Advances in Strained and Interesting Organic Molecules, Volume 8, Volume 8

ADVANCES IN STRAINED AND INTERESTING ORGANIC MOLECULES

Volume 8

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ADVANCES IN STRAINED AND INTERESTING ORGANIC MOLECULES

Editor: BRIAN HALTON School of Chemical and Physical Sciences Victoria University of Wellington

VOLUME 8

Al PRESS INC. Stamford, Connecticut

Copyright © 2000 by JAI PRESSINC 100 Prospect Street Stamford, Connecticut 06904-0811 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: 0-7623-0631-9 ISSN: 1061-8902 Manufactured in the United States of America

CONTENTS LIST OF CONTRIBUTORS

vi i

PREFACE Brian Halton

ix

NATURAL AND NON-NATURAL PLANAR CARBON NETWORKS: FROM MONOMERIC MODELS TO OLlGOMERlC SUBSTRUCTURES Michael M . Haley and W. Brad Wan

1

RECENT DEVELOPMENTS IN STRAINED CYCLIC ALLENES Metin Balci and Yavuz Taskesenligil

43

STRAIN AND STRUCTURE OF STERICALLY CONGESTED TRIPLET CARBENES Hideo Tomioka

83

SYNTHESIS AND CHEMISTRY OF STRAINED CARBOHYDRATES: OXABICYCL0[4.1 .O]HEPTANES Ghislaine S. Cousins and John 0. Hoberg

113

EXPLOITING THE STRAIN IN 12.2.1IBICYCLIC SYSTEMS IN POLYMER AND SYNTHETIC ORGANIC C HEMISTRY Michael North

145

AZlRlNES AND AZlRlDlNES REVISITED Kuriya Madavu Lokanatha Rai and Alfred Hassner

187

INDEX

259

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LIST OF CONTRIBUTORS Metin Balci

Department of Chemistry Middle EastTechnical University Ankara, Turkey

Ghislaine 5. Cousins

School of Chemical and Physical Sciences Victoria University of Wellington Wellington, New Zealand

Michael M. Haley

Department of Chemistry University of Oregon Eugene, Oregon

Alfred Hassner

Department of Chemistry Bar-Ilan University Ramat-Gan, Israel

John O. Hoberg

School of Chemical and Physical Sciences Victoria University of Wellington Wellington, New Zealand

Michael North

Department of Chemistry King's College London, England

Kuriya Madavu Lokanatha Rai

Department of Chemistry University of Mysore Mysore, India

Yavuz Taskesenligil

Department of Chemistry Ataturk University Erzurum, Turkey

Hideo Tomioka

Chemistry Department for Materials Mie University Tsu, Mie, Japan vii

viii W. Brad Wan

LIST OF CONTRIBUTORS Department of Chemistry University of Oregon Eugene, Oregon

PREFACE The past year has seen the publication of the first Supplement to this Series under the editorship of Professor K. K. Laali and this signals our wish to provide timely collections under independent editorship from, for example, important international symposia. Carbocyclic and Heterocyclic Cage Compounds and Their Building Blocks appeared in June as the first Supplement of the past century---I sincerely hope that the next Supplement will not be the last of the present one! The present Volume, which comprises six chapters and involves ten authors from seven countries, provides a pot pourri of interesting strained and not so strained molecules and their use----or abuseBin the widest sense. Haley and Wan of the University of Oregon give a position summary of planar carbon networks. Their discourse commences with graphite and outlines methods now brought to bear on the synthesis of oligophenylenes, their conversions into polycyclic aromatic hydrocarbons such as "supernaphthalene" and "supertriphenylene," and the need to avoid alkyne "mismatch" that can lead to non-planarizable products. Next come polyphenylenes (or polybiphenylenes) that can be expected to link in a linear and an angular manner; the elegance and beauty of the cobalt-mediated cyclotrimerization of alkynes is displayed in its full glory. Graphynes (from hexaethynylbenzenes and dehydrobenzo[ 12]annulenes) and graphdiynes (which have advanced notably from Haley's in situ protiodesilylation/alkynylation protocol) are followed by the tetraethynylcyclobutadiene and tetraethynylethene motifs in what is a well rounded state-of-the-art summary of the field.

x

PREFACE

Chapter 2 introduces our first authors from Turkey. Balci and his student Taskesenligil address the field of strained allenes by considering the five- to nine-membered ring derivatives and then discuss bicyclic analogues. The developments of the past 10 years are put into good perspective and it is noteworthy that matrix technology has yet to be brought to bear on the characterization of cyclopenta- 1,2-diene. While cyclohexa- 1,2-dienes are easily intercepted by cycloaddition, studies on heteroatom-substituted derivatives are more recent. The synthetic potential of these strained cyclic heteroallenes is explored and the biological activity of the compounds addressed. Metal complexation of cyclohexa- 1,2-dienes provides sufficiently stable crystals for structural study and the results are presented. Despite the lower ring strain of the cyclohepta-1,2-dienes effective routes to the compounds have only recently been developed, but it is only with the higher homologue, 1-tert-butylcycloocta-1,2-diene, that stability at 20 ~ is achieved. The generation of an allene in the bicyclo[3.2.1 ] framework has been accomplished and the details of this elegant chemistry are provided. The contribution by Tomioka of Mie University on his studies of sterically congested triplet carbenes fits well in this Series. He provides an easily understood introduction to the nature of carbene geometry and the use of ESR spectroscopy in deducing carbene structure before describing the relationship of strain to structure in triplet carbenes. First, diarylcarbenes are presented and then the discussion focuses on ortho-subsfltuted diphenylcarbenes before considering more sterically congested polybromo, triptycyl, and anthryl derivatives. The use of strained molecules in the synthesis of important new compounds of a natural and nonnatural nature continues through the remainder of the Volume. My Colleague Hoberg (a recent arrival in the antipodes from the United States) and his student Cousins provide a timely contribution on strained carbohydrates. Their discussion follows the fusion of the carbohydrate with a cyclopropane, an oxirane, an aziridine, and a thiirane (episulfide), and the discussion encompasses the methods of formation and selectivity, as well as the chemistry of the newly formed ring systems. Examples of reactions and rearrangements that lead to new products and naturally occurring materials are presented. In particular, ring expansions into oxepanes and the development of electrophilic opening of the three-membered ring with high diastereoselectivities at the anomeric center show the potential for the cyclopropanes. The stereocontrolled opening of carbohydrate epoxides with oxygen nucleophiles leads to oligosaccharides and nucleosides while nitrogen and carbon analogues likewise effect stereospecific opening that have an obvious appeal in synthesis. Carbohydrate aziridines are comparatively recent but they can lead to amino-sugars through ring cleavage. In comparison the thiirane derivatives are few and this area appears to be one for future exploitation. The fifth chapter, by North of King's College, London, describes recent, elegant work that has led to controlled cleavage of the bicyclo[2.2.1 ]heptene (norbornene)

