by 21 kcal/mol and log k ¼ 15:0 2 38 300=2:3RT (Scheme 8.19).37 Quadricyclane is easily formed photochemically from norbornadiene, so it can act as a transducer of energy.

Scheme 8.19

Finally, 1,2,4,5,6,7-hexamethyltricyclo[3.2.0.02,4]hept-6-ene gave the hexamethylcycloheptatrienes at 1808C.38 Subsequently it was shown that various tricyclohept-6-enes gave bicyclo[3.2.0]hepta-2,6-dienes, cycloheptatrienes, quadricyclanes and norbornadienes depending on the substitution.39 In the simplest case, 2,3,3,4-tetramethyltricyclo[3.2.0.02,4]hept-6-ene gave 3,4,4,5-tetramethylbicyclo[3.2.0]hepta-2,6-diene (38%) and 1,6,7,7-tetramethylcycloheptatriene (62%) at 1448C (Scheme 8.20).

184


Scheme 8.20

The former product slowly gives the latter at higher temperatures suggesting that both are primary products, but the latter is formed probably as described in Section 2.2. Indeed, the two products would appear to be formed by the two pathways described there, namely direct disrotatory opening with cyclopropane bond fission to cycloheptatriene and conrotatory opening and cyclopropane bond fission to the cis,trans,cis-trienes which cyclize to the bicyclic diene. 2.4

2-Ethynyl-1-vinylcyclopropane

Both cis- and trans-2-ethynyl-1-vinylcyclopropane give the dimer of 1,2,5cycloheptatriene upon heating, but with different rates.40 The cis isomer reacts at room temperature with log k ¼ 9:98 2 19 900=2:3RT; and the trans isomer requires heating above 2008C. It would seem reasonable that the cis isomer undergoes a concerted 3,3-shift to the triene while the trans isomer must undergo geometric isomerization, presumably via a biradical intermediate, to the cis isomer before rearranging (Scheme 8.21).

Scheme 8.21

There are a number of questions about the reaction. Is the 3,3-shift reversible? Is the allene planar or chiral? What is the stereochemistry of the dimer(s)? The cis-3-oxa and 3-tert-butylaza analogs have been examined as well. The former gives cis-2-ethynylcyclopropanecarboxaldehyde presumably by another 3,3shift while the latter gives the azepine, presumably by acid-catalyzed double bond isomerization (Scheme 8.22).41

C7H6 – C7H12

185

Scheme 8.22

2.5 Spiro[2.4]heptadiene Geometric Isomerization and Rearrangement Spiro[2.4]heptadiene rearranges to 6-methylfulvene and 5-vinylcyclopentadiene in a 5:4 ratio, respectively, with log k ¼ 12:89 2 43 600=2:3RT:42 Subsequently, it was shown that interconversion of cis- and trans-1,2-dimethyl derivatives preceded the structural isomerization.43 The structural and geometric isomerizations would appear to involve cleavage to a biradical followed by hydrogen shift or reclosure after bond rotation (Scheme 8.23).

Scheme 8.23

Interestingly, spiroheptadiene is the product of bicyclo[3.2.0]hepta-1,3-diene at 2 508C,44 and the[3.2.0]diene is probably involved in the interconversion of 4and 5-methylspiro[2,4]heptadiene by a double ring walk which has an activation energy of 44 kcal/mol and an entropy of activation of 2 e.u. (Scheme 8.24).45

186


Scheme 8.24

In the hope of demonstrating the possibility of reversible generation of planar carbon in this system, three different labeled 1-methylspiro[2.4]heptadienes were pyrolyzed (Scheme 8.25).46

Scheme 8.25

Such a species would be revealed by observation of inversion at the spirocenter but not at C1 or C2 (as possibly observed from syn –anti isomerization of the 4-deuterio derivative). At 2218C, the three phenomenological rate constants were

C7H6 – C7H12

187

determined and dissected into mechanistic rate constants for inversion at a single carbon (single subscript), inversion at two carbons (double subscript). Unfortunately, there are too many possibilities to unambiguously determine the mechanistic rate constants. In the most likely scenario, the C1 – C3 bond is the weakest and most readily cleaved, so that all but k1 ; k3 ; and k1;3 can be ignored initially. But these could not be assigned, even on a relative basis. Nonetheless, if k3 were much larger than k1 then k1;3 would be roughly equal to k3 : It is more likely that all three mechanistic rate constants are comparable. 2.6

1-(cis-2-Methylcyclopropyl)-1,2-propadiene to 1,3,6-Heptatriene

Upon heating 3-(cis-2-methylcyclopropyl)-1,2-butadiene rearranges to cis3-methyl-1,3,6-heptatriene presumably via a homodienyl-1,5-hydrogen shift (Scheme 8.26).47

Scheme 8.26

When the two diastereomers of 1-(cis-2-methylcyclopropyl)-1,2-butadiene were examined, one reacted roughly 10 times faster than the other at 1258C to give cis,cis1,4,6-octatriene while the other gave the cis,trans isomer at 1508C (Scheme 8.27).48 It was argued that steric effects were responsible for the rate difference, and the relative configurations of starting materials were assigned on this basis.

Scheme 8.27

188 2.7


Norpinene Degenerate Rearrangement

At 3258C, 3-deuterionorpinene equilibrates with its 1-deuterio isomer (Scheme 8.28).49 The free energy of activation for this 1,3-shift is roughly 43 kcal/mol, which suggests that it involves biradicals.

Scheme 8.28

2.8

Benzene Oxide

Benzene oxide has attracted attention not just because of its relationship to norcaradiene, but because it is the simplest analog of the intermediates involved in the epoxidation of aromatic hydrocarbons which ultimately leads to phenols, particularly in biological systems. The reaction is known as the NIH shift. Benzene oxide is in equilibrium with small amounts (5 –10%) of oxepine. The rate constant for the interconversions are log kf ¼ 14:4 2 9100=2:3RT and log kb ¼ 12:1 2 7200=2:3RT (Scheme 8.29).50

Scheme 8.29

Benzene oxides also undergo circumambulatory 1,5-oxygen shifts.51 Finally, the conversion of benzene oxide to phenol was proposed52 and subsequently found to proceed via an acid-catalyzed reaction53 part of which involves an intermediate diol.54 2.9 o-, p-Isotoluene, Homofulvene, Methylenebicyclo[2.2.0]hex-2-ene, and Toluene The two isotoluenes, 5-methylene-1,4-cyclohexadiene55 and 3-methyl-1,3-cyclohexadiene,56 were prepared in the early 1960s and found to give toluene upon

C7H6 – C7H12

189

treatment with acid (Scheme 8.30). The former was also reported to give toluene in solution in a second-order reaction57 although only ene-dimers were found in a subsequent study by Gortva.58 Further, p-isotoluene gives toluene and dimers in a second-order reaction with activation parameters suggesting radical reactions. Interestingly, the 1,3-hydrogen shift necessary to convert the ortho isomer to toluene apparently is too high in energy because it is symmetry forbidden to occur in a suprafacial manner, and the allowed 1,5-hydrogen shift in the para isomer apparently is sterically prohibited from occurring.

Scheme 8.30

A third isomer, homofulvene (or m-isotoluene) (Scheme 8.30), has been reported to polymerize on heating,59 although a pentamethyl derivative was found to give aromatic products, albeit in a radical process.60 Various other substituted isotoluenes have been studied and invariably the aromatization process involves a radical pathway.61 Finally, methylenebicyclo[2.2.0]hex-2-ene has been prepared and found to rearrange to o-isotoluene with log k ¼ 12:56 2 24 710=2:3RT in acetonitrile (Scheme 8.31).62

Scheme 8.31

2.10

6-Methylenebicyclo[3.1.0]hex-2-ene to Homofulvene

Derivatives of 6-methylenebicyclo[3.1.0]hex-2-ene were shown to undergo a rapid conversion to homofulvene derivatives.63 However, the most complete study of the reaction involved substitution of a methoxy group at C4 since that provided a stereochemical marker to assess the three-dimensional aspect of the conversions (Scheme 8.32).64

190


Scheme 8.32

Both endo and exo isomers of the 4-methoxy-6-methylene derivative gave predominantly endo-7-methoxy-3-methylenebicyclo[3.1.0]hex-2-ene (methoxyhomofulvene) with log kðendoÞ ¼ 13:3 2 26 660=2:3RT and log kðexoÞ ¼ 13:4 2 2470=2:3RT: Furthermore, neither the two starting materials nor the two possible homofulvene products were interconverted among the pair under the reaction conditions (benzene solvent near its boiling point). It would thus appear that both starting epimer rings open to the trimethylenemethane biradical, which is stabilized by extended conjugation and is most likely planar, and this species cyclizes to the homofulvene observed. The explanation for the preference for formation of the endo product was suspended, pending calculations. The isomerization of the 2,3-benzo derivative at 808C provided another pathway for isomerization. Here, various derivatives gave 2 þ 2 dimers (or adducts with furan) of 3,4-benzobicyclo[3.2.0]cyclohepta-1(2),3-diene derivatives which apparently were formed via a homofulvene with a disrupted benzo unit which undergoes a 1,5-shift (or two 1,3-shifts) (Scheme 8.33).65 The stereochemistry depicted in the scheme was inferred from the stereochemistry of the dimers and the formation of the endo-methoxy homofulvene was as observed in the debenzo derivative above.

Scheme 8.33

C7H6 – C7H12

3 3.1

191

C7H10 cis-, trans-1,2-Divinylcyclopropane to 1,4-Cycloheptadiene

The conversion of cis- and trans-1,2-divinylcyclopropane to 1,4-cycloheptadiene was first reported in 196066 although only the trans isomer was actually isolated, and it was found to rearrange with log k ¼ 13:1 2 34 900=2:3RT:67 The cis isomer was prepared and isolated in 1973, and it was found to rearrange very rapidly below room temperature with DH ‡ ¼ 19:4 kcal/mol and DS‡ ¼ 25:3 e.u.68 The low activation enthalpy and negative activation entropy for the cis isomer strongly suggests a concerted 3,3-shift, necessarily via a boat-like transition state with the vinyl groups over the ring. But the trans isomer apparently isomerizes to the cis compound in the rate-determining step judging by the small positive activation entropy and an activation energy comparable to the bond dissociation energy of the doubly allylic cyclopropane bond (Scheme 8.34).

Scheme 8.34

Optically active trans material was found to rearrange 2.8 times faster than it enantiomerized, which further suggests a biradical intermediate although concern was expressed for the apparently greater tendency of the biradicals to cyclize to the cis isomer (Scheme 8.35).67

Scheme 8.35

192


Deuterium kinetic isotope effects at the terminal carbons of both isomers have been determined. With the cis isomer, kH =kD4 ¼ 1=1:29ð0:1Þ at 108C,69 and for the trans isomer, kH =kD4 ¼ 1:18ð0:1Þ at 1598C.70 When the latter ratio is extrapolated to room temperature, it becomes 1.37. The very different isotope effects, inverse with the cis isomer and normal with the trans isomer, reflect the different ratedetermining steps: the former involving bonding between the vinyl termini and the latter involving cleavage to allyl radicals whose bending frequencies are lower than those of olefinic sp2 carbon (Scheme 8.36).

Scheme 8.36

It should be noted, however, that the extent of bond formation in the cis isomer as judged by comparison of the KIE to the equilibrium isotope effect is less than that in the parent 1,5-diene possibly reflecting both an earlier transition state due to the exothermicity of the reaction, ca. 20 kcal/mol, and a transition state that more resembles allyl radicals than the cyclopropyl-substituted 1,4-cyclohexane diyl. Finally, the assertion that the vinyl groups were required to be positioned over the cyclopropane ring in the 3,3-shift comes from the recognition that cis double bonds must be generated in the seven-membered ring and from the observation that cis-1,2-dipropenylcyclopropane is remarkably stable with respect to formation of cycloheptadiene and, instead, isomerizes to trans-1,2-cis,cis-dipropenylcyclopropane (Scheme 8.37).71

Scheme 8.37

C7H6 – C7H12

3.1.1

193

Heterologs

trans-1,2-Divinylethylene oxide was first prepared in 1963,72 and the cis isomer was isolated in 1964.73 Both isomers were shown to give 4,5-dihydrooxepine upon pyrolysis, but the temperatures necessary were very different.74 Not unexpectedly, the former required heating above 2008C, but the latter rearranged at 1008C (Scheme 8.38).

Scheme 8.38

The cis isomer rearranged with DH ‡ ¼ 24:6 kcal/mol and DS‡ ¼ 211:3 e.u., while the trans isomer rearranged with DH ‡ ¼ 36:0 kcal/mol and DS‡ ¼ 20:4 e.u.74 The rate-determining step in each reaction is probably the same as with the hydrocarbons. However, it is interesting that the cis isomer is less reactive than the hydrocarbon judging by the temperature and activation parameters. This is probably attributed to the increased strength of cyclopropane bonds across electronegative substituents which results from electron withdrawal from the antibonding character at that bond of the Walsh orbital.75 cis-2,3-Divinylaziridine is reported to undergo the 3,3-shift at or below room temperature while the episulfide is relatively stable, not unlike the epoxide.76 3.2

1,1-Divinylcyclopropane to 1-Vinylcyclopentene

1,1-Divinylcyclopropane undergoes a thermal 1,3-shift to 1-vinylcyclopentene77 with log k ¼ 13:53 2 42 450=2:3RT (Scheme 8.39).78

Scheme 8.39

The activation energy is only 7 kcal/mol less than that for the vinylcyclopropane rearrangement itself, consistent with roughly 20 kcal/mol for the pentadienyl radical resonance energy, and suggests that the reaction is not concerted. The kinetic isotope effect for four deuteria at the terminal vinyl positions was found to be 1.08 (0.07) at 2428C, and the preference for vinyl over ring-bound deuterium in the mono-dideuteriovinyl material was found to be 1.07 (0.02) (Scheme 8.40).

194


Scheme 8.40

The KIE with the tetradeuterio material is not inconsistent with formation of a biradical whose bending frequencies are probably lower than those associated with olefinic sp2 carbon. The partitioning isotope effect probably reflects the difficulty in rotating a CD2 group vs a CH2 group out of plane of the pentadienyl radical to form the ring in what must be equilibrating biradicals of the “W” and “sickle” geometry.79 3.3

4-Vinylcyclopentene Degenerate Rearrangement

4-Vinylcyclopentene undergoes what appears to be a 3,3-shift with DG‡ ¼ 55:7 kcal/mol at 3938C (Scheme 8.41).80

Scheme 8.41

The high temperatures required for this reaction would appear to require cleavage to bisallyl biradical intermediates. But the fact that this reaction was no faster than the equivalent 3,3-shift of 4-vinylcyclohexene was interpreted as requiring

C7H6 – C7H12

195

formation of a 1,4-diyl species since the greater strain of the cyclopentene system should have promoted the cleavage relative to the six-membered ring. Some perspective is necessary here. As indicated in Chapter 7, Section 4.1 on the Cope rearrangement, the free energy for formation of a cyclohexane-1,4-diyl is 50 –53 kcal/mol and that for formation of two allyl radicals is roughly 57 kcal/mol. However, in the current system, the diyl is destabilized by roughly 20 kcal/mol due to the bicyclo[2.2.1]ring system that must be generated. Such a species is kinetically inaccessible due, in part, to a substantial negative entropy despite the fact that the activation enthalpy for its formation would appear to be 50 –55 kcal/mol. 3.4

1,3-Dimethylcyclopentane Degenerate Rearrangement

Salazar found that 1,3-dimethylenecyclopentane undergoes a degenerate rearrangement by both 1,3- and 3,3-sigmatropic shifts as judged by a 2:1 kinetic ratio of the appropriate deuterium isomers at 3508C (Scheme 8.42).81

Scheme 8.42

The results suggest formation of a 2,20 -bisallylmethane biradical. The activation free energy is comparable to the BDE of the C4 –C5 bond, which further indicates the likely formation of a biradical assuming a normal pre-exponential factor. However, the biradical is not formed with random stereochemistry about the allylic moieties since very different product distributions result from pyrolysis of the corresponding trans- and cis-4,5-dimethyl derivatives, T and C, respectively (Scheme 8.43).

Scheme 8.43

196


The results suggest roughly a 7-fold preference for conrotatory ring opening with the trans isomer and a 3-fold preference with the cis isomer if only the rate constants for the 3,3-shift products are compared (the 1,3-shift products from the trans isomer also shows roughly a 7-fold preference: (47.2/2 £ 3.2)). Further, pyrolysis of the optically active trans-4,5-dimethyl-1,3-dimethylenecyclopentane gave the 1,3-shift product with 21% net inversion under conditions where the starting trans material was only partly racemized indicating that the biradicals are formed and closed without completely equilibrating by rotation about the original C1 – C2 and/or C2 – C3 bonds.82 The preference for conrotation appears to be a result of the need to couple the ring opening with motions about other ring bonds that generate a nonplanar and, therefore, sterically more favorable orthogonal arrangement of the two allylic units which undergo rotation competitive with bond formation. It is reasonable that the other rotations would occur to preserve overlap as much as possible in forming the biradical; thus, formation of the biradicals of Scheme 8.43 was proposed. 3.5

Bicyclo[3.2.0]hept-2-ene to Bicyclo[2.2.1]hept-2-ene

Berson first showed that bicyclo[3.2.0]hept-2-enes are converted thermally to bicyclo[2.2.1]hept-2-enes,83 however, it was the determination of the stereochemistry of the migrating carbon in this 1,3-shift that captured the attention of organic chemists in the late 1960s.84 There was at least a 19-fold preference for inversion in the pyrolysis of exo-7-deuterio-endo-6-acetoxybicyclo[3.2.0]hept-2-ene at 3078C. This is consistent with an allowed reaction in which the allylic moiety is used in a suprafacial manner (Scheme 8.44).

Scheme 8.44

However, whether the reaction is concerted is not clear since log k ¼ 14:762 48 640=2:3RT for the reaction of the parent compound.85 The activation energy is not very different than what might be expected for the bond dissociation energy of the C1 – C7 bond, so a biradical might be involved. The stereochemistry might result from the usual twisting motions that seem to characterize cyclobutane ring openings (see Chapter 7, Section 3.13) to give a biradical which upon least motion closure with rearrangement gives the observed stereochemistry (Scheme 8.45).

C7H6 – C7H12

197

Scheme 8.45

Alternatively, Carpenter’s non-statistical dynamics argument would suggest that the depyramidalization of C7 with appropriate twisting motions in the ring opening generates a species whose momentum carries it on to the inverted product.86 Steric effects exert significant control of the stereochemistry of the migrating carbon when the C7 substituent is something other than deuterium. With the exo-7methyl derivative, a 9.3:1 ratio of inverted over retained product was formed, but with the endo-7-methyl derivative a 7.2:1 ratio of retained over inverted product was formed.87 This almost suggests that the same, nearly equilibrated biradical was formed in each reaction. Here equilibration might be the result of a decreased rate of closure due to methyl substitution and a strong steric preference for formation of the exo-methyl product. Subsequent work on the parent molecule revealed less inversion at C7. In one study at 2768C, 76 ^ 4% was observed,88 and in the other at 3128C, 89% inversion was observed.89 Both studies examined the pyrolysis of exo,exo-6,7-dideuteriobicyclo[3.2.0]hept-2-ene, and found that starting material did not epimerize significantly at C7 under the reaction conditions (subsequently a small amount was measured90), and that the norbornene produced rapidly underwent a retro Diels –Alder reaction so that at most 2% of it was present at any time (Scheme 8.46).

Scheme 8.46

In the two studies, the ethylene produced had a greater percentage of E isomer, 67 and 82%, respectively. It is likely that the results of both studies are within experimental error of one another, but if not, a unique situation arises where higher stereospecificity occurs at a higher temperature. The results are not inconsistent with

198


a biradical pathway, not unlike that described in Scheme 8.45, and the somewhat lower specificity might be attributable to the lack of an acetoxy group at C6 which could somewhat prejudice the direction of rotation about the C5 –C6 bond leading to a more compact rather than an extended biradical in the acetoxy derivative. 3.6 Bicyclo[2.1.0]pentane-5-spirocyclopropane, Bicyclo[3.2.0]hept-1-ene, 5-Methylenebicyclo[2.1.1]hexane, 1,2,6-Heptatriene, 3-Methylene-1,5-hexadiene and Tricyclo[4.1.0.01,3]heptane Bicyclo[2.1.0]pentane-5-spirocyclopropane rearranges to bicyclo[3.2.0]hept-1-ene with log k ¼ 12:94 2 33 750=2:3RT (Scheme 8.47).91

Scheme 8.47

At higher temperatures, the product rearranged to 5-methylenebicyclo[2.1.1]hexane which at higher temperatures rearranged to 3-methylene-1,5-hexadiene, probably by a 3,3-shift in the retro 2 þ 2 cycloaddition product of the bicyclohexane (Scheme 8.47). The first reaction involves the usual conversion of spiropentanes to methylenecyclobutanes, which most likely begins with cleavage of the very weak fused ring bond. Indeed, double inversion of the fused bond occurred rapidly relative to rearrangement and reversibly as judged by the equilibration of the exo,exo- and endo, endo-2,3-dideuterio material with log k ¼ 14:08 2 29 000=2:3RT (Scheme 8.48).

Scheme 8.48

C7H6 – C7H12

199

The spiropentane rearrangement was examined with cis- and trans-20 ,30 -dimethyl materials. Only a small preference for retention of configuration was found (50% with the cis isomer and 75% with the trans isomer) once the steric effects are taken into account suggesting formation of nearly completely rotationally equilibrated biradicals. The methylenecyclobutane rearrangement also appears to proceed via biradicals since the dimethyl products described above equilibrate prior to rearrangement to the other methylenecyclobutane. Interestingly, the extent of the formation of the ultimate product, 3-methylene-1,5-hexadiene, from the methylenecyclobutane rearrangement of bicyclo[3.2.0]hept-1-ene varied inversely with pressure suggesting that it was produced from vibrationally hot methylenebicyclo[2.1.1]hexane. The 3,3-shift of 1,2,6-heptatriene was found to occur92 at 3008C with log k ¼ 9:97 2 30 470=2:3RT:93 While a concerted, cyclic transition state was proposed for the reaction, the possibility that a cyclohexane-1,4-diyl intermediate stabilized by the additional double bond was examined. Extensive calorimetric and kinetic studies as well as oxygen trapping experiments led to the enthalpy surface of Scheme 8.49, which also includes data on the conversion of 2-methylenebicyclo[2.2.0]hexane to 3-methylene-1,5-hexadiene, back to itself with bridgehead double inversion, and to 1,2,6-heptatriene.94

Scheme 8.49

The enthalpy surface is interesting in that the activation energy for the 3,3-shift is lower than that of 1,5-hexadiene itself, but generates a biradical presumably due to the stabilization of one radical site by allylic resonance. Somewhat deceptive on this enthalpy surface is the fact of differential entropy effects that provide important contributions to the free energies of the reactions. In particular, the cyclic structures are destabilized relative to acyclic ones, and the central biradical is destabilized relative to the transition states forming and destroying it. Thus, the biradical should be in a potential well less deep than that depicted. Of interest in this connection is the fact that oxygen trapping of the biradical was incomplete, even extrapolated to high

200


pressure, so it was argued that as much as half of the reaction was concerted, despite the enthalpy surface generated. Pyrolysis of a methyl-labeled derivative of 1,2,6-heptatriene, namely, optically active R-5-methyl-E-1,2,6-octatriene gave a 4:1 mixture of E and Z-4-methyl-3methylene-1,5-octatriene, but the E isomer was racemized to the extent of 32% while the starting material was optically stable.95 This data is consistent with closure to two stereoisomeric, chair-like 1,4-biradicals in an 84:16 ratio (due to equatorial/ axial preferences) which undergo ring flip and cleavage (Scheme 8.50).

Scheme 8.50

Computational efforts at the CASSCF (8,8), CASPT2N, or (U)B3LYP level of theory using a 6-31Gp basis set and zero point corrections found, relative to 1,2,6heptatriene, an intermediate biradical at 14.3 kcal/mol and two transition states, one forming the intermediate at 25.9 kcal/mol and the one forming product at 22.6 kcal/mol. In addition, a pathway was found that connected the two transition states, and was 9.1– 9.6 kcal/mol above the intermediate.96 This enthalpy surface

C7H6 – C7H12

201

closely resembles that determined experimentally using oxygen trapping. However, subsequent theoretical efforts found a second 1,4-diyl slightly higher in energy than the original biradical from CASPT2N calculations. The CASSCF(8,8)/6-31Gp potential energy surface (no zero point correction or entropy contribution) is given in Scheme 8.51.97

Scheme 8.51

The difference between the calculated and experimental surface is striking. Beside giving transition states that are much too high, the calculations reveal that conjugation produces the diyl in a relatively deep potential well, and apparently no concerted 3,3-shift transition state was found. Nonetheless, trajectory calculations associated with the calculation of the potential surface indicate that some of the molecules passing over the first transition state continued over the second transition state to product on a time scale less than collision frequencies even at high pressures. This might further indicate that the energy of the diyl is not randomly distributed over the diyl, thus making the lifetime substantially longer so that all such species would be trapped at high pressures of oxygen. Finally, it is worthy of note that 3-methylene-1,5-hexadiene is also the product of pyrolysis of tricyclo[4.1.0.01,3]heptane98 with log k ¼ 14:21 2 36 500=2:3RT: A small amount of 2-methylenebicyclo[2.2.0]hexane was also formed with log k ¼ 14:07 2 37 300=2:3RT (Scheme 8.52).99 It would appear as if the 2-methylenecyclohexane-1,4-diyl species were an intermediate in this process formed by initial cleavage of a spiropentane radial bond as opposed to a peripheral bond which is the lower energy pathway in the parent compound (see Chapter 6, Section 2).

202


Scheme 8.52

3.7

Bicyclo[3.2.0]hept-6-ene to 1,3-Cycloheptadiene

Bicyclo[3.2.0]hept-6-ene gives cis,cis-1,3-cycloheptadiene with log k ¼ 14:31 2 45 510=2:3RT:100 This reaction either involves a non-concerted, disrotatory ring opening directly to the cis,cis-diene or a concerted, but high energy, conrotatory ring opening to a cis,trans-diene which undergoes subsequent 1,5-hydrogen shifts to give the cis,cis material (Scheme 8.53).

Scheme 8.53

The activation energy is at least as high as the estimated bond dissociation energy of the central bond, so non-concerted ring opening would appear to be involved. 3.8

Bicyclo[4.1.0]hept-1(2)-ene to 6-Methylenebicyclo[3.1.0]hexane

Pyrolysis of bicyclo[4.1.0]hept-1(2)-ene gives 6-methylenebicyclo[3.1.0]hexane with DH ‡ ¼ 41:7 kcal/mol and DS‡ ¼ 2:9 e.u. (Scheme 8.54).101

Scheme 8.54

C7H6 – C7H12

203

The rearrangement, no doubt, proceeds via a trimethylenemethane biradical which is probably in the singlet state. Through the use of methyl labels the product has been shown to undergo bridgehead double inversion and exo-methylene rotation at 160– 1808 at nearly comparable rates. Significantly, endo-exo-isomerization of ring methyl materials occurs without any deuterium kinetic isotope effect at the exomethylene of the product indicating that there is no rotation of that group in the ring opening reaction. Thus, the product distribution is most readily rationalizable by initial formation of a planar TMM species that gives some (ca. 16%) bridgehead double inversion product without exo-methylene rotation but gives mostly a TMM biradical with the exo-methylene group orthogonal to the planar allyl radical in the six-membered ring (Scheme 8.55).102

Scheme 8.55

3.9

3-Vinylmethylenecyclobutane to 4-Methylenecyclohexene

3-Vinylmethylenecyclobutane rearranges to 4-methylenecyclohexene with log k ¼ 12:70 2 35 670=2:3RT (Scheme 8.56).103 The activation energy for this rearrangement is comparable to the bond dissociation energy of the cyclobutane bond undergoing fission by virtue of formation of two allyl radicals, therefore, the reaction is probably not concerted. If ring opening occurred to give a transoid allylic radical, then that step would be reversible since product formation would be impossible; however, opening to a cisoid allylic radical could give the observed product.

Scheme 8.56

204


3.10

Tricyclo[4.1.0.02,4]heptane to Bicyclo[4.1.0]hept-3-ene

anti- and syn-Tricyclo[4.1.0.02,4]heptane give bicyclo[4.1.0]hept-3-ene with very different rates. The syn material required only 2908C to affect rearrangement while the anti material required 3808C (Scheme 8.57).104 Whether the temperature differential reflects different strain or an inherent stereochemical preference for retrograde 2 þ 2 cycloaddition has yet to be resolved.

Scheme 8.57

3.11

cis,cis-1,3,5-Heptatriene 1,7-Hydrogen Shift

Uncatalyzed, first-order 1,7-hydrogen shifts were first observed in the irradiated ergosterol system (Chapter 7, Section 3.5), but were most carefully studied by Havinga with the simpler analogue, which rearranges with an activation energy of 21 kcal/mol and an entropy of activation of 2 17 e.u. (Scheme 8.58).105 Steric considerations suggested that the reaction proceeded with an antarafacial transfer of hydrogen. Subsequent studies, especially with terminal dialkyl substituted cis,cis-1,3,5hexatrienes, revealed that the hydrogen shift occurs at a reasonable rate above 1008C (see Chapter 5).

Scheme 8.58

4 4.1

C7H12 Bicyclo[4.1.0]heptane

Upon heating bicyclo[4.1.0]heptane gave 1-methylcyclohexane with log k ¼ 14:98 2 64 500=2:3RT; methylenecyclohexane with log k ¼ 15:09 2 65 900=2:3RT, and

C7H6 – C7H12

205

cycloheptene with log k ¼ 14:82 2 64 900=2:3RT (Scheme 8.59).106 Apparently cleavage of both the C1–C7 and C1–C6 bonds occurs, and each is followed by a vicinal hydrogen shift as in the conversion of cyclopropane to propylene (see Chapter 4, Section 2). Cyclohexene oxide gave cyclohexanone with log k ¼ 14:82 2 64 900=2:3RT and

Scheme 8.59

cyclohexen-3-ol with log k ¼ 13:11 2 55 800=2:3RT (Scheme 8.60).107 The latter product would appear to arise by a 1,4-hydrogen shift to oxygen.

Scheme 8.60

4.2

cis-2-Methylvinylcyclobutane Homodienyl Hydrogen Shift

The homodienyl-1,5-hydrogen shift in cis-2-alkylvinylcyclobutane was initially observed in the 2-ethyl case (see Chapter 7, Section 4 – vinylcyclobutane) where it constituted roughly two-thirds of the total reaction. Kinetic studies with 2,2-dimethylvinylcyclobutane revealed that the hydrogen shift had log k ¼ 10:8 2 38 380=2:3RT; but the retro 2 þ 2 cycloaddition had log k ¼ 15:09 2 47 440= 2:3RT and was faster than the hydrogen shift by a factor of three even at the lowest temperature (Scheme 8.61).108

Scheme 8.61

206


Like the homodienyl hydrogen shift in cis-2-alkylvinylcyclopropane systems (see Chapter 7, Section 4.9) it might be expected that the vinyl group must be predisposed over the cyclobutane ring in the transition state to accommodate a concerted reaction. Indeed this is the case with various substituted derivatives.109

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C7H6 – C7H12 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60.

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F.-G. Kla¨rner, Angew. Chem. Int. Ed. Engl., 12, 268 (1974); F.-G. Kla¨rner, S. Yaslak, and M. Wette, Chem. Ber., 112, 1168 (1979). J.E. Baldwin and B.M. Broline, J. Org. Chem., 47, 1385 (1982); See also F.-G. Kla¨rner, and B. Brassel, J. Am. Chem. Soc., 102, 2469 (1980). A. Kless, M. Nendel, S. Wilsey, and K.N. Houk, J. Am. Chem. Soc., 121, 4524 (1999), See also p. 7278 (correction). A.A. Jarzecki, J.J. Gajewski, and E.R. Davidson, J. Am. Chem. Soc., 121, 6928 (1999). M.R. Willcott, III and E. Goerland, Tetrahedron Lett., 6341 (1966). J.E. Baldwin and M.S. Kaplan, J. Am. Chem. Soc., 94, 668 (1972); J.E. Baldwin, and M.S. Kaplan, J. Am. Chem. Soc., 93, 3969 (1971). T. Miyashi, M. Nitta, and T. Mukai, Tetrahedron Lett., 3433 (1967); T. Miyashi, M. Nitta, and T. Mukai, J. Am. Chem. Soc., 93, 3441 (1971). E.W. Otterbacher and J.J. Gajewski, J. Am. Chem. Soc., 103, 5862 (1981). M. Christl and G. Bru¨ntrup, Angew. Chem. Int. Ed. Engl., 13, 208 (1974). W.C. Herndon and L.L. Lowry, J. Am. Chem. Soc., 86, 1922 (1964). R.K. Lustgarten and H.G. Richey, Jr., J. Am. Chem. Soc., 96, 6393 (1974). D.S. Kabakoff, J.-C.G. Bu¨nzli, J.F.M. Oth, W.B. Hammond, and J.A. Berson, J. Am. Chem. Soc., 97, 1510 (1975). E. Mu¨ and H. Kessler, Tetrahedron Lett., 3037 (1968). L.A. Paquette and L.M. Leichter, J. Am. Chem. Soc., 93, 5128 (1971). W.R. Dolbier, Jr., O.T. Garza, and B.H. Al-Sader, J. Am. Chem. Soc., 97, 5038 (1975). N. Manisse and J. Chuche, J. Am. Chem. Soc., 99, 1272 (1977). See also F. BourelleWargnier, M. Vincent, and J. Chuche, Tetrahedron Lett., 283 (1978). J.M.E. Krekels, J.W. de Haan, and H. Kloosterziel, Tetrahedron Lett., 2751 (1970). R.G. Bergman, as quoted in M.R. Willcott, III, R.L. Cargill, A.B. Sears, Prog. Phys. Org. Chem., 9, 25 (1972). N.K. Hamer and M.E. Stubbs, Tetrahedron Lett., 3531 (1972); M. Oda and R. Breslow, Tetrahedron Lett., 2537 (1973). R.A. Clark, W.J. Hayles, and D.S. Young, J. Am. Chem. Soc., 97, 1966 (1975). K.E. Gilbert and J.E. Baldwin, J. Am. Chem. Soc., 98, 1593 (1976). D.E. Minter, G.J. Fonken, and F.T. Cook, Tetrahedron Lett., 711 (1979). D.E. Minter and G.J. Fonken, Tetrahedron Lett., 4149 (1977). K. Dietrich and H. Musso, Chem. Ber., 107, 731 (1974). E. Vogel and H. Gu¨nther, Angew. Chem. Int. Ed. Engl., 6, 385 (1967); R. Wehner and H. Gu¨nther, Chem. Ber., 107, 3149 (1974). P.Y. Bruice, G.J. Kasperek, T.C. Bruice, H. Yagi, and D.M. Jerina, J. Am. Chem. Soc., 95, 1673 (1973); F.-G. Kla¨rner, and E. Vogel, Angew. Chem. Int. Ed. Engl., 12, 840 (1973). D. Jerina, J. Daly, B. Witkop, P. Zaltzman-Nirenberg, and S. Udenfriend, Arch. Biochem. Biophys., 128, 176 (1968). G.J. Kasperek and T.C. Bruice, J. Am. Chem. Soc., 94, 198 (1972). G.J. Kasperek, T.C. Bruice, H. Yagi, N. Kaubisch, and D.M. Jerina, J. Am. Chem. Soc., 94, 7876 (1972). W.J. Bailey and R.A. Baylouny, J. Org. Chem., 27, 3476 (1962). H. Plieninger and W. Maier-Borst, Angew. Chem. Int. Ed. Engl., 3, 62 (1964); H. Plieninger and W. Maier-Borst, Chem. Ber., 98, 2504 (1965). W.D. Graham, J.G. Green, and W.A. Pryor, J. Org. Chem., 44, 907 (1979). J. Gajewski and A.M. Gortva, J. Org. Chem., 54, 373 (1989). M. Rey, U.A. Huber, and A.S. Dreiding, Tetrahedron Lett., 3583 (1968). R. Criegee, D. Scho¨nleber, R. Huber, C. Schweickhardt, R. Wolf, and R. Ramirez, Chem. Ber., 106, 857 (1973).

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61. H. Hart and J.D. DeVrieze, Tetrahedron Lett, 4257 (1968); H. Hart and J.D. DeVrieze, Chem. Commun., 1651 (1968); B. Miller and K.-H. Lai, Tetrahedron Lett., 1617 (1971). 62. D. Hasselmann and K. Loosen, Angew Chem. Int. Ed. Engl., 17, 606 (1978). 63. M.S. Newman and M.S. Vander Zwan, J. Org. Chem., 39, 761 (1974). 64. S. Pikulin and J.A. Berson, J. Am. Chem. Soc., 110, 8500 (1988). 65. V.M. Osterman, G. Schulte, and J.A. Berson, J. Am. Chem. Soc., 111, 8727 (1989). 66. E. Vogel, Angew. Chem., 72, 4 (1960); E. Vogel, K. Heinz, and K. Gajek, Justus Liebigs Ann. Chem., 644, 172 (1961). See also W. von E. Doering and W.R. Roth, Tetrahedron, 19, 715 (1963). 67. M. Arai and R.J. Crawford, Can. J. Chem., 50, 2158 (1972). 68. J.M. Brown, B.T. Golding, and J.J. Stofko, Jr., J. Chem. Soc., Chem. Commun., 319 (1973). 69. J.J. Gajewski, C.N. Hawkins, and J.L Jimenz, J. Org. Chem., 55, 674 (1990). 70. J.J. Gajewski, L.P. Olson, and K.J. Tupper, J. Am. Chem. Soc., 115, 4548 (1993). 71. J.E. Baldwin and C. Ullenius, J. Am. Chem. Soc., 96, 1542 (1974). 72. R.A. Braun, J. Org. Chem., 28, 1383 (1963). 73. E.L. Stogryn, M.H. Gianni, and A.J. Passannante, J. Org. Chem., 29, 1275 (1964). 74. E. Vogel, R. Sundermann, and R. Schubart, unpublished work cited in E. Vogel and H. Gu¨nther, Angew. Chem. Int. Ed. Engl., 6, 385 (1967). 75. P.D. Mollere and K.N. Houk, J. Am. Chem Soc., 99, 3226 (1977). See also Chapter 8, Section 2.1. 76. E.L. Stogryn and S.J. Brois, J. Org. Chem., 30, 88 (1965). 77. W.R. Dolbier, Jr. and J.H. Alonso, J. Am. Chem. Soc., 94, 2544 (1972). 78. W.R. Dolbier, Jr., J.H. Alonso, and H.M. Frey, Int. J. Chem. Kinet., 6, 893 (1974). 79. W.R. Dolbier, Jr. and S.-H. Dai, J. Am. Chem. Soc., 90, 5028 (1968); S.-H. Dai and W.R. Dolbier, Jr., J. Am. Chem. Soc., 94, 3946 (1972). 80. J.E. Baldwin, G.D. Andrews, and D.W. Parker, J. Org. Chem., 52, 676 (1987). 81. J.J. Gajewski and J. Salazar, J. Am. Chem. Soc., 101, 2739 (1979). See also p. 2740. 82. J.J. Gajewski and J. Salazar, J. Am. Chem. Soc., 103, 4145 (1981). 83. J.A. Berson and J.W. Patton, J. Am. Chem. Soc., 84, 3406 (1962). 84. J.A. Berson and G.L. Nelson, J. Am. Chem. Soc., 89, 5503 (1967). J.A. Berson, Acc. Chem. Res., 5, 406 (1972). 85. A.T. Cocks and H.M. Frey, J. Chem. Soc. A., 2564 (1971). 86. B.K. Carpenter, J. Am. Chem. Soc., 117, 6336 (1995). 87. J.A. Berson and G.L. Nelson, J. Am. Chem. Soc., 92, 1096 (1970). 88. J.E. Baldwin and K.D. Belfield, J. Am. Chem. Soc., 110, 296 (1988). See also J.E. Baldwin and K.D. Belfield, J. Phys. Org. Chem., 2, 455 (1989). 89. F.G. Kla¨rner, R. Drewes, and D. Hasselmann, J. Am. Chem. Soc., 110, 297 (1988). 90. J.E. Baldwin and P.A. Leber, J. Am. Chem. Soc., 123, 8396 (2001). 91. W.R. Roth and K. Enderer, Justus Liebigs Ann. Chem., 733, 44 (1970). 92. L. Skattebøl and S. Solomon, J. Am. Chem. Soc., 87, 4506 (1965). 93. H.M. Frey and D.H. Lister, J. Chem. Soc. A., 26 (1967). 94. W.R. Roth, D. Wollweber, R. Offerhaus, V. Rekowski, H. Lennartz, R. Sustmann, and W. Mu¨ller, Chem. Ber., 126, 2701 (1993). 95. T.E. Wessel and J.A. Berson, J. Am. Chem. Soc., 116, 495 (1994). 96. D.A. Hrovat, J.A. Duncan, and W.T. Borden, J. Am. Chem. Soc., 121, 169 (1999). 97. S.L. Debbert, B.K. Carpenter, D.A. Hrovat, and W.T. Borden, J. Am. Chem. Soc., 124, 7896 (2002). 98. L. Skattebøl, J. Org. Chem., 31, 2789 (1966). 99. H.M. Frey, R.G. Hopkins, and L. Skattebøl, J. Chem. Soc. B., 539 (1971). 100. G.R. Branton, H.M. Frey, D.C. Montague, and I.D.R. Stevens, Trans. Faraday Soc., 62, 659 (1966).

C7H6 – C7H12 101. 102. 103. 104. 105. 106. 107. 108. 109.

209

A.S. Kende and E.E. Riecke, J. Chem. Soc., Chem. Commun., 383 (1974). J.J. Gajewski and S.K. Chou, J. Am. Chem. Soc., 99, 5696 (1977). W.R. Dolbier, Jr. and G.J. Mancini, Tetrahedron Lett., 2141 (1975). W. Grimme, unpublished work, cited in W. von E. Doering and W.R. Roth, Angew Chem. Int. Ed. Engl., 2, 115 (1963). J.L.M.A. Schlatmann, J. Pot, and E. Havinga, Recl. Trav. Chim. Pays. Bas., 83, 1173 (1964). M.C. Flowers and D.E. Penny, Int. J. Chem. Kinet., 5, 469 (1973). M.C. Flowers, D.E. Penny, and J.-C. Pommelet, Int. J. Chem. Kinet., 5, 353 (1973). J.S. Chickos and H.M. Frey, J. Chem. Soc., P2, 365 (1987). S.J. Getty and J.A. Berson, J. Am. Chem. Soc., 113, 4607 (1991).

