Advances in Theoretically Interesting Molecules: Volume 3

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ADVANCES IN THEORETICALLY INTERESTING MOLECULES

Volume 3

9 1995

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hv i

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This Page Intentionally Left Blank

ADVANCES IN THEORETICALLY INTERESTING MOLECULES Editor: RANDOLPH P. THUMMEL Department of Chemistry University of Houston VOLUME3

91995

JA! PRESS INC.

Greenwich, Connecticut

London, England

Copyright 91995 by JAI PRESSINC. 55 Old Post Road, No. 2 Greenwich, Connecticut 06836 JAI PRESSLTD. The Courtyard 28 High Street Hampton Hill, Middlesex TW12 1PD England All rights reserved. No part of this publication may be reproduced, stored on a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, filming, recording, or otherwise, without prior permission in writing from the publisher. ISBN: 1-55938-698-3

Manufactured in the United States of America

CONTENTS

LIST OF CONTRIBUTORS PREFACE

Randolph P. Thummel

~176

VII

ix

POLYNUCLEAR AROMATIC HYDROCARBONS WITH CURVED SURFACES: HYDROCARBONS POSSESSING CARBON FRAMEWORKS RELATED TO BUCKMINSTERFULLERENE

Peter W. Rabideau and Andrzej Sygula

CHEMISTRY OF CYCLOPROPARENES

Paul MiJIler

PLANAR DEHYDRO[8]ANNULENES AND OTHER THEORETICALLY INTERESTING MOLECULES

Henry N. C. Wong

INFRARED SPECTROSCOPY OF HIGHLY REACTIVE ORGANIC SPECIES:THE IDENTIFICATION OF UNSTABLE MOLECULES AND REACTIVE INTERMEDIATES USING AB INITIO CALCULATED INFRARED SPECTRA

B. Andes Hess, Jr. and Lidia Smentek

MILLS-NIXON EFFECTS?

Natia L. Frank and Jay S. Siegel

RADICAL CATIONS OF CYCLOPROPANE SYSTEMS: CONJUGATION AND HOMOCONJUGATION WITH ALKENE FUNCTIONS

Heinz D. Roth

37

109

147 209

261

AUTHOR INDEX

303

SUBJECT INDEX

305


LIST OF CONTRIBUTORS

Natia L. Frank

Department of Chemistry University of California-San Diego La Jolla, California

B. Andes Hess, Jr.

Department of Chemistry Vanderbilt University Nashville, Tennessee

PauI M(JIler

Department of Organic Chemistry University of Geneva Geneva, Switzerland

Peter W. Rabideau

Department of Chemistry Louisiana State University Baton Rouge, Louisiana

Heinz D. Roth

Department of Chemistry Rutgers University Piscataway, New Jersey

Jay 5. Siegel

Department of Chemistry University of California-San Diego La Jolla, California

Lidia Smentek

Department of Chemistry Vanderbilt University Nashville, Tennessee

Andrzej Sygula

Department of Chemistry Louisiana State University Baton Rouge, Louisiana

Henry N. C. Wong

Department of Chemistry The Chinese University of Hong Kong Hong Kong, China vii


PREFACE Volume 3 in this series continues in the vein established by the first two volumes. The contributors are distinguished organic chemists whose work helps to bridge the gap between theory and experiment. For many years Peter Rabideau has been active in studying the chemical and conformational properties of 1,4-dihydroaromatics. Together with Andrzej Sygula he has recently extended these interests to include topologically important polynuclear arenes such as corannulene. This bowl-shaped molecule is a component of buckminsterfullerene, hence the pseudonym "Buckybowls." This appears to be a very fertile new area and in the future we can expect to see Peter' s bowl to runneth over. Paul Miiller, together with two other groups, has for many years dominated the field of benzocyclopropene chemistry. His brings us up to date on the chemistry of this unique and smallest member of the benzocycloalkene family. Henry Wong has taken a more personal approach to relating his experiences with dehydro[8]annulenes and other theoretically interesting molecules. We are introduced to many of his co-workers and, through their expertise, many new facets of cyclooctatetraene are revealed. Andy Hess and Lidia Smentek are the most computational contributors to this volume. Their ability to predict the IR spectra of simple yet highly unstable organic molecules and intermediates at the ab initio level has become a reliable and invaluable aid to synthetic chemists in quest of these elusive species. Jay Siegal and Natia Frank have organized and examined in detail a subject close to my own heart: the holy grail of aromaticity as manifested by the controversial Mills-Nixon effect. The concept that purely steric effects could be employed to create a "cyclohexatriene" has for many years been a dominant approach to the

x

PREFACE

quantification of aromaticity. These authors present compelling experimental evidence which is sure to create a flurry from some of their theoretical counterparts. Finally, Heinz Roth has methodically dissected and analyzed the chemistry of cylcopropane radical cations. He examines a variety of their uni- and bimolecular reactions and evaluates the parameters which determine the course of this chemistry. In closing, I should like to observe that the 1994 Nobel Prize in chemistry was awarded to George Olah, a pioneer in the field of physical organic chemistry. The articles in this volume fall squarely into that realm of chemistry and I think we can all feel some small satisfaction that recognition for achievements such as these is forthcoming even at the very highest levels. I warmly thank and congratulate all my contributors for participating in this project. I hope that you the reader will share my sense of enlightenment and please feel free to suggest or volunteer future contributions. Randolph P. Thummel Series Editor

POLYNUCLEAR AROMATIC HYDROCARBONS WITH CURVED SURFACES. HYDROCARBONS POSSESSING CARBON FRAMEWORKS RELATED TO BUCKMI NSTERFU LLEREN E

Peter W. Rabideau and Andrzej Sygula il,lll

1. 2. 3.

4.

ll

i

i

ii

Introduction: Fullerene Related Hydrocarbons . . . . . . . . . . . . . . . . . . Circulenes: To Curve or Not to Curve? . . . . . . . . . . . . . . . . . . . . . . The Corannulene Story . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Synthesis: From 16 to 3 Steps . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Nonplanarity and its Consequences . . . . . . . . . . . . . . . . . . . . . 3.3. Dihydrocorannulenes . . . . . . . . . . . . . . . . . . . . . . . . . . . Ful!erene Related Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Expanding the Corannulene Surface . . . . . . . . . . . . . . . . . . .

Advances in Theoretically Interesting Molecules Volume 3, pages 1-36. Copyright 9 1995 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 1-55938-698-3

i

. i

i

iiii

2 4 5 5 8 10 12 12

2

PETERW. RABIDEAU and ANDRZEJ SYGULA

4.2. Semibuckminsterfullerenes: "Buckybowls"! . . . . . . . . . . . . . . . . 4.3. The Potential Dimerization of Semibuckminsterfullerenes . . . . . . . . 5. Theoretical Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Corannulene and Cyclopentacorannulene: Evaluation of Theoretical Models . . . . . . . . . . . . . . . . . . . . . 5.2. Semibuckminsterfullerenes . . . . . . . /. . . . . . . . . . . . . . . . . . 6. Reduction of Corannulene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Electron Addition to Curved Surface Hydrocarbons: General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. The Corannulene Dianion . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. The Corannulene Tetraanion., . . . . . . . . . . . . . . . . . . . . . . . 6.4. Inversion Barriers in Anionic Species . . . . . . . . . . . . . . . . . . . 6.5. Experimental Studies with Anionic Species . . . . . . . . . . . . . . . . 6.6. A Remarkably Stable Dimer of Tetralithiocorannulene . . . . . . . . . . 7. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Note Added in Proof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15 17 19 19 25 27 27 29 29 30 30 31 33 34 34 34

1. INTRODUCTION: FULLERENE RELATED HYDROCARBONS The discovery, that elemental carbon exists in a new allotropic form of caged clusters called fullerenes has triggered unprecedented research activity in physics, chemistry, and material sciences. 1 The most prominent member of the family, buckminsterfullerene (C60; 1), is a highly symmetrical molecule with the geometry of a truncated icosahedron. Its surface of 60 carbon atoms consists of 20 six-membered and 12 five-membered rings, and the curvature causes the hybridization of the carbon atoms to fall between sp 2 (as in graphite) and sp 3 (as in diamond). Several familiar carbon networks of cyclic hydrocarbons may be identified as substructures on the surface of 1: cyclopentadiene (2), benzene (3), [5]radialene (4), naphthalene ($), acenaphthylene (6), and pyracylene (7) represent a few such examples. Although the importance of substructures like [5]radialene and pyracylene has been recognized in attempts to understand the chemistry of fullerenes, 2 the above fragments do not capture the most intriguing feature of the fullerenes--that is, their curvature. In this chapter we will focus our attention on polynuclear aromatic hydrocarbons that have the following common characteristics: (a) their carbon network, consist-

,

1

Polynuclear Aromatic Hydrocarbons with Curved Surfaces

2

3

4

5

6

3

7

ing of trivalent carbon atoms, can be identified on the surface of icosahedral buckminsterfullerene, and (b) their preferred conformation is bowl-shaped, resembling, in varying degrees, the curvature of the carbon cage surface. There are a number of reasons to study this subclass of polynuclear aromatic hydrocarbons. Several are as follows: 1. It is exciting to study the progression of the chemical and physical properties of these systems as a function of their size to determine at what stage (if any) these hydrocarbons begin to behave like the all-carbon cages. That is, these curved polynuclear aromatics may serve as models for the fullerenes. 2. These hydrocarbons are potential substrates for an organic synthesis of fullerenes, a formidable challenge that is being pursued by a number of research groups. 3-6 Although fullerenes obtained by the extraction of carbon soots are now commercially available, a classical synthesis would provide a unique opportunity for the controlled formation of carbon cages. The formation of fullerenes from graphite in an electric arc involves fragmentation of the substrate into small (most probably C2) units that chaotically reform into larger clusters as demonstrated by the large degree of isotope scrambling during the process. 1 On the other hand, an organic synthesis may facilitate: controlled isotope labeling, introduction of heteroatoms, formation of isomers of higher energy (e g., C60's with symmetries lower than the I h symmetry of 1), or the "stuffing" of the cages with guest atoms or molecules. 3. The bowl-shaped conjugated hydrocarbons represent an attractive system for testing the theory of aromaticity since the curved surface of the carbon network represents a compromise between strain and conjugation. 4. The bowl-shaped hydrocarbons give an opportunity to study convex versus concave preferences for complex formation which relates to the interest in endohedral complexes of fullerenes. 5. Since alkali metal doped fullerenes are conductors, and low-temperature superconductors, one wonders if the curved polynuclear hydrocarbons might exhibit a similar behavior. We will begin our considerations with those hydrocarbons that possess the minimum number of carbons, when arranged properly, to afford curvature, and then proceed to the C30's which represent one-half of the buckminsterfullerene surface. The application of theoretical models to curved surface hydrocarbons will be explored, and also consideration of their electron addition processes by both theoretical and experimental methods.

4

PETERW. RABIDEAUand ANDRZEJSYGULA 2. CIRCULENES: TO CURVE OR N O T TO CURVE?

Polynuclear aromatic hydrocarbons formed by the angular fusion of benzene rings may adopt either planar or nOnplanar minimum energy conformations, as demonstrated by the circulenes 8-10. [6]Circulene (coronene; 9), the earliest known member of the family, prefers a planar structure of D6hsymmetry since the presence of the six-membered ring in the core does not require any distortion from the normal bond lengths and angles for the sp2hybridized carbon atom network. 7 The presence of odd-membered rings in the central cores of [5]circulene [corannulene (8)] and [7]circulene (10) introduces a geometrical requirement for accommodation of the bond lengths and angles that is quite different from the normal values. The resulting strain is partially reduced by a distortion of the molecules from planarity. As demonstrated by X-ray crystal structure determination, both 8 and 10 prefer significantly nonplanar conformations, although of quite different shapes. Corannulene forms a bowl-shaped structure 8,,8 while [7]circulene adopts a saddleshaped conformation 10'. 9 The presence of five-membered rings incorporated into a pattern of fused benzene rings is crucial to the curvature of systems related to the fullerenes, l Indeed, the importance of pentagons in causing a surface to fold has been long recognized by producers of soccer balls (an official soccer ball surface consists of 20 hexagons and 12 pentagons in an arrangement which makes it a perfect model of the C60 molecule). Also, the geodesic domes created by Buckminster Fuller are made to curve by the inclusion of pentagonal elements into a pattern of hexagons. From the assemblies of fused five- and six-membered rings identifiable on the surface of 1, corannulene (8) is the smallest fragment that retains a bowl-shaped conformation. However, there are other known hydrocarbons that adopt bowl (or

8

9

8'

10

10'


11

5

12

cup) shaped conformations due to the strain introduced by the fusion of fivemembered rings. The recent work by Kuck and co-workers provides such exampies: tribenzotriquinacenes U and tribenzo-4,7-ditiydroacepentalenes 12. l~ Such structures, however, do not fall into the category under consideration here since they are not polynuclear aromatic systems (some tetravalent carbon atoms are present), and their carbon frameworks can not be identified on the surface of fullerenes.

3. THE CORANNULENE STORY 3.1. Synthesis: From 16 to 3 Steps The preceding section indicated the importance of corannulene as the smallest member of the family of bowl-shaped, polynuclear aromatic hydrocarbons modeling the surface of buckminsterfullerene. Surprisingly, despite its obvious significance in relationship to the theory of aromaticity, 8 had not attracted the attention of chemists prior to the first report of its synthesis in 1966.11 As Barth and Lawton noted " . . to the time of our first report, it appears this structure had neither adorned the jacket or end cover of any book nor served as a symbol of an international symposium. ''12 More than that, corannulene does not appear to have been suggested in the chemical literature prior to 1966. The Barth and Lawt0n synthesis began with acenaphthene and proceeded by building up one six-membered ring at a time around the core five-membered ring. The expected strain in the corannulene system was reduced by dealing with highly hydrogenated derivatives, eventually leading to the ketone 13 (Scheme 1). The ketone was then dehydrogenated to 8, either directly with Pd/C, or through a two-step reduction involving prior reduction of 13 to the corresponding alcohol. 11,12 The overall yield of 0.4%, together with the enormous amount of hard work

15 steps

Pd/C

~3 Scheme 1. The first s~/nthesis of corannulene.

e

6


HO2CCH2-~CH2CO2H ~ ~ O - ~ O 14

~ HO

15

OH 16

required, discouraged repetition of this synthesis. While the limited supply of corannulene did allow for X-ray crystal structure determination--proving the bowl-shaped conformation to be preferred in the crystalS--as well as some spectroscopic studies, 12-14 no chemistry of this interesting system could be explored. However, this work did draw considerable attention, and several attempts toward a more convenient synthesis followed. Two years later, Craig and Robinson attempted an alternative synthesis of 8 with a more symmetrical pathway starting from derivatives of fluoranthene. 15 Cyclization of fluoranthene-7,10-diacetic acid 14 was attempted to produce diketone 15, expected to enolize to the dihydroxycorannulene 16. Unfortunately, several attempts at cyclization failed, including anhydrous hydrofluoric acid, concentrated sulfuric acid, and polyphosphoric acid. Friedel-Crafts cyclization of the corresponding acid chloride of 14 with aluminum or stannic chloride was similarly unsuccessful. 16 However, although Craig and Robinson were not successful, they developed a convenient synthesis of 7,10-disubstituted fluoranthenes which turned out later to be of premium importance in a new, successful synthesis of corannulene. The failure of conventional cyclization with 14 and related systems is attributed to the large distance between the carbon atoms required to form the two new bonds and the rigid planar structure of the fluoranthene nucleus. In an attempt to avoid this problem by Davy et al. in 1979,17 [2,2](2,7)-naphthalenocyclophanediene 17 and [2,2](2,7)-naphthalenocyclophane 18 were used as precursors with formation of the central five-membered ring put off until the final stage of the synthesis. Unfortunately, neither corannulene nor partially hydrogenated corannulenes could be produced under photochemical or chemical conditions. The decisive break came in 1991 when Scott and co-workers reported the formation of corannulene from 7,10-diethynylfluoranthene 19 under flash vacuum pyrolysis (FVP) conditions. 18The critical features of this approach appear to be the high temperature employed (900-1000 ~ and the gas-phase conditions. The high temperature provides energy for drastic fluctuations in 8 away from its equilibrium geometry, allowing the reactive centers to come close enough to form the new bonds while the gas-phase conditions prevent polymerization (although some polymeri-

17

18

Polynuclear Aromatic Hydrocarbons with Curved Surfaces O MeO2CvJJ,,,,,,,,,C O2Me + MeO2C-'O'-CO2M e

Br

/j ,,,

7

1o

20

f

8

19

(a) glycine (catalyst), norbornadiene (solvent), 120o, 36 hrs, 49%. (b) LiAIH4, THF, 25o, 6 hrs, 95%. (c) pyridinium chlorochromate, THF, 25o, 5 Hrs, 70%. (d) CBr4, Ph3P, Zn, CH2CI2, 25o,41 hrs, 50%. (e) lithium diisopropylamide, THF, -78o, 5 hrs, 80%. (f) FVP, 1000o, ca. 10%. (g) FVP, 1000o, ca. 40%.

Scheme 2. The pyrolysis route to corannulene.

zation occurs in the heated sample chamber before the substrate sublimes and enters the pyrolysis tube). The original Scott synthesis is presented in Scheme 2. It turned out that the tetrabromodivinyl derivative 20 also gives corannulene under FVP condition, and this not only saves one step in the synthesis, but also significantly increases the yield due to the higher resistance of 20 toward polymerization in the sample chamber. Pyrolysis of 20 also produces some bromo- and dibromocorannulene. 19 Recently the Scott group further improved the synthesis, reducing the number of steps to three (Scheme 3), and this currently represents the most convenient route to corannulene allowing for the synthesis of gram quantities. 2~ 0

0

[ ~ +

0

a " [ ~

" b

c

8

(a) glycine (catalyst), norbornadiene/toluene, 110 o, 3 days, 70%. (b) PCI5, benzene, reflux, 3 hrs, 60%. (c) FVP, 1100o, ca. 30%.

Scheme 3. The "improved" pyrolysis route to corannulene.

8

PETER W. RABIDEAU and ANDRZEJ SYGULA

a B Br" ~

b,c O2Sr - Br

O2

~~,,>,.~.~.

t 9 / (a) NBS, CCI4, hv, 80%. (b) Na2S, acetone, 75%. (c) H202, acetic acid, 70%. (d) FVP, 1000o, ca. 18%. (e) vacuum pyrolysis, 400o, then Pd/C, xylenes, reflux, ca. 7%.

Scheme 4. An alternative synthesis of corannulene.

An alternative synthesis of corannulene, also involving a pyrolysis, was reported by Siegel and co-workers (Scheme 4). 21 The tetrabromide may be directly pyrolyzed to corannulene, or it may be converted to a bis-sulfone where extrusion of SO 2 produces a mixture of 8 along with its di- and tetrahydro derivatives, and 5,6-di me thy l-ben zo [g hi] fl uor an the ne.

3.2. Nonplanarity and its ConseqUences X-ray crystal structure determination reveals the significantly nonplanar structure of corannulene. 8 The compound crystallizes in space group P21/c with eight molecules in the unit cell, two of them being symmetry-independent. Molecules form well-defined bowls, but with virtually no bowl-to-bowl stacking. The average distances from the plane of the core five-membered ring to the planes, as defined by five-rim quaternary carbons and 10 CH-rim carbon atoms, are 0.52 and 0.87 ,~, respectively (Figure 1a). The six-membered rings are slightly bent, and the average dihedral angle between the central five-membered ring and the inner portion of the six-membered tings is 26.8 ~ The average bond lengths and angles determined in the Crystal of 8 at-70 ~ are shown in Figure lb. The numbers represent the arithmetic means of the bond lengths and angles expected to be equal under Csv symmetry from two independent molecules of 8. 22 The average values probably represent a reasonable description of the structure, although one has to keep in mind that the individual bond lengths and angles, which are related by symmetry in the isolated molecule, differ in the crystal by as much as 0.05 ,~, and 3 ~ respectively. These differences represent a

Polynuclear Aromatic Hydrocarbons With Curved Surfaces

9

Figure 1. X-ray crystal structure of corannulene (8)

superposition of crystal packing forces effects and the experimental error of the structure determination. Due to its high symmetry, 8 exhibits only one signal in its 1H NMR spectrum (7.81 ppm), i2 and three 13C NMR lines at 127.2 ppm (10 methine rim carbons), 130.8 ppm (5 quaternary rim carbon atoms), and 135.8 ppm (5 quaternary hub carbon atoms). 18 The proton resonance of 8 is significantly shifted upfield, as compared to coronene 9 (8.9 ppm), which may reflect an influence of the curvature of the carbon network on the ring current. The formation of unusuall~r light colored charge-transfer complexes Of 8 with both picric acid and trinitrobenzene may be attributed to the decrease of the interaction between the donor and acceptor molecules due to the curvature of the former. 12 In spite of this substantial curvature, corannulene undergoes rapid bowl-to-bowl inversion in solution as demonstrated by the dynamic NMR behavior of its dimethyl carbinol derivative: a single signal for the methyl groups appears at ambient temperature while the diastereotopic methyls become observable separately below -64 ~ A barrier to inversion was determined to be AG* = 10.2 kcal/mol. In a similar fashion, a barrier of 11.4 kcal/mol was determined for isopropylcorannulene. 23

- 10-11 k c a l / m o l

10

PETERW. RABIDEAU and ANDRZEJ SYGULA 3.3.

Dihydrocorannulenes

Dihydrocorannulene (21) and tetrahydrocorannulene (22) were first produced as side products in the synthesis of corannulene (see Scheme 4). 21 These structures retain the general bowl shape, and NMR inversion barriers were estimated to be 8.5 and 7 kcal/mol, respectively. 21 Dihydrocorannulenes can also be produced by the addition of alkyllithiums to corannulene to afford 1-alkyl-l,2-dihydrocorannulenes (21'). 24 The partially hydrogenated ring of dihydrocorannulene constitutes a 1,3-cyclohexadiene ring, a system that has been well-studied with respect to its geometry and the conformational preferences of substituents. 25 However, the curvature of the corannulene surface introduces an additional stereochemical factor that makes the conformational analysis especially interesting. 1,3-Cyclohexadiene (23) and 9,10dihydrophenanthrene (24) serve as models: they are structurally similar systems, and their stereochemistry and conformational preferences are well documented in the literature. In both cases, the reduced ring adopts a nonplanar, semi-chair conformation of C 2 symmetry. The geometry of the 1,3-cyclohexadiene ring can be described by the torsion angle (x which represents the conformation along the Csp3--Csp3 linkage (C a-Cb-CcCd). Values for this and other important torsional angles have been calculated for all three systems by ab initio 3-21G, MM2(87), and AM1 calculations. 24 While the ab initio and MM2 calculations were found to be in fairly good agreement both with each other and with available experimental results, the AM1 values were

21

22

I. RLi

,ID

2. H20

21'

23

24

21


11

if, = C a " C b " C c " C d

aa b

~

considerably different. In any event, MM2 appears to do a good job in calculating these structures, and calculated and experimental values for several examples appear in Table 1.24 It is apparent from Table 1 that the cyclohexadiene ring in 1,2-dihydrocorannulenes is significantly flatter than either 1,3-cyclohexadiene itself or 9,10-dihydrophenanthrene due to the constriction imposed by the remainder of the bowl-shaped ring system. Nonetheless, the lack ofplanarity means that substituents at Csp3 may adopt either pseudoaxial (pa) or pseudoequatorial (pe) positions. However, the surface of the corannulene system is curved, and, unlike 1,3-cyclohexadiene and 9,10-dihydrophenanthrene, neither the two pa positions nor the two pe positions are equivalent. Thus, substituents in l-R- 1,2-dihydrocorannulenes may be: (1) endo-pseudoaxial (endo-pa), (2) endo-pseudoequatorial (endo-pe), (3) exo-pseudoaxial (exo-pa), and (4) exo-pseudoequatorial (exo-pe), where endo and exo are related to concave and convex orientations, respectively. By molecular mechanics (MM) calculations, alkyl substituents prefer the pe position in 1,3-cyclohexadiene regardless of the size of the group, 24'25 and this behavior is analogous to the well-known equatorial preference in cyclohexanes that is primarily due to destabilization of the pa conformation by 1,3-diaxial interactions. 28 In contrast, 9-monosubstituted dihydrophenanthrenes show a strong preference for thepa substituent location (by both calculation and experiment), a result of the severe repulsive interaction of ape substituent with the peri hydrogen atom on the adjacent aromatic ring. 25 Because of the flatter "cyclohexadiene" ring in dihydrocorannulenes, repulsion of substituents with the peri hydrogens is considerably diminished and energies between pa and pe locations are calculated to be Table 1. Calculated and Experimental Values for Torsion Angle o~in 1,3-Cyclohexadiene, 9,10-Dihydrophenanthrene, and 1,2-Dihydrocorannulene 1,3-Cyclohexadiene 9,10-Dihydrophenanthrene 1,2-Dihydrocorannulene i-Pr- 1,2-Dihydrocorannulene t-Bu- 1,2-Dihydrocorannulene

ab initio

MM2(87)

E,xperimental

44.2a 57.0a 30.9 m --

42.0 54.3 33.8 30.9 34.6

46b 53.4-56.4c

Notes: aRef.26.

bElectronDiffraction(reference25). CX-ray(reference27).

28.2 24.8

12


Figure 2. X-ray crystal structures of 1-i-Pr (left) and 1-t-Bu (right) 1,2-dihydrocoran-

nulene.

modest with small alkyl substituents. On the other hand, large alkyl groups, like i-Pr and t-Bu exhibit significant preference for the pa location in the exo arrangement as predicted by MM2(87) calculations and shown in the crystal structures (Figure 2). 24 However, crystal structure determinations show single conformations that are of lowest energy with respect to intra- and intermolecular interactions, while fast interconversion may be expected in solution affording a statistical distribution of conformers related to the differences in relative stabilities. NMR measurements estimate an inversion barrier of 8.5 kcal/mol for 1,2-dihydrocorannulene, 21 lower than the 10-12 kcal/mol barrier for corannulene itself. Although the pa location of substituents is exclusive in the solid state, the contribution of pe substituents in solution can be detected by NMR. 24 With a substituent in the pa position, the two t r a n s - b e n z y l i c hydrogens have a di-pe relationship, and this leads to a relatively small vicinal coupling constant. For example, with R = t-Bu, the trans coupling is 1.6 Hz while the cis (pe-pa) coupling is 8.7 Hz. When R = i-Pr, the trans coupling increases to 4,8 Hz, and with R = TMS it becomes 7.5 Hz. Clearly the less sterically demanding substituents begin to populate the pe position leading to a trans coupling contribution with a di-pa relationship, and this is known to show a large coupling constant. According to MM2(87) there is no apparent e x o versus endo preference of small substituents in the dihydrocorannulene system since the steric energies are virtually identical for e x o / e n d o pairs of 1-Me-21' in both pa and pe conformations. On the other hand, the e x o preference is predicted for bulky substituents, with about 3 kcal/mol energy difference in the case of 1-t-Bu-21'. Accordingly, both i-Pr and t-Bu were found to be e x o (convex) in relation to the dihydrocorannulene unit by X-ray crystal structure determination (Figure 2). 24

4. FULLERENE RELATED HYDROCARBONS 4.1. Expandingthe Corannulene Surface Corannulene is the simplest example within the class of polynuclear aromatic compounds with curved surfaces that are related by carbon framework to the~ fullerenes. These aromatic hydrocarbons possess bowl-shaped geometries, consist


13

Bowl Depth

0.89 A

1.05 A

Figure 3. Bowl depth from X-ray diffraction as measured from the rim aromatic carbons to the best plane containing the central five-membered ring.

of five- and six-membered rings (Figure 3). As long as these compounds are C20 or larger, they will have curved surfaces, albeit with varying degrees of concavity. As one goes beyond corannulene on the buckminsterfullerene surface, the next carbon framework that will afford a stable hydrocarbon corresponds to the cyclopentacorannulene 25. This is an interesting structure that is also the smallest fullerene-related curved aromatic containing the pyracylene unit which is thought to be responsible for some of the chemistry of buckminsterfullerene. The five-membered ring "pulls" the corannulene system a little tighter resulting in a deeper bowl as indicated in Figure 3. 29,30 The synthesis of 25 was accomplished 29 by the same procedure as for corannulene except that the additional five-membered ring was incorporated in the starting material; that is, the sequence was begun with acenaphthene instead of naphthalene. Hydrogen is lost in the pyrolysis step resulting in the unsaturated compound which can then be hydrogenated to 26. The latter compound is the first example of a curved aromatic that does not invert rapidly through bowl-to-bowl inversion. The e x o - and endo-hydrogens in 26 are distinct in proton NMR, and show no sign of signal coalescence up to 135 ~ The dynamics of 26 were also observed by the spin polarization transfer method at 127 ~ That is, a selective 180 ~ pulse

25 on the surface of 1 C

pyracylene

25

I

FVP 1000~

H2

,.~"

25

26

14


was applied to the AA' (or BB') half of the spectrum to reverse its polarization, followed by a nonselective 90 ~ pulse after increasingly longer delay times. The very limited spin polarization transfer allowed an estimation of the lower limit for inversion of approximately 26 kcal/mol at 127 ~ (or kinv = 0.05 s-l), indicating that the barrier for inversion of 26 is at least twice as large as that for corannulene. 29 A most interesting property of 25 is its crystal packing. 3~While convex--concave stacking may be expected for the minimum energy arrangement of two bowlshaped molecules, since it maximizes attractive van der Waals interactions, this arrangement is not observed for corannulene where very little bowl stacking is detected. In contrast, cyclopentacorannulene exhibits a high degree of long-range stacking in the solid state (Figure 4). 30 Since crystal structure determinations currently exist only for two compounds in this series, it remains to be seen as to whether or not this type of stacking in the solid state will be common.

Figure 4.

Stacked bowls of cyclopentacorannulene.

A C28H14hydrocarbon was recently prepared, also by flash vacuum pyrolysis. This interesting reaction, which may involve a benzyne type intermediate, represents the first synthesis of a benzocorannulene. 31

~

0"~== 0 FVP

O28H14


15

4.2. Semibuckminsterfullerenes: "Buckybowls"! As we proceed further with the process of generating hydrocarbons from carbon frameworks represented on the buckminsterfullerene surface, we encounter a new class of compounds known as semibuckminsterfullerenes. These C30 hydrocarbons (Figure 5), which we will also refer to as "buckybowls", represent one-half of the buckminsterfullerene C60 surface, and share several common characteristics including 30 sp 2 carbon frameworks consisting of multiple fused five- and six-membered tings and bowl-shaped geometries. Although there are a number of C30 hydrocarbons, those of greatest interest will be the ones that possess relatively high symmetry (27-29). And despite considerable interest in their synthesis, 3'4'32 semibuckminsterfullerenes remained elusive until recently. The first synthesis of a semibuckminsterfullerene was accomplished by a procedure analogous to the preparation of corannulene except that 1,2,5,6-tetraketopyracene served as the starting material (Scheme 5). 33a Flash vacuum pyrolysis of the tetrachloro compound led to a remarkable quadruple ring closure to produce the semi-buckminsterfullerene 27. The 13C NMR shows three methine carbons and five quaternary carbons at 125.5, 126.8, 127.3, 128.6, 136.7, 137.3, 142.0, and 145.4 ppm. The latter signals compare favorably to the single resonance observed at 143 ppm for buckminsterfullerene. As would be expected, semibuckminsterfullerene 27 is predicted to adopt a bowl-shaped conformation by ab initio 3-21G calculations. 33a However, while 27 is a fascinating compound, it does not represent a symmetrical half of the buckminsterfullerene surface. That is, when the carbon framework of 27 is "removed" from

Figure 5. C3o Hydrocarbons from the buckminsterfullerene surface.

16


O 9,

I

b'VP r

27

Scheme 5. The synthesis of semibuckminsterfullerene 27.

buckminsterfullerene, the remaining C-30 moiety is not identical with it. Hence the intriguing possibility of dimerization to buckminsterfullerene is not possible for 27, at least not without some reorganization of the carbon framework. At best, an isomer of buckminsterfullerene would be formed: an "imperfect" C60 cage of D2d symmetry. While such a structure would have 20 six-membered and 12 five-membered rings like buckminsterfullerene, it would contain four pairs of adjacent (i.e., edge-sharing) pentagons. According to recent calculations, the energy of this isomer is expected to be 116 kcal/mol higher than icosahedral buckminsterfullerene. 34 On the other hand, buckminsterfullerene can be separated into two molecules of 28 (after hydrogenation), which are related as an enantiomeric pair, and hence this "buckybowl" isomer is especially exciting. Indeed, 28 has been the synthetic target of several groups, and has been suggested as a prime intermediate for an organic synthesis of buckminsterfullerene. 3'4'32 Molecule 28 has also been the subject of theoretical calculations to provide insight into its structure and its potential for bowl-to-bowl inversion (see b e l o w ) . 4'35'36 As with corannulene, conventional routes for the preparation of 28 have been unsuccessful, due no doubt to the strain associated with this molecule; nevertheless,

Polynuclear Aromatic Hydrocarbons with Curved Surfaces CI .C'/ ~ cl~..[~-?k~~ 9c,

OHQ . O H

0

C'2HC.~ jL~/~

CH C, LDA

~

--

n-BulJ

R

_~

- oe-g3.gL

06H6

oCI2HC~~

~

H

H

(R = CI, H)

17

Cl

FVP

28

Scheme 6. Synthesisof semibuckminsterfullerene 28.

a FVP route was once again successful. 33b However, due to the lack of stability of a number of potential precursors, the best route so far involves the pyrolysis of a mixture of trichloro and tetrachlorovinyl compounds (Scheme 6) to produce 28, albeit in rather low yield. Like 27, 28 has considerable curvature as illustrated by the calculated structure.

4.3. The Potential Dimerization of Semibuckminsterfullerenes As mentioned above, an especially interesting possibility for some of the semibuckminsterfullerenes is their possible dimerization to buckminsterfullerene. Molecule 28 is chiral, and while bowl-to-bowl inversion would produce racemization, the barrier is expected to be sufficiently high (see below) that both enantiomers must be considered for the dimerization process; that is, d-C30H12 + d-C30Hl2, and d-C30H12 +/-C30H12. In considering the dimerization of 28 to produce C60, it is assumed that the bonding pattern in the substrate carbon network remains unchanged during the fusion process. Moreover, the new carbon-carbon bonds are expected to be formed in a sequential mode between the carbon atoms losing the hydrogens; e.g., C 1-C 1', C2-C2', C 3 - - C 3 ' . . . or C1--C2', C2-C3', C3-C4' . . . . etc. The fusion of two molecules of 28 will produce 12 new C-C bonds and also 12 new rings: since each half consists of three five-membered rings and seven six-membered tings, the number of rings in the product will be 32 regardless of the stereochemistry or regiochemistry of the process. The AM 1 optimized structures for possible C6o'S formed by the dimerization of 28 appear in Figure 6 (30--35; buckminsterfullerene omitted). 23 The dimerization of two identical molecules of 28 leads to four different isomers of C60, depending

18


30

31

32

33

34

35

Figure 6.

Possible structures resulting from the dimerization of 28.

on the docking regiochemistry (30-33), while the fusion of the two enantiomers of 28 yields three isomers (34, 35) and buckminsterfullerene (1). The AM 1 calculated results for the seven isomers of C60 are shown in Table 2.23 The relative energies of the isomers, considered in terms of their structural features, clearly support the validity of the criteria recently proposed by Schmaltz et al. for the evaluation of the stabilities of elemental carbon cages; 37 that is, destabilizing factors such as four-, seven- and eight-membered rings, abutting five-membered rings, and uneven curvature across the surfaces. It is not surprising that 1 has the lowest energy of all the isomers considered (Table 2). However, two other isomers, 30 and 33, show reasonably low energies, and reoptimization by ab initio STO-3G calculations gave relative energies (1 = 0.0) of 1.45 and 4.01 kcal/mol per single carbon atom for 30 and 33, respectively, in reasonable agreement with the AM 1 results. In contrast with 1, for which only four threefold degenerate modes are active, 30 and 33 exhibit 86 and 45 IR active modes, respectively, 58 and 31 of which are twofold degenerate. The lower symmetry of 30 and 33 as compared to 1 also results in a higher number of nonequivalent carbon atoms producing more expected signals


19

Table 2. AM1 Calculated Results for the Isomers of C60 Formed by the Fusion of Two Halves of Semibuckminsterfullerene 28 symmetry 4-mem. rings 5-mem. rings 6-mem. rings 7-mem. rings 8-mem. rings AHfa rel. Eb 13cC

30

31

32

33

34

35

D3

D3

C1

D3h

Cs

C3

0 12 20 0 0 1048 1.25 10

3 12 14 0 3 1576 10.09 10

6 6 14 6 0 1661 11.46 60

3 6 23 0 0 1214 4.01 6

6 6 17 0 3 1780 13.44 30

3 9 17 3 0 1424 7.52 20

1

/h 0 12 20 0 0 973 0.00 1

Notes: aAM1heatof formation(keal/mol).

bRelativeenergyper carbonatom(keal/mol),as compared to 1. eNumberof nonequivalentcarbonatoms. in their 13C NMR spectra. While 1 shows only a single resonance, 30 and 33 should show 10 and 6 signals, respectively. In contrast to the "random" docking of two molecules as considered above, it has recently been suggested that dimerization of 28 might occur via a series of six [2 + 4] Diels-Alder cycloadditions. 35 This would lead to a C60H24 hydrocarbon that, in principle, could be dehydrogenated directly to 1. The same authors also suggested that the process may be metal-catalyzed, and carried out by MNDO and/or PM3 calculations to determine convex versus concave preference for the location of complexed metals on the surface of 28. While an earlier MNDO study showed a convex preference for Li § on the surface of 28, 4 a more recent study suggests that, while a number of metals may prefer the convex side, Ga § may be complexed on the concave surface. Moreover they suggest that Ga § might serve as a catalyst for the reaction, and may also be encapsulated in the dimer. 35 A number of fullerene-related hydrocarbons, including 29, have been considered as intermediates along the route to buckminsterfullerene. 56 ' Strain energies have been calculated at the MM2 level for 20 "fragments" between C14H8 and CssHIo, and it was shown that the strain energy per carbon gradually increases along C 2 and C 3 symmetry paths, but reaches a maximum at the C40H20 stage along the C 5 symmetry path. 6 This latter path includes corannulene as well as 29.

5.

T H E O R E T I C A L STUDIES

5.1. Corannulene and Cyclopentacorannulene: Evaluation of Theoretical Models Corannulene (8) represents a nonalternant, 20 x-electron system in which 15 electrons formally belong to the periphery, while five electrons reside in the core;

20


8a

the degree of aromaticity of the system becomes an interesting question. To satisfy the Htickel rule, a significant contribution from the polar structure 8a was proposed where the periphery becomes aromatic with 14 electrons resulting in a cyclopentadienyl anion inner core with six x-electrons, ll'12 Moreover, the possibility of relieving strain by adoption of a bowl-shaped conformation adds yet another intriguing feature to this molecule, and it is no surprise that it caught the attention of computational chemists. In 1967, a year after the first report of the synthesis of 8, Gleicher published the results of a combined PPP-type SCF and force-field study. 38 In spite of a number of simplifications employed, the nonplanar conformation was correctly predicted for 8. The calculations performed--with no geometry optimization--for both planar and "idealized" basket-like structures favored the latter by about 84 kcal/mol. Gleicher also concluded that, despite the significant curvature, the molecule retains most of its 7t-binding energy (over 90% as compared to the planar conformation), and it should show appreciable aromatic character. Gleicher did find some separation of charge densities, increasing electron density in the core relative to the rim, and this lends support to contributions from canonical structures like 8a. 38 After the work of Gleicher was confirmed by the X-ray crystal structure determination showing 8 to be nonplanar, there appeared to be little further interest in corannulene except as a part of theoretical studies on large classes of hydrocarbons by molecular mechanics (MM) 39 and graph theory methods. 4~ However, interest in the structure of corannulene was revived in the mid- 1980s in apparent connection with the emerging fullerene chemistry. Since then a significant number of theoretical studies have appeared using a full spectrum of methods: topological graph theory; 42-44 MM calculations; 6'45 p-orbital axis vector analysis (POAV); 46-48 and molecular orbital (MO) methods at various levels of sophistication, including HMO, 46'47 semiempirical, 21 '49-51 and ab initio 21'36'52-55 calculations. All of the theoretical methods that allow for geometry optimization predict the bowl-shaped conformation of 8 to predominate over the planar alternative. However, the calculated energy differences between the two conformations differ drastically and span the range of 0.8 to 84 kcal/mol. Moreover, the calculated energy difference between the planar and bowl-shaped conformers is quite important since it represents the barrier for bowl-to-bowl inversion if: (a) planar corannulene represents the transition state (TS) for the process, and (b) the entropic term in AG* is negligible. Condition (a) seems to be fulfilled, since vibrational frequency calculations give one and only one imaginary frequency for the planar structure


21

(and, of course, no imaginary frequencies for bowl-shaped 8) at the semiempirical AM1 level, 21 as well as at the ab initio level, with both 3-31G and 6-31G* basis sets. 36'54'55 Moreover, the eigenveCtor associated with the imaginary frequency describes the distortion of the planar structure to the bowl-Shaped one. 36 For condition (b), some entropy contribution to the barrier for inversion in corannulene is expected due to the different symrrietries of the planar and bowl-shaped 8. This contribution is equal t o - R In t~* + R In t~, where t~* and ~ denote the symmetry numbers of the TS and the substrate. The symmetry numbers for the planar TS and nonplanar 8 (substrate) are 10 and 5, respectively. Thus AS* due to Symmetry i s - R In 2, or -1.4 cal/mol.deg, which increases AG* by ca. 0.4 kcal/mol at room temperature. 55 Recent ab initio calculations provide a more quantitative approach to this problem giving the difference between ab initio calculated relative energies and AG* to be 0.6 kcal/mol after considering both thermal and zero point energy corrections to AH* as well as vibrational and rotational entropies of the planar and nonplanar conformers. 54 Thus, at least in the gas phase, the calculated relative energy of the planar conformer does indeed represent a reasonable approximation for the barrier to inversion. Also, since 8 is a nonpolar molecule, significant entropy contributions due to differential solvation of the substrate and the TS are not expected. The experimental barrier for boWl-to-bowl inversion is 10.2 kcal/mol at -64 ~ for dimethylcarbinol derivative 3619 and 11.3 kcal/mol at-31 ~ for 1-iPr-corannulene 37. 23 The question arises to what extent the inversion barriers of these derivatives approximate that for 8. In this context, 37 may be a better model than 36 since it is a nonpolar hydrocarbon and the isopropyl group is less bulky than the dimethylcarbinol group. The slightly higher inversion barrier for 37 relative to 36 is interesting, assuming the entropic term is not significantly different, and may reflect solvent effects since AM 1 calculations predict the barriers for 36 and 37 to be very similar. In any event, AM 1 gives an inversion barrier for 37 which is 0.5 kcal/mol lower than the AM1 calculated barrier for 8. Thus if AM1 correctly predicts the trend, the inversion barrier for unsubstituted corannulene may be slightly higher than either 36 (10.2 kcal/mol) or 37 (11.3 kcal.mol), and a value of approximately 12 kcal/mol may be estimated for the parent hydrocarbon, 55 With an experimental estimation of the barrier in hand we can now evaluate the performance of the theoretical methods. Not surprisingly, the oldest and most Me M e,,..~..-OH

36

Me M e,,..~_..H

37

22


approximated methods gave the worst results. The commonly used MM methods predict generally lower barriers than the experimental value: 1.2 (MM2PI), 39 0.8 (MOMM), 45 and 6.5 (MM2(87)) 23 kcal/mol. More elaborate semiempirical calculations at the NDDO level predict barriers of 8.0 (MNDO), 5~ 16.9 (AM1),21 and 15.3 (PM3) 51 kcal/mol, representing more reasonable estimates. The closest estimations come from the most sophisticated ab initio calculations, with the best performance from the ab initio LDF method that calculated an energy difference between planar and nonplanar conformers to be 11.4 kcal/mol, 21 in excellent agreement with experiment. HF ab initio calculations, at various levels of basis set quality, give barriers slightly lower than experiment: ca. 9 kcal/mol. 36'52-55 This may reflect the importance of post-HF electron correlation corrections for the proper description of the barrier since the LDF approach, formulated in terms of electron density rather than electron orbitals, incorporates some electron correlation effects. As we showed recently, the inclusion of correlation effects at the MP2/631G level significantly increases the HF barriers for curved surface hydrocarbons by approximately 5--6 kcal/mol. 36

Table 3. Bond Lengths and Bowl Depth of Corannulene by Theoretical Methods and Experiment

Bond length (,~) Method (year)

a

b

c

d

Depth (.~)

Ref

1.42 1.43 1.42

1.40 1.39 1.38

1.44 1.46 1.44

1.39 1.39 1.38

0.93

39 45 23

MNDO(1991) AM1(1992) PM3(1992)

1.44 1.44 1.43

1.39 1.38 1.38

1.46 1.44 1.44

1.39 1.38 1.38

0.80 0.94 0.94

50 21 51

Ab initio HF/3-21G(1994) HF/6-31G*(1990) LDF(1992)

1.42 1.41 1.41

1.36 1.36 1.38

1.45 1.45 1.44

1.37 1.37 1.38

0.88 0.91

55 53 21

Exp. (X-ray)

1.41

1.38

1.44

1.39

0.87

8

Molecular Mechanics

MM2PI(1977) MOMM(1987) MM2/87 Semi-empirical MO


23

Minimum energy conformations of 8 are reasonably well reproduced by most of the commonly used computational methods. Table 3 presents the bond lengths and bowl depth of 8 calculated at various levels of theory, and shows that a good description of minimum energy conformations may be obtained by relatively inexpensive methods. All of the methods predict the bonds b and d to be shorter than a and c in accord with the principal canonical structure. The differences between the calculated and experimental bond lengths do not exceed 0.02 A, and this is certainly a satisfactory result considering the relatively large differences found for the formally equal bond lengths in the crystal (see Section 3.2). The recently determined X-ray crystal structure of cyclopentacorannulene 25 provides an additional opportunity to test the performance of theory in predicting the minimum energy structures of nonplanar conjugated systems. 3~ Table 4 presents the C-C bond lengths in the crystal and a comparison with ab initio values calculated at HF/3-2 ~1Gand HF/6-31 G* levels. The agreement between experimental and theoretical bond lengths is very satisfactory with root mean square deviaTable 4. Comparison of Experimental Bond Lengthsa with the Ab Initio Results for Cyclopentacorannulene 25 8

9

9a 10

1

C1-C2 C1-C10a C2-C2a C2a-C3 C2a-C10c C3-C4 C6a-C7 C6a-Cl0e

X-ray

3-21G

1.372 1.436 1.431 1.451 1.370 1.368 1.451 1.383

1.367 1.448 1.446 1.451 1.361 1.362 1.464 1.371

6-31G*

1.369 1.448 1.447 1.452 1.363 1.364 1.464 1.373

C7-C7a C7a-C8 C7a-Cl0g C8-C9 C10b-C10c C10b-C10f C10c-C10d C10f-C10g

X-ray

3-21G

1.374 1.467 1.419 1.388 1.417 1.412 1.432 1.344

1.367 1.503 1.425 1.344 1.423 1.416 1.430 1.335

6-31G*

1.371 1.493 1.423 1.344 1.420 1.410 1.427 1.337

Note: aIn A. An arithmetic mean value given for the bonds which are equivalent under assumed Cs molecular symmetry. The experimental standard deviations for individual distances are 0.004-0.005 A.

24


tions of 0.017 and 0.015 A for 3-21G and 6-31G*, respectively. The only significant differences are found for the C7a-C8 and C8-C9 bonds in the external five-membered ring: the former is calculated as too long, and the latter too short by about 0.03 and 0.04 ,~, respectively. If these bonds are excluded from comparison, the root mean square deviation drops to 0.009 (3-21G) and 0.007/~ (6-31G*) which is less than two standard deviations of the experiment. The discrepancy between theory and experiment in the description of the external five-membered ring does not seem to reflect a deficiency of the basis set used since significant improvement of its quality from 3-21G to 6-31 G* shortens the C7a-C8 bond only slightly while leaving the C8--C9 bond unchanged. The influence of post-HF electron correlation effects at the semiempirical PM3 level was also tested, and virtually no bond length dependence was found despite the order of CI used. Hence the origin of the discrepancies between theory and experiment for the external five-membered ring is unknown. However, in view of the above discussion on corannulene, it can be concluded that the performance of the ab initio method is indeed satisfactory. The depth of the bowl of 25 is also well described by theory. The calculated distances of the rim carbon atoms C9, C9a, C 10, C 1, C2, and C3 from the plane of the central five-membered ring are 2.08, 1.24, 1.24, 1.04, 0.96, and 0.89 ,~ at the 3-21G level, and 2.00, 1.19, 1.19, 1.00, 0.93, and 10.85/~ at the 6-31G* level, respectively. This is in excellent agreement with the crystal data (2.02, 1.18, 1.19, 1.03, 0.97, and 0.90/~, respectively). 3~ The minimum energy structures of 25 obtained at the semiempirical level (MNDO, AM l, and PM3), as well as by MM2(87), are also in very satisfactory agreement with the crystal structure data. 23 In analogy with corannulene, the planar conformation of 25 was found to represent the TS for bowl-to-bowl inversion by both semiempirical and ab initio calculations; the energy of the planar conformer is higher by 28.8 kcal/mol (HF/6-31 G*//3-21G). 36Inclusion of electron correlation effects again increases the barrier significantly to a value of 34.4 kcal/mol. Comparison of this number with the barrier for corannulene, calculated within the same approximation (14..4 kcal/mol), suggests a dramatic increase in the stiffness of the bowl in 25. While a barrier has not yet been measured, an NMR estimate of the barrier for 26 is higher than 26 kcal/mol. 29 Table 5 presents the results of the semiempirical MO and MM2(87) calculations of the inversion barriers for both 25 and 26. Inspection of the data reveals significant analogies with the results for corannulene. MM2(87) estimates of the barriers are substantially lower than both the semiempirical and ab initio results. Also the semiempirical barriers "bracket" the ab initio estimate, with the MNDO value being lower, and both AM 1 and PM3 values being higher. Finally, comparison of the theoretical results with the available experimental data allows us to draw the following conclusions: 1. Despite the complications caused by the curvature of the systems considered, the performance of the commonly used computational methods in describing the


25

Table 5. Relative Energies (Kcal/Mol) of the Planar Versus Nonplanar Conformers of 25 and 26

25

26

MNDO AM1 PM3 MM2(87)

27.1 43.7 40.1 21.6

23.7 39.1 35.2 23.1

ab initio a

34.4

Method

exp. Notes:

>26 b aHF/6-31G*//3-21G with MP2/6-31G correction (reference 36). bReference 29.

minimum energy conformations is really quite good. MM2(87) (or later parametrizations of the Allinger program) is highly recommended as a starting point for such studies, and the use of MM2(87) optimized geometries as starting geometries for higher level ab initio calculations significantly reduces necessary computer time. 2. When calculating barriers for bowl-to-bowl inversions, MM methods generally give values that are too low, while semiempirical methods produce results which are either too high (AM1 and PM3)or too low (MNDO). Ab initio methods, on the other hand, give satisfactory results if post-HF correlation effects are included. 5.2. Semibuckminsterfullerenes

Both semibuckminsterfullerenes 27 and 28 are predicted by calculations to exhibit a significant degree of curvature in their minimum energy, bowl-shaped conformations. Interestingly, another local minimum was located on the potential energy surface of 27 by ab initio calculations. 33 However, this structure of C2h symmetry (27') is predicted to be 51 kcal/mol (HF/6-31 G*//3-21G level) higher in energy than the global C2v minimum. Similarly, both MM2(87) and AM1 calculations place 27' over 27 by 27 and 55 kcal/mol, respectively. Thus, the bowl-shaped conformer is expected to solely represent 27 under normal conditions. The planar conformation of 27 with D2h symmetry is calculated to be 76.1 kcal/mol higher in energy than the bowl by ab initio calculations. 33a However, this structure is not the TS for bowl-to-bowl inversion since it exhibits two imaginary

26


Figure 7. Calculated structures of two potential energy minima 27 and 27' (ab initio 3-21G), and the transition state 27TS for inversion (AM1).

frequencies at the HF/3-21G level. The AM1 method gives analogous results, predicting the planar structure to be 55 kcal/mol higher in energy and possessing two imaginary frequencies. Because of the size of the molecule the TS could not be located by ab initio calculations, but instead was calculated by the AM 1 method (Figure 7), supporting the two step pathway 27 ---) 27TS --->27' --> 27 as the lowest energy process for bowl-to-bowl inversion. A combination ofab initio and semiempirical results provided an estimation for the inversion barrier in 27 at about 57 kcal/mol. Consideration of electron correlation effects would likely increase the barrier by approximately 5-6 kcal/mol. Similarly, 28 is expected to exist in a rigid, bowl-shaped conformation with C3 symmetry. The planar C3h structure was found to be 77.3 kcal/mol higher in energy by MNDO calculations, 4 and 100 kcal/mol higher by AM I. 23 However both semiempirical methods found the planar structure to show three negative eigenvalues in vibrational frequency calculations, excluding the C3h structure as the TS for bowl inversion. The TS of C 1 symmetry was located by AM1 calculations; while it showed a significant degree of nonplanarity, there was only a very modest gain in energy (ca. 1 kcal/mol) over the planar C3h conformer. Very recent ab initio calculations suggest, however, that the nonplanarity of the TS is in this case an artifact of the semiempirical methods. 36 At the HF/3-21G level of approximation, the C3h structure of 28 exhibits only one imaginary frequency (124i) associated with the eigenvector of A" symmetry that leads to deformation of the C3h structure to the C3 bowl. So at the HF/3-21G level, the planar conformer represents the TS for bowl inversion. The 3-21G optimized structure of the minimum energy conformation is presented in Figure 8. There are some differences in the calculated bond

Polynuclear Aromatic Hydrocarbons wfh Curved Surfaces

27

Figure 8. Ab initio 3-21G optimized structure of semibuckminsterfullerene 28.

lengths between the MNDO and ab initio optimized structures, but the most important difference is found in the frontier orbitals. The semiempirical methods predict both HOMO and LUMO to be degenerate with E symmetry, while ab initio calculations predict a degenerate HOMO with the LUMO to be of A symmetry.36 The calculated energy difference between the two conformations is 68.4 kcal/mol at the HF/6-31 G*//3-21G level. If electron correlation effects are considered, the estimated difference reaches approximately 74 kcal/mol. 36 This number is even higher than the calculated barrier for 27. With such high barriers, both 27 and 28 would be virtually excluded from inversion making them attractive models for studies of concave versus convex complexation. Some computational studies on the complexation preferences of metal cations have been reported. 4'35These studies, performed at the semiempirical level, predict a surprisingly strong convex preference for small cations.