Preface

xi

ring system thereby providing stereocontroUed access to natural products and to polymer systems. The strain present in the bicyclic framework is used to advantage in providing synthetic polymers via the "Ring Opening Metathesis Polymerization"----or ROMP reaction. In this way amino acid, amino ester, peptide, and nucleic-acid base-containing polymers have been generated in living processes. In comparison, ozonolysis of enantiomerically pure norbornene-derived amido acids has provided a concise synthesis of enantiomericaUy pure cyclopentane derivatives in which all of the substituents are syn to each other and ready for elaboration into target molecules. The final chapter provides the first and a much sought after contribution to the series on small-ring nitrogen heterocycles. Hassner and Rai describe the developments in the chemistry of azirines and aziridines and their contribution fills an obvious gap in-our coverage. The detailed chapter opens with a discussion of the structure and spectroscopic properties before moving on to the synthesis and reactions 1H-azirines, 3,3-dimethylamino-2H-azirines, and aziridines. The preparation of azirines from vinyl azides and oximes is followed by a description of the first chiral synthesis of the cytotoxic (R)-(-)-dysidazirine. A notable strength of the chapter is in the full description of the chemistry of the azirines that is provided. The 3-(dimethylamino)azirines have value because of their easy transformation into a wide variety of heterocycles of particular importance, e.g. imidazolones and oxadiazocines, and their transformation into peptide and depsipeptide derivatives is nicely described. The aziridines are similarly treated and after a good synopsis of the synthetic methodologies the chemistry of the heterocycle that leads to important pseudo-sugars, imino-sugars, and natural products is provided. The section concludes with a discussion on the metabolism and cytotoxicity of natural and synthetic aziridines. Brian Halton Series Editor

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NATU RAL AN D NON-NATU RAL PLANAR CARBON NETWORKS: FROM MONOMERIC MODELS TO OLIGOMERIC SUBSTRUCTURES

Michael M. Haley and W. Brad Wan

1. 2.

3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Subunits via Intramolecular D i e l s - A l d e r Reaction . . . . . . . . . . . . . 2.2 Subunits via Alkyne Cyclotrimerization . . . . . . . . . . . . . . . . . . 2.3 Subunits via Intermolecular D i e l s - A l d e r Reaction . . . . . . . . . . . . . Poly(phenylene) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Biphenylene Dimer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Oligo(phenylene)s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphyne . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Hexaethynylbenzenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Dehydrobenzo[ 12] annulenes . . . . . . . . . . . . . . . . . . . . . . . . Graphdiyne . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Hexabutadiynylbenzenes . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Dehydrobenzo[ 18]annulenes . . . . . . . . . . . . . . . . . . . . . . . .

Advances in Strained and Interesting Organic Molecules Volume 8, pages 1--41. Copyright 9 2000 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 1.7623-0631-9

2 3 5 8 9 13 13 14 17 18 20 24 25 25

2

MICHAEL M. HALEY and W. BRAD WAN

6. 7. 8. 9.

Poly(tetraethynylcyclobutadiene) . . . . . . . . . . . . . . . . . . . . . . . . Poly(tetraethynylethene) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Miscellaneous Ethynyl-Linked Networks . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

31 33 36 37 37 38

1. INTRODUCTION For thousands of years the only recognized crystalline forms of carbon were graphite and diamond. Only recently though have these forms found important technological uses [1]. Graphite, comprised of planar sheets of sp2-hybridized carbon atoms, is used in pencils, as a solid lubricant, as a moderator in nuclear reactors, as a component in electrodes, and as a reinforcement material in carbon fiber composites [2]. Diamond, a three-dimensional network of sp3-hybridized carbon atoms, is used in jewelry and as drilling heads in the petrochemical industry. More recently, diamond thin films generated by vapor deposition methods have been used extensively as protective coatings of tools and materials and in electronic devices [3]. Despite the rapidly emerging importance of these carbon allotropes, the study of other carbon-rich systems has been limited for most of the 20th century. A visionary in the field of nonnatural carbon phases was Alexandru Balaban. In 1968, a group of theoreticians led by Balaban published an article in which they put forth a variety of imaginative and aesthetically attractive two- and three-dimensional carbon allotropes, e.g. 1-3 [4]. Although formation of many of these structures is energetically prohibitive, e.g. 1, some networks seemed more reasonable and thus potentially might be within the realm of synthetic endeavors. Over the next 20+ years, numerous two- and three-dimensional all-carbon networks were proposed in the literature, many of which were predicted to exhibit interesting electrical, optical, and structural properties [5]. Nevertheless, synthetic efforts towards the assembly of these nonnatural carbon phases, as well as toward expanded substructures, were lacking. The landmark event that invigorated the study of carbon allotropes was the isolation and confirmation of the structure of C6o (buckminsterfullerene) in 1990

l

2

3

Planar Carbon Networks

3

[6]. Although postulated by Smalley and Kroto in 1985 [7], it was not until 5 years later that Kr~itschmer and Huffman reported the isolation of macroscopic amounts of material, from which they were able to ascertain the now well-established truncated icosahedron structure [8]. Over the ensuing decade since their report, a plethora of publications has appeared dealing with a burgeoning number of carbon-rich and all-carbon molecules. A vast majority of the research has centered on the construction of monomeric and oligomeric substructures of nonnatural sp 2 and/or sp carbon networks. Several reviews have recently appeared from the leading protagonists in some of the respective subareas [9]; therefore, the scope of this review will be limited to selected key examples and more recent studies of planar carbon allotropes and associated model substructures. Three-dimensional systems, which have received only cursory examination [10], will not be discussed.

2. GRAPHITE Given the aforementioned uses of graphite (4), it is not surprising that this natural allotrope is the most heavily studied all-carbon structure [2]. The class of molecules that most resembles graphite (and thus could be considered as a subunit) is the benzenoid polycyclic aromatic hydrocarbons (PAHs). PAHs themselves have been intensively investigated for well over a century [ 11]. Although a wide array of small and intermediate size PAHs are known, e.g. triphenylene, coronene (5), and ovalene, only recently has access to large PAHs been achieved by MiJllen and co-workers [12]. Using very straightforward chemistry, the Mtillen team has assembled a variety of expanded graphitic topologies with excellent efficiency. The driving force behind their work has been the preparation of materials with interesting electrical and/or optoelectronic properties. The inherent thermal stability of the PAHs has allowed for the physisorption of highly ordered monolayers which may be imaged by means of scanning tunneling microscopy (STM) [13]. Furthermore, the PAHs have been shown to form discotic liquid-crystalline mesophases, which may lead to a wealth of technologically important applications [14]. These model graphite topologies of well-defined structure may lead to a better understanding of the materials science of soot, graphite, fullerenes, and nanotubes.

4

$

4

MICHAEL M. HALEY and W. BRAD WAN

According to Mtillen, there are four basic "modes" or methods of constructing PAHs and these are summarized in Figure 1 [12]. The most direct and intuitive method is the dimerization or fusion of smaller benzenoid PAH fragments (Figure 1, path A); however, PAHs are seldom reactive in this manner. The use of forcing conditions is often necessary and results in nonspecific and/or nonordered linkages between the components, which is clearly undesirable for the construction of well-defined substructures. The addition of a C 2 or a C 4 fragment (Figure 1, paths B and C, respectively) at specific target sites is not difficult. However, in virtually all cases, the result of such an addition is the annelation of a nonaromatic ring, which does not contribute to the overall benzenoid motif. An alternative method for the formation of benzenoid PAHs is intramolecular cyclodehydrogenation of suitable PAH precursors (Figure 1, path D). An ideal oligophenylene precursor has the correct number of benzene rings in close proximity and with acceptable spatial arrangement. This approach, which does not alter the number of carbon atoms in the macromolecular system, has proven to be the most efficient method for PAH assembly (vide infra) [12].

Path A

v

dimerization

+

I

,

PathB

__

Addition of C2 fragment

Addition of C4 fragment

Path D

cyclodehydrogenation

Figure 1. Four possible modes for PAH construction.