9 C8H6 –C8H14

CONTENTS 1 C8H6 1.1 3,4-Diethynylcyclobutene to Benzocyclobutadiene 2 C8H8 2.1 (CH)8 Interconversions 2.1.1 COT Geometry 2.1.2 Bicyclo[4.2.0]octa-2,4,7-triene 2.1.3 Semibullvalene 2.1.4 Tricyclo[3.3.0.02,6]octa-3,7-diene to Semibullvalene 2.1.5 Cyclobutadiene Dimers to Cyclooctatetraene 2.1.6 Degenerate Rearrangement of Cyclooctatetraene 2.1.7 High-Temperature Chemistry of Cyclooctatetraene 2.1.8 Tetracyclo[4.2.0.02,403,5]oct-7-ene 2.1.9 Tricyclo[5.1.0,02,8]octadiene to Cyclooctatetraene 2.1.10 Cubane to Cyclooctatetraene 2.1.11 Pentacyclo[3.3.0.0.2,40.3,706,8]octane to Semibullvalene and to COT and Other (CH)8 Isomers 2.1.12 Tetracyclo[3.3.0.0.2,403,6]oct-7-ene to Dihydropentalenes 2.1.13 Barrelene to Benzene and Acetylene 2.1.14 Enthalpy Surface for (CH)8 Interconversions45 2.2 Benzocyclobutene and o-Xylylene 2.3 Spiro[3.4]octatriene to 6-Vinylfulvene to Dihydropentalene 2.4 1-Methylenespiro[2.4]hepta-2,4-diene to 7-Methylenebicyclo[3.2.0]hepta-1(2),3-diene 2.5 1,2-Diethynylcyclobutane 3 C8H10 3.1 Bicyclo[5.1.0]octa-2,5-diene Degenerate Rearrangement 3.2 Bicyclo[4.2.0]octa-2,7-diene, Bicyclo[4.2.0]octa-2,4-diene, Cyclooctatriene, Bicyclo[3.3.0]octa-2,6-diene, Bicyclo[5.1.0] octa-2,4-diene, Bicyclo[4.2.0]octa-2,7-diene, 1,3,5,7-Octatetraene and Tetracyclooctanes

213 213 214 214 214 216 217 218 219 219 221 222 222 222 223 223 224 224 225 227 227 228 229 229

231

212


3.2.1 3.2.2 3.2.3 3.2.4 3.2.5

1,3,5-Cyclooctatriene to Bicyclo[4.2.0]octa-2,4-diene 1,3,6-Cyclooctatriene to 1,3,5-Cyclooctatriene Bicyclo[5.1.0]octa-2,4-diene Bicyclo[4.1.1]octa-2,4-diene to Bicyclo[5.1.0]octa-2,4-diene cis,cis-1,3,5,7-Octatetraene to 1,3,5-Cyclooctatriene and Bicyclo[4.2.0]octa-2,4-dienes 3.2.6 Tetracyclo[5.1.0.02,40.3,5]octane and Tetracyclo[4.2.0.0.2,40.3,5]octane Pyrolysis 3.2.7 C8H10 Free Energy Surface 3.3 6-Vinylbicyclo[3.1.0]hex-2-ene to Bicyclo[3.2.1]octa-2,6-diene 3.4 Tricyclo[3.3.0.02,8]oct-3-ene to Bicyclo[3.3.0]octa-2,7-diene 3.5 6-Methylenebicyclo[3.2.0]hept-2-ene to 5-Methylenebicyclo[2.2.1]hept-2-ene 3.6 Tricyclo[3.2.1.02,4]oct-6-ene to Tetracyclo[3.2.1.0.2,704,6]octane and Bicyclo[3.2.1]octa-2,6-diene 3.7 Cyclopropylidenespiropentane Rearrangements 3.8 cis- and trans-Tricyclo[5.1.0.02,4]oct-5-ene to 1,3,6-Cyclooctatriene 3.9 Dispiro[2.0.2.2]oct-7-ene Pyrolysis 3.10 1,2,6,7-Octatetraene, 3,4-Dimethylene-1,5-hexadiene, Bicyclo[4.2.0]hexa-1,5-diene, 1,2-Divinylcyclobutene, 2,3-Dimethylenebicyclo[2.2.0]hexane, 1,4-Ethano-2-methylene-spiropentane, 1-Allyl-2-vinylcyclopropene 3.10.1 1,2,6,7-Octatetraene Pyrolysis 3.10.2 2,3-Dimethylenebicyclo[2.2.0]hexane 3.10.3 1,4-Ethano-2-methylenespiropentane 3.11 Tricyclo[4.2.0.01,3]oct-4-ene 3.12 Tricyclo[4.2.0.01,3]octa-4-ene to Tricyclo[4.2.0.01,5]octa-3-ene 3.13 Bis-2,20 -methylenecyclopropanyl to 3,6-Dimethylenecyclohexene and 3-Methylenespiro[2.4]octa-1-ene 4 C8H12 4.1 1,2-Divinylcyclobutane, 4-Vinylcyclohexene, 1,5-Cyclooctadiene, Butadiene, and Tricyclo[4.2.0.02,5]octane 4.1.1 cis-1,2-Divinylcyclobutane 4.1.2 trans-1,2-Divinylcyclobutane 4.1.3 4-Vinylcyclohexene 4.1.4 cis,cis-1,5-Cyclooctadiene 4.1.5 cis – trans-1,5-Cyclooctadiene 4.1.6 Butadiene Dimerization 4.1.7 Enthalpy Surface for the Interconversions 4.1.8 1,2-Divinylcyclobutanes Incapable of Undergoing the 3,3-Shift 4.1.9 Tricyclo[4.2.0.02,5]octane and 1,5-Cyclooctadiene 4.1.10 Tricyclo[3.3.0.02,6]octane to 1,5-Cyclooctadiene 4.2 Bicyclo[4.2.0]oct-7-ene and 1,3-Cyclooctadiene

233 233 235 236 237 238 238 239 241 241 243 245 246 246

247 247 249 249 249 250 250 251 251 251 253 254 255 256 256 258 259 261 262 263

C8H6 – C8H14

4.3 1,2,7-Octatriene, 1-Bicyclo[4.2.0]octene, and 6-Methylenebicyclo[3.2.0]heptane 4.4 6-Octene-1-yne to 2-Vinylmethylenecyclopentane 4.5 [3.2.1]- and [2.2.2]Propellane and Dimethylenecyclohexanes 4.6 Bicyclo[5.1.0]oct-2-ene to 1,4-Cyclooctadiene to Bicyclo[3.3.0]oct-2-ene 5 C8H14 5.1 1,6-Octadiene to 2-Methylvinylcyclopentane 5.2 1,7-Octadiene, cis- and trans-Cyclooctene References

1 1.1

213

264 264 265 267 267 267 268 268

C8H6 3,4-Diethynylcyclobutene to Benzocyclobutadiene

trans-3,4-Bis(phenylethynyl)-1,2,3,4-tetramethylcyclobutene rearranges to a benzocyclobutadiene at 1108C (Scheme 9.1).1

Scheme . 9.1

On the other hand, the corresponding cis isomer gives a cis,trans-dienediyne upon heating (Scheme 9.2).2

Scheme . 9.2

214


Of interest in this connection is the fact that cis,cis-3,5-octadiene-1,7-diyne itself has been observed spectroscopically in the hydrolysis of its 1,8-bistrimethylsilyl precursor.3 However, after only a few minutes at room temperature, a tetracyclic, rearranged, dimer of benzocyclobutadiene was isolated (Scheme 9.3).

Scheme . 9.3

2

C8H8

2.1

(CH)8 Interconversions

Cyclooctatetraene (COT), a 4n non-aromatic hydrocarbon, is only one of the many (CH)8 compounds, but it is a central character. The other isomers, many of which interconvert with COT by thermal and photochemical pathways, include bicyclo[4.2.0]octa-2,3,7-triene (BOT), semibullvalene (SB), barrelene (B), tricyclo[3.3.0.02,6]octa-3,7-diene (TOD), the cyclobutadiene dimers (CBD), tetracyclo[4.2.0.0.2,403,5]oct-7-ene (TOE), cubane (C), tetracyclo[4.2.0.0.2,805,7] octene – the intramolecular Diels – Alder isomer of BOT (IDA), tetracyclo[3.3.0.0.2,403,6]oct-7-ene (TCO), pentacyclo[3.3.0.0.2,40.3,706,8]octane or cuneane (CU), and tricyclo[5.1.0.02,8]octadiene or octvalene (O) (Scheme 9.4).4 2.1.1

COT Geometry

COT, which was first synthesized by Willsta¨tter5 then in quantity by Reppe6 is nonplanar, no doubt due to the angle strain in the planar form. It is a tub-shaped material,7 which undergoes both ring inversion and double bond migration at low temperatures. Anet first showed that the two processes occur with different rates in the low-temperature NMR spectrum of the dimethylcarbinol derivative.8 The slower process is p bond shift and the faster is tub –tub interconversion with a difference in

C8H6 – C8H14

215

Scheme . 9.4

free energy of activation of 2.4 kcal/mol at 2 28C (Scheme 9.5). The p bond shift was argued to occur via the planar COT generated in the ring inversion. More recent MCSCF(8/8)/6-31Gp calculations indicate that the transition state for p bond shift has D8h symmetry and is 4.1 kcal/mol higher in enthalpy (ZPE corrected) than that for ring inversion which has D4h symmetry.9 Further, PE spectroscopy indicates that p bond shift occurs via a species with D8h symmetry.10 Finally, an inverse deuterium kinetic isotope effect ðkH =kD6 ¼ 0:69Þ has been observed in the ring inversion of isopropoxy-COT.11 This was attributed to increased p character in the external orbitals of the planar form thus increasing the CH bending frequencies in the transition state.

Scheme . 9.5

216


Earlier work has revealed that substitution has a dramatic effect on the rate of double bond shift in COT. The best example is that by Paquette who synthesized the two double bond isomers, 1,2,3,4-tetramethyl-COT and 1,2,3,8-tetramethyl-COT, independently, and found that they were interconverted only slowly at 1608C to a 3:7 mixture, respectively (Scheme 9.6).12 Further, the tub –tub interconversion was slowed as well, so much so that the two processes, tub inversion and bond shift, have virtually the same rate.

Scheme . 9.6

Work on substituted derivatives has proceeded in attempts to generate a planar COT. See the last two reviews of ref. 4. 2.1.2

Bicyclo[4.2.0]octa-2,4,7-triene

COT undergoes an electrocyclization to bicyclo[4.2.0]octa-2,4,7-triene (BOT) as evidenced by the structure of the Diels –Alder adduct when COT is treated with dieneophiles. That the dieneophile does not induce this reaction was demonstrated by Huisgen who found that the rate of formation of the adduct with high concentrations of very reactive dieneophiles is independent of dieneophile concentration (Scheme 9.7).13

Scheme . 9.7

Huisgen estimated that roughly 0.01% of BOT is present in equilibrium with COT at 1008C. Shortly later, Vogel reported the preparation of BOT and found that it gives COT with log k1 ¼ 11:96 2 18 700=2:3RT:14 Substituted derivatives have also revealed the equilibration between the mono and bicyclic materials15 although 4a,8b-dihydrobiphenylene (7,8-benzoBOT) gives apparently the more stable benzocyclooctatetraene at 08C with DH ‡ ¼ 18:8 kcal=mol and DS‡ ¼ 1:9 e:u:16

C8H6 – C8H14

2.1.3

217

Semibullvalene

SB was first prepared by Zimmerman through a sensitized photolysis of B.17 What is remarkable about SB is its very rapid, degenerate, 3,3-shift at 2 1508C with an activation energy of only 4.8 kcal/mol ðDG‡ ¼ 5:5 kcal=molÞ (Scheme 9.8).18

Scheme . 9.8

At much higher temperatures, SB gives COT in an equilibrium process, DH8 ¼ 22:37 kcal=mol and DS8 ¼ 3 e:u:19 Further, log k ¼ 13:81 2 39 800=2:3RT: This reaction could proceed via a bicyclo[3.3.0]octadienyl biradical or by a concerted process, although a reasonable estimate of the heat of formation of the biradical places it well below the transition state energy for the reaction (Scheme 9.9).

Scheme . 9.9

The biradical would appear to be an intermediate in the formation of SB from a different precursor (see below), so it apparently has a lower activation energy for reversion to SB than cleavage to COT. By contrast, octamethyl-COT gives the SB at 2408C (Scheme 9.9).20 The methyl – methyl-buttressing in the COT probably is responsible for the instability of this compound relative to the SB. Of related interest is the fact that the activation free energy for the 3,3-shift in octamethylsemibullvalene is only 1 kcal/mol higher than that of the parent.21

218


Substituent perturbation of the 3,3-shift equilibrium in SB has been studied,22 but there has been more of a focus on lowering the barrier so that the transition state for the 3,3-shift might be made more stable than the SB itself. Grohmann made the dimethyl tetraester of Scheme 9.10, which is bright yellow and has a ˚ C2 – C8 bond distance in the solid state suggesting that the delocalized 1.78 A transition state was accessible.23 Subsequently Quast provided more examples,24 and calculations agreed that the bishomobenzene structure for the dimethyl-2,6dicyano-4,8-diphenylsemibullvalene (Scheme 9.10), gives rise to the blue color in this case.25

Scheme . 9.10

2.1.4

Tricyclo[3.3.0.02,6]octa-3,7-diene to Semibullvalene

The strained hydrocarbon, tricyclo[3.3.0.02,6]octa-3,7-diene (TOD) gives SB rapidly at 08C.26 By comparison with another reaction, it was determined that log k ¼ 14:0 2 22 800=2:3RT for this isomerization.27 TOD is roughly 9 kcal/mol less stable than SB by MM2-like calculations and 7 kcal/mol by group additivity considerations which places the transition state above the bicyclo[3.3.0] octadienydiyl described previously although a concerted path has been suggested (Scheme 9.11).28

Scheme . 9.11

C8H6 – C8H14

219

It is interesting that if the biradical is involved, it does not cleave to COT but instead preferentially closes to SB. 2.1.5

Cyclobutadiene Dimers to Cyclooctatetraene

The syn- and anti-dimers of cyclobutadiene (CBD) give COT with log k ¼ 14:22 2 30 490=2:3RT and log k ¼ 14:01 2 32 590=2:3RT; respectively.29 Both reactions appear to involve a “forbidden” disrotatory ring opening to BOT which gives COT. The dimers are roughly 47 kcal/mol less stable than COT by MM2 calculations so they are much less stable than BOT. Further, the rapid interconversion of BOT and COT indicates that the rate-determining step in the pyrolysis of CBD is the ring opening (Scheme 9.12).

Scheme . 9.12

Of some interest in this connection is the fact that luminescence was observed in the conversion of the benzo derivative of the cyclobutadiene dimers to benzo-COT when 9,10-dibromoanthracene was present.30 Finally, interesting polycyclic materials with the cyclobutadiene dimer skeleton have been examined.31 2.1.6

Degenerate Rearrangement of Cyclooctatetraene

At elevated temperatures, COTs undergo carbon scrambling as revealed by substituted materials. In the definitive paper on this topic, Paquette studied the interconversions of the dimethyl-COTs in a flow system at 350 – 4508C with 1– 3 s contact time.32 The most relevant conversions are those of the 1,2-material exclusively to the 1,4-isomer, kinetically, and of the 1,5-isomer exclusively to the 1,4-isomer kinetically. The 1,4- and the 1,3-isomers rearrange with little regiospecificity (Scheme 9.13).

220


Scheme . 9.13

The most reasonable mechanism consistent with the facts is electrocyclization to BOT followed by an intramolecular, reversible 4 þ 2 cycloaddition to give tetracyclo[4.2.0.02,805,7]octene. The carbon sequence resulting from this pathway satisfies the observations if the 1,2-isomer preferentially forms the tetracycle with methyls on the two saturated carbons. Much clearer is the result with the 1,5-isomer which can only lead to reformation of the 1,5-isomer or formation of the 1,4-isomer by the pathway described above (Scheme 9.14).

Scheme . 9.14

Other pathways that are ruled out by the latter observation include a 1,3-shift in BOT33 and 2 þ 2 cyclization to TOD34 or to O (Scheme 9.15), but these pathways might be utilized in other instances which are referenced.

C8H6 – C8H14

221

Scheme . 9.15

2.1.7

High-Temperature Chemistry of Cyclooctatetraene

At 400 –6008C in a flow system, COT gives 3,8-dihydropentalene (Scheme 9.16).35 The most reasonable pathway is via the bicyclo[3.3.0]octadiendiyl which undergoes a vicinal hydrogen shift to a dihydropentalene which undergoes 1,5-hydrogen shifts to the observed product. Of interest is the fact that at still higher temperatures, only benzene and acetylene are formed from COT.35,36

Scheme . 9.16

222 2.1.8


Tetracyclo[4.2.0.02,403,5]oct-7-ene

Tetracyclo[4.2.0.02,403,5]oct-7-ene (TOE) was prepared by Meinwald and found to give COT at 400– 5008C in a flow system. The reaction could proceed via BOT or O (Scheme 9.17).37

Scheme . 9.17

2.1.9

Tricyclo[5.1.0,02,8]octadiene to Cyclooctatetraene

Octavalene (tricyclo[5.1.0,02,8]octadiene, O) gives COT in boiling benzene (Scheme 9.18).38

Scheme . 9.18

The low temperature required suggests that this may be a concerted reaction although a symmetry “allowed” pathway generating the all cis-COT cannot be written unless there is an allowed conversion to bicyclo[4.2.0]octa-2,4,7-triene (BOT) initially. 2.1.10

Cubane to Cyclooctatetraene

Cubane39 gives COT at roughly 1 atm pressure in a first-order process with log k ¼ 14:68 2 43 100=2:3RT (Scheme 9.19).40

C8H6 – C8H14

223

Scheme . 9.19

At lower pressures, benzene, acetylene and the dihydropentalenes (see above) are formed probably as a result of highly vibrationally excited COT. COT was envisioned to occur by a retro 2 þ 2 cleavage to the syn-cyclobutadiene dimer followed by ring opening to BOT and then on to COT. 2.1.11 Pentacyclo[3.3.0.0.2,40.3,706,8]octane to Semibullvalene and to COT and Other (CH)8 Isomers Pentacyclo[3.3.0.0.2,40.3,706,8]octane (or cuneane, CU), the silver ion catalyzed rearrangement product of cubane,41 gives SB at roughly 1 atm pressure in a firstorder reaction with log k ¼ 13:82 2 37 700=2:3RT (Scheme 9.20).42 At lower pressures COT is also formed probably from vibrationally hot SB.

Scheme . 9.20

2.1.12

Tetracyclo[3.3.0.0.2,403,6]oct-7-ene to Dihydropentalenes

Tetracyclo[3.3.0.0.2,403,6]oct-7-ene gives various dihydropentalenes upon pyrolysis with an estimated activation energy of 38 kcal/mol apparently by a retro Diels – Alder reaction followed by hydrogen and carbon shifts (Scheme 9.21).43

Scheme . 9.21

224 2.1.13


Barrelene to Benzene and Acetylene

B, the photochemical precursor to SB, undergoes a retro Diels– Alder reaction to benzene and acetylene with log k ¼ 14:27 2 41 700=2:3RT:44 2.1.14

Enthalpy Surface for (CH)8 Interconversions45

Certain features of this energy surface were confirmed, more or less, by very high level calculations (CASPT2N from CASSCF(8/8)/6-31Gp wave functions) which also carefully delineated the reaction pathways for both the thermal and photochemical isomerizations of the (CH)8 hydrocarbons.46 Perhaps the most informative aspect of the calculations is the finding that cis,cis,cis,trans-COT is only 20 kcal/mol less stable than all cis-COT (Scheme 9.22). Further, it cyclizes

Scheme . 9.22

to a bicyclo[5.1.0]octa-2,5-diene-4,8-diyl which is ca. 25 kcal/mol higher in energy whose only fate is formation of SB by a transition state ca. 66 kcal/mol above SB (Scheme 9.23).

C8H6 – C8H14

225

Scheme . 9.23

On the other hand, the calculations indicate that the bicyclo[3.3.0]octa-3,6-dien2,8-diyl is only 9 kcal/mol less stable than SB which is calculated to be 3 kcal/mol more stable than all cis-COT. The former must be at least 10 kcal/mol less and the latter value is reversed compared to reality although actual free energies were not calculated. 2.2

Benzocyclobutene and o-Xylylene

Benzocyclobutenes have been found to ring open to very reactive o-quinodimethanes (o-xylylenes) which can be trapped by various dieneophiles. Huisgen showed that the cis- and trans-3,4-diphenyl-1,2-benzocyclobutenes give the transand cis-3,6-diphenyl-4,4,5,5-tetracyano-1,2-benzocyclohexanes, respectively, when treated with tetracyanoethylene (TCNE), in a first-order reaction independent of the concentration of TCNE (Scheme 9.24).47

Scheme . 9.24

The overall reaction would appear to involve a rate-determining, allowed, conrotatory ring opening followed by an allowed suprafacial – suprafacial 4 þ 2 cycloaddition. Other investigators have made similar observations.48

226


Roth found that benzocyclobutene itself gave the 4 þ 2 adduct of o-xylylene with maleic anhydride which was first order with log k ¼ 14:45 2 39 900=2:3RT at high concentrations of the dieneophile; therefore ko of Scheme 9.25 was being determined.49 At lower concentrations of dieneophile, the dimer, dibenzocyclooctadiene, was formed. In a separate experiment involving flash photolysis of 5,6dimethylenebicyclo[2.2.1]hept-2-ene-7-one, the o-xylylene could be detected and found too close to benzocyclobutene with log kc ¼ 13:34 2 29 300=2:3RT: Subsequent work involving oxygen trapping provided the rate constants: log ko ¼ 13:67 2 38 400=2:3RT and log kc ¼ 12:12 2 27 200=2:3RT: For the equilibrium, DH 8 ¼ 11:1 and DS8 ¼ 7:1 e:u:50

Scheme . 9.25

In a system where disrotatory ring opening of a benzocyclobutene would appear to be required because of fusion to a cyclopentane log k ¼ 13:96 2 45 000=2:3RT for formation of a hydrogen shifted product (Scheme 9.26).51 Thus, the nonconcerted reaction would appear to be substantially less favorable.

Scheme . 9.26

C8H6 – C8H14

227

Calculations at the CASSCF and CAS/MP2 6-311 þ G(d,p) level indicate a 7 – 8 kcal/mol preference for conrotatory over disrotatory ring opening in benzocyclobutene.52 Indeed, with substituted derivatives, the transition state for disrotatory ring opening was found to be near in energy to that for geometric isomerization about the exo-methylene carbons.53 Finally, the high-temperature pyrolysis of benzocyclobutene gives styrene.54 While this could be the result of cleavage of the stronger cyclobutene bond followed by a hydrogen shift, Chapman and Trahanovsky provided evidence based on carbon and deuterium labeling that reversion to a tolyl carbene and its subsequent rearrangement could be responsible for a substantial amount of product.55 2.3

Spiro[3.4]octatriene to 6-Vinylfulvene to Dihydropentalene

Miller showed that spiro[3.4]octatriene gives 6-vinylfulvene rapidly at 2 4.58C ðt1=2 ¼ 90 min; DG‡ ¼ 21 kcal=molÞ (Scheme 9.27).56 The cyclopentadiene ring is dramatically assisting the cyclobutene ring opening.

Scheme . 9.27

In an earlier study, Cavender found that the blood-red fulvene gives, 3,8dihydropentalene, a pale yellow liquid, at 1108C after 8 h at low pressures (Scheme 9.27).57 This reaction would appear to involve an eight-electron cyclization to the dihydropentalene ring system which undergoes 1,5-hydrogen shifts to the observed product. Attempts to determine the stereochemistry of the cyclization were thwarted by the rapid hydrogen shifts. 2.4 1-Methylenespiro[2.4]hepta-2,4-diene to 7-Methylenebicyclo[3.2.0]hepta-1(2),3-diene Miller photolyzed diazocyclopentadiene in allene at 2 558C in an attempt to prepare 1-methylenespiro[2.4]hepta-4,6-diene (MSD) and found that it apparently rearranged to 7-methylenebicyclo[3.2.0]heptadiene-1,3 (MBD) at 08C (Scheme 9.28).58

228


Scheme . 9.28

Both isomers were characterized by their adducts with perfluoro-2-butyne (PFB) and 2,5-diphenylisobenzofuran (DPIBF), respectively. At 08C, MBD dimerizes in a 4 þ 2 cycloaddition to give an adduct, D which reverts to MBD at 808C which then gives a 2 þ 2 dimer, D2, whose structure is as determined by X-ray analysis. The relatively low-temperature conversion of MSD to MBD can reasonably be attributed to the formation of a trimethylenemethane biradical, presumably in its singlet state, which is further stabilized by a cyclopentadienyl moiety (Scheme 9.29). Of interest is the extent to which this species might be stabilized by electron transfer to form a species reminiscent of the oxaallyl zwitterion from cyclopropanone (see Chapter 5, Section 2). Of further interest is the fact that the dimers formed are not those from a trimethylenemethane triplet state.

Scheme . 9.29

2.5

1,2-Diethynylcyclobutane

Upon thermolysis in a flow system, cis-1,2-diethynylcyclobutane gives 7,8dihydropentalene and some bicyclo[4.2.0]octa-1,5,7-triene, but the trans isomer requires 1008C higher temperature to give a 1:1 mixture of the dihydropentalene and vinylacetylene (Scheme 9.30).59

C8H6 – C8H14

229

Scheme . 9.30

The dihydropentalene could arise by a 3,3-shift from the cis isomer to give the meso-cyclic bisallene which cyclizes to a biradical which is also a vinylcarbene that can insert into an adjacent CH bond. However, the trans isomer apparently must isomerize to the cis material, presumably via a biradical, before undergoing the 3,3shift, and the biradical can also undergo cleavage to vinylacetylene. Also possible from the trans isomer is a 3,3-shift to a threo-cyclic bisallene which could undergo a conrotatory cyclization to the 4.2.0 triene that is formed to a greater extent in the pyrolysis of the trans isomer. Pyrolysis of the 1,2-bis(-1-propynyl)cyclobutanes gave the same results with labeling patterns consistent with the mechanistic proposals above. 3 3.1

C8H10 Bicyclo[5.1.0]octa-2,5-diene Degenerate Rearrangement

The self-interconversion of 3,4-homotropilidene (bicyclo[5.1.0]octa-2,5-diene), most reasonably by a 3,3-shift, was found by Doering to have an activation free energy of 14 kcal/mol.60 Subsequently, Gu¨nther found that log k ¼ 11:6 2 12 600=2:3RT for the rearrangement of the 1,2,3,4,4,5,6,7-octadeuterio derivative by line shape analysis.61 Doering originally suggested and later workers confirmed that the ground-state conformation of homotropilidene is the transoid form.62 However, the rearrangement requires the cisoid form; so on the basis of the known activation energy for tropilidene ring flip63 and the revelation from molecular models that the geometry of the cisoid form of homotropilidene is intermediate between that of barbaralane and dihydrobullvalene whose activation energies for the degenerate 3,3-shift are 8.6 and 12.6 kcal/mol, respectively,64 Gu¨nther proposed the enthalpy surface of Scheme 9.31 for the homotropilidene 3,3-shift.

230


Scheme . 9.31

Remarkably, phenyl substitution at C2 and C6 slowed down the reaction by roughly 2.5 kcal/mol,65 which is consistent with a transition state with little radical character at those carbons and one which more resembles allyl radicals (see Chapter 7, Section 4.1) (Scheme 9.32).

Scheme . 9.32

For other substituent effects, see Oth et al.66 Upon heating at 3058C for 24 h homotropilidiene gives bicyclo[3.3.0]octa-2,6diene,60 a reaction which would appear to occur via the bisallyl biradical above (Scheme 9.33).

C8H6 – C8H14

231

Scheme . 9.33

Of tangential interest is the fact that 3,4-epoxytropilidene undergoes the 3,3-shift with an activation free energy 2 –3 kcal/mol67 higher than that of the hydrocarbon, and the dioxa derivative is stable under NMR conditions,68 although the deuterated material does undergo the 3,3-shift with log k ¼ 13:0 2 23 100=2:3RT 69 via the cisoid conformation70 (Scheme 9.34).

Scheme . 9.34

3.2 Bicyclo[4.2.0]octa-2,7-diene, Bicyclo[4.2.0]octa-2,4-diene, Cyclooctatriene, Bicyclo[3.3.0]octa-2,6-diene, Bicyclo[5.1.0] octa-2,4-diene, Bicyclo[4.2.0]octa-2,7-diene, 1,3,5,7-Octatetraene and Tetracyclooctanes Bicyclo[4.2.0]octa-2,7-diene gives an equilibrium mixture of 1,3,5-cyclooctatriene, its 1,3,6-isomer and bicyclo[4.2.0]octa-2,4-diene at 2008C (Scheme 9.35).71 Subsequently the 1,6-dideuteriobicyclo[4.2.0]octa-2,7-diene was partially pyrolyzed and found to give protium at the C6 suggesting that an antarafacial–antarafacial

232


Scheme . 9.35

3,3-shift had occurred at a rate roughly half as fast as the formation of 1,3,5cyclooctatriene (Scheme 9.36).72

Scheme . 9.36

However, since 3,7-dideuteriobicyclo[3.3.0]octa-2,6-diene did not undergo this type of 3,3-shift even at temperatures where fission of the C1 – C5 bond could occur, it was assumed that the reaction in the 4.2.0 case was due to a conrotatory ring opening to a cis,trans,cis-1,3,5-cyclooctatriene which then could cyclize to give the dideuterio isomer corresponding to the 3,3-shift product (Scheme 9.37).

Scheme . 9.37

C8H6 – C8H14

233

A later study revealed that cis,trans,cis-1,3,5-cyclooctatriene generated by a retro Diels –Alder reaction of its furan adduct gave only bicyclo[4.2.0]octa-2,7diene at 1808C which underwent subsequent ring opening to the all cis-cyclic triene (Scheme 9.38).73

Scheme . 9.38

The experimental free energy of activation for the ring opening is 35 kcal/mol which could easily be due to of a symmetry forbidden, non-concerted disrotatory cyclobutene ring opening. 3.2.1

1,3,5-Cyclooctatriene to Bicyclo[4.2.0]octa-2,4-diene

cis, cis, cis-1,3,5-Cyclooctatriene and bicyclo[4.2.0]octa-2,4-diene equilibrate at 1008C with Keq ¼ 0:2 (Scheme 9.39).74 The rate constant for the back reaction ‡ is log k21 ¼ 12:8 2 25 200=2:3RT giving DH21 ¼ 24:7 kcal=mol and DS‡21 ¼ ‡ ‡ 22:0 e:u: as well as DH1 ¼ 25:7 kcal=mol and DS1 ¼ 23 e:u:75

Scheme . 9.39

3.2.2

1,3,6-Cyclooctatriene to 1,3,5-Cyclooctatriene

cis,cis,cis-1,3,6-Cyclooctatriene gives the all cis-1,3,5-isomer (Roth, 1964) with log k ¼ 11:74 2 28 400=2:3RT (Scheme 9.40), with Keq ¼ 43:7 at 1308C.76

Scheme . 9.40

234


The NMR from extensive pyrolysis of 7,8-dideuterio-1,3,5-cyclooctatriene revealed the presence of deuterium on C3, C4, C7, and C8. This is consistent with only 1,5-hydrogen shifts which would interconvert the two all cis-cyclic trienes. Subsequently 5,8-dideuterio-1,3,6-cyclooctatriene was found to isomerize to the 1,3,5-isomer with an intramolecular kinetic isotope effect of 5.0 (Scheme 9.41).77

Scheme . 9.41

Finally, high-temperature pyrolysis of the equilibrium mixture of the two cyclic trienes, and bicyclo[4.2.0]octa-2,7-diene gave benzene (and ethylene), trans,trans1,3,5,7-octatriene, 5-vinyl-1,3-cyclohexadiene, tricyclo[3.2.1.04,6]oct-2-ene, and, at low pressures, styrene (Scheme 9.42).78

Scheme . 9.42

C8H6 – C8H14

3.2.3

235

Bicyclo[5.1.0]octa-2,4-diene

Bicyclo[5.1.0]octa-2,4-diene or 1,2-homotropilidene unlike its 3,4-isomer, is stable to 2258C where it slowly gave (30% in 18 h) a mixture of 1,3,5-cyclooctatriene, tricyclo[3.2.1.04,6]oct-2-ene, and benzene with the two latter materials probably arising from 1,3,5-cyclooctatriene (Scheme 9.43) (Doering, 1963).

Scheme . 9.43

It is likely that the cyclic triene arises by a homo-1,5-hydrogen shift to the 1,3,6isomer followed by the 1,5-hydrogen shifts described in the previous section. 1,2-Homotropilidiene itself undergoes an interesting degenerate rearrangement. At 1508C, 8,8-diprotio-octadeuteriobicyclo[5.1.0]octa-2,4-diene gave the 5,5-diprotio isomer which then gave the 2,5-diprotio derivative by the usual 1,5-hydrogen shift (Scheme 9.44).79

Scheme . 9.44

The rate constant for the carbon skeletal rearrangement (k1) was found to be 3:15 £ 1024 s21 so the free energy of activation was roughly 32 kcal/mol at this temperature. Subsequent work established that the rearrangement occurred via a transoid stereochemistry, a geometric arrangement that permitted an allowed 2ps þ 2ps þ 2ss reaction (Scheme 9.45).80 The alternative pathway via a cisoid transition state was a substantially slower reaction as evidenced by the racemization of tricyclo[5.3.0.02,10]deca-3,5-diene which is characterized by log k ¼ 13:53 2 37 800=2:3RT: Here, the geometry is forced to be less than ideal for the concerted reaction, and the rearrangement may involve a biradical intermediate.

236


Scheme . 9.45

3.2.4

Bicyclo[4.1.1]octa-2,4-diene to Bicyclo[5.1.0]octa-2,4-diene

7,7-Dimethylbicyclo[4.1.1]octa-2,4-diene gave 8,8-dimethylbicyclo[5.1.0]octa-2,4diene upon heating in a flow system with at least 80% inversion of configuration at C7, the migrating carbon in this 1,5-shift of carbon (Scheme 9.46).81

Scheme . 9.46

The 5.1.0 product was not characterized since it gave homo 1,5- and 1,7-hydrogen shifted products under the reaction conditions, and these revealed a product distribution from the syn-7-trideuteriomethyl starting material in which 80% of the hydrogen shifted material was, in fact, the result of a deuterium shift. Subsequent work in a static pyrolysis system revealed that log k1 ¼ 15:1 2 41 900=2:3RT (for loss of the 4.1.1 material), and log k2 ¼ 11:8 2 29 400=2:3RT (for loss of the 5.1.0 isomer) to give a nearly 1:1 ratio of homodienyl 1,5- and 1,7hydrogen shift products, the latter of which undergoes 1,5-hydrogen shifts to a mixture of isopropenyl-1,3-cyclohexadienyl products.82 The dominance of inversion of configuration at the migrating carbon with the syn-trideuteriomethyl group

C8H6 – C8H14

237

was confirmed and the kinetic isotope effect, kH =kD3 was determined to be 1.26 at 2098C which suggests a hyperconjugative isotope effect. Further, there is a kinetic isotope effect on both the homo- 1,5- and 1,7-hydrogen shifts of 4. 3.2.5 cis,cis-1,3,5,7-Octatetraene to 1,3,5-Cyclooctatriene and Bicyclo[4.2.0]octa-2,4-dienes cis,cis-1,3,5,7-Octatetraene gives 1,3,5-cyclooctatriene at room temperature (Scheme 9.47).83

Scheme . 9.47

Subsequently partial hydrogenation of trans,trans- and of cis,cis-2,8-decadiene4,6-diyne over a Lindlar catalyst resulted in trans- and cis-7,8-dimethyl bicyclo[4.2.0]octa-2,4-diene, respectively (Scheme 9.48).84

Scheme . 9.48

The electrocyclization to the cyclooctatriene apparently occurs in a conrotatory manner and is followed by electrocyclization to the bicyclo[4.2.0]octa-2,4dienes. Concurrent with this work was reported the isolation by recrystallization of the air-sensitive trans,cis,cis,trans-, cis,cis,cis,cis- and trans,cis,cis,cis-2,4,6,8-decatetraenes and their conversion to the cyclic trienes and to the same bicyclo octadienes along with quantitative data on all conversions (Scheme 9.49).85

238


Scheme . 9.49

Subsequently, the conversion of the trans,cis,cis,trans isomer at higher temperatures to the products derived from the trans,cis,cis,cis isomer was observed indicating that the disrotatory reaction was 11.1 kcal/mol higher in energy than the conrotatory, concerted electrocyclization. 3.2.6 Tetracyclo[5.1.0.02,40.3,5]octane and Tetracyclo[4.2.0.0.2,40.3,5]octane Pyrolysis Upon pyrolysis at high temperatures, tetracyclo[5.1.0.02,40.3,5]octane86 and tetracyclo[4.2.0.0.2,40.3,5]octane87 were found to give isomers characteristic of the hightemperature pyrolysis of the cyclooctatriene described above (Scheme 9.50).

Scheme . 9.50

3.2.7

C8H10 Free Energy Surface

An approximate free energy surface at 1508C can be constructed from the data reported using 1,3,5-cyclooctatriene as the reference point (Scheme 9.51).

C8H6 – C8H14

239

Scheme . 9.51

3.3

6-Vinylbicyclo[3.1.0]hex-2-ene to Bicyclo[3.2.1]octa-2,6-diene

endo-6-Vinylbicyclo[3.1.0]hex-2-ene was found to rearrange near room temperature to bicyclo[3.2.1]octa-2,4-diene with log k ¼ 11:67 2 22 900=2:3RT (Scheme 9.52).88

Scheme . 9.52

The ease of this rearrangement is consistent with a concerted 3,3-shift, necessarily out of a boat conformation not unlike that of cis-1,2-divinylcyclopropane (see Chapter 8, Section 3). Not unexpectedly, the exo-isomer required heating to 2008C to give the same 3.2.1 diene product, presumably by first undergoing geometric isomerization to the endo-isomer.89 Subsequently it was shown, using optically active materials, that the geometric isomerization involved epimerization at C6 and not a bridgehead double inversion most likely because cleavage of a cyclopropane bond to generate an allyl radical is more favorable than the alternative (Scheme 9.53).90

240


Scheme . 9.53

Bicyclo[3.2.1]octa-2,6-diene itself was converted to bicyclo[3.3.0]octa-2,6-diene and 1-vinylcyclohexa-1,3-diene in a flow system above 5008C (1 s residence time) (Scheme 9.54).91 Further, it was found that pyrolysis of exo-4-deuteriobicyclo[3.2.1]octadiene gave a 1.85:1 mixture of endo- and exo-4-deuteriobicyclo[3.3.0]octadiene, respectively.

Scheme . 9.54

The origin of the 3.3.0 diene would appear to be a 1,3-shift in the bisallylic biradical which forms the 3.2.1 diene (Scheme 9.55). However, this biradical species must be formed with rotation around the CHD group to give the stereochemical scrambling observed.

Scheme . 9.55

The origin of the vinylcyclohexadiene product can be easily rationalized by a retro homo-1,5-hydrogen shift followed by a retro 4 þ 2 cycloaddition to the vinylcyclohexadiene skeleton, then 1,5-hydrogen shifts will give the thermodynamic product (Scheme 9.56).92

C8H6 – C8H14

241

Scheme . 9.56

3.4

Tricyclo[3.3.0.02,8]oct-3-ene to Bicyclo[3.3.0]octa-2,7-diene

Tricyclo[3.3.0.03,8]oct-3-ene gave bicyclo[3.3.0]octa-2,7-diene upon heating at 2008C (Scheme 9.57).71

Scheme . 9.57

This isomerization is most reasonably a homodienyl-1,5-hydrogen shift (see Chapter 7, Section 4), and the subsequent work by Srinivasan with methyl-labeled materials derived from photoaddition of cis- and trans-2-butene to benzene across the meta positions confirmed that it was the endo hydrogen at C7 which was transferred.93 3.5 6-Methylenebicyclo[3.2.0]hept-2-ene to 5-Methylenebicyclo[2.2.1]hept-2-ene 6-Methylenebicyclo[3.2.0]hept-2-ene rearranged to 5-methylenebicyclo[2.2.1]hept2-ene at 2508C with log k ¼ 13:7 2 39 600=2:3RT:94 The 6-dideuteriomethylene derivative was shown to rearrange to its 7,7-dideuterio isomer with log k ¼ 13:4 2 40 900=2:3RT (a rate factor 8– 10 slower than formation of the 2.2.1 isomers), presumably via a 1,3-shift of carbon. However, the two 2.2.1-dideuterio isomers were formed in a 60:40 and in a 56:44 ratio from the two 3.2.0-dideuterio isomers with the 1,3-shift product dominating over the 3,3-shift product in each case (Scheme 9.58).95 Finally, the two 2.2.1 isomers interconverted very slowly with log k ¼ 13:5 2 45 900=2:3RT:

242


Scheme . 9.58

The similarity in the product ratios from the two 3.2.0 dideuterio isomers would appear to minimize the role of isotope effects here. It is possible that the 1,3- and 3,3-shift pathways are separate, but ring opening of cyclobutanes by the coupled motions depicted in Scheme 9.59 would give conformationally distinct, incompletely equilibrated, bisallyl biradicals which could rationalize the preferences.

Scheme . 9.59

To characterize the stereochemistry of the conversions, the exo-7-methyl-6methylenebicyclo[3.2.0]hept-2-ene derivative was pyrolyzed and found to give 88% inversion at C7 in the 1,3-shift product (mostly) endo-6-methyl-5-methylenebicyclo[2.2.1]hept-2-ene.96 However, the endo-7-methyl 3.2.0 isomer also gave 60% retention in its 1,3-shift to give the endo-6-methyl 2.2.1 product. It would appear that the bisallyl biradical responsible for the major product is formed preferentially from

C8H6 – C8H14

243

both starting stereoisomers or that the two bisallyl biradicals can equilibrate under the reaction conditions possibly, though unlikely, by rotation around the methallyl radical moiety (Scheme 9.60).

Scheme . 9.60

Remarkably, when the syn- and anti-7-ethylidenebicyclo[3.2.0]hept-2-enes were heated, they both gave , 25% more endo-6-methyl 3,3-shift product. So it would appear that there must be a pathway for methallyl radical geometric equilibration. 3.6 Tricyclo[3.2.1.02,4]oct-6-ene to Tetracyclo[3.2.1.0.2,704,6]octane and Bicyclo[3.2.1]octa-2,6-diene endo-Tricyclo[3.2.1.02,4]oct-6-ene undergoes an intramolecular 2 þ 2 cyclization at 2878C with k ¼ 1:5 £ 1024 s21 giving tetracyclo[3.2.1.0.2,704,6]octane. The exoisomer gives mostly bicyclo[3.2.1]octa-2,6-diene at the same temperature with lesser quantitites of the tetracyclic material being formed (Scheme 9.61).97

Scheme . 9.61

Further, 3-deuterio-exo-tricyclo[3.2.1.02,4]oct-6-ene (as a 2:1 syn:anti mixture) gave exclusively bicyclo[3.2.1]octa-2,6-diene-4-d in a 2:1 ratio of endo to exo isomers. This observation rules out simple vicinal hydrogen shifts in a trimethylene

244


biradical or non-concerted formation of endo-6-vinylbicyclo[3.1.0]hex-2-ene which could rearrange to the product. Instead, concerted formation of endo-6-vinylbicyclo[3.1.0]hex-2-ene was proposed, and this material can undergo a 3,3-shift to the observed deuterium isomers (Scheme 9.62).