6.

R E D U C T I O N OF C O R A N N U L E N E

6.1. Electron Addition to Curved Surface Hydrocarbons: General Considerations As early as 1967, corannulene 8 was found to undergo facile reduction both electrochemically and by alkali metals. 14 The high electron affinity of 8 is not surprising since it belongs to the family of nonalternant hydrocarbons, and, as a consequence, its LUMO is relatively low in energy. This is also true for other hydrocarbons considered here. Figure 9 presents the frontier orbital diagram for corannulene 8, cyclopentacorannulene 25, and two semibuckminsterfullerenes 27 and 28, as calculated by the ab initio method. The calculations suggest that, due to the low lying unoccupied orbitals, these systems will readily accept electrons. Moreover, the degeneracy of the LUMO in 8 suggests the possibility of "tetraanion" formation. In 28 there are three low lying LUMO's, and so this molecule may accept up to six electrons in the process of reduction. With 25 and 27 it is not as easy to predict the outcome of reduction based on the MO diagram, but formation of at least a dianion of 25 and a tetraanion of 27 seems reasonable.

8

N

co

1.98-

25

-El

27

-A ’

2.09 1.39-

A

’I

2.42

28

-A ,

l.62-B2 1.51-

1

Bl

- -EA

1.70 1.59-

figure 9. HOMO’Sand low lying LUMO’s as calculated by HF/6-31G*//3-21G. Orbital energies in eV.


29

6.2. The Corannulene Dianion Distribution of the two additional electrons to 8 required for dianion formation among the degenerate LUMO orbitals of E 1 symmetry gives rise to four new states, since, within the C5v symmetry group, the direct product E 1E 1 may be reduced to a sum ofA1, A2, and E 2 irreducible representations. The A 2 state represents a triplet, while A 1 and E 2 are singlet states. The degenerate 1E2 state for the dianion of C5v symmetry is expected to be distorted into two singlet states of lower symmetry, most probably due to the Jahn-Teller effect. Inspection of the topology of the wavefunctions associated with the degenerate LUMO orbitals in 8 suggests a distortion to C s symmetry which splits the E 1 LUMO into a pair of orbitals of A' and A" symmetry, respectively. 55 From the pair of 1A1 and 3A 2 states, the latter is expected to be of lower energy. If Hund's rule is applicable to this system, then the higher spin system is preferred. Also, AM 1 calculations prefer the triplet state at both the HF level and various levels of CI treatment. In view of the above considerations, the ab initio study 55 was limited to three dianionic species: the triplet dianion of C5v symmetry, and two closed-shell singlet dianions of C s symmetry for the molecular network and A' and A" symmetries of the HOMO, respectively. Geometry optimization at the HF level led to global potential energy minima for the dianionic species, which were of lower symmetry than the ones predicted by Htickel theory. Thus, C5~ triplet 8 2- symmetry was lowered to C~, whereas the singlet dianions were found to be of C 1 symmetry, instead of C s. However, inclusion of electron correlation effects reversed the trend, showing the lowering of the molecular symmetries to be an artifact of the HF approximation. Despite the level of ab initio treatment, the energies of all three dianionic species considered were very close to each other. The virtually identical energy of the triplet and singlet states may explain the failure to detect the dianion by NMR during the reduction of 8 by lithium metal. 56

6.3. The Corannulene Tetraanion Both semiempirical and ab initio calculations on the tetraanion have been reported. 54-56 Considering the MO diagram, one could expect the retention of C5v molecular symmetry in g 4-. However, vibrational frequency calculations at the ab initio HF/3-21G level disclosed two degenerate frequencies for the C5v structure, excluding it as the minimum energy conformation at this level of approximation. A new stationary point on the potential energy surface was located with C s symmetry, which was confirmed to be a real minimum. However, by analogy with the dianion study, MP2/3-21G calculations preferred the C5v structure strongly, again proving the failure of the HF approximation for a correct description of the tetraanion. Thus, a more reliable structure of 84- was obtained by geometry optimization at the MP2/3-21G level within Csv symmetry. 55

30


~

=

84-

On the basis of NMR results, an intriguing "anion within the trianion" structure was proposed for 84-. 56 Such a structure would require 3.0 ee excess in the rim, and 1.0 ee excess in the hub, as compared to neutral 8. Analysis of the charge densities calculated at both semiempirical AM1 and ab initio levels, however, predict the hub atoms to gain only about 0.5 ee, while the rim atoms bear more of the charge at 3.5 ee. 55'57

6.4. Inversion Barriers in Anionic Species Calculations show that the most interesting feature of corannulene, its curvature, decreases continuously with increasing negative charge. At the HF/3-21G level the calculated depth of the bowl is 0.88/~ for neutral 8, and 0.71 and 0.37/~ for the dianion triplet and tetraanion, respectively. 5s'57 Hence, despite this trend, the tetraanion still prefers the nonplanar conformation. At the ab initio HF level of approximation, the calculated bowl-to-bowl barriers decrease accordingly from approximately 9 kcal/mol for the neutral, through 3-5 kcal/mol for the dianion, 55 to only 0.3 kcal/mol for the tetraanion. 54'55Electron correlation effects increase the barriers significantly to approximately 14, 8-9, and 3 kcal/mol for the neutral, dianion, and tetraanion, respectively. 55 The importance of post-HF effects is further demonstrated by MP2/3-21G geometry optimization, which suggests 84- to be more curved than predicted by HF calculations (a bowl depth of 0.63/~), but still flattened as compared to both the neutral and the dianionic species. 55

6.5. Experimental Studies with Anionic Species An early electrochemical study of corannulene revealed the presence of two well-defined polarographic waves with half-wave potentials of-1.88 and-2.36 V (t-butylammonium perchlorate in acetonitrile). 14 The first wave represented a reversible, one-electron reduction leading to radical anion formation (emerald green solution ) further characterized by UV-VIS and ESR. The second wave was reported to be associated with the formation of a bright red species "which is not paramagnetic, but it is not believed to be the dianion, but rather some decay product of it." Treatment of THF solutions of 8 with sodium and potassium metals also led to the formation of the same species. 14 Recently, an NMR study of the diamagnetic species resulting from lithium reduction of 8 was reported. 56 The THF solution revealed a single IH NMR peak at 6.95 ppm, and three ~3C NMR lines at 86.8, 95.1, and 112.4 ppm, indicating that the reduced species retains the high symmetry of neutral corannulene. Moreover, consideration of the ~3C NMR chemical shift, as compared to neutral 8, provides a


31

reasonable 180 ppm/charge ratio if the species is considered to be the tetraanion. In further support of a tetraanionic intermediate, quenching of the anionic species produced tetrahydrocorannulene as the major product. 56 However, the tetralithiocorannulene, although stable in THF solutions even at room temperature, does not appear to exhibit any exceptional stability since it is either not formed or does not resist protonation in liquid ammonia. 57

6.6. A Remarkably Stable Dimer of Tetralithiocorannulene The relatively simple NMR spectrum of 84-/4Li+ does not provide much information about dynamic processes or possible states of aggregation. In an attempt to gain this kind of information, the tetralithio derivatives of 1-t-Bu- and 1-i-Pr-corannulenes were also examined, and they appeared to show two different species each in similar but unequal abundance. 58 Moreover, no significant changes were observed in the 1H and 13C NMR from-80 ~ to room temperature suggesting the lack of any dynamic processes. The explanation offered for this unusual spectroscopic behavior is the existence of stable, dimeric aggregates. Due to the symmetry of 1-R-corannulenes, face-toface association results in two topologically different dimers (dl and meso). Moreover, the lack of any signal coalescence in either 1H or 13C NMR up to room temperature suggests that these dimers are unusually stable, and do not dissociate on the NMR time scale. 58 7Li NMR spectroscopy provides further insight into the nature of tetralithiocorannulenes. At low temperatures (-63 ~ THF-d s solutions of the salts show two 7Li signals of equal intensity. In the case of the corannulene salt, one very broad signal appears at --4.5 ppm (relative to external LiBr/THF), while a much narrower band appears at-11.7 ppm. The relatively high field chemical shifts of both 7Li absorptions are due to strong shielding from the diamagnetic ring currents of the aromatic corannulene tetraanion. Upon addition of about 2 equivalents of LiBr to the solution, a new peak appears at-2.5 ppm while the peak at--4.5 ppm disappears and the -11.7 peak remains unchanged. This shows that the latter peak represents lithium cations which are more intimately bound to the aromatic moiety (contact ion pairs), and do not rapidly exchange with the added LiBr. On the other hand, the peak at --4.5 ppm represents more loosely bound, solvent-separated lithium cations which undergo rapid exchange with added LiBr. This, together with the significant differences in chemical shifts for the two groups of lithium cations, allows for the R

meso

R

R

R

dl

32

PETER W. RABIDEAU and ANDRZEJ sYGULA

Figure 10. Schematic of the tetralithiocorannulene dimer.

assignment of the signal at-11.7 ppm to the four lithium cations sandwiched between the two corannulene moieties, while the lower field -4.5 ppm signal is assigned to the exchangeable "exterior" lithium cations (Figure 10). The two 7Li NMR peaks coalesce into a single peak at room temperature (Tc = -8 ~ indicating that the interior and exterior cations are able to exchange places without dissociation of the dimer! The 7Li NMR spectra of both 1-i-Pr and 1-t-Bu corannulenea-/4Li § exhibit quite analogous behavior. Moreover, tetraanion solutions generated from mixtures of corannulene and t-Bu-corannulene show the presence of mixed dimers in the aH NMR spectrum. 58

Figure 11. MNDO calculated minimum energy structure for tetralithiocorannulene

dimer.


33

MNDO calculations suggest a "stacked-bowl" arrangement of the two tetraanionic corannulene moieties in a convex-to-concave geometry with four lithiums inside the sandwich as the global energy minimum (Figure J 1).59 Thus, in agreement with theoretical studies on the isolated tetraanions (see above), the corannulene tetraanion also prefers a bowl-shaped conformation when the countercations are included in the calculations. With this model, it is necessary to assume fast inversion of the two corannulene units within the dimer (or planarity); otherwise the top and bottom bowls would not be equivalent in the NMR spectra. MNDO calculations predict an activation enthalpy of approximately 13 kcal/mol for the concerted inversion process, and this value is inconsistent with the NMR behavior that shows no evidence of "freezing out" such a dynamic process. These calculations, however, do not take solvation of the exterior lithiums into account, and so the exterior lithiums were "removed," and the system was calculated under conditions that would roughly model solvent separation of the lithium cations. Accordingly, the approximate barrier for inversion was lowered to approximately 1 kcal/mol, suggesting that solvation of the exterior lithium cations can indeed greatly reduce the inversion barrier. 58

7. C O N C L U D I N G REMARKS The chemistry of curved, polynuclear aromatic hydrocarbons holds great promise due, in part, to the rampant interests in fullerenes. Of special interest, of course, is their relationship to the fullerenes, and these compounds should serve as good models in many ways since the curvature of the systems already known matches that of buckminsterfullerene, at least at the point of maximum curvature. Using the x-orbital vector analysis method, 6~ (POAV 1), the pyramidalization angle of buckminsterfullerene can be compared to several of the curved surface hydrocarbons considered herein. This angle, defined as O ~ n - 90, is 11.64 for buckminsterfullerene (see Figure 12), and, of course, there is a single value since all of the carbons are identical. In curved surface hydrocarbons, on the other hand, the curvature is expected to vary across the surface. The maximum (and average) pyramidalization angles derived from ab initio calculated structures for several curved surface, polynuclear aromatic hydrocarbons are as follows: corannulene 8 = 8.4 (3.7), cyclopentacorannulene 25 = 11.4 (4.8), semi-buckminsterfullerene 27 = 11.4 (5.2), and semi-buckminsterfullerene 28 = 11.1 (5.3). Hence, as we get beyond corannulene, most of these hydrocarbons approach the curvature of buckminsterfullerene itself in the region of their maximum curvature. And, in addition, these compounds have both accessible convex and concave surfaces. Hence, curved surface, polynuclear aromatic hydrocarbons represent a new and exciting area of research, and the comparison with the fullerenes is an added attraction.

34


POAVI

"

Vector

* "x ! !

1

Figure 12, The g-orbital axis vector analysis (POAV) for nonplanar conjugated

organic molecules.

ACKNOWLEDGMENTS This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, of the U.S. Department of Energy, and by SNCC (Louisiana State University) for allocation of computer time. We would also like to thank our co-workers at LSU who performed a considerable amount of the work that was highlighted in this chapter (A. Abdourazak, H. Folsom, Z. Marcinow, Renata Sygula, and R. Dahr), and we thank Dr. Haddon for the latest version of his POAV3 program. Finally, we thank Professor Larry Scott for helpful discussions, and for opening the door to corannulene and related chemistry.

NOTE ADDED IN PROOF There have been several developments in this area since this manuscript was prepared. The heat of combustion of corannulene was determined by microbomb combustion calorimetry and its gas-phase enthalpy of formation was estimated at 110.8 kcal/mol. 61 All anionic oxidation states of corannulene were observed by optical absorption, EPR, and NMR spectroscopies. 62 More support for the "annulene-within-annulene" model of the corannulene tetraanion was presented. 63 An alternative pyrolysis route to corannulene was reported, 64 as well as some attempts toward the synthesis of bowl-shaped subunits of fullerenes. 65 And in contrast with previous semiempirical studies, 4'35 ab initio calculations predicted a general concave preference for the metal cation binding to semibuckminsterfullerene 28. 66

REFERENCES AND NOTES 1. (a) Kroto, H. W.; Heath, J. R.; O'Brien, S. C.; Curl, R. F. and Srnalley, R. E. Nature 1985, 318, 162; (b) Kr~itschrner,W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R.Nature 1990, 347, 354; (c) Kroto, H. W.; Allaf, A. W.; Balm, S. P. Chem. Rev.'1991, 91, 1213; (d) Hammond, G. S.; Kuck,


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. 33.

34. 35. 36.

35

V. J., Eds. Fullerenes, ACS Symposium Series 481, Washington, DC, 1992; (e) Acc. Chem. Res. (special issue), 1992, 25, 97-175. Wudl, E Acc. Chem. Res. 1992, 25, 157. Diederich, E; Rubin, Y. Angew. Chem. Internat. Ed. Engl. 1992, 31, 1101, and references therein. Faust, R.; Volhardt, K. P. C. J. Chem. Soc., Chem. Commun. 1993, 1471. Mehta, G.; Shah, R. S.; Ravikumar, K. J. Chem. Soc., Chem. Commun. 1993, 1006. Sastry, G. N.; Jemmis, E. D.; Mehta, G.; Shah, S. R. J. Chem. Soc., Perkin Trans. 2 1993, 1867. Clar, E. Polycyclic Hydrocarbons; Academic Press, New York, 1964 and references therein. Hanson, J. C.; Nordman, C. E. Acta Cryst. B. 1976, B32, 1147. Yamamoto, K.; Harada, T.; Okamoto, Y.; Chikamatsu, H.; Nakazaki, M.; Kai, Y.' Nakao, T.; Tanaka, M.; Harada, S.; Kasai, N. J. Am. Chem. Soc. 1988, 110, 3578. Haag, R.; Olhorst, B.; Noltemeyer, M.; Schuster, A.; Kuck, D.; de Meiere, A. J. Chem. Soc., Chem. Commun. 1993, 1727; Ceccon, A.; Gambaro, A.; Manoli, E; Venzo, A.; Kuck, D.; Bitterwolf, T. E.; Ganis, P.; Valle, G. J. Chem. Soc., Perkins Trans. 2, 1991, 233. Barth, W. E.; Lawton, R. G. J. Am. Chem. Soc. 1966, 88, 380. Barth, W. E.; Lawton, R. G. J. Am. Chem. Soc. 1971, 93, 1730. Bramwell, E B.; Gendell, J. J. Chem. Phys. 1970, 52, 5656. Janata, J.; Gendell, J.; Ling, C.-H.; Barth, E. E.; Backes, L.; Mark, H. B., Jr.; ldawton, R. G. J. Am. Chem. Soc. 1967, 87, 3056. Craig, J. T.; Robinson, M. D. W. Aust. J. Chem. 1968, 21, 2237. A number of years later, further attempts at the conventional cyclization of 7,10-disubstituted fluoranthenes would also fail. Jacobson, R. H. Ph.D. Dissertation, University of California, Los Angeles, 1986. Davy, J. R.; Iskander, M. N.; Reiss, J. A.Aust. J. Chem. 1979, 32, 1067. Scott, L. T.; Hashemi, M. M.; Meyer, D. T.; Warren, H. B. J. Am. Chem. Soc. 1991, 113, 7082. Scott, L. T.; Hashemi, M. M.; Bratcher, M. S. J. Am. Chem. Soc. 1992, 114, 1920. Cheng, P.-C. M. S. Thesis, University of Nevada, Reno, 1992. Borchardt, A.; Fuchicello, A.; Kilway, K. V.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 1992, 114, 1921. The data are produced utilizing the Cambridge Crystallographic Centre file and they differ slightly from the ones reported in ref. 8 by Hanson and Nordman. Sygula, A.; Rabideu, P. W., unpublished results. Sygula, A.; Sygula, R.; Fronczek, E R.; Rabideau, P. W. J. Org. Chem., to be published. For a review see: Rabideau, P. W.; Sygula, A. In The Conformational Analysis of Cyclohexenes, Cydohexadienes, and Related Hydroaromatic Compounds; Rabideau, P. W., Ed.; VCH Publishers, New York, 1989; Chapter 3, p. 65. Sygula, A.; Rabideau, P. W. J. MoL Struct. (Theochem) 1992, 262, 117. Cosmo, R.; Hambley, T. W.; Sterrdaell, S. J. Org. Chem. 1987, 52, 3119. Eliel, E. L. Stereochemistry of Carbon Compounds; McGraw-Hill, New York, 1962, Chapter 8. Abdourazak, A. H.; Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc. 1993, 115, 3010. Sygula, A.; Folsom, H. E.; Sygula, R.; Abdourazak, A. H.; Marcinow, Z.; Fronczek, F. R.; Rabideau, P. W. J. Chem. Soc., Chem. Commun. 1994, 2571. Bratcher, M. S.; Scott, L. T., Paper No. 420, Organic Division, 207th ACS National Meeting, San Diego, CA, 1994. Loguercio, D. Phi). Thesis, UCLA, Los Angeles, CA, 1988. (a) Rabideau, P. W.; Abdourazak, A. H.; Folsom, H. E.; Marcinow, Z.; Sygula, A.; Sygula, R. J. Am. Chem. Soc. 1994, 116, 7891. (b) Rabideau, P. W.; Abdourazak, A. H.; Folsom, H. E.; Marcinow, Z.; Sygula, A.; Sygula, R. J. Am. Chem. Soc., in press. Raghavachari, K.; Rohlfing, C. M. J. Phys. Chem. 1992, 96, 2463. Plater, J.; Rzepa, H. S.; Stoppa, F.; Stossel, S. J. Chem. Soc., Perkin Trans. 2 1994, 399. Sygula, A.; Rabideau, P. W. J. Chem. Soc., Chem. Commun. 1994, 1497.

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.

PETERW. RABIDEAU and ANDRZEJ SYGULA Schmaltz, T. G.; Seitz, W. A.; Klein, D. J.; Hite, G. E. J. Am. Chem. Soc. 1988, 110, 1113. G!eicher, G. J. Tetrahedron 1967, 23, 4257. Kao, J.; Allinger, N. L. J. Am. Chem. Soc. 1977, 99, 975. Agranat, I.; Hess, B. A., Jr.; Schaad, L. J. Pure Appl. Chem. 1980, 52, 1399. Randic, M.; Trinajstic, N. J. Am. Chem. Soc. 1984, 106, 4428. Aihara, J.; Hosoya, H. Bull. Chem. Soc. Jpn. 1988, 61, 2657. Moyano, A.; Paniagua, J.-C. J. Org. Chem. 1991, 56, 1858. Cyvin, S. J.; Brendsdal, E.; Brunvoll. J.; Skaret. M. J. Mol. Struct. 1991, 247, 119. Kao, J. J. Am. Chem. Soc. 1987, 109, 3817. Haddon, R. C.; Brus, L. E.; Raghavachari, K. Chem. Phys. Lett. 1986, 131, 165. Haddon, R. C. J. Am. Chem. Soc. 1987, 109, 1676. Haddon, R. C. J. Am. Chem. Soc. 1990, 112, 3385. Kataoka, M.; Nakajima, T. Tetrahedron 1986, 42, 6437. Bakowies, D.; Thiel, W. J. Am. Chem. Soc. 1991,113, 3704; Bakowies, D.; Thiel, W. Chem. Phys. 1991, 151, 309. Matsuzawa, N.; Dixon, D. A. J. Phys. Chem. 1992, 96, 6241. Schulman, J. M.; Peck, R. C.; Disch, R. L. J. Am. Chem. Soc. 1989, 111, 5675. Peck, R. C.; Schulman, J. M.; Disch, R. L. J. Phys. Chem. 1990, 94, 6637. Disch, R. L.; Schulman, J. M. J. Am. Chem. Soc. 1994, 116, 1533. Sygula, A.; Rabideau, E W. J. Mol. Struct. (Theochem), in press. Ayalon, A.; Rabinovitz, M.; Cheng, P.-C.; Scott, L. T. Angew. Chem., Int. Ed. Engl. 1992, 31, 1636. Rabideau, P. W.; Marcinow, Z.; Sygula, R.; Sygula, A. Tetrahedron Lett. 1993, 34, 6351. Ayalon, A.; Sygula, A.; Cheng, P.-C.; Rabinovitz, M.; Rabideau, P. W.; Scott, L. T. Science, in press. A similar concave-to-convex arrangement was recently proposed for the adduct of the radical cation dimer of corannulene with neutral corannulene, formed in a gas-phase. Becker, H.; Javahery, G.; Petrie, S.; Cheng, E-C.; Schwartz, H.; Scott, L. T.; Bohme, D. K. J. Am. Chem. Soc. 1993, 115, 11636. Haddon, R. C., Scott, L. T. Pure Appl. Chem. 1986, 58, 137. Haddon, R. C.Acc. Chem. Res. 1988, 25, 243. Haddon, R. C. Science 1993, 261, 1545. Kiyobayashi, T.; Nagano, Y.; Sakiyama, M.; Yamamoto, K.; Cheng, P-C.; Scott, L. T. J. Am. Chem. Soc. 1995, 117, 3271. Baumgarten, M.; Gherghel, L.; Wagner, M.; Weitz, A.; Rabinovitz, M.; Cheng, P-C.; Scott, L. T. J. Am. Chem. Soc., in press. Zhou, Z. J. Phys. Org. Chem. 1995, 8, 103. Zimmermann, G.; Nuechter, U.; Hagen, S.; Nuechter, M. Tetrahedron Lett. 1994, 35, 4747. (a) Hagen, S.; Nuechter, U.; Nuechter, M.; Zimmermann, G. Tetrahedron Lett. 1994, 35, 7013. (b) Hagen, S.; Nuechter, U.; Nuechter, M.; Zimmermann, G. Polycyc. Arom. Compounds, submitted. Sygula, A.; Rabideau, P. W. J. Chem. Soc., Chem. Commun. 1994, 2271.

CHEMISTRY OF CYCLOPROPARENES

Paul M ller

1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Cycloproparenes . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. General Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Strained, Fused, and Short-Lived Cycloproparenes . . . . . . . . . . . . 2.3. Heterocyclic and Functionalized Cycloproparenes . . . . . . . . . . . . 2.4. Alkylidenecycloproparenes . . . . . . . . . . . . . . . . . . . . . . . . Physical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Structure and Strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Spectral Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. The Polarity of Alkylidenecycloproparenes . . . . . . . . . . . . . . . . Reactions of Cycloproparenes . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Substitutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Addition Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Rearrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Reactions with Transition Metal Reagents . . . . . . . . . . . . . . . . 4.5. Reactions of Alkylidenecycloproparenes . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Advances in Theoretically Interesting Molecules Volume 3, pages 37-107. Copyright 9 1995 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 1-55938-698-3 37

38 40 40 50 59 65 67 67 73 77 78 78 85 93 95 97 100 100

38

PAUL MULLER

1. INTRODUCTION In 1888 the structure of a cycloproparene first appeared in the literature. The paper by Perkin entitled, Synthetical Formation of Closed Carbon-Chains in the Aromatic Series, suggested that very interesting results might be obtained if compounds such as benzocyclopropene (1) or benzocyclobutene (2) could be synthesized. 1 Perkin was unable to prepare derivatives of these compounds at that time, however, but he did prepare derivatives of indan and tetralin. Some 60 years later (1956) Cava and Napier isolated benzocyclobutene (2), 2 the next higher homologue of benzocyclopropene (1), while benzyne (3), which may be considered its next lower homologue was postulated as a reaction intermediate in 1942,3 although formal proof of its occurrence wasn't established until 1953.4 Yet, the physical observation of 3 by trapping it in a matrix at low temperature did not occur until 1973.5 Benzocyclopropene (1) was considered a missing link between 2 and 3 and it was suggested that it should "at least be capable of transient existence and quite probably could exist as a stable entity". 6 The first claims of the successful synthesis of a cycloproparene appeared in the literature in 19307 and in 1953,8 but more recent reinvestigations of these reactions revealed that the products which had been obtained were not cycloproparenes. 9'1~ Ullmann and Buncel 6 attempted to deprotonate Aa-carene-2,5-dione (4) and the benzannelated derivative 5 in the hope of generating the possibly aromatic dianions 6 and 7. However, the dianions (and the monoanions) could not be isolated. On the grounds of deuterium exchange experiments, the strain energy of benzocyclopropene (1) was estimated to at least 45.5 kcal/mol higher than that ofcyclopropane. The synthetic breakthrough in cycloproparene chemistry was achieved by Anet and Anet in 1964 who prepared 1,1-dimethyl-3-methoxycarbonylbenzocyclopropene (9) by irradiation of the substituted 3H-indazole 8.11 The parent benzocyclopropene (1) was obtained a year later by Vogel et al. by retro-Diels-Alder reaction (Alder-Rickert cleavage) of 10.12 The chemistry of cycloproparenes is characterized by the interplay of two contradictory effects, namely aromaticity which is generally known to stabilize compounds, and strain which destabilizes them. The fusion of a cyclopropene to a benzene ring results in geometrical distortions and strain which has consequences on the properties of the resulting cycloproparene. It perturbs the aromatic n-electron

1

2

Scheme 1.

3

Chemistry of Cycloproparenes

39

O

O"

X O

O"

4

OCH3 O

6

O"

x OCH 3 O

O"

5

7

Scheme 2.

system, which leads to enhanced reactivity, and at the same time weakens the single bonds of the cyclopropane moiety. Cycloproparenes are highly strained compounds, and for this reason represent an obvious synthetic challenge. They are of interest to theoreticians for testing the limits of models for bonding and reactivity, and their unusual reactivity makes them attractive precursors for the synthesis of unusual compounds. The chemistry of cycloproparenes has been reviewed regularly in the past by Halton 13-15 and Billups. 16'17 This review will attempt to be comprehensive with emphasis on the more recent developments and on the results from the research of the author. The literature has been searched through January 1994.

MeO

""-

MeO)

O

MeOOC

MeO., O

8

10

Scheme 3.

9

40

PAUL MOLLER

2. SYNTHESIS OF CYCLOPROPARENES 2.1. General Strategies The strain of cycloproparenes is the principal obstacle that must be overcome in their synthesis. Cycloproparenes decompose usually at moderate temperatures, and they undergo ring-opening in the presence of ~electrophi!ic or metallic reagents. In contrast, they support even strongly basic conditions quite well. Accordingly, most successful cycloproparene syntheses use some base-induced elimination in the last step. Alternatively, flash vacuum pyrolytic or photochemical extrusion of a neutral fragment, followed by biradical closure or flash vacuum pyrolysis may be used. In these latter approaches the reaction conditions are neutral, and reactive products may be trapped at low temperatures. The principal synthetic approaches to cycloproparenes, schematically summarized in Scheme 4, are discussed below with representative examples. Specific applications are presented in subsequent sections. The large majority of cycloproparene syntheses use base-induced aromatization of an appropriately functionalized bicyclo[4.2.0]hept-2-ene precursor in the final step. The 1,6-dihalogenated bicycloheptenes 13 are conveniently available from X

X

y

11

Y

12

17

13

1

T 14

X

15

Scheme 4.

u

~

16

18


41

Diels-Alder addition of conjugated dienes 11 to 1,2-dihalogenocyclopropenes 12. Addition of dihalocarbenes to 1,4-cyclohexadienes 14, in turn, furnishes the 7,7-dihalogenated bicycloheptene 15. Alternatively cycloproparenes are accessible by formation of the central double bond in a norcaradiene 16. A conceptually different approach uses closure of one of the lateral cyclopropane single bonds in the last step. This may be achieved via ring closure of a biradical, which is obtained from photochemical or pyrolytic extrusion of a (often 2-atom) fragment from a benzannelated 4-or 5-membered ring 17. Closure of one of the lateral cyclopropane single bonds may also be achiev~ via intramolecular 1/3/elimination of an o-substituted benzylic derivative 18 having appropriate functionalities. Variations of these methods have been developed as required for the various specific applications.

Cycloproparenes via Cyc!oaddition of Dienes to Cyclopropenes The cycloaddition of buta-l,3-diene (1!) to tetrahalogenocyclopropenes, as developed originally by Tobey, 18provides convenient access to tetrahalogenobicyclo[3.1.0]hept-2-enes, which undergo bis-(hydro,chloro)elimination by treatment with a base. This simple scheme was first exploited by Vogel et al. in their synthesis of 1,1-difluorobenzocyclopropene (21) from 11 and 1,2-dibromo-3,3-difluorocyclopropene (19) via 20.19 Subsequent applications of the sequence used the more readily available 1,2-dichloro-3,3-difluorocyclopropene instead of 19. 2~ The cycloadditions between tetrahalogenocyclopropenes and butadienes are usually carried out at about 100 ~ in an inert solvent, in the presence of a small amount of K2CO3 and hydroquinone in order to prevent polymerization of the diene. Originally endo selectivity was assumed for additions of substituted openchain dienes to 1,2-dihalogeno- and tetrahalogenocyclopropenes, 18'22 but more recent X-ray and NMR investigations 23,24 showed that the preferred mode of addition is exo. The cycloadducts are conveniently aromatized to cycloproparenes by reaction with t-BuOK in THF at low temperature, Since 1,1-dihalogenocycloproparenes solvolyze very readily, an anhydrous work-up procedure is usually indicated. The reaction sequence is fairly general and may be used for a variety of cycloproparenes. The limiting factor is the reactivity of the products. Usually, Br

Br

Br 11

19

20

Scheme 5.

21

42

PAUL MULLER Ph

Ph

I

Ph

Ph 22

23

X --- CI, F

25

24

26

Scheme 6.

1,1-difluorocycloproparenes are isolable and can often be distilled and purified by column chromatography, although there are exceptions. 25'26Only a few 1,1-dichlorocycloproparenes such as the 2,5-diphenyl derivative 22, 22 1,1-dichloro-lH-cyclopropa[b]naphthalene (23), 27 and its derivative 24 (prepared by a modified route) 28 have been isolated so far, while all attempts to prepare 1,1-dichlorobenzocyclopropene (25) failed. 22The 1-chloro- 1-fluoro derivative 26 has, however, been synthesized, 29 but no synthesis of 1,1-dibromocycloproparenes by this route has yet been reported. The introduction of 1,2-bromochlorocyclopropene (27) by Billups 3~as a synthon for cycloproparenes constituted a major achievement and made a large number of previously unaccessible cycloproparenes, lacking substituents at C 1, available. The cyclopropene 27 is labile and decomposes at room temperature, but nevertheless undergoes addition to a variety of dienes around-20 ~ In some cases reaction times of several days or even weeks are necessary in order to obtain complete conversion. The stereoselectivity for the addition of 27 to open-chain dienes is usually exo, as in the case of the tetrahalogenocyclopropenes. 23'24Bis-(hydro,halogeno)elimination is carded out with t-BuOK in THE as in the case of the adducts of tetrahalogenocyclopropenes. The first cycloproparene prepared by this scheme was 1H-cyclopropa[b]phenanthrene (30), prepared by Billups et al. from diene 28 via adduct 29 as shown below. 3~ The scheme has been applied to simple cycloproparenes, such as benzocyclopropene (1) itself, 31 and to its 2- and 3-methyl derivatives. 32 More interestingly, a number of highly strained, otherwise inaccessible cycloproparenes have been synthesized by this route. Some of them will be discussed in subsequent sections. A variation of this approach, based on interception of o-quinodimethanes provides access to 1H-cyclopropa[b]naphthalenes. Halton 28 exploited the conrotatory ring-opening of t r a n s - 1,2-diphenylbenzocyclobutene (31) to the o-quinodimethane


43 Br(Cl) _

CI(Br) CI 28

27

29

30

Scheme 7.

32, which occurs reversibly at 25 ~ Addition of 32 to tetrachloro- or tetrabromocyclopropene (33 or 34) in refluxing benzene furnished adducts 35 and 36, respectively, and their aromatization under the usual conditions afforded the substituted cyclopropanaphthalenes 24 and 37. However, the dibromo derivative 37 could not be fully characterized. The reaction conditions (80 ~ used for the addition of 33 and 34 to the o-quinodimethane 32 are incompatible with the presence of 1,2-bromochlorocyclopropene (27), thus the potential of this approach was for quite a while not further exploited. However o-quinodimethanes may be synthesized under much milder conditions, and trapped as reactive intermediates with 27. Thus base-induced isomerization of cis-oct-4-ene-2,7-diyne (38) at -78 ~ leads to bis-allene (39). 34 Upon warming of 39, rearrangement to o-quinodimethane (40) occurs between-20 and -10 ~ this adds smoothly to 27 and furnishes the adduct 41. Conversion of x Ph Ph X Ph 31

32

Ph

33

X=CI

34

X = Br

Ph

Ph

I

X

X Ph

Ph 35

X = CI

24

X = CI

36

X = Br

37

X = Br

Scheme 8.

44

PAUL MOLLER

39

38

40

Y

§

-

N[~(CH3)31

CI 42

41

43

SiMe3

Scheme 9.

41 to 1H-cyclopropa[b]naphthalene (42) may be accomplished in the usual fashion. 35 The tetrahalogenocyclopropenes fail to intercept 40, however. This sequence has been exploited for the synthesis of some methyl-substituted derivatives of 42 and also for that of 1H-cyclopropa[b]anthracene (see below). The o-quinodimethane (40) may also be generated by fluoride-induced 1/4/elimination of trimethyl(2-[(trimethylsilyl)methyl]-benzyl) ammonium iodide (43). 36 When this elimination is carried out at low temperature, the reaction conditions are equally compatible with the presence of 27, and this provides an additional route to 42. 37

R

R

R x

0

~

~

N

N

N

44

R=Ph

46

R=Ph, X=Br, H

48

R=Ph

45

R=H

47

R=H,X=Br

42

R=H

CI

CI Br 49

1

Scheme 10.

50

Br

chemistry of Cycloproparenes

45

One of the drawbacks associated with the cycloadditions based on 27 is the low reactivity of halogenocyclopropenes towards dienes. This may be overcome if furanS or isobenzofurans are used instead. The aromatization of the adducts is, however, not straightforward. For example, reaction of the exo-adduct of 27 to diphenylisobenzofuran (DPHIBE 44) with BuLi affords no cycloproparene, but a product 46, where the bromo substituent is replaced by hydrogen. Aromatization may, however, be effected by use of low-valent titanium. 38 This sequence provides a simple access to 48. The parent 42 is also available by this route from 45 via 47. Exposure of adducts of furans, such as 49, to low-valent titanium leads, however, to mixtures of benzocyclopropene (1) and 1,6-dihalogenocycloheptatriene (51). 39 Cycloaddition of furans to tetrahalogenocyclopropenes may equally be realized, 18 but the adducts decompose in the presence of low-valent titanium and afford no cyclopr0parenes.

Cycloproparenes via Adducts of Dihalocarbenes to Cyclohexa-1,4-dienes Aromatization of dihalocarbene adducts to 1,4-cyclohexadiene or synthetic equivalents is the method Of choice for the synthesis of the parent benzocyclopropene (1). 40'41 The mechanism of the aromatization step of the intermediate 7,7-dihalogenobicyclo[4.1.0]hept-2-ene (51) has been shown by labeling experiments with 51 depleted of 13C at C7, to proceed via a series of elimination and double bond migration steps via cyclopropene- and alkylidenecyclopropane intermediates 52 to 54 with preservation of the original carbon skeleton. 42 The synth~is of the benzannelal~ed homologue, 1H-cyciopropa[b]naphthaiene (42), by the same route confirms these findings. 43 Some skeletal rearrangement has, however, been observed in an isolated case.44 Synthetic equivalents of 1,4-cyclohexadiene, such as 1,3-cyclohexadiene 45-48or 3-bromocyclohexene, 49have also been used for the preparation of 1. The procedure is generally applicable to linearly fused cycloproparenes such as 1H-cyclopropa[b]naphthalene (42), 43'50'51 and many even highly strained cycloproparenes

52

51

s4

Scheme 11.

53

46

PAUL MULLER

CI

CI

57

55

t

56

CI

C/H2CI

59 (9%)

~

58

OtBu

60 (8%)

CI

,H

61 (12%)

Scheme 12.

have been prepared by this route. The principal limitation is the propensity of the intermediate cyclopropenes to undergo rearrangement to vinylcarbenes, rather than double bond migration to cycloproparenes. This occurs, in particular, with angularly fused systems. For example the attempted synthesis of 1H-cyclopropa[a]naphthalene (56) from 55 leads to a mixture of ring-opened products 59--61 which must derive from intermediate vinylcarbenes 57 and ~8. 47 The success of the aromatization of di- or tetrahalogenobicyclo[4.1.0]hept-2enes depends upon the appropriate location of the halogens. If they are placed at the bridgehead positions (C 1 and C6), the elimination proceeds under mild conditions. Bicyclohept-2-enes with halogens at C7 may also be aromatized, but require significantly more vigorous reaction conditions. If the halogens are at positions adjacent to the cyclopropane ring, as in 62, the cycloproparene 23 is not formed, but ring opening to 63 occurs. 52 Similarly 64 and 65, which have the potential leaving groups not at the bridgehead of the cyclopropane, may not be aromatized to cycloproparenes. 53,54 Aromatization is, however, possible when one of the halogens occupies a bridgehead position, and the other is placed in such a way that conjugate elimination, involving opening of the cyclopropane ring, may not take place, as exemplified by the conversion of 66 to 67. 55 Aromatization of Norcaradienes The introduction of the central double bond in a norcaradiene precursor 16 to synthesize a cycloproparene appears at first sight as a simple and convenient


47 Br

••C

Br

cl

cI

I

I

u_

62

23

Br

63

Ph

I

64

Ph

65 F F

F

F

Br

66

67

Scheme 13.

approach. Unfortunately, however, norcaradienes are not readily accessible. They are in rapid equilibrium with the corresponding cycloheptatrienes, and the equilibrium is on the side of the latter, unless C7 of the cycloheptatriene carries strongly electron-attracting substituents. 56 Despite this limitation, the first synthesis of the parent benzocyclopropene (1) used the norcaradiene approach, which involved Alder-Rickert cleavage of the cycloadduct 10 of 1,6-methano[10]annulene to acetylene dicarboxylate or other alkynes, such as dicyanoacetylene or benzyne (see Section 1).12'57In the adduct 10, the seven-membered ring of the cycloproparene to be synthesized is locked in the norcaradiene structure owing to skeletal strain and the missing double bond is efficiently introduced upon heating of 10 to 400 ~ Alder-Rickert cleavage has not been widely used for cycloproparene synthesis, since the preparation of the precursors is often tedious, except for the simple cases like 7,7-difluorobenzocyclopropene (21). 19The approach offers, however, decisive advantages in special situations. If the Alder-Rickert cleavage is carried out under flash-vacuum pyrolysis conditions, the products may be isolated under neutral conditions and at low temperature. Thus the synthesis of the highly reactive 1H-cyclopropa[a]naphthalene (56) by pyrolysis of 68 has been achieved by this approach. 58 Several other approaches to 56 failed.

48

PAUL MOLLER E

68

E

E = CN

56

CI BuLi

Br 50

Br 69

1

Scheme 14.

The presence of a locked norcaradiene is however not a prerequisite for introduction of the central double bond of a cycloproparene. Exposure of the 1,6-diha10genated cycloheptatriene 50 to n-BuLi also results in formation of benzocyclopropene (1) via 69, despite of the unfavorable position of the cycloheptatriene-norcaradiene equilibrium. 39 The method is, however, of limited interest, since the most convenient access to 1,6-dihalogenocycloheptatrienes starts with benzocyclopropene (1).41

Cycloproparenes via Closing of a Cyclopropane (r-Bond Cycloproparenes may be prepared by formation Of one of the lateral cyclopropane o-bonds either vi~/biradical closing, or via 1/3/elimination. The first reported synthesis of a benzocyclopropene derivative (see Section 1) 11 is an application of the former of these approaches. Upon irradiation, 3H-pyrazoles 70 loose N 2, and the intermediate biradical 71 cyclizes to 72. There is evidence that the intermediate biradical is in the triplet state, 59 but an alternative interpretation in favor of an excited singlet state has also been presented. 60 A variety of 1 1-disubstituted benzocyclopropenes has been synthesized by the 3H-indazole route, which is however limited. Cycloproparenes lacking substituents at C 1 are not accessible in this way, because the required indazoles occur in the 1H tautomeric form 73. An extension of this route has been developed by Dtirr et al. 61 It consists in the~ irradiation of 3H-spiropyrazoles 74 which rearrange to 3H-indazoles 75 prior to nitrogen extrusion and biradical cyclization to cycloproparenes 76. As above, only 1,1-disubstituted cycloproparenes (mainly with alkoxycarbonyl groups) are acces-

Chemistry of Cycloproparenes R2

1

R

49 R2

\

H

73

70

72

71

R

"

R

I ~lP.

.

R

-- coo. COOR

R

R

COOR

R~

c o.- oo.

COOR N

75

74

~

76

COOR

Ph 1

Ph

Ph 77

78

Scheme 15.

sible by this route, which afforded, among others, a 1H-cyclopropa[a]naphthalene derivative 7762 and a spirobenzocyclopropene 78. 63The application of the approach to cycloproparenes having phenyl substituents at C1 leads, however, to fluorene derivatives and not to cycloproparenes. 64-67

or ~ 79

~

~ 1

T ~ 81

0 80

0

0

Scheme 16.

0

50

PAUL MULLER

1

82

84

83

Scheme 17.

By analogy, benzocyclopropene (1) is formed in low yield via biradical cyclization upon irradiation of benzocyclobutenone (79), upon its flash vacuum pyrolysis,68 or upon pyrolysis of indan-2,3-dione (80). 69Reaction of phthalide 81 in a RF plasma leads also to 1.7o However, these latter extrusion methods are of no preparative interest. The l/3/elimination of o-substituted benzyl derivatives has been proposed as an alternative synthesis of cycloproparenes. Thus intmmolecular cyclization of obromo-ct-methoxytoluene (82) via lithiation to benzocyclopropene (1) has been claimed to proceed with a yield of about 30%. 71 However, subsequent attempts towards applications of the l/3/elimination approach were disappointing. Apparently, the metallated intermediates undergo dimerization more readily than reaction to the highly strained 1 or its derivatives. 72 Nevertheless, cycloproparenes are indeed formed, but the yields are usually low, about 5%. 73 A recent modification of the procedure using more appropriate leaving groups appears promising. The linear cyclobutabenzocyclopmpene (84, rocketene) was obtained in acceptable yield from the acyclic derivative 83. 74

2.2. Strained, Fused, and Short-Lived Cycloproparenes Once it was recognized that cycloproparenes were isoiable compounds, although highly strained, the synthesis of still more strained cycloproparenes, at the limits of isolability became an obvious synthetic challenge. These limits were tested by lmposing additional strain on the molecules via fusion of additional small rings, via annulation of the cycloproparenes with additional benzene rings, and via out-of-plane deformation of the molecules by bridging of the 2,5-positions of 1. The first synthesis of cyclobuta[3.4]benzocyclopropene (84) was realized in 1975 by the carbene addition route. 75 The angular isomer, cyclobuta[2,3]benzocyclopropene (85), was synthesized in low yield from 86, but exposure of 87 to the same reaction conditions afforded n o 8~. 76 The cycloaddition approach to 84 with 1,2-bromochlorocyclopropene (27) was realized only recently, and the sequence afforded, in addition, the higher homo-


51 CI

86

85

87

F

88

89

(CH2)n~.~/ 90-92

98

n=2-4 CI

~

CI 93

n-2-4

94

95-97

F F

n=2-4

Scheme 18.

logues 88 and 89, as well as their angular isomers. 77 The cycloaddition approach with 27 also produced dicyclobutabenzocyclopropene (90) and the higher dicyclopenta- and dicyclohexa-annulated analogues (91,92). 78The difluoro derivatives 95-97 are accessible in analogy by cycloaddition of 1,2-dichloro-3-3-difluorocyclopropene (94) to the appropriate dienes 93 and subsequent aromatization of the adducts. 79 However, in the series of difluorobenzocyclopropenes, even 98 was not isolable. 26 Apparently the fusion of a benzocyclopropene to two cyclobutanes represents the upper limit for isolable benzocyclopropenes. Several attempts to synthesize dicyclopropabenzenes failed. Thus, attempted aromatization of the carbene adducts of the isomeric cyclohexadienes, 99 and 101 afforded none of the expected 100 and 11)2, respectively, 8~ and attempted aromatization of 1t)3 resulted only in uncharacterizable decomposition products without any evidence for the intermediacy of 104.82 Alder-Rickert cleavage was also examined as a potential route for 1110and 1tl2, but the required precursors could not be synthesized. Bis-methano[ 14] annulene 105 added dicyanoacetylene in the wrong sense, and did not yield 11)6, which could have served as precursor of 1112.83 Similarly, the attempted Diels-Alder reaction of the bridged cyclopropa[10]annulene 107 with dimethoxycarbonylacetylene, which should furnish 1118,produced only decomposed material. 84 This is consistent with the observation of Halton and Russel that 1117does not add to 4-phenyl- 1,2,4-triazoline-3,5-dione. 85 In order to circumvent the low reactivity of 1117 in cycloaddi-

52

PAUL MOLLER H B r ~ ~ ' ~ ' ~ j Br

V

Br

Br~

F F

-"H B

"" SePh

9

99

101

103

100

102

104

COOMe

108

107

E

F F

CI

E=CN

106

COOMe

110

105

109

Scheme 19.

tions, its precursor 109 was treated with the acetylene. The adduct 110 decomposed in the presence of base, and no 108 could be obtained. In the series of o-fused cyclopropanaphthalenes there is a marked difference between the linear compound 42 (1H-cyclopropa[b]naphthalene) and the angular


S02Ph 111

53

S02Ph

cI

112

113

56

Scheme 20.

isomer 56 (1H-cyclopropa[a]naphthalene). As shown above, 42 is accessible by quite a variety of routes, including the carbene addition-aromatization sequence, but 56 may only be obtained via Alder-Rickert cleavage. The synthesis via 1/3/elimination has been attempted without success, 84'86 and the carbene additionaromatization route starting from 1,2-dihydronaphthalene afforded only ringopened products derived from vinylcarbenes (see S c h e m e 12). 46,48 Several schemes were attempted to avoid the formation of cyclopropene intermediates during generation of 56 but they all failed. For example U 1, which has the same functionality pattern as 66, reacts with base only with mono-elimination to 112, and aromatization may not be brought to completion. Compound $6 is, however,

Br Br"

Br

~

"Br

114

115

116

118

119

117

120

121

Scheme 21.

54

PAUL MOLLER

accessible via reduction of the dichloro derivative 113 with LiA1H4/A1CI3. This precursor may be synthesized in analogy to 67. 87 The linear cyclobuta[e]cyclopropa[b]naphthalene (114) was synthesized very early by the carbene addition approach, while the synthesis of the angular isomer failed. 47'88 The most strained of all isolable cycloproparenes, 1,4-dihydrocyclopropa[b,g]naphthalene (115), was obtained originally by the same approach from the tetrabromide 116. 89 The synthesis was recently improved. 9~ An alternative access to 115 via 117 as precursors is also available. 9l The strain energy of 116 is extremely high, and it explodes upon melting (132 ~ 89 In contrast, isomers of 115 have so far not been isolated. An early attempt to generate cyclopropa[a,e]naphthalene (118) failed. 47 More recently, the generation of dicyclopropa[a, c] naphthalene (119) was attempted by reaction of 120 with base. When the aromatization was carried out in the presence of DPIBF (44), stereoisomeric bis-adducts of cyclopropenes were isolated. However, the adducts provide no evidence for the formation of 119 as a reactive intermediate, since they are formed by sequential elimination--cycloaddition via 121. Cyclopropene interception of 121 is faster than further elimination to 119. The failure of the reaction to produce 119 has been attributed to the high strain energy of the product, which is estimated some 28 kcal/mol higher than that expected for two isolated cyclopropene units. 92 1H-3,8-Methanocyclopropa[10]annulene (107) is a very reactive compound. It may be synthesized on a large scale (50-100g) by the carbene addition route from 122 (X = H), but must be used immediately because of its tendency towards polymerization. The 9,9-difluoro-derivative 123 is available by the same approach. 93'94 Reaction of the bis-dichlorocarbene adduct 124 of isotetralin with base affords directly the anti-chloro derivative 125 by a mechanism probably involving x

122

x

X=H,F

x

CI

CI

x

107 X = H 123

CI CI

124

X-F

CI H

CI

CI

125

Scheme 22.

