The chemistry ofcyclobutanes

Part 1

Edited by

ZVI RAPPOPORT

The Hebrew University, Jerusalem

and

JOEL F. LIEBMAN

The University of Maryland, Baltimore County

2005

An Interscience Publication

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The chemistry ofcyclobutanes

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The chemistry ofcyclobutanes

Part 1

Edited by

ZVI RAPPOPORT

The Hebrew University, Jerusalem

and

JOEL F. LIEBMAN

The University of Maryland, Baltimore County

2005

An Interscience Publication

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Includes bibliographical references and indexes.ISBN 0-470-86 400-1 (set : acid-free paper)1. Cyclobutane–Synthesis. I. Rappoport, Zvi. II. Liebman, Joel F. III. Series.QD341.H9C554 2005547′.61–dc22

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Contributing authorsJosé Luis M. Abboud Instituto de Quı́mica Fı́sica ‘Rocasolano’, CSIC,

C/Serrano, 119, E-28006 Madrid, Spain. Fax: +34 91 5642431; e-mail: jlabboud@iqfr.csic.es

Ibon Alkorta Instituto de Quı́mica Médica, CSIC, C/Juan de la Cierva,3, E-28006 Madrid, Spain. Fax: +34 91 564 4853; e-mail:ibon@iqm.csic.es

A. Bashir-Hashemi ERC, Inc., at AFRL/PRS, 10 East Saturn Blvd., EdwardsAFB, CA 93524, USA. Fax: +1 626 334 6714; e-mail:hbashir@aol.com

Nathan L. Bauld Department of Chemistry and Biochemistry, TheUniversity of Texas at Austin, Austin, TX 78731, USA.Fax: +1 512 471 8696; e-mail: bauld@mail.utexas.edu

Ulf Berg Organic Chemistry 1, Department of Chemistry, LundUniversity, P. O. Box 124, S-221 00 Lund, Sweden. Fax:+46 46 222 4119; e-mail: ulf.berg@orgk1.lu.se

Holger Butenschön Institut für Organische Chemie, Universität Hannover,Schneiderberg 1B, D-30167 Hannover, Germany. Fax:+49 05 1176 24616; e-mail:holger.butenschoen@mbox.oci.uni-hannover.de

Thomas Carell Department of Chemistry, LMU München,Butenandtstraße 5–13, Haus F, D-81377 München,Germany. Fax: +49 89 2180 77756; e-mail:Thomas.Carell@cup.uni-muenchen.de

Barry K. Carpenter Department of Chemistry and Chemical Biology, CornellUniversity, Ithaca, NY 14853-1301, USA. Fax: +1 607255 4137; e-mail: bkcl@cornell.edu

Siu-Hin Chan Department of Chemistry, Central Laboratory of Instituteof Molecular Technology for Drug Discovery andSynthesis, The Chinese University of Hong Kong, Shatin,New Territories, Hong Kong SAR, China.

Marvin Charton Chemistry Department, School of Liberal Arts andSciences, Pratt Institute, Brooklyn, NY 11205, USA. Fax:+1 718 722 7706; e-mail: mcharton@pratt.edu

Xudong Chen Department of Chemistry, Dartmouth College, Hanover,NH 03755, USA. Fax: +1 603 646 3946

vii

viii Contributing authors

Juan Z. Dávalos Instituto de Quı́mica Fı́sica ‘Rocasolano’, CSIC,C/Serrano, 119, E-28006 Madrid, Spain. Fax: +34 91 5642431; e-mail: jdavalos@iqfr.csic.es

Jacques Fernandes Dias Departamento de Quı́mica e Ambiental, Faculdade deTecnologia, Universidade Estadual do Rio de Janeiro,Estrada Resende-Riachuelo s/n◦, Resende, Brazil, RJ27523-000. Fax: +55 24 3354 7354; e-mail:jacques@fat.uerj.br

M. Eckert-Maksić Laboratory for Physical-Organic Chemistry, Division ofOrganic Chemistry and Biochemistry, Rudjer BoškovićInstitute, Zagreb, Croatia. Fax: +3 851 4680 195; e-mail:mmaksic@emma.irb.hr

Marcus G. Friedel Department of Chemistry, LMU München,Butenandtstraße 5–13, Haus F, D-81377 München,Germany. Fax: +49 89 2180 77756; e-mail:Thomas.Carell@cup.uni-muenchen.de

Nan-Yan Fu Department of Chemistry, Central Laboratory of Instituteof Molecular Technology for Drug Discovery andSynthesis, The Chinese University of Hong Kong, Shatin,New Territories, Hong Kong SAR, China.