Scheme . 9.62

The endo-tricyclic isomer cannot undergo this concerted ring opening, so apparently it undergoes C2 – C4 bond rupture and intramolecular 2 þ 2 cyclization to relieve more torsional strain according to the authors. Substituted materials have been examined. Various 2,4-diphenyl-endo-tricyclo[3.2.1.02,4]oct-6-enes give intramolecular 2 þ 2 cycloaddition products just like the parent (Scheme 9.63).98

Scheme . 9.63

However, endo-3,3-difluorotricyclo[3.2.1.02,4]oct-6-ene undergoes only isomerization to its exo-isomer at 808C. Then at much higher temperatures, the 2 þ 2 adduct was formed (Scheme 9.64).99

Scheme . 9.64

C8H6 – C8H14

245

In a very early work, it was shown that substituted exo-tricyclo[3.2.1.02,4]oct-6ene derivatives give bicyclo[3.2.1]octa-2,5-dienes (Scheme 9.65).100 Biradical pathways were proposed.

Scheme . 9.65

3.7

Cyclopropylidenespiropentane Rearrangements

Cyclopropylidenespiropentane rearranges to 1-methylenedispiro[2.0.2.1]heptane and 7-methylenedispiro[2.0.2.1]heptane in a 87:13 ratio in 1 h at 2108C (Scheme 9.66).101

Scheme . 9.66

It appears as if two different trimethylenemethane biradical species give rise to the two products albeit in different amounts presumably because the less stable cyclopropyl radical is required in the intermediate giving rise to the minor product.

246


3.8 cis- and trans-Tricyclo [5.1.0.02,4]oct-5-ene to 1,3,6-Cyclooctatriene cis-Tricyclo[5.1.0.02,4]oct-5-ene (or cis-s-bishomobenzene) is an unknown compound which might be expected to give 1,3,6-cyclooctatriene at room temperature by analogy with cis-monohomobenzene oxide,102 cis-benzene bisoxide,103 and cisbenzenediimine104 (Scheme 9.67).

Scheme . 9.67

Interestingly, the corresponding trans isomers are stable at elevated temperatures.105 The rearrangement of the cis isomers is probably a concerted 2ss þ 2ps þ 2ss reaction since the activation energy is so low relative to the estimated BDE of the C1 – C7 bond (ca. 43 kcal/mol). The cis arrangement easily allows the pericyclic interactions, but apparently the trans arrangement does not (Scheme 9.68).

Scheme . 9.68

3.9

Dispiro[2.0.2.2]oct-7-ene Pyrolysis

In an interesting case of an unobserved reaction, pyrolysis of dispiro[2.0.2.2]oct7-ene up to 3008C resulted in no reaction.106 The cyclobutene is probably more stable than the ring-opened diene due to the , 25 kcal/mol extra ring strain in the methylenecyclopropanes of the product relative to normal double bonds and cyclopropane rings (Scheme 9.69).

C8H6 – C8H14

247

Scheme . 9.69

3.10 1,2,6,7-Octatetraene, 3,4-Dimethylene-1,5-hexadiene, Bicyclo[4.2.0]hexa-1,5-diene, 1,2-Divinylcyclobutene, 2,3-Dimethylenebicyclo[2.2.0]hexane, 1,4-Ethano-2-methylenespiropentane, 1-Allyl-2-vinylcyclopropene 3.10.1

1,2,6,7-Octatetraene Pyrolysis

Upon heating 1,2,6,7-octatetraene rearranged to 3,4-dimethylene-1,5-hexadiene at low pressures.107 However, in solution, the bisallene gave a 6:4 mixture of bicyclo[4.2.0]oct-1,5-diene and the dimethylenehexadiene with log k ¼ 9:952 24 800=2:3RT:108 Of importance is the fact that at 1 Torr, a: 25:60 mixture of the two isomers were formed, respectively, along with 15% of 1,2-divinylcyclobutene, a product not found at low or high pressures, but one which reverts to the other two at higher temperatures (Scheme 9.70).

Scheme . 9.70

It would appear that the 2,3-dimethylenecyclohexane-1,4-diyl biradical is involved in this reaction, and it was generated independently by flash photolysis of the appropriate azo compound and ketone and found to give 3,4-dimethylene-1,5hexadiene and bicyclo[4.2.0]octa-1,5-diene in a 1:2: ratio in solution with log k ¼ 16:5 2 24 500=2:3RT (Scheme 9.71).109

248


Scheme . 9.71

Further, various dimers were formed with kðms21 Þ ¼ 4:8 £ 1012 exp 2 ð2200= RTÞ: The very high activation energy for isomerization of the biradical and the high pre-exponential term for its dimerization are surprising. Subsequently, the kinetics of all reactions were determined as well as the heats of formation of the hydrocarbon isomers.110 Thus overall loss of the bisallene had log k ¼ 9:79 2 24 600=2:3RT, and the tetraene gave the bicyclic diene with log k ¼ 10:38 2 33 700=2:3RT; and the back reaction had log k ¼ 14:38 2 41 900=2:3RT (with Keq ¼ 1=4 at , 2508C), and the divinylcyclobutene gave the tetraene and the bicyclic diene with log k ¼ 13:68 2 35 700=2:3RT and log k ¼ 11:48 2 29 700=2:3RT; respectively. An enthapy surface was given; however, it is deceptive particularly because the pre-exponential term for isomerization of the diyl must be too high; so an approximate free energy surface at 2008C assuming 5 e.u. per CC single bond rotation is given in Scheme 9.72.

Scheme . 9.72

C8H6 – C8H14

249

The stereochemistry of the intramolecular cycloaddition was examined, and both meso- and chiral-2,3,7,8-decatetraenes gave similar product distributions suggesting the same 2,3-dimethylidenecyclohexane-1,4-diyl species (with two additional methyl groups) are involved in each case.111 3.10.2

2,3-Dimethylenebicyclo[2.2.0]hexane

2,3-Dimethylenebicyclo[2.2.0]hexane gave a 1:2 mixture 3,4-dimethylene-1,5hexadiene and bicyclo[4.2.0]octa-1,5-diene at 1008C in the gas phase. The reaction appears to proceed via the 2,3-dimethylenecyclohexane-1,4-diyl (Scheme 9.72).112 In solution, dimers were formed that gave rise to CIDNP emission signals when formed in an NMR probe indicating triplet precursors. 3.10.3

1,4-Ethano-2-methylenespiropentane

Pyrolysis of 1,4-ethano-2-methylenespiropentane at 1208C results in a methylenecyclopropane rearrangement to an isomer which apparently reversibly undergoes a retro 4 þ 2 cycloaddition at 1508C.113 This set of isomers gives 3,4-dimethylene1,5-hexadiene and bicyclo[4.2.0]octa-1,5-diene at 1808C (Scheme 9.73).

Scheme . 9.73

3.11

Tricyclo[4.2.0.01,3]oct-4-ene

Tricyclo[4.2.0.01,3]oct-4-ene undergoes a unimolecular isomerization to tricyclo[4.2.0.01,5]oct-3-ene at temperatures above 1808C (Scheme 9.74). 114

250


The equilibrium constant for this vinylcyclopropane to vinylcyclopropane rearrangement was determined to be approximately 15.

Scheme . 9.74

A biradical intermediate was proposed for this interesting isomerization. 3.12

Tricyclo[4.2.0.01,3]octa-4-ene to Tricyclo[4.2.0.01,5]octa-3-ene

Tricyclo[4.2.0.01,3]octa-4-ene undergoes a reversible 1,3-shift to tricyclo [4.2.0.01,5]octa-3-ene ðKeq < 15Þ with an activation free energy of 38 kcal/mol at 2008C (Scheme 9.75).115

Scheme . 9.75

It was suggested that biradicals were involved. 3.13 Bis-2,20 -methylenecyclopropanyl to 3,6-Dimethylenecyclohexene and 3-Methylenespiro[2.4]octa-1-ene Upon heating meso- and threo-bis-2,20 methylenecyclopropanyl interconvert with Keq ¼ 0:85 and log kf ¼ 14:1 2 36 700=2:3RT and log kb ¼ 15:1 2 38 500= 2:3RT:116 The mixture slowly gives 3,6-dimethylenecyclohexene as the major product as well as smaller amounts of 3-methylenespiro[2.4]octa-1-ene and subsequent rearrangement products and dimers (Scheme 9.76). Trimethylenemethane and vinylogous bis-allyl biradicals may be involved in the conversions, although the latter was ruled out as the exclusive intermediate in a higher, cyclic, homologue (see Chapter 10, Section 2.13).

C8H6 – C8H14

251

Scheme . 9.76

4

C8H12

4.1 1,2-Divinylcyclobutane, 4-Vinylcyclohexene, 1,5-Cyclooctadiene, Butadiene, and Tricyclo[4.2.0.02,5]octane 4.1.1

cis-1,2-Divinylcyclobutane

cis-1,2-Divinylcyclobutane rearranges to cis,cis-1,5-cyclooctadiene117 with DH ‡ ¼ 23:1 kcal=mol and DS‡ ¼ 211:7 e:u: in the liquid phase.118 Roughly 3% of cis, trans-1,5-cyclooctadiene was also formed2 with DH ‡ ¼ 25:8 kcal=mol and DS‡ ¼ 210:7 e:u:119 Since the BDE of the C1 –C2 bond is roughly 32 kcal/mol by virtue of cyclobutane ring strain and formation of two allyl radicals, it is likely that the rearrangement is a concerted 3,3-shift. However, it must proceed via a boatlike transition state with the vinyl groups over the ring (Scheme 9.77), in order that cis double bonds be generated in the major product.

Scheme . 9.77

The origin of the minor product in pyrolysis of the parent material may be a distorted chair-like transition state.

252


The secondary deuterium kinetic isotope effect, kH =kD4 ; at the terminal carbons of the parent compound in the 3,3-shift is inverse, but small, namely, 1/1.04 in hexachlorobutadiene solvent at 788C.120 This suggests less bond making between the terminal carbons in the transition state than in the acyclic chair-like transition state (see Chapter 7, Section 4.1). Complete bond making should have led to a KIE of 1/1.57, but complete bond homolysis to allyl radicals would give a normal KIE of 1.25 at 788C. Further evidence for the boat-like transition state for formation of the major product was provided by Berson et al. These investigators found a significant retardation (by a factor of 200) in the rate of the rearrangement of cis-1,2-bis(cispropenyl) cyclobutane (CCC), compared with that of the bis(trans-propenyl) isomer (CTT) (Scheme 9.78).121

Scheme . 9.78

Remarkably, CCC gave almost exclusively trans-3,4-cis,cis-1,5-cyclooctadiene instead of the expected cis-3,4-dimethyl isomer if a boat-like 3,3-shift were to occur. The origin of this material is unclear. One possibility considered was formation of trans-3,4-dimethyl-cis,trans-1,5-cyclooctadiene via a chair-like transition state as described in Scheme 9.77 which then underwent trans to cis isomerization (Scheme 9.79).

Scheme . 9.79

C8H6 – C8H14

253

However, Berson found that one of these diastereomers reverted to cis-3,4dimethyl-cis,cis-1,5-cyclooctadiene presumably via CTT under the reaction conditions so double bond isomerization does not occur with this diastereomer.121 Perhaps the diastereomer formed in the pyrolysis of CTT is the other diastereomer which does undergo the double bond isomerization without reverting to the divinylcyclobutane but this is unlikely on the basis of thermochemical considerations, vide infra. Finally, as a result of pressure studies, it was determined that the activation volume for the rearrangement of cis-1,2-divinylcyclobutane itself was 2 13.4 cc/ mol which is consistent with a concerted 3,3-shift.122 4.1.2

trans-1,2-Divinylcyclobutane

trans-1,2-Divinylcyclobutane rearranges to a mixture of 4-vinylcyclohexene, cis,cis-1,5-cyclooctadiene, and butadiene in a 70:25:5 mol ratio at 1758C with DH ‡ ¼ 34:3 kcal=mol and DS‡ ¼ 21:2 e:u: in the liquid phase.2 Further, more butadiene is formed at the expense of mostly the 1,3-shift product at higher temperatures, and optically active starting material also racemizes with DH ‡ ¼ 36:8 kcal=mol and DS‡ ¼ 4:6 e:u: The relatively high activation enthalpy and the temperature dependence of the product distribution suggest reversible formation of a biradical which closes to the 1,3-shift product, to cis-1,2-divinylcyclobutane which undergoes a 3,3-shift, and to butadiene via cleavage, in addition to reclosing to racemized starting material (Scheme 9.80).

Scheme . 9.80

Hammond and DeBoer also found that partially optically active 1,3-shift product is formed from optically active starting material2 suggesting that the biradical(s) is(are) not thermally equilibrated under the reaction conditions. Further, Berson and

254


Dervan found that the 1,3-shift product is formed with 8% net inversion of configuration at the migrating carbon.123 Optically active trans-1,2-bis(trans-propenyl)cyclobutane (TTT) and trans-1(cis-propenyl)-2-(trans-propenyl)cyclobutane (TCT) were pyrolyzed to determine the extent of involvement of all possible 1,3-shift stereopathways. The results were similar in each case leading to an average of 50.0:45.5:40.5% utilization of the si, sr, ar, and ai pathways, respectively (Scheme 9.81). Remarkably, only trans-propenyl groups can function as the three carbon unit of the 1,3-shift, and the cis-propenyl group apparently only functions as the migrating group.

Scheme . 9.81

These observations not only rule out thermally equilibrated biradicals, but suggest little control of the stereochemistry by Orbital Symmetry Considerations. It appears as if the biradicals generated reclose rapidly to the six-membered ring. However, the biradicals must have at least one allylic unit be generated in a cisoid configuration to give the 1,3-shift or else they must reclose to cyclobutanes or undergo cleavage. It is therefore noteworthy that the 1,3-shift pathway accounts for 70% of the product in the parent case. In the case of trans-1,2-divinylcyclobutane pressure studies reveal an activation volume of þ 4.2 cc/mol which is consistent with formation of a biradical in this case.122 4.1.3

4-Vinylcyclohexene

4-Vinylcyclohexene undergoes a retro Diels– Alder reaction to butadiene with log k ¼ 15:2 2 62 000=2:3RT 124 or 15:7 2 61 800=2:3RT:125 At lower temperatures optically active 4-(exo-dideuteriovinyl)cyclohexene undergoes both racemization and deuterium scrambling to C-3.126 These processes occur with

C8H6 – C8H14

255

log krac ¼ 12:09 2 49 650=2:3RT and log kexc ¼ 13:09 2 52 140=2:3RT: The deuterium shift to C-3, which is roughly 40% faster than racemization, could result from a boat-like 3,3-shift, but not via a chair-like transition state because of poor orbital overlap. The 3,3-shift, however, is not a pathway for racemization. Trisection of the experimental data by determination of the label distribution in the enantiomers from pyrolysis of 4S-4-(exo[14C]vinyl)cyclohexene, SVp, at 3808C revealed that the formal 3,3-shift pathway accounted for 74% of the product, inversion without rearrangement accounted for 23% of the product with the rest (ca. 3%) resulting from inversion with rearrangement (Scheme 9.82).127

Scheme . 9.82

The formation of a retained, label-scrambled material, SAp, is suggestive of a significant contribution from a concerted 3,3-shift, but the formation of some inverted scrambled product, RA p, requires the intermediacy of a cis,cis-bisallylic biradical for its formation. This latter intermediate then is probably responsible for ca. 3% of the 3,3-shift product as well as 3% of inverted, unscrambled material, RV p. However, most of the inverted, unscrambled material probably arises from a cis,trans-bisallylic biradical which cannot give any other 4-vinylcyclohexene except starting material. Thus, biradical reactions appear to compete well with the 3,3-shift in this case, most likely because the 3,3-shift transition state must not only be boat-like, but suffer from steric interactions not unlike those in bicyclo[2.2.2]octane. 4.1.4

cis,cis-1,5-Cyclooctadiene

Doering found that cis,cis-1,5-cyclooctadiene (COD) gave 4-vinylcyclohexene (VCH) and butadiene upon pyrolysis with log k ¼ 15:55 2 52 400=2:3RT and log k ¼ 16:3 2 55 930=2:3RT; respectively.9,128 The reaction may involve generation of the cis,cis-bisallylic biradical which either closes to VCH or cleaves to butadiene.

256


Alternatively, COD may revert to cis-1,2-divinylcyclobutane which, at high temperatures may undergo non-concerted ring opening to the same biradicals generated from trans-1,2-divinylcyclobutane, a likely scenario since the ratio of VCH and butadiene is roughly comparable to that from the latter material (Scheme 9.83).

Scheme . 9.83

4.1.5

cis – trans-1,5-Cyclooctadiene

Leitich found that cis,trans-1,5-cyclooctadiene gives its cis,cis isomer with DH ‡ ¼ 31:8 kcal=mol and DS‡ ¼ 22:2 e:u:3 Not only does cis-1,2-divinylcyclobutane appear to be an intermediate in this reaction, but it is formed in the rate-determining step on the basis of thermochemical considerations (Scheme 9.84) and part 7 of this section.

Scheme . 9.84

The transition-state structure must be the distorted chair-like species which most likely generated the cyclic diene from cis-1,2-divinylcyclobutane. This would, therefore, preserve the stereochemistry of groups attached to the terminal vinyl groups. Thus, this reaction could not be responsible for Berson’s observation with CCC (see part 1 of this section). 4.1.6

Butadiene Dimerization

The 4 þ 2 dimerization of butadiene needs to be considered in relation to the C8H12 isomer interconversion. Upon heating, butadiene give mostly 4-vinylcyclohexene

C8H6 – C8H14

257

with log k ¼ 6:95 2 23 700=2:3RT ðmol-sÞ21 or log k ¼ 8:14 2 26 800=2:3RT ðmol-sÞ21 :129 This Diels –Alder 4 þ 2 cycloaddition was also accompanied by formation of 5% trans-1,2-divinylcyclobutane and 2% cis,cis-1,5-cyclooctadiene (Scheme 9.85).130

Scheme . 9.85

Stephenson also studied the stereochemistry of the dimerization of cis,cis-1,4dideuterobutadiene at 1368C and found that 90% of the 4-vinylcyclohexene was formed by a 4s þ 2s cycloaddition with no preference for endo or exo orientation.13 However, 10% of the 4 þ 2 adduct had the trans stereochemistry at C4 and C5 (Scheme 9.86).

Scheme . 9.86

The origin of the label-inverted product is obscure. If trans,trans biradicals were being generated and revert to butadiene, the recovered butadiene should have been scrambled, but that is not the case. Stephenson proposed that 10% of the cycloaddition could be the result of either a 4a þ 2a allowed process or a 4s þ 2a forbidden reaction. In a similar study, Berson observed that at 1958C, trans,trans-pentadiene-1-d gave 82% of a mixture of endo- and exo-3-methyl-4-trans-propenyl adducts with complete stereochemical retention.131 Also formed was 11% of the 5-methyl-4trans-propenyl adduct of unknown stereochemistry, 6% of cis-3,4-dimethyl-1,5cyclooctadiene, and 1% of the corresponding trans-dimethyl isomer (Scheme 9.87).

258


Scheme . 9.87

4.1.7

Enthalpy Surface for the Interconversions

From the experimental and estimated heat of formation of the C8H12 isomers and the activation enthapies so far discussed, an enthalpy surface for the interconversion can be constructed to reveal the relationships between the isomers (Scheme 9.88).9 It may be that the heats of formation of the stable compounds are uncertain by as much as 2 kcal/mol. However, while there might be ambiguity in the heat of formation of the biradicals, they must be close to that of the transition states forming them since these are not thermally equilibrated.

Scheme . 9.88

C8H6 – C8H14

259

An interesting feature of this energy surface is the fact that the transition state for the butadiene dimerization lies close to that of the biradicals. Yet, the Diels– Alder reaction occurs with some stereospecificity. Judging by Stephenson’s results, the reaction is concerted to the extent of 2– 3 kcal/mol. It must be that there is some energetic benefit to bring two butadienes together with one in a cisoid arrangement so as to make the transition state for concerted 4 þ 2 cycloaddition a more favorable process than that for formation of biradicals. Nonetheless, it should be recognized that even the concerted Diels– Alder transiton state must be highly unsymmetrical in this case. The added conjugation on the dieneophilic moiety would cause the transition state to more resemble the biradical than the transition state for the 4 þ 2 cycloaddition of butadiene and ethylene (see Chapter 7, Section 4). Finally, it should be clear that control over the interconversions must include entropy differences. If reasonable estimates of these at the temperature of 2008C are included, the free energy surface would more resemble that of Scheme 9.89. The effect of entropy is to make it energetically more likely to involve the bisallylic biradicals in the pyrolyses, particularly in the case of Berson’s result with CCC.

Scheme . 9.89

4.1.8

1,2-Divinylcyclobutanes Incapable of Undergoing the 3,3-Shift

In an effort to further characterize the energy surface for the divinylcyclobutane thermal reactions, Doering et al. examined the geometric isomerizations and retro

260


2 þ 2 reactions of the dispiro[5.0.5.2]tetradeca-1,8-dienes from 72 to 1188C in mesitylene solvent.132 These materials cannot undergo the 3,3-shift without giving two trans double bonds in the product. For the anti-stereoisomer, conversion to the syn-isomer and cleavage have activation enthalpies of 30.3 and 32.8 kcal/mol, respectively, and have activation entropies of 0.2 and 8.0 e.u., respectively, and have activation volumes of 14.6 and 12.4 cc/mol, respectively. The activation volume for the corresponding reactions of the syn isomer are 15.0 and 12.8 cc/mol, respectively. These values are simple averages of different ways of determining activation volumes. The activation enthalpies and entropies for reaction of the syn-isomer have very high error limits, so they were not the focus of any discussion. A reasonable working hypothesis is that a biradical is generated reversibly, and the retro 2 þ 2 reaction is nearly equally competitive with the reclosure reaction at the temperatures of the reaction since the entropically favored cleavage reaction has the higher activation energy (Scheme 9.90).

Scheme . 9.90

The activation energies are at least equal if not higher than the BDE of the crucial bond consistent with formation of a biradical. Unfortunately, the difference in the activation volumes is not necessarily consistent with this scheme unless the cleavage reaction has a slightly smaller activation volume. This hypothesis is also consistent with unpublished work cited in the paper. Addition of oxygen inhibited both geometric isomerization and cleavage (Scheme 9.91).133

C8H6 – C8H14

261

Scheme . 9.91

Further, because of the oxygen trapping of the biradical, it was determined that there was a 2.7 kcal/mol activation energy for reclosure of the biradical to either the anti- and syn-isomers. Further, two separate determinations of the activation energies for geometric isomerization of the anti- and syn- isomers led to a 1.2 and a 2 0.1 kcal/mol difference in the two. Finally, the Arrhenius parameters for the cleavage of anti and syn were found to be log k ¼ 12:89½14:41 2 32 400 ½32 400=2:3RT and log k ¼ 12:89½14:62 2 31 100½32 500=2:3RT: The values in brackets were the result of a separate determination. The activation enthalpies for cleavage are higher than those for geometric isomerization, but the activation entropies for cleavage are lower (or higher) than those for geometric isomerization. In either event, the data seems inconsistent with the only conclusion in the manuscript stated in the abstract: “the ratio between two channels of exit from a ‘generalized common biradical’ is not controlled by enthalpy and entropy…, but by entropy alone.”

4.1.9

Tricyclo[4.2.0.02,5]octane and 1,5-Cyclooctadiene

The syn- and anti-isomers of tricyclo[4.2.0.02,5]octane, S and A, respectively, gave cis,cis-cyclooctadiene (COD), upon pyrolysis at 2008C.134 The Arrhenius parameters were determined to be log kðSÞ ¼ 13:60 2 31 77=2:3RT and log kðAÞ ¼ 14:66 2 30 500=2:3RT:135 However, both the cis,cis- and cis,trans isomers of COD are formed in an identical 5:3 ratio from both S and A as determined by Diels –Alder trapping experiments.136 Under the reaction conditions cis,trans COD reverts to the cis,cis isomer in the absence of trapping agent. A reasonable working hypothesis for the interconversions is depicted in Scheme 9.92.

262


Scheme . 9.92

By analogy to the cleavage reaction of bicyclo[2.2.0]hexane, formation of a boat-like biradical followed by rapid conformational interconversion with a chairlike biradical and then cleavage can provide both CODs. The heat of formation of S and A were reported to be 56 and 53 kcal/mol, respectively,137 and if the biradicals are comparable in energy to the transition state producing them, they are roughly 12 kcal/mol less stable, enthalpically, than the bisallylic biradicals which figure so prominently in the other thermal interconversions. Considerations of entropy would make them at least another 5 kcal/mol less stable. Of interest in this connection is the fact that anti-1,2,5,6-tetracyanotricyclo[4.2.0.02,5]octane gives exclusively the tetracyano-cis,trans-COD in the solid state with log k ¼ 15:6 2 34 900=2:3RT:138 This reaction is about 4000 times slower than that of ring opening of 1,4-dicyanobicyclo[2.2.0]hexane suggesting that crystal packing force retards the further conformational flip of a chair-like biradical to cleave to the COD (Scheme 9.93).139

Scheme . 9.93

4.1.10

Tricyclo[3.3.0.02,6]octane to 1,5-Cyclooctadiene

Tricyclo[3.3.0.02,6]octane gave 1,5-cyclooctadiene upon pyrolysis with log k ¼ 15:51 2 55 900=2:3RT (Scheme 9.94).140

C8H6 – C8H14

263

Scheme . 9.94

Also formed subsequent to the cyclic diene are 4-vinylcyclohexene and butadiene. A bicyclo[3.3.0]octan-2,6-diyl species would appear to be involved in the rate-determining step for the initial isomerization. 4.2

Bicyclo[4.2.0]oct-7-ene and 1,3-Cyclooctadiene

Bicyclo[4.2.0]oct-7-ene (BOE) isomerizes to cis,cis-1,3-cyclooctadiene (CC),141 with log k ¼ 14:13 2 43 180=2:3RT:142 The pathway for this conversion could be an Orbital Symmetry forbidden disrotatory ring opening, perhaps via a biradical (see Chapter 5, Section 2.2) or, as was suggested by Bloomfield, an allowed conrotatory ring opening to cis,trans-1,3-cyclooctadiene (CT), followed by a 1,5-hydrogen shift (Scheme 9.95).143

Scheme . 9.95

Fonken showed that CT rearranges quantitatively to BOE at 808C,144 and Bloomfield found that log k ¼ 13:14 2 27 870=2:3RT for this transformation.3 Also observed was a mixture of Diels – Alder adducts with dimethylacetylene dicarboxylate identical to that derived from CT when BOE was heated at 1108C. Further, Bloomfield observed the formation of CT from BOE and found DH ¼ 5:60 kcal=mol and DS ¼ 20:7 e:u: for the BOE, CT equilibrium. Because of this pre-equilibrium, the activation parameters for the 1,5-hydrogen shift in CT must be roughly DH ‡ ¼ 37 kcal=mol and DS‡ ¼ 2 e:u: In contrast, the 1,5-hydrogen shift in CC has activation parameters of DH ‡ ¼ 29:3 kcal=mol and DS‡ ¼ 210 e:u: as determined by Winstein (Scheme 9.96).145

264


Scheme . 9.96

Thus either, the 1,5-hydrogen shift in CT has a much higher activation enthalpy and entropy than that in CC or there is still the possibility of a non-concerted ring opening. Indeed, non-concerted ring opening of 3,4-dialkylcyclobutenes has activation energies of the order of 47 kcal/mol (see Chapter 5, Section 2.2). Thus, the conrotatory ring opening may be a dead-end in the pyrolysis of BOE although the free energy of activation for the 1,5-hydrogen shift pathway is comparable to that for the 1,5-hydrogen shift in CC. 4.3 1,2,7-Octatriene, 1-Bicyclo[4.2.0]octene, and 6-Methylenebicyclo[3.2.0]heptane Pyrolysis of 1,2,7-octatriene at 4108C gives 1-bicyclo[4.2.0]octane and 6methylenebicyclo[3.2.0]heptane (Scheme 9.97).146 This is an example of an intramolecular allene – ethylene [2 þ 2] cycloaddition which may or may not be concerted (see Chapter 6, Section 2.6). Small amounts of 1-hepten-7-yne were also reported to be formed in the reaction.

Scheme . 9.97

4.4

6-Octene-1-yne to 2-Vinylmethylenecyclopentane

Huntsman showed that trans-6-octene-2-yne gives 2-vinylmethlenecyclopentane upon pryolysis at 4008C in a flow system (Scheme 9.98).147 The reaction occurs with log k ¼ 9:84 2 34 100=2:3RT:148 The rearrangement appears to be an intramolecular ene reaction with transfer of hydrogen simultaneous with ring formation as evidenced by a substantial kinetic effect, kH =kD ¼ 2 at 3688C, with 8,8,8-trideuterio material.2

Scheme . 9.98

C8H6 – C8H14

265

This reaction has been used elegantly by Arigoni to generate a chiral methyl group of known absolute configuration in acetic acid (Scheme 9.99).149

Scheme . 9.99

4.5

[3.2.1]- and [2.2.2]Propellane and Dimethylenecyclohexanes

Aue found that [3.2.1]propellane opened to 1,3-dimethylenecyclohexane at 3188C (Scheme 9.100).150 The temperature necessary to affect this retro 2 þ 2 cycloaddition is much higher than that for the retro 2 þ 2 cycloaddition of [2.2.2]propellane to 1,4-dimethylenecyclohexane studied by Eaton which occurs at only 258C.151

Scheme . 9.100

Note should be made that the amide group probably has little effect on the rate of this reaction since one of the products is formed without any bond broken to the carbon bearing the amide group. The relative rates of reaction of the two propellanes are in accord with calculations by Hoffmann.152

266


The product from [2.2.2]propellane also undergoes a degenerate 3,3-shift at much higher temperatures with DH ‡ ¼ 39 kcal=mol and DS‡ ¼ 213:8 e:u:153 A probably better study gave DH ‡ ¼ 43:9 kcal=mol and DS‡ ¼ 24:6 e:u: for this rearrangement (Scheme 9.101).154

Scheme . 9.101

Subsequent work revealed that the cis-3,4-dimethyl derivative equilibrated only with syn-1,4-diethylidenecyclohexane and that the trans-3,4-dimethyl derivative equilibrated not only with anti-1,4-diethylidenecyclohexane but with itself via a 3,3shift as discovered by deuterium labeling (Scheme 9.102).155

Scheme . 9.102

Furthermore, the deuterium kinetic isotope effect for the rearrangement of 1,4-bis(dideuteriomethylene) trans-dimethyl derivative to 3,3,4,4-tetradeuterio antidiethylidenecyclohexane was inverse but much smaller than that for the acyclic 1,5-dienes (Chapter 7, Section 4.1) suggesting much less bond making between the methylene termini in the transition state, Scheme 9.102. For comparison, the equilibrium isotope effect between the two isomers of the trans material favored deuterium on the ring carbons by a substantial amount.

C8H6 – C8H14

267

4.6 Bicyclo[5.1.0]oct-2-ene to 1,4-Cyclooctadiene to Bicyclo[3.3.0]oct-2-ene Grimme first discovered the bicyclo[5.1.0]oct-2-ene to 1,4-cyclooctadiene reversible rearrangement156 and found Keq ¼ 37 at 2008C with log ðk1 þ k21 Þ ¼ 13:34 2 38 600=2:3RT (Scheme 9.103).157

Scheme . 9.103

Further, 8,8-dideuterio material gave exclusively 3,3-dideuterio-1,4-cyclooctadiene. This observation rules out a cyclopropane ring opening followed by a vicinal hydrogen shift and leaves the homo-1,5-hydrogen shift as the most reasonable pathway (Scheme 9.104). The relatively low activation energy suggests concert in the rearrangement since ring opening of vinylcyclopropane itself has an activation energy of , 49 kcal/mol.

Scheme . 9.104

At higher temperatures, bicyclo[3.3.0]oct-2-ene was formed from the equilibrium mixture, and the fact that 8,8-dideuteriobicyclo[5.1.0]oct-2-ene gave 3,3-dideuteriobicyclo[3.3.0]oct-2-ene indicates that cyclopropane ring opening occurs and results in the vinylcyclopropane to cyclopentene rearrangement (see Chapter 6, Section 2.1). 5 5.1

C8H14 1,6-Octadiene to 2-Methylvinylcyclopentane

Derivatives of 1,6-octadiene gave 2-methylvinylcyclopentanes upon heating,158 a reaction that appears to be an intramolecular ene reaction.159 In the simplest case,

268


trans-1,6-octadiene gave a 13.6:1 mixture of cis- and trans-2-methylvinylcyclopentane at 4228C with log k ¼ 9:9 2 38 000=2:3RT which suggests a strong preference for one of the two possible transition states (Scheme 9.105).160

Scheme . 9.105

5.2

1,7-Octadiene, cis- and trans-Cyclooctene

1,7-Octadiene is in equilibrium with cis-cyclooctene at 3208C with the equilibrium favoring the acyclic material at higher temperatures.161 Interestingly, transcyclooctene gave 1,7-octadiene at lower temperatures (2508C). Both reactions would appear to be intramolecular ene reactions (Scheme 9.106).

Scheme . 9.106

trans-Cyclooctene itself was first synthesized by Cope who then prepared the optically active material and studied its racemization.162 The activation parameters for the racemization are DH ‡ ¼ 34:7 kcal=mol and DS‡ ¼ 0: The racemization apparently involves rotation of the trans double bond moiety.163 By comparison, trans-cyclonone racemizes much easier with DH ‡ ¼ 19:4 kcal=mol and DS‡ ¼ 0:164

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272 105. 106. 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133. 134. 135. 136. 137. 138. 139.

Hydrocarbon Thermal Isomerization H. Prinzbach and R. Schwesinger, Angew. Chem. Int. Ed. Engl., 11, 940 (1972); E. Vogel, H.-J. Altenbach, and E. Schmidbauer, Angew. Chem. Int. Ed. Engl., 12, 838 (1973). W.R. Dolbier, Jr., D. Lomas, and P. Tarrant, J. Am. Chem. Soc., 90, 3594 (1968). L. Skattebøl and S. Solomon, J. Am. Chem. Soc., 87, 4506 (1965). W.R. Roth, M. Heiber, and G. Erker, Angew. Chem. Int. Ed. Engl., 12, 504 (1973). See also W. Grimme and H.-J. Rother, Angew. Chem. Int. Ed. Engl., 12, 505 (1973). W.R. Roth, M. Biermann, G. Erker, K. Jelich, W. Gerlartz, and H. Go¨rner, Chem. Ber., 113, 586 (1980). W.R. Roth, B.P. Scholz, H. Breuckmann, K. Jelich, and H.W. Lennartz, Chem. Ber., 115, 1934 (1982). G. Becher and L. Skattebøl, Tetrahedron Lett., 1261 (1979). W.R. Roth and G. Erker, Angew. Chem. Int. Ed. Engl., 12, 503 (1973). W.R. Roth and G. Erker, Angew. Chem. Int. Ed. Engl., 12, 505 (1973). E.S. Koltun and S.R. Kass, J. Org. Chem., 65, 3530 (2000). E.S. Koltun and S.R. Kass, J. Org. Chem., 65, 3530 (2000). K.J. Snellings and E.S. Lewis, J. Org. Chem., 57, 4315 (1992). E. Vogel, Justus Liebigs Ann. Chem., 615, 1 (1958). G.S. Hammond and C.D. DeBoer, J. Am. Chem. Soc., 86, 899 (1964). J. Leitich, Int. J. Chem. Kinet., 11, 1249 (1979). See also H.D. Martin, E. Eisenmann, H. Kunze, and V. Bonacicˇ-Koutecky`, Chem. Ber., 113, 1153 (1980). J.J. Gajewski, C.M. Hawkins, and J.L. Jimenez, J. Org. Chem., 55, 674 (1990). J.A. Berson and P.B. Dervan, J. Am. Chem. Soc., 94, 7597 (1972); J.A. Berson, P.B. Dervan, R. Malherbe, and J.A. Jenkins, J. Am. Chem. Soc., 98, 5937 (1976). W. von E. Doering, L. Birladeanu, K. Sarma, J.H. Teles, F.-G. Kla¨rner, and J.-S. Gehrke, J. Am. Chem. Soc., 116, 4289 (1994). J.A. Berson and P.B. Dervan, J. Am. Chem. Soc., 95, 267 (1973). See also p. 269. W. Tsang, J. Chem. Phys., 42, 1805 (1965). N.E. Duncan and G.J. Janz, J. Chem. Phys., 20, 1644 (1952). W. von E. Doering, M. Franck-Neumann, D. Hasselmann, and R.L. Kaye, J. Am. Chem. Soc., 94, 3833 (1972). W. von E. Doering, and D.M. Brenner, Tetrahedron Lett., 899 (1976). G. Huybrechts, L. Luyckx, Th. Vandenboom, and B. van Mele, J. Chem. Kinet., 9, 283 (1977). G.B. Kistiakowsky and W.W. Ransom, J. Chem. Phys., 7, 725 (1939); D. Rowley and H. Steiner, Discuss. Faraday Soc., 10, 198 (1951). L.M. Stephenson, R.V. Gemmer, and S. Current, J. Am. Chem. Soc., 97, 5909 (1975). J.A. Berson and R. Malherbe, J. Am. Chem. Soc., 97, 5910 (1975). W. von E. Doering, J.L. Ekmanis, K.D. Belfield, F.-G. Kla¨rner, and B. Krawczyk, J. Am. Chem. Soc., 123, 5532 (2001). M. Neumann, PhD Dissertation (Professor W.R. Roth, research director), RuhrUniversita¨t Bochum (1994). This Scheme is not exactly that of the paper, but is consistent with a private communication from Prof. Doering. H. Tanida, S. Teratake, Y. Hata, and M. Watanabe, Tetrahedron Lett., 5341 (1969). L.A. Paquette and M.J. Carmody, J. Am. Chem. Soc., 98, 8175 (1976). H.D. Martin and E. Eisenmann, Tetrahedron Lett., 661 (1975). H.D. Martin, E. Eisenmann, H. Kunze, and V. Bonacicˇ-Koutecky`, Chem. Ber., 113, 1153 (1980). D. Bellu˘s, H.-C. Mez, G. Rihs, and H. Sauter, J. Am. Chem. Soc., 96, 5007 (1974). For other reactions, see H.-D. Martin, M. Hekman, G. Rist, H. Sauter, and D. Bellu˘s, Angew. Chem. Int. Ed. Engl., 16, 406 (1977).

C8H6 – C8H14 140. 141. 142. 143. 144. 145. 146. 147. 148. 149. 150. 151. 152. 153. 154. 155. 156. 157. 158. 159. 160. 161. 162. 163. 164.

273

R. Srinivasan and A.A. Levi, J. Am. Chem. Soc., 86, 3756 (1964). R. Criegee, D. Seebach, R.E. Winter, B. Bo¨rretzen, and H.-A. Brune, Chem. Ber., 98, 2339 (1965). G.R. Branton, H.M. Frey, and R.F. Skinner, Trans. Faraday Soc., 62, 1546 (1966). J.S. McConaghy, Jr. and J.J. Bloomfield, Tetrahedron Lett., 3719 (1969); J.J. Bloomfield, J.S. McConaghy, Jr., and A.G. Hortmann, Tetrahedron Lett., 3723 (1969). K.M. Shumate, P.N. Neuman, and G.J. Fonken, J. Am. Chem. Soc., 87, 3996 (1965). D.S. Glass, R.S. Boikess, and S. Winstein, Tetrahedron Lett., 999 (1966). See also J. Branham and C.J. Samuel, J. Chem. Soc. Chem. Commun., 29 (1989), and J.E. Baldwin, P.A. Leber, and T.W. Lee, J. Org. Chem., 66, 5269 (2001). L. Skattebøl and S. Soloman, J. Am. Chem. Soc., 87, 4506 (1965). W.D. Huntsman and R.P. Hall, J. Org. Chem., 27, 1988 (1962). R.W. Woosley, PhD Thesis, Ohio University, Athens (1967); W.D. Huntsman Intra-Sci. Chem. Rep. 6, 151 (1972). C.A. Townsend, T. Scholl, and D. Arigoni, J. Chem. Soc. Chem. Commun., 921 (1975). D.H. Aue and R.N. Reynolds, J. Org. Chem., 39, 2315 (1974). P.E. Eaton and G.H. Temme, III, J. Am. Chem. Soc., 95, 7508 (1973). W.-D. Stohrer and R. Hoffmann, J. Am. Chem. Soc., 94, 779 (1972) See also M.D. Newton, and J.M. Schulman, J. Am. Chem. Soc., 94, 773 (1972). J.J. Gajewski, L.K. Hoffman, and C.N. Shih, J. Am. Chem. Soc., 96, 3705 (1974). W. von E. Doering, and C.A. Troise, J. Am. Chem. Soc., 107, 5739 (1985). J.J. Gajewski and J.L. Jimenez, J. Am. Chem. Soc., 108, 468 (1986). J.J. Gajewski, and J.L. Jimenez, J. Am. Chem. Soc., 108, 2492 (1986), (correction). W. Grimme, unpublished work, cited by W. von E. Doering and W.R. Roth, Angew. Chem. Int. Ed. Engl., 2, 115 (1963). W. Grimme, Chem. Ber., 98, 756 (1965). H. Pines, N.E. Hoffman, and V.N. Ipatieff, J. Am. Chem. Soc., 76, 4412 (1954); W.D. Huntsman and T.H. Curry, J. Am. Chem. Soc., 80, 2252 (1958); W.D. Huntsman, V.C. Solomon and D. Eros, J. Am. Chem. Soc., 80, 5455 (1958). For an early review see H.M.R. Hoffmann, Angew. Chem. Intl. Ed. Engl., 8, 556 (1969). R.W. Woosley, PhD Thesis, Ohio University, Athens, Ohio (1967) with W.D. Huntsman. W.R. Roth, Chimia, 20, 229 (1966). A.C. Cope and B.A. Pawson, J. Am. Chem. Soc., 87, 3649 (1965). L.P. Olson, Internet J. Chem., 2 (3) (1999), Art. No. 3 JAN 27 1999. A.C. Cope, K. Banholzer, H. Keller, B.A. Pawson, J.J. Whang, and H.J.S. Winkler, J. Am. Chem. Soc., 87, 3644 (1965).