109

t 126


55

a l/4/elimination of an allylic hydron and the chloro substituent endo with respect of the hydron. 95 Access to 107 is also possible via cycloaddition of 27 to 1,6-dimethylenecyclohepta-2,4-diene (126) and aromatization of the adduct 109. 84 The first attempts to synthesize 1H-cyclopropa[b]anthracene (129), based on the carbene addition route, produced only ring-opened products. 14'15'47'88This failure was initially ascribed to some inherent destabilizing feature of 129. However, the successful preparation of the dihalogenated derivatives 130 and 131 and later of the parent 129 shows that this hypothesis does not hold. The cycloprop[b]anthracenes are accessible via the cycloaddition approach, starting from the annulated diene 127 via the adduct 128, which is first aromatized with DDQ and then subjected to base-induced elimination. 96,97 A shorter route to 129-131 was subsequently developed, based on the quinodimethane approach. The required onaphthoquinodimethane 133 was generated from 132 by base-induced isomerization and w~is trapped with tetrahalogenocycl0propenes, which provided direct access to 128. 98 The approach to the preparation of the next higher benzannelated homologue of 129, 1H-cyclopropa[b]tetracene, was also attempted, but trapping of the corresponding o-quinodimethane could not be realized. 31 Surprisingly, even dicyclopropaanthracenes are isolable compounds when prepared under appropriate reaction conditions. Twofold cycloaddition of 1,2-bromochlorocyclopropene (27) to tetraene 134 leads to 135 as a mixture of stereo- and

Y CI 128 X= H,F,CI Y = CI,Br

127

129-31

II

132

134

I

133

/

135

136

137

Scheme 23.

X= H,F,CI

56

PAUL MOLLER

regioisomers. Treatment with a base converts 135 to 137. 1H,5H-Dicyclopropa[b, i] anthracene (136) is obtained upon aromatization of 135 with DDQ and subsequent base-induced elimination. 9~ The synthesis of 1H-cyr (142) presented unexpected difficulties and met many failures. Early approaches used a variety of schemes which were not adequate for this highly reactive compound and invariably produced ring-opened products. Thus irradiation of the substituted indazole 138 resulted in nitrogen extrusion and formation of the biradical 139, which reacted with the solvent, benzene, to form 140. The desired cycloproparene 141 was not formed. 99 Ring contraction of 144, in turn, produced derivatives of 9-phenanthroic acid, the formation of which was shown not to involve phenanthrocyclopropenone (143). 1~176 The attempted l/3/elimination of 145 was similarly unsuccessful and afforded no 142. 86 The carbene route to 142 was investigated with the endo-monochloride 149 which is available by reduction of 146. The compound reacts with strong base to provide cyclopropene 150, which does not isomerize to 142. It may be intercepted with nucleophiles to yield substituted cyclopropanes. 1~ The dichloro analogue 146 reacts with strong base to give a chlorocyclopropene 151, which rearranges to a vinylcarbene 153.1~ If an appropriate leaving group (selenoxide) is placed at the bridgehead position as in 147, syn-elimination to 155 occurs, but the product is

Ph

N

H E

E

138

E = COOR

139

E

140

CH2OCH3 ~ur

E

141

E = COOR

142

E -- H

143

E,E = 0

~

Br

145

Scheme 24.

I,~

144

Br


X

57

c~ CI

146 X=H

150 X,Y = H

153 X = H

147 X= Se(O)Ph

151 X=CI, Y=H

154 X = OCH3

148 X=OCH 3

152 X=CI, Y=OCH 3

O

0

149

142 155

X = H

157

X = CI

COOH

156

[

~ -~~

O

158

Scheme 25.

unstable and ring-opens under the reaction conditions to 9-phenanthroic acid (156). 1~ The dichlorocarbene adduct 148 of 9-methoxyphenanthrene eliminates HC1 instead of MeOH and forms a cyclopropene 152. Ring-opening produces the substituted vinylcarbene 154. The latter inserts intramolecularly into the methoxy group and, after elimination of HC1 from 157, a phenanthrofuran 158 is obtained. The sequence is applicable to substituted furans and even to phenanthrocyclopentadienes. 1~ If the leaving group of the precursor to 142 is placed at the bridgehead position (CI a), the elimination may proceed in both the desired direction to 142 or to the l aH-isomeric 150. Elimination of the dimethylselenium 1~ or the dimethylsul-

58

PAUL MULLER

fonium salts 101 159 with strong base in the presence of furans affords the expected interception products 160-162. Isolation of the cyclopropenes is impossible under these reaction conditions. However, 1H-cyclopropa[/]phenanthrene (142) was finally synthesized by Alder-Rickert cleavage of 163, the synthesis of which required a painstaking effort. The cycloproparene 142 is a highly reactive compound, and it decomposes in solution at -60 ~176 The solid material decomposes over a period of days at -78 ~ No other synthesis of 142 has been realized so far, and it is clear that early approaches to it or to its derivatives failed because their high reactivity was underestimated. Other cyclopropaphenanthrenes are, however, isolable. 3~Recently, Billups et al. described the synthesis of dicyclopropa[b,h]phenanthrene (165) and of its dihydro derivative 166 starting from tetramethylidenecyclohexane 164 by the cycloaddition r o u t e . 91

XMe2

163

,,2,//

159

=

,Se

l

150

o

0 162 142

161

164

165

Scheme 26.

0

166

Chemistryof Cycloproparenes

59

(c~r~Br

X

~

167 n=5,6

( C ~ 168 n=5,6

Scheme 27.

Bridging of the 2,5- or 2,4-positions of a cycloproparene with an alkyl chain would introduce additional strain by out-of-plane deformation. An attempt to synthesize 2,5-bridged cyclopropacyclophanes 168 from 167 has been launched by Garratt and Tsotinis. 1~ Unfortunately, treatment of 167 with t-BuOK in THF or DMSO resulted only in intractable material.

2.3. Heterocyclic and Functionalized Cycloproparenes

Heterocyclic Cycloproparenes Only one heterocyclic cycloproparene having the heteroatom in the ring adjacent to the cyclopropene has been isolated, namely the 1,1-dimethylcyclopropa[c]pyridine derivative 170 which is available in moderate yield upon photochemical extrusion of N 2 from the 3H-indazole 169.1~ In contrast, irradiation of 171 produces no cycloproparene 173, but other products derived from the intermediate biradical 172. Apparently, the radical centers in 172 are too far apart to allow ring-closure. 1~ Two other N-heterocyclic cycloproparenes having the heteroatom further elongated from the cyclopropene moiety have been synthesized. Cyclopropa[g]quinoline (176) was prepared from 174 via interception of the heterocyclic

,,ooN.\

P,ooN.-

-

169

170

9

171

CI

;,.N ~

172

Scheme 28.

N

,~.

~N~'~.~

173

60

PAUL MOLLER 4. NMe3

174

SiMe3

175

176

[ N~x,,

177

c,

I N

178

179

Scheme 29.

o-quinodimethane 175 with bromochlorocyclopropene (27) and subsequent aromatization. 37 The substituted 2,7-dimethyl-lH-cyclopropa[g]isoquinoline 179, in turn, was accessible in low yields from the adduct of the azaisobenzofuran 177 to 27. Aromatization of the adduct 178 with low-valent titanium afforded 179. However, the parent compound, lacking the methyl substituents, could not be obtained by this route. 39'110 The synthesis of cycloproparenes resulting from formal fusion of a cyclopropene to furan and thiophene has been attempted with limited success. Reaction of the dichloro-oxabicyclohexane 180 (X - O) 111 afforded a cyclopropene 181 which ring-opened to a vinylcarbene 182, but the cycloproparene 183 was not produced. Similarly, the thia-analogue 180 (X = S) could not be converted to 184. The intermediate cyclopropenes and/or vinylcarbenes have been trapped. 112'113 A cyclopropathiophene derivative 186 was generated, however, from 185. Although it was not isolable, it afforded a bis-adduct 187 when it was produced in the presence of isobenzofuran (45). ll4 Cycloproparenes in which the benzene ring is fused to a five-membered heterocyclic moiety are accessible by the cycloaddition route from the appropriate dienes 188. Both the furan and thiophene derivatives 189 and 190 are isolable, but both are highly reactive. 115The difluoro derivative 191, prepared by an analogous route, has also been synthesized but decomposed rapidly in solution, while 192 was not isolable. 116 Bridging the o-positions of benzene with heteroatoms does not lead to isolable species, but to transients which may be evidenced either by isotopic labeling or by trapping at low temperature. Flash vacuum pyrolysis of isatin 193 affords 1-cyano-


61

CI

CI

CI

X=O,S

181

182

CI .,=l---X---

183 184

X =O,S

180

X=S

Br,~Br 185

186

187

xCC:

188 X=O,S

189 X = O

191

X =O

190 X = S

192

X=S

Scheme 30. H

~

. NH O

-------ii.

193 0

L

x

198 X = S,Se

199

I

195

194

~

I ~ : NH

x

~NH 196

~x CN

X = S,Se

Scheme 31.

200 X = S, Se

197

62

PAUL MULLER

cyclopentadiene (197), via 194-196. The existence of a symmetrical intermediate 195 was demonstrated by isotopic labeling. 117 No evidence for azabenzocyclopropene was obtained upon photochemical decomposition of 1-alkylbenzotriazoles, lIB Photochemical reaction of thia- and selenaindazoles 198 provides the thiaand selenobenzocyclopropenes 199 which may be trapped by matrix isolation. 119 Upon further irradiation, ring contraction to 200 occurs. The thermal decomposition of benzothiadiazoles does not necessarily lead to thiacycloproparenes, however, and the substitution pattern as well as the reaction conditions are also important. 12~

Functionalized Cycloproparenes Most of the synthetic effort in the cycloproparene field has been directed towards the parent compounds and no systematic study of functionalized cycloproparenes has been undertaken. The syntheses discussed in this chapter refer to approaches in which functionality was introduced into a cycloproparene precursor along the synthetic pathway. Functionalization by substitution reactions of cycloproparenes will be discussed in a subsequent chapter. A large number halogenated cycloproparenes is known, in particular the 1,1-dihalogeno derivatives, which are available by the cycloaddition approach mentioned above. The 2- and 3-halogenobenzocyclopropenes 202121 and 204122-124 are conveniently synthesized by the carbene addition route from the appropriate precursors 201 and 203, respectively. Only a few benzocyclopropenes with oxygen substituents have been prepared: 2-methoxybenzocyclopropene (205) was synthesized via the cycloaddition of 1-methoxybutadiene to 27,125 and 3,6-dimethoxycyclopropa[b]naphthalene (207) (R = H) from 206. The dimethoxynaphthalene 207 was oxidized to the quinone 208 with ceric ammonium nitrate. 126 The synthesis of the 1,1-diphenyl derivative of 207 (R = Ph) was attempted long ago via the 3H-indazole route, but without success since the (hypothetical) cycloproparene rearranged to the corresponding phenylfluorene. 64 While 208 is shelf-stable, the parent benzocyclopropene-p-quinone (210) is capable only of existence as a transient. It may be generated by Alder-Rickert cleavage of 209 under flash-vacuum pyrolysis conditions, and characterized as the anthracene adduct 211. ~27 Several derivatives of 1H-cycloprop[f]indene have been synthesized. The difluoro compounds 212 (X - O,S) were obtained by the cycloaddition route from 188 in analogy to the synthesis of 191 and 192. The cycloaddition of diene 213 to 1,2-bromochlorocyclopropene (27) provided 214, which was aromatized to 217. Twofold Curtius degradation of 214 afforded the ketone 215. While attempted aromatization of 215 with base afforded no cycloproparene, the corresponding alcohol 216 reacted with base to 218 in acceptable yield. The alcohol 218 is also accessible from 219. ll6 Benzocyclopropenone (219) or, more generally, oxocycloproparenes have so far not been isolated under synthetic conditions. Although 1,1-dihalogenocyclo-


x

63

x X,

X

201

202 X = CI,Br

203

204 X = CI,Br

205 X= OMe OMe

OMe

OMe

OMe

206

207 0

o

E

0

209

O

O 208

R = H,Ph

210

211

Scheme 32.

BF

EtOOC r

EtOOC 212 X=O,S

H ~ * ~ ~ ~ ~

HO- ~..--'~.,,~ 219

213

X = (COOEt)2 X =O

216

X = H,OH

EtOOC~ / ~ ~ " ~

CI _.._..1~" H

-cl

214 215

HO~~~~

>

218

Scheme 33.

EtOOC 217

64

PAUL MCiLLER O /COOCH 3 ~

X = H,CI Y = H,NTs"

218

o

~

X= H,CI

I

221

O

o

HF2 222

220

219 X = H,CI

C

0

223

224

225 R = CF 3, CHF2

Scheme 34.

proparenes should be obvious precursors to oxocycloproparenes, they react with ring-opening to carboxylic acid derivatives (see below). The failure to synthesize phenanthrocyclopropenone (143) by a ring contraction approach has been mentioned previously (see Scheme 24). 1~176 Indirect evidence for the intermediacy of benzocyclopropenone (219) was derived from the photochemical decomposition of 218 (X = H, Y = NTs-) which, in the presence of MeOH, led ultimately to methyl benzoate (220). Experiments with 3-substituted 218 (X = CI, Y = NTs-) afforded 2- and 3-substituted 220 (X = CI) and 221 and established the intervention of the ketone intermediate 219 (X = CI). 128 Evidence for 219 as a reactive intermediate was also obtained from lead tetraacetate oxidation Of substituted 218 (X = C1, Y = H) which, in the presence of moisture, afforded a mixture of carboxylic acids corresponding to 220 (X = CI) and 221.129 Benzocyclopropenone (219), generated by lead tetraacetate oxidation of 218 (X = H, Y = H), was trapped and characterized as adduct 222 with pentafluoroacetone. Similarly, the heterocyclic oxocycloproparene 224, which forms upon gas-phase thermolysis of 23, was trapped in the presence of penta- or hexafluoroacetone as 225.13~ When 3-diazobenzofuranone (226) is irradiated in an argon matrix at 8 K, a ketene 227 results as the primary product. This loses CO, and the resulting carbene 228 ring contracts to a species which exhibits an infrared (IR) frequency at 1838 cm -1, which was tentatively assigned to intermediate 219. Upon further irradiation, benzyne (3) is formed. TM The benzocyclopropenone 219 is also generated upon irradiation of benzocyclobutanedione (229) in an Ar matrix at 8 K. 132An analogous sequence occurs upon irradiation of 230 at 16 K, and leads initially to cyclopropaacenaphthenone 231, which decarbonylates to acenaphthyne (232). 133


65 /~0

N2

227

226

228

0

229

219 0

230

231

232

Scheme 35.

2.4.

Alkylidenecycloproparenes

The discovery of alkylidenecycloproparenes as a class of isolable compounds is the most significant event in recent cycloproparene chemistry. Methylidenecyclopropabenzene (234) combines the structural features of benzocyclopropene (1), methylenecyclopropene (233), and trimethylenecyclopropane (235) in one and the same structure. Although the parent 234 has so far not been isolated, many substituted and annulated derivatives have been synthesized and characterized. Their unexpected stability contrasts sharply with that of the benzocyclopropenones 219, which are only observable as transient reaction intermediates. Reviews on the chemistry of alkylidenecycloproparenes are available. 134A35 Only two synthetic approaches to alkylidenecycloproparenes are known. The more general one uses the Peterson olefination of 1" benzocyclopropene is silylated by sequential addition of n-butyllithium 136 and trimethylchlorosilane. A second metallation of 236 followed by reaction with benzophenone affords the alkylidenecyclopropabenzene 237.137 A slightly modified procedure is used for the synthesis of alkylidenecyclopropa[b]naphthalenes (240, 241). It consists of desilylation of 239 with t-BuOK in the presence of a non-enolisable ketone. Alkylidenecyclopropabenzene and naphthalene derivatives 238 and 241, having only one aromatic substituent and one hydrogen at the exocyclic double bond, have also been isolated. The method has been adapted to compounds carrying nitro groups

66

PAUL MULLER

233

235

234

Scheme 36.

on the phenyl substituents by effecting desilylation of 239 with KF and a catalytic amount of tetra-n-butylammonium fluoride. 138 Application of the original sequence to cyclic ketones, such as fluorenone, benzotropone, and dibenzotropone afforded calicene 242 and the triaheptafulvalenes 243, 244.139 Surprisingly, reaction of 1,1-dichloro-2,5-diphenylbenzocyclopropene (22) with n-butyllithium affords the dimeric triafulvene 245. Exposure of the unsymmetrical benzocyclopropene 246 to the same reaction conditions leads by analogy to a 1" 1 mixture of stereoisomeric triafulvenes 247 and 248.140

236

237

R = Ph

238

R= H

SiMe3 SiMe~

R

239

240

R = Ph

241

R= H

1

i

R

R

242

R=

H,benzo

243

R = H,benzo

Scheme 37.

244

R = H,benzo

Chemistry of Cycloproparenes Ph

Ph

@~ I

67 Ph

Ph

R

CN

CN

cl

R

R

22

R .. Ph

245

R --- Ph

246

R=CN

247

R=CN

Ph 248

Scheme 38.

~N NC 249

234

250

Scheme 39.

Other approaches to alkylidenecycloproparenes have been attempted without success. Aromatization of appropriate alkylidenecyclopropanes 137 or their precursors TM could not be realized, and flash vacuum pyrolysis of methylene phthalide and 3-methylene-2-coumaranone afforded rearrangement products rather than alkylidenecycloproparenes via extrusion of CO2 .142 The photochemical 143 or thermal144 decomposition of the sodium salt of benzocyclobutenone p-toluenesulfonyl hydrazone led to products derived from dimerization of the intermediate benzocyclobutenylidene, or from its reaction with the solvent, but no ring-contracted products were observed. When the adduct of methylene- 1,6-methano[ 10]annulene to dicyanoacetylene (249) was subjected to Alder-Rickert cleavage, phenylacetylene (250) was formed, which derives reasonably from the parent 234.145

3. PHYSICAL PROPERTIES 3.1. Structure and Strain The consequences of the distortion of the aromatic benzene ring by fusion to a cyclopropene, as it occurs in cycloproparenes, has been the subject of much discussion and speculation. Much of this debate concerned the question of bond fixation in strained aromatics which has a long historical background. In 1930 Mills and Nixon 146 observed different reactivities towards electrophilic substitution of the o~ and 13 positions in tetralin (251) and indane (252). This observation was

68

PAUL MOLLER

251

'"

252

253

1.472

2

1000 ~ the label in 367 is scrambled over all positions. Pyi'olysis of terminally labelled 367 results also in scrambling of the label. This indicates, that the high-temperature rearrangement is reversible. 235 An analogous skeletal scrambling process occurs in 1 upon ionization by electron impact in mass spectroscopy. 236 Pyrolysis of cyclopropa[b]naphthalene (42) affords 2-vinylidene(2H)-indene (368) 237 and, at higher temperature, 2-ethynylindene (369) 238 by an analogous mechanism. Similar ring contractions have been reported in photochemical rearrangements of substituted benzocyclopropenes. 239

Ring-Expansions When 1-alkoxy- or 1,1-dialkoxycarbonylbenzocyclopropenes are photolyzed or heated above their melting points, furans may be formed via intramolecular interception of the intermediate carbenes. 59'61'239For example, 1,1-dimethoxycarbonyl-2,5-diphenylbenzocyclopropene (263) rearranges at 170-210 ~ to the furan 370. Similar reactions occur with 1,1-diphenyl-substituted benzocyclopropenes, which react to give fluorenes. 28'192

1

366

367

42

368

369

Scheme 65.

94

PAUL MULLER

Ph [

Ph

COOCH3

_COOCH3

OCH3

Ph

Ph

263

370

Scheme 66.

Ring expansion of cycloproparenes to cycloheptatrienes or tropones has been discussed in the context of electrophilic addition to cycloproparenes. When 1,1-dichloro-2,5-diphenylbenzocyclopropene (22) is thermolyzed in refluxing benzene, the dimer 373 is formed as a mixture of E/Z-isomers. It is believed to arise via dimerization of the carbene 372, which, in turn results from an allylic rearrangement of 22 to 371. 240 Alternatively, the biradicals generated upon thermolysis of cycloproparenes may rearrange to styrenes via intramolecular hydrogen abstraction, 59'192as exemplified in the rearrangement of 280 to 374. 61"63

Ph

Ph

C{~c '

! c,

I

Ph 22

Ph

~ C I CI

PhC I I ~

CI Ph

Ph

Ph 372 CI

CI

CI Ph

E- 373

Z - 373

Ph Et Et

280

.~c,

Ph 371 ~ ,

Ph

Ph

Ph 374

Scheme 67.

CI


95

4.4. Reactionswith Transition Metal Reagents Transition metal reagents react with cycloproparenes either via attack at the bridging re-bond, or by reaction with a cyclopropane o-bond. The latter pathway is predominant. Diiron pentacarbonyl affords 375 upon exposure to cyclopropa[b]naphthalene (42). 51 When benzocyclopropene (1) or 42 are exposed to Cr(CO)6, benzocyclobutenone (79), and naphthocyclobutenone (376), respectively, are obtained. TM Benzocyclopropene (1) reacts with Ni(0) complexes via oxidative addition to afford nickelacyclobutabenzenes (377). 242 Analogous products (378) form with 1,1-trimethylsilylbenzocyclopropene (379). 243The course of the reaction of 1 depends markedly upon the ligand of the nickel. With (Et3P)2Ni(COD), (Bu3P)2Ni(COD), (Ph3P)4Ni, and (C2H4)3Ni, the nickelacyclobutabenzene (377) is formed. However, the trimethylphosphane complex (Me3P)2Ni(COD) adds to two molecules of 1 to afford 380 which may react further to 381 via CO-insertion, to the dimeric or tetrameric cycloheptatrienes 382, 383 upon reaction with acid, and

0 __.

~

375

X 377 378

x

X=H,L=R3P X=SiMe3, L=R3P

1 R=H,X=H '"~42 R=benzo, X = H 79 R=H,X=SiMe 3

L/

,,,

o

79 R=H 376 R=benzo

,,

380 L=Me3P

382

Scheme 68.

381

96

PAUL MOLLER

F

F F

iL~ 385

L = Me3P

21

X = F

F ~

3i9

X = SiMe3

386 L = Me3P

L/Pd

~ - SiMe3 SiMea

F~Pd2.l

~~---Pd~

387 L=Me3P

388 L=Me3P

389 L=Me3P

Scheme 69.

to the cyclic tetramer 384 in the presence of (CH3)3P.244 An analogous reaction of (MeaP)2Ni(COD) occurs with 84.163 1,1-Difluorobenzocyclopropene (21) reacts with Ni(0) complexes across the central r~-bond to give bicyclobutanes 385, with concomitant loss of ligands from the metal. With tris(ethene)Ni, and in the presence of tetraethylene diamine, one ethylene unit is inserted and the complex 386 results. 245 Reactions of cycloproparenes with (rl3-allyl)(rlS-cyclopentadienyl)palla dium(III) in the presence of trimethylphosphane affords a bicyclobutane (387) with

R*~~J~~X~

SiMea_......~"

R

-SiMea

~

239 R = H 393 R=benzo

I

Cr(CO)3 390 X= Si Me3 391 X = H

I

Cr(CO)3 394 395

X = Si Me 3 X = H

S c h e m e 70.

I

Cr(CO)a 392

e


97

1,1-difluorobenzocyclopropene (21), and a metallacyclobutane 388 with 1,1bis(trimethylsilyl)benZocyclopropene (379). With 1 the intermediate metallacyclobutane suffers intramolecular hydrogen migration to 389. 246 The trimethylsilyl group offers sufficient protection of the adjacent benzene ring of cycloproparenes and no reaction occurs between 1,1-bis(trimethylsilyl)benzocyclopropene (379) and Cr(CO) 6. With the silylated naphthalene derivative (239), complexation with tricarbonyltris(acetonitrile)chromium occurs at the terminal benzene ring and affords the crystalline complex 390, TM which may be desilylated to 391 by reaction with t-BuOK. 247 The complex 391 is deprotonated at the cyclopropane ring with BuLi, and the resulting anion may be intercepted with MeI to afford 392 as a mixture ofcis- and trans-isomers. 248 Complexation of the terminal benzene ring of 1H-cyclopropa[b]anthracene has been realized in remarkably high yield by analogy from 393 to 394 and 395. The X-ray structures of the Cr(CO) 3 complexes exhibit no significant geometrical variations in the uncomplexed moieties in comparison to those of the parent compounds.

4.5. Reactionsof AIkylidenecycloproparenes Almost all reactions of alkylidenecycloproparenes lead to opening of the cyclopropane ring. A notable exception to this is the reversible electrochemical reduction of 237 and 240 which leads to the stable radical anions 396 and 397, with half-wave potentials of-2.32 and -1.93 V, respectively, and their oxidation to the quasi-stable radical cations 398 and 399 (E1/2(ox) = +0.68 and +0.81).249 The cations may be further oxidized to the corresponding very short-lived dications. In contrast, the photoelectron spectra of 237 and 240 reveal practically identical first-oxidation potentials of both compounds, which indicates that the difference in half-wave potentials for oxidation (in condensed phase) of 237 and 240 does not exist in the gas phase. This has been attributed to structurespecific solvation energies in the radical Cations 398 and 399. 250 The protonation of alkylidenecycloproparenes to give stable benzocyclopropenyl cations (290) has been mentioned in a previous section. Quenching of the cations with H20 affords the expected ethanone derivatives 400. They originate from nucleophilic attack at C 1. Other electrophilic reagents afford styrene or phenylacetylene derivatives. With Br 2 a variety of products may occur, depending upon the reaction conditions, for example dibromobenzocyclobutenes 401 or benzocyR

~

R"~ ~ 396 397

Ph

R

Ph

R

R=H R =benzo

~

Ph Ph

398 R=H 399 R = benzo Scheme 71.

98

PAUL MOLLER 0

CHPh2

R

Ph 237

240

~ R , ~ R

R"

R= H

~

290

R' ' ' ~ ' / R = H, benzo

400

R = benzo

Br

il Br Ph

Ph 401 R = H, benzo

R

0

R

Ph

402

t / / ~

Br

Ph

R = H, benzo

403

Br

/p h

Ph

Scheme 72.

clobutenones (402). An alkylidenecycloheptatriene 403, resulting from attack at the bridge bond, has also been isolated upon reaction of 237 with Br2.TM Alkylidenecycloproparenes react with peracids at the exocyclic double bond and yield hydroxyketones. Photooxygenation of 240, in turn, leads initially to a dioxetane 404, by reaction with the exocyclic double bond. The product composition resulting from breakdown of 404 is consistent with napthocyclopropenone (405) as a reaction intermediate. 252 The photooxygenation of 237 is different, however, and affords the spiro compound 406 in low yield. 253 While electrophilic reactions of alkylidenecycloproparenes are generally rapid, the nucleophilic pathways require more vigorous conditions. The only nucleophilic addition of cycloproparenes known is that of t-butoxide across the bridge bond of 237, 240 and 241 which leads to the corresponding heptafulvenes 407. TM

237 240

R= H R=benzo

$0

404

p~

~

-Ph

405

OtBu

407

R = H, benzo, X = H, Ph

Scheme 73.


99

Ph

~ ~ ~ p h

0 Ph 0

408

Ph

R

Ph 237 R = H 340 R = benzo

HO Ph

Ph

Ph

412

411

Scheme 74.

In principle, alkylidenecycloproparenes are susceptible to cycloaddition either at the exocyclic, or at the bridging double bonds. The naphthalene derivative 240 reacts with a variety of dienes. With DPIBF (44) attack occurs initially in a [2+4] fashion and affords the ring-expanded product 408. The same mode of attack prevails with t~-pyrone and leads to 409. With bisacetylene(phenyl)iodonium triflate a formal [2+2] cycloaddition to the exocyclic double bond takes place, and results in formation of 410, which breaks down to 411. 254 In contrast, 237 is much less prone to undergo cycloadditions. It does not react with ct-pyrone, but with DPIBF a 1:1 endo-adduct 412, resulting from cycloaddition to the bridging re-bond, is formed. Its structure was established by X-ray crystallography. Model calculations (PM3) reveal that the addition to the exocyclic double bond of 240 is significantly more exothermic than cycloaddition to the bridging bond, and that addition to the bridge bond of 237 is more exothermic than that of 240. 255 Alkylidenecycloproparenes form stable complexes with rhodium(I) and platinum(II) reagents. Reaction of 237 or 240 with chlorotris(trisphenylphosphine)rhodium(I) gives 2-alkylidene-l-rhodacyclobutarenes (413, 414) as air-stable crystalline solids in high yields. Exposure of 240 to trans-chlorocarbonylbis(triphenylphosphine)rhodium(I) results in oxidative addition and carbonyl insertion to 415 as the main product. When 414 is exposed to CO, however, 416, a regioisomer of 415, is formed. The partitioning between 414 and 415 is temperature-dependent. Tetrakis(triphenylphosphine)platinum(II), in turn, affords the corresponding platinabutarene 417. 256

1 O0

PAUL MOLLER Ph

Ph

Ph L

,,9

'

237 240

~.

R" R=H R = benzo

~

413 414

,iX '-

CI

R=H,L=PPh 3 R = benzo, L = PPh 3

Ph

~-c,

o

415 L = PPh,., o

Ph Ph

R

L 417

L = PPh 3

CI 416

R = H, benzo,L = PPh 3

Scheme 75.

ACKNOWLEDGMENTS It is a pleasure to acknowledge the contributions of my collaborators who shared with me the excitement and the inevitable frustrations of cycloproparene chemistry over the past years: H.-C. Godoy-Nguyen-Thi, Y. Jacquier, Z. Miao, J. Pfyffer, M. Rey, D. Rodriguez, J.-P. Schaller, and E. Wentrup-Byrne. We gratefully acknowledge financial support of our work by the Swiss National Science Foundation. I wish to thank my colleagues and friendly competitors E. Billups and B. Halton for stimulating discussions. Thanks are due, in addition, to H. Dtirr and E. Vogel, for communication of unpublished material, and to C. Gr~nicher for the preparation of the schemes of this article.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. !1.

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204. Garratt, P. J. Aromaticity; J. Wiley, New York, 1986; Loydk, D. Non-Benzenoid Conjugated Carboxcyclic Compounds; Elseveir, Amsterdam, 1984. 205. MUller, P.; Mtillen, K., unpublished. 206. Minsek, D. W.; Zhang, X.; Logan, C. E; Chen, P. 207thACS National Meeting, San Diego, 1994. 207. Neidlein, R.; Kohl, M. Helv. Chim. Acta 1990, 73, 1497; Neidlein, R.; Constaninescu, T.; Kohl, M. Phosphorous, Sulfur, and Silicon 1991, 59, 165. 208. Neidlein, R.; Christen, D. Helv. Chim. Acta 1986, 69, 1623. 209. Okazaki, R.; O-oka, M.; Tokitoh, N.; Shishido, Y. lnamoto, N.Angew. Chem. Int. Ed. Engl, 1981, 20, 799; Okazaki, R.; O-oka, M.; Tokitoh, N.; Shishido, Y.; Hasegawa, T.; Inamoto, N. Phosphorous and Sulfur 1983, 16, 161. 210. Tokitoh, N.; Okazaki, R.J. Am. Chem. Soc. 1987, 109, 1856; Okazaki, R.; O-oka, M.; Akiyama, T.; Inamoto, N.; Niwa, J.; Kato, S. J. Am. Chem. Soc. 1987, 109, 5413; Okazaki, R.; Hasegawa, T.; Shishido, Y. J. Am. Chem. Soc. 1984, 106, 5271. 211. Vogel, E.; Sombroek, J.; Wagemann, Angew. Chem. Int. Ed. Engl. 1975, 14, 564; Vogel, E. Isr. J. Chem. 1980, 20, 215. 212. Sch6pel, D. Ph.D. Thesis, University of Cologne, 1970. 213. H0gel, H. M.; Kelly, D. P.; Browne, A. R.; Halton, B.; Milsom, P. J.; Woolhouse, A. D.J. Chem. Soc. Perkhl Trans. 1 1977, 2340. 214. Vogel, E.; Ippen, J.; Buch, B. Angew. Chem. Int. Ed. 1975, 14, 566; Ippen, J. Ph.D. Thesis, University of Cologne, 1972. 215. Nitta, M.; Sogo, S.; Nakayama, T. Chem. Lett. 1979, 1431. 216. Neidlein, R.; Kohl, M.; Kramer, W. Heiv. Chin. Acta 1989, 72, 1311. 217. Brinker, U. H.; W0ster, H. Tetrahedron Lett. 1991, 32, 593; Brinker, U. H.; Wtister, H.; Maas, G. Angew. Chem. Int. Ed. EngL 1987, 26, 577. 218. Saito, K.; Ishihara, H.; Kagabu, S. BulL Chem. Soc. Jpn. 1987, 60, 4141. 219. Halton, B.; Russell, S. Aust. J. Chem. 1990, 43, 2099. 220. Madox, M. L. Martin, J. C.; Muchowski, J. M. Tetrahedron Lett. 1980, 21, 7. 221. Neidlein, R.; Tadesse, Heiv. Chim. Acta 1988, 71, 249. 222. Kato, H.; Toda, S. J. Chem. Soc., Chem. Commun. 1982, 510; Kato, H., Toda, S.; Arikawa, Y.; Masuzawa, M.; Hashimoto, M.; Ikoma, K.; Wang, S. Z.; Miyasaka, A.J. Chem. Soc. Perkin Trans. 1 1990, 2035; Kato, H.; Arikawa, Y.; Hashimoto, M.; Masuzawa, M. J. Chem. Soc., Chem. Commun. 1982, 938. 223. Bee, L. K.; Garratt, P. J.; Mansuri, M. M. J. Am. Chem. Soc. 1980, 102, 7076. 224. Shirafuji, T.; Nozaki, H. Tetrahedron 1973, 29, 77. 225. Billups, E. W.; Chow, W. Y.; Smith, C. V. J. Am. Chem. Soc. 1973, 96, 1979. 226. Neidlein, R.; Kr~irner, B. Chem. Bet. 1991, 124, 353. 227. Saito, K.; Ito, K. Oppi Briefs 1991, 23, 196. 228. Kagabu, S.; Saito, K. Tetrahedron Lett. 1988, 29, 675. 229. Durucasu, I.; Sarcoglu, N.; Balci, M. Tetrahedron Lett. 1991, 32, 7097. 230. Ippen, J. Diploma Thesis, University of Cologne, 1972. 231. Kagabu, S.; Saito, K.; Watanabe, H.; Takahashi, K.; Wada, K. J. Chem. Soc. Jpn. 1991, 64, 106. 232. MUller, P.; Godoy-Nguyen Thi, H. C.; Gr~icher C. Pfyffer, J., unpublished. 233. Neidlein, R.; Kr'amer, B.; Krieger, C. Z. Naturforsch. 1990, 45b, 1577. 234. Halton, B.; Randall, C. J. Am. Chem. Soc. 1983, 105, 6310; Apeloig, Y.; Arad, D.; Halton, B.; Randall, C. J. J. Am. Chem. Soc. 1986, 108, 4932. 235. Wentrup, C.; Miiller, P. Tetrahedron Lett. 1973, 2915; Wentrup, C.; Wentrup-Byrne, E.; Mtiller, P. J. Chem. Soc., Chem. Commun. 1977, 210. 236. Wentrup-Byrne, E.; Gtilaqar, E O.; MUller, P. Organic Mass Spectrometry, 1977, 12, 636; Singy, G.; Pfyffer, J.; MUller, P.; Buchs, A. Organic Mass Spectrometry 1976, 11,409. 237. Schulz, R.; Schweig, A.; Wentrup, C.; Winter, H.-W. Angew. Chem. bit. Ed. Engl. 1980, 19, 821. 238. Wentrup, C.; Wentrup-Byrne. E.; MUller, P.; Becker, J. Tetrahedron Lett. 1979, 4249.


107

239. Dtirr, H.; Ahr, H.-J. Tetrahedron Lett. 1977, 1991. 240. Fahey, J. A.; HUgel, H. H.; Kelly, P. D.; Halton, B.; Williams, G. J. B. J. Org. Chem. 1980, 45, 2862; Robinson, W. T.; Ditzel, E. J.; Hiigel, M. M.; Kelly, D. P.; Halton, B. J. Org. Chem. 1981, 46, 5003. 241. MUller, P.; Bernardinelli, G.; Jacquier, Y. Helv. Chim. Acta 1988, 71, 1328. 242. Neidlein, R.; Rufinska, A. Schwager, H.; Wilke, G. Angew. Chem. Int. Ed. EngL 1986, 25, 640. 243. K~ger, C.; Laakman, K.; Schroth, G.; Schwager, H.; Wilke, G. Chem. Ber. 1987, 120, 471. 244. Mynott, R.; Neidlein, R.; Schwager, H.; Wilke, G. Angew. Chem. Int. Ed. Engl. 1986, 25, 367. 245. Schwager, H.; Kriiger, C.; Neidlein, R.; Wilke, G. Angew. Chem. Int. Ed. Engl. 1987, 26, 65. 246. Schwager, H.; Benn, R.; Wilke, G. Angew. Chem. Int. Ed. 1987, 26, 67. 247. Mtiller, P.; Bernardinelli, G.; Ricca, A.Helv. Chim. Acta 1989, 72, 1618. 248. MUller, P.; Bernardinelli, J.; Jacquier, Y. Helv. Chim. Acta 1992, 75, 1995. 249. Ashley, K.; Foley, J. K.; Mei, Q.; Ghoroghchian, J.; Sarfarazi, F.; Cassidy, J.; Halton, B.; Stang, P. J.; Pons, S. J. Org. Chem. 1986, 51, 2089; Ashley, K.; Sarfarazi, E; Buckland, S. J.; Foley, J. K.; Mei, Q.; Halton, B.; Stang, P. J.; Pons, S. Can. J. Chem. 1987, 65, 2062. 250. Koenig, T.; Curtiss, T.; Winter, R.; Asley, K.; Mei, Q.; Stang, P. J.; Pons, S.; Buckland, S. J.; Halton, B.; Rolison, D. J. Org. Chem. 1988, 53, 3735. 251. Buckland, S.J.; Halton, B.; Mei, Q.; Stang, P. J.Aust. J. Chem. 1987, 40, 1375. 252. Buckland, S.; Halton, B.; Stang, P. J.Aust. J. Chem. 1988, 41,845. 253. Gainsford, G. J.; Buckland, S. J.; Halton, B. Acta Cryst. 1990, C46, 2226. 254. McNichols, A. T.; Stang, P. J.; Halton, B.; Kay, A. J. Tetrahedron Lett. 1993, 34, 3131. 255. Halton, B.; Kay, A. J.; McNichols, A. T.; Stang, P. J.; Apeloig, Y.; Maulitz, A. H.; Boese, R.; Haumann, T. Tetrahedron Lett. 1993, 34, 6151. 256. Stang, P. J.; Song, L.; Lu, Q.; Halton, B. Organometallics 1990, 9, 2149.


PLANAR DEHYDRO[8]ANNULENES AND OTHER THEORETICALLY INTERESTING MOLECULES

Henry N. C. Wong

1. 2. 3. 4. 5. 6. 7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Strained Cycloalkynes . . . . . ......................... Benzenoid Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transition Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . Stable Derivatives of Alkyne 27 . . . . . . . . . . . . . . . . . . . . . . . . . Polysubstituted Furans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . 109

110 118 130 133 135

141 141 142

1. INTRODUCTION This c h a p t e r is a r e v i e w o f m y r e s e a r c h e n d e a v o r s in planar d e h y d r o [ 8 ] a n n u l e n e s , w h i c h s u b s e q u e n t l y have b e e n b r o a d e n e d into 1,4-endoxide d e o x y g e n a t i o n mediated b y l o w - v a l e n t titanium, as w e l l as into p o l y s u b s t i t u t e d furans. This r e v i e w by

Advances in Theoretically Interesting Molecules Volume 3, pages 109-146. Copyright 9 1995 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 1-55938-698-3 109

110

HENRY N. C. WONG

no means attempts to cover all the chemistry that has been generated on these subjects. Instead, I have tried to present the material in a readable and also possibly pedagogical way intended for young graduate students. For more detailed information, interested readers are referred to my four published reviews on similar subjects. 1-4

2. STRAINED CYCLOALKYNES My research career began in 1973 under the supervision of Franz Sondheimer at University College London. Franz gave me my first project in his usual way by saying, "Henry, if I were you, I would try to synthesize the bis-allene 3 from dibenzo[a,e]cyclooctene 15,6 via the classical rearrangement of the cyclopropyl carbene 7 generated from the dibromocyclopropane 2" (Scheme 1). I soon learned from other postdoctoral fellows in the laboratory that this same project had been given to almost every new graduate student in Franz's group, and that none of my predecessors had achieved any positive result. As a new graduate student, I did not dare to say no and thus set forth to prepare several grams of 1. 5,6 With a sufficient quantity of 1 in hand, I then treated it with dibromocarbene generated under a variety of conditions. After extensive efforts, it became clear to me that the double bonds of 1 are rather electron-poor and as a result refused to react with electrophilic carbenes. 8-1~ That was the end of my first project in London. However, I must mention that the mono-dibromocyclopropanation of 1 was finally accomplished in 1979, albeit in a meager 2.4% conversion. 11 Notwithstanding that I did not obtain any meaningful compound from my first project, I was somehow fascinated by the chemistry of small ring compounds and later wrote several review articles on this subject. 12-15 Eventually Franz admitted that his bis-allene project had been unfruitful. He then came to the laboratory one day and said to me in his familiar manner, "Henry, if I were you, I would try to brominate 1, then treat the resulting tetrabromide 4 with a strong base and see whether or not you can obtain the b;s-alkyne 5" (Scheme 2). At first I thought Franz was crazy, but soon I realized from the literature that some related molecules did exist as stable crystalline compounds. 16Indeed, notable

Br

Br

:CBr 2

........ - o s 1

2

Scheme 1,

Annulenes and Other Molecules

111 Br

Br

Br2

KO-t-Bu

CCi~

THF

75~

.---

1

5

4

Scheme 2.

landmarks in the field of strained cycloalkynes have been the identification of compounds 6,17 7,18 and 8.19 Before I attempted to generate 5 from the known 4, 2~ I suspected that 5 might be unstable and therefore should be trapped by appropriate dienes. Surprisingly, treatment of 4 with KO-t-Bu, followed by chromatography on neutral alumina, provided 5 in 48% yield as pale yellow plates, which decomposed at approximately 110 ~ on attempted melting point determination. 22'23Franz was extremely excited when he heard the result and he himself actually performed the melting point measurement in the laboratory. The bis-alkyne 5 was relatively stable, although some slight decomposition was observed after 2 days when the crystals were allowed to stand at room temperature without protection from light or air. After our preliminary result on the synthesis of 5 had been published, 22 Franz received many requests from prominent physical chemists all over the world, asking for small amounts of 5. For the next few months, my regular research was seriously interrupted because I had to prepare time and again sufficient amounts of 5 in order to send for various physical measurements. Nonetheless, Franz once commented, "Henry, if I were you, I would not feel badly, because, after all, this is the easiest way to get a publication". Therefore I always regard making theoretically interesting compounds as a benign occupational hazard. The bis-alkyne 5 is theoretically interesting because (a) it contains a pair of highly strained triple bonds with an average angular deformation of 24.2 ~ and (b) its central eight-membered ring is presumably planar with the two benzene rings, as was evidenced by an X-ray crystallographic study. 24'25 Noteworthy also is that the 13C NMR acetylenic chemical shifts of cycloalkynes appear to be closely related to their ring strain. 16 In this connection, the value of 109.3 ppm found for 5 is still a world record for all eight-membered cycloalkynes so far identified. 23 A detailed IH NMR spectralanalysis of 5 yielded a Q value 26'27 of 0.961,28 which gave strong indication to the paratropic character and hence planarity of the conjugated eightmembered ring. In view of the fact that 6 is unstable 17 and that the nonplanar

6

7

8

112

HENRY N. C. WONG

6H5

O

C6H5 Et20 H5C6 ---6--n--~---~~_~ C6H5 10

I CI,CHCOCI

~CsHI2 CI

O

Scheme 3.

cyclooctatetraene 29 has the character of a polyene, the bis-alkyne 5 thus might serve as a surrogate for verifying the Hiickel 4nn rule. 3~ The electronic spectrum Of 5 showed both an extensive bathochromic shift and a hyperchromic effect, as compared with the spectrum of the nonplanar 1. 22,23 The photoelectron spectrum of 5 exhibited the first ionization potential at 7.76 eV, while that of the nonplanar 1 was shifted to higher energy (8.24 eV). 31 Such ease of electron excifation in 5 might also be an indication of the electron delocalization in a planar 4nx system. The strain of the triple bonds of 5 could also be reflected by its chemical reactivity with furan, 1,3-diphenylisobenzofuran, as well as dichloroketene (Scheme 3). 23 Mention should be made of my first useof furans in a Diels-Alder reaction, because this first encounter has sparked my enduring interest in furan chemistry throughoui my research career (Section 6). Naturally, my next target was synthesis of the enyne 13. I began the synthesis by brominating 1 with one molar equivalent of bromine at 0 ~ which affOrded the known dibromide 12. 2~ Dehydrobromination of 12 with KO-t-Bu led to 34% of 13 as golden yellow plates, which decomposed upon rapid heating at approximately 85 ~ on attempted melting point determination. 22'23 The enyne 13 was comparatively less stable than 5 and the crystals decomposed after a few minutes standing at room temperature. The 1H NMR spectrum of 1322,23 showed a reiativel), high field singlet (8 5.45) for the olefinic protons, in comparison to those of the nonplanar 1 (~56.75). 23 This IH NMR upfield shift of the olefinic proton clearly indicated the paratropicity which was inherent in the central eight-membered ring of 13. The electronic spectrum of 13 was quite similar to that of 5. 23 The acetylenic carbon

il 3

Annulenes and Other Molecules Br

(s

Br

KO-t-Bu

K

THF

THF.d8

13

12

[CC

2_

14

Scheme 4.

chemical shift of 13 was found to be 108.5 ppm. 23 Unequivocal evidence for the coplanarity of 13 Was given by a low-temperature X-ray crystallographic study, which revealed an average angular deformation of 26 ~ for the bent triple bond. 32 Peter J. Garratt, then a lectUrer in close collaboration with Franz, was interested in generating the dianions of both 5 and 13. However, Peter discovered that only 13 could be reduced by treatment with a potassium mirror in THF-d 8, yielding a deep green solution of the dipotassium salt of the dianion 14 (Scheme 4). The IH NMR spectrum at -40 ~ of 14 exhibited that the olefinic proton absorptions were shifted: downfield to 8 6.67, which was expected for a 10re-electron diatropic system. 22'23 The strain of 13 was demonstrated most convincingly by its readiness to undergo cycloaddition reactions with various reagents (Scheme 5). 23

N,,N,N"C6H5

18

C6H5N3 Et,~O

.c6H5

C6H5 Et20 16

13

15

CI2CHCOCI Et3N CsHI2

c1

17 Scheme 5.

114

HENRY N. C. WONG

O_ "'O uoO'~ 19

20

21

The ESR spectra of the radical anions of 1, 5, and 13 have been recorded. 33 The corresponding hyperfine coupling constants of these molecules were found to be similar in magnitude. These findings indicated that the radical anion of 1 was also planar, thus resolving a 10-year controversy between Katz 34 and Carrington. 35 Electrochemical reduction of 1, 5, and 13 by cyclic voltammetry was also performed, and the results again lent support to the coplanarity of the radical anion of

1.36

The observation that the bis-alkyne 5 was more stable that the enyne 13 led us to believe that the non-annelated bis-alkyne 19 might be a stable molecule. Our preliminary experiment to synthesize 19 was similar to the method used by Krebs. 17 Unfortunately, dehydrobromination of the dibromide 2037,38 with an excess of KO-t-Bu only gave an unseparable mixture of 21. 38 There is therefore no direct support for the formation of 19 as an intermediate. As an alternative approach to our program for the realization of 19 or its derivatives, we sought to make use of the kinetic stabilization provided by one annelated benzene ring. Thus, the bis-alkyne 23 was designed as our next target. The precursor for 23 was obviously the known dibromide 22, which had been prepared by Barton. 39'40 Interestingly, Barton also dehydrobrominated 22 with KO-t-Bu in the presence of 2,3,4,5-tetraphenylcyclopentadienone (TC) to give the adduct 24, without isolating the possible intermediate 23. 41 I repeated Barton's experiment in the absence of TC and indeed 23 was obtained as a yellow oil after low-temperature column chromatography (Scheme 6). 42'43 The bis-alkyne 23 decomposed very rapidly even at 0 ~ and was also unstable in solution. However, the 1H NMR spectrum of 23 at -20 ~ showed a sharp olefinic singlet at ~54.93, which gradually disappeared on standing. The location of the olefinic signal at relatively high field (the corresponding olefinic protons of the nonplanar 22 gave

H5C6

C6H5

H5C6

nr

~6H5 C6H5

--

KO-t-Bu THF

H5C 6

L ~ L ~

C6H 5

-CO

22 24

Scheme 6.


115

Br2

cc~

KO-t-Bu r

THF 27

25 26

OtBu 28

~O 29

30 Scheme 7.

an absorption at ~55.75) 43confirmed the paratropicity of 23, as expected for a planar 4nr~ carbocycle. The identity of the strained bis-alkyne 23 was subsequently substantiated by its cycloaddition with TC, affording the known 24, after extruding carbon monoxide at 40 ~ Encouraged by the isolation and identification of 23, I turned my attention to the kinetic stabilization effect afforded by three annelated benzene tings on 6. It is toward this goal that alkyne 27 was designed. The starting material 25 had been prepared in low yield by a tedious procedure starting from tetraphenylene. 44 I, nevertheless, prepared 25 from 15 by using a multistep procedure. 38 [The remarkable difficulty in the preparation of 25 prompted me to aim at other routes and it was because of this necessity that I later developed my low-valent titanium-mediated endoxide deoxygenation protocol for the construction of novel benzenoid compounds (Section 3)]. After I secured a sufficient quantity of 25, I then brominated it and the resulting dibromide 26 was again subjected to KO-t-Bu dehydrobromination. To my disappointment, only a mixture of t-butoxy compound 28 and ketone 29 was obtained (Scheme 7). 43 However, in the presence of 1,3-diphenylisobenzofuran, the same reaction afforded an adduct 30. Despite very careful workup, no sign of the expected 27 was observed. The results reported by Meier were in full agreement with mine. 45 The instability of 27 is in fact not surprising. Inspection of a planar model of 27 revealed that it would suffer from severe peri-H-H nonbonded interactions (Scheme 7). It is of interest to note that it eventually took me another 15 years to remove these detrimental peri H-H interactions and to prepare two relatively stable derivatives of 27 (Section 5).

116

HENRY N. C. WONG

KO-t-Bu THF

~

, _ . . . _

OtBu 31

32

33

Me

H5C6--~ -

C6H5

34

Scheme 8.