Planar Carbon Networks

5

Oxidative cyclodehydrogenation can be achieved using a variety of Lewis acid catalysts combined with oxidants, e.g. vanadium(V) or thallium(llI) with 0 2 [15], zero-valent metals, e.g. K or Pt [16], as well as by photochemical routes [17]. Another method employs an A1CI3 melt that is combined with atmospheric oxygen in order to fuse aromatic components [18]. Most of these traditional routes have s~gnificant drawbacks. High temperatures are required to drive these reactions, and the result is often decomposition and/or unpredictable rearrangement. The reactions often result in incomplete cyclization with difficulty in separating the partially cyclized materials. In the 1960s Kovacic was able to polymerize benzene and its derivatives under relatively mild conditions, utilizing A1C13or FeC13 in concert with atmospheric oxygen to catalyze the formation of aryl-aryl bonds [ 19]. This method was somewhat successful but not very selective; the result was an uncontrolled mixture of ortho and meta branching. By subtle modification of Kovacic's mild conditions (use of CuC12/A1C13 or CuSO2CF3/AIC13 as the catalytic system), the Mtillen team has been able to complete the synthesis of a family of large graphitic, all-benzenoid PAH structures [ 12].

2.1

S u b u n i t s via

Intramolecular Diels-Alder

Reaction

Mtillen and co-workers have developed three fundamental approaches of synthesizing oligophenylenes, which serve as the precursors for PAHs. The first approach involves an intramolecular [4+2] cycloaddition of suitable phenylenevinylene derivatives. A representative example of this strategy is the preparation of 6 (Scheme 1) [20]. The coupling partners 7 and 8 were constructed with relative ease via Wittig reaction of 2-iodobenzyltriphenylphosphonium chloride with benzaldehyde and cinnamaldehyde, respectively. The individual components were heated briefly in toluene at 110 ~ with a catalytic amount of iodine and in the absence of light to convert the mixture of isomers into the thermodynamically more stable

b 1oo%

+

75% 7

8

9

e _

d 77%

75%10

11

Scheme I. Reagents: (a)i] 7, BuLi, THF,-78 ~

6

ii] ZnBr2, THF,-78 ~ iii] 8, Pd2(dba)3, THF, -78 to 25 ~ (b) toluene, 100 ~ (c) DDQ, benzene, 78 ~ (d) AICI3, CuCl2, CS2, 25 ~

6

MICHAEL M. HALEY and W. BRAD WAN

(E) form. Aryl-aryl cross-coupling of 7 with 8 was accomplished according to the method of Knochel et al. in 75% yield [21]. Once constructed, intermediate 9 was heated at 100 ~ in toluene to induce an intramolecular Diels-Alder reaction, providing i0. The central ring was aromatized upon treatment with DDQ to give PAH precursor 11 in 75% yield. Oxidative cyclodehydrogenation of 11 could be accomplished by reduction with a potassium mirror for several days followed by oxidation with iodine in toluene in 47% yield; a superior method was use of modified Kovacic conditions (CuCI2/AIC13 in CS2), achieving the same transformarion in 77% yield. More importantly, this latter method was amenable to scaled-up reactions. Utilizing analogous chemistry, polyene 12 could be assembled from 8 and diiodide 13 (Scheme 2) [20]. In order to avoid the possibility of premature Diels-Alder reaction, the cross-coupling step was performed at a lower temperature and thus resulted in the lower isolated yield (32%). Heating 12 at 100 ~ in toluene induced the double [4+2] cycloaddition and subsequent aromatization with DDQ gave 14 in 92% yield. Oxidative cyclodehydrogenation furnished a hydrocarbon that was characterized by its molecular weight from laser desorption time-of-flight mass spectrometry (LDTOF-MS). Two factors, however, complicated PAH identification: (1) the starting arene can exist as two rotomers (14a,b) which upon cyclodehydrogenation can lead to the formation of two different C54

,T

r '

) 12

Scheme 2. Reagents: (a)i] 8, BuLi, THF, -78 ~

14b

Ifi

ii] ZnBr2, THF, -78 ~ iii] 13, Pd2(dba)3, THF, -78 to 25 ~ (b) toluene, 100 ~ (c) DDQ, 1,1,2,2-tetrachloroethane, 140 ~ (d) AICI3, CuCI2, CS2, 25 ~

Planar Carbon Networks

7

topologies (15 and 16). This dilemma illustrates the need for careful consideration of the starting polyene structure, and (2) PAHs are known to exhibit extremely poor solubility. This recurring problem made it impossible to determine the ratio of 15 to 16. In fact, poor solubility limited most PAH product characterization primarily to TOF-MS experiments and solid-state techniques. In order to assure the formation of 15 exclusively, an alternate synthesis was devised in which the central core of the arene precursor is in a fixed geometry prior to cyclodehydrogenation (Scheme 3) [22]. Suzuki cross-coupling of dibromide 17 with 18 gave stilbenoid 19 in excellent yield (92%) and with no evidence of premature cyclization. Heating 19 in 1,1,2,2-tetrachloroethane induced the double intramolecular Diels-Alder reaction, providing 20 in essentially quantitative yield. Treatment with DDQ gave 21 in 96% yield. Unlike 14, the central benzene moiety of this PAH precursor cannot rotate and so oxidative cyclodehydrogenation provided PAH 15 as the sole compound in essentially quantitative yield. Judicious placement of the ene-diene moieties in the starting stilbenoids can lead to a variety of topologies. For example, reversal of the substituents in 17 and 18 provided 22, which upon similar elaboration gave the C54 isomer 23 [22]. Although traditional spectroscopic techniques were generally ineffective due to low solubility, other methods for structural characterization could be utilized as the PAHs displayed remarkable thermal stability. For example, deposition of 15 at 450 ~ under ultrahigh vacuum onto a MoS 2 surface produced well-ordered layers of

I Br

a

b

92%

17

18

e

I

98%

19

~

d

v

96%

20

99%

21

15

Scheme3. Reagents:(a) Pd(PPh3)4,toluene, EtOH, H20, reflux; (b) 1,1,2,2-tetrachlo-

roethane, 135 oC; (c) DDQ, 1,1,2,2-tetrachloroethane, 135 oC; (d) AICI3, CuCI2, CS2,

25 ~

8

MICHAEL M. HALEY and W. BRAD WAN

22

23

the PAH from which the unit mesh could be derived by low-energy electron diffraction [22]. The area of the mesh (ca. 200 A) corresponded roughly to the van der Waals area of the planar PAH. STM analysis of the deposited material clearly showed planar molecules of rhombic shape closely packed in an ordered arrangement on the substrate.