11 C10H6 –C10H16

CONTENTS 1 C10H6 1.1 Miscellaneous Isomerizations 2 C10H8 2.1 Azulene, Naphthalene, and Benzofulvene Interconversion 2.2 Tricyclo[6.2.0.03,6]deca-1,4,7,9-tetraene to Bicyclo[6.2.0]deca-1,3,5,7,9-pentaene 2.3 Benzobicyclo[2.2.0]hexa-2,5-diene to Naphthalene 2.4 1,2,4,6,7,9-Cyclodecahexaene to Naphthalene 3 C10H10 3.1 (CH)10 Interconversions – Introduction 3.2 cis-, trans-9,10-Dihydronaphthalene and the 1,3,5,7,9-Cyclodecapentaenes 3.3 Bullvalene and Lumibullvalene to cis-9,10-Dihydronaphthalene 3.4 Isobullvalene to Lumibullvalene 3.5 Tetracyclo[5.3.0.0.4,605,10]deca-2,8-diene to cis-9,10-Dihydronaphthalene 3.6 Bicyclo[4.2.2]deca-2,4,7,9-tetraene to Tetracyclo[4.4.0.0.2,1005,7]deca-3,8-diene to cis-9,10-Dihydronaphthalene 3.7 Basketene to syn-Tricyclo[4.4.0.02,5]deca-3,7,9-triene to Nenitzescu’s Hydrocarbon 3.8 Nenitzescu’s Hydrocarbon to cis-9,10-Dihydronaphthalene, 1,2-Dihydronaphthalene and 1-Phenylbutadiene 3.9 Bicyclo[6.2.0]deca-2,4,6,9-tetraene to trans-9,10-Dihydronaphthalene 3.10 Tetracyclo[4.4.0.0.2,507,10]deca-3,8-diene to trans-9,10-Dihydronaphthalene 3.11 Snoutene Degenerate Rearrangement and Conversion to cis-9,10-Dihydronaphthalene 3.12 Hypostrophene Degenerate 3,3-Shift and 1,3-Shift to Pyrohypostrophene

325 325 326 326 328 329 329 329 329 332 336 337 338

338 339 340 342 342 343 345

324


3.13 Diademane to Triquinacene to Azulene 3.14 Tetracyclo[5.3.0.0.2,1003,6]deca-4,8-diene 3.15 Tetracyclo[5.3.0.0.2,803,6]deca-4,9-diene to Tetracyclo[5.3.0.0.2,1003,6]deca-4,8-diene and Tricyclo[5.3.0.02,8]deca-3,5,9-triene to Isobullvalene to Lumibullvalene 3.16 Miscellaneous (CH)10 Hydrocarbons by Photolysis 3.17 Summary of (CH)10 Interconversions 3.18 9-Methylenebicyclo[6.1.0]nona-2,4,6-triene Pyrolysis 3.19 3-Vinylbenzocyclobutene to 1,2-Dihydronaphthalene 4 C10H12 4.1 Aromatic Claisen Rearrangement and All Carbon Analogues 4.1.1 Attempts at an All-Carbon Claisen Rearrangement 4.2 1,6-Trimethylenenorcaradiene 4.3 cis-, trans-Bicyclo[6.2.0]deca-2,4,6-triene to 1,3,5,7-Cyclodecatetraene and Tetrahydronaphthalene 4.4 Dispiro[2.2.2.2]deca-4,9-diene and Dispiro[2.0.2.4]deca-7,9-diene 4.5 Dispiro[2.0.2.4]deca-1,5-diene and Tetramethylene Dewar Benzene 4.6 Pentacyclo[5.3.0.02,60.3,508,10]decane and 1,5-Bishomocyclooctatetraene 4.7 1,2,6,7-Cyclodecatatetraene to 2,3-Divinyl-1,3-cyclohexadiene 4.8 Bishomobarrelene Pyrolyses 4.9 endo-Tetracyclo[5.3.0.0.2,1003,6]deca-8-ene to Tricyclo[5.2.1.04,10]deca-2,5-diene 4.10 syn-Tricyclo[4.2.1.12,5]deca-3,7-diene to syn-Tricyclo[5.3.0.02,6]deca-3,9-diene 4.11 6-Allyl-3-methylene-1,4-cyclohexadiene to Butenylbenzene 4.12 2,5-Tetramethylene Dewar Benzene to 4-meta-Cyclophane to Tetralin and Tetramethyleneprismane to Tetralin and Tetramethylene Fulvene 5 C10H14 5.1 5-Vinylbicyclo[2.2.2]octa-2-ene to Bicyclo[4.4.0]deca-2,8-diene and Retro 4 þ 2 Cycloaddition 5.2 Bicyclo[6.2.0]deca-2,6-diene Degenerate Rearrangement 5.3 cis-, trans-4-Methylenebicyclo[5.2.0]nona-2-ene to 5-Methylene-1,3-cycloheptadiene and 2-Methylenebicyclo[3.2.2]nona-6-ene 5.4 1,2,8,9-Decatetraene Pyrolysis 5.5 9-Methylenebicyclo[6.1.0]nona-2-ene to 3-Methylene-1,4-cyclononadiene 5.6 2-(2-Methylene-3-butenyl)methylenecyclobutane to 1-(3-Methylene4-pentenyl)cyclobutene to 3,6-Dimethylene-1,7-octadiene 5.7 3,6-Dimethylene-1,7-octadiene Pyrolysis and Bicyclo[4.2.2]deca-1,5-diene

346 347

347 348 349 349 351 351 351 354 354 355 356 358 359 360 362 363 363 363

364 365 365 367

368 368 369 369 370

C10H6 – C10H16

5.8 1,3,7,9-Decatetraene Potentially Degenerate 5,5-Shift 5.9 Bornadiene pyrolysis 5.10 Tetracyclo[4.2.1.1.1,602,5]decane to 2,4-Dimethylenebicyclo[3.2.1]octane 5.11 cis-1,2,4-Cyclodecatriene to trans-Bicyclo[4.4.0]deca-2,4-diene 5.12 cis-Bicyclo[7.1.0]deca-2,3-diene to Tricyclo[5.3.0.02,4]deca-5-ene 6 C10H16 6.1 1,5-Cyclodecadiene 3,3-Shift 6.2 1,6-Cyclodecadiene Intramolecular 2 þ 2 Cycloaddition 6.3 trans-1,3,9-Decatrienes to Bicyclo[4.4.0]deca-2-enes References

1 1.1

325 371 372 373 373 373 374 374 375 375 376

C10H6 Miscellaneous Isomerizations

Synthesis of the highly substituted syn,syn-diethynyldimethylenecyclobutene in Scheme 11.1 was reported to give the benzodicyclobutadiene upon reflux in toluene.1 However, it was the syn,anti isomer which was formed from mesobisallene that was subjected to pyrolysis, and it must have undergone a geometric isomerization to give the necessary syn,syn relationship between the acetylenic linkages. In fact, the syn,syn isomer, which was formed from heating the racemic bisallene, could not be isolated; only the benzodicyclobutadiene was formed, presumably via an eight-electron cyclization followed by the 2 þ 2 cyclization of the cyclic bisallene.2

Scheme 11.1

326


The solid-state structure of the tricyclic product reveals strong bond alternation consistent with two quinoid resonance structures like that of Scheme 11.1 without a ˚ long while the other central benzene-like ring. Indeed, the fusion bonds are 1.55 A 3 ˚ bonds in the central ring are 1.40 A long. Calculations at the MP2/6-31Gp level, however, indicated that the central ring aromatic structure was more stable than the quinoid structure by ca. 10 kcal/mol.4 Later calculational efforts at both the MP2/631Gp and BL3LYP/6-31Gp levels revealed a 3 –4 kcal/mol preference for the quinoid structure with a reasonable reproduction of the solid-state bond lengths.5 In an attempt to synthesize a bisdehydrocyclodecapentaene, an o-diethynylbenzene derivative was isolated as well as a naphthalene whose extra hydrogens came from the solvent (Scheme 11.2).6

Scheme 11.2

This reaction is not unrelated to the 1,2-diethynylethylene degenerate rearrangement via a benzene-1,4-diyl and represents perhaps the first example of it (see Chapter 7, Section 1). 2 2.1

C10H8 Azulene, Naphthalene, and Benzofulvene Interconversion

At high temperatures, azulene is converted to naphthalene7 with DH ‡ ¼ 48:8 kcal/ mol and DS‡ ¼ 25:3 e.u. (Scheme 11.3).8

Scheme 11.3

C10H6 – C10H16

327

Subsequently, 2-methylazulene was shown to give nearly a 2:1 mixture of b-methyl and a-methylnapthalene, but more importantly, it was found the reaction had an induction period suggesting that it was a radical chain process.9 Further, shock wave pyrolysis of azulene allowed determination of log k ¼ 12:93 2 63 000=2:3RT for its unimolecular conversion to naphthalene.10 Furthermore, the rate constant calculated at the temperatures involved in the initial reports makes it clear that the unimolecular process is 10 000 times slower than what was observed. So radical chemistry is most likely involved in the early reports. Nonetheless, a unimolecular pathway involving reversible bicyclobutane formation was proposed to account for the azulene – naphthalene conversion and could account for formation of the b-methyl isomer but not the a-isomer (see asterisk of Scheme 11.4).11

Scheme 11.4

Moreover, it was shown that a-13C naphthalene interconverted with b-13C naphthalene at 10008C with a free energy of activation of 86 kcal/mol. It was pointed out that this is approximately the sum of the energy difference between azulene and naphthalene and the observed activation free energy for the azulene to naphthalene conversion suggesting that azulene could be an intermediate in the label scrambling of naphthalene (Scheme 11.5).

Scheme 11.5

328


However, this pathway could not account for the formation of a-methyl naphthalene formed from 2-methylazulene unless naphthalene was produced sufficiently vibrationally hot to undergo some isomerization. It now appears that benzofulvene, which gives naphthalene under conditions of flash vacuum pyrolysis,12 is an intermediate in the automerization of naphthalene.13 Thus benzofulvene labeled in the methylene position gave a 79:21 ratio of b- and a-labeled naphthalene consistent with competing aryl and vinyl shifts to carbene precursors to the naphthalenes (Scheme 11.6).

Scheme 11.6

Lastly, it is not obvious that benzofulvene could be an intermediate in the conversion of azulene to naphthalene. 2.2 Tricyclo[6.2.0.03,6]deca-1,4,7,9-tetraene to Bicyclo[6.2.0]deca-1,3,5,7,9-pentaene Tricyclo[6.2.0.0 3,6]deca-1,4,7,9-tetraene gives bicyclo[6.2.0]deca-1,3,5,7,9pentaene in a first-order process at 1008C (Scheme 11.7).14

Scheme 11.7

This would appear to be analogous to the cyclooctatetraene (COT) – bicyclo[4.2.0]octa-2,4,7-triene interconversion (See Chapter 9, Section 2). The product, pentaene, is remarkably stable, dimerizing only slowly at 1008C.

C10H6 – C10H16

2.3

329

Benzobicyclo[2.2.0]hexa-2,5-diene to Naphthalene

Benzobicyclo[2.2.0]hexadiene gives naphthalene with log k ¼ 12:3 2 23 680= 2:3RT (Scheme 11.8).15

Scheme 11.8

A disrotatory, non-concerted cyclobutene ring opening to give a biradical was proposed for the reaction pathway. 2.4

1,2,4,6,7,9-Cyclodecahexaene to Naphthalene

Treatment of 5,5,10,10-tetrabromotricyclo[7.1.0.04,6]deca-2,7-diene with methyllithium at 2 788C gave naphthalene (Scheme 11.9).16

Scheme 11.9

While ionic pathways can be drawn for this conversion, a possible thermal path is via meso-1,2,4,6,7,9-cyclodecahexaene, which can undergo an electrocyclization in an “allowed” six-electron disrotatory fashion to naphthalene.

3 3.1

C10H10 (CH)10 Interconversions – Introduction

The central molecules from this saga of the 1960s are the potentially aromatic cyclodecapentaenes and bullvalene, a compound of 10/3! faces. Efforts directed to the synthesis of these compounds involved thermal, photochemical, and metalcatalyzed processes which not only accomplished the major task but also generated

330


a vast array of new isomers and reaction pathways to embellish the chemical landscape. Having had its degenerate schizophrenia exposed by Doering,17 bullvalene succumbed to synthesis by thoughtful serendipity. Schro¨der, working on the structure of the thermal dimers of COT, recognized the potential photochemical cleavage of one of these to benzene and THE (CH)10 hydrocarbon, and was able to produce it in quantity (Scheme 11.10).18

Scheme 11.10

As predicted,17 the NMR spectrum of bullvalene coalesces to a single line at high temperatures due to 3,3-shifts of the cis-divinylcyclopropane moieties, and subsequent work established log k ¼ 14:0 2 13 900=2:3RT for the conversion.19 At high temperatures, bullvalene (BV) gives cis-9,10-dihydronaphthalene, (C9,10) (Scheme 11.11),20 an isomer which had been prepared previously in the unfulfilled hope of thermalizing it to 1,3,5,7,9-cyclodecapentaene.21

Scheme 11.11

This, however, was just the beginning of the interconversions. Only two other isomers were known prior to 1966, namely, tricyclo[4.2.2.02,5]deca-3,7,9-triene or Nenitzescu’s hydrocarbon (NHC)22 and triquinacene (TRQ)23 (Scheme 11.12).

C10H6 – C10H16

331

Scheme 11.12

In the subsequent 6 years, a small number of research groups provided the vast array of isomers and pathways shown in Scheme 11.13.24

Scheme 11.13

The purely thermal processes that are involved include the formation of cis-9,10dihydronaphthalene (C9,10) from bullvalene (BV), bicyclo[4.2.2]decatetrene (422) and its intramolecular Diels– Alder adduct, tetracyclo[4.4.0.0.2,1005,7]decadiene (TCD), from lumibullvalene (LBV), isobullvalene (IBV), tricyclo[5.3.0.02,8]deca3,5,9-triene (TDT), and isolumibullvalene (ILBV), from basketene (BSK), NHC, TRQ, and the syn and anti isomers of tricyclo[4.4.0.02,5]decatriene (S and A, respectively), and from all cis-cyclodecapentaene (C5), snoutene (SNT), and diademane (DIA). A more recent review includes many heats of formation and enthalpies of activation.25

332


The trans isomer of 9,10-dihydronaphthalene (T9,10) is formed from cis,cis,cis,cis,trans-cyclodecapentaene (C4T) and bicyclo[6.2.0]deca-2,4,6,9-tetraene (620), and NHC may also give T9,10 (Scheme 11.14).

Scheme 11.14

Other (CH)10 isomers that have been prepared are pteradactyladiene (PTD), hypostrophene (HPS), pentaprismane (PPM), pyrohypostrophene (PHPS), tetracyclo[5.3.0.0.2,1003,6]deca-4,8-diene (TD), the cyclopropyl and methane-capped benzene isomer (CMB), one of the lumi products of bullvalene, namely, lumiisobullvalene (LIBV), and barettane (BAR) (Scheme 11.15).

Scheme 11.15

More extensive lists of isomers are available from graph theory.26 3.2 cis-, trans-9,10-Dihydronaphthalene and the 1,3,5,7,9-Cyclodecapentaenes Most of the (CH)10 isomers ultimately give C9,10 or T9,10 upon heating at high temperatures, but every valley on this energy surface is a potential resting site on the descent from these compounds to the thermodynamic sink, and so it is difficult,

C10H6 – C10H16

333

particularly in light of modest quantitative information on relative stabilities of isomers, to describe the paths with certainty. Nonetheless, it is worthwhile trying starting with the reaction of the most stable of the isomers, C9,10. C9,10, the initial product from pyrolysis of BV in a flow system at 3508C, is converted to 1,4-dihydronaphthalene, 1,2-dihydronaphthalene, and naphthalene at 4008C in a ratio of 4:2:1.20 The origin of the 1,4-dihydronaphthalene is unclear but 1,2-dihydronaphthalene most reasonably results from two successive 1,5-hydrogen shifts (Scheme 11.16).

Scheme 11.16

In a flow system at 5108C (tr , 1 s), 2,3-dideuterio-C9,10 scrambles deuterium rather indiscriminately just prior to conversion to the pyrolysis products reported above.27 Reversible formation of C5 was suggested as a likely pathway for the reaction. A series of sequential, but unknown, antara,antara-3,3-sigmatropic shifts was also considered (see Chapter 7, Section 4). A third possibility involving cross ring bond formation to give two allyl radicals was suggested28 and is shown in Scheme 11.17 along with the two other pathways.

Scheme 11.17

A distinction between the three processes is possible if the deuterium distribution could be determined at short reaction times.

334


It was also found that 9,10-dimethyl-C9,10 gives a 54:46 ratio of 1,5- and 1,9dimethyl-C9,10 with log k ¼ 12:0 2 26 000=2:3RT (Scheme 11.18).29

Scheme 11.18

While it was argued that the former isomer could arise from dimethyl-C5, and the latter via a 1,5-vinyl shift, the suggestion of cross ring bond formation (Alder) might seem more attractive. However, this latter proposal has energetic demands that appear to remove it from consideration. s bond formation between the ends of two diene systems to give a bisallylically stabilized biradical is uphill by , 7 kcal/mol, but here a four-membered ring is generated which raises the endothermicity to , 33 kcal/mol. Thus the ring opening pathway via C5 is the likely one. Furthermore, C5 appears to have been isolated from low temperature photolysis of C9,10 followed by a 2 808C chromatography and was found to give C9,10 with an activation enthalpy of only 17 kcal/mol although the cis,trans,cis,trans,cis isomer (CTCTC) might also be involved (Scheme 11.19).30

Scheme 11.19

C10H6 – C10H16

335

trans-9,10-Dihydronaphthalene (T9,10)31 gives cis-1-phenylbutadiene, 1,2dihydronaphthalene, and naphthalene upon pyrolysis at 2808C in a flow system (Scheme 11.20).32

Scheme 11.20

A 1,5-hydrogen shift followed by a retro-electrocyclization is probably responsible for the phenylbutadiene, and a second 1,5-hydrogen shift after the first gives the 1,2-isomer, and naphthalene is always formed from the 1,2-isomer upon heating. Related to this is the observation that cis,cis,cis,cis,trans-cyclodecapentaene (C4T), isolated from low temperature photolysis of C9,10 followed by 2 808C chromatography (to free it from other10 annulenes – see earlier), gives T9,10 at 2 258C with an activation enthalpy of only 17 kcal/mol in what appears to be a disrotatory electrocyclization (Scheme 11.21).30

Scheme 11.21

Finally, C4T was shown to have temperature-dependent NMR spectra from 2 89 to 2 308C indicating conformational interconversions.33 Extensive computational efforts have helped clarify the structural and energetic landscape for the [10] annulenes. At the highest level of theory, a nearly planar, aromatic, Cs ; C4T is the most stable structure although it has a low barrier for interconversion with a non-planar conformer which is roughly 7 kcal/mol higher in energy at the MP2/TZ2P//MP2/DZd and B3LYP/TZP level (Scheme 11.22).34

336


Scheme 11.22

Remarkably, the C2 symmetric conformation of CTCTC is roughly comparable in energy to the less stable conformer of C4T, but more stable than the Cs symmetric, tub-like conformer of all cis isomer, C5, by , 5 kcal/mol at the DZSCF/MP2/ZPVE level.35 However, the most stable arrangement of all planar C5 is one with D10h symmetry, but it is , 15 kcal/mol less stable than the tub-like geometry. Finally, suggestion has been made that the compound isolated in the photolysis of C9,10 is C5 which can give C9,10 rapidly, but CTCTC is known to give C9,10 much too fast to be isolated.34 3.3

Bullvalene and Lumibullvalene to cis-9,10-Dihydronaphthalene

Bullvalene (BV) gives cis-9,10-dihydronaphthalene (C9,10) at high temperatures20 with DH ‡ ¼ 45 kcal/mol and DS‡ ¼ 2 e.u.36 A suggested pathway20 involved tricyclo[5.3.0.04,8]deca-2,5,9-triene or lumibullvalene (LBV), the vinyl cyclopropane rearrangement product of BV. The stereochemistry required for this rearrangement represents a “forbidden” pathway. However, the Eact is sufficiently higher than the BDE of the cyclopropane bond in BV that a bisallylic biradical derived from cyclopropane bond fission in BV can easily be formed, and it can close to LBV (Scheme 11.23).

Scheme 11.23

C10H6 – C10H16

337

LBV is known to give C9,10 at 2808C in a flow system with residence times of 5 s, which corresponds to a free energy of activation of roughly 35 kcal/mol.37 The likely pathway for this transformation would appear to be a retro Diels– Alder reaction to all cis-cyclodecapentaene (C5), which electrocyclizes to C9,10 (see Section 3.2). LBV has also been shown to undergo a degenerate rearrangement, most likely via non-concerted 1,3-shifts, at a rate comparable to the retro Diels– Alder reaction.38 In this pathway, the most stabilized biradical is produced, and it, most likely, is an intermediate in the retro Diels– Alder reaction (Scheme 11.24).

Scheme 11.24

3.4

Isobullvalene to Lumibullvalene

Tricyclo[5.3.0.02,10]deca-3,5,8-triene or isobullvalene (IBV) apparently gives lumibullvalene (LBV) at 08C, since a rational synthesis of the former resulted in the latter (Scheme 11.25).39

Scheme 11.25

Subsequently IBV was isolated from reaction of the cyclononatetraenide ion, methylene chloride, and n-butyl lithium at 2 308C, and it gave LBV with DH ‡ ¼ 19:5 kcal/mol and DS‡ ¼ 10 e.u. (Scheme 11.26).40

338


Scheme 11.26

Furthermore, at 2 608C both C4T and C9,10 were found in the reaction mixture,33 but C5 was not found despite its stability under the reaction conditions. Speculation on the origin of C4T focused on tricyclo[7.1.0.02,10]deca-3,5,7-triene, a possible precursor to IBV as well (Scheme 11.26). Finally, lumiisobullvalene (LIBV), the photoproduct of isobullvalene at 2 1008C was also found in the reaction mixture.33 3.5 Tetracyclo[5.3.0.0.4,605,10]deca-2,8-diene to cis-9,10-Dihydronaphthalene Tetracyclo[5.3.0.0.4,605,10]deca-2,8-diene or isolumibullvalene (ILBV), so named because it is a potential di-p-methane photoisomer of LBV, gives C9,10 upon heating with a free energy of activation of , 29 kcal/mol at 1008C.41 A retro Diels – Alder reaction is the likely pathway for this isomerization (Scheme 11.27). Naphthalene was also produced in this reaction.

Scheme 11.27

3.6 Bicyclo[4.2.2]deca-2,4,7,9-tetraene to Tetracyclo[4.4.0.0.2,1005,7]deca-3,8-diene to cis-9,10-Dihydronaphthalene Upon heating at 2458C for 4 h, bicyclo[4.2.2]deca-2,4,7,9-tetraene (422), which is a photoproduct of C9,10, reverts to C9,10 (20%) and goes on to naphthalene (80%)42

C10H6 – C10H16

339

since it was later demonstrated that at low conversions, C9,10 is the major product formed with DH ‡ ¼ 34 kcal/mol and DS‡ ¼ 25 e.u. (Scheme 11.28).36

Scheme 11.28

422 also undergoes a degenerate rearrangement as judged by deuterium label scrambling via its intramolecular Diels– Alder isomer, tetracyclo[4.4.0.0.2,1005,7]deca-3,8-diene (TCD), at least 10 times faster than the formation of C9,10.43 Subsequently, the reaction was studied by dynamical NMR resulting in DG‡ , 25 kcal/mol at 1408C (Scheme 11.29) (Seidner, 1970).

Scheme 11.29

TCD was also isolated from low temperature photolyses of 620, C9,10, and T9,10 and was found to give 422 at 2 158C with DH ‡ ¼ 21 kcal/mol and DS‡ ¼ 5 e.u.44 Thus, TCD is only 5 kcal/mol less stable than 422 and most likely, undergoes a nonconcerted, forbidden retro 4 þ 4 cleavage to C9,10 with an activation free energy around 31 kcal/mol. Numerous substituted derivatives of 422 have been studied45 including the benzo derivative,32 which requires higher temperatures to give the corresponding benzoC9,10.46

3.7 Basketene to syn-Tricyclo[4.4.0.02,5]deca-3,7,9-triene to Nenitzescu’s Hydrocarbon Pentacyclo[4.4.0.0.2,50.3,804,7]deca-9-ene or basketene (BSK) gives tricyclo[4.2.2.02,5]deca-3,7,9-triene or Nenitzescu’s hydrocarbon (NHC) upon heating with DH ‡ ¼ 29 kcal/mol and DS‡ ¼ 1 e.u. (Scheme 11.30).47

340


Scheme 11.30

The double bond was found to be essential for the reaction. Moreover when 9,10dideuterio BSK was heated, 1,8-dideuterio NHC was formed ruling out simple retro 2 þ 2 cycloaddition. It was suggested that a retro Diels– Alder reaction occurred to give syn-tricyclo[4.4.0.02,5]deca-3,7,9-triene (S), and this material underwent a 3,3shift to NHC (Scheme 11.31).

Scheme 11.31

Support for this pathway was provided when it was found that S rearranged to NHC with log k ¼ 11:5 2 23 500=2:3RT:48 3.8 Nenitzescu’s Hydrocarbon to cis-9,10-Dihydronaphthalene, 1,2-Dihydronaphthalene and 1-Phenylbutadiene NHC has been reported20 to give 1,2-dihydronaphthalene, C9,10, and 1phenylbutadiene in a flow system at 3008C in the ratio 10:3:1. A different distribution, 84:7:8, was reported at 2808C,39 and the reaction has log k ¼ 15:5 2 46 400=2:3RT (Scheme 11.32).49

Scheme 11.32

C10H6 – C10H16

341

It is possible that NHC undergoes a forbidden non-concerted ring opening to 422 which is known to give C9,10 (Section 3.6). However, in the flow system 422 gives C9,10 as the major product unlike NHC, so it was argued that a different path is required. It was suggested that NHC undergoes a stepwise or concerted 3,3-shift to S (or its anti isomer A) – see Section 3.7 – which gives some C9,10 as well as 620 which gives C4T. C4T gives T9,10 which gives 1,2-dihydronaphthalene and 1-phenylbutadiene (Scheme 11.33).

Scheme 11.33

Support for the intermediacy of 620 is given in Section 3.9. Evidence from deuterium scrambling results indicates that NHC also undergoes a degenerate rearrangement prior to structural isomerization (Scheme 11.34).47

Scheme 11.34

This is consistent with reversible formation of the 3,3-shift product, S, and supports, but does not require, the entire mechanism of Scheme 11.33.

342


3.9 Bicyclo[6.2.0]deca-2,4,6,9-tetraene to trans-9,10-Dihydronaphthalene Bicyclo[6.2.0]deca-2,4,6,9-tetraene (620) gives T9,10 with DH ‡ ¼ 25 kcal/mol and DS‡ ¼ 0 e.u. (Scheme 11.35).50

Scheme 11.35

This reaction was assumed to proceed via an allowed conrotatory opening to C4T which closes to T9,10 in an allowed disrotatory electrocyclization utilizing the trans double bond. Further, 9-deuterio-620 gives C9,10 with an olefin to alkyl proton ratio of 4.6,44 which suggests that conformational equilibration and double bond equilibration is complete within C4T before cyclization, e.g. Scheme 11.36 (see Section 3.2).

Scheme 11.36

3.10 Tetracyclo[4.4.0.0.2,507,10]deca-3,8-diene to trans-9,10-Dihydronaphthalene anti,anti-Tetracyclo[4.4.0.0.2,507,10]deca-3,8-diene or pteradactyladiene (PTD) has been reported to give naphthalene above 1308C presumably via trans9,10-dihydronaphthalene (T9,10) formed from C4T which is formed from 620 (Scheme 11.37).51

C10H6 – C10H16

343

Scheme 11.37

Earlier work with the 1,6-dicarbomethoxy derivative revealed that it gave a dicarbomethoxy-T9,10 derivative at 508C.52 It was proposed that non-concerted central bond ring opening occurred to give a cyclohexane-1,4-diyl stabilized by carbomethoxy groups, and this species ring opened to a bicyclo[6.2.0]deca2,4,6,9-tetraene which underwent an (in this case a doubly) allowed conrotatory ring opening to a cis,cis,cis,trans-cyclodecapentaene which is known to give T9,10 (Scheme 11.38). In the parent case, it may be more likely that a cyclobutene bond ruptures first.

Scheme 11.38

3.11 Snoutene Degenerate Rearrangement and Conversion to cis-9,10-Dihydronaphthalene Pentacyclo[3.3.2.0.2,40.3,706,8]deca-9-ene or snoutene (SNT), the silver ion catalyzed rearrangement product of basketene (BSK), undergoes a remarkable

344


degenerate rearrangement. Upon pyrolysis at 5008C in a flow system, the 9,10dideuterio material gives SNT with deuterium on the cyclopropane carbons remote from the double bond before formation of C9,10 (Scheme 11.39).53

Scheme 11.39

An allowed 2ss þ 2sa þ 2pa process was suggested as responsible for this skeletal rearrangement although a non-concerted process could not be ruled out. The C9,10 produced from the dideuterio-SNT had all the hydrogens scrambled as evidenced by a 2:2:1 ratio of the protons. The product was known to scramble deuterium under these conditions (see Section 3.2). Nonetheless, two pathways were proposed for the SNT to C9,10 conversion. One involved the 1,4-biradical that may also be on the pathway for the degenerate rearrangement of SNT that cleaves instead of adding to the other double bond (Scheme 11.40). However, this pathway would not give deuterium on the bridgehead position of C9,10.

Scheme 11.40

The second pathway involved a retro Diels– Alder reaction to 7-(30 -cyclopropenyl)norcaradiene which could open to the tropilidene which could undergo a series of 3,3-shifts that would scramble deuterium at all positions, then undergo a 2 þ 2 cycloaddition, which would undergo a 3,3-shift to TCD that is known to give C9,10 at high temperatures (see Section 3.5) (Scheme 11.41).

C10H6 – C10H16

345

Scheme 11.41

3.12 Hypostrophene Degenerate 3,3-Shift and 1,3-Shift to Pyrohypostrophene Tetracyclo[5.3.0.0.2,603,10]deca-4,8-diene or hypostrophene (HPS) undergoes what appears to be a degenerate 3,3-shift as evidenced by deuterium scrambling at 08C (Scheme 11.42).54

Scheme 11.42

An alternative pathway, namely, intramolecular 2 þ 2 cycloaddition to give pentaprismane (PPM) which undergoes a retro 2 þ 2 cycloaddition at a different cyclobutane ring is also possible. MMX calculations55 suggest that pentaprismane is only 19 kcal/mol less stable enthalpically than HPS although pentaprismane also has a lower entropy due to symmetry and rigidity than HPS. However, under normal circumstances, non-concerted formation of a 1,4-biradical from two ethylenes is

346


uphill enthalpically by about 35 kcal/mol, so this pathway may be inaccessible kinetically. Further, upon heating to 808C, a new isomer was produced, namely, tetracyclo[5.2.1.0.2,603,10]deca-4,8-diene, a 1,3-sigmatropomer of HPS, here named pyrohypostrophene (PHPS) (Scheme 11.42). 3.13

Diademane to Triquinacene to Azulene

The methyne-capped cis,cis,cis-trishomobenzene, diademane (DIA), was found to give triquinacene (TRQ) at 808C with log k ¼ 13:3 2 28 300=2:3RT (Scheme 11.43).56

Scheme 11.43

The reaction is formally a retro 2 þ 2 þ 2 cycloaddition and is probably concerted. At higher temperatures, TRQ gives azulene, and at still higher temperatures, naphthalene is formed most likely by isomerization of azulene (see Chapter 11, Section 2).57 It was suggested that TRQ first loses dihydrogen, then undergoes a 1,5vinyl shift resulting in ring contraction to a spiro compound whose ring opens to azulene (Scheme 11.44).

Scheme 11.44

Remarkably, at still higher temperatures, 1,2-dihydronaphthalene is also formed. It was suggested that TRQ undergoes a retro vinylcyclopropane rearrangement (most likely by rate-determining formation of a biradical – possibly a higher entropy process to account for the temperature dependence) to isobullvalene (IBV). IBV is known to give C9,10 which gives 1,2-dihydronaphthalene at high temperatures (Section 3.2) (Scheme 11.45).

C10H6 – C10H16

347

Scheme 11.45

3.14

Tetracyclo[5.3.0.0.2,1003,6]deca-4,8-diene

Tetracyclo[5.3.0.0.2,1003,6]deca-4,8-diene (TD) is converted to naphthalene and tetracyclo[5.3.0.0.2,1003,6]deca-4-ene as well as its rearrangement product, tricyclo[5.3.0.02,10]deca-3,5-diene (dihydroisobullvalene) in a 2:2 ratio, respectively, at 2008C to the extent of 20% after 1 h (Scheme 11.46).58

Scheme 11.46

The reaction is first order in TD, and since the starting material could be prepared from benzene-d6 , it was observed that deuterium was distributed everywhere in the product. It was suggested that TD opens to C5 in the rate-determining step, and it reacts with the starting material to produce a dimer which disproportionates to the observed products. 3.15 Tetracyclo[5.3.0.0.2,803,6]deca-4,9-diene to Tetracyclo[5.3.0.0.2,1003,6]deca-4,8-diene and Tricyclo[5.3.0.02,8]deca-3,5,9-triene to Isobullvalene to Lumibullvalene At high temperatures tetracyclo[5.3.0.0.2,8 03,6 ]deca-4,9-diene gave small isolated amounts of the exo isomer of tetracyclo[5.3.0.0.2,1003,6]deca-4,8-diene (Scheme 11.47).59

348


Scheme 11.47

The reaction would appear to be a 1,3-shift of the bicyclo[2.1.1]hexene variety (see Chapter 7, Section 3). What is interesting about this pyrolysis is that the starting material does not undergo a disrotatory ring opening to tricyclo[5.3.0.02,8]deca-3,5,9-triene, TDT. This triene was shown to give tricyclo[5.3.0.02,10]decatriene (isobullvalene), IBV, at 208C with a half-life of only 74 min, and this material gives lumibullvalene, LBV, with the same half-life at the same temperature (see Chapter 7, Section 3.4). Deuterium labeling, as shown in Scheme 11.48, suggests the intermediacy of an allylic –pentadienylic biradical which closes to two possible deuterium isomers of IBV ultimately giving two deuterium isomers of LBV.60

Scheme 11.48

3.16

Miscellaneous (CH)10 Hydrocarbons by Photolysis

The cyclopropyl-methyne-capped benzene derivative (CMB) is formed upon photolysis of tetracyclo[5.3.0.0.2,1003,6]deca-4,8-diene (TD) presumably via a 2 þ 2 cycloaddition (Scheme 11.49).61

C10H6 – C10H16

349

Scheme 11.49

Photolysis of triquinacene (TQR) gives among other products hexacyclo[4.4.0.0.2,40.3,100.5,807,9]decane or barettane (BAR) (Scheme 11.50).62 Also formed is pentacyclo[5.2.1.0.2,60.3,504,10]deca-8-ene (PDE) which was prepared independently.63 Finally, pentaprismane (PPM) has been synthesized.64

Scheme 11.50

3.17

Summary of (CH)10 Interconversions

Scheme 11.51 contains all the compounds examined here with the appropriate pathways and experimental65 (in bold) or calculated heat of formation of each by different techniques, either RHF/6-31Gp coupled with isodesmic reactions66 (in parentheses) or AM167 (in square brackets) or MMX molecular mechanics (in italics).68 Also included are the experimental enthalpies of activation and, where known, approximate free energies of activation (in bold). 3.18

9-Methylenebicyclo[6.1.0]nona-2,4,6-triene Pyrolysis

A rational synthesis of 9-methylenebicyclo[6.1.0]nona-2,4,6-triene by pyrolysis of an amine oxide led to 4-methylenebicyclo[5.2.0]nona-2,5,8-triene which upon further pyrolysis gave cis-1-methylene-8,9-dihydroindene (Scheme 11.52).69 It was suggested that the expected elimination product underwent a 3,3-shift to the observed 520 product, and formation of the dihydroindene occurred via a, possibly

350


Scheme 11.51

Scheme 11.52

C10H6 – C10H16

351

non-concerted, cyclobutene ring opening to 9-methylenecyclo-1,3,5,7-nonatetraene which then electrocyclized. 3.19

3-Vinylbenzocyclobutene to 1,2-Dihydronaphthalene

3-Vinylbenzocyclobutene gives 1,2-dihydronaphthalene upon pyrolysis at 2508C most reasonably via an appropriate geometric isomer of an o-xylylene which electrocyclizes (Scheme 11.53).70

Scheme 11.53

Presumably, the other stereoisomer of the o-xylylene recloses to the starting material. Interestingly, trans-3-propenylcyclobutene gives the methyldihydronapthalene, but the cis-propenyl derivative gives 1-o-tolyl-1,3-butadiene presumably via a 1,7-hydrogen shift in the o-xylylene (Scheme 11.54).

Scheme 11.54

4 4.1

C10H12 Aromatic Claisen Rearrangement and All Carbon Analogues

The isomerization of allylphenyl ether to o-allylphenol, the Claisen rearrangement, forms an important historical touchstone for 3,3-sigmatropic shifts and

352


still attracts mechanistic interest. The 3,3-shift product dienone is , 5 kcal/mol less stable than the starting material and is not usually observed since it undergoes a rapid, and most likely, catalyzed tautomerization to the phenol (Scheme 11.55).71

Scheme 11.55

The reaction occurs above 1508C and is first order with DH ‡ ¼ 32 kcal/mol and DS‡ ¼ 211 e.u.72 Experiments designed to distinguish between chair and boat transition states usually find the former although the difference may not be large.73 Concern over the possibility of ionic character in the 3,3-shift transition state led to an examination of the rates of rearrangement of substituted phenylallyl ethers with the observation of a linear free-energy correlations with sþ giving rþ values between 2 0.5 and 2 0.7 depending slightly on solvent polarity.74,75 It was found that the reaction is , 30 times faster in 28.5% ethanol – water mixtures than in tetradecane (White) and it has been reported to be 105 times faster in trifluoroacetic acid than in non-polar solvents76 suggesting hydrogen bonding if not proton transfer to the oxygen in the rate-determining 3,3-shift. Perhaps the best insight into transition state structure here is provided by deuterium kinetic isotope effects. At 1858C in methyl salicylate solvent, kH =kD2 at the a position was determined to be 1.18 and that at the g position was 1/1.06.77 Subsequent work confirmed these and provided heavy atom isotope effects which could be reproduced by B3LYP/6-31Gp calculations.78 The chair-like transition ˚ and the forming C –C bond was state structure had a C –O bond length of 2.19 A ˚ long which indicate a “loose” transition state more resembling an allyl 2.12 A radical and phenoxy radical rather than a cyclic diyl structure as in the Cope rearrangement of 1,5-hexadiene (see Chapter 7, Section 4.1). Finally, a multiparameter correlation79 of the meager solvent effect data indicates that solvent polarity as measured by the Kirkwood– Onsager function, ð1 2 1Þ=ð21 þ 1Þ; and hydrogen bonding as measured by the free energy of transfer of chloride ion between the solvents and gas phase both contribute to the modest solvent effect observed. The dominant effect is appropriate to each solvent,

C10H6 – C10H16

353

i.e. the Kirkwood– Onsager term in DMSO and hydrogen bond donation in ethylene glycol and water. Two other reactions have been observed in the pyrolyses of allylphenyl ethers, both subsequent to the 3,3-shift. One is the “abnormal Claisen” reaction of the product phenol from g-alkyl allylphenyl ethers. Here, a retro homo-1,5-dienyl hydrogen shift utilizing the hydroxyl proton in the product phenol results in a spirocyclopropyl dienone which undergoes the homo-1,5-dienyl hydrogen shift utilizing hydrogen from the original g-alkyl group. This reaction then slowly scrambles the original a- and g-carbons (Scheme 11.56).80

Scheme 11.56

The second “other” reaction is the ortho –ortho rearrangement which may scramble the original a- and g-carbons even in an unsubstituted case. The reaction is thought to proceed via an intramolecular Diels –Alder reaction and retro 2 þ 2 cycloaddition (Scheme 11.57).81 Finally, a spectacular degenerate oxo-Claisen rearrangement has been observed.82

Scheme 11.57

354 4.1.1


Attempts at an All-Carbon Claisen Rearrangement

Attempts to demonstrate an all-carbon analog of the Claisen rearrangement led to failure. In 1933, almost a decade before the discovery of the 3,3-shift of 1,5-hexadienes, 4-phenyl-1-butene was found to give only toluene and propylene at 5008C under conditions where the expected product, o-allyltoluene, is stable (Scheme 11.58).83

Scheme 11.58

Subsequent work with radical stabilizing substituents on the allyl group did not result in a 3,3-shift.84 Still later, it was pointed out that not only is the 3,3-shift substantially uphill, but the tautomerization may also not occur easily in an allcarbon system; so the parent system was pyrolyzed in the presence of strong base (potassium tert-butoxide), but only double bond isomerization products were isolated.85 Still later, labeled 4-(o-allylphenyl)-2-butene was heated at 3508C, but no label scrambling, possibly due to a reversible 3,3-shift, was observed. It was estimated that the activation energy for the 3,3-shift would be 32 – 35 kcal/mol if the activation entropy was 2 30 e.u. (Scheme 11.59).86

Scheme 11.59

The only early examples of an all-carbon Claisen rearrangement are in phenylvinylcyclopropane systems where relief of ring strain assists the bond cleavage.87 4.2

1,6-Trimethylenenorcaradiene

1,6-Trimethylenenorcaradiene was prepared in order to test the prediction that the norcaradiene – tropilidene equilibrium in this case would favor the diene.88 Indeed, this is the case from spectra and chemical reactions. Upon heating the diene above

C10H6 – C10H16

355

2008C, 1,2-trimethylenetropilidene (among other isomers) was obtained presumably via 1,5-alkyl shift of the cyclopropylmethylene (Scheme 11.60).

Scheme 11.60

4.3 cis-, trans-Bicyclo[6.2.0]deca-2,4,6-triene to 1,3,5,7-Cyclodecatetraene and Tetrahydronaphthalene cis-Bicyclo[6.2.0]deca-2,4,6-triene (cBDT) gives almost exclusively trans-1,2,9,10tetrahydronaphthalene at 1508C with DH ‡ ¼ 32:2 kcal/mol and DS‡ ¼ 3:4 e.u. (Scheme 11.61).89

Scheme 11.61

Only traces of the cis isomer were formed suggesting that the free energy for formation of it is at least 4 kcal/mol higher. Pyrolysis of cBDT with deuterium labeling on the saturated carbons resulted in initial products with deuterium still on the saturated carbons indicating that hydrogen shifts did not occur in the initial reactions (Scheme 11.61). This was of concern since the trans product was found to undergo a 1,5-hydrogen shift to 1,2,8,9-tetrahydronaphthalene under the reaction conditions. The initial isomerization is formally a 1,3-sigmatropic shift with allowed stereochemistry, but it was proposed that the reaction proceeded by an allowed conrotatory ring opening to a cis,cis,cis,trans-1,3,5,9-cyclodecatetraene which electrocyclized in an allowed disrotatory fashion to the major product (Scheme 11.62).

356


Scheme 11.62

Earlier it was found that photolysis of cBDT gave its trans isomer which was in rapid thermal equilibrium with what appeared to be trans,cis,cis,trans-cyclo1,3,5,7-decatetraene formed by an allowed conrotatory ring opening with DG‡ ¼ , 15 kcal/mol (Scheme 11.63).90

Scheme 11.63

Of interest is the fact that the ring opening of trans isomer did not occur to give the all cis isomer of the cyclodecatetraene despite the fact that this too is an allowed process. Even at higher temperatures, this isomer was not formed; instead, a 3,3-shift product, trans-5,6-divinylcyclohexadiene, was formed possibly by a chair-like transition state. Finally, a more recent report that all-trans-1,3,5,7,9-decapentaene is the primary product from photolysis of cBDT at all temperatures91 must be in error since the previous observations were repeated with the same result.92 4.4

Dispiro[2.2.2.2]deca-4,9-diene and Dispiro[2.0.2.4]deca-7,9-diene

Tsuji reported that at 1938C in di- and triglyme, dispiro[2.2.2.2]deca-4,9-diene gives p-diethylbenzene (Scheme 11.64).93

Scheme 11.64

C10H6 – C10H16

357

Fascinating is the observation that when the reaction is performed in the probe of an NMR spectrometer, the cyclopropyl protons of the starting material increase in intensity initially while the vinyl protons’ intensity decreased and even became an emission signal. The absorption signals decrease in intensity at about the same rate as the consumption of the starting material. All the ethyl protons of the product showed enhanced adsorption while all the aromatic protons gave emission signals (Scheme 11.64). It was suggested that two biradicals could be envisioned in the reaction: one resulting from initial cleavage of the C1 – C3 bond and the second resulting from the first by cleavage of the other cyclopropane ring. The intersystem-crossing in the unsymmetrical biradical could be responsible for the nuclear polarization from S ! To mixing.94 Closs noted that normal S ! To mixing should have produced a multiplet effect in the ethyl group of the products and suggested that the polarization was due to S ! T2 mixing, which should produce enhanced adsorption from the protons near the radical site in the products from the singlet species.95 In support of this contention, triplet-sensitized photolysis of the dispirodiene resulted in the starting material and p-diethylbenzene both exhibiting emission from the aliphatic protons, and no polarization of the olefinic or aromatic protons was found. It was further suggested that the weak emission from the olefinic and aromatic protons resulted from a nuclear Overhauser effect. Tsuji later pointed out that the polarization must have resulted from the pdiethanobenzene biradical in order that the aromatic and olefinic protons not be polarized.96 Subsequently, Tsuji reported rate constants for the disappearance of the spirodiene in triglyme but they increased in the presence of thiocreosol indicating reversible formation of the biradicals described above.97 Further, the dienes react at 1608C to give [8]paracycloph-4-enes although not in the presence of thiols. With TCNE, tetracyano[3.3]paracyclophanes are formed at 608C. In a related case, de Meijere found that dispiro[2.0.2.4]deca-7,9-diene gives o-ethylstyrene and tetralin in an 87:13 ratio upon heating at 1408C with a half-life of 29 min (Scheme 11.65).98 This reaction occurred much faster than that of the spiro[2.5]octa-4,6-diene which gives styrene itself suggesting that the second cyclopropane ring is involved in the rate-determining step, possibly sterically or electronically.