Taking advantage of my knowledge of the kinetic stabilization provided by benzannelation, it was of interest to assess the possible existence of alkyne 32, which can be viewed as a benzannelated derivative of 6. The presence of a methyl group in 32 was expected to facilitate the recognition of any paratropicity by 1H NMR spectrometry. Experimentally, the precursor 31 was subjected to dehydrobromination with KO-t-Bu, giving 33 as the sole product. 46 The assumption that alkyne 32 was an intermediate was confirmed by a similar dehydrobromination of 31 in the presence of 1,3-diphenylisobenzofuran. This reaction led to the adduct 34 (Scheme 8). 46 Interestingly, the observed diminished stability of the benzannelated 32, as compared with that of the benzannelated 23, was in keeping with the relative stabilities of 13 and 5. The presence of a methyl group, on the other hand, might also contribute to the instability of 32 due to some nonbonded interaction in its presumably planar geometry. Dibenzo[a,c]cyclooctene 35 has been known since 1963. 47 In a continuation of my interest in dehydro[8]annulenes, the conversion of 35 to the bis-alkyne 36 w ~ also attempted. 48 However, it was apparent that the presence of the 1,3-diyne unit, together with the expected peri-H-H interaction, would render the planar 36 highly

35

36

H5C6 43

44


117 Br Br_-,

Br

KO-t-Bu

CCh

Br

THF

35

37

Br

Br

Br 38

_

.

.

~

39

~

~

[

// ,4 u 40

41

tBu 42

Scheme 9.

unlikely to exist as a stable compound. Later results indeed showed that my worry was well-grounded. In practic e, bromination of 35 gave the tetrabromide 37, which was dehydrobrominated carefully to the dibromide 38. Further dehydrobromination with an excess of KO-t-Bu furnished only a mixture of 40 and 42. It was likely that the dehydrobromination pathway was of a stepwise manner, going through 39 and 41 as intermediates (Scheme 9), although none of which could be trapped. 48 The presence of 39 and41 was indirectly proved by the isolation of an adduct 43 through a dehydrobromination of 38 in the presence of 1,3-diphenylisobenzofuran. 48 It was quite clear that the instabilities of 39 and 41 were also due to the severe peri-H-H interaction, as indicated in 44. After achieving some modest success in the chemistry of dehydro[8]annulenes, I wanted to reinvestigate the synthesis of the bis-allene 3 (Scheme 1), using a different approach. I reasoned that the bis-alkyne 47 would be an ideal target because it was capable of rearranging under basic condition to 3. This avenue was explored by allowing the dilithium salt 45 to react with an equimolar amount of the bis-tosylate 46. 49 However, no evidence for the existence of the bis-alkyne 47 nor the bis-allene 3 had been obtained. The only identifiable product proved to be 49, whose formation posed an intriguing mechanistic problem (Scheme 10). I finally reasoned that both 47 and 48 were intermediates, and 48 subsequently abstracted two hydrogen atoms from the solvent. 49 This possibility was confirmed by repeat-

118

HENRY N. C. WONG Li THF TsOCH2-" 45

Li

v

-lO~

46

47 R

R

48

49R-H 50R=D

Scheme 10.

ing the coupling reaction in THF-d 8, whereby the corresponding dideuterio analog 50 was obtained. 49 Our diradical mechanism was in agreement with the observations made earlier by both Bergman 5~ and Masamune. 51 At that time, none of us realized that these diradical intermediates would have important pharmaceutical implications. 52 Now it is all too clear that the highly reactive diradical species generated from the so-called enediyne antitumor antibiotics are responsible for the damaging of DNA. 53-55 The mechanism involved in such DNA cleavage is likely a diradical hydrogen abstraction process, with DNA being a hydrogen atom donor. 56 It is noteworthy that Jones 5~and Darby, 51 who pioneered the enediyne rearrangement, now known as the Bergman rearrangement, and Nicolaou, who achieved the first total synthesis of the enediyne antibiotic calicheamicin ~ 55were under the joint supervision of Profs. Franz Sondheimer and Peter Garratt. It might seem that all researchers in the enediyne field have been influenced by Sondheimer and Garratt. It is also of interest to note that Wai-Kee Li, who performed the calculation on the diradical p-benzyne, 57 as well as Wei-Min Dai, who coauthored the informative review article 54with Nicolaou, are now my colleagues at the Chinese University ofHong Kong and the Hong Kong University of Science and Technology, respectively.

3. BENZENOID COMPOUNDS Later at the Shanghai Institute of Organic Chemistry I was joined by two research assistants, Yi De Xing and Yong Fan Zhou. As mentioned previously, there was as yet no efficient way to synthesize tribenzo[a,c,e]cyclooctene 25 (see Scheme 7). Furthermore, our research effort in the quest of the angle-strained alkyne 27 was halted at the last dehydrobromination step, from which only 28 and 29 were obtained (Scheme 7). The peri-H-H repulsion of the expected planar 27 was considered to be responsible for its instability. We reasoned that a planar derivative of 27, tailored to eliminate this unfavorable interaction, might sustain some degree


15

119

H2 5oh Pd-C

P205 H3PO4

EtOAc

150~

51

25

Scheme 11.

of stability. It is toward this goal that an efficient way to prepare 25 was sought. Hopefully this method could also be subsequently extended to the preparation of some derivatives of 25, which might eventually lead to a stable derivative of 27. With a reliable access to 15 at my disposal (see Scheme 5), the construction of 25 was first attempted. Thus, 15 was carefully hydrogenated over 5% Pd-C to afford the dihydro-product 51, which expectedly dehydrated to give 25 in 32% overall yield (Scheme 11).58'59 It was apparent that the least hindered double bond had been hydrogenated preferentially. Not surprisingly, it was difficult to convince myself that the conversion of 15 to 25 via 51 was the ultimate solution of my long-standing problem. As an alternative approach, I was encouraged by the general applicability of the construction of benzenoid systems via 1,4-endoxide deoxygenation. 6~We then embarked on soliciting appropriate oxygen extrusion reagents in this connection. After some experimentation, it was found that the low-valent titanium reagent, 61-63 generated by in situ reduction of TiCI 4 with LiA1H4 in Et3N and THE was extremely effective for such purpose. Some results with this reagent are summarized in Scheme 12, which demonstrates that 15 and 9 were converted, respectively, to 25 and 52 in fair yields, and that both the trifluoromethyl and methoxycarbonyl functionalities were tolerable in the conversion of 53 to 54. 64,65 Similar uses of low-valent forms of iron and tungsten as well as titanium were earlier reported by Hart. 66 It was my responsibility to uplift my co-worker, saying: "Ms. Xing, if I were you, I would not be distressed because, after all, we were only narrowly beaten by an organic chemist of high renown in the 1,4-endoxide field." A zinc--copper couple was later found to be effective for reducing TiC14 to low-valent-titanium. The reagent generated in this manner was also applied to endoxide deoxygenation. 67 Encouraged by the versatility of the low-valent-titanium reagent in the synthesis of polyfunctional arenes, we took one step further to extend its application to the preparation of cyclohexadiene systems. The precursor 55 was prepared by hydrogenating 53a over Pd-C. In practice, treatment of 55 by using the optimal reducing condition (molar ratio of endoxide:TiCla:LiAlHa:Et3N = 1 "7:2.5" 1) resulted merely in a mixture of 54a and 56. 68 We reasoned that the formation of 54a was attributed to an acid-induced dehydration reaction due to an insufficient amount of the proton scavenger Et3N in the reaction mixture. Accordingly, to ensure a nonacidic medium, 25 molar equivalents of Et3N were used for the deoxygenation of 55. To our

120

HENRY N. C. WONG

TiCh LiAIH4 Et3N

THF 15

25

TiCI4 LiAIH4

C ~ ~

Et3N THF

9

52

R~

R 3~~, , . . , ~

R~

R2

)o' IT

R 3 / ~ R 2

TiCh LiAIH4 Et3N THF

Ira-

R3~ ~ , , . T('~T / ~ ~ R3

53

~R 2

R2

a. R 1 = R3 = H, R2 = CO2Me b. R l = Me, R2 = CO-~Me,R 3 = H c. R l = H, R2 = R3 = CO2Me d. R l = Me, R2 = R3 = CO2Me e. R i = R3 = H, R2 = CF3 f. Rl = Me, R2 = CF3, R3 = H

54

Scheme 12. surprise, only 54a was obtained. 68 On the other hand, 56 was yielded from 55 as the sole isolable product when a similar reaction was performed in the absence of Et3N. 68 The question was still open whether or not Et3N played a very important part in the reduction mechanism, and a more detailed examination is discussed later on its quantitative involvement. As I pointed out earlier, a planar derivative of 27, having the detrimental peri-hydrogens removed, might sustain some degree of stability. In order to test this strategy in a pragmatic manner, I enlisted my first graduate student, Xue Long Hou, to bring forward a viable solution. Accordingly we designed two target molecules 57 and 58, for which 59 and 60 appeared to be the two obvious starting materials. The obstacle encountered by Hou was that there was hitherto no existing method for constructing substituted tribenzo[a,c,e]cyclooctene, not to mention compounds such as 59 and 60, which were likely very synthetically demanding. Notwithstanding, Hou overcame the first hurdle by synthesizing several 1-substituted and 1,4-disubstituted derivatives of 25 utilizing as a general protocol two

~CO~Me IMe ~:

TiCh LiAIH4

~CO2Me

Et3N

v 54a

"CO

TiCh LiAIH4 THF

(25 equiv.)

-.,,v

THF

55

-CO2M e

Scheme 13.

~ C O 2

Me

P

~ 56

-CO2M e


121 0a .

.

.

.

s7

o

s8 It .

.

59

.

.

tl

60

pivotal steps, i.e. the formation of 1,4-endoxides and their subsequent deoxygenation with the low-valent titanium reagent, 69 The realization of the presumably planar 59 and 60 still remained a formidable challenge. Hou eventually prepared 59 and 60, whose respective syntheses are shown in Schemes 14 and 15.70'71

~

M

Me--Me ,'~0%Me ~

r 13

TiCl4

_ LiAIH4 EtaN THF

61 B

NBS CC14

.

.

)

r

NaOMe MeOH

-~

63

62

eoH2c- cH,o e A ~ 7 v

~

v

~

Ay~-~N]

~

EtOAc

RuCI3"H20 NalO4 CCI4.MeCN H,O

65

M _

. e Polyphosphoric

acid O

66

NBS "~

cC14 (C6H5CO2)2

67

O KO-t-Bu THF

59 Scheme 14.

~

122

HENRY N. C. WONG

Me

e MeLi Et20 66

HO~~~-~OH OZ L(~~ v

x__/ 69

NBS CC14 (C6H5CO2)2

....... 90~ H2SO4

"70

KO-t-Bu THF 60 Scheme 15.

In view of the high reactivity of an angle-strained cycloalkyne, 16 enyne 1322,23 was chosen as the starting material. However, somewhat to our dismay, of a number of 2,5-disubstituted furans only 2,5-dimethylfuran underwent cycloaddition with 13 to afford endoxide 61. Deoxygenation of 61, as expected, furnished 62, which was an 1,4-dimethyl-derivative of 25. Treatment of 62 with NBS provided a separable mixture of dibromide 63 and a small amount of a monobromide. Williamson ether synthesis transformed dibromide 63 to ether 64. It was necessary to hydrogenate the olefinic bond of 64 to provide 65, prior to oxidation with RuO 4. Otherwise, oxidative ring cleavage at the olefinic bond of 64 took place. The ester 66 thereby obtained served as a common intermediate in our synthetic schemes for 59 (Scheme 14) and 60 (Scheme 15). 70'71 Without the interference of an olefinic bond, the cyclization of 66 was accomplished smoothly, furnishing the diketone 67. Introduction of a bromo group to 67 was brought about by treatment with NBS, from which only a monobromide 68 was isolated. Without purification, 68 was dehydrobrominated with KO-t-Bu to afford our first target molecule 59. 70'71 Compound 59 presumably manifested a planar geometry, which was evidenced by an appreciable paratropic contribution to the ring current, showing an unusually high 1H NMR signal for the olefinic protons at 155.83. 70'71 The olefinic protons of the nonplanar and atropic 62, on the other hand, showed absorption at 156.60. 69 Unfortunately, all attempts to utilize 59 as a precursor for 57 were seriously hampered by its sparing solubility in most organic solvents. A separate effort was therefore made to prepare 60, which was expected to be a more soluble compound. As can be seen in Scheme 15,70'71 66 was allowed to react with an excess of MeLi to give 69, which was cyclized to 70 by treatment with sulfuric acid. Again, bromination of 70 provided monobromide 71, which was dehydrobrominated with KO-t-Bu to form our second target molecule 60. The 1H NMR signal for the olefinic protons at 155.91, as compared with those of 62 at 156.60, 69 also suggested that the


72

123

73

74

n-skeleton of 60 was essentially planar. The planar structure of 60 was eventually substantiated by an X-ray crystallographic analysis.71 Despite the fact that Hou had synthesized both 59 and 60, 70'71 he did not proceed two more steps forward to generate my ultimate goals 57 and 58. The realization of these two molecules is covered later in Section 5. Later at the Chinese University of Hong Kong, I hoped that my cycloadditiondeoxygenation protocol could be put to test in a more systematic and rigorous manner. Novel and theoretically interesting benzenoid molecules, whose benzene rings could be constructed via my approach, were obviously my prime targets. In light of this dibenzo[2.2]paracyclophane 72 stood out as a unique target molecule, because its fixed geometry for orthogonal benzenes should likely be supplied by virtue of its rigid molecular skeleton. In this manner, 72 illustrates an impeccable model for the study of classically conjugated but orbitally unconjugated systems. Due to its remarkable structure, 72 was listed in an organic chemistry textbook as a challenging target molecule, awaiting conquest. 72 The task of assaulting 72 was bestowed upon Chin Wing Chan whose undergraduate research work was under my direction. The key feature of our route to 72 doubtlessly revolved around the combined use of the Diels-Alder reaction in conjunction with low-valent-titanium deoxygenation. Thus, the known 7573 was brominated with NBS to afford the dibromide 76, which was then dehydrobrominated in the usual way to give presumably the strained alkyne 77. As expected, all efforts to isolate 77 as a stable molecule were fruitless. However, 77 could be trapped by furan to form the endoxide 78. Low-valent-titanium deoxygenation convened 78 to the desired molecule 72 (Scheme 16).74,75 Br NBS

KO-t-Bu

CC14

THF

75

76

O

77 TiCl4 LiAIH4 Et3N THF

78

72

Scheme 16.

1 24

HENRY N. C. WONG

The electronic spectrum of 72 showed intense charge-transfer bands in the 400-500 nm region, 74'75 indicating that the rc-basicity of the para-linked benzene rings of 72 was larger than that of the ortho-fused benzene nuclei. The distortion of the para-linked "benzene" rings in 72 into face-to-face boat conformations was revealed by the absorptions of these benzene protons at an unusually high field position of 8 6.69. Moreover, such distortion was also substantiated by an X-ray crystallographic analysis. 76 In collaboration with Fabian Gerson at the University of Basel, the radical anion of 72 was also studied by ESR, ENDOR, and triple-resonance spectroscopy. 77 The results obtained thus far indicated that the odd electron of the radical anion of 72 was delocalized exclusively between the para-linked benzene tings, and was consistent with the result procured from a similar study on the parent [2.2]paracyclophane. 77-79 Similar trapping experiments of 77 with isobenzofuran and 2-methylfuran provided the corresponding endoxides, which were deoxygenated to furnish 7374,75 and 74, 75 respectively. The structure of 73 was also attested by an X-ray crystallographic study. 8~ Another method of functionalizing 72 was initiated by catalytically hydrogenating 78 to 79. Thermolysis of 79 at 220 ~ under vacuum resulted in an ethylene extrusion to generate furan 80. Diels-Alder reaction of 80 with dimethyl acetylenedicarboxylate (DMAD) provided endoxide 81, which on subsequent deoxygenation with low-valent titanium gave ester 82 (Scheme 17). 75 Photoelectron spectroscopy studies 81 on 72 and 74 were carried out by another Basel chemist, Edgar Heilbronner. Tentative orbital energy assignments of 72 were made by him on the basis of a simple MO model calculation and the crystal structural data of 72. 76 These assignments were unambiguously verified by the empirical results obtained from the less symmetrical 74. 81

220~ 0.5mmHg

H2 5%Pd-C C6H6-EtOH 78 DMAD ~ 150Oc CtH5Me v

79

80

TiCl4 ~ C O 2 M e CO2Me LiAIH4 N~/

~

"CO2Me THFE3tN

v

N~(.~y v 82

81

Scheme 17.

-CO2Me


125

220~

-N,

s~

.

-Se

83

C6H5 Tetracyclone -CO

9, ~

C6H5

v

,,,(( ) y y

-C6H5

C6H5 84

Scheme 18.

In order to confirm the existence of the strained cyclophyne 77, 82 Chan also generated 77 by an emphatic route, 78 using the 1,2,3-selenadiazole 83 as a starting material. 83 Thus, pyrolysis of 83 at 220 ~ in the presence of tetraphenylcyclopentadienone (tetracyclone) gave 84, after extrusion of CO. The existence of cyclophyne 77 as an intermediate seemed evident (Scheme 18). Less than two years after Chan and I disclosed the synthesis of 72 and 73, TM a more practical synthesis of 72 based on palladium-mediated reactions was reported by Armin de Meijere. 84'851was deeply impressed by the highly efficient way Armin and his co-worker, Oliver Reiser, were preparing 72 as well as its derivatives. Immediately, I realized that it was impractical to compete with them. I therefore quit the dibenzo[2.2]paracyclophane field, with the strong conviction that many other unknown theoretically interesting molecules were still waiting for our pursuit. The tactics for the synthesis of 72 were also applied to the synthesis of dibenzo[2.2]metaparacyclophanes 89 and 91 (Scheme 19). 86,87 By using exactly

Br KO-t-Bu~ THF Br

Br2 ID

cch 85

R

~ ' ~

[

87

86

TiCh LiAIH4

R

r

Et3N 9 THF R

R 89 R = H 91 R=Me

88 R = H

90 R=Me

Scheme 19.

126

HENRY N. C. WONG

the same method as described above, 8586,87 was convened to dibromide 86 by treatment with bromine. Dehydrobromination of 86 in the presence of either furan or 2,5-dimethylfuran yielded endoxides 88 86'87 and 90, 87 respectively. The expectedly elusive cyclophyne 87 was presumably an intermediate in these experiments. Low-valent-titanium reagent was applied once again to effect the deoxygenation of 88 and 90, yielding cyclophanes 89 and 91, respectively. 86'87 The most unusual phenomenon in the 1H NMR spectrum of 89 was the barely observable appearance at 24 ~ of a broad signal at/5 6.95 for the four benzenoid protons of the para-linked benzene ring. s7 This broad signal can be explained by a conformational inversion process, 88-95 which has been well established for the [2.2]metaparacyclophane 92. 96 In this process, the signals of the four para-linked benzene ring protons of 89 coalesced at 24 ~ due to the conformational change of its meta-bridged benzene ring. To my knowledge, conformationally mobile molecules whose IH NMR signals show room temperature coalescence are rare. It is of my own nostalgic motivation to note that [18]annulene synthesized by Sondheimer97-99 belongs to this elite family. In order to acquire a more quantitative understanding of the flipping process, Timothy Wong carried out a series of variable-temperature 1H NMR studies on 89 and 85, 86'87 whose pertinent data, together with those reported previously for 92, 88-.92 93, 86'87'100 and 94, l~ are summarized in Table 1. As can be seen, the AG~ at Tc appears to have an intrinsic correlation with the bond types of the carbon--carbon bridges, l~176 In 92, the torsional strain (Pitzer strain) resulting from the eclipsed methylene bridge protons in its transition state (C2v symmetry) was expected to play a significant role. On the other hand, regardless of its shorter olefinic bonds, the AG~ of 94 was still lower than that of 92. This observation is most likely due to (a) the conjugative stabilization at its C2v symmetry transition state between the two vinyl bridges and the meta-bridged benzene ring, and (b) the relief in molecular strain by widening of the bridge angles to 120 ~ Compound 89, though also benefiting from the aforementioned stabilizing factors of its C2vsymmetry transition state, suffered, however, from the unfavorable peri-H-H repulsion between the meta-linked benzene and the ortho-bridged ones. As a result, 89 displayed an intermediate AG~ value between those of 92 and 94. Similar arguments were also applicable to 85 and 93, and was expectedly predominant in 91 where an even larger nonbonded interaction between the C 1 methyl group and the meta-bridged benzene ring proton was involved. Chi Wai Hung and Chi Hung Wong, who later joined my research team, were instrumental in introducing cyclopentadienyltitanium trichloride and dicyclopentadienyltitanium dichloride as TiCl 4 substitutes for homogeneous deoxygenation variations. 103Their results showed that, after reduction with LiA1H4, these organotitanium reagents could reduce endoxides to their corresponding arenes in fair yields. 1~ Hung, moreover, reinvestigated the quantitative involvement of Et3N in endoxide deoxygenation (Section 3; Scheme 13). To our surprise, the relative proportion of


127

Table 1. Coalescence Temperatures (Tc), Free Energies of Activation for the Conformational Flipping Process (AG*c),and NMR Operating Frequencies for Measurement of Compounds 85, 89, 91-94. AG~c kJ/mol

Tc ~

NMR Operating Frequency MHz

Ref.

Me > 140

250

85

91

140 146 187

84 87 92

60 60 100

88,89 91,92 90

100 116

76 75

90 250

100 86,87

24

57

250

86,87

-41

44

250

86,87

-96

35

100

101,102

92

93

119

85

94

128

HENRY N. C. WONG

~CO2Me "~

"CO2Me

LiAI~ TiCl4

9

~,CO2Me + ~

EtBN THF

COEMe

55

trans-95

~CO2Me v

-C02M e 96

TiCI4 LiAIH4 Et3N THF

~

'" ~'CO2Me

-,,4/ "',/CO2Me endo, cis-95

"r x a f ""~CO2Me 97

Scheme 20.

Et3N unquestionably played an important role. In the presence of 0.5 molar equivalent of this amine, endoxide 55 was reduced by low-valent titanium reagent (molar ratio of endoxide: TiCI4: LiAIH4: Et3N = 1: 6.9: 2.7: 0.5) to a mixture of products, namely endo,cis-95 and trans-95 in a ratio of 1:5 (Scheme 20). 1~176 In many cases, this reducing system would reduce enedicarboxylates to succinates, while other unactivated olefinic functionalities in the molecules would remain intact. 1~ A fitting example was the conversion of 96 to 97 as shown in Scheme 20.1~ Yuet Ming Man, a later student, was given the task of synthesizing several benzo-fused derivatives of tetraphenylenes with potential clathrate inclusion capabilities? The rationale was based on the 1943 observation that tetraphenylene 52 (see Scheme 12) was able to form 2:1 adducts with a variety of solvent molecules. 1~ Before Man started his research, Thomas C. W. Mak (an X-ray crystallographer colleague at the Chinese University of Hong Kong) and I had already carried out systematic studies of the clathrates formed by 52 and a wide variety of guest species, ranging in size from CH2CI 2 to cyclohexane. 106--108 Taking into consideration that twofold molecular symmetry had long been recognized to assume a preponderant role in the architecture of clathration lattices consolidated by van der Waals attraction and/or hydrogen bonding, 1~176 Man purposely prepared two benzofused derivatives of tetraphenylene, namely 9811J'112 and 99.112 For 52, 98, and 99 we reasoned that a secondary C 2 axis passing through the centers of a pair of

> >

> >

52 98

99


129

O 101 C6H 14-C6I-I6

100

102

TiCI4 LiAIH4 Et3N THF

ID

98

S c h e m e 21.

opposite carbon-carbon single bonds, which gave the molecules their twisted shapes, instead of their principal C 2 axis of their respective point groups, would be a prerequisite component of inclusion behavior. Employing our general approach to arenes by deoxygenation, we had already prepared 52 (Section 3; see Scheme 12) by reducing 9 (Section 1, Scheme 3) with iow-valent titanium. 64 Likewise, 98 was prepared by Man in a similar way. As depicted in Scheme 21, alkyne 100111'112was allowed to react with isobenzofuran 101,113'114 giving endoxide 102. Low-valent titanium reduction of 102 predictably gave 98.111'1i2 The realization of 99 was not trivial. The five-step route (Scheme 22) commenced from the cycloaddition of 103112 with furan. The resulting adduct 104 was reduced by low-valent titanium to 105, which was brominated to afford 106. Dehydrobromination of 106 in the presence of 1011!3'114provided endoxide 107. Again, low-valent titanium deoxygenation converted 107 to 99 (Scheme 22). 112 In agreement with our aforementioned prediction, 98 and 99, both of which comprehend a secondary C 2 axis, functioned as effective hosts for benzene, p-xylene, and 1,2,4-trimethylbenzene. 111'112On the contrary, the other benzo-fused derivatives of tetraphenylene, which are deprived of a secondary C 2 axis, did not manifest any inclusion capacity in all solvents tested. 112 Several attempts have been made to determine the barrier to ring inversion in 52.115-119 Although a rather low barrier had been claimed, 115 high barrier of 936

130

HENRY N. C. WONG

O

TiCI4 Et3N THF

103

1114 KO-t-Bu THF

ar,

C6H14"C6H6

106

101

> TiCI4 LiAIH4 Et3N THF

)

99

107

Scheme 22.

kJ/mol was predicted by a CNDO/2 calculation. 117 Experimentally, a lower limit of 190 kJ/mol was, however, established. 119 By the advantage of comprising a C 2 axis as the only symmetry element in its molecular array, 99 was resolved by the Swedish chemist Jan Sandstrrm and his co-worker Parviz Rashidi-Ranjbar at University of Lund into its pure optical antipodes by repeated chromatography on microcrystalline, swollen triacetylcellulose. 12~On heating to 600 ~ 99 could be partly racemized with a rate corresponding to a free energy barrier for ring inversion of about 283 kJ/molJ 2~To our best knowledge, this is the highest conformational barrier ever recorded. We are also confident that this value should be approximately valid for 52.

4. TRANSITION METAL COMPLEXES The syntheses of planar, benzo-fused cyclooctatetraenes embedded in small tings have always been my favorite research objectives. 7~ The major advances in this area have been the syntheses of the reportedly planar derivatives of dibenzo[a, c]cyclooctene 35, namely 108,121 109,122 and 110.123 In addition to eight-membered carbocycles, the unusual properties of transitionmetal complexes containing 2,2'-diazabiaryl ligands also attracted a good deal of


131

H

108

109

110

f"

t

J

113

my attention. 124'125 To this end, it would be interesting to amalgamate cyclooctatetrene with 2,2'-diazabiaryl frameworks, forming novel molecules such as 111126'127 and 112.127'128It is noteworthy that Randy Thummel, the Editor of this series, is a pioneer in the same area. 129-131 Structurally, it would be of interest to examine the geometry of the transitionmetal complexes 113, formed from 111 and 112. Like 109 and 110, complex 113 should conceivably also be planar. Nevertheless, from a realistic viewpoint, the syntheses of 111 and 112, together with their subsequent metal complexation, seemed to be too ambitious a project. Fortunately, in 1986, Xiu Chun Wang, from the Shenyang College of Pharmacy in the northeastern part of Mainland China, was assigned to synthesize and to complete the follow-up transformations of 111 and 112 to 113. Wang first focused her attention on 111, whose synthesis is outlined in Scheme 23.126'127 As can be seen, Friedl~nder's procedure 132was employed to construct the quinoline skeleton of 111. Thus, condensation between the known diketone 115133 and the aminoaldehyde 114 TM afforded 116, which was brominated to give dibromide 117. Dehydrobromination with DBN converted 117 to our first target 111 (Scheme 23). 126'127 In order to test the practicability of 111 as a chelating ligand and to observe the structural change of 111 due to transition-metal complexation, 111 was subjected to reaction with copper(II) perchlorate, from which a 2:1 copper(I) complex 118 was realized. The structure of 118 was established by an X-ray crystallographic study. 126'127The much smaller N-C--C-N torsion angle of 38 ~ in 118, as compared to that of 63 ~ in 111, unquestionably indicated a significant deformation of the diquinoline moiety resulting from metal coordination. However, it was also shown by this same study that the cyclooctatetraene ring in 118 did not undergo a similar flattening, and was geometrically akin to the eight-membered ring in 111.126'127

132

HENRY N. C. WONG

Br KOH EtOH 114

Br., . CHCI3 ~ ~ - . - ~ ' ~ B r

I15

116

Cu(ClO4)2

DBN

C_~HsMe ~

117

,, - ,,

EtOH80oC :--- 2(Cu

118

III

Scheme 23.

At this point it should be stressed that the low solubility of 111 in common solvents posed a problem. To circumvent this, the more soluble 112 was also assembled by Wang via the route as shown in Scheme 24.127A28 Bromination of the previously prepared 119131 with NBS furnished dibromide 120. Direct dehydrobromination of 120 provided our second target 112 (Scheme 24). 127'128On treatment with Mo(CO)6 , 112 generated a 1:1 complex 121, whose structure was also confirmed by X-ray crystallography. 127'128 It was again found that the dipyridine ligand suffered a severe distortion from coordination, while the geometry of the eight-membered ring was not perturbed. Unlike 109 and 110, the original presump-

Br NBS

KOH EtOH

COt4 119

120

Mo(CO)6 p.C6H4Me2 140-150~

p

" (CO)4M~ ~.

112

/~

121

Scheme 24.

Annulenes andOther Molecules

133

tion of producing planar derivatives of 35 by metal complexation approach was incorrect. Although planar structures for 111 and 112 were not attained, it is still likely that these novel diazabiaryls can serve as chiral ligands with C 2 symmetry. 135In support of this likelihood, the resolution of 112 was accomplished recently on swollen, microcrystalline triacetylcellulose by Jan Sandstr6m. The free energy barrier to ring inversion of 112 was found to be about 101 kJ/mol, through a thermal racemization process using chiral 112.136

5. STABLE DERIVATIVES OF ALKYNE 27 As discussed in Section 3, Xue Long Hou was unable to tackle the synthesis of alkynes 57 and 58. In 1989, Xiao Min Wang was recruited for this last assault. Wang prepared on a large scale the starting materials 59, 67 (see Scheme 14)70'71 and 70 (see Scheme 15). 70'71 By brominating 70 with NBS using boiling chlorobenzene as solvent, Wang was able to isolate a mixture of dibromides 122, which was not separated, and was treated with KO-t-Bu to generate expectedly the alkyne 5871'137 (Scheme 25). On the other hand, treatment of 59 with bromine and the subsequent dehydrobromination of the resulting dibromide 123 gave 5771 (Scheme 25). Thus, 15 years after my own failure to procure 27 as a stable molecule (Section 2, Scheme 7), Wang was triumphant at last in producing two stable derivatives of 27. Needless to say, I was elated because my own logic for the stabilization of 27 by removal of the detrimental per-H-H interactions was verified after all. Surprisingly, alkyne 58 was very stable and no apparent decomposition was detected after standing for several days at room temperature without protection from light or air. It formed light-yellowish crystals which melted between 203 and 205 ~ It is clear that 58 must be planar and belong to the C2vpoint group because

o. .

.

.

.

,,i

oa .

NBS

~

.

.

KO-t-Bu

(C6H5CO2)2 C6H~CI 70

.

THF ._._

135vc

58 122

0

0

Br2

KO-t-Bu

CHC!3 O~

THF

59

57 123

Scheme 25.

~l

134

HENRY N. C. WONG

all the methyl protons showed only one signal at 8 t.42 in its 1H NMR spectrum. It is likely that 58 should be paratropic because all its proton signals experienced upfield shifts. The appearance of only one absorption for all four methyl carbons in the 13C NMR spectrum of 58 further confirmed its planarity. Of particular interest was the downfield shift of the absorptions for the alkynyl carbons at 8 108.4, as compared to the chemical shifts of other linear sp-hybridized alkynes. 16This effect might be due to the angle strain in 58, which caused a largerp-orbital involvements in the hybridization of the alkynyl carbons. 71'137 The planar structure of 58 was substantiated by an X-ray diffraction study.71 Wang also found that the strained triple bond of 58 permitted Diels-Alder cycloadditions. 137 Alkyne 57, like 58, was very stable in the solid state. The paratropic character of 57, as reflected by the upfield shift of all its proton signals in the IH NMR spectrum, indicated that it should possess a planar skeleton. The sparing solubility of 57 in most organic solvents, nonetheless, gave Wang some difficulty. He was therefore unable to obtain a 13C NMR spectrum for 57. Due to its low solubility, further experimental efforts on 57 were seriously hampered as well. In addition to the synthesis of stable derivatives of 27, I also looked into the possibility of stabilizing 23 (Section 2, Scheme 6) and 19. The molecules I designed for this course were 124 and 125, in which the aromatic rings were expected to contribute a certain degree of stability. Furthermore, the triple bonds in 124 and 125 should be kinetically protected by the protons as illustrated. Most importantly, the length of the C9--C10 bonds in the phenanthrene moieties should be approximately 1.34/~,138 which was exactly equal to the length of a carbon--carbon double bond (1.34/~). A combination of all these factors should furnish stable 124 and 125, which retain all the basic structural characteristics of 23 and 19, respectively. Prior to the generation of 124, one of my students, Samuel K.T. Cheng, in a separate study on the preparation ofclathrate inclusion hosts, had prepared a perfect precursor 131 (Scheme 26). The synthetic routes leading towards 131139 and 12414~ are shown in Scheme 26. As can be seen, a rather synthetically oriented program based on the initial construction 21 of 128 was carried out by Cheng, who achieved this goal by treatment of a DMF solution of dibromides 126 TM and 127139 with H

@

23

O 19


135

Zn DMF 126

PCC CH2C12

BrCH2" " ~ 127 O2N~

OH

O

NO2

v I-.SC1 Et3N CICH2CH2CI

NaBH4 THF

Br2 CCI4 80~

KO-t-Bu THF Br"

132

Br

Scheme 26.

activated zinc. 142Compound 128 was then converted to 129 by benzylic oxidation. Reduction of 129 gave the key intermediate 130, which was transformed to 131139 by utilizing an efficient method in which allyl alcohols could be converted to 1,3-dienes via a 1,4-dehydration. 143-145Another student, Chun Yip Leung, completed the project by brominating 131 to tetrabromide 132, which on dehydrobromination yielded the bis-alkyne 124140 (Scheme 26). In good agreement with our presumption, 124 was also very stable. It melted with decomposition at 183 ~ The two pairs of symmetrical alkynyl carbons gave two signals in the 13C NMR spectrum at 8 109.7 and 113.1, in keeping with a distortion of the otherwise linear sp-hybridized triple bonds. 16The result of an X-ray crystallographic analysis of 124 verified that the bond length of the phenanthrene C9--C10 double bond was 1.34/~. In this manner, Leung was successful in kinetically stabilizing 23 by our phenanthrene protocol. Although we have not yet embarked on the preparation of 125, which is presumably a stable surrogate of 19, I firmly believe the same synthetic strategy should be useful.

6. POLYSUBSTITUTED FURANS I wish to relate my continued collaboration with Fabian Gerson of the University of Basel in Switzerland on ESR studies. One of our recent cooperative explorations involved the radical anions of the nonplanar tribenzo[a,c,e]cyclooctene 25 as well as its nonplanar and planar derivatives. 146During this investigation, which actually lasted for many years, Gerson required some deuterated derivatives for an explicit

136

HENRY N. C. WONG

D D

D

D

133

134

13

Scheme 27. assignment of coupling constants. For this reason, he inquired if it was possible to prepare 133 whose structure is shown in Scheme 27. Based on my experience in syntheses of tribenzo[a~c,e]cyclooctenes, I recommend a retrosynthetic pathway for 133 (Scheme 27). However, to my surprise, the recorded synthesis of 3,4-dideuteriofuran 135147 was very tedious and was therefore laborious to repeat. As it turned out, a careful scrutiny of the relevant literature revealed that even the syntheses of 3,4-disubstituted furans were not trivial. 148-151 Over the last 20 years much effort has been devoted to the synthesis of 3,4-disubstituted furans, and today these molecules can be prepared from acyclic precursors 152'153 as well as from cyclic precursors. 154-157 These approaches could not effectively yield 3,4-disubstituted furans with elaborate substituents, nor even a furan as simple as 135. An entirely different access to 135 was obviously necessary. In this context, I reasoned that the bis-silylated furan 136 could be readily converted to 135, with the silyl groups being potential ipso-directors. Well known for their t~-donating character, the silyl groups are able to stabilize a 13-carbocation through the so-called (p-t~)~ overlap, and as a result contribute to the ipso-substitution pattern. 158-16~In my long-term planning, 136 might also be converted to the otherwise difficult to acquire 3,4-disubstituted furans. Alternatively, the bis-stannylated furan 137 could also lead to 135 as well as other 3,4-disubstituted furans, due primarily to an even larger kinetic 13-effect exhibited by the stannyl groups. 161 We did ultimately prepare both 136 and 137. Their syntheses and reactions will be discussed below. Because the problem concerning deuterated 25 had been resolved earlier by other means, 146 the transformations of 136 and 137 to 135 have thus never been attempted. It is, however, inspiring to note that furan 136 and 137 played instead pivotal roles as building blocks for 3,4-disubstituted furans in another synthetic program, which has lately become one of my most productive ones.

Me3Si~SiMe3

nBu3Sn~SnnBu3

136

137


)])

SiMe 3

C6H5

O//~

SiMe3

137

Et3N

Me3Si

SiMe3

. 250~

~,~ N

-C6H~CN

139

136

138

Scheme 28.

Mei Sing Ho was responsible for the synthesis of 136 (Scheme 28), 162'163which was based on the Diels-Alder reaction of oxazoles. 155 An acceptable yield of 136 was obtained by heating oxazole 139164 with bis(trimethylsilyl)acetylene 138, with subsequent expulsion of benzonitrile. Ho also completed certain cycloadditions and acylations on 136,162'163 but failed to uncover crucial ways to 3,4-disubstituted furans. The role of 136 as a versatile building block for 3,4-disubstituted furans was intensely exploited to a notable level by Zhi Zhong Song from Lanzhou University who first secured a reliable and high yield access to 136 by adding a small amount of triethylamine to the mixture of 138 and 139, prior to thermolysis, in order to suppress an undesirable acid-catalyzed rearrangement of the resulting 136 (Scheme 28).163,165

After some painstaking experimentation, Song was finally successful in a regiospecific ipso-displacement 166,167 of one of the silyl groups with boron trichloride, furnishing presumably a dichloroborane (Scheme 29). The dichloroborane was not isolated and was hydrolyzed directly with either a dilute acid or a dilute

OMe3Si

SiMe3

Me3Si~x

1. BCI3, CH2CI2 .,O.

136

2.5ek HCI or 2 M Na2CO3

Me3Si

O.B..O ~SiMe3

Rz

,x pd[P(C6Hs)3h 1 M Na2CO3 MeOH-C6HsMe 120~

141

140

R1 1. BC13,CH2C12 0~

2.1 M Na2CO3

R1

O.,B., O ~ R

R 1 = aryl, benzyl R 2 = aryl, benzyl, vinyl X=Br, I

R2

"X pd[P(C6H5)3]4

,,O,.

1

142

Scheme 29.

2 M Ya2CO3 MeOH-C6HsMe 120-150~

143

SiMe3

138

HENRY N. C. WONG

base to afford boroxine 140 in an almost quantitative yield. 163'165Though it was unprecedented for boroxines to undergo palladium-catalyzed coupling reactions, it was found that 140 was converted readily to 141163,165 through the regular Suzuki reaction conditions. 16s The substitution of the remaining silyl group in 141 with boron trichloride required more rigorous conditions, giving 142 in inferior yield. Likewise, 142 was convened to 143 (Scheme 29), 163'165and it should be noted that R l must be insensitive to Lewis acid. The choice of R 2, on the other hand, depended solely on the limitation of the Suzuki reaction. 168 A mechanistic dilemma concerning many organometallic reactions, in particular palladium-catalyzed reactions, is that the rate of 13-elimination is always faster than transmetallation. 169Consequently, those R 1and R 2 with an sp 3 C-H 13to the carbon beating the leaving group X are notably absent from the list of R IX and R2X. In order to overcome this limitation, so that 3,4-dialkyl furans could also be prepared, Song undertook an independent route through an ipso-iodination of one silyl group of 136. The resulting iodide 144 was converted to 145 by a nickel-catalyzed cross-coupling with n-butyl magnesium bromide (Scheme 30). 17~In this way, an alkyl group with ~-hydrogens was certainly tolerable. Having tried unsuccessfully to directly iodinate 145, an indirect iodination route was sought by Song. Thus, boroxine 146 was prepared as usual from 145. To our relief, iodination of 146 gave regiospecifically iodide 147 (Scheme 30). 17~Although Song did not proceed further to prepare 148 through a second nickel-catalyzed cross-coupling, we believe that our original ambition of preparing 3,4-dialkyl furans was for a major part fulfilled. Under the Suzuki reaction condition and in the presence of 2,3-bis(bromomethyl)quinoxaline 149, Song unexpectedly discovered that 140 was able to provide the self-coupling product, namely bifuran 150 in excellent yield (Scheme 31).~7~ Similarly, 150 produced quateffuran 152 via 151, from which terfuran 153 was also obtained via a cross-coupling with 144 under identical conditions (Scheme 31). A variety of symmetrical and unsymmetrical furan-3,4-diyl oligomers were

Me3Si,

SiMe3

136 1.BCI3.CH2CI2 -78~ 2.2 MNa2CO3

12 I SiMe3 nBu SiMe3 " AgO2CCF3 ~ ~ . B n'u.MgBr THF NiCI2[P(CrH5)3]2 -78~ THF 144 145 O nBu~ -'~ nBu R ~ B " O ' B ~~xO AgBF4 I2 n B ~ I

6.B.6

~

-Ts~ nBu

146 Scheme 30.

.

147

.

.

.

148

Annulenes and Other Molecules O-

139

Me3Si~-'~x

~B"O"B '~~~O Pd[ P(C6HS)3]4 2 MNa:CO3 ~ Me3Si Me3Si

O "B ,,O

MeOH'1C 5~6HsMel siMe3 ~ ~ ) ~

O~

v

140

1.BCI3 CH2C12

-78 -o~

SiMe3 2. 2 MNa2CO3

~

CH2Br

"~O/150

"N" "CH2Br 149

~I B ~ O SiMe3

Me3Si,j~

BI O,..,,.O

10

Pd[P(C6H5)3]4

~

O

2 MNa2CO3 Me3Si MeOH-C6HsMe" ~

~

SiMe3

CH,Br 0 ---u

SiMe3 ~

151

I

SiMe3

144

152

-N-''-CH2Br 149

3. Pd[P(C6Hs)3]4 2 MNa2CO3 MeOH-C6HsMe

1. BCI3 CH2C12 -78 -0~ 2.2 MNa2CO3

pd[P(C6Hs)3]4 2 MNa2CO3 MeOH.C6HsMe

115~

115~

149

0 Me3Si~ ~ 0 ~ $ I O

iMe3

0

0

(0)

MeaSi ~

153

SiMe3

154

Scheme 31.

synthesized in this manner. 171 It is likely that these processes will allow the synthesis of quinquefurans, sexifurans, and septifurans. To put such suggestion to test, 152 was successfully transformed in two reactions to octifuran 154 (Scheme 31),171 which is the longest furan-3,4-diyl oligomer known. It is believed that the oligomeric furans can serve as prototypic structures for the study of novel furan3,4-diyl polymers. The self-coupling mechanism still remains unclear, although its synthetic options appear to be rather encouraging. Mechanistically, it is likely that Pd(IV) intermediates were involved. 171 Similar to 136, furan 137 was also prepared via the oxazole route. 155 Yun Yang, a student from Guangzhou Institute of Chemistry, thermolyzed oxazole 139164 and bis(tri-n-butylstannyl)acetylene 155 in a sealed tube to afford a separable mixture

140

HENRY N. C. WONG SnnBu3

Sn IBu3

nBu3Sn

139

snnBu3

137

SnnBu3

156

155 Scheme 32.

of the desired 137 and 156, albeit in low yield (Scheme 32). 172 The initial palladium-catalyzed cross-coupling of 137 with aryl bromides gave only symmetrical diarylfurans 157 (see Scheme 33). 172 Encouraged by the recent work on the conversion of 3-trimethylstannylfuran or 156 to 3-acylfurans, 156'173-175the palladium-catalyzed reaction was carried out by Yang between 137 and benzoyl chloride, yielding 158 (Scheme 33).172 This par-tial acylation therefore provided a direct entry to unsymmetrical 3,4-disubstituted fui'ans. A fitting example to demonstrate such an application was the conversion of 158 to 159 as shown in Scheme 33.172 Another route from which unsymmetrical 3,4-disubstituted furans could be obtained was by utilizing a tin-lithium exchange pathway, 176as depicted in Scheme 34.177 To achieve a complete exchange of one stannyl group in 137, generating 160, 2.2 equivalents of n-butyllithium were needed. Smaller amounts of n-butyllithium merely led to incomplete exchange, while larger amounts gave no sign of exchange with the second stannyl group. Such a seemingly unfavorable restriction, however, guided Yang to the synthesis of various unsymmetrical 3,4-disubstituted furans. One of the transformations for 160 was effected with cyclohex-2-en-1-one, which o nBu3Sn

137

SnnBu3 C6HsCOCi H 5 C 6 ~ PdCl2[P(C6Hs)312 THF 65-80~

ArBr Pd[P(C6Hs)3]4 HMPA air 65-80~ Ar

/6H5 B

Ar

SnnBu3

158 [(C3H5)PdCI]2 HMPA 60~

o H5C6--~

159

157a Ar = C_~H5

157bAr= C-6H4-p-NO2 157cAr= C.61-I4-p-COMe Scheme 33.

C6H5


"Bu3Sn

141

SnnBu3

"Bu3Sn n-BuLi (2.2 equiv.)

Li

:.

THF -78~

~ 161

OH

....

[(C3H5)PdCI]22 .DMF

DMPU

162

Scheme 34.

furnished alcohol 161. Again, a palladium-catalyzed cross-coupling reaction converted 161 to 162 an unsymmetrical 3,4-disubstituted furan. With 136 and 137 functioning as versatile building blocks, we believe that we have duly elaborated several synthetic routes to symmetrical as well as unsymmetrical 3,4-disubstituted furans with diverse substitution patterns.

7. CONCLUSION The design, syntheses, and study of non-natural molecules with theoretical implications have been intellectually challenging. I cannot agree more with Phil Eaton of the University of Chicago, Who remarked in the preface for a Symposium-inPrint devoted to non-natural products, 178that "a non-natural product is the invention of one's mind . . . . The justification for non-natural product synthesis lies not only in the desire to answer the challenge of the synthesis itself, but also in the need to answer questions about the product. Non-natural products, if intelligently conceived, are sought after, for example, to test a theory, or to probe a particular phenomenon, or to try an idea for a new and useful compound, perhaps a new polymer or a new drug . . . . The non-natural product world remains full of real challenges and holds the promise of further great rewards". By way of appreciating these statements, it is sincerely wished that I have presented in this chapter sufficient delineations on relevant highlights concerning some theoretically interesting mole' cules.

ACKNOWLEDGMENTS Thanks are due to Drs. Tze-Lock Chan and Wai-Kee Li who assisted in the preparation of this account. Their advice and suggestions are gratefully acknowledged. I am indebted to The Chinese University of Hong Kong and the Ramsay Memorial Fellowships Trust for awarding to me the Shell Scholarship for Postgraduate Study in the United Kingdom and the Ramsay Memorial (British) Fellowship, respectively. Part of the works described in this

142

HENRY N. C. WONG

account was financially supported by two Hong Kong Research Grants Council earmarked grants (Acc. No. 221300110 and 221600120). I also wish to acknowledge the award of a fellowship by the Alexander von Humboldt-Stiftung, which enabled me to visit the Institut ftir Organische Chemie at Universit~it zu KSln as a guest of Prof. Emanuel Vogel for one enjoyable and inteUectually-rewarding month, during which I was able to conceive the layout and contents of this article. Last but not least, I wish to thank all my collaborators, whose names have been mentioned in the text as well as in the literature citations, for their extraordinary contributions, without which this account would not have materialized.

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145

127. Wang, X. C.; Wong, H. N. C. Pure Appl. Chem. 1990, 62, 565-568. 128. Wang, X. C.; Cui, Y. X.; Mak, T. C. W.; Wong, H. N. C. J. Chem. Soc., Chem. Commun. 1990, 167. 129. Thummel, R. P.; Lefoulon, F.; Cantu, D.; Mahadevan, R. J. Org. Chem. 1984, 49, 2208. 130. Thummel, R. P.; Lefou!on, F. J. Org. Chem. 1985, 50, 666. 131. Thummel, R. P.; Lefoulon, E; Mahadevan, R. J. Org. Chem. 1985, 50, 3824. 132. Claeng, C. C.; Yah, S. J. Org. React. 1982, 28, 37. 133. Yates, P.; Lewars, E. G.; McCabe, P. H. Can. J. Chem. 1972, 50, 1548. 134. Smith, L. I.; Opie, J. W. Org. Syn. Coll. VoL 1955, 3, 56. 135. Rosini, C.; Franzini, L.; Raffaelli, A.; Salvadori, P. Synthesis 1992, 503. 136. Rashidi-Ranjbar, P.; Sandstr6m, J.; Wong, H. N. C.; Wang, X. C. J. Chem. Soc., Perkin Trans. 2 1992, 1625. 137. Wang, X. M." Wang, R. J.; Mak, T. C. W.; Wong, H. N. C. J. Am. Chem. Soc. 1990, 112, 7790. 138. Kay, M. I.; Okaya, Y.; Cox, D. E. Acta Crystaliogr. 1971, B27, 26. 139. Cheng, S. K. T.; Wong, H. N. C. Syn. Commun. 1990, 20, 3053. 140. Leung, C. Y.; Mak, T. C. W.; Wong, H. N. C., unpublished experiments. 141. Cava, M. P.; Deana, A. A.; Nuth, K.J. Am. Chem. Soc. 1959, 81, 6458. 142. Cava, M. P.; Napier, D. R. J. Am. Chem. Soc. 1957, 79, 1701. 143. Reich, H. J.; Wollowitz, S. J. Am. Chem. Soc. 1982, 104, 7051. 144. Kharasch, N. Langford, R. B. Org. Syn. Coll. VoL 1973, 5, 474. 145. Paquette, L. A.; Wang, T. Z.; Luo, J. M.; Cottrell, C. E.; Clough, A. E.; Anderson, L. B. J. Am. Chem. Soc. 1990, 112, 239. 146. Felder, P.; Gerson, E; Gescheidt, G.; Heckendorn, R." Tong, T. H." Wang, X. M." W0ng, H. N. c.; Hou, X. L. Helv. Chim. Acta 1991, 74, 644. 147. Chadwick, D. J.; Chambers, J.; Hodgs0n, P. K. G.; Meakins, G. D.' Snowden, R. L. J. Chem. Soc., Perkin Trans. 1 1974, 1141. 0 ~.rg 148. Gschwend, H. W.; Rodriguez, H.R. React. 1979, 26, 1 149. Dean, F. M. Adv. HeterocycL Chem. 1982, 30, 167. 150. Dean, E M. Adv. HeterocycL Chem. 1982, 31,237. 151. Donnelly, D. M. X.; Meegan, M. J. In Comprehensive Heterocyclic Chemistry; Bird, C. W.; Cheeseman, G. W. H. Eds.; Pergamon Press, Oxford, 1984, Vol. 4, Part 3, pp. 657-712. 152. Garst, M. E.; Specer, T. A.J. Am. Chem. Soc. 1973, 95, 250. 153. Reich, H. J.; Olson, R. E. J. Org. Chem. 1987, 52, 2315. 154. Corey, E. J.; Crouse, D. N.; Anderson, J. E. J. Org. Chem. 1975, 40, 2140. 155. K6nig, H.; Graf, F.; Webernd6rfer, V. Liebigs Ann. Chem. 1981, 668. 156. Keay, B. A.; Bontront, J.-L. J. Can. J. Chem. 1991, 69, 1326. 157. Danso-Danquah, R. E.; Scott, A. I.; Becker, D. Tetrahedron 1993, 49, 8195. 158. Ushakov, S. N.; Itenberg, A. M. Zh. Obshch. Khim. 1937, 7, 2495. 159. Chan, T. H.; Fleming, I. Synthesis 1979, 761. 160. Thomas, S. E. Organic Synthesis. The Role of Boron and Silicon; Oxford University Press, Oxford, 1992, pp. 47-50. 161. Lambert, J. B.; Wang, G. T.; Teramura, D. H. J. Org. Chem. 1988,53, 5422. 162. Ho, M. S.; Wong, H. N. C. J. Chem. Soc., Chem. Commun. 1989, 1238. 163. Song, Z. Z.; Ho, M. S.; Wong, H. N. C. J. Org. Chem. 1994, 59, 3917. 164. Bredereck, H.; Gompper, R. Chem. Ber. 1954, 87, 700. 165. Song, Z. Z.; Zhou, Z. Y.; Mak, T. C. W.; Wong, H. N. C. Angew. Chem. hit. Ed. Engl. 1993, 32, 432. 166. Effenberger, E: Hfibich, D. Liebigs Ann. Chem. 1979, 842. 167. Lambert, J. B. Tetrahedron 1990, 46, 2677. 168. Suzuki, A. Pure Appl. Chem. 1991, 63, 419. 169. Stille, J. K.Angew. Chem. hit. Ed. EngL 1986, 25, 508.