2.2 Subunitsvia Alkyne Cyclotrimerization As previously mentioned, the ideal precursor for an all-benzenoid PAH is an oligophenylene with the benzene rings in good spatial agreement. One of the most elementary compounds that matches this description is hexaphenylbenzene (24, R = H). The preparation of 24 via hexaaryl substitution of hexahalobenzene is exceedingly difficult [23]. A much more convenient route to 24 is the cobaltmediated cyclotrimerization [24] of 1,2-diphenylethyne (also known as tolane, 25), as shown in Scheme 4 [13]. The cyclization conditions are sufficiently mild to tolerate both halogen and alkyl substituents on the phenyl rings. The tolane derivatives are readily assembled by Pd-catalyzed alkynylation of an appropriate iodobenzene and allow the introduction of solubilizing substituents. This method is restricted, however, to producing identically substituted PAH precursors. Another

3

R~~j~,R _ a 92%

_

R

b ~

R

49%

] 25

24

26

(R ==H, ~.~-~, tBu)

(R - H. n-alkyl,tBu)

Scheme 4. Reagents: (a) Co2(CO)8, dioxane, 100 ~

C52, 25 ~

5 d; (b) Cu(OSO2CF3)2, AICI3,

Planar Carbon Networks

9

drawback is that inclusion of the alkyl groups complicates the cyclodehydrogenation reaction. The substituted oligophenylenes also undergo alkyl group migration, dealkylation, and chlorination. Fortunately these deleterious side reactions can be suppressed by use of a weaker Lewis acid. For example, the yield of 26 increased from 49% using A1C13 to 95% using FeC13 [25]. In addition to standard spectroscopic techniques, PAH 26 (R = C12H25) was visualized using STM [ 12]. As with 15, the macromolecule packed in a highly ordered manner. Interestingly, several long-chain alkyl derivatives of 26 displayed discotic liquid crystalline behavior, an unusual result for a pure hydrocarbon. The columnar mesophases formed by 26 exhibited extremely broad temperature ranges [13]. ,

2.3 Subunits via Intermolecular Diels-Alder Reaction The construction of differently substituted derivatives of 26 required an alternative synthesis of 24. The Mtillen team developed an intermolecular Diels-Alder approach in which cycloaddition of tetraarylcyclopentadienones (tetracyclones, e.g. 27) with tolanes and subsequent extrusion of carbon monoxide led directly to the hexaphenylbenzene structure 24 (Scheme 5) [25]. The requisite tetracyclones are readily prepared via well-established transformations [26] and are much more

R

~

r

a, b ~ 60%

2 R R

d

27

2

2 R

[~

R

R

R

R

R

r

>95% R

R

26

24

(R, R', R2- 14,n-,dkyi,tBu, halollcn) Scheme 5. Reagents: (a) BuLi, THF, -78 ~

(b) 1,4-dimethylpiperazine-2,3-dione; (c) Fe(CO)5, NaOH, CH2CI2, H20, dodecyltrimethylammonium bromide; (d) KOH, EtOH, reflux, 3 h; (e) 25, Ph20, 250 ~ 3-6 h; (f) Cu(OSO2CF3)2,AICI3, CS2, 25 ~

Z

78

METIN BALCI and YAVUZ TASKESENLIGIL

calculated strain energies for the ground state conformations of both diastereomers are reported to have similar values.

REFERENCES [1] Johnson, R. P.,Chem. Rev., 89 (1989) 11 II. [2] Johnson, R. P. In Thummel, R. P. (Ed.)Advances in Theoret~'caUyInterestingMolecules, Vol. I, JAI Press Inc.:Greenwich, CT, 1989, p. 401. [3] Landor, S. R. (Ed.) The Chemistry of the Allenes,Vols. I-3, Academic Press:New York, 1982. [4] Dykstra, C. E., Schaefer,H. F. In Patai,S. (Ed.),The Chemistry ofKetenes, Allenes, and Related Compounds, Wiley: New York, 1980, p. I. [5] Greenberg, A., and Liebman, J.L.,StrainedOrganic Molecules, Academic Press:New York, 1978, p. 126. [6] Blomquist, A. T., Burger, R. E. Jr.,Liu, L. H., Bohrer, J. C., Sucsy, A. C., and Kleis, C., J. Am. Chem. Soc., 73 (1951) 5510. [7] Ball, W. J. and Landor, S. R., Proc. Chem. Soc., London, (1961) 143. [8] Wisser, J. P. and Ramakers, J. E., J. Chem. Soc., Chem. Commun. (1972) 178. [9] Balci, M. and Jones, W. M., J. Am. Chem. Soc., I02 (1980) 7607. [10] Bottini,A. T., Corson, E P.,Fitzgerald,R., and Frost,K. A., Tetrahedron, 28 (1972) 2883. [11] Bottini,A. T., Hilton,L. L., and Plott,J.,Tetrahedron, 31 (I975) 1997. [12] Moore, W. R. and Moser, W., J. Am. Chem. Soc., 92 (I970) 5469. [13] Dillon, P. W. and Underwood, G. R., J. Am. Chem. Soc., 96 (1974) 779. [14] Wentrup, C., Gross, G., Maquestiau, A., and Flammery, R., Angew. Chem., Int.Ed. Engl., 27 (1983) 542. [15] Schmidt, M. W., Angus, R. O., and Johnson, R. P., J. Am. Chem. Sot., 104 (1982) 6838. [16] Angus, R. O., Schmidt, M. W., and Johnson, R. P., J. Am. Chem. Soc., 107 (1985) 532. [17] Lam, B. and Johnson, R. P., J. Am. Chem. Sot., 105 (1983) 7479. [18] Andres, J., Cardenas, R., and Tapia, O., J. Chem. Sot., Perkin Trans. 2, (1985) 363. [19] Yavari, I., J. Mol. Struct. (Theochem.), 65 (1980) 169. [20] Favorskii, A. E., J. Gen. Chem. USSR (Engl. Transl.), 6 (1936) 720. [21] Ceylan, M., Secen, H., and Stitbeyaz, Y., J. Chem. Res. (S), (1997) 293. [22] SUtbeyaz,Y., Ceylan, M., and Secen, H., J. Chem. Res. (S), (1993) 70. [23] Taskesenligil, Y., TOmer, E, and Balci, M., Turk. J. Chem., 19 (1995) 305. [24] Tolbert, M. L., Islam, M. N., Johnson, R. P., Loisella, P. M., and Shakespeare, W. C., J. Am. Chem. Soc., 112 (1990) 6416. [25] Tolbert, M. L. and Siddiqui, S., J. Am. Chem. Soc., 106 (1984) 5538. [26] Balci, M., Ceylan, M., Secen, H., and Stitbeyaz, Y., unpublished results. [27] Shakespeare, W. C. and Johnson, R. P., J. Am. Chem. Sot., 112 (1990) 8578. [28] Wittig, G. and Fritze, P., Angew. Chem., Int. Ed. Engl., 5 (1966) 846. [29] Wittig, G. and Fritze, P., Justus Liebigs Ann. Chem., 82 (1968) 711. [30] Miller, B. and Sift, X., J. Am. Chem. Sot., 109 (1987) 578. [31] Christi, M., Braun, M., and Mtiller, G., Angew. Chem., Int. Ed. Engl., 31 (1992) 473. [32] Hamos, S., Tivakornpannarai, S., and Waali, E. E., Tetrahedron Lett., 27 (1986) 3701. [33] Christi, M. and Schreck, M., Chem. Bet., 120 (1987) 915. [34] Christi, M. and Braun, M., Chem. Ber., 122 (1989) 1939. [35] Janoschek, R., Angew. Chem., Int. Ed. Engl., 31 (1992) 476. [36] Danheiser, R. L., Gould, A. E., Praclilla, E, and Helgason, A., J. Org. Chem., 59 (1994) 5514. [37] Burrell, R. C., Daoust, K. J., Bradley, A. Z., DiRic.o, K. J., and Johnson, R. P., J. Am. Chem. Sot., 118 (1996) 4218. [38] Schreck, M. and Christi, M., Angew. Chem., Int. Ed. Engl., 99 (1987) 692.