Scheme 11.65

358


4.5 Dispiro[2.0.2.4]deca-1,5-diene and Tetramethylene Dewar Benzene The pyrolysis of dispiro[2.0.2.4]deca-1,5-diene gives tetralin, benzocyclobutene, and styrene in a 2:2:1 ratio, respectively, along with traces of 4-ethynylspiro[2.5]hept-1-ene (Scheme 11.66).99

Scheme 11.66

A cyclopropene ring opening to vinylcarbene followed by intramolecular addition to tetramethylenebenzvalene was proposed to occur on the pathway to the major products (Scheme 11.67).

Scheme 11.67

As an aside, silver ion catalyzes the conversion of the dispirodiene to both 1,2and 1,4-tetramethylene Dewar benzene; the former gives tetralin at room temperature, but the latter requires heating to 1408C to give p-xylylene-like polymers (Scheme 11.68).100

Scheme 11.68

C10H6 – C10H16

359

The latter reaction may proceed through [4]p-cyclophane which was calculated (DZ þ dTCSCF CISD þ Q) to be 9.3 kcal/mol less stable than the 1,4tetramethylene Dewar benzene.101 Further, the computed transition state enthalpy for the interconversion was calculated to be 31.2 kcal/mol above the Dewar benzene. 4.6 Pentacyclo[5.3.0.02,60.3,508,10]decane and 1,5-Bishomocyclooctatetraene At 1008C, anti,syn,anti-pentacyclo[5.3.0.02,60.3,5.08,10]decane gives trans-1,5bishomo-COT with log k ¼ 13:94 2 31 380=2:3RT: This isomerization might appear to be a retro 2 þ 2 cycloaddition although it would not account for the trans stereochemistry of the product. The structure of the product was assigned on the basis of a temperature-dependent NMR, which indicated a degenerate conformational interconversion which had an activation free energy of 16.4 kcal/mol. At higher temperatures the product underwent a vinylcyclopropane rearrangement to tricyclo[5.3.0.02,4]deca-5,9-diene (Scheme 11.69).102

Scheme 11.69

In contrast, the anti,anti,anti-pentacyclodecane isomer required higher temperatures to give not only trans-bishomo-COT but also polymeric material with log k ¼ 14:12 2 36 520=2:3RT (Scheme 11.70).

Scheme 11.70

Similar observations were made with the diepoxides corresponding to the pentacyclodecanes; in fact, the activation parameters were very similar, a detail that led to the speculation that one of the central bonds underwent initial homolysis in each case rather than a bicyclo[2.1.0]pentane bond. These then ring open to different conformations of anti-tricyclo[5.3.0.08,10]deca-2,5-diene; only one of this could undergo a 3,3-shift to trans-bishomo-COT (Scheme 11.71).

360


Scheme 11.71

Finally, in a footnote, Ph. D. work at Ko¨ln (W.W. Frey with E. Vogel, 1966) was cited in which cis-1,5-bishomo-COT was found to give cis-b-ethylstyrene and syn-tricyclo[5.3.0.08,10]deca-2,5-diene (besides two unknowns) at 2508C (Scheme 11.72).

Scheme 11.72

The styrene may have come from the tricyclodecadiene via a cyclodecatetraene. 4.7

1,2,6,7-Cyclodecatetraene to 2,3-Divinyl-1,3-cyclohexadiene

1,2,6,7-Cyclodecatetraene was shown to give 2,3-divinyl-1,3-cyclohexadiene upon heating at 2008C (Scheme 11.73).103

Scheme 11.73

The reaction appears to be a 3,3-shift, and subsequently, it was demonstrated that it was the meso form that was involved which therefore requires a boat-like transition state.104 The kinetics for rearrangement of the meso isomer were determined and gave log k ¼ 12:53 2 29 750=2:3RT:105 Furthermore, oxygen trapping experiments lead to the conclusion that this reaction proceeded by rate-determining formation of a fused bicyclic bisallylic biradical which not only is 17 kcal/mol more stable than starting material and therefore has a barrier to reformation of starting material of roughly 47 kcal/mol, but also has an 18 kcal/mol barrier to ring open to

C10H6 – C10H16

361

the divinylcyclohexadiene product. Furthermore, 2,3-divinylcyclo-1,3-hexadiene isomerizes to 1,2-divinyl-1,3-cyclohexadiene with log k ¼ 12:27 2 37 830=2:3RT: This would appear to involve a 1,5-hydrogen shift, and this tetraene rapidly undergoes an electrocyclization to bicyclo[4.4.0]-deca-1(2),5(6),7-triene (Scheme 11.74).

Scheme 11.74

The meso and dl-trans-4,9-dimethyl-1,2,6,7-cyclodecatetraenes reacted with similar rates and the dl derivative was found to give two 3,3-shift isomers apparently via a chair transition state (with methyls either diequatorial or diaxial) with log k ¼ 10:98 2 29 580=2:3RT: However, oxygen trapping studies suggested that an intermediate was formed from the dl material with log k ¼ 12:38 2 28 900=2:3RT: Further, optically active 3,3-shift products were formed from optically active dl material. It was suggested that both concerted and biradical pathways were involved, and the former pathway involved a chair-like transition state (Scheme 11.75).105 Alternatively, the chair-like diyl might be involved, and it is trapped only inefficiently. For other chemistry involving the cyclic bisallene see ref. [106].

Scheme 11.75

362 4.8


Bishomobarrelene Pyrolyses

Pyrolysis of syn,anti-bishomobarrelene (tetracyclo[3.3.2.0.2,406,8]deca-9-ene) in a flow system at 6008C gives a different distribution of products than the anti,anti isomer even though both are slowly interconverted under the reaction conditions (Scheme 11.76).107

Scheme 11.76

Conformation-different trimethylene biradicals were proposed with the biradical from the syn,anti isomer giving more vicinal hydrogen shifted material than that from the anti,anti isomer, which gives mostly intramolecular 2 þ 2 product. Both also gave tetralin and cis-b-methylstyrene. The intramolecular 2 þ 2 product itself (anti-pentacyclo[5.2.1.0.2,90.3,506,8]decane) is the photolysis product of the syn,anti starting material and gives some retro 2 þ 2 product, hydrogen shifted product, and the aromatic compounds above (Scheme 11.77).108

Scheme 11.77

C10H6 – C10H16

363

2,10 3,6

4.9 endo-Tetracyclo[5.3.0.0. 0 ]deca-8-ene to Tricyclo[5.2.1.04,10]deca-2,5-diene endo-Tetracyclo[5.3.0.0.2,1003,6]deca-8-ene gives tricyclo[5.2.1.04,10]deca-2,5diene at 3008C (Scheme 11.78).109

Scheme 11.78

The reaction appears to be a homo-1,5-alkyl shift involving a cyclobutane ring carbon. Evidence that this is a concerted reaction comes from the observation of retention of configuration of the migrating carbon starting with cis-4,5-dimethyl material (Scheme 11.78).110 4.10 syn-Tricyclo[4.2.1.12,5]deca-3,7-diene to syn-Tricyclo[5.3.0.02,6]deca-3,9-diene syn-Tricyclo[4.2.1.12,5]deca-3,7-diene rearranges to syn-tricyclo[5.3.0.02,6]deca3,9-diene with DH ‡ ¼ 26:1 kcal/mol and DS‡ ¼ 22 e.u. (Scheme 11.79).111

Scheme 11.79

The reaction is a 3,3-shift necessarily via a boat-like transition state. A biradical path was ruled out since the 1,3-shift product, the standard cyclopentadiene dimer, was not observed. The transition state, however, probably closely resembles the biradical since the activation entropy is not strongly negative as is usually the case with chair-like 3,3-shifts (see Chapter 7, Section 4.1). 4.11

6-Allyl-3-methylene-1,4-cyclohexadiene to Butenylbenzene

Various 6-allyl-3-methylene-1,4-cyclohexadienes have been found to give butenylbenzenes upon standing at room temperature (Scheme 11.80).112

364


Scheme 11.80

Besides allyl, benzyl, and methyl groups also migrate.113 In every case, the reaction is intermolecular with an induction period involving equilibration of the allylic termini (in that case) and is inhibited by thiophenol, and related chain-transfer agents clearly indicating free radical processes.

4.12 2,5-Tetramethylene Dewar Benzene to 4-meta-Cyclophane to Tetralin and Tetramethyleneprismane to Tetralin and Tetramethylene Fulvene Flash vacuum pyrolysis of 2,5-tetramethylene Dewar benzene gives tetralin. However, the intermediacy of 4-meta-cyclophane and its benzvalene equivalent has been implicated in the reaction with deuterium labeling being consistent with the observations (Scheme 11.81).114

Scheme 11.81

C10H6 – C10H16

365

Interestingly, photolysis of the Dewar benzene led to a tetramethyleneprismane which upon standing at room temperature gave tetralin and a fulvene.115 The former was presumed to occur via a 1,2-tetramethylene Dewar benzene, but the latter product, when derived from the deuterated material of Scheme 11.81, had a deuterium distribution that was rationalized by formation of two different tetramethylene benzvalenes. These benzvalenes then were assumed to give the fulvene by bond cleavages to give a cyclopentadienyl carbene. Unfortunately, the major fulvene product from deuterium labeling was argued to be derived from a benzvalene, which was indicted as responsible for tetralin in the flash vacuum pyrolysis of 2,5tetramethylene Dewar benzene. Perhaps this is the result of the temperature differential between the two reactions, although the temperature independence of the ratio in the room temperature pyrolysis of the tetramethyleneprismane was noted. 5

C10H14

5.1 5-Vinylbicyclo[2.2.2]octa-2-ene to Bicyclo[4.4.0]deca-2,8-diene and Retro 4 1 2 Cycloaddition Pyrolysis of 5-endo-vinylbicyclo[2.2.2]octa-2-ene gives the 3,3-shift product, bicyclo[4.4.0]deca-2,8-diene, with log k ¼ 12:8 2 44 200=2:3RT: Pyrolysis of the corresponding exo isomer gives both the 4.4.0 diene and the retro Diels– Alder fragments in nearly a 1:1 ratio at roughly one-fourth the rate of the endo isomer (Scheme 11.82).116

Scheme 11.82

A temperature study revealed that the exo isomer had a DH ‡ roughly1.8 kcal/mol higher than the endo isomer. It was suggested that the reactions of both isomers involve biradical-like transition states. The 3,3-shift must involve a boat-like

366


transition state, and these are usually “looser,” or more dissociative, than chair-like 3,3-shifts (see Chapter 7, Section 4.1). Pyrolysis of the 5-hydroxyl-substituted derivatives gives similar results except that the enols produced give ketones and the exo-vinyl derivative undergoes a transannular hydrogen shift, possibly via a biradical rather than undergoing the retro 4 þ 2 cycloaddition (Scheme 11.83).117

Scheme 11.83

That enols were products from pyrolysis of the alcohols was verified by pyrolysis of the corresponding methyl ethers. The major product from the endo-vinyl derivative was the 3,3-shift valence tautomer while elimination of methanol was the major pathway from the exo isomer although again, both the 3,3-shift product and the retro 4 þ 2 products were also formed (Scheme 11.84).118

Scheme 11.84

C10H6 – C10H16

367

The results of pyrolysis of the alcohols stand in stark contrast to those of the corresponding conjugate bases. Thus pyrolysis of the potassium salt of the 4-methoxy-endo-5-vinyl alcohol occurred rapidly in boiling tetrahydrofuran containing one-equivalent of 18-crown-6 with log k ¼ 11:5 2 18 200=2RT:119 This suggests a 1017-fold rate increase in the 3,3-shift relative to the conjugate acid. Of importance is the fact that the exo-vinyl-endo-alkoxide did not rearrange (Scheme 11.85).

Scheme 11.85

The origin of the rate effect is in the enormous weakening of the doubly allylic bond by the oxy-anion.120 Thus the transition state for this 3,3-shift, which more resembles two allyl radicals because it is boat-like, is accelerated by the decreased strength of the s bond (see Chapter 7, Section 4.1).

5.2

Bicyclo[6.2.0]deca-2,6-diene Degenerate Rearrangement

Bicyclo[6.2.0]deca-2,6-diene undergoes a 3,3-shift resulting in a degenerate skeletal rearrangement at 1858C. However, the temperature necessary to affect the rearrangement is about 1008C higher than that for the 3,3-shift in cis-1,2divinylcyclobutane itself. Moreover, stereospecific deuterium labeling of C4 and C5 lead to stereospecific labeling of the cyclobutane ring in a manner that suggests a “laid back” conformation for the 3,3-shift transition state, not unlike that observed with the parent system (see Chapter 9, Section 4). However, in this bicyclic case, steric effects would destabilize this transition state making it higher in energy than in the parent case (Scheme 11.86).121

Scheme 11.86

368


5.3 cis-, trans-4-Methylenebicyclo[5.2.0]nona-2-ene to 5-Methylene-1,3-cycloheptadiene and 2-Methylenebicyclo[3.2.2]nona-6-ene Pyrolysis of cis- and trans-4-methylenebicyclo[5.2.0]nona-2-ene in a flow system at 5008C gave the retro 2 þ 2 cycloaddition product, 5-methylene-1,3-cycloheptadiene, as well as the 1,3-shift product, 2-methylenebicyclo[3.2.2]nona-6-ene (Scheme 11.87).122

Scheme 11.87

A biradical was suggested as an intermediate for both reactions. At higher temperatures, the 1,3-shift product also underwent a retro 4 þ 2 reaction. An allowed 2s þ 2a pathway was considered for the retro 2 þ 2 cycloaddition mechanism but was discarded on the basis of the supposition that the trans material would give substantially more of this product than the cis isomer. 5.4

1,2,8,9-Decatetraene Pyrolysis

1,2,8,9-Decatetraene gives 7,8-dimethylene-cis-bicyclo[4.2.0]octane and 3,4dimethylenecyclooctene in a 1:3 ratio, respectively, at temperatures well above 3008C in a flow system (Scheme 11.88).123

Scheme 11.88

C10H6 – C10H16

369

The reaction would appear to be an intramolecular allene dimerization since both products could reasonably be derived from bicyclo[6.2.0]deca-1,7-diene (Scheme 11.88). The kinetics gave log k ¼ 9:53 2 31 200=2:3RT: Further both cis- and trans-7,8dimethylenebicyclo[4.2.0]octane were prepared and found to be stable under the reaction conditions, although at higher temperatures, the triene was formed presumably via bicyclo[6.2.0]deca-1,7-diene (Scheme 11.89).

Scheme 11.89

5.5 9-Methylenebicyclo[6.1.0]nona-2-ene to 3-Methylene-1,4-cyclononadiene Pyrolysis of the appropriate cyclopropylcarbinyldimethylamine oxide at 1508C to synthesize 9-methylenebicyclo[6.1.0]nona-2-ene resulted in the formation of 3-methylene-1,4-cyclononadiene (Scheme 11.90).124

Scheme 11.90

It would appear that the synthesis was successful, but the product rearranged via a homodienyl 1,5-hydrogen shift (see Chapter 7, Section 4). 5.6 2-(2-Methylene-3-butenyl)methylenecyclobutane to 1-(3-Methylene-4-pentenyl)cyclobutene to 3,6-Dimethylene-1,7-octadiene At 1708C, 2-(2-methylene-3-butenyl)methylenecyclobutane undergoes a reversible 3,3-shift to 1-(3-methylene-4-pentenyl)cyclobutene which irreversibly ring opens to 3,6-dimethylene-1,7-octadiene (Scheme 11.91).125 An independent pyrolysis of the cyclobutene allowed calculation of the rate constants of Scheme 11.91.

370


Scheme 11.91

For comparison, the 3,3-shift in the triene was 91 times faster than that of 2isobutenylmethylenecyclobutane indicating substantial stabilization of a cyclohexane-1,4-diyl transition state by a radical-stabilizing group at the 2-position of this species (see Chapter 7, Section 4.1). 5.7 3,6-Dimethylene-1,7-octadiene Pyrolysis and Bicyclo[4.2.2]deca-1,5-diene Pyrolysis of 3,6-dimethylene-1,7-octadiene at 1908C gave two intramolecular 2 þ 2 cycloadducts, probably via biradicals, as well as 2,6-dimethylenebicyclo[3.3.0]octane (Scheme 11.92).126

Scheme 11.92

The latter material is formed from one of the cycloadducts, 5-vinyl-2methylenebicyclo[3.2.0]heptane, possibly via a 3,3-shift of an intermediate bridgehead bisolefin formed from a biradical precursor to vinylmethylenebicyclo[3.2.0]heptane. Remarkably, at 4008C an intramolecular 4 þ 2 adduct is formed, namely, 6methylenebicyclo[3.3.1]nona-1-ene, and this equilibrates with its 1,3-shift isomer, 2,5-dimethylenebicyclo[2.2.2]octane (Scheme 11.93).127

C10H6 – C10H16

371

Scheme 11.93

In a related observation, it was found that the bridgehead bisolefin, bicyclo[4.2.2]deca-1,5-diene gave 2,5-dimethylenebicyclo[4.2.0]octane at room temperature with an activation energy of 19.6 kcal/mol, presumably via a 3,3-shift (Scheme 11.94).128

Scheme 11.94

5.8

1,3,7,9-Decatetraene Potentially Degenerate 5,5-Shift

In an effort to uncover a 5,5-sigmatropic shift of trans,trans-1,37,9-decatetraene, the meso- and threo-5,6-dimethyl derivatives were pyrolyzed in a flow system at 3758C (residence time , 1 s). A complicated mixture of tetrenes was formed which was virtually the same from both meso and threo starting materials (Scheme 11.95).129

Scheme 11.95

The products were the result of 3,5-, 1,5-, and 5,5-shift processes in roughly equal amounts. Furthermore, pyrolysis of a mixture of dideuterio and perproptio starting materials gave the 3,5- and 5,5-shift products with the monodeuterio isomer predominating, indicating a cleavage-recombination reaction path and not concerted, intramolecular reactions.

372


The 3,5-shift products were also found to be unstable under the reaction conditions and gave the 1,5- and 5,5-shift products, as well as starting material. This observation suggests that the potential 3,3-shift products might not be stable under the reaction conditions and may give two a-methylpentadieneyl radicals which combine in various ways (Scheme 11.96).

Scheme 11.96

The pyrolysis conditions suggest a free energy of activation of , 43 kcal/mol for the reaction at 3708C. For comparison the 3,3-shift of 3-methyl-1,5-hexadiene has a free energy of activation of 41 kcal/mol at 2208C, and the expected BDE of the C4 – C5 bond in 1,3,7,9-decatetraene is approximately 42 kcal/mol. 5.9

Bornadiene Pyrolysis

Bornadiene undergoes a thermal conversion to 1,7,7-trimethylcycloheptatriene, p-cymene, and 1,5,5-trimethylcyclopentadiene (Scheme 11.97).130

Scheme 11.97

The latter product is the result of a retro Diels –Alder reaction, but the former two would appear to be formed from C1 –C7 cleavage to a biradical which either cyclizes to caradiene, then ring opens to the seven-membered ring or undergoes a vicinal hydrogen shift to p-cymene. Alternatively, the first step might be a concerted 1,3-shift to the caradiene. To examine the stereochemistry, (2 )-(7R)-8-deuteriobornadiene was pyrolyzed and the trimethylcycloheptatriene formed was hydroxylated at the C3 methyl and converted to the hydrotropyl ester to make the C7 methyls diastereotopic to allow the determination that deuterium was incorporated into both methyls to an equal extent. A control experiment verified that the bornadiene was stable under the reaction conditions. Thus the observation is

C10H6 – C10H16

373

consistent with formation of a Cs symmetric biradical intermediate formed after rate-determining ring opening of bornadiene (Scheme 11.97).131 5.10 Tetracyclo[4.2.1.1.1,602,5]decane to 2,4-Dimethylenebicyclo[3.2.1]octane Tetracyclo[4.2.1.0.1,602,5]decane undergoes a retro 2 þ 2 cycloaddition at high temperatures (3258C) in contrast to the same reaction of [2.2.2]propellane which reacts at room temperature (see Chapter 9, Section 4) (Scheme 11.98).132

Scheme 11.98

5.11

cis-1,2,4-Cyclodecatriene to trans-Bicyclo[4.4.0]deca-2,4-diene

Pyrolysis of cis-1,2,4-cyclodecatriene at 1008C gives trans-bicyclo[4.4.0]deca-2,4diene (Scheme 11.99).133

Scheme 11.99

This remarkably stereospecific conversion was attributed to a 1,5-hydrogen shift to trans,cis,cis-1,3,5-cyclodecatriene which electrocyclizes in an allowed disrotatory fashion. The stereoselectivity of the hydrogen shift was attributed to an allowed suprafacial process via the conformation depicted in Scheme 11.99. 5.12

cis-Bicyclo[7.1.0]deca-2,3-diene to Tricyclo[5.3.0.02,4]deca-5-ene

Only one of the two diastereomers of cis-bicyclo[7.1.0]deca-2,3-diene rearranged upon heating to 2508C in a flow system, and the major product was found to be tricyclo[5.3.0.02,4]deca-5-ene.133 The stereochemistries of the reactant and product were reasonably assigned on the basis of the stereoelectronic requirements for

374


the steps depicted in Scheme 11.100, namely a homodienyl-1,5-hydrogen shift to trans,cis,cis-1,3,6-decatriene followed by an intramolecular Diels– Alder reaction.

Scheme 11.100

6 6.1

C10H16 1,5-Cyclodecadiene 3,3-Shift

Pyrolysis of cis,trans-1,5-cyclodecadiene gives greater than 99% cis-1,2-divinylcyclohexane at 1208C with a half-life of 7 h.134 The reaction stereochemistry is consistent with a chair-like transition state (See Chapter 7, Section 4.1) (Scheme 11.101).

Scheme 11.101

By contrast, pyrolysis of trans,trans-1,5-cyclodecadiene gave trans-1,2-divinylcyclohexane upon heating to 408C with a half-life of 144 min.135 This reaction too requires a chair-like transition state (Scheme 11.102). Unpublished work indicated that this reaction is exothermic by 6.7 kcal/mol.136

Scheme 11.102

C10H6 – C10H16

6.2

375

1,6-Cyclodecadiene Intramolecular 2 1 2 Cycloaddition

Pyrolysis of trans,trans-1,6-cyclodecadiene at 2008C gives cis-bicyclo[5.3.0]deca-2-ene (72%) and both syn- and anti-tricyclo[5.3.0.02,6]decane (24 and 4%, respectively).137 A similar product distribution was obtained from cis,cis-1,6cyclodecadiene, but higher temperatures (3208C) were required. It is reasonable that a similar biradical is involved in both reactions (Scheme 11.103).

Scheme 11.103

6.3

trans-1,3,9-Decatrienes to Bicyclo[4.4.0]deca-2-enes

trans-1,3,9-Decatriene was found to give a nearly 1:1 ratio of the intramolecular Diels – Alder products,138 cis- and trans-bicyclo[4.4.0]deca-2-ene (octahydronaphthalene), upon heating to 1908C (Scheme 11.104).139

Scheme 11.104

The activation parameters for the formation of the cis and trans products are log k ¼ 7:06 2 25 410=2:3RT and log k ¼ 7:11 2 25 560=2:3RT; respectively,140 with activation volumes of 2 37.6 and 2 35.0 cc/mol, respectively.141 Activation of the terminal mono-olefin by ester substitution in either an E or Z fashion increased the rate but did not alter the stereoselectivity; however, Lewis acids not only dramatically increased the rate of cycloaddition of the E ester, but also increased the stereoselectivity to . 99% preference for the trans isomer.138 On the other hand, simple methyl substitution at C3 of the parent hydrocarbon gave almost exclusively the trans-octahydronaphthalene presumably due to steric

376


destabilization of the transition state leading to the cis-fused product in which the saturated chain is in a chair-like arrangement (Scheme 11.105).142

Scheme 11.105

Activation of the 1,3,9-decadienes by oxo substitution at C7 in the form of a ketone or an ester or an amide leads mostly to cis-fused products via a boat-like arrangement of the six-membered ring133 and this is also favored by ab initio and DFT calculations (Scheme 11.106).143

Scheme 11.106

cis-1,3,9-Decatrienes require much higher temperatures to react in not always a predictable fashion144 although Lewis acid catalysis has led 4 þ 2 reactions in heavily substituted cases.145

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Hydrocarbon Thermal Isomerization G.B.M. Kostermans, P. van Dansik, W.H. de Wolfe, and F. Bickelhaupt, J. Org. Chem., 53, 4531 (1988). G.B.M. Kostermans, M. Hogenbirk, L.A.M. Turkenburg, W.H. de Wolf, and F. Bickelhaupt, J. Am. Chem. Soc., 109, 2855 (1987). J.A. Berson and E.J. Walsh, Jr., J. Am. Chem. Soc., 90, 4730 (1968). J.A. Berson and M. Jones, Jr., J. Am. Chem. Soc., 86, 5017 (1964), see also p. 5019. J.A. Berson and E.J. Walsh, Jr., J. Am. Chem. Soc., 90, 4729 (1968). D.A. Evans and A.M. Golob, J. Am. Chem. Soc., 97, 4765 (1975). M.L. Steigerwald, W.A. Goddard, III, and D.A. Evans, J. Am. Chem. Soc., 101, 1994 (1979). See also J.J. Gajewski and K.R. Gee, J. Am. Chem. Soc., 113, 967 (1991). W. Grimme, J. Am. Chem. Soc., 94, 2525 (1972). L.A. Paquette, R.P. Henzel, and R.F. Eizember, J. Org. Chem., 38, 3257 (1973). R. Sato, MS Thesis, Indiana University (1973). See also L. Skattebøl and S. Solomon, J. Am. Chem. Soc., 87, 4506, (1965). P. Radlick, W. Fenical, and G. Alford, Tetrahedron Lett., 2707 (1970). K.J. Shea and S. Wise, J. Org. Chem., 43, 2710 (1978). K.J. Shea and S. Wise, Tetrahedron Lett., 2283 (1978). K.J. Shea and S. Wise, J. Am. Chem. Soc., 100, 6519 (1978). J.R. Wiseman and J.J. Vanderbilt, J. Am. Chem. Soc., 100, 7730 (1978). D.H. Gibson and R. Pettit, J. Am. Chem. Soc., 87, 2620 (1965). M.R. Willcott, III and C.J. Boriack, J. Am. Chem. Soc., 90, 3287 (1968). M.R. Willcott, III and C.J. Boriack, J. Am. Chem. Soc., 93, 2354 (1971). D.H. Aue and R.N. Reynolds, J. Org. Chem., 39, 2315 (1974). D.E. Minter, G.J. Fonken, and F.T. Cook, Tetrahedron Lett., 711 (1979). P. Heimbach, Angew. Chem. Int. Ed. Engl., 3, 702 (1964). P.S. Wharton and R.A. Kretchmer, J. Org. Chem., 33, 4258 (1968). D.J. Johnson, Ph.D. Thesis, University of Wisconsin, Madison (1968), cited by S.F. Nelson, J.P. Gillespie, J. Am. Chem. Soc., 94, 6237 (1972). W.R. Roth, Chimia, 20, 229 (1966). Reviews: B. Ciganek, Org. React., 32, 1 (1984); D. Craig, Chem. Soc. Rev., 16, 287 (1987); W.R. Roush, Comprehensive Organic Synthesis (B.M. Trost, ed.), Pergamon Press, Oxford 5, 513– 550 (1991). Y.-T. Lin and K.N. Houk, Tetrahedron Lett., 2269 (1985). M.K. Diedrich, F.-G. Kla¨rner, B.R. Beno, K.N. Houk, H. Senderowitz, and W.C. Still, J. Am. Chem. Soc., 119, 10255 (1997). M.K. Diedrich, and F.-G. Kla¨rner, J. Am. Chem. Soc., 120, 6212 (1998). S.R. Wilson and D.T. Mao, J. Am. Chem. Soc., 100, 6289 (1978); S.R. Wilson and D.T. Mao, J. Org. Chem., 44, 3093 (1979). D.J. Tantillo, K.N. Houk, and M.E. Jung, J. Org. Chem., 66, 1938 (2001); G.A. Jones, M.N. Paddon-Row, M.S. Sherburn, and C.I. Turner, Org. Lett., 4, 3789 (2002). W. Oppolzer, Comprehensive Organic Synthesis (B.M. Trost, ed.), Pergamon Press, Oxford, Vol. 5, pp. 315– 398 (1991). For an earlier analysis see H.O. House, and T.H. Cronin, J. Org. Chem., 30, 1061 (1965). N. Yakelis, and W.R. Roush, Org. Lett., 3, 957 (2001).

12 C11H10 – C11H16

CONTENTS 1 C11H10 1.1 Methano[10]annulene and Tricyclo[4.4.1.01,6]undeca2,4,7,9-tetraene 1.2 Spiro[4.6]undecapentaene to Bicyclo[6.3.0]undecapentaene and Phenylcyclopentadiene 1.3 2,3-Benzonorcaradiene, Benzocycloheptatrienes, and 2,3-Benzonorbornadiene to 1,2-Benzocycloheptatriene 2 C11H12 2.1 1-Vinylspiro[2.6]nonatriene to Bicyclo[5.4.0]undeca-1,3,5, 9-tetraene and Phenyl Analogues 2.2 1-Phenyl-2-vinylcyclopropane to 4-Phenylcyclopentene and 3,3-Shift 2.3 7-Carbethoxytricyclo[5.4.0.02,11]undeca-3,5,9-triene 2.4 o-Allylstyrene to 1-Methyl-1,2-dihydronaphthalene and Benzonorpinene 2.5 Tricyclo[4.2.2.12,5]undeca-3,7,9-triene and Pentacyclo[5.4.0.0.2,50.3,904,8]undeca-10-ene Rearrangement and Cleavage 2.6 Bicyclo[4.4.1]undeca-2,4,7,9-tetraene to cis-Bicyclo[5.4.0]undeca-2,4,8,10-tetraene 2.7 9-Vinylbicyclo[6.1.0]nona-2,4,6-triene 3 C11H14 3.1 1,2- and 1,4-Pentamethylene Dewar Benzene to Benzocycloheptene, 8-Methylenespiro[4.5]deca-6,9-diene and 5-meta-Cyclophane 3.2 7-(3-Butenyl)cycloheptatriene Intramolecular Diels –Alder Reactions 4 C11H16 4.1 5-(50 -Penten-1-yl)-1,3-cyclohexadiene Pyrolysis References

382 382 383 384 388 388 389 391 391

392 393 393 394

394 395 396 396 397

382 1


C11H10

1.1 Methano[10]annulene and Tricyclo[4.4.1.01,6]undeca-2,4,7,9-tetraene One of the spectacular achievements in the syntheses of conjugated p electron systems is that of 1,6-methano[10]annulene.1 The material is aromatic as evidenced by chemical reactivity and by NMR chemical shifts which are independent of temperature.2 Interestingly, the annulene was prepared by synthesis of its valence tautomer, tricyclo[4.4.1.01,6]undeca-2,4,7,9-tetraene, a bridged norcaradiene (Scheme 12.1).

Scheme 12.1

An estimate of the relative energies of the two tautomers favored the pentaene by 4.5 kcal/mol.3 Interestingly, geminal-dimethylsubstitution on the one carbon bridge results in an NMR spectrum that is temperature-dependent suggesting that the two tautomers are in equilibrium with the annulene being the more stable (Scheme 12.2).4

Scheme 12.2

Finally, the 13C NMR spectrum of 11-cyano-11-methyl[10]annulene indicates an upfield shift of C1 and C6 as the temperature is lower suggesting that the norcaradiene form is more stable in this case (Scheme 12.3) (see Chapter 8, Section 2 and ref. 2).

Scheme 12.3

C11H10 – C11H16

383

1.2 Spiro[4.6]undecapentaene to Bicyclo[6.3.0]undecapentaene and Phenylcyclopentadiene Upon photolysis of diazocyclopentadiene in benzene, a norcaradiene-like adduct, spiro[4.6]undecapentaene, was obtained which gave bicyclo[6.3.0]undecapentaene at 708C in 24% yield along with polymeric material (Scheme 12.4).5

Scheme 12.4

The reaction appears to be a 1,5-shift of a cycloheptatriene (the retroelectrocyclization product of the norcaradiene) ring carbon over the cyclopentadiene system which is followed by a 1,5-hydrogen shift. The equilibrium between the norcaradiene and the cycloheptatriene is sensitive to substituents, and, further, the interconversion is fast at room temperature.6 Finally, dimethyl substitution at the migration termini on the cyclopentadiene frustrate the rearrangement so that dimethylphenylcyclopentadienes are formed.7 Presumably, this occurs by cleavage of the other bond of the norcaradiene cyclopropyl moiety followed by a vicinal hydrogen shift to the aromatic compound. This then undergoes 1,5-hydrogen shifts in the cyclopentadienyl ring (Scheme 12.5).

Scheme 12.5

384


1.3 2,3-Benzonorcaradiene, Benzocycloheptatrienes, and 2,3-Benzonorbornadiene to 1,2-Benzocycloheptatriene One of the first isolable norcaradienes was that formed by addition of methylene to the a,b-bond of naphthalene to give 2,3-benzonorcaradiene.8 At around 1808C in the NMR, the methylene protons of this norcaradiene become equivalent, and an approximate line shape analysis allowed determination of log k ¼ 11:79 2 19 400=2:3RT for this conversion which presumably involves the 2,3-benzocycloheptatriene (Scheme 12.6).

Scheme 12.6

At higher temperatures, 2,3-benzonorcaradiene gave 1,2-benzocycloheptatriene.9 Pyrolysis of the 7,7-dideuterio derivative gave the 1,2-benzocycloheptatriene with deuterium distributed over C3, C4, C6, and C7 in a 1/3:1/2:1/2:2/3 ratio, respectively (Scheme 12.7), without any scrambling of deuterium in the starting material.10

Scheme 12.7

Two possible pathways were envisioned for the reaction: (a) cyclopropane to propylene-like rearrangement followed by 1,5-hydrogen shifts, that can equilibrate C4 and C6 as well as C3 and C7 with a slower 1,5-deuterium shift (due to the primary isotope effect) and (b) the second pathway would involve a retroelectrocyclization to a cycloheptatriene destroying the aromatic p system in the process, and this undergoes a 1,5-deuterium shift to the 1,2-benzocycloheptatriene which subsequently undergoes a 1,5-hydrogen shift to equilibrate C4 and C6 but also must equilibrate C3 with C7. Further, a 1,5-deuterium shift in the 7-deuterio material gives the isomer from path (a) (Scheme 12.8). Either pathway can account for the deuterium distribution. If equilibration were complete, there would be one deuterium distributed between C3 and C7 in a 2:1 ratio and there would be one deuterium distributed equally between C4 and C6. (This is best seen if the two hydrogens distributed between C3 and C7 were labeled.) If, however, equilibration were complete only for the first 1,5-hydrogen shift in each

C11H10 – C11H16

385

Scheme 12.8

pathway with no deuterium shift, then by path (a) the ratio of deuterium at C7 and C3 would be 2:1 as would that at C6 and C3; however, by path (b) while the ratio of deuterium at C7 and C3 would be 2:1, the ratio at C6 and C3 would be 1:2. Since there was a 1:1 ratio of deuterium at C3 and C6, the 1,5-deuterium shift must have occurred at a rate comparable to the hydrogen shift thus removing evidence that might distinguish between the two paths. At still higher temperatures, 1,2-benzocycloheptatriene gives naphthalene, and both a- and b-methylnapthalene and 1,2-benzo-1,3-cycloheptadiene apparently by a radical chain process.11 3,4-Benzocycloheptatriene gives its 1,2-isomer at 4208C with extensive scrambling of deuterium when starting with the 1,6-dideuterio-3,4-benzo material.12 In addition, recovered starting material had substantial quantities of deuterium distributed over the benzene ring (Scheme 12.9).

Scheme 12.9

386


The scrambling of deuterium in 3,4-benzo starting material was ascribed to a 1,5-hydrogen shift followed by reversible norcaradiene –cycloheptatriene electrocyclization, and subsequent 1,5-hydrogen shifts could account for the 1,2-benzo isomer (Scheme 12.10).

Scheme 12.10

However, the equilibration of the first row of the scheme could not be complete or else deuterium would be incorporated into other positions on the original cycloheptatriene ring. To rationalize the formation of the 1,2-benzo product, a 1,5-hydrogen shift in the first intermediate of the first row of the scheme would lead to another intermediate which could undergo a 1,5-hydrogen and a 1,5-deuterium shift and account for labeling at C3, C4, C6, and C7. However, because none of these 1,5-hydrogen shift pathways could give deuterium at C5 of the product 1,2benzo isomer, a norcaradiene –norcaradiene rearrangement was also proposed as a competing pathway. Returning to the deuterium equilibration in the 3,4-benzo starting material, the interesting spirobisnorcaradiene of Scheme 12.10 was isolated as its diphenyl derivative, and it was found to give 3,4-benzocycloheptatriene with DH ‡ ¼ 15:2 kcal=mol and DS‡ ¼ 221:2 e:u:; which indicates a rapid reaction at ambient temperatures (Scheme 12.11).13

C11H10 – C11H16

387

Scheme 12.11

Finally, 2,3-benzonorbornadiene was found to give 1,2-benzocycloheptatriene at 3808C.10 The 5,6-dideuterio isomer gives initially deuterium mostly at C4, C5, and C6 which subsequently scrambles by 1,5-hydrogen and deuterium shifts (Scheme 12.12).

Scheme 12.12

A 1,3-sigmatropic shift to the 2,3-benzonorcaradiene followed by rearrangement to the 1,2-benzocycloheptatriene which could undergo 1,5-hydrogen shifts would account for an initial deuterium distribution of 0.5:1.0:0.5 at C4, C5, and C6, respectively. The further scrambling may occur by 1,5-deuterium shifts in 2,3benzocycloheptatriene because it had been shown earlier that 3,5,7,7-tetradeuterio1,2-benzocycloheptatriene scrambles deuterium to C4 and C6 at 3808C, which may be interpreted in terms of the intermediacy of 2,3-benzocycloheptatriene (Scheme 12.13).14

388


Scheme 12.13

2

C11H12

2.1 1-Vinylspiro[2.6]nonatriene to Bicyclo[5.4.0] undeca-1,3,5,9-tetraene and Phenyl Analogues The photolysis products of diazocycloheptatriene and trans- and cis-piperylene undergo a 3,3- (or a 3,7-) shift at or slightly above room temperature, respectively, to give a mixture of 8-methylbicyclo[5.4.0]undeca-1,3,5,9-tetraenes (Scheme 12.14).15

Scheme 12.14

The adduct of cis-piperylene undergoes the rearrangement with DH ‡ ¼ 24 kcal=mol and DS‡ ¼ 24:8 e:u: It was argued that biradicals were involved since the activation enthalpy is about that estimated for the strength of the cyclopropane bond if the resonance energy of the cycloheptatrienyl radical is 21 kcal/mol or greater.16

C11H10 – C11H16

389

Further, the double bond termini which must interact in a concerted reaction are not close to one another. Finally, since mixtures of stereoisomers were formed, biradical intermediates seem appropriate. Interestingly, the adduct of cycloheptatrienylidene and styrene gave 9phenylbicyclo[5.2.0]nona-1,3,5-triene at 758C,15 a reaction that appears to be a 1,7-shift but may also involve biradicals. At higher temperatures this product gave 2-phenylindane presumably via a norcaradiene which is also a bicyclo[2.1.0]pentane which could undergo central bond homolysis followed by a vicinal hydrogen shift (Scheme 12.15).

Scheme 12.15

Other derivatives have been examined.17

2.2 1-Phenyl-2-vinylcyclopropane to 4-Phenylcyclopentene and 3,3-Shift In an attempt to provide an example of an all-carbon aromatic Claisen rearrangement, cis-1-phenyl-2-vinylcyclopropane was pyrolyzed. However, at 1508C only geometric isomerization was observed with DH ‡ ¼ 32 kcal=mol and DS‡ ¼ 25:8 e:u:18 At still higher temperatures the 1,3-shift product, 4-phenylcyclopentene, was formed with log k ¼ 13:38 2 41 000=2:3RT (Scheme 12.16).19

Scheme 12.16

More recent work established that the ratio of trans- to cis-1-phenyl-2vinylcyclopropane at 216.48C is roughly 4, and that the latter gives the 1,3-shift product with a rate constant roughly 2.3 times faster than the former.20 Furthermore,

390


optically active trans material gave nearly racemic cis material kinetically, and careful analysis of the 1,3-shift products from optically active E-2-deuteriovinyltrans-perdeuterio material indicated that the reaction proceeds via the four stereopathways, si, ai, sr, ar in the ratio 58:10:24:8, respectively (Scheme 12.17).

Scheme 12.17

The potential 3,3-shift product, which might be unstable with respect to starting material, could also be reluctant to tautomerize to the still more stable aromatic derivative so the pyrolyses were also conducted in deuterated tert-butyl alcohol solvent in the presence of its conjugate base.21 Under these circumstances, 1-phenyl1,3-pentadiene was formed having deuterium distributed over the non-aromatic carbons (Scheme 12.18).

Scheme 12.18

It was suggested that the 3,3-shift had occurred, but, the conjugate base of the product underwent a retro-electrocyclic reaction and then incorporated deuterium. Regardless, the approximate free energy of activation for the overall reaction is , 33 kcal/mol, which is roughly the expected BDE of the cyclopropane bond undergoing reaction, so if the reaction is a 3,3-shift, it probably involves biradicals. In a related case, cis-1-(3-hydroxyphenyl)-2-vinylcyclopropane gave a 3,3shift product in added phenol with DH ‡ ¼ 26:7 kcal=mol and DS‡ ¼ 217:5 e:u: (128C temperature range!). The rate was independent of added phenol concentration suggesting that the 3,3-shift here is rate determining (Scheme 12.19).21

C11H10 – C11H16

391

Scheme 12.19

2.3

7-Carbethoxytricyclo[5.4.0.02,11]undeca-3,5,9-triene

In an attempt to prepare a precursor to interesting (CH)12 hydrocarbons, the potential 3,3-shift of 7-carbethoxytricyclo[5.4.0.02,11]undeca-3,5,9-triene was examined. At 508C it instead gave a 1,3-shift product with DH ‡ ¼ 31 kcal=mol and DS‡ ¼ 13 e:u:22 At higher temperatures some evidence for the 3,3-shift product was obtained (Scheme 12.20).