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INFRARED SPECTROSCOPY OF HIGHLY REACTIVE ORGANIC SPECIES: THE IDENTIFICATION OF UNSTABLE MOLECULES AND REACTIVE INTERMEDIATES USING AB INITiO CALCULATED INFRARED SPECTRA

B. Andes Hess, Jr. and Lidia Smentek

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Theoretical Background of Infrared Spectroscopy . . . . . . . . . . . . . . 2.1. Description of the Model . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Method of Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Reliability of the Model . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Experimental Polarizations . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Highly Reactive Organic Molecules . . . . . . . . . . . . . . . . . . . . . . . 3.1. Cyclobutadiene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Methylenecyclopropene . . . . . . . . . . . . . . . . . . . . . . . . . .

Advances In Theoretically Interesting Molecules Volume 3, pages 147-208. Copyright 9 1995 by JAI Press Inc. All Hghts of reproduction in any form reserved. ISBN: 1-55938-698-3

147

148 .149 149 155 156 162 166 166 166 171

148

B. ANDES HESS, JR. and LIDIA SMENTEK

3.3. o-Benzyne . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Cyclopentadienylideneketene . . . . . . . . . . . . . . . . . . . . . . . 3.5. Thiirene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Reactive Intermediates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Methylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Cyclopropenylidene and Its Isomers . . . . . . . . . . . . . . . . . . . 4.3. Alkoxymethylenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Trimethylenemethane . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

171 181 182 186 186 187 194 199

205

1. INTRODUCTION Infrared (IR) spectroscopy was the first modern spectroscopic method which became available to chemists for use in the identification of the structure of organic compounds. Not only is IR spectroscopy useful in determining which functional groups are present in a molecule, but also with more careful analysis of the spectrum, additional structural details can be obtained. For example, it is possible to determine whether an alkene is cis or trans. With the advent of nuclear magnetic resonance (NMR) spectroscopy, IR spectroscopy became used to a lesser extent in structural identification. This is because NMR spectra typically are more easily interpreted than are IR spectra. However, there was a renewed interest in IR spectroscopy in the late 1970s for the identification of highly unstable molecules. Concurrent with this renewed interest were advances in computational chemistry which allowed, for the first time, the actual computation of IR spectra of a molecular system with reasonable accuracy. This chapter describes how the confluence of a new experimental technique with that of improved computational methods led to a major advance in the structural identification of highly unstable molecules and reactive intermediates. For a spectroscopic observation to be understood, a theoretical model must exist on which the interpretation of a spectrum is based. Ideally one would like to be able to record a spectrum and then to compare it with a spectrum computed theoretically. As is shown in the next section, the model based on the harmonic oscillator approximation was developed for interpreting IR spectra. However, in order to use this model, a complete force-constant matrix is needed, involving the calculation of numerous second derivatives of the electronic energy which is a function of nuclear coordinates. This model was used extensively by spectroscopists in interpreting vibrational spectra. However, because of the inability (lack of a viable computational method) to obtain the force constants in an accurate way, the model was not initially used to directly compute IR spectra. This situation was to change because of significant advances in computational chemistry. In the 1970s a new technique, low temperature (5-20 K) isolation, became available to organic chemists that allowed the preparation and "isolation" of highly reactive species which previously could not be directly observed. Typically a precursor was condensed in a matrix of an inert substance (e.g., argon) and then

IR Identification of Reactive Organics

149

photolyzed to convert it to the desired product. This allowed the spectroscopic study of the reactive molecule produced. Since IR spectroscopy was generally the method of choice, there was renewed interest in being able to fully interpret IR spectra of simple systems, At about the same time, ab initio methods had advanced to the point where one could actually compute the complete force-constant matrix of a molecule and therefore its IR spectrum. These calculations were long and tedious, often requiring numerous single-point energy calculations of a system. Nevertheless, for the first time it was feasible to compute in an ab initio way the IR spectra of molecules. Fortunately the initial successes in the ability to computationally reproduce observed spectra prompted theoreticians to develop better and much simpler (at least for the user) programs. Because of the current availability of such programs, we thought it would be useful to provide the organic chemist with examples of how the method might be used, as well as to describe, in hopefully understandable terms, the model in which the calculation of IR spectra is based. In Section 2 the model is introduced and its reliability is demonstrated by comparison of the computed spectra with those observed for several well-known molecules. In Section 3 examples of the use of computed IR spectra in the identification of reactive molecules are presented, and in Section 4 those of reactive intermediates.

2. THEORETICAL BACKGROUND OF INFRARED SPECTROSCOPY 2.1. Description of the Model Since the beginning of the century, when together with the development of quantum mechanics the foundations of theoretical spectroscopy were established, there has been great demand for a model which relates spectra to molecular structure (that is the geometry) and possible internal motions among the individual atoms. In such a model the known features of molecules (e.g., mass, number of atoms, and their relative positions) and characteristic parameters (e.g., bond distances and bond angles) can be used to determine the energy of inner states, the difference of which yields transition energies that can be measured experimentally. The first attempt to explain the characteristic properties of molecular spectra in terms of the quantum mechanical equation of motion was undertaken by Born and Oppenheimer. The method presented in their famous paper of 19271 forms the theoretical background of the present analysis. The discussion of vibronic spectra is based on a model that reflects the discovered hierarchy of molecular energy levels. In most cases for molecules, there is a pattern followed in which each electronic state has an infrastructure built Of vibrational energy levels, and in turn each vibrational state consists of rotational levels. In accordance with this scheme the total energy, Eivj, has three distinct components of different orders of magnitude, givj

=

Eel

vib

rot

i + Ei, v + Ei,v,j

1 50


with the assumption that:

~l

>> z~vib >> ,~Erot

The equations of internal motion defined within the local (molecular) fixed coordinate system have to be transformed to the laboratory fixed coordinate system in which all experiments are performed. Thus, introducing Euler's angles, O, 0, and Z, the coordinates of the electrons (e) and nuclei (n) are transformed in the following way:

7e= (rex, ~y' ~)loc - S((I)0X)~e ~z-St -~ (Re, ~y, Re)lab rn _=(~, ~, ~), (~),0~Z)~n _=(Rx, Ry, Rz),ab

1

O C~

n

n

tl

As a consequence of the transformation, the equation of motion depends on three extra coordinates which describe the orientation in space of the rotating local system. Furthermore, there are additional terms in the Hamiltonian which represent uncoupled momenta of the nuclear and electronic motion and moment of inertia of the molecule. In general, the Harniltonian has a structure which allows for separation of electronic and vibrational motions. The separation of rotations however is not obvious. Following the standard scheme of the various contributions to the energy, one may assume that the momentum and angular momentum of internal motions vanish. Thus, the Hamiltonian is simplified to the following form,

H = Te(R e) + Tn(R n) + V(Re, R n)

(I)

where the energy of electron motion is defined as,

N r

1

-

Te(Re) = ~

~=

a(xYz)

(2)

with L a denoting the angular momenta of electrons and ).t(~13is determined by the components of the tensor of the total momenta of inertia. The second operator of Equation 2 represents the rotational part of the kinetic energy and is often neglected (in particular in the case of calculations based on the variational principle). The kinetic energy of the nuclei has a simple form and because of the separation of the vibronic and rotational states it is defined by a single operator, namely,

r.(rr>-Xg

(3)

i=1 m

V(R e, R") in Equation 1 denotes the operators of the Coulomb interaction between all particles of a molecule. It should be pointed out that all operators contributing


151

to the Hamiltonian defined by Equation 1 have such a simple form as a result of the assumption that the total tensor of inertia is well approximated by its part produced by the nuclei. This means that the electronic part of a tensor of inertia is neglected. This approximation is justified by the fact that the nuclear and electron masses differ by orders of magnitude. In order to separate the electronic and nuclear coordinates in an eigenvalue problem for the Hamiltonian defined by Equation 1, the adiabatic approximation in the version of a Born--Oppenheimer model is used. In general, the eigenfunction defined within the adiabatic approximation is defined as a linear combination,

t~(gegn) = Z k

Xk(-Rn)~gk(ReRn)

(4)

where ~gk are the solutions of the electronic eigenvalue problem and Xk are the unknown coefficients which depend on the nuclear positions. The particular choice of ~k leads to various versions of the adiabatic approximation. In the case of the Born-Oppenheimer method, ~Bo(-~e-~n)is a linear combination of ~g0(~,,)) which are the solutions of the electronic equation: [T~(~") + V(~", g~ )]Vk(R. 0 -~:~,, e )-~kl(-en)~k(-ge-R n)

(5)

The electronic equation is defined for a "momentary position" of the nuclei, and it has to be solved for various choices of ~n. As a consequence, the electronic energy depends in a parametric way on the nuclear positions and the energy curve (surface) ~,l(~n) is the general solution of Equation 5. The wave function assumed in the form of Equation 4 is used in the eigenvalue problem of the total Hamiltonian:

Ht~B~

n) = Et~BO(-Re-Rn)

(6)

When the orthogonality of various functions ~0 is taken into account, Equation 7 for the unknown functions Xj (R")) has to be satisfied,

[Te(-Rn) + E~jI(-R")IXj(-Rn) + 2 (W?('Re-Rn)lTn(-Rn)l~k(-Re~)")elXk(-~n) EXj(-~n) - -

(7)

k where the integration in the matrix element is performed over the electronic coordinates. The solutions of Equation 7 (which, in fact, are the coefficients of the linear combination in Equation 4) are interpreted as vibronic wavefunctions. The matrix elements in Equation 7 represent the mixing of vibrational states with the electronic states via the dynamic part of nuclear motion. The degree of this mixing is determined by the value of these matrix elements. At the same time, the sum of these matrix elements describes the coupling of various vibrational states through the nuclear motion operator. If there is no degeneracy or closeness of

152


electronic states, the sum of these matrix elements is small enough to be neglected. This approximation leads to the Born-Oppenheimer model of nuclear vibrations. If the equation of nuclear motions contains the diagonal matrix elements of the sum in Equation 7, the approximation is know in the literature as the Born-Huang model. 2 There are several possible ways of introducing the Born-Oppenheimer model 3-5 and here the most descriptive way has been chosen. 6 It is worth mentioning, however, that the justification for the validity of the Born-Oppenheimer approximation, based on the smallness of the ratio of the electronic and nuclear masses used in its original formulation, 1has been found irrelevant. Actually, Essrn 7 started his analysis of the approximate separation of electronic and nuclear motions with the virial theorem for the Coulombic forces among all particles of molecules (nuclei and electrons) treated in the same quantum mechanical way. In general, quantum chemistry is dominated by the Born-Oppenheimer model of the theoretical description of molecules. However, there is a vivid discussion in the literature which is devoted to problems characterized by, for example, Monkhorst's article of 1987, 8 "Chemical Physics without the Born--Oppenheimer Approximation..." In summary, the adiabatic approximation defined within the Born-Oppenheimer model leads to the equation,

+

EXj( ")

(8)

where the dynamic coupling of the electronic states is neglected. Equations 5 and 8 form the basis for the further discussion. It is seen from Equation 8 that the electronic energy plays the role of a potential in the nuclear motion equation. This potential is given by the energy curve depending on the nuclear positions. Assuming that the electronic eigenvalue problem is solved and the general solution E~jl(-Rn) is known, two possible models of vibrations might be adopted. Namely, there is the harmonic oscillator approximation and the model in which anharmonicity is taken into account. In the results presented here the analogy of vibrations of atoms in a molecule to a spring has been used, and following Hooke's law the harmonic model of vibrations has been applied. If the nuclear displacements from the equilibrium positions are small, the potential U(R")= Uji(Rn) is expanded as a Taylor series around the equilibrium position of the nuclei Ro, -

u(e") =

+

1

o -. -. -. . kij(R i - ROi)(R ) - -RO) ) +..,

(9)

i,j

where: (10)


153 m

The electronic energy curve reaches a minimum in R~) and therefore the first derivative of Eel(-Rn) in Equation 9 vanishes. The factor k~ (Equation 10) is interpreted classically as a force constant. Actually, according to Hooke's law, the force is proportional to the displacement, m

F = -k(R - Requ.)

(11)

and the energy is of a quadratic form:

E = ~ k1 ( ] ~ -

-g,.qu.) ,, 2

(12)

In terms of mass-weighted coordinates: r = ~]~-(~n _ R~)

(13)

The potential of Equation 8 has the form (the energy is scaled in order to obtain zero energy for the equilibrium positions),

1

U(r) =~ E kijrirj i,j

(14)

and kij = k~ 1/z. When the normal coordinates are used, the potential has a diagonal form, and Equation 8 is split into a set of uncoupled equations for linear harmonic oscillators. In the language of quantum mechanics, the energy of a harmonic oscillator is quantized, and it is determined by the expression (in units of cm-l),

where v is the vibrational quantum number and v = 0, 1, 2... It is interesting to note that the lowest vibrational energy level is obtained for v = 0, and it has a non-zero value (the so-called zero-point energy). This means that the atoms can never be placed in fixed positions. This result shows the main difference between the quantum mechanical and classical description of molecular vibrations. The total vibrational energy is a sum of energies of 3N-6 distinct harmonic oscillators. Indeed, 3N-6 is the final number of coordinates in Equation 8. Namely, the number of 3N+3 coordinates of the initial equation has been reduced by three through the elimination of internal rotations. Furthermore, the equation of nuclear motion (mainly its potential) has to be invariant under rotations and translations of a molecule as a whole (which is equivalent to the momentum and angular momentum preservation laws). The latter requirement leads to a further reduction of the number of coordinates by six (five in the case of linear molecules for which there are only two possible rotations).

154


The general form of the energy of the harmonic oscillator indicates that the vibrational energy levels are equally spaced. Due to the vector character of the dipole transition operator, the transition between vibronic energy levels is allowed only if the following selection rule is satisfied: AV =+1

(16)

Actually, the dipole moment of a molecule consists of two components, = ae(~,e) + anCR'-'n)

(17)

where de and d. are created by charges of electrons and nuclei, respectively. Thus, the dipole transition moment defined within the Born-Oppenheimer model is determined by a sum of the matrix elements, Dfi

._ D~! + /-~vib ,..,fi

(18)

where the electronic part has the form, D ~ l -=

I

<x:(Te) l

I v,P)>

(19)

and the vibronic component of the transition moment is given by:

Dvib = <x:P) l

l

l

(20)

It is seen from Equation 19 that the electronic transitions take place without changing the equilibrium positions of the nuclei, and the electronic component of the dipole transition moment is non-zero only if there is no change of the vibronic state during this transition, r)vib ,--fi is non-zero only if the transitions occur between the vibronic states within one electronic state, and the selection rules of Equation 16 are derived from the conditions for a non-vanishing matrix element in nvib sJfi 9 The intensity of the observed line is determined by the dipole transition moment. Thus, in order to evaluate the intensities, knowledge of the electron and vibrational functions is required. Furthermore, it is important to take into account that the nuclear part of the dipole operator has the form,

I(~'de(ge) 1

d"(R")=~. [~, 0Rn f .

+eZ i

}R n

(21)

Roi where the extra term, the first derivative of the electronic dipole operator (i.e., the first term of the Taylor series), is caused by the fact that the nuclear vibrations alter the interelectron distances and therefore ~ -= ~e(~,,). In such a situation de(~) in Equations 17 and 19 represents the electronic operator taken for the equilibrium positions of nuclei R(]. It should be mentioned that not all transitions allowed by the selection rule of Equation 16 give rise to the characteristic features of vibrational spectra. The internal vibrations are reflected in the observed spectra only if the

JR Identification of Reactive Organics

155

vibration causes a change of the dipole moment. Therefore a vibrational mode which corresponds to a "symmetric stretch" is "infrared inactive", whereas an "antisymmetric stretch", which alters the dipole moment, is "infrared active". 9 Finally, one may conclude that the frequency of the spectral line absorbed or emitted is determined directly by the energy difference of two neighboring states: Vspectroscopic = Ev~---v+l = ~osc"

Summarizing, one should keep in mind that the theoretical analysis presented here is based on a model which is defined within the following framework: 1. Adiabatic approximation of the Born-Oppenheimer method. 2. Separation of rotational and vibrational states. 3. Harmonic oscillator approximation. This model is known in the literature as the Wilson-Decius-Cross method, and its complete version has been presented in reference 9. It should be pointed out that in the case of a large class of problems of modem quantum chemistry this method is treated as a historical one. However, as is presented below, this model turns out to still be a useful tool in the calculation of IR spectra for the identification of highly reactive organic molecules and reactive intermediates.

2.2. Method of Calculation In order to evaluate the vibrational frequencies defined within the model described in Section 2.1, the second derivative of the electronic energy with respect to the nuclear coordinates (usually the normal coordinates) must be evaluated. There are three different methods of evaluation of the second derivative; namely, it is possible to perform numerical second differentiation, numerical first differentiation of analytical derivatives, or direct analytical second differentiation. These derivatives provide the matrix of force constants which when diagonalized gives frequencies of the IR transitions as well as their normal modes (the degree and direction of the motion of each atom for a particular vibration), l~ The discussion presented in the subsequent parts of this chapter is based on the results of ab initio calculations of the electronic energy of molecular systems. Details about this kind of calculation are described in reference 11. In connection with this procedure, two major questions have to be addressed. The first is the choice of the wavefunction (basis set) to be used in the calculation, and the second whether or not to include electron correlation. In the early calculations of IR spectra of molecules, small basis sets (e.g., STO-3G, 3-21G, or 4-31G) were used because of limitations of computational power. At present typically a basis set consists of split valence functions (double zeta) with polarization functions placed on the heavy atoms (i.e., non-hydrogens) of the molecule (the so-called DZ+P or 6-31G*'basis set). Such basis sets have been

156


found to be of sufficient quality to provide useful results for the interpretation of IR spectra of organic systems. While calculations at the Hartree-Fock level provide results which can be used for the adequate interpretion of the IR spectra of most organic molecules, the practice today is to include electron correlation in the calculation for molecules containing up to 6 to 10 first row atoms. Inclusion of electron correlation provides some improvement in the agreement between the observed and calculated spectra. The most widely used method for including correlation is the M011er-Plesset method limited to the second order (MP2). The calculations described in this chapter were performed using one of the Gaussian programs from Pople's group 12 and the CADPAC program of Handy. 13 The current versions of these programs calculate the second derivative of the energy analytically.

2.3. Reliability of the Model In the previous section a model was introduced by which one can calculate the ab initio IR spectra of molecules and reactive intermediates. In the early development of the computation of vibrational spectra, the main goal was to reproduce with the greatest possible accuracy the vibrational frequencies of very small systems. This was achieved by using very large basis sets and extensive inclusion of electron correlation. In such a way, the frequencies were reproduced within a few wavenumbers of the experimental harmonic frequencies. However, for the calculations to be of interest to the experimentalist, vibrational calculations would have to be performed for molecules larger than di- or triatomics. Unfortunately, even today it is not feasible to reproduce frequencies to this accuracy for molecules with more than four or five atoms (and even then several of these would have to be hydrogen). In addition, the early calculations focused almost solely on evaluation of the frequencies with little or no attention paid to the calculation of the intensities which (as shown below) are of utmost importance in assisting the experimentalist in interpreting IR spectra. It is interesting to note that when vibrational calculations were fi~t included in the major ab initio packages, only frequencies were calculated and not intensities. Presented below are three examples designed to give the reader some idea of what one can expect from the theoretical analysis Of vibrational spectra based on the simple harmonic oscillator model. Systems have been chosen whose structures have been know for many years and, in fact, were known prior to the availability of IR spectroscopy. Hence their spectra have previously been well characterized and these serve as a "test of the method".

Ethylene Ethylene contains six atoms and therefore has 12 possible lines in its vibrational spectrum [(3 x 6) - 6 = 12]. However it is a molecule with relatively high symmetry


157

(D2h), and as a result only five bandsmthose of Blu, B2u, arid B3u symmetrynare observed in its IR spectrum. Of the remaining seven bands, six are Raman active (Ag, Big, and B2g), and one (An) is neither IR nor Raman active. In Figure 1 the experimental IR spectrum is compared to that calculated witti the 6-31 G* basis set, with and without inclusion of electron correlation. It is seen that the spectrum calculated at the SCF level (lower spectrum in Figure 1) reproduces very nicely the overall pattern of the experimental bands. The most intense band is correctly predicted to be the band of second lowest frequency. The relative inteiasities of the other bands are reasonably reproduced by the calculation. The lowest band is predicted to be of very low intensity, the band that occurs experimentally at 1472 cm -1 is correctly predicted to be of weak intensity, and the bands due to C-H stretches are of medium intensity. When electron correlation is included (MP2mtop spectrum) there is significant improvement in the accuracy of the frequencies, but still there remains large error in the predicted frequencies of the C-H stretches. Generally it is found that spectra calculated with the SCF/6-31 G* basis set have an error o f - 1 0 % in the frequencies, and with the same basis set with inclusion of

I I I I I I I I I I I I I I I I

I

I

I

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i

!

I

I

I

I

i

I

I

I

MP2/6-3 !G*

1

EXPERIMENT

II

t SCF/6-3 I G*

t

II i

,

I

t t 3200

t

t

it 2800

t

t

t t 2400

I

tit 2000

I 1800

I

.

I 1600

I

I 1400

I

t 1200

t

I 1000

I

I 800

I

I 600

v(cm")

Figure 1. Comparison of computed and observed IR spectra of ethylene.

158

B. ANDES HESS,JR. and LIDIA SMENTEK

correlation (MP2) the error is reduced to --4-5%. It has been found that the percentage error in intensities is normally greater than that of the frequencies. Errors as high as 100% in absolute intensities are not uncommon, even with inclusion of electron correlation. However, when the frequencies are normalized with respect to the intensity of the strongest band in both, the calculated and observed spectra are often in quite good agreement (as is seen in Figure 1), possibly because of error cancellation. There have been numerous efforts made to "improve" the accuracy of calculated spectra to bring them into better agreement with experiment. These procedures range from simple scaling l~ (a single scale factor, e.g., in Figure 1 one might scale the MP2 frequencies by multiplying all frequencies by a factor of0.95)to elaborate schemes. In the latter case, for example, force constants are scaled by different factors depending upon the origin of the force constant (e.g., a C-H diagonal force constant would be scaled, by a different factor than an H - C - H bend). 14-23 The "scaled" force constant matrix is then diagonalized to give "scaled" frequencies. While the latter scheme is able to accurately reproduce IR spectra of "standard" molecules, it should be realized that this type of scaling has limited utility when one is dealing with highly strained or otherwise structurally unusual systems. 1~The first type of scaling mentioned above may be of importance in helping the experimentalist to locate, for example, an especially weak absorption in a complex spectrum. But generally if one computes both frequencies and intensities, the results are sufficiently useful to the experimentalist without having to resort to scaling. All spectra presented in this chapter are devoid of any scaling.

Ethylene Oxide The next molecule that will be considered is ethylene oxide. As will be seen, the addition of one more atom (oxygen) results in a much more complex spectrum than that for ethylene. The number of vibrations (15) is larger because of the addition of one atom, but the number of IR absorptions is significantly increased over that of ethylene because of the reduced symmetry (C2v)of ethylene oxide. All but three vibrations are IR-active which leads to a total of 12-IR active absorptions. In Figure 2 the observed 24 and calculated 25'26 IR spectra of ethylene oxide are presented. It is seen that both calculated spectra nicely reproduce the pattern of the observed spectrum. This good agreement allows an easy assignment of the symmetries and the origin of all bands in the observed spectrum with the exception of three very weak bands near 1100 cm -1. Even with the inability to assign these three weak bands, if one had made ethylene oxide for the first time, it would have been possible to identify this system with reasonable certainty through comparison of the observed IR spectrum with that of the calculated spectra. It has been known for many years that the IR spectra of isotopomers can greatly aid in the interpretation of IR spectra. The comparison of the observed IR spectra of isotopomers with those calculated can also greatly enhance the ability to assign a structure to an observed

IR Identification of Reactive Organics 'lll'"lll"l'i'IIIl""lll'l

I

I

159 I .....

I"1

I

I

I

MP2/6-3 I G*

I

I'1

I

I

I

j 9.

II

EXPERIMENT

I] SCF/6-31 G*

9

~_

I

I

I

I

I I 3200

I

t

I I,.i 2800

t

t I 2400

t

I

I 2000

I

I 1800

9

I

I ! 600

I

I

I i 400

HI

1

-

I ! 200

I

I, 1000

1,

I 800

I

I 600

v(cm t)

Figure 2. Comparison of computed and observed IR spectra of ethylene oxide.

IR spectrum. Fortunately for the theoretician, obtaining spectra of isotopomers is a trivial task (once the spectrum of the parent molecule has been calculated). To demonstrate the usefulness of the spectra of isotopomers the experimental spectrum of ethylene oxide-d4 is compared with its calculated spectra in Figure 3. Full deuterium substitution is seen to dramatically change the observed spectrum of ethylene oxide. Not only are all bands significantly shifted in frequency, but there is a rather dramatic change in the relative intensities with many of the bands increasing in intensity. These changes are nicely reproduced by the calculated spectra. An analysis of the isotopic shifts of the various bands is presented in Table 1 for ethylene oxide. It is seen that the isotopic shifts of all the observed bands are well reproduced by both methods of calculation with the MP2/6-31 G* results being somewhat better than the SCF/6-31G* results. A comparison of the computed and observed relative intensities, as well as the isotopic shifts of the three bands observed between 1100 and 1200 crn-1, allows an unambiguous assignment of the symmetry of these three bands. The computed spectrum contains three very weak bands (-1300 cm -1 in the computed spectrum) in this region of spectrum. They

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J EXPERIMENT

[I

SCFI6-3 IG**

t

I

t

t I 3200

t

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t

i

]

I I 2400

t

t

I 2000

.

I

t

! 1800

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t 1600

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i 1400

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I 1200

,I

.

jl

.

It

I

I I000

t

t 800

I

1,600

v(cm" I)

Figure 3. Comparison of computed and observed IR spectra of ethylene oxide-d4.

Table 1. Comparison of Computed and Observed Isotopic Shifts in Ethylene Oxide SCF/6-31 G* Symmetry

BI B2 A1 A1 B1 B2 A1 B2 A1 B2 A1 B1

v

887 974 990 1309 1290 1293 1424 1652 1705 3290 3304 3391

Obsd. A

-254 -20 -158 -166 -286 -275 -299 -437 -217 -915 -889 -861

V

808 822 877 1120 1147 1159 1270 1470 1497 2978 3005 3065

160

MP2/6-31G * A

-231 -10 -125 -158 -249 -259 -257 -387 -185 -801 -801 -743

V

843 870 920 1187 1187 1167 1333 1559 1597 3172 3179 3278

D

-242 -14 -141 -161 -266 -250 -269 -414 -214 -884 -857 -832


161

each have a different symmetry (B l, B 2, and Al). Examination of the normal modes of the computed spectra (MP2/6-31G*) allows one to assign the A 1 band in the parent molecule (1187 crn-1) to the 1026 cm -1 A 1 band in the isotopomer. The relative intensity of this band is predicted to increase dramatically on deuterium substitution (from 99% for indan. 47 The bromination of annelated thiophene derivatives appears to quite substratespecific. While bromination of 3,4-dimethylthiophene (54) leads to substitution, the bromination of cyclobutathiophene derivatives (54-56, Figure 7) 53 leads exclusively to addition products. It has been speculated that annelation induces localiza-

Mills-Nixon Effects?

221

Table 5. Halogenation of Benzocycloalkenes Substrate

%r

%~

[,~A,,

14

86

6.1

a

i

71 75 58 20 70

2.4 3 1.4 0.2 2.3

b c

j J

d

J

e

k

~

29 25 42 80 30

a

i

56 61 48 70 93

1.3 1.6 0.9 2.3 13

b

J

c

J

d

J k

~

44 39 52 30 7 22 16 27

78 84 73

3.5 5.2 2.7

b e d

J

-

100

-

e

k

125.4

114.7

168.5

128.8

159

d,e

46

~ ~

151.9

121.7

156.5

124.39

159.6

g

~

B3 122.8

110.0

169

B4 145.5 143.4

117.3

160.2

67

B4 143.1

118.9

158.7

68

B5 142.1 B4 142.9

122.9

157.3

65

~

B6 135.6 69

(~~

141.9

120.0

155.1

70

(~~

B5 141.5

124.8

153.8

B6 134.7 129.3

134.1

152.3

71

154.0

72 6~

n

n

n

m

(continued)


229

Table 8. (Continued)

Structure

~(CI,2)

~(C3,6)

Ij(13C3.H)

8(C4,5)

1j(13C4-H ) Note

73

~

161.0

f

74

~

C5 156.8

f,l

75

~

C4 159.4 156.9

f,l f

76

-~

156.1

f

77

( ~

!32.5

h

k__2

78

~

137.5

h

79

~

138.5

h

8O

~

134.2

i

81

~

136.2

j

82

~

135,9

k

Notes:

(a) Chemical shifts downfield from TMS. Aromatic carbon; B4, B5, B6 = bridgehead carbon fused to four-, five-, or six-membered ring. (b) Ref. 83. (c) Ref. 84. (d) Ref. 78. (e) Ref. 79. (f) Ref. 80. (g) Ref, 81a. (h) In CDCI3, Ref. 85. (i) Ref. 86. (j) In CDCI 3, Ref. 87. (k) Ref. 88. (1) JcH where C4 and C5 are C-H bonds ortho to four- and five-membered rings, respectively. (m) Ref, 68b. (n) With cyclopropa- and cyclobutaannelation designated as 1-2 positions, and 3-4 positions, the chemical shifts (coupling constants) for positions (1--6) are (1) 126.0, (2) 119.6, (3) 135.9, (4) 148.0, (5) 121.0(162 Hz), (6) 112.4 (166 Hz).

An important correlation with regard to m-deactivation is revealed by plotting JCH versus the pKa of a series of pyridines with the nitrogen in the analogous site for which JCHwas derived. 16bAs Jcrl increases, the pKa value decreases. Given that the larger JCH also correlates with greater s-character in the bonding orbital, this implies that a change in hybridization correlates to a loss of electron density in the position. As such, the NMR coupling constant data supports the rehybridization model for the deactivation of the o~position toward attack by electrophilic reagents. JCH data are unable, however, to elucidate any bond fixation effects.

230

NATIA L. FRANK and JAY S. SIEGEL

Carbon-Carbon Couplings. In principle, the Cipsoto Ca coupling could reveal changes in bond order comparable to those required for bond fixation. Among benzocycloalkenes a "standard" value for Jcc of 58 _+ 2 Hz can be established; slightly higher than in benzene itself (56.0 Hz). Cyclopropabenzene, however, deviates significantly from the norm (Jcc = 87 Hz). 89aThe deviation in cyclopropabenzene is explained by the unusual bonding in cyclopropane which induces rehybridization at the Cipso-position. A recent study on cyclobutabenzene reveals a small but systematic trend in the C-C coupling constants. 89b Hydrogen-Hydrogen Coupling. Three-bond or vicinal couplings are affected by (1) the torsion angle between C-H bonds, (2) the C-C bond order, and (3) electronegativity of neighboring group atoms. 9~ For planar aromatics the torsion angle is not a significant variant. Jnn couplings decrease between H1 and H2 along the series 83, indan, cyclobutabenzene, and cyclopropabenzene (Table 9). 90-96 The increasing electronegativity of Cipsoshould cause a decrease in HI-HE but should not alter H2-H3. Fixation of double bond character between Cipso-Cl Table 9. H-H Coupling Constants in Benzocycloalkenes

Ha

Compound

3J12

4,/13

5J14

3J23

Conditions

O

7.6

1.4

0.7

7.6

b

5

(~

7.6

1.4

0.5

7.5

a,c

62

(~

8.1

1.6

0.3

7.0

c,d

58

(~

6.0

0.3

1.8

7.6

e,c

38

(~

7.4

1.0

1.0

7.8

e,c

1

(~

7.6

1.2

0.5

7.2

e,c

83

~

7.6

1.3

0.6

7.3

e,c

40

~

7.3

1.2

0.6

7.6

f,c

Notes: Conditions: (a)Calculatedfromtoluene,assumingadditivityofcouplingconstants.Co) Ref.92. (c)Ref.93.(d)Ref.9lb. (e)CI~I3, 100MI-Izand220MHz.(f) CS2,60MHz.


231

would also cause a decrease in the vicinal coupling between HI-H2, but in addition, it would cause an increase in the vicinal coupling between HE-H3. The vicinal coupling between H1-H2 in 83 is roughly 1.51 _+ 0.2 Hz larger than that in cyclopropabenzene, and the vicinal coupling between H2-H3 is slightly changed (0.3 +_0.2 Hz). On the basis of the change in JHH with decrease in ring size, one can conclude that Cipso is effectively more electronegative in cyclopropabenzene than in tetralin (Table 9). This increase in electronegativity at Cipso is more indicative of rehybridization than of bond fixation. Long-range meta- and para-splittings are very small and their differences are difficult to interpret reliably given no more than 0.5 Hz precision. 93-96 In this series, four-bond couplings range from 0.3 Hz in cyclopropabenzene to 1.2 Hz in indan, and five-bond para-couplings range from 1.9 Hz in cyclopropabenzene to 0.5 Hz in indan (Table 9). These changes are consistent with common angle distortions and proximity effects without any need to resort to bond fixation. Thus, long-range proton couplings provide no compelling evidence for bond fixation. All the trends for these hydrogen-hydrogen couplings are in line with the general paradigm that strained annelation induces rehybridization at Cipso. The variations in the data are on the borderline of being significant, but it is clear that in no case is bond fixation necessary to explain the data.

Aromatic Chemical Shifts Carbon Chemical Shifts. In general, because carbon chemical shift values are dominated by the paramagnetic term, they are sensitive to a variety of factors such as hybridization, inductive, electric field, "steric", resonance, and neighboring anisotropic effects. 97 In many instances trends can be found for a series of compounds, but interpretation of the results is difficult. Therefore, it can be more enlightening to simply look for where changes in chemical shift do and do not occur and correlate them to better characterized data. It is revealing that changes in the chemical shift of Cipso in the benzocycloalkene series parallel the Calkene shifts in simple cycloalkenes (see Table 8). 78-88'98 This parallel behavior indicates that the aromatic ring causes no special effect on the chemical shift All the evidence we have seen so far points to the ct-carbon as a site where significant changes are occurring across the series tetralin to cyclopropabenzene while the 13-site remains relatively constant. It is not surprising, therefore, that the lac chemical shifts for the (z- and 13-site show this same relative behavior; i.e. the chemical shift of the (~-site decreases (ca. 15 ppm) monotonically across the series (2, 1, 38, 58) but only a small change is detected for the 13-site (ca. 4 ppm). All 13C chemical shift data are thus consistent with the common picture of strain induced rehybridization. Proton Chemical shifts. Hydrogen chemical shifts are affected by a number o f factors, but ring current and heteroatom effects are particularly noticeable.

232

NATIA L. FRANK and JAY S. SIEGEL Table 10. Aromatic Proton Chemical Shifts in Annelated Aromatics

3

5

7.05 c

1

38

H3,6 - 7.01 a

2

H3,6 - 7.07 a

H3,6 - 6 . 7 6 a

H3,6 - 7.15 a

58

H4,5 - 6 . 9 3

H4,5 - 6 . 9 9

H4,5-6.91

H 4 , 5 - 7.19

47

48

46

84

7.1 b

7.05 b

6.92 c

7.00 d

7.21

72

85

65

66

67

69

6.90 e

7.15 f

6.85 h

6.64 e

6.91 e

7.08 e

CC:

101

~

273

283

270

279

267(380)

272(400)

222sh(4169) 260(295)

b,m

a,j

80

~

81

~

262(242)

a,k

82

~

265(370)

a,I

Notes:

(a) in 95% ethanol.(b) Pentane.(c) Hexane.(d) Cyclohexane.(e) Ref. 80. (f) Ref. 85. (g) Ref. 116.(h) Ref. ll0b, 68a.(i) Ref.66a.(j) Poef.86. (k) Ref.87a.(1)Ref.88. (m)Ref. 113.(n) Ref.66c.(o) Ref.68b. (p) Ref. 109. 236


237

[a,c,e] trisannelations have absorptions around 270 nm for monocyclic anneiations and 260 nm for bicyclic annelations (Table 11). 85-87a'112-1~4The ~max for trisbicyclo[2.2.2]octabenzene, trisbicyc1012.2.1]heptabenzene, and trisbicyclo[2.1.1 ]hexabenzene are 260, 262, and 265 nm, respectively. The HOMO/LUMO gap in benzene is relatively insensitive to in-plane angular distortion of the hydrogens. Thus, angular distortions and bond fixation are not likely to be manifested in the UV spectrum of trisbicyciohexabenzene. A strong bathochromic shift is seen in the alkene series by the difference between the UV spectrum of norbornene (197 nm) and bicycl0hexene (217 nm). One possible explanation for this invokes the interaction between the cyclobutane orbitals of the bicyclohexyl fragment and the n-system. 1~5This interaction has been estimated to be destabilizing to the HOMO when the n-system is ethylene-like and stabilizing when the n-system is butadiene-like. Table ~2. Ionization Energies of Annelated Benzenesd

5

57

64

39

2

1

38

58

I V,1

8.57

8.51

8.42

8.44

8.44

8.46

8.66

8.82

i V,2

9.10

9.0

8.91

8.96

8.96

9.04

9.20

9.48

72

69

67

66

76

75

74

73

I v,1

8.07

8.13

8.i9

8.22

8.29

8.24

8.38

8.35

I v,2

8.68

8.70

8.77

8.91

8.57

8.77

8.71

8.69

95

77

78

79

7.8

7.7

7.8

8.1

9.5

10.1

10.0

I.p.c

I v,1 I v,2

Notes: (a) Ref. 117b. (b) Ref.109. (c) Ref. 85. (d) Verticalionizationpotentialsreportedhere.

238


PES: Ionization Potentials The ionization potentials of cycloannelated, biscycloannelated, 1~ and triscycloannelated 85 benzenes have been investigated for ring strain effects (Table 12). Comparison with the ionization energies obtained for simple cycloalkenes 117 reveals a similar trend in both annelated benzenes and the analogous cycloalkenes. As ring strain increases, a shift to higher ionization potential results. The magnitude of the shifts are largest in cycloalkenes (8 --- 0.9 eV) and smallest for triscycloannelated benzenes (8 ~ 0.2 eV). Hyperconjugative interactions between the benzene (or alkene) x-orbitals and the cycloalkene t~-orbitals are responsible for the variation observed. It has been suggested that if partial bond localization existed in triscycloalkenobenzenes, shifts on the order of only ~0.1 eV would occur. 85 Thus, any contribution from bond localization would be hidden by effects due to hyperconjugation. 3.3.

Redox Potentials and ESR

Polarography and ESR data provide important information about the energies and electron distribution of the excited states of annelated benzenes. 118-121 By incorporating rehybridization effects into the Htickel model of electron densities, a correlation between ring strain, experimental spin densities, and redox potentials is obtained for a series of naphthalenes and naphthoquinones. 119'12~These studies provide further support for ring-strain induced rehybridization. The half-wave reduction potentials for a series of annelated 1,4 naphthoquinones (102-106) increase upon alkylation, and decrease as ring size decreases (Table 13). 119 The more cathodic reduction potentials of 2,3-dimethylnaphtho-l,4-quinone (106, 0.846 V) and 1,2,3,4-tetrahydro-9,10-anthroquinone (105, 0.854 V) as compared to 1,4-naphthoquinone (0.685 V) are expected from inductive electron donation of alkyl groups. A decrease in reduction potential from 105 to 2,3cyclobutanaphtho-l,4-quinone (103) (0.695 V) as ring size decreases is observed such that the reduction potential of 103 is only slightly higher than the parent 1,4-naphthoquinone.

The trend toward decreased reduction potential with ring size and the similarity of values for the parent and cyclobuta-annelated naphthoquinone can be understood by taking into account strain induced rehybridization . The strain present in cyclobutane induces a rehybridization primarily at the position ortho to annelation toward greater s-character, increasing the effective electronegativity at this position. Incorporation of this model into the Htickel framework leads to a modification of the coulomb integrals to include increased electronegativity at the ortho-position (ct = o~o + 0.213) and decreased electronegativity at the ring fusion carbons (tx = ~o - 0.21~).119Using parameters derived from ESR data in conjunction with perturbation theory predicts no change in the energy of the lowest unoccupied molecular orbital of 1,4-naphthoquinone upon cyclobuta-annelation. This prediction results from essentially identical Htickel coefficients at the ortho-(C2 = 0.374) and


239

Table 13. Reduction and Oxidation Potentials a Structure

-El/2, (dme)V

Structure

10 2

O

0.685

10 7

10 3

o 0

0.695

88

0.748

108

0.854

109

0.846

110

~

~

-El/2, (vs. sce)V

+El/2, (vs. sce)V

2.58

1.70

2.62

1.59

2.66

1.55

2.68

1.54

o 104

O

~

o 105

~

106

o O

~

("~

A

,(y.)

2.65

o

Note: aRef. 119.

fused-position (C1 = 0.332). As ring size increases, hybridization effects lessen and the Coulomb integrals for the ortho-position approach that of a standard value, while the Coulomb integrals at the fused position increase due to simple alkyl donation. A steady increase in the reduction potential is therefore observed. 119The changes in half-wave reduction potentials and oxidation potentials for annelated naphthalenes are also correlated by incorporation of rehybridization into the Hfickel framework. 119 Further support for the rehybridization model was provided by analysis of the spin densities of annelated 1,4-naphthoquinones, naphthalenes, and cyclobutabenzene radical anions by ESR. 12~The hyperfine coupling constants at the methylene position decrease as ring strain increases. Calculation of spin densities using the rehybridization model leads to an excellent fit of experimental spin densities, while calculation of spin densities using either the Coulson-Crawford hyperconjugation model or INDO do not accurately correlate experimental values. Such results suggest that rehybridization can account for the observed changes in spin densities.

240


4. GEOMETRIES OF ANNELATED BENZENES: THEORY VERSUS EXPERIMENT Direct observation of molecular structure is the most conclusive way of addressing the question of bond fixation. Bond fixation in its purest sense could be invoked in the distortion of benzene (D6h) to cyclohexatriene (D3h); the expected bond lengths would be uniformly 1.395/~ in the former and alternately 1.33/1.50 A in the latter. These two situations define the extremes to which all other structures must be compared. 122 By symmetry, any annelated benzene with a threefold axis will have two sets of bonds that must be different at some level of accuracy, even in the absence of chemical significance; for lower symmetries each bond length may be different due to independent chemical phenomena. For our discussion, it is important to define a parameter and a threshold value for a structure tO be considered "significantly bond-fixed". For symmetric trisannelated benzenes, we can define uniquely two sets of bonds, exo and endo to the annelations, and a parameter 5, defined as the difference in bond lengths" ~5- doexo(,~)-dendo(A), (Figure 13) In the range of the aromatic bond length (1.395 A) a difference of 0.05 A ('5 pm) corresponds to a change in Pauling bond order of roughly one-quarter of a bond; this value is also large enough to rule out any no'rmal error due to precision of the diffraction experiment. At the cyclohexatriene limit 8 = 1.50-1.33/~ = 0.17,4, (17 pm). For less symmetrical arenes an analogous partitioning allows the derivation of 1~51values. In addition, given the wealth of data supporting rehybridization as a ramification of severe angle strain, we must remain aware of structural effects which occur independent of the aromatic nature of the molecules we examine. 4.1. Experimental Structures Benzocycloalkenes manifest ~5under 2 pm; there is no evidence for bond fixation in any of these compounds (Figure 14; see Table 14). 123-126The primary distortion mode of strained cycloannelation affects the internal angles of the benzene ring. As ring size decreases, the internal benzene angle at the ipso-position (~) and the internal angle at the position meta to ring fusion (7), increase from 120.0 ~ and 120.4 ~ in tetralin (2) to 124.0 ~ and 123.0 ~ in cyclopropabenzene (58), respectively (Figure

( f - ~ endo /_\

.... /

Figure 13. Criteria for determining the extent of bond length alternation: ~5= d,,,o(A) - dendo(A).


241

58

38

1

2

87 n=l 88 n=2

112

6i

73

66

65

Figure 14. Mono- and bisannelated benzenes.

15). Accompanying this trend is a monotonic decrease in the internal angle at the adjacent position (13), from 119.6 ~ in tetralin to 113.0 ~ in cyclopropabenzene (Figure 16). The bond lengths at the ring fusion are demonstrably shorter in cyclopropabenzenes. This has caused some discussion of bond fixation in cyclopropabenzene; however, parallel effects are seen for cyclopropane and cyclopropene among cycloalkanes and cycloalkenes, respectively. 122 Thus, the shorter bonds in cyclopropabenzene are expected or rationalized by simple rehybridization independent of any bond fixation. The bisannelated compounds have two isomeric forms, [a,c] annelation (meta) and [a,d] annelation (para) (Figure 14; see Table 15). 124c'125c-fNone of these forms display a ~i greater than 3 pm. The [a,d] bisannelation places the strained tings in a position to amplify angular distortions, but attenuate bond fixation. Rehybridization effects in strained cycloannelated benzenes are roughly additive, hence the geometries of [a,d] bisannelated aromatics show particularly severe angular distortions at the position adjacent to ring fusion. The proximal angles ~ and 7 widen up to 125 ~ With a commensurate closing down of the remote angle 13 to 109 ~ in [a, d] -cyclobutacyclopropabenzene; these large angular deformations are consistent with rehybridization. 124cAs with the monocyclic structures, the cyclopropa systems bear the short bonds predicted from the rehybridization model.

Figure 15. The structural parameters used to define significant bond lengths (a-d) and bond angles (~-7) in monoannelated benzenes.

242

N A T I A L. F R A N K a n d JAY S. SIEGEL

Angle Dependence on Ring Size 130

I

'

'

'

I

,

,

,

I

,

,

,

I

,

,

I

,

127-~124-4,

, , , . 9 ~ +

13

o

y

-

-

..4

o.

121

'-

118-

-"

~

~

$

$

w '

'

~

_ sp 2

.4

115-112109

"

~ ~

.

2

,

,

4

'

i

'

'

'

6

'

8

I

'

'

'

10

I

sp 3

12

Figure 16. T h e d e p e n d e n c e o f i n t e r n a l a n g l e (z, 13, a n d y o n r i n g size, w h e r e n is d e f i n e d as the a n n e l a t e d r i n g size, i.e., n = 3 for c y c l o p r o p y l . As r i n g size d e c r e a s e s , the a n g l e [3 d e c r e a s e s , w h i l e bridization.

o~ a n d y i n c r e a s e , s u g g e s t i n g

ring-strain

induced

rehy-

If annelation by simple strained cycloalkanes induces significant bond alternation in benzene, this effect would be maximized in trisannelated benzenes, where D3h symmetry is enforced (Figure 17). Only recently have the strained trisannelated benzenes been structurally characterized (see Table 16). 125fSymmetrical trisannela-

113

99

78

97

Figure 17.

Trisannelated

79

77 benzenes.


243

80

114

Figure 18.

115

Structurallycharacterizedbicycloannelatedbenzenes.

tion minimizes internal angular distortions; the internal angles now being all roughly 120 ~ Again no appreciable bond fixation appears among the cycloalkyl series. Indeed, the structures of triscyclobutabenzene (79) and perfluorotriscyclobutabenzene (79F) exhibit a 15of less than 3 pm. 12sf-h Less is known about bicyclic annelations than monocyclic ones, although there is evidence that this area is rich in new chemistry. Recently, the structure of 114127 was determined and found to have a small degree of bond alternation (Figure 18; see Table 16); an averaging of exo- and endo-bond lengths of the central ring leads to a bond alternation on the order of 2.2 pm (eXO(avg) = 1.380/~,; endo(av.) = 1.402 /~). The crystal structure of trisbicyclo[2.2.2]octabenzene (80) 12s exhibits a bond alternation of only 1.5 pm in the aromatic ring. The crystal structure of one maverick structure, [a,c]bisbicyclo[2.2.1]oxaheptabenzene (115), 129 displays a ~i of 5 pm.

TMS TMS

TMS_4~~TMS TMS TMS 116

117

118

119

,,••t-Bu 120

Figure 19.

121

122

Benzocyclobutadienederivatives116-123.

123

244


This suggests that bond alternation in the trisannelated system would be quite significant. The structures of two related series of compounds show significant differences in dexo and dendo: 1) starphenylene (116), angular[3]phenylene (118), biphenylene (122); 2) triphenylene (124), phenanthrene (125), naphthalene (126) (Figure 19, see Table 14). The bond lengths exocyclic to annelation in 116 and 118 are shortened to 1.34 and 1.35 ,~,, respectively, while the endocyclic bonds are lengthened to 1.49 and 1.45 ,~,, respectively. 13~ These bond lengths correspond to large degrees of bond fixation on the order of 16 pm and 10 pro, respectively. In the starphenylene series dex o < dendo, whereas and in the triphenylene series dendo < dex o. To elucidate the nature of this behavior, bond fixation mechanisms can be dissected into r~ and ~ effects with the aid of computational studies.

4.2. Computational Studies The earliest theoretical treatments of the Mills-Nixon effect were carried out by Longuett-Higgins and Coulson, TM and Sutton and Pauling, 8 with opposing results. Longuett-Higgins and Coulson134 calculated by a simple MO treatment the geometry, bond orders, and electron densities in the aromatic ring of indan. A slight bond alternation in the reverse direction to that predicted by Mills and Nixon was suggested. Sutton and Pauling examined the two resonance structures in indan and tetralin by simple valence bond wave functions, 8 and concluded that simple annelation could not induce a large enough perturbation to overcome the great resonance stabilization of benzene. The advent of modern computational methods allowed this phenomenon to be reinvestigated in significantly finer detail. In order to expand on the experimental structural data on the annelated benzenes, a large number of computational investigations were undertaken. 135-142A body of literature in favor of strong bond fixation developed from studies done with semiempirical hamiltonians, or minimal to low-level split-valance ab initio basis sets. Unfortunately, basis sets below split-valance ab initio with polarization [6-31G(D)] were inadequate to produce experimentally consistent results. As a result, the question of bond fixation became a battleground for some computational chemists. High level Hartree-Fock (HF) basis sets and local density functional (LDF) computations involving electron correlation ultimately gave excellent experimental agreement and provided great insight to the problem.

The Stanger Demonstration and o-Effects In an insightful investigation, Amnon Stanger demonstrated that at 3-21G(D) the structure of triscyclobutabenzene was not strongly "bond-fixed," but that an artificially constrained benzene structure, with CCH angles held at 90 ~ was. 137Stanger's distorted benzene displays bond lengths of 1.32 and 1.52/~ (3-21G). This work clearly established a correspondence between angular distortion and bond fixation


245

....

I ....

l , , , , l , , , , I , , , , l ~ , , , I , , , , l , , , ,

0.25 1 0.2-

%

".....