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Moore, W. R. and Moser, W. R., J. Am. Chem. Sor 92 (1970) 5469. Moore, W. R., Ward, H. R., and Merritt, R. E, J. Am. Chem. Sor 83 (1961) 2019. K6brich, G. and Goyert, W., T e t r ~ n , 24 (1968) 4327. Backes, J. and Brinker, U. In Eegitz, M. (Ed.), Methoden der Organischen Chemie (Houben-Weyl), Thieme Verlag: Stuttgart, E19b, 1989, p. 400. [82] Taylor, K. G., Hobbs, W. E., Clark, M. S., and Chancy, J., J. Org. Chem., 37 (1972) 2436. [83] Warner, P. M., Herold, R. D., Chu, I., and Lessman, J., J. Org. Chem., 53 (1985) 942. [84] Bettinger, H. E, Schleyer, P. v. R., Schreiner, P. R., and Schaefer III, H. E,J. Org. Chem., 62 (1997) 9267. [851 Bettinger, H. E, Schleyer, P. v. R., Schreiner, P. R., and Schaefer IIl, H. E, J. Phys. Chem., 100 (1996) 16147. [86] Yildiz, Y. K., Sefen, H., Krawiec, M., Watson, W. H., and Balci, M., J. Org. Chem., 58 (1993) 5355. [87] Jelinek-Fink, H., Christi, M., Peters, E. M., Peters, K., and Schnering, H. G. v., Chem. Ber., 124 (1991) 2569. [88] Harris, J. W. and Jones, W. M., J. Am. Chem. Sot., 104 (1982) 7329. [89] Matzinger, S., Bally, T., Patterson, E. V., and McMahon, R. J., J. Am. Chem. Sot., 118 (1996) 1535. [90] Lu, Z., Abboud, K. A., and Jones, W. M., Organometallics, 12 (1993) 1471. [91] Pietruska, J., K6nig, W. A., Maelger, H., and Kopf, J., Chem. Ber., 126 (1993) 159. [92] Price, J. D. and Johnson, R. P., Tetrahedron Lea., 27 (1986) 4679. [93] Christi, M., Rudolph, M., Peters, E. M., Petrs, K., and Schnering, H. G. v., Angew. Chem., Int. Ed. Engl., 34 (1995) 2730. .~ [94] Kreuzholz, R. and Szeimies, G., Liebigs Ann./Recueil, (1997) 1131. [95] Pietruska, J., Hochmuth, D. H., Gehrcke, B., Icheln, D., Runge, T., and K6nig, W. A., Tetrahedron: Asymmetry, 3 (1992) 661. [96] Roth, W. R. and Bastigkeit, T., Liebigs Ann., (1996) 2171. [97] Hemandez, S., Kirehhoff, M. M., Swartz, S. G., and Johnson, R. P., Tetrahedron Lett., 37 (1996) 4907. ,~ [98] Johnson, R. P., Org. Photochem., 7 (1985) 75. [99] Price, J. D. and Johnson, R. P., J. Org. Chem., 56 (1991) 6372. [lOO] Stierman, T. J., Shakespeare, W. C., and Johnson, R. P., J. Org. Chem., 55 (1990) 1043. [lOl] Harusawa, S., Kase, N., Yoneda, R., and Kurihara, T., Tetrahedron Lett., 35 (1994) 1255. [1o2] Harusawa, S., Moriyama, H., Ohishi, H., Yoneda, R., and Kurihara, T., Heterocycles, 38 (1994) 1975. [los] Skatteb~l, L. and Solomon, S., J. Am. Chem. Soc., 87 (1965) 4506. [lO4] Christi, M., Moigno, D., Peters, E-M., Peters, K., and Schnefing, H. G. v., Liebigs Ann./Recueil, (1997) 1791. ~, : [lO51 Kwetkat, K., Riches, B. H., Ropsset, J-M., Breeknell, D. J., Byriel, K., Kennard, C. H. L., Young, D. J., Schneider, U., Mitchell, T. N., and Kitehing, W., Chem. Commun., (1996) 773. [lO6] Ward, H. R. and Karafiath, E., J. Am. Chem. Soe., 91 (1969) 7475. [107] Betridge, J. C., Fotrester, J., Foulger, B. E., and Gilbert, A., J. Chem. Soe., Perkin Trans. 1, (1980) 2425. [1081 Stierman, T. J. and Johnson, R. E, J. Am. Chem. Soe., 107 (1985) 3971. [~091 Stierman, T. J., Shakespeare, W., and Johnson, R. E, J. Org. Chem., 55 (1990) 1043. [110] Bergman, R. G. and Rajadhyaksha, V. J., J. Am. Chem. Soe., 92 (1970) 2163. [1111 Balei, M. and Jones, W. M., J. Am. Chem. Soe., 103 (1981) 2874. [112] Balei, M. and Harmandar, M., T e t r ~ n Lea., 25 (1984) 237. [1131 Taskesenligil, Y., Kashyap, R. E, Watson, W. H., and Bald, M., J. Org. Chem., 58 (1993) 3216. [1141 Tilrner, E, Taskesenligil, Y., and Balei, M., unpublished results. [1151 TUrner, E, Taskesenligil, Y., Dastan, A., and Balci, M., Aust. J. Chem., 49 (1996) 599.

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[116] Taskesenligil, Y., Tamer, E, Kazaz, C., and Balci, M., Turk. J. Chem., in press. [117] Cossu, S., De Lucchi, O., Lucchini, V., Valle, G., Bald, M., Dastan, A., and Demirci, B., Tetrahedron Lett, 38 (1997) 5319. [1181 Farina, M. and Morandi, C., Tetrahedron Lett., 30 (1974) 1819. [119] Harusawa, S., Moriyama, H., Kase, N., Ohishi, H., Yonexta, R., and Kurihare, T., Tetrahedron, 51 (1995) 6475. [120] Sugai, M., Tanino, K., and Kuwajima, I., Synlett, (1997) 461. [121] Reynolds, K. A., Dopico, P. G., Sundermann, M. J., Hughes, K. A., and Finn, M. G., J. Org. Chem., 58 (1993) 1298. [122] Brody, S. B., Williams, R. M., and Finn, M. G., J. Am. Chem. Soc., 119 (1997) 3429. [123] Shimizu, T., Kamigata, N., and Ikuta, S., J. Mol. Struct. (Theochem.) 369 (1996) 127. [124] Dehmlow, E. V. and Stiehm, I'., Tetrahedron Lea., 31 (1990) 1841. [125] Yavari, I., Baharfar, R., and Asghari, S., J. Mol. Struct. (Theochem.) 283 (1993) 277. [126] Yavari, I., Asghari, S., and Shaabani, A., J. Mol. Struct. (Theochem.) 309 (1994) 53. [127] Yavari, I., Baharfar, R., and Nori-Shargh, D., J. Mol. Struct. (Theochem.) 393 (1997) 167.