Scheme 12.20

Other derivatives behave similarly making this one of the few cis-1,2divinylcyclopropanes which does not undergo a 3,3-shift even though favorably arranged geometrically. Thermodynamic factors may be involved since the 3,3-shift product in one case has been demonstrated to revert to the tricyclo 5.4.0.0 system which then gives the 1,3-shift product (Scheme 12.21).23

Scheme 12.21

2.4 o-Allylstyrene to 1-Methyl-1,2-dihydronaphthalene and Benzonorpinene o-Allylstyrene undergoes an intramolecular ene reaction to 1-methyl-1,2-diydronaphthalene at 3308C in 13 h. In addition, a small amount of benzonorpinene was also formed (Scheme 12.22).24

392


Scheme 12.22

A biradical intermediate was suggested to be involved since examination of models suggested that the necessary orientation for a concerted reaction25 could not be achieved. 2.5 Tricyclo[4.2.2.12,5]undeca-3,7,9-triene and Pentacyclo[5.4.0.0.2,5 0.3,9 04,8]undeca-10-ene Rearrangement and Cleavage Tricyclo[4.2.2.12,5]undeca-3,7,9-triene gives benzene and cyclopentadiene at 1408C, but does so via a 3,3-shift followed by a reversible six-electron electrocyclization to give an anti isomer followed by a retro 2 þ 2 cycloaddition (Scheme 12.23).26

Scheme 12.23

Interestingly, pyrolysis of pentacyclo[5.4.0.0.2,50.3,904,8]undeca-10-ene, or homobasketene, at 1808C gave the same cleavage products presumably by the same pathway after a retro Diels – Alder reaction to the 3,3-shift product from the tricyclo 4.2.2.1 material.27 The cleavage reaction in this case has log k ¼ 12:76 2 35 300=2:3RT: It was pointed out that the retro 4 þ 2 reaction is rate determining and has an activation energy only 5 kcal/mol higher than that of basketene itself when at least 16 kcal/mol less strain is relieved in the homobasketene cleavage. Thus the transition state reflects the different exothermicities only to a small extent.

C11H10 – C11H16

393

2.6 Bicyclo[4.4.1]undeca-2,4,7,9-tetraene to cis-Bicyclo[5.4.0] undeca-2,4,8,10-tetraene 1,6-Dimethylbicyclo[4.4.1]undeca-2,4,7,9-tetraene rearranges to 5,7-dimethyl-cisbicyclo[5.4.0]undeca-2,4,8,10-tetraene at room temperature (Scheme 12.24).28

Scheme 12.24

This reaction is formally a 1,5-shift of a butadiene bridge However, it was suggested that transannular bond formation to a highly stabilized biradical followed by, or less likely coupled with, C1 –C2 bond fission could account for the product.28 2.7

9-Vinylbicyclo[6.1.0]nona-2,4,6-triene

anti-9-Vinylbicyclo[6.1.0]nona-2,4,6-triene gives anti-9-vinyl-cis-bicyclo[4.3.0]nona-2,4,7-triene at 1108C in an irreversible reaction which probably occurs via a pathway resembling that of the parent triene (see Chapter 10, Section 2) (Scheme 12.25).29

Scheme 12.25

However, the syn isomer rearranges to bicyclo[4.3.2]undeca-2,4,7,10-tetraene at only 608C presumably via a 3,3-shift. The tetraene apparently undergoes an intramolecular Diels– Alder reaction to a polycyclic hydrocarbon which undergoes another 3,3-shift to give tetracyclo[5.4.0.0.2,1104,10]undeca-5,8-diene with log k ¼ 13:65 2 28 440=2:3RT (Scheme 12.26).29 There is a second possible intramolecular Diels –Alder product which was calculated to be more stable than that involved in the observed pathway; however, it

394


Scheme 12.26

and the 3,3-shift products from the second intramolecular 4 þ 2 adduct were calculated by molecular mechanics to be less stable than the ultimate product. Similar reactions had been noted previously with oxygen substituted derivatives.30 3

C11H14

3.1 1,2- and 1,4-Pentamethylene Dewar Benzene to Benzocycloheptene, 8-Methylenespiro[4.5]deca-6,9-diene and 5-meta-Cyclophane 1,4-Pentamethylene Dewar benzene gives benzocycloheptene at 1208C with a halflife of 88 min in chloroform. This product is also formed quantitatively from 1,2pentamethylene Dewar benzene at 498C (Scheme 12.27).31

Scheme 12.27

The latter reaction is a disrotatory ring opening common to Dewar benzene (see Chapter 7, Section 2), but the former reaction is surprising. Further, the 1,4-isomer gives 8-methylenespiro[4.5]deca-6,9-diene in a flow system at 2808C in addition to the benzocycloheptene (Scheme 12.28). It was suggested that the 1,4-isomer ring opens to [5]-p-cyclophane which then undergoes benzylic bond fission and recombination to the spiro compound. However, formation of the cyclophane by photolysis of the 1,4-Dewar benzene isomer at 2 208C led to polymer.32 A rationalization for the disparate behavior of the system is the reversible formation of the [5]-p-cyclophane at all temperatures with

C11H10 – C11H16

395

Scheme 12.28

polymerization at high concentrations, but only at the higher temperatures could it give the spiro compound in a reaction with a larger entropy of activation compared with that for the possible 1,3-shift of the pentamethylene group to the 1,2pentamethylene Dewar benzene precursor to benzocycloheptene. However, it is also possible that the [5]-p-cyclophane gives the [5]-m-cyclophane via a benzvalene since [5]-m-cyclophane was also found to give benzocycloheptene possibly via a benzvalene at 1508C (Scheme 12.29).33

Scheme 12.29

3.2 7-(3-Butenyl)cycloheptatriene Intramolecular Diels – Alder Reactions 7-(3-Butenenyl)cycloheptatriene gives a variety of intramolecular Diels– Alder adducts subsequent to 1,5-hydrogen shifts in the cycloheptatrienyl moiety at 2158C after 4 days (Scheme 12.30).34 Similar results were obtained with allylcycloheptatrienyl ether35 and with 2-(3-butenyl)dihydrocycloheptatrienone.36

396


Scheme 12.30

4 4.1

C11H16 5-(50 -Penten-1-yl)-1,3-cyclohexadiene Pyrolysis

At 2108C, 5-(50 -penten-1-yl)-1,3-cyclohexadiene undergoes an intramolecular Diels– Alder reaction as well as a 1,5-hydrogen shift to 4-(50 -penten-1-yl)-1,3cyclohexadiene which also undergoes an intramolecular Diels –Alder reaction to give a 3.5:1 ratio of syn and anti isomers (Scheme 12.31).37

Scheme 12.31

C11H10 – C11H16

397

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

26. 27. 28. 29. 30. 31. 32.

E. Vogel, Pure Appl. Chem., 20, 237 (1969). H. Gu¨nther, H. Schmickler, W. Bremser, F.A. Straube, and E. Vogel, Angew. Chem. Int. Ed. Engl., 12, 570 (1973). W.R. Roth, F.-G. Kla¨rner, G. Siepert, and H.-W. Lennartz, Chem. Ber., 125, 217 (1992). R. Bianchi, A. Mugnoli, and M. Simonetta, J. Chem. Soc., Chem. Commun., 1073 (1972). D. Scho¨nleber, Chem. Ber., 102, 1789 (1969). H. Du¨rr, H. Kober, V. Fuchs, and P. Orth, J. Chem. Soc., Chem. Commun., 973 (1972). H. Du¨rr and H. Kober, Tetrahedron Lett., 1259 (1972). H. Du¨rr and H. Kober, Tetrahedron Lett., 1255 (1972); H. Du¨rr, H. Kober, and M. Kausch, Chem. Ber., 107, 3415 (1974). E. Vogel, D. Wendisch, and W.R. Roth, Angew. Chem. Int. Ed. Engl., 3, 443 (1964). E. Muller, H. Fricke, and H. Kessler, Tetrahedron Lett., 1525 (1964). M. Pomerantz and G.W. Gruber, J. Org. Chem., 33, 4501 (1968). M. Pomerantz, A.S. Ross, and G.W. Gruber, J. Am. Chem. Soc., 94, 1403 (1972); M. Pomerantz and A.S. Ross, J. Am. Chem. Soc., 97, 5850 (1975). G.W. Gruber and M. Pomerantz, Tetrahedron Lett., 3755 (1970). H. Du¨rr, M. Kausch, and H. Kober, Angew. Chem. Int. Ed. Engl., 13, 670 (1974). M. Pomerantz and G.W. Gruber, J. Am. Chem. Soc., 89, 6798 (1967). See also p. 6799. E.E. Waali and W.M. Jones, J. Am. Chem. Soc., 95, 8114 (1973). G. Vincow, H.J. Dauben, Jr., F.R. Hunter, and W.V. Volland, J. Am. Chem. Soc., 91, 2823 (1969). T. Mukai, T. Nakazawa, and K. Isobe, Tetrahedron Lett., 565 (1968); A.S. Kende and P.T. MacGregor, J. Am. Chem. Soc., 86, 2088 (1964). E.N. Marvell and C. Lin, Tetrahedron Lett., 2679 (1973); E.N. Marvell and C. Lin, J. Am. Chem. Soc., 100, 877 (1978). J.M. Simpson and H.G. Richey, Jr., Tetrahedron Lett., 2545 (1973). J.E. Baldwin and S.J. Bonacorsi, Jr., J. Am. Chem. Soc., 118, 8258 (1996). E.N. Marvell and S.W. Almond, Tetrahedron Lett., 2777 (1979). E. Vedejs, W.R. Wilber, and R. Twieg, J. Org. Chem., 42, 401 (1977). R. Aumann, Angew. Chem. Int. Ed. Engl., 15, 376 (1976). See also T. Miyashi, H. Kawamoto, T. Nakajo, and T. Mukai, Tetrahedron Lett., 155 (1979). J.B. Lambert and J.J. Napoli, J. Am. Chem. Soc., 95, 294 (1973). H.M.R. Hoffmann, Angew. Chem. Int. Ed. Engl., 8, 556 (1969). For more recent reviews of the ene reaction see K. Mikami and M. Shimizu, Chem. Rev., 92, 1021 (1992) and B.B. Snider and B.M. Trost, Comprehensive Organic Synthesis, Pergamon Press, Oxford, 5, 1 (1991). W. Grimme, W. Mauer, and G. Reinhardt, Angew. Chem. Int. Ed. Engl., 18, 224 (1979). W. Mauer and W. Grimme, Tetrahedron Lett., 1835 (1976). J. Frank, W. Grimme, and J. Lex, Angew. Chem. Int. Ed. Engl., 17, 943 (1978); T. Sato, and S. Ito`, Tetrahedron Lett., 1051 (1979). W. Grimme and G. Wiechers, Tetrahedron Lett., 5249 (1988). J.T. Groves and K.W. Ma, Tetrahedron Lett., 5335 (1973); M.J. Goldstein and S.-H. Dai, Tetrahedron Lett., 535 (1974). J.W. van Straten, I.J. Landheer, W.H. de Wolf, and F. Bickelhaupt, Tetrahedron Lett., 4499 (1975). L.W. Jenneskens, F.J.J. De Kanter, P.A. Kraakman, L.A.M. Turkenburg, W.E. Koolhaus, W.H. de Wolf, F. Bickelhaupt, Y. Tobe, K. Kakiuchi, and Y. Odaira, J. Am. Chem. Soc., 107, 3716 (1985).

398 33. 34. 35. 36. 37.

Hydrocarbon Thermal Isomerization J.W. van Straten, W.H. de Wolf, and F. Bickelhaupt, Tetrahedron Lett., 4667 (1977). See also Y. Tobe, Top. Current Chem., 172, 1 (1994). C.A. Cupas, M.-S. Kong, M. Mullins, and W.E. Heyd, Tetrahedron Lett., 3157 (1971). C.A. Cupas, W. Schumann, and W.E. Heyd, J. Am. Chem. Soc., 92, 3237 (1970). C.A. Cupas, W.E. Heyd, and M.-S. Kong, J. Am. Chem. Soc., 93, 4623 (1971). A. Krantz and C.Y. Lin, Chem. Commun., 1287 (1971).

13 C12H10 – C12H18

CONTENTS 1 C12H10 1.1 Heptalene p-Bond Shift 1.2 6,60 -Bisfulvene Electrocyclization 2 C12H12 2.1 [12]Annulene to cis-Bicyclo[6.4.0]dodeca-2,4,6,9,11-pentaene to cis,anti,cis-Tricyclo [6.4.0.02,7]dodeca-3,5,9,11-tetraene to Benzene, and Pentacyclo[6.4.0.0.2,70.3,1206,9]dodeca-4,10-diene to cis,syn,cis-Tricyclo[6.4.0.02,7]dodeca-3,5,9,11-tetraene to Benzene 2.2 Tricyclo[8.2.0.02,9]dodeca-3,5,7,11-tetraene to Tricyclo [4.4.2.02,5]dodeca-3,7,11-tetraene and Benzene 2.3 Tetracyclo[5.5.0.0.2,403,10]dodeca-5,8,11-triene to Pentacyclo[5.5.0.0.2,120.3,906,8]dodeca-4,10-diene and Benzene 2.4 Pentacyclo[6.4.0.0.2,50.3,1004,9]dodeca-6,11-diene to Benzene 2.5 Tetracyclo[6.2.2.0.2,703,6]dodeca-4,9,11-triene to Benzene 2.6 Tetracyclo[5.3.2.0.2,506,8]dodeca-3,9,11-triene to Tricyclo [5.3.2.04,8]dodeca-2,5,9,11-tetraene 2.7 9-(30 -Cyclopropenyl)bicyclo[6.1.0]nonatriene to Pentacyclo[6.4.0.0.2,40.3,1005,9]dodeca-6,11-triene 2.8 Tricyclo[5.5.0.02,8]dodeca-3,5,9,11-tetraene to Tricyclo[5.5.0.06,10]dodeca-2,4,8,11-tetraene 2.9 Miscellaneous (CH)12 Preparations, Compounds, and Summary of Interconversions 2.10 3,4,7,8-Tetramethylene-anti-tricyclo[4.2.0.02,5]octane to 3,4,7,8-Tetramethylene-1,5-cyclooctadiene to 3,4,7,8-Tetramethylenetricyclo[3.3.0.02,6]octane (1,2,5,6-Tetramethyl Derivatives) 2.11 1,5,9-Cyclododecatriyne to [6]Radialene 3 C12H16 3.1 1,4-Hexamethylene Dewar Benzene to [6]Paracyclophane

400 400 400 401

401 402 403 403 404 405 405 406 406

410 411 412 412

400


4 C12H18 4.1 5,6-Divinylcyclooctene to cis,cis,trans- and cis,trans,trans-Cyclododeca-1,5,9-triene References

1 1.1

413 413 413

C12H10 Heptalene p Bond Shift

Dehydrogenation of bicyclo[5.5.0]dodecapentaene gives heptalene, a hydrocarbon prone to polymerization presumably because it does not have a 4n þ 2 p electron periphery.1 p Bond alternation is common in these cases, but in heptalene, the p bond shift is fast as evidenced by 13C NMR and can be slowed only below 2 1208C. An activation free energy of 3.5 kcal/mol for the p bond shift could be established from line shape analysis (Scheme 13.1).

Scheme 13.1

1.2

6,60 -Bisfulvene Electrocyclization

Bisfulvene gives tricyclo[7.3.0.02,6]dodeca-3,5,7,9,11-pentaene at only 308C with a half-life of 20 min (Scheme 13.2).2

Scheme 13.2

The stereochemistry was assigned to be trans from the proton coupling constant, but with some reservation. The reaction is a 12-electron electrocyclization and should proceed in a conrotatory manner if it is concerted, and that would result in the trans fusion. At 808C, the pentaene undergoes the usual cyclopentadiene 1,5hydrogen shifts to give the symmetrical isomer of dihydro-a,s-indacene.

C12H10 – C12H18

2

401

C12H12

2.1 [12]Annulene to cis-Bicyclo[6.4.0]dodeca-2,4,6,9,11-pentaene to cis,anti,cis-Tricyclo[6.4.0.02,7]dodeca-3,5,9,11-tetraene to Benzene, and Pentacyclo[6.4.0.0.2,70.3,1206,9]dodeca-4,10-diene to cis,syn,cis-Tricyclo[6.4.0.02,7]dodeca-3,5,9,11-tetraene to Benzene Central to the interconversions of the (CH)12 hydrocarbons are the [12]annulenes, which are non-aromatic, and the benzene dimers. The cis,trans,cis,trans,cis,trans[12]annulene appears to have been generated by photolysis of cis,syn,cistricyclo[8.2.0.02,9]dodeca-3,5,7,11-tetraene at 2 1008C (Scheme 13.3).3

Scheme 13.3

At 2 1008C, the 1H NMR of the annulene has two singlets at d 6.92 and d 5.93 in a 1:1 ratio, but at 2 1708C, two broad singlets at d < 7:8 and d < 5:9 are present in a 1:3 ratio. This was interpreted in terms of conformational interconversion of the C3v [12]annulene without p bond shift because even at 2 408C the signals do not collapse to a single line. The rate of the interconversion was determined to be log k ¼ 10:7 2 4100=2:3RT; which gives DG‡ ¼ 5:5 kcal/mol at 2 1008C. At 2 408C, the [12]annulene rearranges irreversibly to cis-bicyclo[6.4.0]dodeca2,4,6,9,11-pentaene with DG‡ ¼ 17:4 kcal/mol, and at 308C, this bicyclic species is converted to cis,anti,cis-tricyclo[6.4.0.02,7]dodeca-3,5,9,11-tetraene, the anti-o, o0 -benzene dimer, with DG‡ ¼ 23:3 kcal/mol (Scheme 13.4).

Scheme 13.4

It was suggested that cis,cis,cis,trans,cis,trans-[12]annulene was an intermediate in the formation of the tricyclo 6400 tetraene. Then this annulene could undergo two electrocyclizations to the a,o,o 0 -tetraene (Scheme 13.4). Finally, extended pyrolysis at 308C gave benzene in a “forbidden” retro 2 þ 2 cycloaddition, no doubt, assisted by the stability of the biradical intermediates, relief of ring strain, and the product stability. The tricyclic tetraene has been synthesized by an alternative route and

402


shown to give benzene4 at 308C with DH ‡ ¼ 24:1 kcal/mol and DS‡ ¼ 22:23 e.u. in cyclohexane, and the activation parameters are the same in acetone and acetonitrile. Finally, the reaction is weakly chemiluminescent.5 The syn-o,o 0 -benzene dimer is formed from the pentacyclic dimer of benzene, pentacyclo[6.4.0.0.2,7,0.3,1206,9]dodeca-4,10-diene, at 408C.6 This dimer is more stable than the anti dimer and gave benzene with DH ‡ ¼ 22:5 kcal/mol and DS‡ ¼ 214 e.u. (Scheme 13.5).7

Scheme 13.5

A benzene triplet state might be generated in the latter reaction which may account for the very negative entropy of activation in a cleavage reaction. This retro 2 þ 2 reaction is entropically worse than that of the anti dimer which actually has a higher activation enthalpy. 2.2 Tricyclo[8.2.0.02,9]dodeca-3,5,7,11-tetraene to Tricyclo[4.4.2.02,5]dodeca-3,7,11-tetraene and Benzene syn-Tricyclo[8.2.0.02,9]dodeca-3,5,7,11-tetraene gives exo-tricyclo[4.4.2.02,5]dodeca3,7,9,11-tetraene and benzene in nearly equal quantities at 1208C in 2 h (Scheme 13.6).8

Scheme 13.6

The stereochemistry of the former product was not assigned, but later, the exoisomer was synthesized and found to be identical to the product generated from the 8200 material.9 Benzene might arise from the bicyclo[6.4.0.]dodeca-2,4,6,9,11pentaene which could be formed by a forbidden disrotatory retro cyclobutene electrocyclization of starting material, and at least two paths to the tricyclo[4.4.2.0]tetraene can be envisioned. These are a 1,5-shift of the cyclobutene ring and

C12H10 – C12H18

403

a divinylcyclobutane 3,3-shift (Scheme 13.6). A non-concerted reaction was ruled out because of the high stereospecificity. Subsequently, a carbene reaction which generated both the exo- and endo-tricyclo[4.4.2.0]dodecatetraene possibly via the syn-tricyclo 8200 tetraene utilized the 1,5-hydrogen shift pathway as deduced from deuterium labeling.10 The potential circumambulatory shift of the cyclobutene ring around the periphery of the eight-membered ring in the tricyclo 8200 material by 3,7-sigmatropic shifts has not been examined (Scheme 13.7).

Scheme 13.7

Finally, the exo,exo- and exo,endo-tetracyclo[4.4.2.0.2,507,10]dodeca-3,8,11trienes were both found to give exo-tricyclo[4.4.2.02,5]dodeca-3,7,9,11-tetraene in a flow system at 5008C (Scheme 13.8).9

Scheme 13.8

2.3 Tetracyclo[5.5.0.0.2,403,10]dodeca-5,8,11-triene to Pentacyclo[5.5.0.0.2,120.3,906,8]dodeca-4,10-diene and Benzene At 1208C, tetracyclo[5.5.0.0.2,403,10]dodeca-5,8,11-triene gives pentacyclo [5.5.0.0.2,120.3,906,8]dodeca-4,10-diene presumably via an “allowed” 3,3-sigmatropic shift.11 Both isomers give benzene above 1608C, and it was suggested that the pentacyclo material reverts to the tetracyclo compound, which undergoes a retro Diels– Alder reaction to bicyclo[6.4.0]dodeca-2,4,6,9,11-pentaene which is known to give benzene probably by electrocyclization to a tricyclo[6.4.0.02,7]dodeca-3,5,9,11-tetraene, the o,o 0 -benzene dimer, which undergoes the retro 2 þ 2 reaction (Scheme 13.9).

Scheme 13.9

404 2.4


Pentacyclo[6.4.0.0.2,50.3,1004,9]dodeca-6,11-diene to Benzene

Pentacyclo[6.4.0.0.2,50.3,1004,9]dodeca-6,11-diene, the o,o 0 :o,p 0 dimer of benzene, also called ansaradiene, gives benzene with log k ¼ 14:1 2 36 900=2:3RT (Scheme 13.10).12

Scheme 13.10

It was suggested that a retro Diels –Alder reaction occurred to give the syn-o,o 0 dimer of benzene which might reasonably give benzene under the reaction conditions just as does the anti-o,o 0 isomer. 2.5

Tetracyclo[6.2.2.0.2,703,6]dodeca-4,9,11-triene to Benzene

anti-Tetracyclo[6.2.2.0.2,703,6]dodeca-4,9,11-triene gives benzene in a first-order process at 1108C with a free energy of activation of 31.5 kcal/mol (Scheme 13.11).13

Scheme 13.11

It was suggested that the cyclobutene ring opened to give an o,p 0 -benzene dimer, which underwent a retro Diels –Alder reaction to benzene. Support for this proposal was the isolation of an iron tricarbonyl derivative of the benzene dimer when the reaction was run in the presence of iron pentacarbonyl. Subsequently, the o,p 0 benzene dimer was prepared independently and found to readily give benzene with log k ¼ 11:78 2 14 700=2:3RT; so it is a likely intermediate in the overall reaction.14

C12H10 – C12H18

405

2,5 6,8

2.6 Tetracyclo[5.3.2.0. 0 ]dodeca-3,9,11-triene to Tricyclo[5.3.2.04,8]dodeca-2,5,9,11-tetraene It was noted that tetracyclo[5.3.2.0.2,506,8]dodeca-3,9,11-triene gave tricyclo[5.3.2.04,8]dodeca-2,5,9,11-tetraene at 4808C in a flow system (Scheme 13.12).15

Scheme 13.12

While a two-step pathway was proposed, it was subsequently pointed out that an allowed 2s þ 2s þ 2s process could also account for the product (Scheme 13.12).16 2.7 9-(30 -Cyclopropenyl)bicyclo[6.1.0]nonatriene to Pentacyclo[6.4.0.0.2,40.3,1005,9]dodeca-6,11-triene 9-(30 -Cyclopropenyl)bicyclo[6.1.0]nona-2,4,6-triene gives pentacyclo [6.4.0.0.2,403,1005,9]dodeca-6,11-diene upon heating in a flow system at 3008C (Scheme 13.13).10

Scheme 13.13

It was argued that 9-(30 -cyclopropenyl)-cis 4-cyclononatetraene was formed by a disrotatory ring opening and closed to the two stereoisomeric cis-(30 cyclopropenyl)indenes, one of which gave the product via an intramolecular Diels –Alder reaction.

406


2.8 Tricyclo[5.5.0.02,8]dodeca-3,5,9,11-tetraene to Tricyclo[5.5.0.06,10]dodeca-2,4,8,11-tetraene The spiro symmetrical tricyclo[5.5.0.02,8]dodeca-3,5,9,11-tetraene gives tricyclo[5.5.0.06,10]dodeca-2,4,8,11-tetraene at 1028C with a half-life of 63 min (Scheme 13.14).17

Scheme 13.14

Deuterium labeling at the carbons adjacent to the bridgehead carbons revealed that the reaction was formally a concerted 1,3-shift and did not involve a label scrambling biradical. Interestingly, reported in the same paper was the conversion of 9,10-dimethylenetricyclo[5.3.0.02,8]deca-3,5-diene to 8,9-dimethylenetricyclo[5.4.1.02,10]deca-2,4diene at 808C with a half-life of 65 min. This appears to be a 1,5-shift of carbon (Scheme 13.15).

Scheme 13.15

The 1,5-carbon shift product from the 5500 dodecatetraene was calculated to be uphill by molecular mechanics, but downhill in the dimethylene case. Further, the subsequent 3,3-shift product from the 1,5-shift product of the 5500 dodecatetraene, namely, the p,p 0 -benzene dimer, was also calculated to be less unstable than the starting material. For still another comparison, see Chapter 11, Section 3.15.

2.9 Miscellaneous (CH)12 Preparations, Compounds, and Summary of Interconversions The (CH)12 hydrocarbons of Scheme 13.16 have been prepared, and the interconversions are noted.18

C12H10 – C12H18

407

Scheme 13.16

Hexahydro-3,47-methanocyclopenta[a]pentalene was prepared from 7-norbornadienyl chloride and thallium cyclopentadienide (Scheme 13.17).19

Scheme 13.17

This reaction would appear to involve formation of 7-cyclopentadienylnorbornadiene which undergoes an intramolecular Diels– Alder reaction. Dihydro-anti,synindacenes were also formed. Tricyclo[7.3.0.04,12]dodeca-2,5,7,10-tetraene was prepared in the hope of revealing a degenerate 3,3-shift, which in this case must occur via a boat-like transition state. However, upon heating the material at 1418C, no proton or carbon interconversion was found in the NMR (Scheme 13.18).20

408


Scheme 13.18

An estimate of the activation energy is , 35 kcal/mol which, even assuming no entropy price, would suggest that the material must be heated to 2008C to affect the reaction with a reasonable laboratory rate. Pyrolysis of 2,6-tetramethylene Dewar benzene ultimately gives [4,4]metacyclophane.21 However, intermediate products include the two tetramethylene bridged bis-basketenes. It was suggested, based on deuterium labeling studies, that 4-meta-cyclophane was formed and dimerized in a 4 þ 2 fashion to give two bistetramethylene bridged tricyclo[6.2.2.0 2,7]dodeca-3,4,8,10-tetraenes, which undergo intramolecular 4 þ 2 cycloaddition to the two bis-tetramethylene bridged bis-basketene derivatives. These then undergo a different retro 4 þ 2 cycloaddition to the ultimate product (Scheme 13.19).

Scheme 13.19

The interesting p,p 0 -benzene dimer, tricyclo[4.2.2.22,5]dodeca-3,7,9,11-tetraene, has apparently not been synthesized, but tetramethylene and benzo derivatives have been prepared.22 These undergo a 3,3-shift to the syn,o,o 0 -benzene dimer at room temperature (Scheme 13.20).

C12H10 – C12H18

409

Scheme 13.20

A compendium of all possible (CH)12 hydrocarbons is available.23 RHF/6-31Gp calculations of the energies of the more symmetrical (CH)12 hydrocarbons have been performed and are given, relative to two benzene molecules, in Scheme 13.21.24

Scheme 13.21

MMX force-field calculations are also given in italics in Scheme 13.21.25 A summary of all the reactions reported in the (CH)12 series is given in Scheme 13.22 along with the RHF/6-31Gp calculations and MMX force-field calculations of the heat of formation relative to two benzene molecules. The experimental activation free energies are given over the arrows.

410


Scheme 13.22

2.10 3,4,7,8-Tetramethylene-anti-tricyclo[4.2.0.02,5]octane to 3,4,7,8-Tetramethylene-1,5-cyclooctadiene to 3,4,7,8-Tetramethylenetricyclo[3.3.0.02,6]octane (1,2,5,6-Tetramethyl Derivatives) 1,2,5,6-Tetramethyl-3,4,7,8-tetramethylene-anti-tricyclo[4.2.0.02,5]octane gives 1,2,5,6-tetramethyl-3,4,7,8-tetramethylene-1,5-cyclooctadiene upon heating to 2408C in a flow system.26 At 3808C, 1,2,5,6-tetramethyl-3,4,7,8-tetramethylenetricyclo[3.3.0.02,6]octane is formed (Scheme 13.23).

C12H10 – C12H18

411

Scheme 13.23

The reactions appear to be forbidden 2 þ 2 retro- and cycloaddition. The latter reaction probably proceeds via a bicyclo[3.3.0]diyl. In the case of tricyclo[3.3.0.02,6]octa-3,7-diene, isomerization to a semibullvalene occurs at only 08C indicating either the [3.3.0]diyl is easily accessible or a forbidden 1,3-shift occurs easily. Relief of ring strain probably plays a role in these cases. However, a semibullvalene cannot be formed in the current case, so the biradical probably has no choice but to reclose to the tricyclic system which apparently is more stable than the cyclooctadiene. 2.11

1,5,9-Cyclododecatriyne to [6]Radialene

Pyrolysis of 1,5,9-cyclododecatriyne at 6508C in a flow system appears to give [6]radialene (Scheme 13.24).27

Scheme 13.24

No intermediates could be isolated or trapped. One suggested pathway was via 2 þ 2 þ 2 cyclization to triscyclobutenobenzene which undergoes cyclobutene ring openings to the hexane. An alternative mechanism involved a series of 3,3-shifts, (see Scheme 13.24). Subsequent labeling established that the reaction results from a series of 3,3-shifts.28

412


In this connection, it should be noted that the tris-(benzocyclobuteno)benzene has been prepared and found to have a heat of hydrogenation of the central ring of 2 71.4 kcal/mol (Scheme 13.25).29

Scheme 13.25

Given that the resulting central ring was calculated to have roughly 13 kcal/mol of strain relative to cyclohexane itself, the central ring would appear to be non-aromatic by virtue of the four-membered rings forcing the p bonds to localize between them and not in them. Interestingly, the estimated heat of hydrogenation of “cyclohexatriene” using three cyclohexenes as a model is virtually identical to the experimental value above corrected for the ring strain in the product.

3 3.1

C12H16 1,4-Hexamethylene Dewar Benzene to [6]Paracyclophane

A collaborative effort demonstrated that 1,4-hexamethylene Dewar benzene, upon heating, gives [6]paracyclophane with log k ¼ 9:8 2 20 900=2:3RT (Scheme 13.26).30

Scheme 13.26

C12H10 – C12H18

413

By comparison, 1,5-pentamethylene Dewar benzene is, in fact, more stable than [5]paracyclophane.31 4

C12H18

4.1 5,6-Divinylcyclooctene to cis,cis,trans- and cis,trans,trans-Cyclododeca-1,5,9-triene In an important study of the 3,3-shift, the cis- and trans-5,6-divinycyclooctenes were pyrolyzed, and the activation parameters were determined for the chair and boat-like pathways as judged by the relative amounts of the stereoisomeric products obtained (Scheme 13.27).32

Scheme 13.27

With reference to the scheme, log k1 ¼ 12:0 2 36 900=2:3RT; log k2 ¼ 12:7 2 44 300=2:3RT; log k3 ¼ 13:5 2 43 200=2:3RT; and log k4 ¼ 11:9 2 35 800=2:3RT: It is interesting to note the higher activation enthalpy and entropy for the boat processes relative to the chair processes. This general feature of 3,3-shifts is addressed in Chapter 7, Section 4.1.

REFERENCES 1. 2. 3. 4. 5. 6. 7.

E. Vogel, H. Ko¨nigshofen, J. Wassen, K. Mu¨len, and J.F.M. Oth, Angew. Chem. Int. Ed. Engl., 13, 732 (1974). H. Sauter and H. Prinzbach, Angew. Chem. Int. Ed. Engl., 11, 296 (1972). H. Ro¨ttele, W. Martin, J.F.M. Oth, and G. Schro¨der, Chem. Ber., 102, 3985 (1969). J.F.M. Oth, H. Ro¨ttele, and G. Schro¨der, Tetrahedron Lett., 61 (1970); J.F.M. Oth, J.-M. Gilles, and G. Schro¨der, Tetrahedron Lett., 67 (1970). J.A. Berson and R.F. Davis, J. Am. Chem. Soc., 94, 3658 (1972). T. Noh, H. Gan, S. Halfon, B.J. Hrnjez, and N.C. Yang, J. Am. Chem. Soc., 119, 7470 (1997). N.C. Yang and M.G. Horner, Tetrahedron Lett., 543 (1986). N.C. Yang, B.J. Hrnjez, and M.G. Horner, J. Am. Chem. Soc., 109, 3158 (1987).

414


8. G. Schro¨der and W. Martin, Angew. Chem. Int. Ed. Engl., 5, 130 (1966). 9. L.A. Paquette and J.C. Stowell, Tetrahedron Lett. 4159 (1969); L.A. Paquette and J.C. Stowell, J. Am. Chem. Soc., 93, 5735 (1971). 10. D.G. Farnum, M. Ghandi, S. Raghu, and T. Reitz, J. Org. Chem., 47, 2598 (1982). 11. E. Vedejs and R.A. Shepherd, J. Org. Chem., 41, 742 (1976). 12. H.-D. Martin and P. Pfo¨hler, Angew. Chem. Int. Ed. Engl., 17, 847 (1978); H.-D. Martin, P. Pfo¨hler, T. Urbanek, and R.E. Walsh, Chem. Ber., 116, 1415 (1983). 13. W. Grimme and E. Schneider, Angew. Chem. Int. Ed. Engl., 16, 717 (1977). 14. R. Braun, M. Kummer, H.-D. Martin, and M.B. Rubin, Angew. Chem. Int. Ed. Engl., 24, 1059 (1985). See also W. Grimme, P. Ho¨ner, H.T. Ka¨mmerling, R. Waldraff, and J. Wirz, Angew. Chem. Int. Ed. Engl., 28, 1353 (1989). 15. G. Schro¨der, Chem. Ber., 97, 3131 (1964); J.N. Labows, J. Meinwald, H. Ro¨ttele, and G. Schro¨der, J. Am. Chem. Soc., 89, 612 (1967). 16. L.T. Scott and M. Jones, Jr., Chem. Rev., 72, 181 (1972). 17. J. Dressel, P.D. Pansegrau, and L.A. Paquette, J. Org. Chem., 53, 3996 (1988). 18. L.A. Paquette and M.J. Kukla, J. Chem. Soc., Chem. Commun., 409 (1973); L.A. Paquette, M.J. Kukla, S.V. Ley, and S.G. Traynor, J. Am. Chem. Soc., 99, 4756 (1977). 19. M.A. Battiste and J.F. Timberlake, J. Org. Chem., 42, 176 (1977). 20. D.G. Farnum and A.G. Hagedorn, III, Tetrahedron Lett., 3987 (1975). 21. G.B.M. Kostermans, P. van Dansik, W.H. de Wolf, and F. Bickelhaupt, J. Am. Chem. Soc., 109, 7887 (1987). 22. H. Gan, J.L. King, and N.C. Yang, Tetrahedron Lett., 1205 (1989). 23. M. Banciu, C. Popa, and A.T. Balaban, Chem. Scr., 24, 28 (1984). 24. G.W. Schriver and D.J. Gerson, J. Am. Chem. Soc., 112, 4723 (1990). 25. PCMODEL version 4.0, Serena Software, P.O. Box 3076, Bloomington, IN, USA. 26. W.T. Borden and A. Gold, J. Am. Chem. Soc., 93, 3830 (1971). 27. A.J. Barkovich, E.S. Strauss, and K.P.C. Vollhardt, J. Am. Chem. Soc., 99, 8321 (1977). See also L.G. Harruff, M. Brown, and V. Boekelheide, J. Am. Chem. Soc., 100, 2893 (1978). 28. W.V. Dower and K.P.C. Volhardt, Tetrahedron, 42, 1873 (1986). 29. H.-D. Beckhaus, R. Faust, A.J. Matzger, D.L. Mohler, D.W. Rogers, C. Ru¨chardt, A.K. Sawhney, S.P. Verevkin, K.P.C. Vollhardt, and S. Wolff, J. Am. Chem. Soc., 122, 7819 (2000). 30. S.L. Kammula, L.D. Iroff, M. Jones, Jr., J.W. van Straten, W.H. de Wolf, and F. Bickelhaupt, J. Am. Chem. Soc., 99, 5815 (1977). 31. J.W. van Straten, I.J. Landheer, W.H. de Wolf, and F. Bickelhaupt, Tetrahedron Lett., 4499 (1975). 32. R. Riena¨cker and N. Balcioghi, Justus Liebigs Ann. Chem., 650 (1975).

14 C13H10 – C16H16

CONTENTS 1 C13H10 1.1 Naphthocyclopropane to Phenalene 2 C13H12 2.1 Tricyclo[7.3.10.2,804,12]trideca-5,10-diene Racemization 3 C14H12 3.1 7,8-Benzobicyclo[4.2.2]deca-2,4,7,9-tetraene Degenerate Rearrangement 3.2 1,8-Divinylnaphthalene Intramolecular Cycloadditions 3.3 Spiro[benzocyclobutene-1,70 -cyclohepta-10 ,30 ,50 -triene] to 9a,10-Dihydrobenz[a]azulene 4 C14H18 4.1 3-(5-Hexenyl)benzocyclobutene to trans-1,2,3,4,4a,9,10, 10a-octahydrophenanthrene 5 C16H12 5.1 2,3;7,8-Dibenzobicyclo[4.2.0]octa-2,4,7-triene Degenerate Rearrangement via Dibenzocyclooctatetraene 6 C16H14 6.1 9,10-Benzotricyclo[6.2.2.02,7]dodeca-3,5,9,11-tetraene to Benzene and Naphthalene 7 C16H16 7.1 [16]Annulene 7.2 Cyclooctatetraene Dimers 7.3 [2.2]Paracyclophane References

416 416 416 416 417 417 417 419 420 420 421 421 421 421 422 422 422 424 424

416 1 1.1


C13H10 Naphthocyclopropane to Phenalene

The naphthocyclopropane derivative of Scheme 14.1 not only was found to give phenalene, but it underwent bridgehead double inversion upon heating. 1 These reactions most reasonably occur via cleavage of the more substituted cyclopropane bond followed by a vicinal hydrogen shift. Subsequently, it was shown that the reactions were not affected by the presence of oxygen which indicates that the biradical is probably in its singlet state.2

Scheme 14.1

Low-temperature matrix photolysis of the cyclopropane derivative of Scheme 14.1 gives a species which has a triplet ESR spectrum assigned to the triplet state of 1,3peri-naphthadiyl.3 Though controversy surrounds the singlet –triplet gap in the diyl, it is true that it gives phenalene with log k ¼ 4:5 2 4500=2:3RT.3 The low preexponential term for the isomerization has been attributed to matrix effects and to a required spin forbidden triplet – singlet interconversion.3 This is an example of the formation of 1,8-naphthoquinodimethanes which have been reviewed.4 2 2.1

C13H12 Tricyclo[7.3.10.2,804,12]trideca-5,10-diene Racemization

Optically active tricyclo[7.3.1.0.2,804,12]trideca-5,10-diene racemizes at 408C with log k ¼ 15:6 2 28 900=2:3RT (188C range).5 The molecule is predisposed to a boatlike 3,3-shift, and undergoes the reaction almost a billion times faster than 1,5hexadiene itself undergoes the boat-like 3,3-shift (see Chapter 7, Section 4.1) (Scheme 14.2). Both favorable orientation and strain relief appear to contribute to the relatively low activation free energy for this rearrangement.

C13H10 – C16H16

417

Scheme 14.2

3

C14H12

3.1 7,8-Benzobicyclo[4.2.2]deca-2,4,7,9-tetraene Degenerate Rearrangement 7,8-Benzobicyclo[4.2.2.]deca-2,4,7,9-tetraene undergoes a degenerate rearrangement as determined from methyl-labeled derivatives. It occurs via a reversible intramolecular Diels– Alder reaction using a p bond of the benzene ring with overall log kf ¼ 12:81 2 43 000=2:3RT and log kb ¼ 12:83 2 41 200=2:3RT (Scheme 14.3).6

Scheme 14.3

At longer reaction times isomerization to 9-methyldihydrophenanthrene occurred with log k ¼ 11:2 2 41 000=2:3RT as determined from the vinylmethyl derivative. A 1,5-shift of the etheno bridge followed by cyclobutene ring opening and hexatriene cyclization can be responsible for this product. 3.2

1,8-Divinylnaphthalene Intramolecular Cycloadditions

Pyrolysis of 1,8-divinylnaphthalene gives a 3:1 mixture of the two intramolecular 2 þ 2 cycloadducts, peri-naphthobicyclo[3.1.1]heptene and peri-naphthobicyclo [3.2.0]heptene, respectively with a free energy of activation of 30 kcal/mol (Scheme 14.4).7,8

418


Scheme 14.4

It was argued that a biradical intermediate was involved since trans, trans-1,8distyrylnaphthalene gave almost exclusively the endo, exo-diphenyl derivative of the 3.1.1 product which is the most stable material derivable from the more stable biradical derived from the syn conformation of starting material.7 The stereochemistry in the parent case was examined with deuterium substitution, which also suggested intermediacy of a biradical (Scheme 14.5).

Scheme 14.5

The 3.2.0 product is presumably derived from the other possible 1,4-biradical, 1,4-perinaphthadiyl resulting from bond formation at the termini of the vinyl groups. Interestingly, this species has been generated by photolysis of the appropriate azo compound at 77 K and gives the 3.2.0 material (Scheme 14.6).9

Scheme 14.6

The diyl appears to be a ground-state triplet or nearly so, and flash photolysis of the azo compound generated the diyl whose first-order decay kinetics yield log k ¼ 6:9 2 4500=2:3RT: The low A factor would appear to reflect the requisite spin flip in the triplet to achieve closure.