0.15 _

oo

0.1-

_

_

_

0.05 0-

_

_

-0.05

,

85

,'",

,

1 ,

90

,

,

,

1 ,

95

,',

,

1,

100

,

,,"1,,

105

,

,

i

,

110

,

,

,

i

,

115

,',

,

i

,

120

,',

,

125

CCH Angle / o

Figure 20. The dependence of bond alternation (6) in benzene on the CCH angle, as calculated at 6-31G(D)/MP2 level. Ref. 112.

in benzene. A full structure correlation of CCH angle versus 5 for benzene from 120 to 90 ~ at 6-31G(D)/MP2 shows a cubic dependence with a limiting value of 8 = 15.7 pm; with CCH = 90 ~ the bond lengths are 1.35 and 1.48/~ (Figure 20). 112 Thus, even at higher basis sets and including electron correlation the angular dependence of [i is maintained. 112 Reinvestigation of the trisannelated benzenes with ring sizes 6-3 (77, 78, 79, 113) [at 6-31 (G(D) and LDF levels] reveals that there is no systematic increase in bond alternation 05) !22 In the case of triscyclobutabenzene, ~5is predicted to be ca. 2 pm, and for tricyc!opropabenzene there is a uniform contraction of all the aromatic bonds to i.35/~ making 8 virtually 0. These calculations created a paradox between the distortions of benzene and the minimum energy structures of strained benzene derivatives. Computations on the bicyclo series (80-82, 127-136) shed some light on this problem (Figure 21, see Table 16). The structures of tris[2.2.2]bicyclooctabenzene (80), tris[2.2.1 ]bicycloheptabenzene (81), and tris[2.1.1 ]bicyclohexabenzene (82) display an increase in [i with decreasing ring size. When these bicycloannelated benzenes are compared to the Stanger curve at similar angular distortions, identical values of 8 are found. Thus, the bicyclo series seems to express the pure angular effect on bond fixation. The largest value of ~i is predicted for trisbicyclo[2.1.1]hexabenzene; 6 pm at 63 1G(D)/]V[P2.112'122'151

246


80

81

127

X

128-BH 129-CH2 130.NH 131.O

Figure21.

132-B 82-CH 133.N

134-BH 135-NH 136.O

Bicycloannelated benzenes.

A novel correlation between 8 and the JC-H one-bond coupling constant for the carbon attached to the aromatic ring connects the monocyclo and bicyclo series. It has long been recognized that s-orbital character, expressed as one-bond C-H coupling constants at sp 3 carbon is related to distortions of the tetrahedral geometry and quite refined correlations of JC-H as a function of degree of distortion have been forwarded. 143'144Conversely, coupling constant changes among a series of similar hydrocarbons can be implemented as a gauge of degree of tetrahedral distortion. A plot of 8 as a function of JC-H over the six mono- and bicyclic annelated benzene studied reveals a demystifying correlation which points to the s-character of the o~-carbons as an important, though not unique factor leading to bond-localized geometries in annelated benzenes. 112'122'151 Ideally, the Stanger demonstration should be an expression of the stretch-bend interaction in the D3hB E normal mode in benzene. 145 Thus, it is instructive to examine the nature of the stretch-bend interaction in typical monocyclic and bicyclic alkenes. 145 The IR spectra of monocyclic and bicyclic alkenes shift hypsochromically for exomethylenes and bathochromically for endocyclic alkenes with decreasing ring size. 146 This effect has been shown to be dominated by mode-coupling for the monocyclic series, but consistent with a pure angular effect or the bicyclic series. 146'147 Further investigation in this vein is likely to resolve the paradox initiated by Stanger's work.


247

Electronegativity Versus Hiickel Effects Bond angle/bond length relationships do not readily account for the bond localization noted for starphenylene (116) and triphenylene (124). A cursory examination of the structures reveals that in these cases the annelated cycles can have an aromatic character of their own. 122'136 In starphenylene the cycle would contain four electrons and be antiaromatic, whereas in triphenylene the cycle would have six electrons and be aromatic. From the simple Htickel rule, the antiaromatic cycle should be disfavored. In such a case, structural stabilization can be accomplished by greater contribution from the resonance form that has single-bond character at the endo-bond. The reverse is expected for the aromatic cycle. This model is simple, predictive, and accurate! In tricyclopropabenzene (113) it has been shown that no significant bond fixation is present, and no direct Htickel character can be assigned to the annelated cycle. 122 In trisboracyclopropabenzene (141), where an empty p-filled x-interaction is enhanced, bond shortening of the endocyclic bond to 1.362 A and subsequent lengthening of the exocyclic bond to 1.450/~, (8 = 8.8 pm) is observed at the 6-31G(D) level (Figure 22; see Table 14).122 On the contrary, an energetically unfavorable filled p-filled x-interaction would cause a decrease in rc character of the endocyclic bond, as is observed in the geometries of trisazacyclopropabenzene (142): 1.489/~ and trisoxacyclopropabenzene (143) 1.386/~ (6-31G(D) consistent with the significant role of ~-x-interactions in causing bond fixation in benzene. 122 In accordance with the Htickel paradigm, 15is positive for the NH derivative and negative for the B H derivative. Effects on benzene geometry stemming from substituent electronegativity are dominated by changes in internal angles. 148 Nonetheless, the idea that the electronegative character of an annelation can induce bond fixation has been forwarded. 135a'eIn order to resolve the situation, a direct test of x-donor character (i.e.

137

138

B---'-~--~B

141

139

N

140

Si

142

143

i

144

Figure22. Heterocyclopropabenzene derivatives.

248


Structural Data of Annelated Benzenes~

Table 14.

Bond Lengths

Bond Angles

a

b

c

d

a

e

Method

1.399

i.393

1.384

1.383

119.6

120.1

120.2

117.2

exp a

f

1.399

1.392

1.390

1.383

120.0

119.6

120.4

115.9

exp a

g

1

1.393 1.393

1.382 !.388

1.391 1.386

1.381 1.384

120.8 120.6

118.2 118.6

120.9 120.8

108.0 110.4

exp a exp a

f g

38

1.391 1.383 1.383 1.387

1.385 1.391 1.386 1.376

1.400 1.395 1.396 1.399

1.399 1.392 1.389 1.392

122.3 122.6 122.6

116.0 121.7 1 1 5 . 1 122.3 115.2 122.2

93.5 93.8 93.4

exp exp a exp a 6-31G

h f g i

2

58

~

~1

Ref

1.334

1.363

1.387

1.390

124.5

113.2

122.4

63.6

exp

j

1.342 1.355 1.332

1.394 1.379 1.370

1.407 1.384 1.440

1.390 1.388 1.395

126.0 124.0 124.6

109.2 113.0 113.0

124.5 123.0 122.3

63.2 61.8 63.5

exp a exp a 6-31G(D)

f g k,l

120

1.435 1.426

1.360 1.339

1.444 1.438

1.376 1.358

122.6

155.6

88.5

3-21G 3-21G

m,n o

121

1.401

1.367

1.445

1.370

122.1

115.6

122.2

89.1

exp

p

122

1.432 1.426 1.418

11372 1.428 1.372 1.423 1.359 1.418

1.370 1.385 1.377

122.5 122.6

115.0 115.2

122.5 122.2

90.0

exp exp 6-31G

q r s

137

1.374

1.414

1.360

1.434

122.2

115.6

122.2

62.2

6-31G(D)

k

138

1,369 1.327

1.329 1.352

1.467 1.427

1.353 1.384

126.5

110.5

123.0

60.37

6-31G(D) 6-31G

k t

139

1.311 1.310

1.336 1.347

1.448 1.434

1.369 1.383

128.2

108.4

123.4

62.60

6-31G(D) 6-31G

k t

140 87

1.372 1.368

1.415 1.337

1.357 1.437

1.465 1.439

124.9

114.7

120.5

62.9

6-31G(D) exp b

1 u

88

1.407

1.354

1.423

1.432

122.6

117.3

120.1

93.0

exp b

v

tSee Table 16 for notes and references.


249

Table 15. Structural Data of Bis-Annelated Benzenes t

d Bond Lengths

B o n d Angles a

b

c

d

7

~.

Method

Ref.

112

1.361

1.373

1.389

1.424

117.9

o~

125.9

[~

116.1

62.8

MP2/6-31G*

w

61

1.351

1.363

1.368

1.401

120.0

116.3

123.7

63.1

exp b

x

118.1

124.3

117.7

93.8

exp b 6-31G b

x i

3-21G

o

104.5

exp

y

exp

z

63.4

exp b

aa

94.0

exp b

bb

93.4

exp 6-31G

h i

3-21G

o

e = 1.384 f = 1.393 73

1.402 1.397

1.385 1.367

1.392 1.378

1.413 1.408

120

1.484

1.313

1.326

1.487

116

1.412

1.357

1.374

1.439

118.4

123.7

117.5

119

1.449

1.345

1.348

1.446

117.9

124.6

117.8

65

1.349

1.385

66

1.35 1.399 1.392

1.38 1.394 1.386

1.392

1.381

124

1.405

1.402

126.3

109.2

126 124.0

108 112.1

124.4

tSee Table 16 for notes and references.

Htickel effects) versus electronegative character can be designed for substituted tricyclopropabenzene. The structures of 113 and 141-144 have been computed at 6-31G(D) and show a range of ~5values (Table 16). 112'122A plot of 15versus electronegativity of the X group display a nonmonotonic relationship with poor ability to be fit to a linear approximation. The same 8 values plotted against (~R for the group X gives a monotonic increasing relationship between 8 and (~Rwith a linear correlation of ca. 0.95. The failure of the electronegativity versus the Htickel postulate is clearly illustrated by the effect predicted by the two theories for CH 2 versus CF 2. The substitution of fluorines for hydrogen should increase the net electronegativity of the group and diminish its OR character. On the basis of these changes in electronic character, the electronegativity postulate predicts 8 should be positive, but the aromatic postulate predicts 8 should be negative. The computed value of 8 is negative. A further test of the origin of electronic effects can be evaluated when carbon is replaced by boron or nitrogen in the bicyclic series. 112 For this series, the residual

250

NATIA L. FRANK and JAY S. SIEGEL Table 16,

Structural Data of Tris-Annelated B e n z e n e s

C.9 Bond Angles a

b

1.385 1.392

1.410 1.375

1.393

1.385

1.406 1.391 1.408 1.413

1.368 1.378 1.380 1.390

79F

1.386 1.383 1.370 1.406

1.392 1.391 1.396 1.396

120.1 119.2

119.9

99

1.363

1.372

1 . 4 1 1 1.385 121.9

117.7

113

1.359 1.378

1.356 1.370

116

1.494

1.335

117

1.524

114

1.418 1.407 1.402 1.406 1.410 1.404 1.408

81

82

77 78

79

80

c

d

Bond L e n g t h s o~

~

~

E

Method

Ref

6-31G(D) exp

cc dd

6-31G(D)

cc

6-31G 6-31G(D) LDF expe

i cc cc x,ee

93.6 94.3 93.6

exp exp 6-31G(D) LDF

ff gg cc cc

63.2

expb

x

63.0 62.8

6-31G(D) MP2/631G(D)

cc cc

88.3

expc

hh

1.309

3-21G

o

1.363 1.369 1.380 1.380 1.390 1.388 1.393

112.7 112.8 112.6 112.9 112.5

6-31G(D) drool exp d 6-31G(D) dmol dmol(fc) exp d

k k ii k k k jj

1.416 1.417 1.415 1.417

1.364 1.374 1.374 1.379

106.2 6-31G(D) 106.1 dmol 106.3 dmol(fc) exp

k k k kk

1.440 1.442 1.430 1.428 1.433 1.431 1.438

1.344 1.340 1.373 1.366 1.356 1.353 1.349

101.8 101.8 102.1 102.0 101.8 102.5 102.3

121.2

93.4 120

120

93.0

120.4

6-31G(D) k TZV(D) k MP2/631G(D) k dmol k dmol(fc) k nonlocal k exp 11 (continued)

251

Mills-Nixon Effects? Table 16. (Continued) a

b

127

1.429 1.411

1.353 1.370

111.3 112.1

6-31G(D) dmol(fc)

k k

128

1.407

1.369

109.5

6-31G(D)

k

129

1.433 1.424

1.384 1.364

105.5 105.8

6-31G(D) dmol(fc)

k k

130

1.436

1.341

104.0

6-31G(D)

k

131

1.438

1.340

103.0

6-31G(D)

k

132

1.517

1.337

94.5

6-31G(D)

k

133

1.449

1.332

103.0

6-31G(D)

k

134

1.484

1.474

111.2

6-31G(D)

k

135

1.333

1.444

97.6

6-31G(D)

k

136

1.451

1.338

99.2

6-31G(D)

k

141

1.362 1.403

1.450 1.431

62.5 61.9

6-31G(D) MP2/6-31G(D)

cc k

142 (D3h)

1.489

1.280

6-31G(D)

k

1.389

1.336

62.3

MP2/6-31G(D)

k

1.396

1.303

60.5

6-31G(D)

k

1.389

1.336

62.3

MP2/6-31G(D)

k

143

1.386 1.396

1.289 1.316

60.5 61.6

6-31G(D) MP2/6-31G(D)

k k

144

1.369 1.406

1.422 1.405

67.8 67.4

6-31G(D) MP2/6-31G(D)

k k

142 (C3v)

Notes:

c

d

ct

[I

T

e,

Method

Ref.

(a) Average values of bond lengths and bond angles derived from the Cambridge Crystallographic Database. (b) In bisannelated derivatives, bond lengths and bond angles are defined with respect to [a] = smallest ring, and [e] or [f] = second annelation. Bond [a] is then defined as geometric parameter "a". (e) Geometries are defined with respect to the central benzene ring. (d) Bond lengths "a" and "b" are reported here as the average of the endo and exo bond lengths in the crystal structure. (e) Geometric parameters of the low temperature structure are reported here. (f) Ref. 123a. (g) Ref. 123b. (h) Ref. 125e. (i) Ref. 135h. (j) Ref. 124d. (k) Ref. 112. (1) Ref. 135e. (in) Ref. 135c. (n) Ref. 135g. (o) Ref. 136. (p) Ref. 132c. (q) Ref. 132a. (r) Ref. 132b. (s) Ref. 135f. (t) Ref. 135a. (u) Ref. 124e. (v) Ref. 125e. (w) Ref. 135d. (x) Ref. 125f. (y) Ref. 129. (z) Ref. 131b. (aa) Ref. 124c. (bb) Ref. 125d. (cc) Ref. 122. (rid) Ref. 126b. (ee) Ref. 149. (if) Ref. 125g. (gg)Ref. 125h. (hh) Ref. 131a. (ii) Ref. 127a. (jj) Ref. 128. (kk) Ref. 150. (ll) Ref. 151.

252


p-orbital of the heteroatom is orthogonal to the x-system of the arene; the eleetr0negativity effect should be independent of this orientation, whereas the Htickel paradigm should be inoperative due to the orthogonality of the orbitals. The values of all three compounds lie close to the Stanger curve, implying that only angular effects are important here. This is exactly consistent with the x-effect being "turned off" by the conformational constraint and there being at most an inconsequential electronegativity effect with regard to bond fixation. 5.

CONCLUSIONS

A historical review of the original work of Mills and Nixon reveals that the effect beating their name is a phantom founded on an erroneous model for the structure of benzene. Their work, predicated in the work of van 't Hoff and Kekulr, relied on the unrealistic picture of benzene as a mixture of tautomeric angularly distorted cyclohexatrienes and did not take into consideration the theories of Pauling or Htickel that benzene was a single symmetrical structure. As such, it is wholly improper to refer to modem structural studies as confirming or refuting this postulate which is based on a nonexistent paradigm. Nonetheless, two important chemical pursuits spun off from this work: 1) to find an explanation for the increased C~ to C a selectivity as a function of decreasing ring size in benzocycloalkenes and benzobicycloalkenes; 2) to develop an understanding of what factors if any can induce benzene to take on significant cyclohexatriene character (i.e., is bond fixation in benzene possible). Over the last 60 years, both of these pursuits have been largely confused as leading to the same grail, due to the unrecognized mistakes of Mills and Nixon. Separation of these goals and reexamination of the literature data has greatly simplified the understanding of this area. The empirical data for electrophilic aromatic substitution on benzocycloalkenes over a variety of reactions and conditions show a consistent trend of increased CI3 selectivity due primarily to C a deactivation, with some indication that C~ activation occurs in benzobicycloalkenes. Acidity work on the benzocycloalkenes and related pyridines demonstrates clearly the extent of C a deactivation. The rehybridization model of Finnegan and Streitweiser has been postulated to account for the C a deactivation. Thummel's correlation of C a JC-H to pK a provided the necessary link between rehybridization and deactivation. Theories involving bond fixation in the Wheland intermediate deserve some further consideration but are not essential to an understanding of the present empirical data. Aside from NMR, general spectroscopic data have not had great impact on either the bond fixation or the reactivity question. This is probably because the magnitude of the energy necessary to cause either the reaction selectivities or bond fixation is close to the error in these methods. The NMR studies stand out from this generalization. Chemical shift data have been useful in demonstrating that the effects of strained annelations are localized to the C a- and ipso-carbons. Coupling constant


253

data have been instrumental in establishing the correlation between rehybridization and C a deactivation. Pure computational results indicate that it should be possible to observe bond fixation in angularly distorted benzenes. Structural studies, either experimental or high-level computational, show that simple cyclic annelations are ineffective at inducing such bond fixation. Computational investigations of arenes with bicyclic annelations predict that these systems will show the desired bond fixation, trisbicyclo[2.1.1]hexabenzene, is the favored structure. EffectS other than angular distortions can lead to bond fixation. A simple model based on Htickel aromatic theory accounts for a large number of such cases (e.g., starphenylene and triphenylene). Ideas that electronegative groups in the annelation are responsible for the bond fixation are shown to be inconsistent when tested against a significant sample of data. There has clearly been a overworking of the benzocycloalkenes and cognates, due to unwarranted following of the work of Mills and Nixon. Sorely missing are studies into the benzobicycloalkene systems and their cognates. The small group of such molecules which have been studied show the largest magnitudes of effects among any systems investigated. They are the only compounds where pure angularly dependent bond fixation might be realized. We see this as an area in need of attention and expect it tO be rich in new and novel aromatic chemistry.

NOTE ADDED IN PROOF During the time this review was sent to press, significant progress in the structural characterization of strained bicycloannelated derivatives has been made; the experimental structures of the most strained members of the trisannelated series, trisbicyclo[2.1.1 ]hexabenzene (82) and trisbicyclo[2.2.1]heptabenzene (81) have been determined. 15~ Significant bond localization is observed in both with a ~5 of 9 pm and 4 pm, respectively; this data is included in Table 16. Access to and further investigation of these compounds and related strained aromatics have enhanced and evolved our understanding of the bond fixation process. This review is our first assessment of the area and may be open to reinterpretation as we acquire new information. Ideas for the mechanism of bond localization are being explored through reactivity and spectroscopic studies of bicycloannelated aromatics. We hope to present a more mature analysis in the future.

REFERENCES AND NOTES 1. (a) Mills, W. H.; Nixon, I. G. J. Chem. Soc. 1930, 2510; (b) Badger, G. M. Q. Rev. Chem. Soc. 1951, 5, 147; (c) Berthier, G.; Pullman, A. BulL Soc. Chim. France 1950, 88; (d) Siegel, J. S. Angew. Chem. Int. Ed. Engl. 1994,33, 1721. 2. Kekulr,F. A. Liebigs Annl Chem. 1872, 162, 77. 3. The ~ position herein is defined as the position ortho to annelation, and 13defined as meta to annelation.

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4. (a) Schroeter, G. Liebigs Ann. Chem. 1922, 426, 83; (b) Smith, C. J. Chem. Soc. 1904, 85, 728; (c) Borsche, W.; Bodenstein, A. Chem. Ber. 1926, 59, 1909; (d) Diepolder, E. Chem. Ber. 1909, 42,2916. 5. Early theory of electrophilic aromatic substitution: (a) Allen, J.; Oxford, A. E.; Robinson, R.; Smith, J. C. J. Chem. Soc. 1926, 401; (b) Ingold, C. K.; Ingold, E. H. J. Chem. Soc. 1926, 1310; (c) Ing, H. R.; Robinson, R. J. Chem. Soc. 192@ 1655; (d) Goss, E R.; Ingold, C. K.; Wilson, I. S.J. Chem. Soc. 1926, 2440. 6. Holleman, A. E Die direkt Einfi~rung von Substituenten in den Benzolkern. Veit, Leipzig, 1910. 7. Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University, Ithaca, NY, 1964; Chapter 14. 8. Sutton, L. E.; Pauling, L. Trans. Faraday Soc. 1935, 31,939. 9. For recent examples, see: (a) Olofson, R. A.; Vander Meer, R. K.; Hoskin, D. H.; Bernheim, M. Y.; Stournas, S.; Morrison, D. S. J. Org. Chem. 1984, 49, 3367; (b) Paquette, L. A.; Wang, T.-Z. J. Am. Chem. Soc. 1988,110, 8192; (c) Daly, C. M.; Iddon, B.; Suschitzky, H.; Jordis, U.; Sauter, E J. Chem. Soc. Perkin Trans. 11988, 1933; (d) Li, Y.; Rubin, Y.; Diederich, E; Houk, K. N. J. Am. Chem. Soc. 1990, 112, 1618; (e) Roth, W. R.; Langer, R.; Ebbrecht, T.; Beitat, A.; Lennartz, H.-W. Chem Ber. 1991, 124, 2751; (f) Anstead, G. M.; Srinivasan, R.; Peterson, C. S.; Wilson, S. R.; KatzeneUenbogen, J. A. J. Am. Chem. $oc. 1991, 113, 1378; (g) Hahn, S. E; Martin, S. J.; McKelvy, M. L. Macromolecules 1992, 25, 1539; (h) Taylor, R.; Walton, D. R. M. Nature 1993, 363, 685; (i) Mart/nez, A.; Jimeno, M. L.; Elguero, J.; Fruchier, A. New J. Chem. 1994, 18, 269. 10. Hounshell, W. D.; Iroff, L. D.; Iverson, D. J.; Wroczynski, R. J.; Mislow, K. lsr. J. Chem. 1980, 20, 65. 11. Mislow, K. Chimia 1986, 40, 395. 12. (a) Berg, U.; Liljefors, T.; Roussel, C.; Sandstr6m, J.Acc. Chem. ICes. 1985, 18, 80; (b) Roussel, C.; Balaban, A. T.; Berg, U.; Chanon, M.; Gallo, R.; Klatte, G.; Memiaghe, J. A.; Metzger, J.; Oniciu, D.; Pierrot-Sanders, J. Tetrahedron 1983, 39, 4209. 13. Frank, N. L.; Siegel, J. S., University of California at San Diego, unpublished results. 14. The high 13-selectivity in bicycloannelated benzenes toward nitration has previously been partially attributed to increased steric effects. See Section 2.2. For previous attempts to establish relative steric effects in benzocycloalkenes, see: Arnold, R. T.; Webers, V. J.; Dodson, R. M. J. Am. Chem. Soc. 1952, 74, 368 and references therein. 15. (a) The values for o + in indan and tetralin are roughly the same (--0.41), see: Baker, R.; Eaborn, C.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1972, 97; (b) For a previous determination by a different reference reaction, see: Jaff6, H. H. Chem. Rev. 1953, 53, 191. Since each set of these constants is based on a single determination, no test of their constancy and usefulness is available. 16. (a) Godar, E.; Madella, R. P. J. Am. Chem. Soc. 1957, 79, 1402; (b) Thummel, R. P.; Kohli, D. K.J. Org. Chem. 1977, 42,2742; (c) Thummel, R. P.; Kohli, D. K. J. Org. Chem. 1978, 43, 4882; (d) Thummel, R. P.; Kohli, D. K. Tetrahedron Lett. 1979, 143. 17. Although there are several naming systems for annelated pyridines, we have adopted the nomenclature system where the bond between positions 1,2 is the [a] bond; 2, 3 is the [b] bond; 3, 4 is the [c] bond; and 4,5 is the [d] bond. 18. Similar effects are observed in the basicities of annelated quinolines. See: (a) Markgraf, J. H.; Katt, R. J. Tetrahedron Lett. 1968, 6067; (b) Markgraf, J. H.; Antin, J. H.; Walker, E J.; Blatchley, R. A. J. Org. Chem. 1979, 44, 3261" (c) Markgraf, J. H.; Scott, W. L. J. Chem. Soc., Chem. Commun. 1967, 296. 19. Studies of the effect of ring strain on hydrocarbon acidity reveal a general increased acidity in the position ortho (r to strained ring annelation. See: Streitwieser, A. Jr.; Ziegler, G. R.; Mowery, P. C.; Lewis, A.; Lawler, R. G. J. Am. Chem. Soc. 1968, 90, 1357. 20. The formation of solely 0c-metallated species in benzocyclobutene after equilibration suggests an increased thermodynamic acidity at the ocposition. See: Finnegan, R. A.J. Org. Chem. 1965, 30, 1333, and references therein.

M ills-N ixon Effects?

255

21', Vaughan, J.; Wright, G. J. J. Org. Chem. 1968, 33, 2580. 22. Stroud, M. M. A.; Taylor, R. J. Chem. Res. (S), 1978, 425. 23. It has been noted that the ortho:para ratio for protodetritiation ofp-[aH] toluene varies with acid

medium, and although side reactions are present with both sulphuric acid and Lewis acids, they are usually absent in trifluoroacetic acid. See: (a) Taylor, R. ElectrophilicAromatic Substitution.; J. Wiley and Sons, New York, 1990, pp. 61-64; (b) Baker, R.; Eaborn, C.; Taylor, R. J. Chem. $oc. 1961, 4927. 24. A general increase in rates of substitution at the [3position with annelation is also observed in the rates of benzoylation of benzocycloalkenes. See: Jensen, E R.; Maciel, G. J. Org. Chem. 1960, 25, 640. 25. Taylor, R.; Wright, G. J.; Holmes, A. J. J. Chem. Soc. (B) 1967, 780. 26. Bassindale, A. R.; Eaborn, C.; Walton, D. R. M. J. Chem. Soc. (B) 1969, 12. 27. Hydrogen exchange studies have been extended to larger aromatic systems in which ring strain directed regioselectivity toward 13-selectivityis also observed. See refs. 26, 28-30. 28. Biphenylene and fluorene: (a) Blatchley, J. M.; Taylor, R. J. Chem. Soc. 1964, 4641; (b) Taylor, R. J. Chem. Soc. (B) 1971, 536; (c) Baker, R.; Bott, R. W.; Eaborn, C. J. Chem. Soc. 1963, 2136; (d) Baker, R.; Bott, R. W.; Eaborn; C. Greasley, P. M. J. Chem. Soc. 1964, 627. 29. Dibenzofuran, dibenzothiophene, and anisole derivatives: (a) Baker, R.; Eaborn, C. J. Chem. Soc. 1961, 5077; (b) Taylor, R. J. Chem. Soc. (B) 1968, 1559; (c) ref. 15a. 30. Acenaphthene and perinaphthane: Ansell, H. V.; Taylor R. Tetrahedron Lett. 1971, 4915. 31. Similar effects have been demonstrated in the protodetritiation of ~-hydroxy-substituted and 13-methoxy-substituted benzocycloalkenes. See: (a) Selander, H.; Nilsson, J. L. G. Acta Chem. Scand. 1971, 25, 1182; (b) protodetritiation of l~eteroaromatic systems: Serebryanskaya, A. I.; Ertsov, A. V.; Shatenshtein, A. I.J. Org. Chem. USSR (Engl. Transl.) 1967, 343; Zh. Org. Khim. 1967, 3, 358; (c) Czernohorsky, J. H.; Richards, K. E.; Wright, G. J. Aust. J. Chem. 1972, 25, 1459; (d) Selander, H.; Nilsson, J. L. G.Acta Chem. Scand. 1972, 26, 3377. 32. The nitration of heteroannelated benzenes have also been examined. See: (a) Utley, J. H. P.; Vaughan, T. A.J. Chem. Soc., Perkin Trans. 2 1972, 2343;(b) Takakis, I. M.; Hadjimihalakis, E M. J. Heterocycl. Chem. 1991, 28, 625, and references therein. 33. A greater selectivity toward nitration in the (z-position in acidic media is accounted for by substantial ipso attack followed by rearrangement to the (z-nitro product in the series indan, tetralin, and benzocycloheptene. In the absence of nucleophilic trapping, the percentage of (z product corresponds to the percentage of ipso-attack. (a) Myhre, E C. J. Am. Chem. Soc. 1972, 94, 7921; (b) Fischer, A.; Ramsay, J. N.J. Chem. Soc., Perkin Trans. 2 1973, 237; (c) Hahn, R. C.; Strack, D. L. J. Am. Chem. Soc. 1974, 96, 4335; (d) Fischer, A.; Greig, C. C.; Wilkinson, A. L.; Leonard, D. R. A. Can. J. Chem. 1972, 50, 2211; (e) Fischer, A.; Leonard, D. R. A. Can. J. Chem. 1972, 50, 3367; (f) Hahn, R. C.; Groen, M. B. J. Am. Chem. Soc. 1973, 95, 6128; (g) Galley, M. W.; Hahn, R. C. J. Org. Chem. 1976, 41, 2006; (h) Galley, M. W.; Hahn, R. C. J. Am. Chem. Soc. 1974, 96, 4337. 34. Nitration of indan and tetralin leads to ct/13product ratios of 1:1 in acetic acid or sulfuric acid media. See: (a) Tanida, H.; Muneyuki, R. Tetrahedron Lett. 1964, 2787; (b) Tanida, H.; Muneyuki, R. J. Am. Chem. Soc. 1965, 87, 4794; (c) Vaughan, J.; Welch, G. J.; Wright, G. J. Tetrahedron 1965, 21, 1665. 35. Ref. 33g. 36. The nitration of benzocyclobutene under neutral conditions has not been examined, and nitration under acidic conditions leads to considerable ring cleavage. See: Homer, L; Schmelzer, H.-G.; Thompson, B. Chem. Ber. 1960, 93, 1774; (b) Ref. 46. 37. Fischer, A.; Packer, J.; Vaughan, J.; Wright, G. J. J. Chem. Soc. 1964, 3687. 38. Cristol, S. J.; Lewis, D. C. J. Am. Chem. Soc. 1967, 89, 1476. 39. Klanderman, B. H.; Perkins, W. C. J. Org. Chem. 1969, 34, 630. 40. Tanida, H.; Ishitobi, H. Tetrahedron Lett. 1964, 807.

256


41. This value was estimated by applying the free energy relationship AG = - R T In K where K

-

42. Enhanced reactivity at the [3 position must be caused either by internal strain or o-bond participation. A correlation between the partial relative rates of nitration at the [3-position and solvolyses of the corresponding 1-(13-benzocyclenyl)ethyl chlorides provides support or increased electron density meta to bicycloannelation. See ref. 34b. 43. A large steric interaction between the bridging methylene and nitro group of the o-complex has been suggested to account for decreased oc-reactivity: "the fused-ortho effect", though analysis of the sterics in bicycloannelated derivatives show this is unlikely (Section 2.1). For discussion of the "fused-ortho effect", see ref. 34a,b. 44. For the nitration of substituted fluoranthene derivatives, see: Oliver, M. J.; Patney, H. K.; Paddon-Row, M. N. Aust. J. Chem. 1980, 33, 795. 45. Lloyd, J. B. E; Ongley, P. A. Tetrahedron 1965, 21,245. 46. Lloyd, J. B. E; Ongley, P. A. Tetrahedron 1964, 20, 2185. 47. Sket, B.; Zupan, M. J. Org. Chem. 1978, 43, 835. 48. Zajc, B.; Zupan, M. Tetrahedron 1989, 45, 7869. 49. Cava, M. P.; Napier, D. R. J. Am. Chem. Soc. 1958, 80, 2255. 50. For the halogenation of annelated heteroaromatics, see: (a) ref. 29b. (b) Berrier, C.; Jacquesy, J. C.; Jouannetaud, M. E; Renoux, A. New J. Chem. 1987, 11, 605. 51. Similar selectivities are seen in the bromination of hydroxy substituted benzocycloalkenes with bromine in carbon tetrachloride. See: (a) Nilsson, J. L. G; Selander, H.; Sievertsson, H.; Sk~mberg, I.; Svensson, K.-G. Acta Chem. Scand. 1971, 25, 94; (b) Pascual, O. S. Chem. Abs. 1964, 60, 10516f. 52. The remainder of products ( ca. 10%) are caused by electrophilic ring opening, see: ref. 45. In strong acids, such as liquid HE polymerization of benzocyclobutene is observed. See ref. 49. 53. (a) Garratt, P. J.; Nicolaides, D. N. J. Org. Chem. 1974, 39, 2222; (b) Garratt, E J.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1972, 94, 7087; (c) For an examination of the effect of annelation on the properties of thiophene, see: Zwanenburg, D. J.; de Haan, H.; Wynberg, H. J. Org. Chem. 1966, 31, 3363; Wynberg, H.; Zwanenburg, D. J. J. Org. Chem. 1964, 29, 1919. 54. Cerfontain, H. Mechanistic Aspects inAromatic Sulphonation and Desulphonation; Interscience: New York, 1968. 55. Kilpatrick, M.; Meyer, M. W.; KilpatricL M. L. J. Phys. Chem. 1960, 64, 1433. 56. Cerfontain, H.; Sixma, E L. J.; Vollbracht, L. Recl. Trav. Chim. Pays-Bas 1963, 82, 659. 57. deVries, H.; Ceffontain, H. Red. Tray. Chim. Pays-Bas 1967, 86, 873. 58. Prinsen, A. J.; Ceffontain, H. Red. Trav. Chim. Pays-Bas 1969, 88, 833. 59. Cerfontain, H.; Nienhuis, Z. R. H.; Zwart Voorspuy, W. A. J. Chem. Soc., Perla'n Trans. 2 1972, 2087. 60. Cerfontain, H.; Koeberg-Telder, A.; van Kuipers, E. J. Chem. Soc., Perkin Trans. 2 1972, 2091. 61. Koeberg-Telder, A.; Cerfontain, H. J. Chem. Soc., Perkin Trans. 2 1974, 1206. 62. Gore, E H. In Friedel-Crafts and Related Reactions; Olah, G. A., Ed.; John Wiley and Sons, 1964, Vol III, Part I, p. 1; Homer, L.; Muth, K.; Schmelzer, H.-G. Chem. Ber. 1959, 92, 2953. 63. Streitwieser, A., Jr.; Owens, E H.; Wolf, R. A.; Williams, J. E., Jr. J. Am. Chem. Soc. 1974, 96, 5448. 64. (a) Taylor, R. J. Chem. Soc. B. 1968, 1402; (b) Ref. 25. 65. (a) Eckert-Maksi~, M.; Maksi~, Z. B.; Klessinger, M. J. Chem. Soc., Perla'n Trans. 2 1994, 285; (b) Eckert-Maksi~, M.; Maksi~, Z. B.; Klessinger, M. Int. J. Quant. Chem. 1994, 49, 383. 66. (a) Halton, B. Chem. Rev. 1973, 73, 113; (b) Billups, W. E. Acc. Chem. Res. 1978, 11, 245; (c) Billups, W. E.; Rodin, W. A.; Haley, M. M. Tetrahedron 1988, 44, 1305; (d) Halton, B.; Stang, E J. Ace. Chem. Res. 1987, 20, 443.

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67. (a) Wentrup, C.; Wentrup-Byrne, E.; Mtiller, P. J. Chem. Soc., Chem. Comm. 1977, 210; (b) Dtirr, H.; Ahr, H.-J. Tetrahedron Lett. 1977, 1991; (c) Liiddecke, E.; Rau, H.; Diirr, H.; Schmitz, H. Tetrahedron 1977, 33, 2677. 68. (a) Davalian, D.; Garratt, P. J.; Koller, W.; Mansuri, M. M. J. Org. Chem. 1980, 45, 4183; (b) Davalian, D.; Garratt, P. J.; Mansuri, M. M. J. Am. Chem. Soc. 1978, 100, 980. 69. Bee, L. K.; Garratt, P. J.; Mansuri, M. M. J. Am. Chem. Soc. 1980, 102, 7076. 70. (a) Kumar, A.; Tayal, S. R.; Devaprabhakara, D. Tetrahedron Lett. 1976, 863; (b) Ippen, J.; Vogel, E. Angew. Chem. Int. Ed. Engl. 1974, 13, 736; (c) Billups, W. E.; Chow, W. Y.; Smith, C. V. J. Am. Chem. Soc. 1974, 96, 1979. 71. Billups, W. E.; Rodin, W. A.J. Org. Chem. 1988,53, 1312. 72. Okazaki, R.; Masaharu, O-o.; Tokitoh, N.; Shishido, Y.; lnamoto, N.Angew. Chem. Int. Ed. Engl. 1981, 20, 799. 73. Schwager, H.; Benn, R.; Wilke, G. Angew. Chem. Int. Ed. EngL 1987, 26, 67. 74. (a) Schwager, H.; Kriiger, C.; Neidlein, R.; Wilke, G.Angew. Chem. Int. Ed. Engl. 1987, 26, 65; (b) Neidlein, R.; Rufifiska, A.; Schwager, H.; Wilke, G. Angew. Chem. Int. Ed. Engl. 1986, 25, 640; (c) Mynott, R.; Neidlein, R.; Schwager, H.; Wilke, G. Angew. Chem. Int. Ed. EngL 1986, 25, 367; (d) Krtiger, C; Laakmann, K.; Schroth, G.; Schwager, H.; Wilke, G. Chem. Ber. 1987, 120, 471. 75. Cotton, E A.; Troup, J. M.; Billups, W. E.; Lin, L. P.; Smith, C. V. J. Organomet. Chem. 1975, 102, 345. 76. Hanlan, A. J. L.; Ugolick, R. C.; Fulcher, J. G.; Togashi, S.; Bocarsly, A. B.; Gladysz, J. A. Inorg. Chem. 1980, 19, 1543. 77. (a) Muller, N.; Pritchard, D. G. J. Chem. Phys. 1959, 31,768; (b) Shoolery, J. N. J. Chem. Phys. 1959, 31, 1427; (c) For a discussion of the variation of one-bond C-H coupling constants in polycyclic aromatic hydrocarbons, see: Hansen, P. E. Org. Magn. Reson. 1979, 12, 109. 78. GUnther, H.; Jikeli, G.; Schmickler, H.; Prestien, J. Angew. Chem. Int. Ed. EngL 1973, 12, 762. 79. (a) Thummel, R. P.; Nutakul, W. J. Org. Chem. 1978, 43, 3170; (b) Thummel, R. P. Isr. J. Chem. 1982, 22, 11. 80. Thummel, R. P.; Nutakul, W. J. Org. Chem. 1977, 42, 300. 8i. (a) For assignments of the chemical shifts, see: Reitz, T. J.; Grunewald, G. L. Org. Mag. Res. 1983, 21,596; (b) For measurement of coupling constants, Frank, N. L.; Siegel, J. S., University of California at San Diego, unpublished results. 82. Rizvi, S. Q. A.; Gupta, B. D.; Adcock, W.; Doddrell, D.; Kitching, W. J. Organomet. Chem. 1973, 63,67. 83. Motell, E. L.; Lauer, D.; Maciel, G. E. J. Phys. Chem. 1973, 77, 1865. 84. Adcock, W.; Gupta, B. D.; Khor, T. 12.; Doddrell, D.; Kitching, W. J. Org. Chem. 1976, 41,751. 85. Heilbronner, E.; Kova~, B.; Nutakul, W.; Taggart, A. D.; Thummel, R. P. J. Org. Chem. 1981, 46, 5279. 86. Komatsu, K.; Aonuma, S.; Jinbu, Y.; Tsuji, R.; Hirosawa, C.; Takeuchi, K.-i. J. Org. Chem. 1991, 56, 195. 87. (a) Gassman, P. G.; Gennick, I. J. Am. Chem. Soc. 1980, 102, 6864. 88. Frank, N. L.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 1995, 117, 2102. 89. (a) Giinther, H.; Herrig, W. J. Am. Chem. Soc. 1975, 97, 5594; (b) Butensch6n, H.; Gabor, B.; Mynott, R.; Wey, H. G. Z. Naturforsch. 1995, 50b, 483. 90. Cooper, M. A.; Manatt, S. L. J. Am. Chem. Soc. 1969, 91, 6325. 91. (a) Castellano, S.; Sun, C. J. Am. Chem. Soc. 1966, 88, 4741; (b) Castellano, S.; Kostelnik, R. Tetrahedron Lett. 1967, 5211. 92. Giinther, H. Tetrahedron Lett. 1967, 2967. 93. Cooper, M. A.; Manatt, S. L. J. Am. Chem. Soc. 1970, 92, 1605. 94. Collins, M. J.; Gready, J. E.; Sternhell, S.; Tansey, C. W. Aust. J. Chem. 1990, 43, 1547. 95. Patrick, T. B.; Patrick, P. H. J. Am. Chem. Soc. 1973, 95, 5192.

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96. (a) Nair, P. M.; Gopakumar, G. Tetrahedron Lett. 1964, 709; (b) Cheung, C. S.; Cooper, M. A.; Manalt, S. L. Tetrahedron 1971, 27, 701. 97. Alger, T. D.; Grant, D. M.; Paul, E. G. J. Am. Chem. Soc. 1966, 88, 5397. 98. Incorrect assignments of C3,6 and C4,5 carbons have been published (ref. 83) and corrected (ref. 84). These values for a series of benzocycloalkenes are tabulated in their correct form in Table 8. Assignments have been made using deuterium substitution, when feasible, and GUnther's "fingerprint" method in the IH-coupled spectrum. See: Giinther, H.; Schmickler, H.; Jikeli, G. J. Magn. Reson. 1973, 11,344. 99. Pomerantz, M. J. Am. Chem. Soc. 196@ 88, 5349. 100. (a) Thummel, R. E; Nutakul, W. J. Am. Chem. Soc. 1978, I00, 6171; (b) Billups, W. E.; Chow, W. Y. J. Am. Chem. Soc. 1973, 95, 4099. 101. (a) Patel, D. J.; Howden, M. E. H.; Roberts, J. D.J. Am. Chem. Soc. 1963, 85, 3218; (b) Bernett, W. A. J. Chem. Ed. 1967, 44, 17. 102. Figeys, H. E; Defay, N.; Martin, R. H.; McOmie, J. E W.; Ayres, B. E.; Chadwick, J. B. Tetrahedron 1976, 32, 2571; (b) Fraenkel, G.; Asahi, Y.; Mitchell, M. J.; Cava, M. E Tetrahedron 1964, 20, l 179. 103. For suggestions that annelation perturbs the aromatic ring current as observed by proton chemical shifts, see: (a) Bastiaan, E. W.; Wegman, M. A. M.; MacLean, C. Org. Mag. Reson. 1987, 25, 817; (b) Godfrey, R.J. Chem. Soc., Perkin Trans. 2 1978, 1019; (c) Meier, H.; Mtiller, Eu.; Suhr, H. Tetrahedron 1967, 23, 3713. 104. Johnson, C. E., Jr.; Bovey, E A. J. Chem. Phys. 1958, 29, 1012. 105. (a) Mitchell, R. H.; SIowey, P. D.; Kamada, T.; Williams, R. V.; Garratt, E J.J. Amer. Chem. Soc. 1984, 106, 2431-2432; (b) Mitchell, R. H.; Vivekanantan, S. I.; Khalifa, N.; Mahadevan, R.; Venugopalan, S.; Weerawarna, S. A.; Zhou, E J. Am. Chem. Soc. 1995, 117, 1514. 106. Pascal, R. A., Jr.; Grossman, R. B.; Van Engen, D. J. Am. Chem. Soc. 1987, 109, 6878. 107. Rao, C. N. R. Ultra-violet and Visible Spectroscopy, Chemical Applications, 3rd ed.; Butterworths, London, 1975. 108. Matsen, B.; Robertson, W. W.; Chuoke, R. L. Chem. Rev. 1974, 41, 273. 109. In addition, there is a slight increase in intensity with increasing strain. This has been attributed to a larger split in the degeneracy of the two configurations caused by hyperconjugation, and slight molecular distortions induced by cyclobutyl annelation. See: Santiago, C.; Gandour, R. W.; Houk, K. N.; Nutakul W.; Cravey, W. E.; Thummel, R. P. J. Am. Chem. Soc. 1978, 100, 3730. 110. (a) Davalian, D.; Garratt, P. J. J. Am. Chem. Soc. 1975, 6883; (b) Saward, C. J.; Vollhardt, K. P. C. Tetrahedron Lett. 1975, 4539. 111. The para-fused annelated systems generally display greater extinction coefficients from those of the meta-fused isomers. Comparison with the tetraalkyl analogues, prehnitene (1,2,3,4-tetramethylbenzene e = 301,244, 235) and durene (1,2,4,5-tetramethylbenzene e = 712, 654, 727) reveal a similar increase in extinction coefficients. In benzene itself, the lowest excited singlet state is an equal admixture of the [al-al] and [bE-b2] configurations, which have equal and opposite transition dipoles, and so is electronically forbidden. CNDO-S calculations predict a greater split in the degeneracy of excited states in durene over prehnitene, causing an increase in the transition moment, and an increase in the observed extinction coefficient. See: ref. 109. 112. Frank, N. L.; Baldridge, K. K.; Siegel, J. S., University of California at San Diego, manuscript in preparation. 113. Billups, W. E.; Arney, B. E. Jr.; Lin, L-J. J. Org. Chem. 1984, 49, 3436. 114. Meier, H.; Heiss, J.; Suhr, H.; Miiller, Eu. Tetrahedron 1968, 24, 2307. 115. (a) Jorgensen, W. L.; Borden, W. T. J. Am. Chem. Soc. 1973, 95, 6649; (b) Gleiter, R.; Bischof, P.; Gubernator, K.; Christi, M.; Schwager, L.; Vogel, P. J. Org. Chem. 1985, 50, 5064. 116. Arnett, E. M.; Bollinger, J. M.J. Am. Chem. Soc. 1964, 86, 4729. 117. (a) Bischof, E; Heilbronner, E. Helv. Chim. Acta 1970, 53, 1677; (b) Brogli, E; Giovannini, E.; Heilbronner, E.; Schurter, R. Chem. Ber. 1973, 106, 961.

M9i l l s - N i x o n

Effects?

259

118. Breslow, R.; Murayarna, D. R.; Murahashi, S.-I.; Grubbs, R. J. Am. Chem. Soc. 1973, 95, 6688. 119. (a) Rieke, R. D.; Rich, W. E.; Ridgway, T. H. J. Am. Chem. Soc. 1971, 93, 1962; (b) Rieke, R. D.; Rich, W. E.; Ridgway, T. H. Tetrahedron Lett. 1969, 4381. 120. (a) Rieke, R. D.; Rich, W. E. J. Am. Chem. Soc. 1970, 92, 7349; (b) Rieke, R. D.; Bales, S. E.; Meares, C. E; Rieke, L. I.; Milliren, C. M. J. Org. Chem. 1974, 39, 2276; (c) Rieke, R. D.; Bales, S. E.; Hudnall, P. M.; Meares, C. E J. Am. Chem. Soc. 1971, 93, 697; (d) Rieke, R. D.; Meares, C. E; Rieke, L. I. Tetrahedron Lett. 1968, 5275. 121. Davies, A. G.; Ng, K. M. J. Chem. Soc., Perkin Trans. 2 1992, 1857. 122. Here we are intentionally restricting from our discussion bond fixation of the type found in cross-conjugated accepter-donor substituted benzenes. For a discussion of these effects, see: Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 1992, 114, 9583. 123. Average geometries for monoannelated benzenes collected from the Cambridge Crystallographic Database. See (a) Allen, E H. Acta Crystalogr., Sect. B 1981, B37, 900; (b) Benassi, R.; Ianelli, S.; Nardelli, M.; Taddei, E J. Chem. Soc., Perkin Trans. 2 1991, 1381. 124. Experimentally determined cycloproparene derivatives: (a) Allen, E H. Tetrahedron 1982, 38, 645; (b) Hardgrove, G. L.; Templeton, L. K.; Templeton, D. H. J. Phys. Chem. 1968, 72, 668; (c) Bliiser, D.; Boese, R.; Brett, W. A.; Rademacher, P.; Schwager, H.; Stanger, A.; Vollhardt, K. P. C.Angew. Chem. Int. Ed. Engl. 1989, 28, 206; (d) Neidlein, R.; Christen, D.; Poignte, V.; Boese, R.; Bltiser, D.; Gieren, A.; Ruiz-Ptrez, C.; HUbner, T. Angew. Chem. Int. Ed. Engl. 1988, 27, 294; (e) Billups, W. E.; Chow, W. Y.; Leavell, K. H.; Lewis, E. S.; Margrave, J. L.; Sass, R. L.; Shieh, J. J.; Werness, P. G.; Wood, J. L. J. Amer. Chem. Soc. 1973, 95, 7878; (f) Carstensen-Oeser, E.; MUller, B.; DUrr, H. Angew. Chem. Int. Ed. Engl. 1972, 11,422. 125. Experimentally determined cyclobutarene derivatives: (a) Allen, E H.; Trotter, J. J. Chem. Soc. (B) 1970, 916; (b) Allen, F. H.; Trotter, J.J. Chem. Soc. (B) 1970, 1551; (c) Boese, R.; Blliser, D. Angew. Chem. Int. Ed. Engl. 1988, 27, 304; (d) Lawrence, J. L.; MacDonald, S. G. G. Acta Crystailogr., Sect. B 1969, B25, 978; (e) Crawford, J. L.; Marsh, R. E. Acta Crystallogr., Sect. B 1973, B29, 1238; (f) Boese, R.; Bl~er, D.; Billups, W. E.; Hale),, M. M.; Maulitz, A. H.; Mohler, D. L.; Vollhardt, K. P. C.Angew. Chem. Int. Ed. Engl. 1994, 33, 313; (g) Thummel, R. P.; Korp, J.D.; Bernal, I.; Harlow, R. L.; Soulen, R. L. J. Am. Chem. Soc. 1977, 99, 6916; (h) CobbledicL R. E.; Einstein, F. W. B. Acta Crystallogr., Sect. B 1979, B32, 1908. 126. Cyclopentyl Derivatives: (a) ref. 9f; (b) Boyko, E. R.; Vaughan, P. A.Acta Crystallogr. 1964,17, 152. 127. (a) Crystal structure determination: Venugopalan, P.; BUrgi, H. B.; Frank, N. L." Baldridge, K. K.; Siegel, J. S. Tetrahedron Lett. 1995, 36, 2419; (b) For a previous attempt at solving the structure, see: Huebner, C. F.; Puckett, R. T.; Brzechffa, M.; Schwartz, S. L. Tetrahedron Lett. 1970, 359; (c) For synthesis, see: Hart, H.; Shamouilian, S.; Takehira, Y. J. Org. Chem. 1981, 46, 4427. 128.. Komatsu, K.; Jinbu, Y.; Gillette, G. R.; West, R. Chem. Lett. 1988, 2029. 129. Kohnke, E H.; Mathias, J. P.; Stoddart, J. E; Slawin, A. M. Z.; Williams, D. J. Acta Crystallogr., Sect. C 1992, C48, 663. 130. The C~Car bond for a hexasubstituted benzene is 1.397 A, with a value of o = 0.009 A; o for all benzenes is 0.013 A. These values are taken from the Cambridge Crystallographic Database as tabulated by: Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem. Soc., Perkin Trans 2 1987, S 1. 131. (a) Starphenylene: Diercks, R.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1986, 108, 3150; (b) angular-phenylene: Diercks, R.; Vollhardt, K. P. C. Angew. Chem. Int. Ed. Engl. 1986, 25, 266. 132. Experimentally determined structure of biphenylene: (a) Yokozeki, A.; Wilcox, C. F., Jr.; Bauer, S. H.J. Am. Chem. Soc. 1974, 96, 1026; (b) Fawcett, J. K.; Trotter, J.Acta Crystallogr. 1966, 20, 87; (c) Experimentally determined structure of a cyclobutadienobenzene derivative: Winter, W.; Straub, H.Angew. Chem. Int. Ed. Engl. 1978, 17, 127. 133. Triphenylene: Filippini, G. J. Mol. Struct. 1985, 130, 117.