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STRAI N AN D STRUCTU RE OF STERICALLY CONGESTED TRIPLET CARBENES

H ideo Tom ioka

1. 2. 3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relationship between Structure and Ground-State Multiplicities . . . . . . . . . EPR Spectroscopy of Triplet Carbenes . . . . . . . . . . . . . . . . . . . . . . 3.1 The Zero-Field Splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Triplet Carbene Structure and ZFS Parameters . . . . . . . . . . . . . . . Strain and Structure of Triplet Carbenes . . . . . . . . . . . . . . . . . . . . . 4.1 Diarylcarbenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Diphenylcarbenes Bearing ortho Substituents . . . . . . . . . . . . . . . 4.3 Effect of Triptycyl Group . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Advances in Strained and Interesting Organic Molecules Volume 8, pages 83-112. Copyright 9 2000 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 1-7623-0631-9 83

84 84 86 86 90 94 95 97 106 109 110 110

84

HIDEO TOMIOKA

1. INTRODUCTION Since bis(m-phenylenecarbene) was shown to have a quintet ground state in 1967 [1], triplet diphenylcarbene units have served as the source of electron spins in constructing high-spin molecules as models for purely organic ferromagnetics [2]. Thus, poly(m-phenylenecarbenes) have been studied most systematically inter alia and are accepted as the highest spin organic molecules [3,4]. The highly transient nature of the species [5], however, is an inherent drawback for further extension to usable magnetic materials. In this light, it is highly desirable to synthesize persistent triplet carbenes [6] and then to connect them in a ferromagnetic fashion with an appropriate topological coupler [7]. In order to achieve this purpose, it is important to reveal the relationship between the structure and stability of triplet carbenes. Electron paramagnetic resonance (EPR) spectroscopy in a rigid matrix at low temperature is an especially useful technique in this regard as it is expected to provide valuable information on the molecular and electronic structures of triplet carbenes [8]. In this chapter, we wish to describe the research along this line, where EPR data of a series of triplet carbenes are systematically analyzed and discussed in terms of structural features so as to understand how the structures of triplet carbenes are affected by electronic and steric effects. We will first discuss briefly the relationship between structure and ground-state multiplicities of carbenes, and then describe the type of information that can be obtained from matrix isolation EPR spectroscopy. This will be followed by the main topics, namely the structures of triplet carbenes that are "forced" to become less bent mainly by steric crowding around the carbenic center. 11

RELATIONSHIP BETWEEN STRUCTURE AND GROUND-STATE MULTIPLICITIES

The carbene carbon atom is linked to two adjacent groups by covalent bonds, and it possesses two nonbonding electrons that may have their spins antiparallel (singlet state) or parallel (triplet state) [9]. If the carbene unit were linear, it should have two degenerate p orbitals, and Hund's first rule would predict a triplet ground state. If the carbene unit is not linear the two orbitals become different. The orbital perpendicular to the plane defined by the three atoms is designated as p, while that parallel to this plane is called o. Actually most carbenes are not linear and so the ground-state multiplicity depends upon the relative energy of the singlet and triplet states. The four lowest energy configurations of a carbene have an electronic configuration described as olp 1, o2, or p2. The triplet state has a olp 1 configuration, while the oa is generally thought to be the lowest configuration for the singlets (Figure 1). In a singlet state, the electron-electron coulombic repulsion would be severe, since two electrons are constrained to the same small molecular orbital (MO). On

Congested TripletCarbenes

85

H

H ~

H

,

T H

Linear

-t-CBent

(~2

T o~pl

Figure 1. Linear methylene with two degenerate p orbitals and bent methylene with a (~ and a p orbital.

the other hand, the triplet configuration is stabilized by relief of the coulomb repulsion, although one pays a price for separation of the electrons into different MOs. Thus, the magnitude of the energy difference between the triplet and singlet states (the singlet-triplet splitting, AGsT) is roughly equal to the electron-electron repulsion minus the energy required to promote an electron from the a- to the p-nonbonding orbital. In other words, as the energy separation between a and p states increases, the promotion energy becomes large enough to overcome the repulsion energy, while if the spacing is small, the species will still have a triplet ground state. The small difference between the energies of SOand T 1 may easily be overturned by the effects of substituents on the carbene center. The factors that influence the spacing can be analyzed in terms of electronic and steric effects. An electron-donating substituent raises the energy of the p orbital, thereby increasing the separation of the p and a orbitals. The ground state of such a carbene becomes a singlet. Many carbenes in this class are known. The most familiar ones are the halocarbenes. On the other hand, an electron-withdrawing group lowers the p - o gap and the ground state for such carbenes is expected to be T 1. From a valence-bond viewpoint, this means that electron-donor groups stabilize the electrophilic singlet carbene more than they do the radical-like triplet, and that electron-withdrawing groups destabilize the singlet and lead to a greater AGsT [ 10]. The magnitude of AGsT is expected to be sensitive to the carbon-carbene-carbon bond angle. A linear carbene has two degenerate p orbitals and this is calculated to provide the maximum value of AGsT.Bending of the bond angle removes the orbital degeneracy and reduces AGsT.As the carbon-carbene-carbon bond angle is further contracted, the o orbital picks up more s-character and consequently moves even lower in energy. The smaller the bond angle, the more energy it takes to promote an electron from the c~ to the p orbital and the smaller AGsT becomes. This is shown more quantitatively by calculation for methylene. The calculations predict that the energy of singlet methylene will drop below that of the triplet state for carbenes with bond angles less than about 906. On the other hand, theory also suggests that opening of the central angle strongly destabilizes the singlet state, but

86

HIDEO TOMIOKA

requires very little additional energy for the triplet, thus making AGsT larger [11 ]. In accordance with this prediction, cyclopropenylidene has been shown to have a singlet ground state [12], while di(adamantyl)carbene has been shown to have a triplet ground state [ 13].

3. EPR SPECTROSCOPY OF TRIPLET CARBENES 3.1 The Zero-Field Splitting The electron has the spin S = 1/2, which in a magnetic field is allowed to take the spin quantum number m s = +1/2. For one or more unpaired electrons, one defines the total spin multiplicity as 2S + 1. One unpaired electron has the spin multiplicity 2.1/2+1 = 2, and the system is called a doublet, corresponding to the two values of m s. Two unpaired electrons, as in triplet carbenes, have the spin multiplicity 2.1 + 1 = 3 (triplet) and it has three values of the spin magnetic quantum number, ms=l, 0, and -1. The three components (1, 0, -1) of the triplet state correspond to three different possibilities for the magnetic quantumnumber m s. Each of these possibilities corresponds to a stable state, i.e. one of the three components of a triplet, which is defined as T§ - o~1~ 2, T_ - 131~2' and T O- tx1132, for m s = 1, -1, and 0, respectively, where ct and 13 refer to the spin vectors. This means that the m s = 1 and m s = -1 states correspond to the two electronic magnets being oriented in the same direction where they repel each other, while the m s = 0 state behaves much like a singlet state, the two electronic magnets being antiparallel and therefore attracting each other. In contrast to the two energy levels of a doublet (radical), which are degenerate unless an external field is applied, the three levels of a triplet are split even in the absence of an external magnetic field since a dipole coupling of the two spins creates an internal magnetic field in the molecule, which splits the energy levels. The ms=0 level is stabilized and the m s = 1 and -1 levels are destabilized. This energy separation is called D (Figure 2). Each electron moves in the magnetic field of the other. If the molecule has less than cylindrical symmetry there will be two different values of the internal field. These fields do not affect the m s = 0 level, which has no overall spin. They cause, however, a further splitting of the m s = 1 and m s = -1 levels. This splitting is called E. In contrast to the energy levels of a doublet, which are degenerate unless an external magnetic field is applied, these splittings persist in the absence of an external magnetic field, and hence the constants D and E are termed zero-field splitting (ZFS) parameters (Figure 2). Each energy level is associated with a principal magnetic axis in the molecule because its energy will not change with magnetic field when the field is parallel to that axis. Triplet EPR spectroscopy provides a direct measure of the distribution of spins in the molecule. The magnitude of this splitting is, however, very small and so an external field is applied to bring the transition into the microwave range. When an external magnetic field H is applied, the energy difference between the m s = 1 and m s = -1