C13H10 – C16H16

419

Finally, a material related to the 3.1.1 product was the result of pyrolysis of cisdivinyldihydroacenaphthene, presumably via the 3,3-sigmatropomer, 1,6-(l0 ,80 naphthalene)-1,5-hexadiene (Scheme 14.7).8

Scheme 14.7

3.3 Spiro[benzocyclobutene-1,70 -cyclohepta-10 ,30 ,50 -triene] to 9a,10-Dihydrobenz[a]azulene The adduct of benzocyclobutenylidene with benzene rearranges to 9a,10dihydrobenz[a]azulene with log k ¼ 14:14 2 30 100=2:3RT (Scheme 14.8).10

Scheme 14.8

Ring opening to the o-xylylene was proposed as the rate-determining step, and it occurs roughly 2:7 £ 106 times faster than that of benzocyclobutene itself (see Chapter 9, Section 2). Thus it was argued that the transition state had more biradical character than in the parent case. The subsequent electrocyclization involves 10 electrons so it should be a disrotatory closure. However, with a methyl on the cyclobutene ring and with the assumption that the ring opening places the methyl “outside” a trans-relationship was found between the methyl and the sevenmembered ring which indicates a conrotatory electrocyclization to what is probably the more stable product (Scheme 14.9).

420


Scheme 14.9

Furthermore, given the steric congestion in the o-xylylene moiety, the disrotatory cyclization transition state might be too high in energy relative to the, no doubt, non-concerted conrotatory one.

4

C14H18

4.1 3-(5-Hexenyl)benzocyclobutene to trans-1,2,3,4,4a,9,10,10a-octahydrophenanthrene 3-(5-Hexenyl)benzocyclobutene undergoes a remarkable transformation to trans1,2,3,4,4a,9,10,10a-octahydrophenanthrene in boiling decane (Scheme 14.10).11

Scheme 14.10

This reaction probably involves ring opening to the o-xylylene followed by an intramolecular Diels– Alder cycloaddition. The stereospecificity probably results from a conrotatory ring opening with “outward” rotation of the side chain to generate a transoid diene system which undergoes the cycloaddition as shown in Scheme 14.10. This synthetic methodology was used to prepare (þ )-estrone in 5 steps and 12.5% yield from 2-methylcyclopentenone where the benzocyclobutene was prepared by a cobalt-mediated cyclization of 1,5-diynes and acetylenes. The reaction type was also used in an alkaloid synthesis.12

C13H10 – C16H16

5

421

C16H12

5.1 2,3;7,8-Dibenzobicyclo[4.2.0]octa-2,4,7-triene Degenerate Rearrangement via Dibenzocyclooctatetraene 2,3;7,8-Dibenzobicyclo[4.2.0]octa-2,4,7-triene undergoes a degenerate rearrangement at 1008C as determined from deuterium-labeling experiments which also suggest that ring opening to dibenzocyclooctatetraene is involved (Scheme 14.11).13

Scheme 14.11

Here apparently the two fully aromatic benzene rings in the tetracyclic compound make it more stable than the “less” aromatic benzene rings of dibenzocyclooctatetraene. The activation free energy is 29.5 kcal/mol which is roughly 10 kcal/mol higher than that for conversion of the parent bicyclo[4.2.0]octatriene to cyclooctatetraene reflecting the differential stabilization (see Chapter 9, Section 2.1).

6

C16H14

6.1 9,10-Benzotricyclo[6.2.202,7]dodeca-3,5,9,11-tetraene to Benzene and Naphthalene 9,10-Benzotricyclo[6.2.202,7]dodeca-3,5,9,11-tetraene gives benzene and naphthalene near room temperature with log k ¼ 12:07 2 19 550=2:3RT (Scheme 14.12).14

Scheme 14.12

The reaction would appear to be a simple retro Diels –Alder reaction which has an activation energy 3– 4 kcal/mol higher than that for the retro Diels– Alder reaction

422


of the o,p 0 -benzene dimer described in Chapter 13, Section 2. It was argued that the relative stabilities of the products exert control over the transition state enthalpy. 7 7.1

C16H16 [16]Annulene

[16]Annulene has a temperature-dependent NMR spectrum,15 which was interpreted to involve slow equilibrium of two isomers with a free energy of activation of 11.5 kcal/mol and an equilibrium constant of 2.9 at 2 608C (Scheme 14.13).16

Scheme 14.13

In the major isomer, there are eight different arrangements of the hydrogens on four contiguous carbons, and these interconvert at 2 608C by rotation of the trans double bonds with a free energy of activation of 8.95 kcal/mol and p bond shift with a free energy of activation of 8.99 kcal/mol. Thus, the former process is 10% faster than the latter one. In the minor isomer, olefin rotation and p bond shift are equal in rate and these are 40 times faster than those in the major isomer. Despite the complexity of the NMR temperature-dependence, the chemical shifts of the olefinic protons at the lowest temperature suggest that [16]annulene is nonaromatic. Furthermore, the material undergoes a facile intramolecular cyclization at 208C with a half-life of 44 h.17 7.2

Cyclooctatetraene Dimers

Cyclooctatetraene (COT) was reported to give two dimers upon heating at 1508C in 1948.18 These had melting points of 43 and 148C and were reported to be formed in a 3:1 ratio, respectively. The 438C dimer was converted to the 148C dimer above 2008C. Two different dimers were obtained from COT upon heating at 1008C. These had melting points of 53 and 768C and were formed in a 5:1 ratio, respectively.19

C13H10 – C16H16

423

Further, these two did not give the originally reported dimers upon heating. The structures for these dimers are given in Scheme 14.14, but these structures are not those originally proposed.

Scheme 14.14

The structures of the 53 and 768C dimers were assigned20 on the basis of degradation and NMR with confirmation of the structure of the 768C dimer in the form of its photochemical cleavage to bullvalene, THE molecule of the 1960s (see Chapter 11, Section 3.1).21 The 438C dimer was assigned a structure22 on the basis of its spectra and the likely possibility that a vinylcyclopropane rearrangement would convert it to the 148C dimer whose structure was determined by X-ray crystallographic analysis.23 It was also suggested22 that the 438C dimer could be formed from the cis-isomer of the 768C dimer by an intramolecular Diels– Alder reaction thus implying that the 768C dimer has the trans stereochemistry. The pathways for formation of the 768C dimer focused21 on the structure originally proposed to be the 538C dimer,19 namely the 4 þ 2 adducts of two COTs which can undergo an intramolecular 4 þ 2 cycloaddition followed by a six-electron shift to give the precursor to both the 76 and the 438C dimer (Scheme 14.15).

Scheme 14.15

424 7.3


[2.2]Paracyclophane

[2.2]Paracyclophane is known to undergo disproportionation and polymerization at 400– 6008C (Scheme 14.16).24

Scheme 14.16

Both reactions reasonably proceed via initial fission of the bisbenzylic bond. There is evidence that this is reversible since optically active 4-carbomethoxy [2.2]paracyclophane racemizes upon heating to 2008C with a free energy of activation of ca. 38 kcal/mol.25 There was no polar solvent effect indicating a homolytic process, and rotation around the substituent-bearing phenyl in starting material was viewed as unlikely. Reversible one bond cleavage was considered for the racemization since crossover experiments with disubstituted materials indicated an intramolecular process. Further evidence for the biradical intermediate was provided by trapping in hydrogen-donating solvents and non-stereospecific addition of dimethyl maleate and fumarate. Trapping the biradical suggests that cleavage to the p-xylylene is energetically demanding and perhaps occurs only at higher temperatures because of the favorable entropy. Further, the strain energy of [2.2]paracyclophane was estimated to be 31.3 kcal/mol26 and that plus the resonance energy of two benzyl radicals would suggest that the biradical is roughly 12 kcal/mol more stable than the transition state forming it, so trapping is not unreasonable. It is, therefore, surprising that the photolysis of [2.2]paracyclophane at low temperatures was reported to give the biradical which upon warming to 83 K resulted in p-xylylene.27

REFERENCES 1. W.R. Roth and K. Enderer, Justus Liebigs Ann. Chem., 730, 82 (1969). 2. R.M. Pagni, M.N. Burnett, and H.M. Hassaneen, Tetrahedron, 38, 843 (1982). 3. J.-F. Muller, D. Muller, H.J. Dewey, and J. Michl, J. Am. Chem. Soc., 100, 1629 (1978); M. Gisin, E. Rommel, J. Wirz, M.N. Burnett, and R.M. Pagni, J. Am. Chem. Soc., 101, 2216 (1979).

C13H10 – C16H16 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

425

M.S. Platz, Diradicals (W.T. Borden, ed.), Wiley, London, pp. 222– 255 (1982). W. Grimme and S. Krauthaüser, Tetrahedron, 53, 9903 (1997). W. Grimme, T. Grommes, W.R. Roth, and R. Breuckmann, Angew. Chem. Int. Ed. Engl., 31, 872 (1992). J. Meinwald and J.A. Kopecki, J. Am. Chem. Soc., 94, 6235 (1972). S.F. Nelsen and J.P. Gillespie, J. Am. Chem. Soc., 94, 6237 (1972); S.F. Nelsen and J.P. Gillespie, J. Am. Chem. Soc., 94, 6238 (1972). M. Gisin, E. Rommel, J. Wirz, M.N. Burnett, and R.M. Pagni, J. Am. Chem. Soc., 101, 2218 (1979). M.A. O’Leary, G.W. Richardson, and D. Wege, Tetrahedron, 37, 813 (1981). R.L. Funk and K.P.C. Vollhardt, J. Am. Chem. Soc., 102, 5245 (1980). See also p. 5255. W. Oppolzer, J. Am. Chem. Soc., 93, 3833 (1971), see also p. 3834; W. Oppolzer and K. Keller, J. Am. Chem. Soc., 93, 3836 (1971). W. Grimme, J. Lex, and T. Schmidt, Angew. Chem. Int. Ed. Engl., 26, 1268 (1987). W. Grimme and H.G. Ko˝ser, Angew. Chem. Int. Ed. Engl., 19, 307 (1980). F. Sondheimer and Y. Gaoni, J. Am. Chem. Soc., 83, 4863 (1961). J.F.M. Oth and J.-M. Gilles, Tetrahedron Lett., 6259 (1968). For studies of substituted [16]annulenes, see G. Schro˝der, G. Kirsch, J.F.M. Oth, Chem. Ber., 107, 460 (1974). W. Reppe, O. Schlichting, K. Klager, and T. Toepel, Justus Liebigs Ann. Chem., 560, 1 (1948). W.O. Jones, Chem. Ind. (Lond), 16 (1955). G. Schro¨der and W. Martin, Angew. Chem. Int. Ed. Engl., 5, 130 (1966). G. Schro¨der, Chem. Ber., 97, 3131 (1964). H.W. Moore, J. Am. Chem. Soc., 86, 3398 (1964). S.C. Nyburg and J. Hilton, Acta Crystallogr., 12, 116 (1959). J.R. Schaefgen, J. Polym. Sci., 15, 203 (1955); W.F. Gorham, J. Polym. Sci., Polym. Chem. Ed., 4, 3027 (1966). H.J. Reich and D.J. Cram, J. Am. Chem. Soc., 91, 3517 (1969). R.H. Boyd, Tetrahedron, 22, 119 (1966). G. Kaupp, E. Teufel, and H. Hopf, Angew. Chem. Int. Ed. Engl., 18, 215 (1979).

Subject Index [1-13C,1-2H]acetylene at 13 [1.1.1]propellane 58 [2.1.1]propellane 133 [2.2]paracyclophane 424 [2.2.2]propellane 265, 266, 373 [3.2.1]propellane 265 [4]p-cyclophane 359 [4,4]meta-cyclophane 408 [5]-m-cyclophane 395 [5]-p-cyclophane 394, 395 [5]paracyclophane 413 [6]paracyclophane 412 [6]radialene 411 [8]paracycloph-4-enes 357 [9-13C,3-2H]-6-methylenebicyclo[3.2.1]oct-2-ene 306 [12]annulenes 401 [16]annulene 422 [2+2] cycloaddition 264 (+)-estrone 420 (–)-(7R)-8-deuteriobornadiene 372 (4,4)CASPT2/6-31G* 104 (DZ+dTCSCF CISD +Q) 359 (RZ)- and (SE)-4-methyl-1,2,5-heptatriene 116 (RZ)-4-methyl-2-hepten-6-yne 116 (SE)-4-methyl-2-hepten-6-yne 116 (U)B3LYP 200 1(4)-bicyclo[2.2.0]hexene 121 1,1,2,2-tetramethylcyclopropane 29 1,1,2,2-tetramethylenecyclobutane 50 1,1,3-trideuterioindene 277 1,1,4,4-tetradeuterio-1,3-butadiene 160 1,1,6,6-tetrafluoro-1,5-hexadiene 146 1,1-dicyano-2-methyl-anti-3-ethylidenecyclobutane 77 1,1-dideuterio-1,5-hexadiene 140 1,1-dideuterioallene 78, 121 1,1-dideuterioallyl mesylate 149

1,1-dideuterioethylene 159 1,1-dimethylallene 122 1,1-divinylcyclopropane 193 1,2,3,4,4a,9,10,10a-octahydrophenanthrene 420 1,2,3-trideuteriocyclopropane 26 1,2,4,5,6,7-hexamethyltricyclo[3.2.0.02,4]hept-6-ene 183 1,2,4,6-cycloheptatetraene 175 1,2,4-heptatriene-6-yne 174 1,2,4-trideuterio-5,5-dimethylbicyclo[2.1.0]pentene 56 1,2,4-trimethylspiropentanes 73 1,2,5,6-tetramethyl-3,4,7,8-tetramethylene-1,5cyclooctadiene 410 1,2,5,6-tetramethyl-3,4,7,8-tetramethylenetricyclo[3.3.0.02,6]octane 410 1,2,5,7-octatetraene 116 1,2,5-cycloheptatriene 184 1,2,5-hexatriene 114–116 1,2,6,7-cyclodecatetraene 360 1,2,6,7-octatetraene 123, 247 1,2,6-heptatriene 199, 200 1,2,7,8-nonatetraene 308 1,2,7-octatriene 264 1,2,8,9-decatetraene 123, 368 1,2,8,9-tetrahydronaphthalene 355 1,2,8-nonatriene 316 1,2- and 1,3-dimethylenecyclobutane 124, 133 1,2-benzazulene 14 1,2-benzo-1,3-cycloheptadiene 385 1,2-benzocycloheptatriene 385, 387 1,2-bis(-1-propynyl)cyclobutanes 229 1,2-bis(1-cyclopentenyl)ethane 139 1,2-bis(dideuteriomethylene)cyclobutane 121 1,2-cyclononadiene 122, 316 1,2-di-tert-butyl-3-(nonadeuterio-tert-butyl)cyclobutadiene 34 1,2-dideuterio-3,3-dimethylenecyclobutene 95 1,2-diethynylcyclopropane 172

428

Subject Index

1,2-diethynylethene 88 1,2-diethynylethylene 326 1,2-dihydronaphthalene 333, 335, 340, 341, 346, 351 1,2-dihydrophthalate esters 89 1,2-dimethyl-1,2-bis(trideuteriomethyl)cyclopropane 29 1,2-dimethylcyclobutanes 48 1,2-dimethylcyclopropane 29 1,2-dimethylenecyclobutane 118, 123, 124, 125 1,2-dimethylenecyclopropane 57, 58 1,2-divinyl-1,3-cyclohexadiene 361 1,2-divinylcyclobutene 247 1,2-hexadien-5-yne 97 1,2-homotropilidene 235 1,2-pentamethylene Dewar benzene 394, 395 1,2-tetramethylene Dewar benzene 365 1,2-trimethylenetropilidene 355 1,3,3-trimethylcyclopropene 21 1,3,5,7,9-cyclodecapentaene 330 1,3,5-cycloheptatriene 176, 180, 182 1,3,5-cyclononatriene 300 1,3,5-cyclooctatriene 231 –233, 235, 237, 238, 298 1,3,5-hexatrienes 108, 115 1,3,6-cyclononatriene 314 1,3,6-cyclooctatriene 233, 246 1,3,7,9-decatetraene 372 1,3,9-decadienes 376 1,3- and 3,3-shift 118, 242, 293, 305 1,3- and 3,3-sigmatropic shifts 195 1,3-butadiene 38, 154 1,3-carbon shift 174, 286, 313 1,3-cyclohexadiene 108, 110, 113–115 1,3-cyclopentadiene 53 1,3-cyclopentane biradical 66 1,3-dimethylallyl radicals 138 1,3-diethyl benzene 112 1,3-dimethylcyclopropene 21, 22 1,3-dimethylenecyclobutane 123 1,3-dimethylenecyclohexane 265 1,3-dimethylenecyclopentane 195 1,3-hexadien-5-yne 96, 99 1,3-hydrogen shift 21, 23, 30, 115, 189 1,3-pentadienes 22 1,3-peri-naphthadiyl 416 1,3-shift 20, 61, 69, 70, 105, 119, 156, 183, 190, 193, 196, 220, 240, 250, 253, 254, 288, 292, 302, 303, 305, 307, 337, 348, 363, 368, 391, 395, 406, 411 1,3-shift isomer 370 1,3-shift of carbon 241, 281 1,3-shift product 389

1,3-sigmatropic shift 101, 157, 355, 387 1,3-sigmatropomer 346 1,4,4-trimethylbicyclo[3.2.0]hepta-2,6-diene 180 1,4,6-bicyclo[3.2.0]heptatriene 172 1,4,7-cyclononatriene 299 1,4- and 1,3-cyclohexadiene 106 1,4-bicyclo[2.2.0]hexadiene 98 1,4-cycloheptadiene 191 1,4-cyclohexadiene-1,4-diyl 98 1,4-cyclohexadiene 87, 112, 113 1,4-cyclohexane diyl 140, 141, 192 1,4-cyclononadiene 315 1,4-cyclooctadiene 267 1,4-dicyanobicyclo[2.2.0]hexane 262 1,4-diethylbenzene 112 1,4-dihydronaphthalene 333 1,4-dimethylenecyclohexane 43, 265 1,4-diphenylbicyclo[2.1.0]pentane 67 1,4-diyl 195 1,4-ethano-2-methylenespiropentane 249 1,4-hexamethylene Dewar benzene 412 1,4-hydrogen shift 205 1,4-pentadiene 66, 68 1,4-pentamethylene Dewar benzene 394 1,4-perinaphthadiyl 418 1,4-tetramethylene Dewar benzene 359 1,5,5-trimethylcyclopentadiene 372 1,5,9-cyclododecatriyne 411 1,5-alkyl shift 308, 355 1,5-bishomocyclooctatetraene 359 1,5-carbon shift 174 1,5-cyclononadiene 315 1,5-cyclooctadiene 262 1,5-deuterium and hydrogen shifts 277 1,5-deuterium shifts 54 1,5-dimethyl-5-deuteriomethyl-CPD 55 1,5-hexadiene 97, 138, 140, 141, 152, 154, 352, 354 1,5-hexadiene 3,3-shift 134 1,5-hexadiyne 95 1,5-hydrogen 53, 387 1,5-hydrogen shift 79, 80, 96, 99, 110, 118, 173, 176, 178, 180, 189, 202, 221, 227, 234–236, 240, 263, 264, 279, 280, 289–291, 299, 300, 307, 316, 317, 333, 335, 355, 361, 373, 383, 384, 386, 387, 395, 396, 400, 403 1,5-pentamethylene Dewar benzene 413 1,5-shift 190, 383, 393, 402, 406 1,5-shift of carbon 236, 279, 307, 406 1,5-silyl shift 55 1,5-vinyl shift 334, 346 1,6,7,7-tetramethylcycloheptatriene 183

Subject Index 0

0

1,6-(l ,8 -naphthalene)-1,5-hexadiene 419 1,6-dideuterio-1,5-hexadiene 95 1,6-dideuteriobicyclo[4.2.0]octa-2,7-diene 231 1,6-dimethylbicyclo[4.4.1]undeca2,4,7,9-tetraene 393 1,6-octadiene 267 1,6-trimethylenenorcaradiene 354 1,7,7-trimethylcycloheptatriene 372 1,7 carbon ring walk 285 1,7-hydrogen shift 108, 110, 112, 204, 237, 351 1,7-octadiene 268 1,7-shift 287, 389 1,7-vinyl shift 280 1,8-divinylnaphthalene 417 1,8-naphthoquinodimethanes 416 1-(3-methylene-4-pentenyl)cyclobutene 369 1-(cis-2-methylcyclopropyl)-1,2-butadiene 187 1-bicyclo[4.2.0]octane 264 1-butene 46 1-chlorobicyclo[2.2.0]hexa-2,5-diene 87 1-deuterio-1-methyl-3-tert-butylindene 278 1-deuterioindene 277 1-deuteriopropene 15 1-deuteriopropyne 24 1-dimethylaminobicyclo[2.2.0]hexa-2,5-diene 87 1-ethylcyclopropene 21 1-ethynyl-2-methylcyclopropane 115 1-hepten-7-yne 264 1-hexen-5-yne 115, 116 1-isopropyl-3,3-dimethylcyclopropene 21 1-isopropylidene-2-vinylcyclopropane 130 1-methoxybicyclo[3.2.0]hepta-3,6-dienone 180 1-methyl-1,2-diydronaphthalene 391 1-methyl-1,3-cyclohexadiene 111 1-methyl-3-isopropenyl-1,4-cyclohexadiene 177 1-methylcyclohexane 204 1-methylcyclopentene 160, 161 1-methylenedispiro[2.0.2.1]heptane 245 1-methylenespiro[2.4]hepta-2,4-diene 227 1-methylspiro[2.4]heptadienes 186 1-nonen-8-yne, cis-bicyclo[4.3.0]nona-7-ene 316 1-o-tolyl-1,3-butadiene 351 1-pentene 80 1-phenyl-1,3-pentadiene 390 1-phenyl-6-deuterio-1,3-hexadien-5-yne 100 1-phenylbutadiene 340, 341 1-phenylspiro[2.6]nonatetraene 280 1-tert-butyl-3-methylindene 278 1-vinylcyclohexa-1,3-diene 240 1-vinylcyclopentene 193 1-vinylnortricyclene 314 1-vinylspiro[2.4]hepta-4,6-diene 290 10/10 MCSCF 36

429

11-cyano-11-methyl[10]annulene 382 12 e-/10 O CASSCF 68 2+2 cleavage 223 2+2 cyclization 220, 243, 244, 325 2+2 cycloaddition 46, 77, 99, 132, 154, 198, 204, 205, 265, 340, 344, 345, 348, 353, 359, 368, 373, 375, 392, 401 2+2 cycloadducts 370, 417 2+2 dimer 190, 228 2+2 dimerization 121, 281 2+2 process 34, 174, 304 2+2 product 362 2+2 reaction 49, 259, 402, 403 2+2 reaction to allene 72 2+2 retro- and cycloaddition 411 2+2+2 cyclization 312, 411 2+2+2 cycloaddition 311, 346 2,2-dideuterio-biscyclopropylidene 124 2,2-dideuteriomethylenecyclobutane 68 2,2-dimethylallylidenencyclopropane 129 2,2-dimethylvinylcyclobutane 205 2,20 -bisallyl biradical 118, 121, 123 2,20 -bisallylmethane biradical 195 2,20 -bismethylenecyclopropanyl 298 2,3,3,4-tetramethyltricyclo[3.2.0.02,4]hept-6-ene 183 2,3-benzonorbornadiene 384, 387 2,3-benzonorcaradiene 384 2,3-diazabicyclo[2.2.1]hept-2-ene 67 2,3-dimethyl-3-hexene 15 2,3-dimethylbutadiene 35 2,3-dimethylenebicyclo[2.2.0]hexane 123, 249 2,3-dimethylethylbenzene 111 2,3-dimethylindene 279 2,3-divinyl-1,3-cyclohexadiene 360 2,3-divinylcyclo-1,3-hexadiene 361 2,3-divinylcyclopentane 310 2,3-pentadiene 19 2,3;7,8-dibenzobicyclo[4.2.0]octa-2,4,7-triene 421 2,4-dimethyl-1,4-pentadiene 68 2,4-dimethyl-1-carbethoxyspiropentanes 75 2,4-diphenyl-endo-tricyclo[3.2.1.02,4]oct-6-enes 244 2,4-hexadiene 123 2,5-dimethylenebicyclo[2.2.2]octane 370 2,5-dimethylenebicyclo[4.1.0]hepta-2-ene 298 2,5-dimethylenebicyclo[4.2.0]octane 371 2,5-diphenyl-1,5-hexadiene 141 2,5-diphenylisobenzofuran 228 2,5-tetramethylene Dewar benzene 364, 365 2,6-dimethylenebicyclo[3.3.0]octane 370

430

Subject Index

2,6-tetramethylene Dewar benzene 408 2,7-dimethyl-7-carbomethoxycycloheptatriene 178 2,7-dimethyl-7-cyanocycloheptatriene 178 2-(2-methylene-3-butenyl)methylenecyclobutane 369 2-(3-butenyl)dihydrocycloheptatrienone 395 2-allyl-1,3-cyclohexadiene 304 2-allyl-1-ethynylcyclopropane 116 2-butene 48, 107 2-deuterioindene 277 2-ethynyl-1,3-butadiene 97, 98 2-isobutenylmethylenecyclobutane 370 2-methoxymethylmethylenecyclopropane 43 2-methyl-1-propenylcyclobutanes 156 2-methyl-3-ethylidenecyanocyclobutane 78 2-methyl-3-isopropenyl-1,4-cyclohexadiene 177 2-methylazulene 327 2-methylcyclopentenone 420 2-methylenebicyclo[2.1.0]pentane 114, 115 2-methylenebicyclo[2.2.0]hexane 199, 201 2-methylenebicyclo[3.2.2]nona-6-ene 368 2-methylenecyclohexane-1,4-diyl 201 2-methylvinylcyclopentanes 267 2-phenyl-1.5-hexadiene 145 2-phenyl 141 2-phenylindane 389 2-phenylindene 280 2-phenylmethylenecyclopropane 46 2-vinyl-4-methylenecyclohexene 296 2-vinylmethlenecyclopentane 264 2/2 CASSCF 74 2s+2s+2s 405 3,3-dideuterio-1,4-cyclooctadiene 267 3,3-dideuteriobicyclo[3.3.0]oct-2-ene 267 3,3-divinyl-1-methylenecyclobutane 296 3,3-shift 94– 97, 134, 135, 137–140, 142–144, 146, 172, 180–182, 184, 191–194, 196, 198, 199, 201, 217, 218, 229, 231, 232, 239, 241, 244, 251–253, 255, 259, 260, 266, 281, 283–285, 287, 293–295, 297, 298, 301, 302, 305, 306, 310, 311, 315, 330, 340, 341, 344, 345, 352–354, 356, 359–361, 363, 365–367, 369–372, 374, 388, 390, 391, 393, 394, 403, 406 –408, 411, 413, 416 3,3-shift of 1,4-dimethylenecyclobutane 138 3,3-shift product 243 3,3-sigmatropic shift 115, 132, 141, 333, 351, 403 3,3-sigmatropomer 419 3,30 -biscyclopropenyl 90, 91, 94 3,30 -dimethylbiscyclopropenyl 89

3,4,4,5-tetramethylbicyclo[3.2.0]hepta-2,6-diene 183 3,4,4-trideuterio-a-thujene 105 3,4-benzobicyclo[3.2.0]cyclohepta-1(2),3-diene 190 3,4-benzocycloheptatriene 385 3,4-bis(trimethylsiloxy)tricyclo[4.2.1.02,5]nonadiene 288 3,4-dideuterio-cis-tetrahydropyridazine 47 3,4-diethynylcyclobutene 213 3,4-dimethylene-1,5-hexadiene 123, 126, 247, 249 3,4-dimethylenecyclobutene 95, 96 3,4-dimethylenecyclooctene 368 3,4-dimethylenecyclopentanone 120 3,4-epoxytropilidene 231 3,4-homoheptafulvene 286 3,4-homotropilidene 229 3,6-dimethylene-1,7-octadiene 369, 370 3,6-dimethylenecyclohexene 250 3,7,7-cycloheptatriene 177 3,7,7-trimethylcycloheptariene 180 3,7-shift 388 3,7-dideuterio-1,3,6-cyclononatriene 314 3,7-dideuteriobicyclo[3.3.0]octa-2,6-diene 232 3,7-dideuteriobicyclo[3.3.1]nona-2,6-diene 313 3,7-dimethyl-7-methoxymethylcycloheptatriene 178 3,7-dimethylenetricyclo[4.1.0.02,4]heptane 298 3,7-sigmatropic shifts 403 3,8-dihydropentalene 221 3,9,9-trideuterio-6-methylenebicyclo[3.2.1]oct-2ene 305 3- and 4-methylenecyclopentene 114 3-(5-hexenyl)benzocyclobutene 420 3-(cis-2-methylcyclopropyl)-1,2-butadiene 187 3-allyl-2-methyl-1,3-diphenyl.cyclopropene 132 3-allyl-3-methyl-1,2-diphenylcyclopropene 132 3-allylcyclopropene 132 3-carbomethoxy-2,2-dideuterio-1-carbomethoxymethylenecyclobutane 71 3-deuterio-exo-tricyclo[3.2.1.02,4]oct-6-ene 243 3-deuteriocyclobutene 40 3-deuterionorpinene 188 3-deuteriopropyne 24 3-ethylfulvenallene 173 3-ethylidenecyclohexene 15 3-hydroxy-3-methyl-1,5-hexadiene 146 3-isopropyl-3-methylcyclobutene 40 3-isopropylidenecyclopentene 129, 130 3-methyl-1,3-cyclohexadiene 188 3-methyl-1,5-hexadiene 372 3-methyl-2-cyano-ethylidenecyclopropane 46 3-methylcyclopentene 63

Subject Index 3-methylene-1,4-cyclononadiene 369 3-methylene-1,5-hexadiene 198, 199, 201 3-methylenebicyclo[3.2.0]oct-6-ene 310 3-methylenecyclobutene 58 3-methylenecyclobutylidene 59 3-methylenecyclopentene 117, 127 3-methylenecyclopentylidene 133 3-methylenespiro[2.4]octa-1-ene 250 3-methylenetetracyclo[3.2.1.0.2,7 04,6]octane 295 3-methylenetricyclo[3.2.1.02,4]oct-6-ene 295 3-vinylcyclobutene 106, 107 3-vinylmethylenecyclobutane 203 3R-3-phenyl-3-methyl-E-1,5-heptadiene 137 3S-3-methyl-6-phenyl-E-1,5-heptadiene 137 4+2 addition 160 4+2 adduct 226, 257, 277, 370, 394, 423 4+2 cycloaddition 159, 160, 220, 225, 228, 240, 249, 257, 259, 301, 304, 366, 408, 423 4+2 Diels –Alder-like 34 4+2 dimerization 256 4+2 fashion 408 4+2 reaction 158, 182, 368, 376, 392 4+4 cleavage 339 4+4 cycloaddition 174 4,4,7-trimethylbicyclo[3.2.0]hepta-2,6-diene 180 4,4-dimethyl-3-methylenecyclopentene 130 4,5-dihydrooxepine 193 4,5-dimethyl-3-ethylidenecyclopentenes 130 4,5-dimethylene-3,3,6,6-tetramethyl-3,4,5,6tetrahydropyridazine 122 4,7-dihydroindene 290, 291 4- and 5-methylspiro[2,4]heptadiene 185 4-(50 -penten-1-yl)-1,3-cyclohexadiene 396 4-(exo-dideuteriovinyl)cyclohexene 254 4-(o-allylphenyl)-2-butene 354 4-31G+DZP 20 4-cyano-1-methylcyclopentene 65 4-ethylcyclohexene 155 4-ethynylspiro[2.5]hept-1-ene 358 4-meta-cyclophane 364, 408 4-methyl-3-methylenecyclopentene 129 4-methylbicyclo[3.1.0]hexenes 101 4-methylcyclopentene 64, 163 4-methylenebicyclo[5.1.0]oct-2-ene 309 4-methylenebicyclo[5.2.0]nona-2,5,8-triene 349 4-methylenecyclohexene 203 4-methylenepyrazoline 43, 44 4-phenyl-1-butene 354 4-phenylcyclopentene 389 4-vinylcyclohexene 194, 253 –257, 263 4-vinylcyclopentene 194

431

4a,8b-dihydrobiphenylene 216 4S-4-(exo[14C]vinyl)cyclohexene 255 5,5,10,10-tetrabromotricyclo[7.1.0.04,6]deca-2,7-diene 329 5,5,5-trideuterio-cis-1,3-pentadiene 79 5,5-diethyl-1,3-cyclohexadiene 111 5,5-shift 372 5,5-sigmatropic 371 5,6-dimethyl-2,3-diazobicyclo[2.2.2]oct-2-enes 154 5,6-dimethylenebicyclo[2.2.1]hept-2-ene-7-one 226 5,6-divinylcyclooctene to cis,cis,trans- 413 5,7-dimethyl-cis-bicyclo[5.4.0]undeca-2,4,8,10tetraene 393 5,8-dideuterio-1,3,6-cyclooctatriene 234 5-(50 -penten-1-yl)-1,3-cyclohexadiene 396 5-allyl-1,3-cyclohexadiene 304 5-endo-vinylbicyclo[2.2.2]octa-2-ene 365 5-isopropylidenebicyclo[2.1.0]pentane 44 5-methyl-1,3-cyclohexadiene 110 5-methyl-cis-1,2,4-hexatriene 118 5-methylene-1,3-cycloheptadiene 368 5-methylene-1,4-cyclohexadiene 188 5-methylenebicyclo[2.1.1]hexane 198 5-methylenebicyclo[2.2.1]hept-2-ene 241 5-methylenebicyclo[2.2.2]octane 305 5-methylenebicyclo[2.2.2]octene 292 5-protio-1,2,3,4,5-pentadeuteriocyclopentadiene 54 5-trimethylsilyl-CPD 55 5-vinyl-1,3-cyclohexadiene 234 5-vinyl-2-methylenebicyclo[3.2.0]heptane 370 5-vinyl-2-norbornene 301 5-vinylcyclopentadiene 185 5S-2-deuterio-6-methyl-2-trans-4-cis-octadiene 79 6,60 -bisfulvene electrocyclization 400 6,7-bis(trimethylsiloxy)-8,9-dihydroindene 288 6,7-dimethylenebicyclo[3.2.0]heptane 308 6-311G(2d,2p) 144 6-311Gpp 11, 15 6-31G* 6e–/6 orbital CASSCF 128 6-31G* CASPT2N 44 6-31G* MCSCF 119 6-31G* MP3 93 6-31G* 34, 48, 62, 74, 111, 179, 300 6-allyl-3-methylene-1,4-cyclohexadienes 363 6-methyl-1,2-diphenyltricyclo[3.1.1.02,5]hexane 132 6-methylenebicyclo[3.1.0]hex-2-ene 189 6-methylenebicyclo[3.1.0]hexane 202

432

Subject Index

6-methylenebicyclo[3.2.0]hept-2-ene 241 6-methylenebicyclo[3.2.0]heptane 264 6-methylenebicyclo[3.2.1]oct-2-ene 305 6-methylenebicyclo[3.3.0]oct-2-ene 309 6-methylenebicyclo[3.3.1]nona-1-ene 370 6-methylfulvene 185 6-vinylfulvene 227 7,7-dimethylbicyclo[4.1.1]octa-2,4-diene 236 7,8-benzobicyclo[4.2.2.]deca-2,4,7,9-tetraene 417 7,8-dideuterio-1,3,5-cyclooctatriene 234 7,8-dihydropentalene 228 7,8-dimethylene-cis-bicyclo[4.2.0]octane 368 7,10-diethynylfluoranthene 100 7-(3-butenenyl)cycloheptatriene 395 7-(30 -cyclopropenyl)norcaradiene 344 7-carbethoxytricyclo[5.4.0.02,11]undeca3,5,9-triene 391 7-cyclopentadienylnorbornadiene 407 7-deuteriocycloheptatriene 176 7-isopropylidene-2,3-diazabicyclo[2.2.1]hept2-ene 44 7-methylene-1,4-cyclooctadiene 309 7-methylene-bicyclo[3.2.0]hept-2-ene 120 7-methylenebicyclo[3.3.0]oct-2-ene 310 7-methylenebicyclo[4.2.0]oct-2-ene 292, 305 7-methylenebicyclo[3.2.0]hepta-1(2),3-diene 227 7-methylenedispiro[2.0.2.1]heptane 245 7-norbornadienyl chloride 407 7-tert-butoxynorbornadiene 183 7-vinylcycloheptatriene 294 7-vinylnorbornene 301 8,8-dideuteriobicyclo[5.1.0]oct-2-ene 267 8,8-dimethylbicyclo[5.1.0]octa-2,4-diene 236 8,8-diprotio-octadeuteriobicyclo[5.1.0]octa2,4-diene 235 8,9-dihydroindene 290 8,9-dimethylenetricyclo[5.4.1.02,10]deca-2,4diene 406 8-methylbicyclo[5.3.0]deca-1(2),9-diene 131 8-methylbicyclo[5.4.0]undeca-1,3,5,9-tetraenes 388 8-methylenebicyclo[3.2.1]oct-6-ene 309 8-methylenespiro[4.5]deca-6,9-diene 394 8-phenylbicyclo[5.2.0]nona-1,3,5,8-tetraene 280 9,9-dichloro-trans,trans-bicyclo[6.1.0]nona-4-ene 310 9,9-dicyanobicyclo[6.1.0]nonatriene 285 9,9-methylbicyclo[6.1.0]nonatriene 284 9,10-benzotricyclo[6.2.202,7]dodeca-3,5,9,11tetraene 421

9,10-dibromoanthracene 219 9,10-dihydronaphthalene 332 9,10-dimethylenetricyclo[5.3.0.02,8]deca-3,5diene 406 9-(30 -cyclopropenyl)-cis4-cyclononatetraene 405 9-(30 -cyclopropenyl)bicyclo[6.1.0]nona2,4,6-triene 405 9-cyanobicyclo[4.2.1]nonatrienes 286 9-deuterio-cis-bicyclo[6.1.0]nonatriene 281 9-methyldihydrophenanthrene 417 9-methyl-cis-bicyclo[6.1.0]nonatriene 283 9-methylenebicyclo[6.1.0]nona-2,4,6-triene 349 9-methylenebicyclo[6.1.0]nona-2-ene 369 9-methylenecyclo-1,3,5,7-nonatetraene 351 9-oxabicyclo[6.1.0]nona-2,6-diene 299 9-phenylbicyclo[5.2.0]nona-1,3,5-triene 389 9a,10-dihydrobenz[a]azulene 419

a- and b-methylnapthalene 385 a-13C naphthalene 327 a-thujene at 104 ab initio 376 abnormal Claisen 353 acetic acid 265 acetylene 182, 221, 223, 224 acetylene dyotropic reactions 13 acrylonitrile 78 allene 22, 24, 68, 77, 79, 121, 175, 227 allene dimerization 369 allene racemization 19 allene–ethylene 264 allyl (1-trimethylsilylvinyl) ether 149 allyl cation 77 allyl radical 15, 17, 69, 107, 125, 149, 203, 239, 308, 316, 352 allyl radicals 134, 141–144, 146, 182, 192, 195, 203, 230, 251, 297, 333, 367 allylbenzene 297 allylcycloheptatrienyl ether 395 allylic radical 65, 69, 203 allylidenecyclopropane 127, 131 allylidenecyclopropane rearrangement 128 allylphenyl ether 351, 353 allylvinyl ethers 146 AM1 63, 349 ansaradiene 404 anti,anti-tetracyclo[4.4.0.0.2,507,10]deca3,8-diene 342 anti- and syn-6-methylbicyclo[2.1.1]hexenes 101 anti-1,2,5,6-tetracyanotricyclo[4.2.0.02,5]octane 262 anti-1,5-bishomocycloheptatriene 314

Subject Index anti-9-vinyl-cis-bicyclo[4.3.0]nona-2,4,7-triene 393 anti-9-vinylbicyclo[6.1.0]nona-2,4,6-triene 393 anti-pentacyclo[5.2.1.0.2,90.3,506,8]decane 362 anti-tetracyclo[6.2.2.0.2,703,6]dodeca-4,9,11triene 404 anti-tricyclo[3.1.0.02,4]hexane 113 anti-tricyclo[5.3.0.08,10]deca-2,5-diene 359 azulene 326, 346

b-13C naphthalene 327 b-methyl and a-methylnapthalene 327 B3LYP 11, 35, 111, 144, 180, 300 B3LYP (UHF) 179 B3LYP(RHF) 179 B3LYP/6-31+G* 146 B3LYP/6-31G* 352 B3LYP/6-31G 80, 96, 97 B3LYP/6-311G(3d,2p) 12 barbaralane 229, 294 barettane 332, 349 barrelene 214, 224 basketene 331, 339, 343, 392 Becke3LYP/6-31G* 150 benzene 88, 92, 95, 96, 98, 221, 223, 224, 235, 241, 297, 304, 392, 402, 404, 421 benzene dimer 401, 403, 404, 406, 422 benzene dimertricyclo[4.2.2.22,5]dodeca-3,7,9,11tetraene 408 benzene oxide 188 benzene topomerization 92 benzene-1,2-bis-13C 92 benzene-1,4-biradical 87 benzene-1,4-diyl 326 benzobarbaralane 294 benzobicyclo[2.2.0]hexadiene 329 benzocyclobutadiene 213, 214 benzocyclobutene 173, 225–227, 358, 419 benzocyclobutenylidene 419 benzocycloheptatrienes 384 benzocycloheptene 394, 395 benzocyclopropene 173 benzodicyclobutadiene 325 benzofulvene 328 benzonorpinene 391 benzvalene 88, 89, 91, 93, 364, 395 benzyl radicals 424 bicyclo[1.1.0]butane 35, 58 bicyclo[1.1.1]pentane 68 bicyclo[2.1.0]pentane 30, 66, 67, 113, 282, 292, 312, 389 bicyclo[2.1.0]pentane bond 359 bicyclo[2.1.0]pentane-5-spirocyclopropane 198

433

bicyclo[2.1.0]pentene 56, 280 bicyclo[2.1.1]hexane 101, 154 bicyclo[2.1.1]hexene 102, 348 bicyclo[2.2.0]hex-2-ene 114 bicyclo[2.2.0]hexane 152, 262, 290 bicyclo[2.2.1]hept-2-ene 143, 196 bicyclo[2.2.1]heptadiene 182 bicyclo[2.2.2]octane 255 bicyclo[3.1.0]hex-1,3-diene 93 bicyclo[3.1.0]hexane 30, 161 bicyclo[3.1.0]hexene 101, 103, 106, 159 bicyclo[3.2.0]hept-1-ene 198, 199 bicyclo[3.2.0]hept-2-ene 196 bicyclo[3.2.0]hept-6-ene 37, 202 bicyclo[3.2.0]hepta-1,3,6-triene 174 bicyclo[3.2.0]hepta-1,3-diene 185 bicyclo[3.2.0]hepta-2,6-diene 180, 182, 183 bicyclo[3.2.0]hepta-3,6-dien-2-one 140 bicyclo[3.2.1]octa-2,4-diene 239 bicyclo[3.2.1]octa-2,5-dienes 245 bicyclo[3.2.1]octa-2,6-diene-4-d 243 bicyclo[3.2.1]octa-2,6-diene 240, 243 bicyclo[3.2.2]nona-2,5,7-triene 294 bicyclo[3.2.2]nonatriene 294 bicyclo[3.3.0]diyl 411 bicyclo[3.3.0]oct-2-ene 267 bicyclo[3.3.0]octa-2,6-diene 140, 181, 230, 240 bicyclo[3.3.0]octa-2,7-diene 241 bicyclo[3.3.0]octa-3,6-dien-2,8-diyl 225 bicyclo[3.3.0]octadiendiyl 221 bicyclo[3.3.0]octadienyl biradical 217 bicyclo[3.3.0]octan-2,6-diyl 263 bicyclo[3.3.1]nona-2,6-diene 313 bicyclo[3.3.1]nona-2,7-diene 308 bicyclo[4.1.0]hept-1(2)-ene 202 bicyclo[4.1.0]hept-3-ene 204 bicyclo[4.1.0]heptane 204, 312 bicyclo[4.1.0]hepta-2,4-6(7)-triene 175 bicyclo[4.1.1]octa-2,4-diene 236 bicyclo[4.2.0]oct-1,5-diene 247 bicyclo[4.2.0]oct-7-ene 263 bicyclo[4.2.0]octa-1,5,7-triene 228 bicyclo[4.2.0]octa-1,5-diene 123, 126, 249 bicyclo[4.2.0]octa-2,3,7-triene 214 bicyclo[4.2.0]octa-2,4,7-triene 216, 222, 328 bicyclo[4.2.0]octa-2,4-diene 231, 233, 237 bicyclo[4.2.0]octa-2,7-diene 231, 233, 234 bicyclo[4.2.0]octatriene 421 bicyclo[4.2.1]nonatriene 287 bicyclo[4.2.2]deca-1,5-diene 371 bicyclo[4.2.2]deca-2,4,7,9-tetraene 338 bicyclo[4.2.2]decatetrene 331 bicyclo[4.3.0]nona-1,6-diene 307