260


134. Longuett-Higgins, H. C.; Coulson, C. A. Trans. Faraday Soc. 1946, 42, 756. 135. (a) Eckert-Maksir M.; Maksi~, Z. B.; Hodo~ek, M.; Poljanec, K. Int. J. Quant. Chem. 1992, 42, 869; (b) Maksir Z. B.; Eckert-Maksi~, M.; Kova~ek, D.; Hodo~k, M.; Poljanec, K.; Kudnig, J.J. Mol. Struct. 1991, 234, 201; (c) Eckert-Maksi~, M.; Lesar, A.; Maksi~, Z. B. J. Chem. Soc., Perkin Trans. 2 1992, 993; (d) Koch, W.; Eckert-Maksir M.; Maksi~, Z. B. J. Chem. Soc., Perkin Trans. 2 1993, 2195; (r Maksi~, Z. B.; Eckert-Maksi~, M.; Pfeifer, K-H. J. Mol. Struct. 1993, 300, 445; (f) Eckert-Maksir M.; Hodo~ek, M.; Kova~ek, D.; Maksir Z. B.; Poljanec, K. Chem. Phys. Lett. 1990, 171, 49; (g) Maksir Z. B.; Eckert-Maksi~, M.; Kova~ek, D.; Margeti~, D. J. Mol. Struct. 1992, 260, 241; (h) Eckert-Maksie, M.; Kovacek, D.; Hodo~k, M.; Miti~, D.; Poljanec, K.; Maksi~, Z. B. J. Mol. Struct. 1990, 206, 89. 136. Faust, R.; Glendening, E. D.; Streitwieser, A.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1992, 114, 8263. 137. Stanger, A. J. Am. Chem. Soc. 1991, 113, 8277. 138. Apeloig, Y.; Arad, D.J. Am. Chem. Soc. 1986, 108, 3241. 139. Schulman, J. M.; Disch, R. L. J. Amer. Chem. Soc. 1993, 115, 11153. 140. Hiberty, E C.; Ohanessian, G.; Delbecq, E J. Am. Chem. Soc. 1985, 107, 3095. 141. Aihara, J-i. Bull. Chem. Soc. Jpn. 1990, 63, 1956. 142. Glidewell, C.; Lloyd, D. Tetrahedron 1984, 40, 4455. 143. (a) Foote, C. S. Tetrahedron Lett. 1963, 9, 579; (b) Mislow, K. M. Tetrahedron Lett. 1964, 22, 1415; (c) Baum, M. W.; Guenzi, A.; Johnson, C. A.; Mislow, K. Tetrahedron Lett. 1982, 23, 31; (d) Szalontal, G. Tetrahedron 1983, 39, 1783. 144. For analysis of coupling constants in bicyclic systems, see: (a) Barfield, M.; Della, E. W.; Pigou, P. E.; Walter, S. R. J. Am. Chem. Soc. 1982, 104, 3549; (b) Barfield, M.; Brown, S. E.; Canada, E. D., Jr.; Ledford, N. D.; Marshall, J. L.; Walter, S. R.; Yakali, E. J. Am. Chem. Soc. 1980, 102, 3355; (c) Barfield, M.; Della, E. W.; Pigou, P. E. J. Am. Chem. Soc. 1984, 106, 5051. 145. Bellamy, L. J. The Infra-red Spectra of Complex Molecules, 3rd ed.; Chapman and Hall, London, 1975, Chapter 3.2. 146. Wiberg, K. B.; Nist, B. J. J. Am. Chem. Soc. 1961, 83, 1226. 147. (a) Kohlrausch, K. W. F.; Seka, R.; Tramposch, O. Chem. Ber. 1942, 75, 1385; (b) Kohlrausch, K. W. E; Seka, R. Chem. Ber. 1936, 69, 729. 148. (a) Domenicano, A.; Vaciago, A.; Coulson, C. A.Acta Crystallogr, Sect. B 1975, B31, 1630; (b) Domenicano, A.; Mazzeo, P.; Vaciago, A. Tetrahedron Lett. 1976, 1029; (c) Domenicano, A.; Murray-Rust, P. Tetrahedron Lett. 1979, 2283. 149. For the synthesis of tricyclobutabenzene, see: Nutakul, W.; Thummel, R. P.; Taggart, A. D. J. Am. Chem. Soc. 1979, 101,770. 150. Frank, N. L.; Baldridge, K. K.; Gantzel, P.; Siegel, J. S. Tetrahedron Lett. 1995, in press. 151. Btirgi, H.-B.; Baldridge, K. K.; Hardcastle, K.; Frank, N. L.; Siegel, J. S.; Ziller, J.Angew. Chem. Int. Ed. Engl. 1995, in press.

RADICAL CATIONS OF CYCLOPROPANE SYSTEMS: CONJUGATION AND HOMOCONJUGATION WITH ALKENE FUNCTIONS

Heinz D. Roth

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Spectroscopic Techniques for Studying Cyclopropane Radical Cations: EPR, CIDNP, and Electronic Spectroscopy . . . . . . . . . . . . . . . . . . . 3. Cyclopropane Radical Cation Structures . . . . . . . . . . . . . . . . . . . . . 4. Alternative Cyclopropane Radical Cation Structures . . . . . . . . . . . . . . 5. Radical Cations of Vinylcyclopropane Systems . . . . . . . . . . . . . . . . . 6. Electron Transfer Photochemistry of Cyclopropane Systems: Radical cation Reactivities . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Nucleophilic Capture of Cyclopropane Radical Cations: Correlations between Radical Cation Structure and Reactivity . . . . . . . . . 8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Advances in Theoretically Interesting Molecules Volume 3, pages 261-302. Copyright 9 1995 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 1-55938-698-3

261

262 266 269 276 281 286 295 299 299

262

HEINZ D. ROTH

1. INTRODUCTION Radical cations derived from strained ring systems have been the focus of intense research for over two decades and continue to be the target of detailed scrutiny by various techniques. Strained ring systems have unusual properties and their oneelectron oxidation results in a variety of interesting reactions. Strained-ring hydrocarbons have long been known to quench the fluorescence of excited state electron acceptors efficiently. 1'2 Since a correlation between the quenching efficiencies and oxidation potentials 3 or vertical ionization potentials 4 was documented, charge transfer was recognized as the likely mechanism. 4-7 The lower oxidation potentials of strained ring compounds compared to less strained isomers can be interpreted as evidence that a significant fraction of the strain energy is released upon oxidation. Therefore the structural details of the radical cations have been of major interest. Cyclopropane, the simplest strained ring system, poses an interesting structure problem. Its photoelectron spectrum, investigated over 20 years ago, shows Gaussian shaped bands near 11 eV and a broad signal near 13 eV without fine structure. These bands have been assigned to a 2E' state, and their large splitting, which is also observed for numerous cyclopropane derivatives, ascribed to Jahn-Teller (JT) distortion (Figure 1).8

I.P.

2~

1A~ I

Figure 1. Potential energy diagram for cyclopropane in its ground state (1A1,) and the two lowest states of its radical cation (2A1 and 2B2). The vertical transition yields two bands (Jahn-Teller split) in the photoelectron spectrum.

Radical Cations of Cyclopropane Systems

263

S

A A

I I

1

2

Figure 2. The degenerate pair of cyclopropane HOMOs, 5 and A, and radical cation structures 1 and 2 resulting from removing an electron from one of the orbitals.

The two states resulting from the JT split correspond to two different structures, 2B2 and 2A1, which can be described as a ~-complex between methylene and an olefin radical cation (2B2), or as a trimethylene species with overlapping termini (2A1). The structural duality poses interesting questions" which structure is of lower energy?; can the 2B2 species undergo dissociation?; can the 2A 1 species undergo ring opening?; how is the ring opened trimethylene radical cation related to the propene radical cation?; how do substituents affect the relative energies of the 2B2 and 2A1 structure types? For molecules with a pair of degenerate HOMOs S and A, suitable substitution may lift the degeneracy and favor one structure over the other (Figure 2). Qualitative predictions of the favored structure can be based on a frontier molecular orbital (FMO)/perturbational molecular orbital (PMO) approach. 9'1~ The substrates are dissected into molecular fragments and the potential interactions of the component FMOs are considered. According to PMO theory, 9 the strength of the fragment perturbation is approximately proportional to S2/AE, where S is the overlap integral between the components and AE is the difference between the FMO energies. The S2 term will depend mainly on three factors, including the FMO symmetry (where present); the magnitude of the coefficients at the point(s) of union; and the orientation of the fragments relative to each other. 1~ In general, cyclopropane systems with substituents at a single carbon might most effectively stabilize the antisymmetrical HOMO, whereas substitution at two carbons is expected to stabilize the symmetrical orbital. Systems containing olefinic moieties in addition to a cyclopropane ring pose the interesting question whether one of these functionalities is the exclusive electron donating site because of a favorable FMO energy, or whether the FMO energies are comparable. The structures of the radical cations resulting in the two cases will

Table 1. Electron Transfer Induced Reactions of Cyclopropane Derivatives

/c~R

Nucleophilic Capture and Aromatic Substitution

H~C~

,~R

\'~R

+

Nc

.

C'H~R

"'"~'R 3

3

OCH3 Intramolecular Nucleophilic Capture Followed by Aromatic Substitution

~oo

,,

Deprotonation

Additionto an UnsaturatedBond A

Ar Ar

1,2-Hydrogen Shift

Ar

Ar

Ar Ar

-

,~+ Ar

Ar

Ar

A 264

(continued)

265

Radical Cations of Cyclopropane Systems Table 1. (Continued)

Geometric Isomerization .C6H5 6H5

~

C6H5

Ar

9

C6H5

Ar

Molecular Rearrangement

/

H3C

= "I"~

differ substantially. In the case of comparable FMO energies, a delocalized radical cation would be formed, whereas a significant difference in FMO energies would favor a species with localized spin and charge. Cyclopropane and derivatives containing olefinic moieties are also of interest because they undergo a rich variety of unimolecular and bimolecular reactions upon photo-induced electron transfer. Among the bimolecular reactions, nucleophilic capture generates a neutral radical; 11-13 typically, this reaction occurs with ring opening and release of ring strain. Deprotonation also forms a neutral radical; 14this reaction is often aided by the geminate radical anion, especially when the latter is a strong base. Finally, addition to an unsaturated bond may give rise to a new, bifunctional radical cation. 15 Unimolecular reactions include ring opening via a 1,2-hydrogen shift (--->propene radical cation) 12'15 as well as geometric isomerizations 16-18 and molecular rearrangements. 19-23 In addition, alcohol 13 or carboxylic acid functions 24'25 present in the substrate may lead to intramolecular nucleophilic capture. Some representative examples of electron transfer-induced reactions of cyclopropane derivatives are shown in Table 1. The primary "reaction products," i.e., the secondary intermediates mentioned above, are reactive and undergo further reactions. In fact, few of the intermediates

266

HEINZD. ROTH

have been observed spectroscopically. Typically, the intermediates are invoked and their structures based on the structure of reaction products. The obvious question whether the structure and stereochemistry of the reaction products can, in fact, provide insight into the nature of the radical cations and the mechanisms of their reactions will be discussed in detail below. In this chapter we will discuss three different approaches to the structure and reactivity of radical cations derived from substrates containing a cyclopropane function, as well as additional electron-donating functions. First, we will consider experimental results that may reveal the spin density distribution, viz., electron paramagnetic resonance (EPR) spectra in frozen glasses and chemically induced dynamic nuclear polarization (CIDNP) effects observed in solution. Second, we will discuss the structure problem in the light of ab initio molecular orbital (MO) calculations; this approach provides geometries, free energies, and spin and charge densities. Finally, the reaction products of electron transfer-induced photoreactions of cyclopropane systems in solution may reveal structural features of the radical cations. We begin with the parent system, then introduce simple alkyl and aryl substituents, and conclude with more complex derivatives bearing unsaturated functions in rigid geometries.

2. SPECTROSCOPICTECHNIQUES FOR STUDYING CYCLOPROPANE RADICAL CATIONS: EPR, CIDNP, AND ELECTRONIC SPECTROSCOPY The structure of radical cation intermediates can be characterized by geometric parameters such as bond lengths and angles, as well as dihedral angles, which provide a convenient measure for assessing changes in molecular geometry upon ionization. However, no experimental method is, as yet, available to determine such parameters for the short-lived intermediates discussed here; the assignment of geometric parameters typically resorts to MO calculations. Similarly, the distribution of the positive charge in the intermediate will be one of the factors determining the reactivity of the species. Finally, the spin-density distribution manifests itself in spectroscopically observable data. The spin density determines the hyperfine coupling patterns of radical ions, which can be observed directly, as EPR-splitting patterns, or may be reflected in the CIDNP patterns. CIDNP effects constitute indirect evidence; they are induced in the radical ion intermediate and then transferred to a diamagnetic product. The electron spin resonance (ESR) experiment records transitions between different levels of a "multi"-spin system consisting of one electron and several nuclear (ill) spins. 26.27 The splittings, so-called hyperfine couplings (hfcs), are related to carbon spin densities by different mechanisms of interaction. For ~-radicals, there are two principal mechanisms involving either an exchange interaction (ma-polarization) or hyperconjugation (n,a-delocalization). Protons attached directly to carbon atoms bearing positive spin density have negative hfc's because of


267

Figure 3. (Left) Preferred configuration of electron spins in the (~-orbital connecting a hydrogen atom to an sp2-hybridized carbon atom bearing unpaired g spin density. (Right) "Molecular g-orbital" consisting of two carbon Pz orbitals and an H2 "group orbital" generated by hyperconjugative interaction of an sp2-hybridized C atom bearing unpaired spin with a CH2-R group. the preferred exchange interaction between the unpaired rt spin density and the carbon ~-electron (Figure 3, left). Positive hfc's, on the other hand, are usually observed for protons which are one C--C bond removed from a carbon beating positive spin density. The positive sign is due to a hyperconjugative interaction which delocalizes the n-spin density on carbon into an H n "group orbital" (cf. Figure 3, right). The variations of radiofrequency intensities occurring in these experiments as a function of magnetic field are observed directly. Their interpretation in terms of a hfc pattern and the assignment of the nuclei causing the splitting is secondary and subject to ambiguities. The typical ESR experiment reveals the absolute magnitude, but not the sign of hfcs, and it identifies the number of nuclei so coupled but not their identity. Significantly, the positive charge is not observed, but is assigned based on the chemical intuition of the investigator and on appropriate secondary experiments. The ambiguities of such an assignment will be illustrated for the secondary intermediate obtained upon warming a matrix containing the tetramethylcyclopropane radical cation generated by 7-radiolysis. A technique related to EPR, electron nuclear double resonance (ENDOR), 28 allows the assignment of the individual hfcs to particular nuclei and, with reasonable assumptions, will also identify the sign of the interaction. The only obvious drawback of this technique lies in the fact that it requires sophisticated instrumentation, which is, so far, available in only a few laboratories. Applications to strained ring systems, viz., cyclobutene, 29 bicyclobutane, 3~ or a tricyclic derivative, 31 have been reported. However, applications to simple cyclopropane systems have not been reported to date. CIDNP 32-35 is a nuclear resonance method based on the observation of transient signals, substantially enhanced in either absorption or emission. These effects are induced as a result of magnetic interactions in radical or radical-ion pairs on the nanosecond timescale. This method requires acquisition of an NMR spectrum during (or within a few seconds of) the generation of the radical-ion pairs. The theory underlying this effect depends critically on two selection principles: (1) the nuclear spin dependence of intersystem crossing in a radical pair, and (2)

268

HEINZ D. ROTH

the electron-spin dependence of the rates of radical pair reactions. Combined, these selection principles cause a "sorting" of nuclear spin states into different products and result in characteristic non-equilibrium populations in the nuclear spin levels of geminate reaction products (whose formation is allowed for singlet pairs but spin forbidden for triplet pairs) and in complementary non-equilibrium populations in the spin levels of free-radical ("escape") products (whose formation is electron-spin independent). The transitions between these levels will be in the direction towards restoring the normal Boltzmann population; their intensities will depend on the extent of non-equilibrium population. The observed effects are optimal for radical pairs with lifetimes in the nanosecond range. On a shorter time scale, hyperfineinduced intersystem crossing is negligible, whereas on a longer timescale the polarization decays due to spin lattice relaxation in the radicals. The quantitative theory of CIDNP 32-35 is developed to a state where the intensity ratios of CIDNP spectra can be computed on the basis of reaction and relaxation rates and the characteristic parameters of: the radical pair (initial spin multiplicity, kt); the individual radicals (electron g factors, hfcs, a); and the products (spin-spin coupling constants, J). On the other hand, the patterns of signal directions and intensities observed for different nuclei of a reaction product can be interpreted in terms of hfcs of the same nuclei in the radical cation intermediate. The CIDNP method is an indirect method since the hyperfine pattern of a paramagnetic intermediate is derived from the unusual NMR intensities of a diamagnetic product derived from it. This method also has limitations and potential sources of misassignments. Similar to the EPR technique, the CIDNP method only documents the unpaired spin; additional evidence for the nature of the paramagnetic intermediate, particularly for the presence of the charge, is derived typically on the basis of mechanistic considerations and from supporting secondary experiments. Any CIDNP-based assignment of the sign and relative magnitude ofhfcs is valid only if the radical pair mechanism (RPM) 32-35 is operative; they become invalid if an alternative process is the source of the observed effects. The triplet--Overhauser mechanism (TOM) is based on electron nuclear cross-relaxation. 36-39 For effects induced via the TOM, the signal directions depend on the mechanism of cross-relaxation and the polarization intensities are proportional to the square of the hfc. Thus, they do not contain any information related to the signs of the hfcs. A third source of misassignment has its roots in the existence of nuclear-nuclear cross- relaxation. 4~ Again, depending on the mechanism of cross-relaxation and on the polarization of the originally polarized nucleus, this may result in enhanced absorption or emission. This process induces nuclear spin polarization in nuclei without hfc, or alters the nuclear spin polarization of nuclei with weak hfcs. On the other hand, the magnitude of these effects may be quite small and fall below the threshold of chemical significance. Optical spectroscopy is also an important method for studying organic radical ions. Its strength lies in the detailed kinetic information it provides; the time resolution of this method has reached a state where nanosecond resolution is

269

Radical Cations Of Cyclopropane Systems

routine. 44-47Thus the generation and decay of appropriate radical cations, generated by photoinduced electron transfer, can be followed conveniently. On the other hand, optical spectroscopy does not necessarily reveal structural details, because typical optical spectra in condensed media consist of broad bands without identifying features. Structural information is limited to systems having banded spectra, or to the conditions of high resolution laser spectroscopy. 48 Other restrictions, especially for the species discussed here, are due to the fact that customary rapid-response detectors are limited to the spectral range of 370-800 nm, excluding many interesting species from scrutiny. Finally, optical methods are not suitable for the study of degenerate processes such as electron exchange between a radical cation and its neutral precursor. Optical spectroscopy has merits in identifying radical cations, particularly when their spectra are known independently. For example, the radiolysis of quadricyclan e led to the observation of the known spectrum of norbornadiene radical cation. 49'5~ In another study, irradiation of cyclooctatetraene radical cation caused the color of the sample to change from bright red to royal blue, 51 suggesting the conversion to a different species, the previously identified semibullvalene radical cation. 52 Further irradiation of the latter led to a characteristic banded (vibrationally resolved) spectrum; the nature of this spectrum suggested that the rearranged species may be a linear conjugated radical cation and helped in its identification as 1,4-dihydropentalene radical cation. 53

3. CYCLOPROPANE RADICAL CATION STRUCTURES The cyclopropane radical cation can be prepared by 7-radiolysis in rigid matrices. At temperatures as low as 4.2 K, its ESR spectrum shows evidence for static Jahn-Teller distortion, resulting in a structure of the (ring-closed) trimethylene (ZA1) type ' 1-+ 54,55

~ 1

Exposure of several methyl-substituted derivatives to T-radiolysis at 77 K in cryogenic matrices gave rise to a family of radical cations of the same structure type, 56'57 some of which had been previously identified on the basis of CIDNP results. 58'59 We begin with a discussion of the CIDNP investigations, since they preceded the ESR studies of all species but the prototype. The first CIDNP results attributed to a cyclopropane radical cation were observed during the photoreaction between 1,4-dicyanonaphthalene and cis-l,2-diphenylcyclopropane. 16 However, the nature of the cyclopropane radical cation was characterized by CIDNP effects observed during the reaction of chloranil with cis- and trans-1,2-diphenylcyclopropane. 17

270

HEINZ D. ROTH H

H

F-

H

r

l

1

2.5

l__t

I

1__1_

2.0

J

J

!

I

).

4.5

I

.i

i

i

I

1.0

Figure 4. PMR spectra (90 MHz) observed during the irradiation of chloranil (0.02 M) in acetonitrile-d 3 solutions containing 0.02 M trans- (top) or cis-diphenylcyclopropane (bottom), respectively. 17

The pattern ofbenzylic and geminal polarization (Figure 4) supports radical ions with spin density on the benzylic carbons. Strictly, the results do not differentiate a priori between a "closed" and an "open" radical cation. The "closed" structure was assigned because the reaction did not cause geometric isomerization, suggesting that the stereochemistry at the key carbons was conserved in the intermediate. These results establish local minima on the radical cation potential surface. However, we emphasize that these data have no bearing on the possible existence of additional minima with altered stereochemistry, nor do they allow any conclusion concerning the global minimum on the radical cation potential energy surface. Radical cations of the same general structure type as those derived from cis- and trans-diphenylcyclopropane have been established for numerous cyclopropane


271

6H5

C6H5 C6H5

C6H5

cis-1,3-diphenyl-1 "+

trans-l,3-diphenyl-1 "+

derivatives, including the parent, 1,2-di-, 1,1,2-tri-, and 1,1,2,2-tetramethylcyclopropane (Table 2). Two of these systems provide a direct comparison between the results of CIDNP and ESR experiments. In both cases, the ESR spectra observed by Williams and co-workers following T-radiation in frozen solutions 56'57 show splitting patterns supporting the presence of spin density on two carbon centers, thus confirming the structure type (1"+; 1,1,2,2-tetramethyl- and 1,1,2,trimethyl-) assigned on the basis of CIDNP results. This assignment is also supported by ab initio calculations. Cyclopropane radical cations have been the target of theoretical investigations for over two decades. Early pioneering studies by Haselbach, 6~ Rowland, 61 and Collins and Gallup 62 were followed by. ab initio calculations by Wayner and colleagues, 63'64 Borden and co-workers, 65'66 and Krogh-Jespersen and Roth. 67 Borden and co-workers performed the most thorough ab initio calculations on the cyclopropane radical cation, 1-+, evaluated its potential ring opening to the trimethylene radical cation, 3-+, and considered its further rearrangement to propene cation radical 4 -+.

1 "+

3 "+

4

"+

It is hardly surprising that the propene radical cation 4-+ was found to be the

intermediate of lowest energy on the C3H~ potential surface. At the unrestricted Hartree-Fock (UHF)/6-3 1G*//MP2 level (Cs symmetry), 4 .+ lies --10 kcal mo1-1 below the radical cation, 1-+, in which a single C-C bond is lengthened relative to the parent molecule. A vibrational analysis of structure 1"+ shows only positive frequencies, thus identifying this species as a local minimum. In contrast, the ring-opened trimethylene species, 3-+, does not appear to be a minimum, since a 6-3 1G* vibrational analysis (C2v symmetry) showed one imaginary and one low frequency. Additional geometries in which the terminal CH 2 groups were rotated

3-(0,0)

8-(0,90)

3-(90,90)

272

HEINZ D. ROTH

Table 2. 1H Hyperfine Coupling Constants [G] for Radical Cations of Selected Cyclopropanes _

i

,

,

R a d"i c a l C a t i o n s

,

Calculation !

,

.

CIDNP

.

.

.

.

.

.

.

.

.

ESR

b

a'd

no result

a b

+

12.5 21.0

no result

a b c

(-) (+) (+)

10.4 20.5 20.5

no result

a b c

(-) (+) (+)

11.9 21.8 21.8

a b c d

+ + +

a b c d

(-) (+) (+) (+)

9.8 14.5 20.6 17.9

a

+

b

+

a b

(+) (+)

15.0 18.7

a b c

(-) (+) (-)

5.1 6.6 2.0

c

a

c

a

.,,

d ~

a

|11

b ar

r

b A r ~ A r

a

b

+

a

b a

b a

+

;'Ar r

a

~

a b c

(-) (+) (-)

10.5 7.9 4.0

a b c

(-) (+) (+)

10.6 9.9 12.6

a

b

+

C

-

a

m

c

'1

nab

b

+

C

+


273

between the plane of the three-carbon unit (orientation "0") and planes perpendicular to it (orientation "90") likewise failed to qualify as local minima. Finally, the calculations revealed a substantial energy difference (-22 kcal mo1-1) between 3 .+ (CEv, 0,0 geometry) and 1.+ but failed to indicate a chemically significant barrier for the conversion of 1.+ to 4.+ (90,90 geometry). Both results argue strongly against the possible ring opening of 1.+ to 3 "+. Several attempts have been undertaken to provide theoretical support for a cyclopropane radical cation of the ZBz structure type. For example, Ki"oghJespersen and Roth 67 investigated JT-type distortions in a series of methyl-substituted cyclopropane cations, including the parent cation (a), the 1-methyl (b) 1,1-dimethyl (c), the trans-2,3-dimethyl (d), cis-2,3-dimethyl (e), and the 2,2,3,3tetramethyl (f) substituted species. Two sets of radical cationic structures were considered, reflecting the first-order JT distortions occurring in the parent Cation (a) from a doubly degenerate 2E' (D3h symmetry) ground state to nondegenerate states of 2Ai and ZB2 symmetry (C2v point group). States of the "ZAl-type" possess one long and two short-ring C-C bonds (1) and are always structural minima on their respective potential energy surfaces. They represent the minimum energy structures for la, ld, le, and If. The potential existence of the 2B2 structure type (2) and its stabilization by methyl substituents was specifically addressed. The "2B2-type" states are structurally characterized by two long-and one short-ring C-C bonds. For both mono- and gem-dimethyl derivatives, the 2Bz-type structures are the preferred first-order JT type distorted structures (2b, 2c); their energies lie below the 2A1 structures (lb, lc). However, these species are transition states and undergo a second-order JT distortion to unsymmetrical (scalene) triangular structures with one very long C-C bond which represent the absolute minima for (5b, 5c; Figure 5). These structures may be viewed as distorted from the "ZB2-type" geometries via a second-order JT-type mechanism or, alternatively, as "2Al-type" with the substituents at the "wrong" carbon atom. The calculations suggest that the radical cation state preference can be fine-tuned by appropriate substituents and predict substantial differences in spin-density distributions. These predictions should be verifiable by an appropriate spectroscopic technique (ESR or CIDNP) and might be probed via the chemical reactivity of the radical cations (vide infra). Although the results of high-level ab initio calculations argue strongly against the possible ring opening of 1"+ to 3"+, this reaction was postulated for several cyclopropane radical cations to rationalize low-temperature ESR observations in CF2CI--CFC12 matrices at cryogenic temperatures. 56'57Upon warming the matrices, the primary ESR spectra decayed irreversibly at temperatures characteristic for the

,, + \ 1

"4.

2

"4.

", 5

"4.

274

HEINZ D. ROTH

\!

6.9

7.2

7.2

/

Figure 5. Minima and transition states on th.e potential energy surface of the radical cationic states of 1-methylcyclopropane (PMP4/6-311G*//UMP2/6-31G* + A ZPE; relative energies in kcal/mol). 64

substrate, typically near 100 K. A second type of spectrum emerged, in which the protons at one cyclopropane center no longer interact with the electron spin. This coupling pattern was interpreted as evidence for a ring-opened trimethylene species, 3 "+, in which one terminal carbon has rotated into an orthogonal orientation. 56'57 Obviously, the conclusions derived from the ESR results for the various cyclopropane radical cations are incompatible with the ab initio calculations on the parent system. Accordingly, the postulated structure of the ring-opened species and the mechanism of its formation were challenged on the basis of several arguments. 63'68 Interestingly, the putative rearrangement was found to be limited to two matrices, CFC12CF2C1 and CF2C1C2CI, posing an intriguing question concerning the role of the matrix in the "rearrangement." Several alternative explanations were advanced, including structures in which the cationic center of 1"+or 3 .+ has captured a chloride ion (----~6"),63'64 interacts with a matrix molecule to form a chloronium substituted

1,1,3,3-tetramethyl-1

9

1,1,3,3-tetramethyl--3"+

1,1,3,3-tetramethyl--6


275

free radical, 6s or interacts with several nearby matrix molecules. 56 If the actual species were the 13-chloroalkyl radical (6 ~ formed by nucleophilic ring opening by chloride ion, one possible role of the matrix may lie in preventing the approach of the ion at the lower temperatures. However, the ionic diameter of the chloride ion (-363 pm) would require a rather tight matrix, perhaps not a very likely assumption after hours of radiolysis. Another problem with the existence of species 3.+ is its failure to undergo a hydrogen shift, generating the known and stable propene radical cation. 69'7~Hydrogen shifts have been observed in numerous electron transfer induced reactions, 71-76 and are known to occur in matrices at cryogenic temperatures under the very conditions that give rise to the putative 3"+. 71-74 The hydrogen migrations will be discussed in more detail below (see Section 6). A more detailed evaluation of the diverse structures proposed for the secondary species goes beyond the scope of this review. We merely emphasize that the ESR results provide detailed evidence for the nature of the radical center, but fail to elucidate the cationic site. The identity of this center is left to secondary considerations or speculation. We also note that any alternative structure has the virtue of not contradicting the ab initio calculations; the potential capture of chloride ion has precedent in the nucleophilic substitution at a cyclopropane carbon (see Section 7). Another type of ring-opened structure has been postulated as an intermediate in the aminium radical cation catalyzed rearrangement of 1-aryl-2-vinylcyclopropanes (see Section 5). The structure type related to the eyclopropane S HOMO prevails also for radical cations derived from substrates with two conjugated cyclopropane rings---for example prismane (7), quadricyclane (8), and tetraeyclooctane (9). Quaddcyclane and its radical cation have long been targets of interest, and the principal features of their structures are well established. The antisymmetrical combination of the two cyclopropane S HOMOs becomes the HOMO for quadricyelane. 49'77-sl Many attempts to observe 8"+ directly have failed; some of them resulted in rearranged radical cations. 49'5~ CIDNP results s4's5 and ab initio MO calculations a6 indicated that the spin (and charge) density of 8 .+ is delocalized over the four cyclobutane carbons, causing the protons attached to these carbons to have negative hfcs via n,a-spin polarization. 26'27 On the other hand, the 1H nuclei in the bridgehead positions have strong positive hfcs; these are caused by the spin density at the cyclobutane carbons via r~,o-spin delocalization. 26'27These assignments have been confirmed by a recent time-resolved (TR) ESR study, s7 which identified hfcs of 5.1 (4H; cyelobutane 1H), 6.6 (2H; bridgehead 1H), and 2.0 G (bridge 1H).

7

8

9

276

HEINZ D. ROTH

A similar structure was assigned to the hexamethylprismane radical cation (7 .+) and to its hexamethyl derivative (hexamethyl-7.+) based on ab initio calculations and CIDNP effects, respectively. 88 Also, the higher homolog of 8 .+ the tetracyclo[4.2.0.02'8.05'7]octane radical cation 9.+, was characterized by EPR spectroscopy with hfcs of 5.7 (4H; cyclobutane IH) and 2.7 G (2H; bridgehead 1H); the hfcs of the bridge protons were not resolved. 89 In comparing the spectral data for 8"+ and 9.+, it is interesting to note the similarity of the cyclobutane hfcs, yet the significant differences in the bridgehead hfcs. The difference can be ascribed to different dihedral angles between the spin-beating p-orbitals and the C1-H1 and C4-H4 axes.

4. ALTERNATIVE CYCLOPROPANE RADICAL CATION STRUCTURES Since there is ample evidence for radical ions derived from the prototype of 2A1 symmetry (vide supra), cyclopropane radical cations with the alternative, antisymmetrical singly occupied (SO) MO appeared be ofparticular interest. In view of the negative results of ab initio calculations on simple methyl-substituted derivatives, we approached this structure type by studying substrates in which the cyclopropane moiety is aligned rigidly relative to one or two olefinic or aromatic functions, so that either one of its degenerate HOMOs or the other can interact with the olefinic/aromatic p-orbitals. For example, we consider the norcaradiene system 10 containing the cyclopropane function in an orientation relative to a butadiene group that may allow an interaction with the antisymmetric cyclopropane HOMO.

Other systems studied include: the spiroheptadiene system (13) in which the cyclopropane moiety lies in the nodal plane of the butadiene HOMO; [ 1:2,9:10]bismethano[2.2]paracyclophane (15), in which two cyclopropane moieties are joined with two benzene rings in such a way that tertiary-tertiary cyclopropane bonds lie parallel to the aromatic n system; 7-methylenequadricyclane radical cation (16.+), in which a pair of cyclopropane groups lie orthogonal to an olefinic moiety; and the 7-spirocyclopropanenorbornadiene radical cation (18"+), in which a pair of nonconjugated ethene groups and a cyclopropane moiety are joined. The structure of the corresponding radical cations should be determined by the symmetry of the two fragment FMOs at the points of union. The butadiene HOMO is antisymmetric at the positions of attachment; in the norcaradiene framework, it may interact with the antisymmetric cyclopropane HOMO (as shown above). Indeed, norcaradiene derivatives provide the most promising examples of radical


277

cations containing a cyclopropane function of the 2B2 structure type. This is born out by three types of evidence, including ab initio calculations for the parent radical cation, CIDNP effects observed for the benzo derivative 11, and the results of the electron transfer photochemistry of the 1,6-trimethylene-bridged tricyclo[4.3.1.01'6]deca-2,4-diene (12).

While ab initio calculations fail to support 2B2 type structures for the simple methyl-substituted cyclopropanes, 67 a radical cation of this structure type was indicated for the norcaradiene radical cation 10"+.90 This more highly strained valence isomer of cycloheptatriene is accessible only in appropriately substituted derivatives, such as 11 or 12. Ab initio calculations on 10"+at the MP2-6-31 G* level of theory support a structure featuring elongated C1-C7 and C6--C7 bonds and bearing spin density in the 2-, 4-, and 7-positions. These results suggest that the butadiene fragment interacts with the cyclopropane moiety by homoconjugation. However, the spin density in the 2- and 4-positions is three times as large as that at C7, indicating a less than perfect match of orbital energies for the two fragments. Attempts to elucidate the chemical reactivity of these radical cations will be discussed below. The assignment of an antisymmetrical cyclopropane SOMO to the radical cation of benzonorcaradiene (10) 58 is based on a comparison of CIDNP effects (Figure 6) with those for cis-1,2-diphenylcyclopropane. While the nuclei of the aromatic segments show the identical signal directions, the cyclopropane protons show characteristic differences. This suggests significantly different spin-density distributions for the cyclopropane moieties of the two species and, thus, different structures. 58 Like the norcaradiene HOMO, the styrene HOMO is antisymmetric at the positions of attachment, suggesting preferred interaction with the antisymmetric cyclopropane HOMO. In the norcaradiene system, the "natural" structure (2A1) of the cyclopropane radical cation is altered by the interaction with the diolefin entity. Interestingly, no EPR studies have been published to date, even though the CIDNP effects have been known for a decade, and the synthesis of 11 and 12 are straightforward. The lack of symmetry in 11 and the prospect of unraveling as many as 10 different hfcs may serve as a deterrent. For the CIDNP investigation, on the other hand, the existence of magnetically distinct nuclei is a decided advantage, as it allows a detailed and unambiguous account of the spin-density distribution. In the case of spiroheptadiene (13), the cyclopropane moiety lies in the nodal plane of the butadiene HOMO, causing the interaction with either cyclopropane HOMO to be negligible. On the other hand, the dibenzo derivative of 13, spiro[cy-

278

HEINZ D. ROTH

HBHA

it

HA..../HB ' ~/H c C6H S ' ~ " " Hc C6H5

i

Hc

HA..../HB

B

~

A

Z I5

0.0

-0.5

Figure 6. 1H CIDNP spectra (cyclopropane resonances) observed during the electron transfer photoreaction of chloranil with cis-1,2-diphenylcyclopropane (top) and benzonorcaradiene (bottom). The opposite signal directions observed for analogous protons in the two compounds constitute evidence that the two radical cations belong to two different structure types. ;7's5

clopropane-l,9'-fluorene] (14) 91 has more closely spaced orbitals. CIDNP effects observed for this system suggest a radical cation in which the 2B2 FMO of the cyclopropane ring interacts weakly with the second highest MO of the biphenyl segment causing the less common radical cation structure.

While the tertiary-tertiary cyclopropane bond of benzonorcaradiene (11) lies in a plane perpendicular to the aromatic n system, a parallel arrangement of these


279

elements is featured in [ 1:2,9:10]bismethano[2.2]paracyclophane (15). 92 The photoreaction of this obviously strained compound with chloranil gives rise to CIDNP effects not unlike those observed for cis-diphenylcylopropane (cis- 1,2-diphenyl-1). However, the relative signal intensities of 15 are noticeably distorted. The (secondary) endo-proton (H n, 1.2 ppm) is more strongly enhanced (indicating a larger hyperfine coupling) than both the exo (H x, 2.0 ppm) and the benzylic protons. This finding was interpreted in terms of different dihedral angles, Onn and Onx, between the singly occupied orbital and the two geminal C-H bonds. Nonidentical dihedral angles for the secondary cyclopropane protons can result if the "flap" angle and the distance between the benzylic cyclopropane carbons is increased in the radical cation relative to the parent hydrocarbon. The endo-proton, which in the diamagnetic molecule lies in the shielding cone of the aromatic moieties, has in the radical cation the smaller dihedral angle 19 with the benzylic "r~"-orbital and, therefore, the greater hyperfine coupling. 93 l-ln

15 The comparably weak enhancement of the benzylic protons and the relatively strong ZaC polarization for the benzylic carbons can be explained if the benzylic carbon adopts a pyramidal structure. Although radicals containing pyramidal carbon are relatively rare, the existing examples are derived from strained-ring systems. 94-97 It is an interesting question whether the radical cation of lb .+ is localized on a single cyclopropane entity or whether it is delocalized over both strained rings. The CIDNP method does not lend itself to decide this issue because the two cyclopropane groups are magnetically equivalent in the substrate/product. The ESR method, on the other hand, is well suited to elucidate the precise nature of the intermediate since a radical cation involving only one cyclopropane group will have fewer interacting nuclei. In the case of two interacting cyclopropane groups, it is possible, in principle, to distinguish a fully delocalized system from rapidly equilibrating "localized" ones. 98'99However, this method may not be applicable to distorted strained-ring systems, as pointed out recently. 1~176 Another interesting interaction is that between a pair of cyclopropane groups, such as those of quadricyclane and an olefinic moiety arranged rigidly near them, as realized in the 7-methylenequadricyclane radical cation 16.+. Since the exomethylene p-orbital is arranged orthogonal to the HOMO of the quadricyclane fragment, one might expect negligible interaction between the two functions and, thus, a spin-density distribution much like that in 8"+. As expected, the CIDNP effects observed during the photoinduced electron transfer reaction of 16 suggest

280

HEINZ D. ROTH

that the spin density is located in the four carbons of the cyclobutane frame. However, the results also indicate strong negative spin density for the exomethylene carbon (C8) of 16.+. This finding is tentatively ascribed to an exchange interaction between the SOMO of the 8 fragment and the FMO of the ethene unit. This type of interaction is operative typically in systems which are aligned orthogonal to each other. 1~ The principal features of the hfc-coupling pattern derived from the CIDNP results were reproduced by ab initio molecular orbital calculations on the radical cations 16 .+ (at the MP2/6-31 G* level).

We have also considered the converse case, the interaction between a pair of nonconjugated ethene groups, such as those of norbornadiene (17), and a cyclopropane moiety, once again arranged rigidly near them. This case is realized in the 7-spirocyclopropanenorbomadiene radical cation (18). The structure of the norbornadiene fragment 17 .+ rests on the polarization pattern of the CIDNP spectrum, 84'85 ab initio MO calculations, 86 and the hfc pattern from ESR and ENDOR data. 1~176 All three methods agree that spin (and charge) is delocalized over the four equivalent olefinic carbons. The bridgehead protons ofN ~ lie in the nodal plane of the corresponding SOMO, causing the n,a-spin delocalization mechanism (hyperconjugation) typically expected for 13-protons26 to be inefficient. The weak negative hfc suggested for these protons was ascribed to "residual" r~,t~-polarization, 1~ an assignment born out by ESR/ENDOR results, 1~ CIDNP effects, 84'85 and calculations 86 alike. This type of interaction is usually obscured by the much stronger n,t~-spin delocalization mechanism. The CIDNP effects for the spirocyclopropane signals of 18 are considerably weaker than those for 16. The homoconjugative and exchange interactions between the 17 SOMO and the cyclopropane A FMO must be relatively weak, which can be ascribed to the less than perfect match between the respective orbital energies.

18

Radical Cations of Cyclopropane Systems 0

281

RADICAL CATIONS OF VINYLCYCLOPROPANE SYSTEMS

Having delineated several structure types for various cyclopropane radical cations, we turn to systems contai'ning a single olefinic moiety conjugated with the cyclopropane function. The simplest system containing both functions is the vinylcyclopropane radical cation 19.+ (see Figure 7), which has not been adequately characterized by either experiment or calculation. The reactivity of 19.+ has been studied in the gas phase. 1~ Changes in the mass spectrum of the resulting ion(s) as a function of varying modes of ionization were interpreted as evidence that 19.+ suffers ring opening to penta-l,3-diene radical cation. The interesting question whether this type of chemistry also occurs in solution is discussed in Section 6. An

Figure 7. Calculated bond lengths (pm) and hyperfine couplings (G) for the syn- and anti-conformers of vinylcyclopropane radical cationic states (MP4SDQ/6-31G*).TM

282

HEINZ D. ROTH

STO-3G calculation for 19"+ was carded out as a model for tricyclo[5.3.1.01'7]un deca-2,4,9-triene with the vinyl group and two cyclopropane carbons held coplanar; 1~ this approach surely was dictated by the need to minimize the required computation time. To our knowledge, only one vinylcyclopropane system, bicyclo[4.1.0]hept-2-ene, was studied by EPR. However, the prominent hfc (A ~ 50 G) reported, and assigned "to a single axial 13-proton in the C4 position" only serves to identify C3 as a center bearing major spin density, and does not permit any conclusion concerning the delocalization of spin and charge onto the cyclopropane ring. 1~ Perhaps the most thorough study of a radical cationic system containing the vinylcyclopropane fragment involved the geometric isomerizations induced in 1-aryl-2-vinylcyclopropanes (20) upon reaction with aminium radical cations. 18'23 The stereochemical course of these reactions was explained via ring-opened cyclopropane radical cations. Of course, the phenyl moiety present in these systems may veil the "true" nature of the vinylcyclopropane radical cation by taking part in delocalizing of spin and charge. We suggest that the interpretation of the experimental findings allow some latitude. If the potential surface of this radical cation contains several shallow minima separated by low barriers, the isomerization products may offer little information about any one minimum. In fact, even ESR results, could they be obtained at -90 ~ might not differentiate unambiguously between a planar ring-opened radical cation (22-*) and a rapidly equilibrating pair of orthogonal bifunctional intermediates (e.g., 21"*). Perhaps the most likely explanation involves a pair of isomeric cyclopropane radical cations with one very weak bond (viz. 20-*), which might allow either or both tertiary carbon to rotate. The bifunctional structures shown below, either orthogonal (e.g., 21-*) or planar (22-*) may signify minima or transition structures.


283

On the other hand, the CIDNP approach and recent ab initio calculations have provided what we consider unambiguous evidence for the nature of the vinylcyclopropane radical cation as well as for some simple derivatives. We have carried out ab initio calculations on the prototype 19.+ and several simple derivatives, and probed by CIDNP the structures of three simple vinylcyclopropane radical cations in which the two functionalities are locked in a syn configuration. Calculations of the vinylcyclopropane radical cation, carried to the MP4SDQ/631 G* level of theory, support the existence of two minima, syn- and anti-19 "+.Both structures feature two lengthened cyclopropane bonds: 1.611 and 1.612/~ for the syn- anti-isomer, respectively. The spin density is located mainly in the terminal vinyl carbon: p = 0.881 and 0.853 for the syn- anti-isomer, respectively, with lesser spin densities in the tertiary, 9 = 0.258 and 0.224, and secondary cyclopropane carbons, p = 0.113 and 0.134/~, respectively, for syn- and anti-19 ~247 Correspondingly, large negative hfcs are calculated for the terminal olefinic (-33 - - 3 5 G) and tertiary cyclopropane protons (-- -10 G). Previous experience with radical cations of various structure types 67'86'88'107'108suggests that these Values are likely overestimated by a factor o f - 2 . Interestingly, the hfcs calculated for the secondary cyclopropane protons syn and anti to the vinyl group have opposite signs: -2.3 G (-1.6 G) for Hsy,, vs. +8.0 G (+5.6 G) for Hanti (Figure 7). This interesting manifestation of stereoelectronic effects is as yet unconfirmed by experimental results for 19.+ but has precedence in the CIDNP effects observed for bicyclo[3.1.0]hex-2-ene (23) discussed below. We expect the positive hfcs to be predicted accurately by the ab initio calculations. 67'86'88'107'108 We have also applied the CIDNP method to probe the hfc patterns and structures of three rigid vinylcyclopropane radical cations, with the two functionalities locked in a syn configuration. Spin polarization effects observed during electron transfer reactions of bicyclo[3.1.0]hex-2-ene (23), bicyclo[4.1.0]hept-2-ene (norcarene, 24), and 3,7,7-trimethylbicyclo[4.1.0]hept-2-ene (carene, 25) suggest two significantly different structure types in which either the internal cyclopropane bond (as in 24.+) or a lateral one (e.g., 23 .+ or 25.+) is involved in delocalizing spin and charge. 1~ These findings elucidate the electronic and stereochemical requirements for conjugation between the two functionalities. Precedent suggested that three structure types be considered for these radical cations: in addition to the well-documented structure with one lengthened C--C bond (23-25a.+), we considered species of a structure type with two lengthened bonds (23-25b.+), as well as the ring-opened bifunctional structures (23-25e.+).

Q 23a

9 +

23b

"+

23c

"+

284

HEINZ D. ROTH

H3

H2

H1

H6anti AH6syn

H4syn H4an

9

9

I

6.0

w

9

i

l

I

5.0

|

3.0

i

H5

I

i

!

!

2.0

!

!

i

9

i

1.0

!

i

i

!

!

I

i

0.0

Figure8. 11--INMR spectra (250 MHz) ofacetone-d6 solutions containing 20 mM each of bicyclo[3.1.0]hex-2-ene and chloranil in the dark (bottom)and during UV irradiation (top).The identity of the CIDNP signals is assigned in the Figure; the signals of an impurity (2.8 ppm) and of the residual protons of the solvent (2.0 ppm) have been omitted in both traces for reasons of clarity.1~ The latter structures have the advantage of minimized strain energies, albeit at the expense of diminished delocalization. The effects observed for 23 (Figure 8) are particularly clear-cut, since the IH spectrum is fully resolved. 110The key to the structure of 23 .+ lies in the prominent enhanced absorption signals Of H3 (5.2 ppm) and H6syn(--0.2 ppm) and the strong emission of H4anti(2.4 ppm), H4s~.n (2.2 ppm), and H5 (1.5 ppm). This polarization pattern supports a sp~ies with spin density on C3 and C6, indicating the delocalization of spin and charge into the lateral cycl0propane bond. Weakly enhanced absorption observed for H2 (5.8 ppm), n6anti (0.8 ppm), and H1 (1.75 ppm), and weak emission for H5 further support this structure type. z~ We rationalize the involvement of the less substituted lateral bond of 23 instead of its more highly substituted internal bond as reflecting the need for orbital overlap. The bicyclohexene system has limited mobility, allowing the lateral cyclopropane bond more significant orbital overlap with the alkene p-orbitals (23.+). The partici-


285

pation of the lateral bicyclohexene bond in delocalizing spin and charge is also supported by ab initio calculations, carried to the MP2/6-31G* level of theory. The lateral cyclopropane bond is lengthened (C1-C6 = 1.748 A), and carbons C3 and C6 carry prominent spin density, with lesser spin density at C2 and C1.111 CIDNP effects observed for norcarene 112'113 support a radical cation 24 '+ with spin and charge delocalized between the olefinic group and the Walsh orbital of the (more highly substituted) internal cyciopropane bond. Enhanced absorption signals for H3, H 1, and H6 indicate spin density on C3, C 1, and C6, which is confirmed by the emission signals for the multipiets of the adjacent protons, H4syn,anti, H5svnanti, and H7syn.anti.114 The bicycloheptene system 24.+ appears to be more flexil~le than 23.+, allowing either the internal or the lateral cyclopropane bond to aligri with the alkene p-orbitals. Accordingly, delocalization of spin and charge into the more highly substituted bond is preferred (24"+).

23 +

24 +

25 +

Finally, the CIDNP effects for carene suggests a radical cation structure 25 .+ in which spin and charge are delocalized between the olefinic group and the Walsh orbital of the (more highly substituted) laieral cyclopropane bond. The change in structure (25.+ vs. 24 .+) caused by the introduction of the geminal methyl groups at C7 reflects a more stable radical cation due to the hyperconjugative interaction with the methyl groups. Thus, for both 24.+ and 25.+, the more highly substituted cyclopropane bond participates in delocalizing spin and charge. The different results obtained for the three radical cations, 23.+, 24.+, and 25 "+, raise interesting questions concerning the preferred site and the steretichemistry of nucleophilic capture (Section 6). Actually, the earliest derivative of a vinylcyclopropane radical cation was a serendipitous discovery. It was formed by an unusual hydrogen shift upon photoinduced electron transfer oxidation of tricyclo[4.1.0.02'7]heptane (26). 112-114This result has been questioned on the grounds that the same rearrangement was not observed in a Freon matrix. 1~ However, there is no basis for the assumption that radical cation reactions in frozen matrices at cryogenic temperatures should follow the same course as those at room temperature in fluid solution and in the presence of a radical anion, which is potentially a strong base. In several cases, matrix reactions have taken a decidedly different course from those in solution. For example, radiolysis of 8 in a Freon matrix generated the bicyclo[3.2.0]hepta-2,6diene radical cation (27.+),82 or caused retro-Diels-Alder cleavage; 82 yet, the

286

HEINZ D. ROTH

electron transfer photochemistry of g in solution showed no traces of products due to these reactions. 115'116 Of course, one rationale frequently offered for matrix isolation studies is the opportunity to generate reactive species under exclusion of reagents that would be present in fluid solution.

27

94.

8

"4-

17

"4.

The [4 + 2] dimer of spiro[4.2]heptadiene (28) allows an interesting comparison between the stabilization by conjugation versus homoconjugation. Each of the 1,1-disubstituted cyclopropane groups of 28 is rigidly oriented relative to a 1,2dialkyl-substituted olefin. Two "localized" radical cations derived from 28 can be stabilized either by homoconjugation with the norbomene fragment or by conjugation with the cyclopentene n-orbital. CIDNP results 117'118 identify the resulting radical cation as a "vinylcylopropane" species. Apparently, stabilization by conjugation outweighs the possible stabilization by homoconjugation. For species 18"+, the cyclopropane Walsh orbital interacts with two olefinic moieties and is, therefore, stabilized by twice the homoconjugation energy available for 28 .+.