Congested Triplet Carbenes

87

Energy o -

ms=

t 2z I n s = ",

1 -1

%%%%%~ /D

ms=0

Figure 2. Zero-field splitting in the triplet state.

levels will increase, but the energy of the ms = 0 level remains unchanged. Thus the three levels split as shown in Figure 3. The selection rule that allows transitions usually has ~trns = +1. One would expect then two EPR transition for a triplet [14,15]. However, the exact transition frequency seen depends on the orientation of the internal principal magnetic axes of the triplet carbene with respect to the external magnetic field. Thus, one might expect an infinite number of possible orientations for a randomly oriented sample. Fortunately, though, one observes selectively those structures that have one of their principal magnetic axes nearly aligned with the external field. Since there are three magnetic axes, and two transitions for each of the canonical orientations, one expects 3 x 2 = 6 lines for a triplet spectrum. The lines appear in pairs, according to whether they arise from structures which have the x, y or z principal magnetic axis nearly aligned with the external field.

/

J

ms =

I (Tt)

m, = 0 (To)

~ ~

4 ~n = I

ms =-I (T.z) I

o

Figure 3. Energy levels of the triplet in a magnetic field (H).

-

//

88

HIDEO TOMIOKA

Figure 4 shows the theoretical EPR spectrum of a randomly oriented triplet species [ 15]. The values of D and E are calculated from the values of x, y, and z by using the relationships x = D I 3 - E, y = D I 3 + E, a n d z = - 2 D I 3 . D and E are obtained in gauss from the spectrum, but usually they are converted to energy units (cm -l) and reported as ID/hcl and IE/hcl. The absolute value signs indicate the fact that D can be positive or negative, and the sign is not known. b)

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I

4iooo s,ooo

G Figure 4. Theoretical EPR spectrum of randomly oriented triplet species. (a) E~ O, (b) E = O.

Congested Triplet Carbenes

89

The E value describes the smallest splitting and relates to the molecular symmetry of the triplet. In a structure with threefold or higher symmetry, two of the triplet sublevels will be degenerate, producing a value of zero for E. In this case, the middle pairs of lines collapse to one pair and produce a four-line spectrum [Figure 4(b)]. In addition to the six (E ~ 0) or four (E = 0) lines described above, an extra line sometime appears at much lower frequencies as indicated by dashed line in Figure 4. This line corresponds to Ams = 2 transitions between the two outer triplet sublevels. It is a critical line because its presence unambiguously signals that one is observing a triplet state. In a simple model, the ZFS parameters D and E for a triplet depend on the distance between electrons with parallel spins as given by Eq. 1: D = ~" g2gt2 ~

, E = ~- g21a2

(1)

In this equation r is the distance between the two spins and x, y, and z are the component of r along the x, y, and z axes, respectively. As mentioned before, these parameters describe the separation of the three energy levels when no external magnetic field is present. Each energy level is associated with a principal magnetic axis in the molecule because its energy is not changed with magnetic field when the field is parallel to that axis. The parameter D measures the magnetic dipole interaction along thez axis and is related to the average 1/r3 as shown above. A high value of D implies a large spin-spin interaction and a close proximity of the two spins. The parameter E, on the other hand, is a measure of the difference between similar magnetic dipole interactions along the x and y axes. A consequence of this is that a molecule with three different axes should have a finite E, whereas this quantity vanishes for linear molecules with degenerate p orbitals. More plainly, the more the two electrons are delocalized in carbenes with conjugated r systems, the smaller the value the repulsive interaction D will be. On the other hand, increasing the bond angle at the carbene center leads to a higher p-orbital contribution and a smaller value for E. Although the values D and E depend on the electronic distribution, it has been shown that there is a good correlation between the E/D ratio and the bond angle at the divalent carbon atom. However, one should be aware that the ratio of E/D is not always a reliable guide to the structure of triplet carbenes. For instance, diphenylcarbene and fiuorenylidene have very different geometries but almost exactly the same D and E values [8]. If one really wants to learn about the geometry, one must label the carbene carbon with t3C and measure the hyperfine coupling constants. The other difference between triplet and doublet EPR spectroscopy concerns the nature of the samples used. Free radical spectra are typically taken in fluid media and are thus nicely isotropic. However, triplet spectra generally require rigid samples. In order to obtain the appropriate information on the interaction of the

90

HIDEO TOMIOKA

unpaired spins in the triplet state molecule, it is necessary to prevent rapid rotation of the molecule. Carbenes are usually generated by photolysis of an appropriate precursor, e.g. the diazo compound, in a suitable inert rigid matrix at low temperature. Here, the matrix is expected to prevent the highly reactive molecules from reacting either with themselves or the carbene precursor. The low temperature also prevents rapid free rotation and suppresses the reactivity of the triplet species. The intensity of an EPR signal depends on the magnitude of the magnetic susceptibility, Z. For a simple paramagnetic system, a Curie plot, a plot of Z versus 1/T, should be linear. This means that the intensity becomes smaller as the temperature is raised. Deviations from linearity could indicate a temperature dependent equilibrium between a triplet and a singlet. Conversely, a linear Curie plot is taken as evidence for a triplet ground state, but this requires that the carbene be stable to a certain extent at elevated temperature.

3.2 Triplet Carbene Structure and ZFS Parameters From the above it can be seen that ZFS parameters provide information on the molecular and electronic structures of triplet carbenes. We will see here how the parameters change systematically by examining a series of sterically less perturbed triplet carbenes.

3.2.1 Effects of Carbenic Substituents Some D and E values for typical triplet carbenes are collected in Table 1 [ 16-18]. The ZFS parameters for methylene [16], the parent compound of all carbenes, clearly indicated that it has bent structure. A bond angle of 136 ~ is estimated [16b] which is in good agreement with most theoretical calculations [9b]. Introduction of aryl groups on methylene results in a significant decrease in D values; thus D decreases from 0.69 to 0.515 on going from methylene to phenylcarbene (PC) [17]. The D values decrease further as the aromatic ring is changed from phenyl to naphthyl to anthryl (Table 1) [18]. These trends are interpreted in terms of an increase in spin delocalization into the aromatic rings. It is interesting to note here that there are only small changes in E/D values among the monoarylcarbenes listed, indicating that the central bond angle of the carbenes is not affected significantly by change in the nature of the aromatic ring.

3.2.2 Effects of Remote Substituents Effects ofpara-subsfituents on the ESR spectrum of triplet diphenylcarbenes (1; 3DPCs) have been investigated (Table 2) [19]. Two trends become obvious when the D values are compared for para- and para, para'-disubstituted DPCs. First, substitution generally causes a decrease in D from that in the parent molecule. This is obviously due to extended g delocalization of spin density. Second, the decrease in D is largest when 3DPC is substituted with one p-electron-withdrawing group

Congested Triplet Carbenes

91

Table 1. Zero-Field Splitting Parameters a for Some Typical Triplet Carbenes D(cm- I)

Carbenes H,~," H

~H

E(cm- t)

E/D

Ref.