434

Subject Index

bicyclo[4.3.0]nona-2,4-diene 300 bicyclo[4.3.0]nonadiene 55, 307 bicyclo[4.3.2]undeca-2,4,7,10-tetraene 393 bicyclo[4.4.0]-deca-1(2),5(6),7-triene 361 bicyclo[4.4.0]deca-2,8-diene 365 bicyclo[5.1.0]oct-2-ene 267 bicyclo[5.1.0]octa-2,4-diene 235, 236 bicyclo[5.1.0]octa-2,5-diene 229 bicyclo[5.1.0]octa-2,5-diene-4,8-diyl 224 bicyclo[5.2.0]nona-1,3,5,8-tetraene 280 bicyclo[5.2.0]nona-1,3,5-triene 292 bicyclo[5.2.0]nona-2,4,8-triene 283 bicyclo[5.2.0]nona-2,5,8-triene 281 bicyclo[5.2.0]nona-2,5-diene 298 bicyclo[5.5.0]dodecapentaene 400 bicyclo[6.1.0]nona-2,4-diene 299 bicyclo[6.1.0]nona-2,5-diene 314 bicyclo[6.1.0]nona-2,6-diene 298 bicyclo[6.1.0]nona-2-ene 315 bicyclo[6.1.0]nona-3,5-diene 299 bicyclo[6.1.0]nonatriene 286 bicyclo[6.2.0]deca-1,3,5,7,9-pentaene 328 bicyclo[6.2.0]deca-1,7-diene 369 bicyclo[6.2.0]deca-2,4,6,9-tetraene 332, 342 bicyclo[6.2.0]deca-2,6-diene 367 bicyclo[6.3.0]undecapentaene 383 bicyclo[6.4.0]dodeca-2,4,6,9,11-pentaene 402, 403 bicyclobutane 36, 89, 327 bicyclopropenyl 89 biphenyl-2-ylacetylene 14 bis-allyl biradicals 250 bis-basketenes 408 bisallene 95 bisallyl biradical 119, 120, 122, 242, 243, 250, 262, 302 bisallyl radical 295 biscyclopropenyl 88 biscyclopropyl 160, 161 biscyclopropylidene 124, 127 biscyclopylidenes 126 bisfulvene 400 bishomobenzene 218 BL3LYP/6-31G* 326 BLYP 147 bond dissociation energy (BDE) 3 bornadiene 372 bullvalene 143, 293, 329–331, 336, 423 butadiene 35, 43, 159, 253 –257, 263, 301 butalene 87 butane 17 butanone enolate 30 butenylbenzenes 363

c,t,c,c-nonatetraene 283 calicheamicin 88 carbene 173, 403 carbene–carbene rearrangement 173 4 £ 4 CAS MCSCF 48 CAS (6,6)/6-311G* 58 CAS(12, 12)PT2/6-31G(d) 133 CAS/MP2 6-311+G(d,p) 227 CASPT2 68 CASPT2N 74, 176, 179, 200, 201, 224, 286 CASPT2N 6-31G*//CASSCF(2,2)/6-31G* 175 CASPT2N/CASSCF(8/8)/c-31G(d) 57 CASSCF 69, 144, 147, 180, 227, 285 CASSCF (8,8) 200 CASSCF (8/8) 179 CASSCF 3-21G* 143 CASSCF(4,4).T2ZP 30 CASSCF(8,8)/6-31G* 201 CASSCF(8/8)/6-31G 224 CASSCF/3-21G 57 CASSCF/6-31G 96, 98 CASSCF/6-31G* 312 CASSCF/CASPT2 153 CCSD 15 CCSD/6-311G* 58 chemiluminescent 402 chorismate 151 chorismate mutase 152 chorismic acid 151 CI-SD/6-31Gpp 15 CIDNP 123, 249 circumambulatory 178, 179 circumambulatory 1,3-shifts 56 cis-1,2,3,4,5,6-hexadeuteriocyclohexenes 159 cis-1,2,3,4-tetradeuteriocyclobutane 48 cis-1,2,3,4-tetramethylcyclobutene 38 cis-1,2,4-cyclodecatriene 373 cis-1,2,4-hexatriene 118 cis-1,2,6-cyclononatriene 310 cis-1,2-bis(cis-propenyl) cyclobutane 252 cis-1,2-diethynylcyclobutane 228 cis-1,2-diethynylethene 86 cis-1,2-dimethylcyclobutane 50 cis-1,2-dipropenylcyclopropane 192 cis-1,2-divinylcyclobutane 251, 253, 256, 367 cis-1,2-divinylcyclohexane 374 cis-1,2-divinylcyclopentane 315 cis-1,2-divinylcyclopropane 143, 239, 391 cis-1,3,5-hexatriene 106, 108, 118 cis-1,3,8-nonatriene 317 cis-1,3,9-decatrienes 376 cis-1,3-cyclohexadiene 37 cis-1,3-pentadiene 79, 118

Subject Index cis-1,4-pentadiene 162 cis-1-(3-hydroxyphenyl)-2-vinylcyclopropane 390 cis-1-ethynyl-2-methylcyclopropane 115 cis-1-methylene-8,9-dihydroindene 349 cis-1-phenyl-2-vinylcyclopropane 389 cis-1-phenylbutadiene 335 cis-2,3-dideuterio-trans-1-(Z-2-deuterio-1-tertbutylethenyl)cyclopropane 64 cis-2,3-dideuteriobicyclo[2.1.0]pentanes 66 cis-2,3-dimethyl-anti-1-chlorocyclopropane 77 cis-2,3-dimethyl-trans-1-butenylidenecyclopropane 130 cis-2,3-divinylaziridine 193 cis-2-(2-propyl)-1(E)-propenylcyclopropane 162 cis-2-alkylvinylcyclobutane 205 cis-2-alkylvinylcyclopropane 206 cis-2-butene 49 cis-2-deuterio-1-allenylcyclopropane 117 cis-2-ethynylcyclopropanecarboxaldehyde 184 cis-2-ethylvinylcyclobutene 155 cis-2-methyl-1,3,5-hexatriene 118 cis-2-methyl-1-(trans- and cis-1-propenyl)cyclobutane 155 cis-2-methylvinylcyclopropane 162 cis-3,4-dimethyl-1,5-cyclooctadiene 257 cis-3,4-dimethyl-3,4,5,6-tetrahydropyridazines 49 cis-3,4-dimethyl-cis,cis-1,5-cyclooctadiene 253 cis-3,6-diphenyl-4,4,5,5-tetracyano-1,2-benzocyclohexanes 225 cis-3-methyl-1,3,6-heptatriene 187 cis-3-methyl-2-vinylmethylenecyclopropane 129 cis-5,6-dideuterio-1,3-cyclohexadiene 114 cis-5,6-dimethyl-1,3-cyclohexadiene 109 cis-7,8-dimethylenebicyclo[4.2.0]octane 123 cis-7-butenylidenebicyclo[4.1.0]heptanes 131 cis-8,9-dihydrobenzoindene 294 cis-8,9-dihydroindene 283, 287, 294 cis-9,10-dihydronaphthalene 330, 331, 336, 340 cis- and trans-1,2-dideuteriospiropentane 72 cis- and trans-1,2-divinylcyclopropane 191 cis- and trans-1,3-hexadiene-5-yne 98 cis- and trans-2-ethynyl-1-vinylcyclopropane 184 cis- and trans-2-methylvinylcyclopentae 268 cis- and trans-2-methylvinylcyclopropane 63 cis- and trans-3,4-diphenyl-1,2-benzocyclobutenes 225 cis- and trans-4,5-dimethyl-1-carbethoxyspiropentanes 74 cis- and trans-4-methylenebicyclo[5.2.0]nona2-ene 368 cis- and trans-5,6-divinycyclooctenes 413

435

cis- and trans-6,9-dimethylspiro[4.4]nona-1,3diene 307 cis- and trans-7,8-dimethylenebicyclo[4.2.0]octane 369 cis- and trans-8,9-dihydroindene 281 cis- and trans-bicyclo[4.3.0]nona-2,4-diene 300 cis- and trans-bicyclo[4.3.0]nona-2-ene 317 cis- and trans-bicyclo[4.4.0]deca-2-ene 375 cis- and trans-oxatrishomobenzene diesters 311 cis,anti-2,3-dideuterio-1-vinylcyclopropane 60 cis,anti-2,3-dimethylbicyclo[2.1.0]pentanes 66 cis,anti,cis-tricyclo[6.4.0.02,7]dodeca-3,5,9,11tetraene 401 cis,cis-1,3,5,7-octatetraene 237 cis,cis-1,3,5-heptatriene 204 cis,cis-1,3-cycloheptadiene 202 cis,cis-1,3-cyclooctadiene 263 cis,cis-1,4,6-octatriene 187 cis,cis-1,4-dideuterobutadiene 257 cis,cis-1,5-cyclooctadiene 251, 253, 255, 257 cis,cis-1,6-cyclodecadiene 375 cis,cis-2,8-decadiene-4,6-diyne 237 cis,cis-3,5-octadiene-1,7-diyne 214 cis,cis,cis-1,3,6-cyclooctatriene 233 cis,cis,cis,cis-2,4,6,8-decatetraenes 237 cis; cis; cis; cis; trans-cyclodecapentaene 332, 335 cis,cis,cis,trans-1,3,5,9-cyclodecatetraene 355 cis,cis,cis,trans-[5.5.5.6]tribenzofenestrane 11 cis,cis,cis,trans-cyclodecapentaene 343 cis,cis,cis-trishomobenzene 346 cis,cis-cyclooctadiene 261 cis,syn,cis-tricyclo[8.2.0.02,9]dodeca-3,5,7,11tetraene 401 cis,trans,cis-1,3,5-cyclooctatriene 232, 233 cis,trans,cis-1,3,5-cycloheptatriene 180 cis,trans,cis,cis-1,3,5,7-cyclononatetraene 281 cis,trans-1,3-cycloheptadiene 37 cis,trans-1,3-cyclooctadiene 263 cis,trans-1,5-cyclodecadiene 374 cis,trans-1,5-cyclooctadiene 251, 256 cis,trans-3,4-dimethyl-2,4-hexadiene 38 cis,trans-muconic acid 38 cis,trans,trans-1,4,7-cyclononatriene 312 cis,trans,trans-cyclododeca-1,5,9-triene 413 cis- or trans-1,2-dideuterioethylene 160 cis-(3 0 -cyclopropenyl)indenes 405 cis-b-ethylstyrene 360 cis-b-methylstyrene 362 cis-s-bishomobenzene 246 cis-anti-1,2-dimethyl-cis-3,4-dideuteriocyclobutane 49 cis-benzene bisoxide 246 cis-benzenediimine 246

436

Subject Index

cis-bicyclo[4.3.0]nona-2-ene 317 cis-bicyclo[4.3.0]nona-3,7-diene 301 cis-bicyclo[5.3.0]deca-2-ene 375 cis-bicyclo[6.1.0]nonatriene 281, 282 cis-bicyclo[6.2.0]deca-2,4,6-triene 355 cis-bicyclo[6.4.0]dodeca-2,4,6,9,11-pentaene 401 cis-bicyclo[7.1.0]deca-2,3-diene 373 cis-cyclobutene-3,4-dicarboxylate 38 cis-cyclodecapentaene 331, 337 cis-cyclononatetraene 284, 289, 290 cis-cyclooctene 268 cis-dideuterioethylene 47 cis-dihydroindene 282 cis-divinylcyclopropane 330 cis-divinylcyclopropane moiety 293 cis-divinyldihydroacenaphthene 419 cis-monohomobenzene oxide 246 cis-tricyclo[5.1.0.02,4]oct-5-ene 246 cis-trishomobenzene 312 CISD(T)/DZP//B3LYP/DZP 93 CISD(T)/DZP//B3LYP/DZP+ZPVE 92 Claisen 149, 151 Claisen rearrangement 146, 351, 354, 389 cohesive energy density 148 conservation of orbital symmetry 30, 39, 49, 89, 131, 135 Cope rearrangement 134, 180, 195 Cope rearrangement CASSCF 116 corannulene 100 cubane 214, 222, 223 cuneane 214, 223 cyclobutadiene 33, 34, 219 cyclobutadiene dimers 214 cyclobutane 46, 47, 68, 242 cyclobutanone 28 cyclobutene 36, 38, 66, 110, 140, 246, 288, 329, 351, 369, 404, 411, 417 cyclobutene electrocyclization 402 cyclobutene ring opening 174, 180, 233, 281 cyclobutylidene 295 cyclodecapentaenes 329 cycloheptatetraene 174 cycloheptatriene 182, 183 cycloheptatrienyl radical 388 cycloheptatrienylidene 175, 176, 389 cycloheptene 205 cyclohexadiene 106 cyclohexadienyl radical 151 cyclohexane diyl 152 cyclohexane-1,4-diyl 141, 195, 199, 295, 343, 370 cyclohexanediyl 154 cyclohexanone 205

cyclohexatriene 88, 92, 412 cyclohexene 154, 158, 160–162 cyclohexene oxide 205 cyclohexene-1,4-diyl 116 cyclohexenyl radical 305 cyclononatetraenide anion 282, 337 cyclononyne 316 cyclooctatetraene 214, 219, 221, 222, 328, 421, 422 cyclooctatetraene dimers 422 cyclopentadiene 53, 182, 301, 392 cyclopentadiene dimer 301, 363 cyclopentadienyl radical 173 cyclopentane-1,3-diyl 67 cyclopentane-1,3-singlet 68 cyclopentene 30, 59, 66 cyclopropane 25, 60, 160, 162, 384 cyclopropane structural isomerization 29 cyclopropane to propylene 205 cyclopropanone 228 cyclopropene 20, 24, 173, 297, 358 cyclopropyl radical 125, 126, 245 cyclopropylallene 117 cyclopropylidene 20 cyclopropylidenespiropentane 245 density functional theory 62 deuterium 192 deuterium isotope effect 22, 27, 121 deuterium kinetic isotope effect 15, 54, 56, 62, 203, 215, 266, 352 Dewar benzene 88 –91, 94, 394 DFT 35, 58, 65, 80, 88, 116, 144, 147, 176, 300, 376 DFT-B3LYP 312 diademane 311, 331, 346 diazocycloheptatriene 280, 388 diazocyclopentadiene 227, 383 dibenzocyclooctadiene 226 dibenzocyclooctatetraene 421 dideuteriocyclobutadiene 33 dideuterioethylene 15 dideuteriomethylenecyclobutane 69 dideuteriovinylcyclopropane 161 dielectric constant 148 Diels–Alder 112, 159, 160, 176, 214, 257, 331 Diels–Alder adducts 263, 395 Diels–Alder cycloaddition 420 Diels–Alder fragments 365 Diels–Alder isomer 339 Diels–Alder products 182, 317, 375 Diels–Alder reaction 126, 158, 197, 223, 224, 233, 254, 259, 287, 288, 294, 297, 301, 317,

Subject Index 337, 338, 340, 344, 353, 372, 374, 392, 393, 396, 403 –405, 407, 417, 421, 423 diethynyl epoxide 173 difluorocarbene 161 dihydro-a,s-indacene 400 dihydrobullvalene 229 dihydroindene 288, 289 dihydropentalene 221, 223, 227– 229 dimethanospiro[2.2]alkaplanane 12 dimethyl-2,6-dicyano-4,8-diphenylsemibullvalene 218 dimethyl maleate 77 dimethylacetylene dicarboxylate 263 dimethylallyl radicals 79 dimethylcyclohexenes 155 dimethylenecyclobutanes 127 dimethylprismane 94 diphenylisobenzofuran 87 dispiro[2.0.2.2]oct-7-ene 246 dispiro[2.0.2.4]deca-1,5-diene 358 dispiro[2.0.2.4]deca-7,9-diene 356, 357 dispiro[2.2.2.2]deca-4,9-diene 356 dispiro[5.0.5.2]tetradeca-1,8-dienes 260 distal-1,4-dimethylspiropentane 73 double Zeta 34 dynamic correlation 116 dynamical control 102 dynamical effects 28, 104 dynamical factors 286 dyotropic reactions 17 DZP-TCSCF 28 DZSCF/MP2/ZPVE 336 E-1-deuterionortricyclylethene 314 E-1-deuteriobutadiene 40 E and Z-4-methyl-3-methylene-1,5-octatriene 200 E; E- and Z; Z-2,6-octadiene 134 E; E-propenyl-2-butaenyl ether 147 E; Z-2,6-octadiene 134 electrocyclic 390 electrocyclization 176, 220, 237, 238, 282, 300, 329, 335, 342, 361, 383, 384, 386, 392, 403, 419 electrocyclize 337, 351, 355, 373 emission signals 357 endo-3,3-difluorotricyclo[3.2.1.02,4]oct-6-ene 244 endo-5,6-diphenylbicyclo[3.1.0]hexene 105 endo-5-carbomethoxy-1,5-dimethylbicyclo[2.1.0]pentene 57 endo-6-methyl-5-methylenebicyclo[2.2.1]hept2-ene 242 endo-6-methylbicyclo[3.1.0]hexenes 101 endo-6-vinylbicyclo[3.1.0]hex-2-ene 239, 244

437

endo-7-methoxy-3-methylenebicyclo[3.1.0]hex2-ene 190 endo,endo-4,5-dideuteriobicyclo[6.1.0]nona-2,6diene 299 endo,endo-8,9-dideuteriobicyclo[5.2.0]nona-2,5diene 299 endo- and exo-1-ethyl-1-methyl-trans-dihydroindene 284 endo- and exo-3-methyl-4-trans-propenyl 257 endo-tetracyclo[5.3.0.0.2,1003,6]deca-8-ene 363 endo-tricyclo[3.2.1.02,4]oct-6-ene 243 ene isomer 317 ene reaction 81, 264, 267, 268, 304, 316, 391 equilibrium isotope effect 192 ergosterol system 204 Erying absolute rate theory 3 erythro-3,4-dimethyl-1,5-hexadiene 134 esperamicin 88 ESR 43, 44, 67, 120, 416 ethane 16, 17 ethane cleavage reaction 17 ethane dyotropic 16 ethyl carbene 30 ethylene geometric isomerization 15 ethylene 46, 48, 68, 72, 77, 78, 80, 107, 154, 197, 234, 345 ethynylcyclopentadiene 172, 173 etracyclo[4.4.0.0.2,1005,7]decadiene 331 exo-2,3,5,6-tetradeuteriobicyclo[2.2.0]hexane 152 exo-4-deuteriobicyclo[3.2.1]octadiene 240 exo-4-deuteriobicyclo[3.3.0]octadiene 240 exo-7-deuterio-endo-6-acetoxybicyclo[3.2.0]hept2-ene 196 exo-7-methyl-6-methylenebicyclo[3.2.0]hept-2ene 242 exo-9-methylbicyclo[6.10]nona-2,6-diene 299 exo- and endo-1-carbethoxy-cis-8,9-dihydroindene 284 exo- and endo-tricyclo[4.4.2.0]dodecatetraene 403 exo-tetracyclo[4.3.0.0.2,405,7]nonene 289 exo-tricyclo[3.2.1.02,4]oct-6-ene 245 exo-tricyclo[4.4.2.02,5]dodeca-3,7,9,11-tetraene 402, 403 exo,cis-2,3-dicarbomethoxybicyclo[2.2.0]hexane 153 exo,cis-5,6-dideuteriobicyclo[2.2.0]hex-2-ene 114 exo,exo-6,7-dideuteriobicyclo[3.2.0]hept-2-ene 197 exo,exo- and exo,endo-tetracyclo[4.4.2.0.2,507,10]dodeca-3,8,11-trienes 403 extended Hu¨ckel theory 25

438

Subject Index

fall off behavior 2 Feist’s ester 45 fenestranes 11 fulvenallene 172, 173 fulvene 93, 95, 96, 365 furan 190, 233 G2 35, 91 g-alkyl allylphenyl ethers 353 geometric isomerization 25 GVB/6-31G* 27 heavy atom tunneling 33 hemicarceplexes 175 hemicarcerand 34 heptalene 400 hexacyclo[4.4.0.0.2,40.3,100.5,807,9]decane 349 hexadiene 416 hexafluorovinylcyclopropane 161 hexahydro-3,47-methanocyclopenta[a]pentalene 407 hexahydroindenes 317 hexalin 108 hexamethyl Dewar benzene 90 hexamethylbenzene 90 hexamethylbenzvalene 90 hexamethylprismane 90 hexatriene 86, 417 HF/3-21G level 56, 69 Hinshelwood 2 Hoffmann 4 homo 1,5- and 1,7-hydrogen shift 236, 237 homo-1,5-alkyl shift 363 homo-1,5-hydrogen shift 63, 64, 75, 115, 235, 267, 287, 294, 296 homo-3,3-shift 289 homobasketene 392 homodienyl 1,5- and 1,7- hydrogen shift products 236 homodienyl-1,5-hydrogen shift 162, 177, 187, 205, 206, 241, 296, 299, 308, 309, 314, 315, 374 homofulvene 189, 190 homoheptafulvene 287 homotropilidiene 230 Hu¨ckel p overlap 7 Hu¨ckel aromatic transition state 16, 17 hydrogen bond 148, 353 hydrogen bonding 352 hydrogen shift 30, 98, 106, 185, 297, 302, 353 hydrophobic-like 149 hyperbolic paraboloid surface 144

hyperconjugative isotope effect 237 hypostrophene 332, 345 indane 277, 279, 280, 290, 292, 307 isobullvalene 331, 337, 338, 346 isoindane 291 isoindene 277 isolumibullvalene 331, 338 isopropenylcyclobutane 155 isopropenylcyclopropane 160 isopropylidenecyclooctatetraene 296 isotoluenes 188, 189 isotope effect 124, 149, 151, 297 Kassel 2 ketene 78 kinetic effect 264 kinetic isotope effect 143, 147, 193, 234, 237, 314 kinetic or equilibrium deuterium isotope effect 134 Kirkwood–Onsager 148, 352, 353 least motion 79 Lewis acids 149 Lindemann 2 lumibullvalene 331, 336, 337, 348 lumiisobullvalene 332, 338 luminescence 89, 94, 219 lumitropolone methyl ether 180 maleic anhydride 226, 277 MCSCF 15, 62, 63, 88 MCSCF(6,6)/3-21G-CI 120 MCSCF(8/8)/6-31G 215 meso,cis,trans-1,3,4,6-tetradeuterio-1,5-hexadiene 152 meso; E; Z-1,3,4,6-tetradeuterio-1,5-hexadiene 138 meso- and threo-bis-2,2 0 methylenecyclopropanyl 250 meso- and chiral-2,3,7,8-decatetraenes 249 meso-1,2,3,4-cycloheptatetraene 172 meso-1,2,4,6,7,9-cyclodecahexaene 329 meso-3,4-dicarbomethoxy-1,5-hexadiene 154 meso-3,4-dimethyl-1,5-hexadiyne 95 meso-bis-2-methylenecyclopentyls 139 meta-toluene diyl 174 methallyl radical 243 methane 11 methano[10]annulene 382 methyl radicals 17 methyl-7,7-dicyanonorcaradienes 178 methylene 161, 384 methylene-d2 161

Subject Index methylenebicyclo[2.1.1]hexane 199 methylenebicyclo[2.2.0]hex-2-ene 189 methylenecyclobutane 68, 72 methylenecyclobutane rearrangement 199 methylenecyclobutene 58, 59 methylenecyclohexane 204 methylenecyclopropane 41, 246, 296 methylenecyclopropane rearrangement 129, 249 methylenespiropentane 124–127 MINDO 143, 160 MINDO 3 88, 91, 143, 160 MINDO/3 36, 141 MM2 219 MM2-like 218 MMX 297, 309, 345, 349, 409 Mo¨bius p overlap 7 Mo¨bius aromatic transition state 16 molecular mechanics 406 MP2/6-311+G(2d,p) 12 MP2/6-31G(d) 12 MP2/6-31G 80 MP2/6-31G* 326 MP2/TZ2P//MP2/DZd and B3LYP/TZP 335 MP4(SDQ) 147 MP4(SDQ)/6-31G* 69 mr-ci+Q/cc-pVDZ/DFT 99 MRCI-CASSCF 48 MRCI–MCSCF 20 MROPT2 179, 180 N; N; N-trimethyl-cis-triazatrishomobenzene 313 naphthalene 326, 328, 329, 333, 335, 338, 342, 346, 347, 384, 385, 421 naphthocyclopropane 416 Nenitzescu’s hydrocarbon 330, 339, 340 neocarzinostatin 174 NIH shift 188 non-statistical 102 non-statistical dynamics 68, 197 norbornadiene 182, 183 norbornatriene 174 norcaradiene 176, 179, 188, 292, 382, 383, 386, 389 norcaradiene–tropilidene 354 norpinene 188 o-allylphenol 351 o-allyltoluene 354 o-ethylstyrene 357 o-quinodimethanes 225 o-xylylene 225, 226, 351, 419, 420 octadienydiyl 218 octamethylsemibullvalene 217 octavalene 222

439

octene 214 orbital symmetry control 178 oxepine 188 oxy-anion 367 oxygen 44, 120, 201, 260, 416 oxygen trapping 88, 99, 199, 201, 261, 360, 361 p-benzyne 86, 88 p-cymene 177 p-diethanobenzene biradical 357 p-diethylbenzene 356, 357 p-xylylene 358, 424 PE spectroscopy 15, 215, 300 pentacyclo[3.3.0.0.2,40.3,706,8]octane 214, 223 pentacyclo[3.3.2.0.2,40.3,706,8]deca-9-ene 343 pentacyclo[4.4.0.0.2,50.3,804,7]deca-9-ene 339 pentacyclo[5.2.1.0.2,60.3,504,10]deca-8-ene 349 pentacyclo[5.3.0.02,60.3,508,10]decane 359 pentacyclo[5.4.0.0.2,50.3,904,8]undeca-10-ene 392 pentacyclo[5.5.0.0.2,120.3,906,8]dodeca-4,10-diene 403 pentacyclo[6.4.0.0.2,403,1005,9]dodeca-6,11-diene 405 pentacyclo[6.4.0.0.2,50.3,1004,9]dodeca-6,11-diene 404 pentacyclo[6.4.0.0.2,7,0.3,1206,9]dodeca-4,10-diene 402 pentadienyl radical 193, 297 pentaprismane 332, 345, 349 perfluoro-2-butyne 228 peri-naphthobicyclo[3.1.1]heptene 417 peri-naphthobicyclo[3.2.0]heptene 417 phenalene 416 phenanthrene 14 phenol 188, 390 phenoxy radical 352 phenyl acetylene 14, 280 phenyl carbene 175 phenylalanine 151 phenylallyl ethers 352 phenylcyclopropane 29 phenylvinylcyclopropane 354 phenylvinylidene 14 photoelectron spectrum 88 planar carbon 11 prephenic acid 151 previtamin D2 108 primary deuterium kinetic isotope 79, 118, 278 primary isotope effect 384 prismane 88, 89, 91, 94 propylene 25, 30, 46, 48, 80, 162, 304, 354 propyne 20, 22, 24 pseudoindene 277

440

Subject Index

2R; 3R-cis-2,3-dicarbomethoxy-1-dideuteriomethylenecyclobutane 71 R-2,3-pentadiene 78 R-5-methyl-E-1,2,6-octatriene 200 R; R-4,5-dimethyl-Z; Z-1,1,1,8,8,8-hexadeuterio2,6-octadiene 137 radical chain 113 radical chain process 55, 327, 385 radical process 189, 364 radical reactions 189 Ramsperger 2 RHF/3-21G 80 RHF/6-21G 96 RHF/6-31G* 349, 409, 410 Rice 2 ring walk 185 ROHF+(3 £ 3)-CI 27 rotational isotope effects 117, 125

spiropentane 72 spiropentane/methylenecyclobutane 77 STO-3G 34 styrene 173, 227, 234, 357, 358, 389 syn-1,4-diethylidenecyclohexane 266 syn-4,4-dideuterio-cis-1,2-dimethylspiropentane 73 syn-9-carbethoxy-cis-bicyclo[6.1.0]nonatriene 284 syn- and anti-7-ethylidenebicyclo[3.2.0]hept-2enes 243 syn- and anti-9-ethyl-9-methylbicyclo[6.1.0]nonatriene 284 syn- and anti-tricyclo[5.3.0.02,6]decane 375 syn,anti-bishomobarrelene 362 syn,anti-isomer of 3,4-diethylidenecyclobutene 95 syn-cyclobutadiene dimer 223 syn-E-2,3,2 0 -trideuteriovinylcyclopropane 61 syn-tricyclo[4.2.1.12,5]deca-3,7-diene 363 syn-tricyclo[4.4.0.02,5]deca-3,7,9-triene 340 syn-tricyclo[5.3.0.02,6]deca-3,9-diene 363 syn-tricyclo[5.3.0.08,10]deca-2,5-diene 360 syn-tricyclo[8.2.0.02,9]dodeca-3,5,7,11-tetraene 402 syn-Z-2,3,2 0 -trideuteriovinylcyclopropane 61

SCF 179 SCF/6-31G* 110 SCF-MP2/6-31G* 40 secondary deuterium isotope effect 27, 41, 42, 63 secondary deuterium kinetic isotope 40, 110, 141, 160, 252 semibullvalene 143, 214, 217, 218, 293, 411 shift 53, 173, 387 shikimic acid 151 snoutene 331, 343 solvent kinetic isotope effect 149 solvolysis reaction 149 spiro[2,4]hepta-1,4,6-triene 174 spiro[2.4]heptadiene 185 spiro[2.5]octa-4,6-diene 357 spiro[2.6]nona-4,6,8-triene 292 spiro[2.6]nonatetraene 280 spiro[3.4]octatriene 227 spiro[4.4]nona-1,3-diene 55, 279, 307 spiro[4.4]nona-1,3,6-triene 279, 291 spiro[4.4]nona-1,3,7-triene 291 spiro[4.4]nonatetraene 279 spiro[4.4]nonatrienes 290 spiro[4.6]undecapentaene 383 spiro[benzocyclobutene-1,70 -cyclohepta-10 ,30 ,50 triene] 419

t,c,c,c-nonatetraene 284 TCNE 281 TCSCF/DZ 27 tetra-tert-butylcyclobutadiene 35 tetra-tert-butylcyclopentadienone 35 tetra-tert-butyltetrahedrane 35 tetra-trimethylsilyltetrahedrane 35 tetracyanoethylene 225 tetracyano[3.3]paracyclophanes 357 tetracyclo[3.2.1.0.2,704,6]octane 243 tetracyclo[3.3.0.0.2,403,6]oct-7-ene 214, 223 tetracyclo[3.3.2.0.2,406,8]deca-9-ene 362 tetracyclo[4.2.0.0.2,40.3,5]octane 238 tetracyclo[4.2.0.0.2,403,5]oct-7-ene 214, 222 tetracyclo[4.2.0.02,805,7]octene 220 tetracyclo[4.2.1.0.1,602,5]decane 373 tetracyclo[4.2.1.0.2,805,7]nonene 287 tetracyclo[4.3.0.0.2,507,9]nonene 290 tetracyclo[5.1.0.02,40.3,5]octane 238 tetracyclo[5.2.1.0.2,603,10]deca-4,8-diene 346 tetracyclo[5.3.0.0.2,1003,6]deca-4,8-diene 332, 347, 348 tetracyclo[5.3.0.0.2,603,10]deca-4,8-diene 345 tetracyclo[5.3.0.0.2,803,6]deca-4,9-diene 347 tetracyclo[5.3.0.0.2,1003,6]deca-4-ene 347 tetracyclo[5.3.0.0.4,605,10]deca-2,8-diene 338

pteradactyladiene 332, 342 pyro- and isopyrocalciferol 108 pyrohypostrophene 332, 346 quadricyclane 182, 183 quadricyclo[2.2.0.02,5,03,6] hexane 88

Subject Index 2,5 6,8

tetracyclo[5.3.2.0. 0 ]dodeca-3,9,11-triene 405 tetracyclo[5.4.0.0.2,1104,10]undeca-5,8-diene 393 tetracyclo[5.5.0.0.2,403,10]dodeca-5,8,11-triene 403 tetradeuteriocyclohexene 159 tetrahedrane 33 tetrahydroindene 301 tetralin 357, 358, 362, 364, 365 tetramethylbutane 17 tetramethylene benzvalene 358, 365 tetramethylene biradical 47 tetramethylene Dewar benzene 358 tetramethyleneethane (TME) 118, 120, 298 tetramethyleneprismane 365 thallium cyclopentadienide 407 threo-3,4-dimethyl-1,5-hexadiene 134 threo-meso-bis-2-methylenecyclopentyls 139 TME 124 TMM 42, 44, 45, 125, 203 TMM singlet 41 TMM triplet 43 toluene 174, 176, 182, 188, 189, 354 torquoselectivity 40 trans,cis,cis,cis-2,4,6,8-decatetraenes 237 trans,cis,cis,cis-cyclononatetraene 282 trans,cis,cis-1,3,5-cyclodecatriene 373 trans,cis,cis-1,3,5-cycloheptatriene 180 trans,cis,cis-1,3,6-decatriene 374 trans,cis,cis,trans-2,4,6,8-decatetraenes 237 trans,cis,cis,trans-cyclo-1,3,5,7-decatetraene 356 trans,cis,trans-2,4,6-octatriene 109 trans,trans-1,3,5,7-octatriene 234 trans,trans-1,3,7,9-decatetraene 371 trans,trans-1,5-cyclodecadiene 374 trans,trans-1,6-cyclodecadiene 375 trans,trans-1,8-distyrylnaphthalene 418 trans,trans-pentadiene-1-d 257 trans- and cis-1-cyano-2-trans-propenylcyclobutane 156 trans- and cis-2-deuteriovinylcyclopropane 60 trans- and cis-2-isopropenyl-1-cyanocyclopropane 65 trans- and cis-7,8-dimethyl bicyclo[4.2.0]octa2,4-diene 237 trans- and cis-piperylene 388 trans-1,2,3,4-tetramethylcyclobutene 39 trans-1,2,9,10-tetrahydronaphthalene 355 trans-1,2-bis(trans-propenyl)cyclobutane 254 trans-1,2-cis,cis-dipropenylcyclopropane 192 trans-1,2-dicarbomethoxyindane 292 trans-1,2-dideuteriocyclobutane 47 trans-1,2-dideuteriocyclopropane 26

441

trans-1,2-dideuterioethylene 159 trans-1,2-dimethyl-cis-3,4-dideuteriocyclobutane 49 trans-1,2-dimethylspiropentane 73 trans-1,2-divinylcyclobutane 253, 254, 256, 257 trans-1,2-divinylcyclohexane 374 trans-1,2-divinylethylene oxide 193 trans-1,2-trans,trans- and trans,cis-bispropenylcyclobutanes 156 trans-1,3,5-hexatriene 106, 107 trans-1,3,8-nonatriene 317 trans-1,3,9-decatriene 375 trans-1,6-octadiene 267 trans-1-(cis-propenyl)-2-(trans-propenyl)cyclobutane 254 trans-1-ethynyl-2-methylcyclopropane 115 trans-2,3-dicarbethoxymethylenecyclopropane 45 trans-2,3-dimethyl-trans-1-butenylidenecyclopropanes 130 trans-2-butene 15, 241, 48 trans-2-ethylvinylcyclobutane 155 trans-2-methyl-1-(trans-propenyl)cyclopropane 64 trans-2-methylvinylcyclopropane 36, 163 trans-3,4-bis(phenylethynyl)-1,2,3,4-tetramethylcyclobutene 213 trans-3,4-cis,cis-1,5-cyclooctadiene 252 trans-3,4-dimethyl-1,2-dimethylenecyclobutane 119 trans-3,4-dimethyl-cis,trans-1,5-cyclooctadiene 252 trans-3,6-dideuterio-1,4-cyclohexadiene 112 trans-3-methyl-2-vinylmethylenecyclopropane 129 trans-3-propenylcyclobutene 351 trans-4,5-dimethyl-1,1-dicyanospiropentane 76 trans-4,5-dimethyl-1,3-dimethylenecyclopentane 196 trans-4,9-dimethyl-1,2,6,7-cyclodecatetraenes 361 trans-5,6-dimethyl-1,3-cyclohexadienes 110 trans-5,6-divinylcyclohexadiene 356 trans-6-octene-2-yne 264 trans-7-butenylidenebicyclo[4.10]heptanes 131 trans-8,9-dihydroindene 289 trans-9,10-dihydronaphthalene 335, 342 trans-bicyclo[4.3.0]nona-2-ene 316 trans-bicyclo[4.3.0]nona-3,7-diene 312 trans-bicyclo[4.4.0]deca-2,4-diene 373 trans-bicyclo[6.1.0]nonatriene 286 trans-cyclooctene 268 trans-dideuterioethylene 47

442

Subject Index

trans-diethynylethene 173 trans-dihydroindene 282, 284 trans-octahydronaphthalene 375 trans-trishomobenzene 312 transition state theory 3 tri-tert-butyl trimethylsilyl tetrahedrane 35 tricyclo[3.1.0.02,6]hexane 37 tricyclo[3.2.0.02,4]hept-6-ene 182 tricyclo[3.2.1.04,6]oct-2-ene 234, 235 tricyclo[3.2.2.02,4]nona-6,8-diene 297 tricyclo[3.3.0.02,6]octa-3,7-diene 214, 218, 411 tricyclo[3.3.0.02,6]octane 262 tricyclo[3.3.0.03,8]oct-3-ene 241 tricyclo[3.3.1.02,7]nona-3-ene 304 tricyclo[3.3.1.02,8]nona-3-ene 308 tricyclo[3.3.1.02,8]nonadiene (barbaralane) 293 tricyclo[4.1.0.01,3]heptane 201 tricyclo[4.1.0.02,4]heptane 204 tricyclo[4.1.0.02,7]hept-3-ene 182 tricyclo[4.1.0.02,7]heptane 37 tricyclo[4.1.0.02,7]heptene 180 tricyclo[4.2.0.01,3]oct-4-ene 249 tricyclo[4.2.0.01,3]octa-4-ene 250 tricyclo[4.2.0.01,5]oct-3-ene 249 tricyclo[4.2.0.01,5]octa-3-ene 250 tricyclo[4.2.0.02,5]octane 261 tricyclo[4.2.1.02,5]nonadiene 287 tricyclo[4.2.2.02,5]deca-3,7,9-triene 330, 339 tricyclo[4.2.2.12,5]undeca-3,7,9-triene 392 tricyclo[4.3.0.02,9]nona-4,7-diene 287 tricyclo[4.3.0.07,9]non-3-ene 312 tricyclo[4.4.0.02,5]decatriene 331 tricyclo[4.4.1.01,6]undeca-2,4,7,9-tetraene 382 tricyclo[5.1.0.02,8]octadiene or octvalene 214 tricyclo[5.1.0,02,8]octadiene 222 tricyclo[5.2.1.04,10]deca-2,5-diene 363 tricyclo[5.3.0.02,4]deca-5,9-diene 359 tricyclo[5.3.0.02,4]deca-5-ene 373 tricyclo[5.3.0.02,8]deca-3,5,9-triene 331, 348 tricyclo[5.3.0.02,10]deca-3,5,8-triene 337 tricyclo[5.3.0.02,10]deca-3,5-diene 235 tricyclo[5.3.0.02,10]deca-3,5-diene (dihydroisobullvalene) 347 tricyclo[5.3.0.02,10]decatriene (isobullvalene) 348 tricyclo[5.3.0.04,8]deca-2,5,9-triene 336 tricyclo[5.3.2.04,8]dodeca-2,5,9,11-tetraene 405 tricyclo[5.5.0.02,8]dodeca-3,5,9,11-tetraene 406 tricyclo[5.5.0.06,10]dodeca-2,4,8,11-tetraene 406 tricyclo[6.1.0.04,9]nona-2,6-diene 297 tricyclo[6.4.0.02,7]dodeca-3,5,9,11-tetraene 403 tricyclo[7.3.0.02,6]dodeca-3,5,7,9,11-pentaene 400

tricyclo[7.3.0.04,12]dodeca-2,5,7,10-tetraene 407 tricyclo[7.3.1.0.2,804,12]trideca-5,10-diene 416 tricyclo[6.2.0.03,6]deca-1,4,7,9-tetraene 328 tricyclo[7.1.0.02,10]deca-3,5,7-triene 338 trimethylene 27, 28, 30 trimethylene biradical 25, 243, 362 trimethylenemethane 41, 57, 120, 126, 250 trimethylenemethane biradical 127, 130, 190, 203, 228, 245, 296 triquinacene 311, 330, 346, 349 tris-(benzocyclobuteno)benzene 412 tris-oxacyclononatriene 313 triscyclobutenobenzene 411 trishomobenzene 311 trishomoconjugation 300 tropilidene 176, 344 tunneling 54, 110, 118 TZ+2P 15 UB3LYP+ZPE 179 UB3LYP/6-31G* 179 vicinal (1,5-) hydrogen shift 53 vicinal hydrogen 173, 372 vicinal hydrogen atom shift 29 vicinal hydrogen shift 20, 23, 25, 30, 66, 106, 115, 176, 205, 221, 243, 267, 277, 383, 389, 416 vicinal hydride shift 59 vinylacetylene 228, 229 vinylcarbene 20, 23, 24, 229, 358 vinylcyclobutane rearrangement 157 vinylcyclobutane 108, 154, 155, 158 vinylcyclopropane rearrangement 63, 66, 117, 163, 193, 294, 336, 346, 359, 423 vinylcyclopropane 29, 59, 161, 250, 267 vinylidene 14, 15, 20–23 vinylidene rearrangement 99, 317 vinylidenecyclopropane 57, 58 vinylmethylenebicyclo[3.2.0]heptane 370 vinylmethylenecyclopropane 127 vinyltrimethylenemethane biradical 128 vitamin D 118 vitamin D2 108 Walsh orbital 176 Woodward 4 Z-2-methylethylidenecyclobutane 70 Z,E-2,3,4,5-tetraphenyl-2,4-hexadiene 40 ZPVE 93 zwitterion 175

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