6. ELECTRON TRANSFER PHOTOCHEMISTRY OF CYCLOPROPANE SYSTEMS: RADICAL CATION REACTIVITIES The electron transfer-induced reactions of cyclopropane derivatives containing olefinic moieties have been studied in some detail. 11-25 In the following section, we will delineate the reactivity of a range of cyclopropane compounds and examine the results for any features that may allow insight into structures of the corresponding radical cations. Because radical cations contain both an unpaired spin and a positive charge, one might expect these intermediates to display a dual reactivity, characteristic for one of these functions or for the other. Among the wide range of reactions of radical cations derived from strained-ring systems, various rearrangements of the carbon skeleton have been reported, typi-

287


cally resulting in the release of ring strain. Several cyclopropane derivatives undergo geometric isomerization upon reaction with photo-excited singlet acceptors. Although CIDNP studies have clearly demonstrated the steric integrity of cis- and trans-1,2-diphenylcyclopropane radical cations during the radical ion pair lifetime (when they are generated by reaction with triplet sensitizers), 17 cis-trans isomerization results from the electron transfer reaction with several singlet sensitizers. 16'17This conversion has been ascribed to reverse electron transfer in pairs of triplet spin multiplicity, a process which populates a (ring-opened) triplet state with two orthogonal p-orbitals. The structure of this intermediate lies between the geometries of cis- and trans-isomer, and can decay to regenerate either isomer. sHs

~C6Hs C6Hs

~ "~ CsHs

cis-diphenyl-1

trans-diphenyl-1

Cyclobutane cycloreversions are among the typical electron transfer reactions of strained ring compounds. For example, quadricyclane 8, a system containing three adjoining ("cumulated") strained rings, can be converted to norbornadiene (17). In this system, the driving force for the cyclobutane to diolefin conversion is increased by the breaking of two cyclopropane bonds, releasing their strain energy; 19-22it has a lifetime in the 100 ns range. 87 The lower and higher homologs of 8 "+, 7 "+, and 9 .+ show an interesting gradation of reactivities. The radical cation 9.+ was generated by irradiating its valence isomer 29.+, whereas the most highly strained member of the group 7.+ has only a fleeting existence; it could be identified only tentatively by a weak CIDNP spectrum. 88

9

29 .+

hv

9 .+

Other rearrangements, including the ring enlargement with dehydrogenation (30 ---> 32), observed upon the electron transfer sensitized irradiation of allyl- or benzylcyclopropene systems, 119 or the puzzling conversion of dimethyldicyclopropenyl to dimethylbenzvalene, 12~are part of the rich chemical lore of strainedring radical cations without, however, providing insight into the corresponding structures.

288

HEINZ D. ROTH

Ph

30

Ph

Ph

31

Ph

32

Vinylcyclopropane radical cations can undergo two rearrangements that would relieve its ring strain; the vinylcyclopropane rearrangement would generate a cyclopentene system, whereas hydrogen (hydride) transfer would lead to pentadiene derivatives. There is precedent for both vinylcyclopropane rearrangement and hydride transfer in radical cations. For example, the electron transfer-induced chemistry of various p-anisyl derivatives revealed several rearrangements of the vinylcyclopropane type. 18'23 Hydrogen migrations have been demonstrated or invoked in several instances, although the detailed course of these reactions has not been fully elucidated. The involvement of hydrogen shifts is postulated based on experimental data, such as EPR or CIDNP spectra, the structure of reaction products, or on theoretical calculations (Section 3). 65,66 Although some migrations clearly involve a hydride shift across several C--C bonds, TM the shifts observed for cyclopropane radical cations clearly are 1,2-migration from the "central" carbon to one of the carbons of the one-electron bond. 71-74,112.113 For example, EPR evidence showed that cyclohexane-l,4-diyl, generated by radiolysis of hexadiene, rearranged to cyclohexene radical cation. 71-73 Similarly, anti-5-methylbicyclo[2.1.0]-pentane radical cation (33) rearranged to 1-methylcyclopentene radical cation (34) via a 1,2-shift of the syn-5-hydrogen. 74

33

34

The electron transfer induced rearrangement of tricyclo-[4.1.0.02'7]heptane (26) to bicyclo[4.1.0]hex-2-ene (24) has been mentioned; 112-113 this conversion was recognized with the help of CIDNP data. A related ring opening reaction was observed for benzonorcaradiene 11; the chloranil photosensitized reaction of 11 showed two polarized reaction products: 1- and 2-methylnaphthalene (35, 36) 58 via hydrogen migration from a tertiary to a secondary carbon. Apparently, either bond linking the cyclopropane methylene group with the dihydronaphthalene skeleton may be broken. The driving force of the net conversion lies in the relief


11

289

35

36

of ring strain and the formation of an aromatic system. Mechanistic details have not been established. As mentioned above, the conversion of cyclopropane to propene radical cation has been investigated by ab initio calculations. 65'66 The general course of this reaction was confirmed, or anticipated, by product studies in the electron transfersensitized conversion of 1,1,2,2-tetraphenylcyclopropane (37) to 1,1,3,3-tetraphenylpropene (38). 15The sequence of the key steps, migration versus ring opening cannot be derived from the results. In the case of 37"§ the four phenyl substituents may actually favor a ring-opened bifunctional radical cation. Ph ~ . ~ ~ / P h ph/

Ph~H

Ph

~ Nph

--

Ph

37

Ph

38

We have studied the electron transfer-induced isomerization of two vinylcyclopropanes, in which the two functionalities are locked in the anti- (sabinene 39) or syn-orientation (t~-thujene 41). Both systems are converted to less strained, fully conjugated products; the reaction of (1R,5R)-(+)-39 gives rise to (S)-(+)-~-phellandrene (40), 12 whereas (1S,5S)-(+)-thujene (2) is converted to (S)-(+)52). These considerations account for the preference of 51 over 52 as well as for the suppression of the diastereomer.

294

HEINZ D. ROTH

Ar

52

50

5CH3

Electron transfer-induced nucleophilic addition to several other cyclopropane compounds was also studied. The nucleophilic addition of methanol to quadricyclane radical cation 8"+ produces the two methanol adducts 53 and 54. The stereochemistry of the methoxy groups in these structures identifies the preferred direction of nucleophilic attack upon the intermediate radical cations 8"*. Detailed NOE experiments delineate the structure of 53 and establish conclusively that the norbornene derivative 54 contains a 7-anti-methoxy group. The stereochemistry of both is compatible with stereospecific nucleophilic attack exclusively from the exo-position. 132 7-Methylenequadricyclane also is attacked exclusively from the exo-face. 133 These results can be explained via backside attack with inversion of configuration.

/OCH3

F 8

Nu

.+

53

54

Another ring-opening substitution reaction was observed for tricyclane 55; the attack occurred exclusively at the tertiary carbon, not at the quaternary one. 134 T h e chiral isomer 56, of the symmetrical 55, has two 30--4~ bonds either of which may be the site of spin and charge, possibly in an equilibrium. Regardless of the actual structure of the radical cation, it appears that the attack of the nucleophile is less hindered at the carbon further removed from the dimethyl-substituted bridge (approach a). The isolated product 57 is optically active, and formed by backside attack on the less hindered carbon. 135 Nu

.3co

Nu Nu 55

0+

56

o+

57

Cyclopropane derivatives of various structure types are involved in electron transfer-induced oxygenation reactions. For example, the photoreaction of 9,10-dicyanoanthracene with 1,2-diarylcyclopropanes generates a mixture of cis- and

295


trans-3,5-diaryl-1,2-dioxolanes (58). 136 Adducts of the same structure-type could be observed upon irradiation of charge transfer complexes between tetracyanoethylene and tetraarylcyclopropane systems. 137-14~In some of these systems, geometric isomerization was more efficient than oxygenation. The solvent polarity dependence of product yields suggests the involvement of radical cations which, depending on the reduction potential of the acceptor, will couple to superoxide or molecular oxygen. hv

""tt,~6H5

HsC6~

DCA/02

trans-diphenyl-1

CsHs

0--0 58

7. NUCLEOPHILIC CAPTURE OF CYCLOPROPANE RADICAL CATIONS: CORRELATIONS BETWEEN RADICAL CATION STRUCTURE AND REACTIVITY The various reactions of cyclopropane radical cations discussed in the preceding section have elucidated several facets of their reactivity. The results raise questions concerning the factors that determine the products observed. More significantly, we will consider whether the structures, the stereochemistry, and the chirality of the products can be related unambiguously to the structures of the radical cationic intermediates, particularly to their spin- and charge-density distributions. Many of the reactions discussed are not suitable to establish such a relationship, either because of the general stereochemical course of the reaction type, or because of the inherent symmetry of the substrate. Any reaction in which the pyramidal sp3 hybridized cyclopropane centers are converted to planar sp2 hybridized ones will lose any memory of the radical cation chirality or stereochemistry, unless it is transferred to a new chiral center generated in the course of the conversion. On the other hand, there are several reaction types which, given appropriate substrates, may be used to probe the stereochemical course of a cyclopropane radical cation reaction. For example, several hydrogen migrations have shown elements of stereoselectivity. 12'74 Similarly, oxygenation reactions may have the potential to reveal some degree of stereoselectivity. However, electron transfer-induced photoreactions in the presence of nucleophiles have attracted by far the greatest attention; a rich variety of cyclopropane systems have been subjected to these reaction conditions. We will consider several factors that may affect the structure of the radical cations as well as the stereo- and regiochemistry of their nucleophilic capture. Factors to be considered include: (1) the spin and charge density distribution in the cyclopropane radical cation (the educt); (2) the spin density distribution in the free-radical product; (3) the extent of

296

HEINZ D. ROTH

conjugation in both educt and product; (4) steric factors determining the approach of the nucleophile to the educt and, thereby, affecting the free energy of the transition state; and (5) the release of ring strain upon forming the product. The significance of these factors can be evaluated in appropriate systems. The consideration of structural aspects of starting material (i.e., the radical cation; educt) as well as product (the free radical resulting from nucleophilic capture) may require some justification. The rate of a chemical reaction is generally recognized to be determined by the transition state of the conversion. It is a distinguished tradition of physical organic chemistry to identify those features of either the educt or the product which affect the structure and free energy of the transition state. We mention the Hammond postulate designating a transition state as "early" or "late", respectively, if it reflects the features of the educt or the product. TM It is also a well-established practice to specifically differentiate "kinetically controlled" reactions, whose products are formed via a favorable (low-lying) transition state, from "thermodynamically controlled" reactions, which yield the more stable products (having lower free energies). Accordingly, it is an accepted practice to consider factors that stabilize either educt or product and to consider their contribution to the stability of the transition state. In the special case of radical cations, it has been shown that the barriers for many of their reactions may be significantly reduced. 18'23'129'142-144 We suggest that the transition states of these reactions may reflect favorable (stabilizing) aspects of both educts and products. Of course, it is not obvious whether the conversion of radical cations to free radicals by nucleophilic capture of methanol occurs in a single step or in several discrete steps. The analogy to the well-known solvolysis reactions suggests that at least the deprotonation of the oxonium function is a separate step. In addition, the involvement of a complex, in which the oxygen is loosely bound to the r~-system or an appropriate hydrogen atom of the radical cation, may have to be considered. For example, ab initio calculations support the existence of a hydrogen-bridged water-ethene radical cation complex, C H 2 = C H . . . H § 9.OH2, on the potential energy surface of the ethanol radical cation and its [3-distonic isomer. 145,146 If the nucleophilic capture of radical cations indeed occurs via a reaction sequence of several steps, the reaction rate and the products formed may be determined by different reaction parameters. The relationship between these factors has not been probed exhaustively. The range of nucleophilic substitutions discussed above allow us to evaluate several factors affecting radical cation reactivity. For example, the balance between ring strain, on the one hand, and the degree of delocalization, on the other, appears to be subtle. For the structures of the educt, delocalization appears to be very important, even at the expense of ring strain. In spite of some notions to the contrary, there is little doubt that many cyclopropane radical cations maintain a significant degree of bonding and, as a consequence, retain some ring strain. On the other hand, many reactions of cyclopropane radical cations, either intra- or intermolecular, appear to be driven by the release of ring strain. For example, essentially all


297

nucleophilic capture reactions as well as hydrogen shifts fully release the ring strain present in the educt radical cations. At the same time, delocalization of unpaired spin in the free-radical product appears to be important for the course of the substitution reaction. For example: hydrogen shift in sabinene radical cation 39a "+ leads to a conjugated system (40"+); nucleophilic attack on 1-aryl-2-alkylcyclopropane radical cations 43 .§ or 47 .+ produces benzylic radicals; nucleophilic attack on 39a "+ generates an allylic species; and attack on the tricyclane radical cations 55 .+ or 56 .+ forms tertiary radicals. Apparently, formation of delocalized or otherwise stabilized free radicals is preferred. Concerning steric factors, 43 .+ is attacked in the most hindered position ("inverse effect of substitution"); 129'13~likewise, 39.+ is attacked at the most hindered carbon. Obviously, the transition states for the formation of 44o or 50o show limited sensitivity to the degree of substitution, and the relief of ring strain is a more significant factor than the steric hindrance in the transition state. On the other hand, steric factors are important in systems such as 13-phellandrene radical cation 40.+ which is attacked at the exo-methylene carbon (most easily accessible), or the tricyclane radical cation 56"+ which is attacked at the less hindered 3 ~ carbon further removed from the dimethyl-substituted bridge (approach a). 135 Both reactions also benefit from the formation of the most highly substituted, hyperconjugatively stabilized free radicals. So far in our discussion, we have delineated three factors that play a significant role in determining the course of nucleophilic capture in radical cations. The release of ring strain, the formation of the more delocalized product and, perhaps to a lesser extent, minimal steric hindrance all play a role in determining the products of these reactions. However, these factors fail to account for one significant detailJthe complete absence of products due to nucleophilic capture of radical cations such as 43"+ or 39 -+ at the secondary cyclopropane positions. The generation of free radicals 59o and 60o by attack on these carbons appears exceptionally favorable; it would occur with release of ring strain, formation of a conjugated n-system, and with minimal steric hindrance. Indeed, this general type of attack was reported for 47"+.131 In order to account for the absence of products derived from 59o and 60o we consider the charge density distribution in 43.+ and 39.+, particularly on C3 of 43.+ and on C6 of 39"+. The spin and charge densities of these radical cations are delocalized only into the more highly substituted C1-C2 and C1-C5 bonds, respectively, whereas the charge densities at C3 and C6, respectively, are insufficient to attract the nucleophile. Correspondingly, stereoelectronic effects and charge stabilization factors were invoked to explain the attack at the 2 ~ center of 47~ TM an explanation born out by our CIDNP study ofbicyclo[3.1.0]hex-2-ene. 1~ Accordingly, the distribution of spin and charge in the educt is relevant for the course of radical cation substitutions.

298

HEINZD. ROTH

HsC6

C6Hs

Nu

9

~

43

OCH3 59

,_ ~ ~ f , " ~ N u "..

39a

,

iK

~.OCH3

,

?-

.§

60

The significance of individual governing principles can be evaluated in systems designed in such a way that different pathways are favored by different principles. Thus, ~-thujene (61) was chosen to probe the relative significance of delocalization in the product and of steric factors. If the stability of(he product radical is important, nucleophilic capture should occur at the tertiary (C5) or secondary center (C6), not at the quaternary one (C 1). Because reduced steric crowding is anticipated for the corresponding transition states, these reactions may be faster than those of sabinene radical cation 39 "+, or o~-thujene radical cation 41 "+. As a result, the hydride shift, which competes efficiently in the case of sabinene 12 and a-thujene, 12~may be less favorable. Stereoelectronie effects of the kind discussed by Hixson TM and supported by our CIDNP study of bicyclo[3.1.0]hex-2-ene 1~ may favor conjugation with the lateral bond, directing attack to C6. Any "inverse effect of substitution," on the other hand, would direct the attack towards the quaternary carbon (C 1); the corresponding reaction rate should be unchanged, and the hydride shift would remain competitive. The results observed in this system unambiguously support free-radical stability as a significant governing principle. By far the major product (62) is derived by attack on C5; a minor product (63) is due to attack on C6 and hydrogen migration is effectively eliminated. These results show clearly that steric hindrance is of some importance in these reactions, although the transition states for some of these reactions appear to be remarkably "early."

st,,.,

61

.4.

62

OCH3

B"OCH3

n,o..~ 63

Radical Cations o f Cyclopropane Systems

299

8. CONCLUSION Radical cations derived from simple cyclopropane systems and derivatives containing olefinic moieties have been studied by a variety of techniques. Structural features have been probed by ab initio calculations and by spectroscopic techniques that may reveal the distribution of unpaired spin density over the carbon framework. In general, spin and charge are delocalized over olefinic and cyclopropane moieties. With few possible exceptions, the cyclopropane radical cations maintain a degree of bonding sufficient to conserve the stereochemical integrity of the cyclopropane function. Among a variety of uni- and bimolecular reactions, the combination with (or capture by) nucleophiles have been studied in particular detail. Four reaction parameters have been identified that determine the course of these reactions. The release of ring strain in the cyclopropane skeleton and the generation of a delocalized or otherwise stabilized free radical are of major significance, whereas steric factors are of lesser importance. The charge density distribution in the radical cations also has been recognized as a significant factor. While intuitively obvious, the significance has been fully realized only recently.

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AUTHOR IN DEX Anet, F.A.L., 38, 100 Anet, R., 38, 100 Arnold, D.R., 271,300 Barth, W.E., 5, 34, 35 Barton, J.W., 114, 143 Bergman, R.G., 118, 143 Billups, E.W., 39, 42, 58, 101,102, 103, 171,206, 207 Borden, W.T., 166, 206, 271,300 Born, M., 149-152, 205 Boyd, R.J., 271,300 Buncel, E., 38, 100 Carrington, A., 114, 143 Cava, M.P., 38, 100 Chan, C.W., 123, 144 Chapman, O.L., 166-171, 173, 206, 207 Cheng, S.K.T., 134-135, 145 Collins, J.R., 271,300 Coulson, C.A., 244, 259 Craig, J.T., 6, 35 Dai, W.-M., 118, 143 Darby, N., 118, 143 Davidson, E.R., 166, 206 Davy, J.R., 6, 35 Dowd, P., 200, 208 Du, P., 271,300 Durr, H., 48, 102

Eaton, P.E., 141,146 Eggers, D.F., 162 Essen, H., 152, 206 Ewig, C.S., 166, 206 Gallup, G.A., 271,300 Garratt, P.J., 59, 103, 113, 118 Gerson, F., 124, 135-136, 144, 145 Gleicher, G.J., 20, 35 Grimme, W., 38, 41, 101 Halton, B., 39, 42-43, 51,100, 101,102 Hart, H., 119, 143, 243, 259 Hart, P., 166, 206 Haselbach, E., 271,300 Heilbronner, E., 124, 144 Hess, B.A.Jr., 166, 206 Hixson, S.S., 298, 302 Ho, M.S., 137, 146 HoUeman, A.F., 211,253 Hou, X.L., 120-121,133, 143 Hrovat, D.A., 271,300 Huang, K. 152, 205 Hung, C.W., 126, 144 Jones, R.R., 118, 143 Katz, T.J., 114, 142 Kekule, F.A., 210-253 Kesselmayer, M.A., 194-198, 208 Kollmar, H., 166, 206 303

304

Komatsu, K., 243, 259 Korte, S., 38, 41,101 Krantz, A., 166-171,206 Krebs, A., 114, 142 Krogh-Jesperson, K., 271,273, 300 Kuck, D., 4, 34 Law, D.F.C., 41,101 Lawton, R.G., 5, 34, 35 Leopold, D.G., 173, 207 Leung, C.Y., 135, 145 Li, W.-K., i18, 143 Longuett-Higgins, H.C.,. 244, 259 Maier, G., 171,187, 190, 199-205, 208 Mak, T.C.W., 128, 144 Man, Y.M., 128, 144 Masamune, S., 118, 143, 167, 206 Meier, H., 115, 143 de Meijere, A., 125, 144 Michl, J., 167, 170, 206 Mills, W.H., 67-72, 104, 209-253 Monkhorst, H.J., 152, 206 Napier, D.R., 38, 100 Nicolaou, K.C., 118, 143 Nixon, I.G., 67-72, 104, 209-253 Oppenheimer, R., 149-152, 205 Pascal, R.A. Jr., 234, 258 Pauling, L., 211,240, 244, 252, 253 Perkin, W.H., Jr., 38, 100 Pettit, R., 166, 206 Pople, J.A., 156, 206 Qin, x.z., 271,300 Rashidi-Ranjbar, P., 130, 145 Reiser, O., 125, 144 Roberts, J.D., 171,207 Robinson, M.D., 6, 35 Roth, H.D., 271,273, 300 Rowland, C.G., 271,300 Russel, S.G.G., 51,102

AUTHOR INDEX-

Sandstrom, J., 130; 133, 145 Schaad, L.J., 166, 206 Schaefer, III, H.F., 173, 181, 187, 194, 207, 208 Scheiner, A.C., 181,207 Schmaltz, T.G., 18, 35 Schweig, A., 173, 207 Scott, L.T., 6-7, 35, 36 Sheridan, R.S., 194-198, 208 Siegel, J.S., 8, 35 Snow, L.D., 271,300 Sondheimer, F., 110-111, 118, 126, 142, 143, 144 Song, Z.Z., 137-138, 146 Staemmler, V., 166, 206 Staley, S.W., 171,206 Stanger, A., 244-246, 259 Strausz, O.P., 182, 207 Streitweiser, A., 252, 256, 259 Sutton, L.E., 211,244, 253 Thummei, R.P.131,145, 252, 254, 257, 258, 259 Tobey, S.W., 41,101 Tsotinis, A., 59, 103 Ullmann, E.F., 38, 100 van't Hoff, J.H., 252 Vogel, E., 38, 41,101 Wang, X.C., 131-132, 145 Wang, X.M., 133-134, 145 Wayner, D.D.M., 271,300 West, R., 243, 259 Wiberg, K.B., 1611,206 Williams, F., 271,300 Wong, C.H., 126, 144 Xing, Y.D., 118, 143, 298, 302 Yang, Y., 139-141,146 Yum, T.Y., 162 Zhou, Y.F., 118, 143

SUBJECT INDEX Acenaphthylene, 2 (see also "Hydrocarbons with curved surfaces...") Alder-Rickert cleavage, 47, 51-53, 58, 62 (see also "Cycloproparenes... ") Alkoxymethylenes, 194-199 Benzene, 2 (see also "Hydrocarbons with curved surfaces...") Benzocyclopropene, 38 (see also "Cycloproparenes... ") o-benzyne, 171-181 Buckminsterfullerene, 2 (see also "Hydrocarbons with curved surfaces...") "Buckybowls," 15-16 (see also "Fullerenes") Chloromethoxymethylene, 198-199 Circulenes, 4 (see also "Hydrocarbons with curved surfaces...") Corannulenes, 4, 5-12 (see also "Hydrocarbons with curved surfaces...") dianion, 29 dihydro- and tetrahydro-, l0 flash vacuum pyrolysis, 6-8 models of, theoretical, evaluation of, 19-25

synthesis, 5-8 tetraanion, 29 Curtius degradation, 62 Cyclobutadiene, 166-171 Cyclopentacorannulene, 13 models of, theoretical, evaluation of, 19-25 X-ray crystal structure, 23-24 Cyclopentadiene, 2 (see also "Hydrocarbons with curved surfaces...") Cyclopentadienylideneketene, 181182, 183-185 Cyclopropenylidene, 187-194 Cycloproparenes, chemistry of, 37107 introduction, 38-39 aromaticity and strain, interplay of, 38-39 benzocyclopropene, 38 Synthetical Formation of Closed Carbon-Chains in the Aromatic Series, 38 physical properties, 67-78 alkylidenecycloproparenes, polarity of, 77-78 benzocyclopropene, 68-72, 76 bond fixation, 72 cyclopropa[b]naphthalene, 73 dicyclopropanapthalene, 72, 73 305

306

SUBJECTINDEX

1,1-difluorobenzocyclopropene, 69-73 fulvalenes, 77-78 HOMO, 72 Mills-Nixon effect, 67-72 spectral characteristics, 73-77 starphenylene, 68 structure and strain, 67-73 terphenylene, 68 tricyclobutabenzene, 68 reactions of, 78-99 addition, 85-93 alkylidenecycloproparenes, 8384, 97-99 benzocyclopropene, 87-88, 95 benzocyclopropenyl cation, 7883, 90 cycloheptatrienide, 83 1H-cyclopropa[blanthracene, 97 cyclopropa[alnaphthalene, 86 cyclopropa[b]naphthalene, 9091, 93, 95 cyclopropa[/lphenanthrene, 86, 87 cycloproparynes, generation and interception of, 92-93 cyclopropenyl cations, 79, 83 1,1-difluorobenzocyclopropene, 80, 85, 96-97 1,1-dihalogenocyclopropa[blanthracenes, 80 1,1-dihalogenocyclopropa[b]napthalenes, 80

1,1-dihalogenocycloproparenes, 89-90 electrophilic substitution, 84 halogens, 85-93 Htickel calculations, 78-79, 83 metallation/electrophilic substitution, 83-84 3,8-methanoaza[ 10]annulene, 87 nickelacyclobutabenzenes, 95 NMR spectra of ions, 80-81

nucleophilic substitution, 78-83 pseudohalogens, 85-93 rearrangements, 93-94 ring-contraction, 93 ring-expansions, 93-94 substitutions, 84 TCNE, 91 transition metal ions, 91 with transition metal reagents, 95-97 tropylium ion, 78-79 Wheland intermediate, 89 X-ray crystallography, 99 synthesis of, 40-67 Alder-Rickert cleavage, 47, 5153, 58, 62, 67 alkylidenecycloproparenes, 65-67 alkylidenecyclopropa[b]naphthalenes, 65 azabenzocyclopropene, 62 azaisobenzofuran, 60 benzocyclobutanedione, 64 benzocyclobutenone, 50 benzocyclopropene, 42, 45 benzocyclopropene-p-quinone, 62 benzocyclopropenone, 62-64, 65 1,2-bromochlorocyclopropene, 42-44, 50-51, 55-56, 60 via closing of cyclopropane obond, 48-50 Curtius degradation, 62 via cycloaddition of dienes to cyclopropenes, 41-45 cyclobuta[3,4]benzocyclopropene, 50 1,4-cyclohexadiene, 45-46 1H-cycloprop[f]indene, 62 1H-cyclopropa[ b ]anthracene, 44, 55 cyclopropacyclophanes, 59 IH-cyclopropa[b ]naphthalenes, 42-44, 45-46, 47, 49, 52-54

Subject Index

1H-cyclopropa[b ]phenanthrene, 42 1H-cyclopropa[/]phenanthrene, 56, 58 cyclopropa[g]quinoline, 59-60 cyeloproparenes, strained, fused and short-lived, 50-59 cyclopropathiophene, 60 dicyclobutabenzocyclopropene, 51 1H,5H-dicyclopropa[b,/]anthracene, 56 dieyclopropabenzenes, 51 dieyelopropa[b,h]phenanthrene, 58 1,1-difluorobenzoeyclopropene, 41 7,7-difluorobenzoeyelopropene, 47 via dihalocarbene adducts to cyclohexa-1, 4-dienes, 45-46 1,1-dihalogenoeyeloproparenes, 41, 62-64 1,4-dihydrocyelopropa[b,g]naphthalene, 54 3,6-dimethoxyeyelopropa[b]naphthalene, 62 1,1-dimethyleyelopropa[c]pyridine, 59 funetionalized, 62-65 halogenobenzoeyelopropenes, 62 heteroeyelie, 59-62 3H-indazoles, 48 isatin, 60-62 1H- 3,8-methanoeyelopropa[10lannulene, 54 2-methoxybenzoeyclopropene, 62 methylideneeyelopropabenzene, 65 o-naphthoquinodimethane, 55

307

norcaradicnes,aromatization of, 46-48 oxocycloproparencs,64 Peterson olefination,65 phenanthrocyclopentadienes,57 phenanthrocyclopropenone, 56, 64 phenanthrofuran, 57 phenylacetylene,67 3H-pyrazoles,48 3H-spiropyrazoles,48 o-quinodimethanes, 42-44 spirobenzocyelopropene,49 strain,overcoming, 40 strategies,general,40-50 tetrahalogenoeyelopropenes,41, 42, 44, 45, 55 tetramethylideneeyelohexane, 58 titanium, low-valent, 45, 60 vinylcarbenes, rearrangement to, 46, 56, 60 Dehydro[8]annulenes, planar, and other theoretically interesting molecules, 109-146 alkyne 27, stable derivatives of, 133-135 benzylic oxidation, 135 clathrate inclusion hosts, 134 dehydrebromination, 133 Diels-Alder cyeloadditions, 134 phenanthrene, 134-135 benzenoid compounds, 118-130 barrier to ring inversion, 129130 ~r-basicity, 124 C2v symmetry, 126 charge-transfer bands, 124 r barrier, 130 conformational inversion pro= cess, 126 conjugative stabilization, 126

308

SUBJECT INDEX

cyclooctatetraenes, benzo-fused, 130 cyclopentadienyltitanium trichloride, 126 cyclophyne, 125-126 dehydrobromination, 118, 126, 129 deoxygenation, 119-124, 126, 129 dibenzo[2.2lparacyclophane, 123, 125-126 dicyclopentadienyltitanium dichloride, 126 Diels-Alder reaction, 123, 124 dimethyl acetylenedicarboxylate (DMAD), 124 electronic spectrum, 124 1,4-endoxide deoxygenation, 119-123, 126-129 flipping process, 126, 127 furans, 126 2,5-disubstituted furans, 122 isobenzofuran, 124 [2.2]metaparacyclophane, 126 2-methylfuran, 124 peri-H-H repulsion, 118-119, 126 photoelectron spectroscopy, 124 Pitzer strain, 126 1,2,3-selenadiazole, 125 spectroscopy, triple-resonance, 124 tetraphenylcyclopentadienone, 125 tetraphenylenes, 128 titanium, low-valent, 119, 120, 123-124, 126, 128, 129 torsional strain, 126

tribenzo[a,c,e]cyclooctene, 118121 Williamson ether synthesis, 122 conclusion, 141

cycloalkynes, strained, 110-118 bathochromic shift, 112 benzannelation, 116 benzenoid compounds, 115 Bergman rearrangement, 118 bis-alkyne 5, 111-112 bromination, 117 calicheamicin, 118 cyclooctatetraene, 112 dehydrobromination, 112-118 deoxygenation protocol, 115 dibenzo[a,c]cyclooctene, 116117 dichloroketene, 112 Diels-Alder reaction, 112 1,3-diphenylisobenzofuran, 112, 115, 116, 117 diradical hydrogen abstraction process, 118 DNA cleavage, 118 eight-membered, planarity of, 111 enediyne rearrangement, 118 furan, 112 Htickel 4nzr rule, 112 hyperchromic effect, 112 ionization potential, 112 kinetic stabilization, 114-116 peri-H-H nonbonded interactions, 115-117, 118 pharmaceutical implications, 118 planar 4nrr system, 112, 115 tetraphenylene, 115 2,3,4,5-tetraphenylcyclopentadienone (TC), 114-115 titanium, low-valent, 115, 119, 120 X-ray crystallography, 113 furans, polysubstituted, 135-141 benzonitrile, 137 benzoyl chloride, 140 bifuran, 138

Subject Index

bis(trimethylsilyl)acetylene, 137 boron trichloride, 137, 138 boroxine, 138 n-butyl magnesium bromide, 138 n-butyllithium, 140 diarylfurans, 140 3,4-dideuteriofuran, 136 Diels-Alder reaction, 137 3,4-disubstituted furans, 136137, 140-141 furan-3,4-diylpolymers, 139 ipso-iodination, 138 ipso-substitution pattern, 136, 137 nickel-catalyzed cross coupling, 138 octifuran, 139 (p-o),~ overlap, 136 oxazoles, 137, 139-140 palladium-catalyzed coupling reactions, 138, 140-141 quaterfuran, 138 quinquefurans, 139 self-coupling, 138-139 septifurans, 139 sexifurans, 139 Suzuli reaction, 138 terfuran, 138 tin-lithium exchange pathway, 140 tribenzo[a,c,e]cyclooctene, 135136 triethylamine, 137 introduction, 109-110 1,4-endoxide deoxygenation, 109 transitional metal complexes, 130133 copper(II) perchlorate, 133 and cyclooctatetrene, 131 dehydrobromination, 131-133 2,2'-diazabiaryl ligands, 130-131 thermal racemization process, 133

309

Diels-Alder reaction, 112, 123, 124, 134 retro-Diels-Alder cleavage, 285286 Dihydrocorannulenes, 10-12 (see also "Hydrocarbons with curved surfaces...") Electron nuclear double resonance (ENDOR), 267 Electron spin resonance (ESR), 266, 269-276 Electronic spectroscopy, 266-269 (see also "Radical cations...") ENDOR, 267 EPR, 266-269 (see also "Radical cations...") ESR, 266, 269-276 Flash vacuum pyrolysis (FVP), corannulene and, 6-8, 14 Friedel-Crafts substitution, 223 (see also "Mills-Nixon effect") Fuller, Buckminster, geodesic domes of, 4 FuUerenes, 2-36 (see also "Hydrocarbons with curved surfaces...") fullerene related hydrocarbons, 23, 12-19 bowl depth, 13 "buckybowls," 15-17 13C NMR spectra, 19 corannulene surface, expanding, 12-14 cyclopentacorannulene, 13 crystal packing, 14 dimerization of semibuckminsterfullerenes, 16, 17-19 semibuckminsterfullerenes, 1517 1,2,5,6-tetraketopyracene, 15 Furans, 112

310

Hooke's law, 152-153 H tickel theory, 20, 29, 78-79, 199, 247-252 aromatic theory, 253 electron densities, model of, 238 4nrr rule, 112 thiirene, 182-186 Hund's rule, 29 Hydrocarbons with curved surfaces, polynuclear aromatic, 1-36 circulenes, 4 corannulene, 4 coronene, 4 geodesic domes, 4 pentagons, importance of, 4 soccer balls, principle incorporated in, 4 tribenzo-4,7dihydroacepentalenes, 4 tribenzotriquinacenes, 4 concluding remarks, 33 pyramidalization angle, 33 corannulene, 4, 5-12 ab initio calculations, 10, 20-25 AM 1 calculations, 10 bowl-to-bowl stacking, none, 89 cyclization, conventional, 6 1,3-cyclohexadiene ring, 10 dihydrocorannulenes, 10-12 dihydrophenanthrene, 11 exo versus endo presence, 12 flash vacuum pyrolysis (FVP), 6-8, 14 inversion, barrier to, 9 MM2 calculations, 10- I 1, 20 models of, theoretical, evaluation of, 19-25 nonplanarity and its consequences, 8-9 pseudoaxial or pseudoequatorial positions, 11, 12 synthesis, 5-8

SUBJECT INDEX

tetrahydrocorannulenes, 10-12 X-ray crystal structure of, 9, 12, 23-24 corannulene, reduction of, 27-33 "anion within trianion" structure, 30 anionic species, 30-31 considerations, general, 27 dianion, 29 dimeric aggregates, 31-33 electron addition, 27 frontier orbital diagram, 27-28 HOMO and LUMO, 27-28 inversion barriers in anionic species, 30 Jahn-Teller effect, 29 7LiNMR spectroscopy, 31-32 nonalternant hydrocarbons, 27 tetraanion, 29-30 tetralithiocorannulene dimer, 31-33 fullerene related, 2-3, 12-19 (see also "Fullerenes") "buckybowls," 15-17 ~3C NMR spectra, 19 corannulene surface, expanding, 12-14 dimerization of semibuckminsterfuUerenes, 16, 17-19 semibuckminsterfuUerenes, 1517 introduction: fullerene related, 2-3 acenaphthylene, 2 benzene, 2 buckminsterfullerene, 2 carbon cages, 3 characteristics, common, 2-3 cyclopentadiene, 2 naphthalene, 2 pyracylene, 2 radialene, 2 reasons for study, 3

Subject Index

theoretical studies, 19-27 ab initio calculations, 20-25 AM1 calculations, 24-25 aromaticity of system, 20 conclusions, 24-25 electron correlation corrections, 22 graph theory methods, 20 HOMO, 27, 28 LDF method, 22 LUMO, 27, 28 MM2 calculations, 22, 24-25 MNDO, 22, 24-25, 31-33 models of corannulene and cyelopentacorannulene, evaluation of, 19-25 molecular orbital (MO) methods, 20 p-orbital axis vector analysis (POAV), 20, 34 PM3, 22, 24-25 semibuckminsterfuUerenes, 2527 topological graph theory, 20 Infrared spectroscopy of highly reactive organic species, 147-208 background of, theoretical, 149-166 basis set, 155-156 Born-Huang model, 152 Born-Oppenheimer model, 151155 calculation, method of, 155-156 cyclopropene model, 161-162, 163-165 description of model, 149-155 dipole moment, 154 electron correlation, 156 ethylene model, 1.56-158 ethylene oxide model, 158-161 Hamiltonian, 150 harmonic oscillator approximation, 152, 155

311

Hooke's law, 152-153 inertia, total moments of, 150151 kinetic energy, 150-151 MoUer-Plesset method, 156 nuclear motion equation, 152 polarization, experimental, 162166 reliability of model, 156-162 rotational levels, 149-150 summary, 166 vibrational energy levels, 149-151 vibrational quantum number, 153 vibrational spectra, 154-155 Wilson-Decius-Cross method, 155 'zero point energy, 153 highly reactive organic molecules, 166-186 o-benzyne, 171-181 cyclobutadiene, 166-171 cyclopentadienylideneketene, 181-182, 183-185 methylenecyclopropene, 171, 172 Moller-Plesset theory, 167 thiirene, 182-186 intermediates, reactive, 186-205 alkoxymethylenes, 194-199 chloromethoxymethylene, 198199 cyclopropenylidene, 187-194 methylene, 186-187, 188 propargylene, 187-190 trimethylenemethane, 199-205 vinylidenecarbene, 190, 191-193 introduction, 148-149 cis or trans alkene, 148 for highly unstable molecules, 148 low temperature (5-20 K) isolation, 148-149 model, theoretical, 148, 149-166

312

Jahn-Teller effect, 29 distortion, 262-263, 269 Kekule tautomer, 210-211 (see also "Mills-Nixon effect") oscillation hypothesis, 210 7LiNMR spectroscopy, 31-32 Methylene, 186-187, 188 Methylenecyclopropene, 17l, 172 Mills-Nixon effect, 209-260 conclusions, 252-253 Hiickel aromatic theory, 253 geometries of annelated benzenes, 240-252 aromatic character, 247 bond fixation, 240-252 computational studies, 244-252 coupling constant, 246 cyclohexatriene, 240 o-effects, Stanger demonstration and, 244-246 electronegativity versus Hiickel effects, 247-252 experimental structures, 240-244 Hartree-Fock (HF) basis sets, 244 Hiickel effects, electronegativity, 247-252 local density functional (LDF) computations, 244 rehybridization, 240-241 Stanger demonstration and o-effects, 244-246 stretch-bend interaction, 246 structural data of, 248-251 introduction, 210-212 cyclohexatriene-like geometries, 211 in eleetrophilie aromatic substitution, 211-212 goals, research, 212

SUBJECT INDEX

indan, 210, 244 Kekule oscillation hypothesis, 210 Kekule tautomer, 210-21 l, 244, 252 resonance, concept of, 211 van't Hoff tetrahedra, 210, 252 reactivity effects, 212-226 acid-base equilibria, 212, 214218 annelated pyridines, 214 n--basicity, 217-218 benzenes, annelated, nitration of, 219 bond localization, 224 conformational equilibria, 212 cyclobutabenzene, 223 cyclopropabenzene, 218-231 cycloproparenes, ring opening of, 224-226 deprotonation/carboxylation reaction, 216 electronic effects, 214-218 electrophilic attack, 225-226 electrophilic halogenation, 224226 electrophilic metal insertions, 226 electrophilic reactions, 218-224 electrophilic substitution, 224 Friedel-Crafts substitution, 223 H2S207, 222 H2SO4, 222 halogenation, 220-221 halogenation, electrophilic, 224226 HOAcBr+, 220 hyperconjugative release, 214 inductive effects, 214-218 o-insertions, 225 ipso-attack, 223 ipso-substitution, 220 isopropyl conformations, 214

Subiect Index

kinetic acidity, 214, 216 molecular mechanics computations, 212 nitration, 218-220 protodesilylation, 217 protodetritiation, 214 reactivity/selectivity principles, 20 rehybridization, 224 resonance effects, 214-218 ring opening of cycloproparenes, 224-226 steric effects, 212-214 sulphonation, 221-223 thermodynamic acidity, 216 Wheland intermediates, 224, 252 spectroscopy, 227-239 angular distortions, 237 antiaromatic o-ring current, 233 aromatic chemical shifts, 231-233 bond fixation effects, 229-231, 233, 237 carbon-carbon couplings, 230 carbon chemical shifts, 231 carbon-proton coupling constants, 227-230 s-character, 229 chemical shifts, aromatic, 231233 Coulomb integrals, 239 Coulson-Crawford hyperconjugation model, 239 coupling constants, 227-231 dimethyldihydropyrenes, 233 electronegativity, 230 ESR data, 238 extinction coefficient, 235 Hiickel model of electron densities, 238-239, 252 hybridization, 227-230 hydrogen-hydrogen coupling, 230-231 hyperconjugation, 234

313

nuclear magnetic resonance (NMR), 227-233 paramagnetic term, 233 photoelectron (PES), 227, 238 polarography, 238 proton chemical shifts, 231-233 rehybridization, 230-231 ring currents, 231-233 ring strain-induced rehybridization, 238 twisted benzenes, 234 ultraviolet-visible (UV-vis), 227, 234-238 Molecular mechanics (MM) calculations, 10-11 Moller-Plesset method, 156, 167 Naphthalene, 2 (see also "Hydrocarbons with curved surfaces...") Pentagons, importance of in fullerenes curved systems, 4 (see also "Hydrocarbons with curved surfaces...") Propargylene, 187-189 Pyracylene, 2 (see also "Hydrocarbons with curved surfaces...") Pyrolysis to produce corannulene, 6-8 Radialene, 2, (see also "Hydrocarbons with curved surfaces...') Radical cations of cyclopropane systems, 261-302 conclusion, 299 electron transfer photochemistry, 286-295 ab initio calculations, 288-295 allyl-benzylcyclopropene systems, 287

314

SUBJECTINDEX

aromatic substitution product, 291,293 benzobicyclo[3.1.0]hexene, 292 benzonorcaradiene, 288 bicyclo[4.1.0]hex-2-ene, 288 r nucleophilic capture of, 290-291 CIDNP, 287-295 eyclohexene, 288 cyeloreversions, 287 1,2-diaryleyr 294295 9,10-dieyanoanthraeene, 294295 1,4-dieyanobenzene, 292 dimethylbenzvalene, 287 dimethyldieyelopropenyl, 287 1,2-diphenyleyelopropane cations, 287 electron transfer, 287 EPR, 288 geometric isomerization, 287 hydrogen-shift reactions, 289290 hydrogen (hydride) transfer, 288 1-methylcyelopentene, 288 7-methylenequadricyr 294 N-methylphthalimide, 291 NOCAS reaction, 291 norbomadiene, 287 nueleophilie attack, 292, 293 nucleophilie capture of earbocations, 290-291 nucleophilic cleavage, 291 nucleophilic substitution-ring opening, 292 optical activity, retention of, 289 orbital overlap, 292 phellandrene, 289 B-phellandrene, 293, 297 phenylcyclopropane, 291,293 quadricyclane, 287, 294

rearrangements, 286-287 reverse electron transfer, 287 ring enlargement, 287 ring-opened cyclopropane radicall cation, 291,292 ring strain, release of, 287, 290 SN2 reaction, 291 sabinene, 289, 292, 293 stereochemistry, importance of, 291-294 strained-ring, 287 tetraarylcyclopropane systems, 295 1,1,2,2-tetraphenylcyclopropane, 289 three-dimensional integrity, 292 a-thujene, 289-290 tricyclane, 294 tricyclo-[4.1.0.02'7]heptane, 288 triplet state, 287 vinylcyclopropane, 288-290, 292 introduction, 262-266 bifunctional, 265 charge transfer, 262 cyclopropane, 262-266 delocalized, 265 fluorescence, 262 frontier molecular orbital (FMO) approach, 263-265 geometric isomerizations, 265 intramolecular nucleophilic capture, 265 Jahn-Teller (JT) distortion, 262263 molecular rearrangements, 265 perturbational molecular orbital (PMO) approach, 263 release of ring strain, 262, 265 strained ring hydrocarbons, 262 vertical ionization potentials, 262 nucleophilic capture of, 295-298 ab initio calculations, 296 bicyclo[3.1.0]hex-2-ene, 297-298

Subject Index

CIDNP study, 297-298 conjugation, 296 /3-distonic, 296 electron transfer-induced photoreactions, 295 free-radical stability, 298 Hammond postulate, 296 hydride shift, 298 hydrogen migrations, 295, 297 hyperconjugatively stabilized, 297 inverse effect of substitution, 297-298 kinetically controlled reactions, 296 methanol, 296 nucleophilic substitutions, 296297 oxonium function, deprotonation of, 296 B-phellandrene, 297 radical cation substitution, 297 release of ring strain, 296-297 sabinene, 298 spin and charge density, 295,297 stereoelectronic effects, 297 steric factors, 296, 297 steric hindrance, role of, 297298 stereoselectivity, 295 thermodynamically controlled reactions, 296 /3-thujene, 298 tricyclane, 297 spectroscopic techniques for studying, 266-269 CIDNP, 266-276 cyclooctatetraene radical cation, 269 1,4-dihydropentalene radical cation, 269 electron nuclear double resonance (ENDOR), 267

315

electron nuclear crossrelaxation, 268 electronic spectroscopy, 266-269 EPR, 266-269 ESR, 266 hyperfine coupling patterns (hfcs), 266-267, 272 norbornadiene radical cation, 269 nuclear-nuclear crossrelaxation, 268 optical, 268-269 quadricyclane radical cation, 269 radical pair mechanism (RPM), 268 semibullvalene radical cation, 269 spin-density distribution, 266 triplet-Overhauser mechanism (TOM), 268 structure and reactivity, correlation between, 295-298 (see also "...nucleophilic capture...") structures, 269-276 1-aryl-2-vinylcyclopropanes, 275 ab initio calculations, 271-277 CIDNP, 266-276 "closed-ring" and "open-ring" cation, 270, 274 1,2-diphenylcyclopropane, 269271 EPR spectroscopy, 276 ESR, 269-276 hexamethylprismane radical cation, 276 hyperfine coupling constants, 272 Jahn-Teller distortion, 269-273 matrix molecule, interaction with, 274-275 prismane, 275

316

quadricyclane, 275, 279 ring-opened species, 270, 274 tetracyclooctane, 275, 276 structures, alternative, 276-280 ab initio calculations, 277-280 benzonorcaradiene, 277, 278-279 [1:2,9:10]bismethanol[2.21paracyclophane, 279 ~3C polarization, 279 CIDNP, 277-280 cycloheptatdene, 277 1,2-diphenylcyclopropane, 277, 278 ESR, 279-280 FMO, 278, 280 hfc-coupling pattern, 280 methyl-substituted, 277 7-methylenequadricyclane radical cation, 276, 279 norcaradiene system, 276-277 spin-density distributions, 277 spiro [cycloprop ane- 1,9'fluorenel, 278 7-spirocyclopropanenorbornadiene radical cation, 280 spiroheptadiene system, 276, 277-278 tricyclo[4.3.1.0~' 6]deca-2,4diene, 277 of vinylcyclopropane systems, 281286 ab initio calculations, 281-286 1-aryl-2-vinylcyc!opropanes, 282 bieyclo[4.1.0]hept-2-ene, 282, 285 bieyclo[3.2.0]hepta-2, 6-diene, 285-286 bicyclo[3.1.0]hex-l-ene, 284

SUBJECT INDEX

carene, 285 CIDNP, 283-286 EPR, 282-286 geometric isomerization, 282 norcarene, 285 nucleophilic capture, stereochemistry of, 285 orbital overlap, 284 retro-Diels-Alder cleavage, 285286 rigid cations, three, 283 ring-opened cyclopropane, 282 spin density, 283 spiro[4.2]heptadiene, 286 stereoelectronic effects, 283 tricyclo[4.1.0.02'Tlheptane, 285 tricyclo[5.3.1 ff'7]undeca-2,4,9triene, 282 Walsh orbital, 285-286 Semibuckminsterfullerenes, 15-17 (see also "Fullerenes") dimerization to buckminsterfullerene, 16, 17-19 studies, theoretical, 25-27 Suzuki reaction, 138 Tetrahydrocorannulenes, 10-12 (see also "Hydrocarbons with curved surfaces...") Thiirene, 182-186 Trimethylenemethane, 199-205 van't H off tetrahedra, 210 Vinylidenecarbene, 190, 191-193 Walsh orbital, 285, 286 Williamson ether synthesis, 122 Wilson-Decius-Cross method, 155

Advances in Theoretically Interesting Molecules Edited by Randolph P. T h u m m e l , Department of Chemistry, University of Houston This series presents review articles pertaining to molecular systems which are of interest due primarily to the presence of structural features or characteristics about which prior predictions can be made based on present knowledge of chemical theory. These molecules often test the outer limits of what can be prepared and studied with regard to theromodynamic stability and kinetic reactivity. Studies of such systems provide an excellent test for existing chemical theory and thus furnish perhaps the best opportunity for advancing the frontiers of chemical knowledge. The articles contained in this series will discuss in detail the work originating from the author's laboratory. This work will be placed in the context of other similar work which has already been accomplished or is currently in progress. The format will be both explicit and pedagogical so that these articles should be of interest to novices as well as experts in the field. It is expected that a certain amount of historical background will be provided as well as speculation on possible new directions which the chemistry under discussion might follow. Volume 1, 1989, 467 pp. ISBN 0-89232-869-X

$97.50

CONTENTS: Introduction to the Series: An Editor's ForewOrd, Albert Padwa. Preface, Randolph P. Thummel. Isobenzofurans, Bruce Rickbom. Dihydropyrenes: Bridged [14] Annulenes Par Excellence. A Comparison with Other Bridged Annulenes, Richard H. Mitchell. [l.m.n.] Hericenes and Related Exocyclic Polyenes, Pierre Vogel The Chemistry of Pentacyclo [5.4.0.02'6.03'1~ Undecane (PCUD) and Related Systems, Alan P. Marchand. Cyclic Cumulenes, Richard P. Johnson. Author Index. Subject Index.

.1 A 1 P R E S S

,~

.1 A l P R E S S

Volume 2, 1992, 223 pp. ISBN 0-89232-953-X

$97.50

Volume 2 in this series continues in the vein established by the first volume. The contributors are distinguished organic chemists whose work helps bridge the gap between theory and experiment. Leo Paquette has been intrigued for nearly two decades by the dynamics of cyclooctatetraene (COT) ring inversion. Using elegantly designed synthetic techniques, he has substituted COT in such a way as to force it to reveal these dynamics quite clearly. The reader who studies this story carefully will find himself rewarded by a veritable banquet of truly classic physical organic chemistry. Ken Shea is the world's foremost leader in the battle to shatter Bredt's Rule. Dealing with bridgehead olefins, which exist at the limit of what structural theory will allow, helps to shed light on structure-activity relationships for this most fundamental functional group. His chapter is an attempt to quantify and organize the data available in this field and relate these data to molecular mechanics approximations and chemical reactivity. Bill Hemdon attacks the notion that planarity is a strict requirement for the so-called phenomenon of aromaticity. He tackles the problem from a graph-theoretical point of view and offers bright prospects for the eventual characterization of highly nonplanar condensed aromatic systems. Ron Warrener has extended the benzene valence-isomer concept to heterocyclic systems, most notably furan. The synthetic protocols that he has developed for the preparation of these elusive molecules have led to an intriguing and general application of acetylene stacking and transfer technology to the preparation of unsaturated cyclic compounds. --From the Preface

CONTENTS: List of Contributors. Introduction to the Series: An Editor's Forward, Albert Padwa. Preface, Randolph P. Thummel. Cyclooctatetraenes: Conformational and ~-Electronic Dynamics Within Polyolefinic [8] Annulene Frameworks, Leo A. Paquette. A Compilation and Analysis of Structural Data of Distorted Bridgehead Olefins and Amides, Timothy G. Lease and Kenneth J. Shea. Nonplanarity and Aromaticity in Polycyclic Benzenoid Hydrocarbons, William C. Herndon and Paul C. Nowak. The Dewar Furan Story, Ronald N. Warrener. Author Index. Subject Index.

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