0.69

0.003

0.004 b

16

0.5150 0.518

0.0251 0.024

0.04873 c 0.046 d

17

(Z') 0.4347 (E) 0.4555

0.0208 0.0202

0.0478 0.0443

18 18

(Z) 0.4926 (E) 0.4711

0.0209 0.0243

0.0424 0.0516

18 18

0.3008

0.0132

0.0439

18

0.4055

0.0194

0.0478

17

(PC)

"FH

~

(1-NC) H (2-NC)

(AC)

(DPC) Notes: "Measured in benzophenone at 77 K unless otherwise noted.

bMeasured in xenon at 4.2 K. CMeasured in p-dichlorobenzene at 77 K. dMeasured in fluorolube glass at 77 K.

and one p'-electron-donating group. The decrease in D in these unsymmetrically disubstituted 3DPC is always larger than that predicted by taking the sum of the effects in the independent mono-substituted derivatives. These observations are explained in terms of merostabilization, a term first suggested by Katritzky [20] to describe increased delocalization in radicals for which reasonable charge-separated resonance structures can be drawn (Scheme 1). These charge-separated resonance structures contribute only in unsymmetrically disubstituted DPCs containing strong electron-withdrawing and electron-donating groups.

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Table 2. Zero-Field Splitting Parameters a for para, para'-Disubstituted Diphenylcarbenes (1) [19]

Substituent X

Y

D(cm-I)

E(crn-I)

H

H

H OMe H CN H NMe 2 H NO 2 CN CN NO 2 NO 2

OMe OMe CN CN NMe 2 NMe 2 NO 2 NO 2 OMe NMe 2 OMe NMe 2

0.4088 0.4043 0.4022 0.3906 0.3879 0.3876 0.3 748 0.3778 0.3773

0.0170 0.0191 0.0189 0.0193 0.0178 0.0168 b 0.0180 c 0.01 73 d 0.01 77 b 0.0172 0.0163 d 0.0172 b 0.0164 b

0.3774 0.3.518 0.3611 0.3351

Notes: aMeasuredin 4 19 methylcyclohexane isopentane 9 matrix unless otherwise noted. bMeasured in tetrahydrofuran matrix. CMeasured in 4 19methylcyclohexane tetrahydrofuran 9 matrix. ~ salt was used.

M e 2 N . ~ ] ~ ~

NO2

Me2

I

0

§

Me2

+

.I

0

Scheme 1.

0

=0

93

Congested Triplet Carbenes 3.2.3 Geometrical Isomerism

Triplet carbenes whose divalent carbon atom is substituted with a n sp 2 hybridized carbon atom may exist in two rotomeric forms which are stable at very low temperatures. In favorable cases, ESR spectra of these carbenes exhibit two sets of triplet signals with sufficiently different ZFS parameters. The magnitude of the ZFS parameters D is largely determined by the spin-spin dipolar interaction of the two electrons at the divalent carbon atom. Accordingly, the fraction of the g spin density located at the carbenic center can be estimated from the D value of the carbene. In spite of the predominance of this one-center interaction, the spin density at atoms several bonds removed from the divalent carbon atom can also have a significant effect on the ZFS parameters. This was first-observed for the pairs of 1- and 2-naphthylcarbenes in 1965 [18], and since then reports of isomerism in triplet carbenes have appeared with increasing frequency, where two sets of triplet signals having similar but non-identical ZFS parameters are observed [21 ]. The spectra are assigned to the two conformations of the carbene in which the a orbital at the divalent carbon and the aromatic moiety are coplanar. When the distribution of the spin in the n orbital is unsymmetric, the dipole spin-spin interaction of the ~ electron with the electron localized in the a orbital is different for the two conformations. Consequently, the ZFS parameters will be different and in cases where the differences are sufficiently large, it is possible to observe the spectra of the two isomers. The assignment of the ZFS parameters to specific conformer is made possible by a point spin model [21], with individual contributions, D i, due to n spin densities, Pi, at the individual carbon atoms, C i, viz.,

where r i is the distance between the divalent carbon, Cdi, and a carbon, C i, bearing = spin density and zi is the z coordinate of C i. As mentioned above, carbenes will have noticeable secondary contributions to ID/hcl due to the nearest carbons with g-electron spin densities. Take 2-naphthylcarbene for instance (Scheme 2). The n-electron density at the 1-position is twice that at the 3-position (03 = 1/2Pl). For

(E)-2-NC

(Z)-2- NC Scheme 2.

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HIDEO TOMIOKA

the (E)-isomer, C1 lies close to the z axis so that r is approximately parallel to z. Hence, Zl = rl, and D i o~ Pi/-~l~i = Pi

(3)

causing a noticeable negative contribution to ID/hcl. A similar inspection of the (Z)-isomer shows that C3 lies close to the z axis (% = r3), whereas C1 is far from the z axis (z 1 > rl). Therefore, the negative contribution to ID/hcl is smaller. Based on these considerations, the (E)-conformer must have the smaller ID/hcl value (see Table 1).

3.2.4 Effects of Host Matrix The effects of the host matrix on the geometry of a triplet carbene have been studied extensively for 3DPC [ 17,22]. The ZFS parameters obtained in a series of matrices indicate that the magnitudes of D and E show a small dependence on the nature of the host matrix. The ZFS parameters from measurement in single crystals of benzophenone and 1,1-diphenylethene differ appreciably from each other. These observations are interpreted as suggesting a possible difference in the geometry of the diphenylcarbene in these two matrices. However, it is noted that use of the linear diphenylethyne as the matrix results in a set of ZFS parameters having essentially identical values to those in other hosts. Further, the use of other precursors for DPC, such as diazidodiphenylmethane [23] and tetraphenyloxirane [24], have provided essentially the same ZFS parameters in the ESR experiments. Thus the influence of the host is not overwhelming at least for usual triplet carbenes.

4. STRAIN AND STRUCTURE OF TRIPLET CARBENES Bending of the molecule would remove the degeneracy of the p orbitals and convert them to A 1 and B 1 orbitals of the C2v group. Therefore, bent molecules could be singlet states. However, it is not at all obvious that the energy minimum of the triplet state would inevitably have the linear configuration, although such has sometimes been assumed to be the case. Because bending would be accompanied by rehybridization, the relative amounts of s and p character in both the bonding and the nonbonding orbitals would change with consequential changes to the binding energies of both the bonding and the nonbonding electrons. If the levels of the A 1 and B 1 orbitals are close together, relief of electron-electron repulsion might very easily be enough to compensate for the promotion energy required to put one unshared electron in the higher orbital allowing the bent molecule to be a triplet [llc]. In accordance with those theoretical considerations, the ZFS parameters of typical triplet carbenes thus far described suggest that most of these carbenes favor

Congested Triplet Carbenes

95

a bent structure at least in a rigid matrix. This is also true in solution at ambient temperature. In other words, a linear structure is thermodynamically less stable for most triplet carbenes but becomes favored when perturbation is large enough to compensate for the hybridization energy. We will see in this section how the structure of triplet carbenes is perturbed either sterically or electronically.

4.1 Diarylcarbenes INDO calculations on DPC have been carried out [ 11 a] on several geometrical conformations by changing the values of three variable parameters; these are the angle ((x) between the two bonds joining the carbene carbon to the phenyl tings ( C - C - C ) and two angles (01 and 02) each being the dihedral angle between one of the rings and the central plane, i.e. the plane containing (x. The state of 3DPC calculated to be the most stable is a linear triplet ((x = 180 ~ with rings at 90 ~ to each other. However, an energy of