Johannes Gierlich Department of Chemistry, LMU München,Butenandtstraße 5–13, Haus F, D-81377 München,Germany. Fax: +49 89 2180 77756

Hiroyuki Higuchi Department of Chemistry, Faculty of Science, ToyamaUniversity, 3190 Gofuku, Toyama, Toyama 930-8555,Japan. Fax: +81 076 445 6616; e-mail:higuchi@sci.toyama-u.ac.jp

Henning Hopf Institute of Organic Chemistry, Technical University ofBraunschweig, Hagenring 30, D-38106 Braunschweig,Germany. Fax: +49 53 1391 5388; e-mail:h.hopf@tu-bs.de

William M. Horspool Department of Chemistry, The University of Dundee,Dundee, DD1 4HN, Scotland, UK. e-mail:william.horspool@tesco.net

Richard P. Johnson Department of Chemistry, University of New Hampshire,Durham, NH 03824, USA. Fax: +1 603 862 4278; e-mail:rpj@cisunix.unh.edu

Dietmar Kuck Fakultät für Chemie, Universität Bielefeld,Universitätsstraße 25, D-33501 Bielefeld, Germany. Fax:+49 52 1106 6417; e-mail: dietmar.kuck@uni-bielefeld.de

Edward Lee-Ruff Department of Chemistry, York University, Toronto,Ontario, M3J 1P3, Canada. Fax: +1 416 736 5936;e-mail: leeruff@yorku.ca

David M. Lemal Department of Chemistry, Dartmouth College, Hanover,NH 03755, USA. Fax: +1 603 646 3946; e-mail:david.m.lemal@dartmouth.edu

Contributing authors ix

Joel F. Liebman Department of Chemistry and Biochemistry, University ofMaryland, Baltimore County, 1000 Hilltop Circle,Baltimore, MD 21250, USA. Fax: +1 410 455 2608;e-mail: jliebman@umbc.edu

Z. B. Maksić Quantum Organic Chemistry Group, Division of OrganicChemistry and Biochemistry, Rudjer Bošković Institute,Zagreb, Croatia. Fax: +385 1 4561 118; e-mail:zmaksic@spider.irb.hr

Arunkumar Natarajan Department of Chemistry, Tulane University, NewOrleans, LA 70118, USA. Fax: +1 504 865 5596; e-mail:murthy@tulane.edu

H. Mark Perks Department of Chemistry and Biochemistry, University ofMaryland, Baltimore County, 1000 Hilltop Circle,Baltimore, MD 21250, USA. Fax: +1 410 455 2608;e-mail: perks@umbc.edu

Esther Quintanilla Instituto de Quı́mica Fı́sica ‘Rocasolano’, CSIC,C/Serrano, 119, E-28006 Madrid, Spain. Fax: +34 91 5642431; e-mail: esther q@iqfr.csic.es

V. Ramamurthy Department of Chemistry, Tulane University, NewOrleans, LA 70118, USA. Fax: +1 504 865 5596; e-mail:murthy@tulane.edu

Peter Rudolf Seidl Departamento de Processos Orgânicos, Escola deQuı́mica, Universidade Federal do Rio de Janeiro, Centrode Tecnologia, Bloco E, Ilha do Fundão, Rio de Janeiro,Brazil, RJ 21949-900. Fax: +55 21 2562 7567; e-mail:pseidl@eq.ufrj.br

Hans-Ullrich Siehl Abteilung Organische Chemie I der Universität Ulm,Albert Einstein Allee 11, D-89069 Ulm, Germany. Fax:+49 07 3150 22787; e-mail:Ullrich.Siehl@chemie.uni-ulm.de

Suzanne W. Slayden Department of Chemistry, George Mason University, 4400University Drive, Fairfax, VA 22030, USA. Fax: +1 703993 1055; e-mail: sslayden@gmu.edu

Amnon Stanger Department of Chemistry, The Lise Meitner-MinervaCenter for Computational Quantum Chemistry and TheInstitute of Catalysis, Science and Technology,Technion—Israel Institute of Technology, Haifa 32000,Israel. Fax: +9 72 4 829 3944; e-mail:stanger@tx.technion.ac.il

J. M. Tanko Department of Chemistry, Virginia Polytechnic Instituteand State University, Blacksburg, VA 24061, USA. Fax:+1 540 231 3255; e-mail: jtanko@vt.edu

Kenneth B. Wiberg Department of Chemistry, Yale University, New Haven,CT 06520-8107, USA. Fax: +1 203 432 5161; e-mail:kenneth.wiberg@yale.edu

x Contributing authors

Henry N. C. Wong Department of Chemistry, Institute of Chinese Medicineand Central Laboratory of Institute of MolecularTechnology for Drug Discovery and Synthesis, TheChinese University of Hong Kong, Shatin, NewTerritories, Hong Kong SAR, China. Fax: +86 852 26035057; e-mail: hncwong@cuhk.edu.hk

ForewordThis is another volume in “The Chemistry of Functional Groups” series which deals withthe chemistry of cyclic hydrocarbons and their derivatives. Earlier volumes dealt with “TheChemistry of Alkanes and Cycloalkanes” and “The Chemistry of the Cyclopropyl Group”.The Cyclobutyl group occupies an intermediate position between these two groups.

The two parts of the present volume contain 23 chapters written by experts from 10countries. They deal with theoretical and physical organic chemical aspects of cyclobutanederivatives including the aromaticity/antiaromaticity of derived unsaturated species, withtheir stereochemical aspects, their thermochemistry, and with the acidity and basicity ofselect derivatives. There are chapters on NMR, IR and mass spectra of cyclobutanes, onintermediates such as carbocations and cation radicals containing the cyclobutyl moietyand on the directing and activating effects of cyclobutane derivatives.

Several chapters deal with synthetic aspects of formation and use of cyclobutane deriva-tives, as well as with their rearrangements, their photochemistry, their organometallicderivatives and with their formation by solid state dimerization of olefins. The biomedi-cally interesting pyrimidine dimers and their relevance to DNA damage is also discussed.

Special topics include highly unsaturated derivatives, arenocyclobutenes andcyclobutadiene, fluorocyclobutanes and polycyclic systems containing cyclobutanes suchas cubanes, prismanes, ladderanes, bicyclo [2.1.0]pentanes and bicyclo [2.2.0]hexanes andother species. Unfortunately, two planned chapters, on cyclobutyl carbanions and anionradicals, and on structural chemistry did not materialize.

The literature coverage is up to 2004.We would be grateful to readers who draw our attention to mistakes in the present

volume, or to the omission of important chapters, which deserve to be included in sucha treatise.

Jerusalem and Baltimore ZVI RAPPOPORTAugust, 2004 JOEL F. LIEBMAN

xi

TheChemistryofFunctionalGroupsPreface to the seriesThe series ‘The Chemistry of Functional Groups’ was originally planned to cover ineach volume all aspects of the chemistry of one of the important functional groups inorganic chemistry. The emphasis is laid on the preparation, properties and reactions of thefunctional group treated and on the effects which it exerts both in the immediate vicinityof the group in question and in the whole molecule.

A voluntary restriction on the treatment of the various functional groups in thesevolumes is that material included in easily and generally available secondary or ter-tiary sources, such as Chemical Reviews, Quarterly Reviews, Organic Reactions, various‘Advances’ and ‘Progress’ series and in textbooks (i.e. in books which are usually foundin the chemical libraries of most universities and research institutes), should not, as a rule,be repeated in detail, unless it is necessary for the balanced treatment of the topic. There-fore each of the authors is asked not to give an encyclopaedic coverage of his subject,but to concentrate on the most important recent developments and mainly on material thathas not been adequately covered by reviews or other secondary sources by the time ofwriting of the chapter, and to address himself to a reader who is assumed to be at a fairlyadvanced postgraduate level.

It is realized that no plan can be devised for a volume that would give a completecoverage of the field with no overlap between chapters, while at the same time preservingthe readability of the text. The Editors set themselves the goal of attaining reasonable cov-erage with moderate overlap, with a minimum of cross-references between the chapters.In this manner, sufficient freedom is given to the authors to produce readable quasi-monographic chapters.

The general plan of each volume includes the following main sections:(a) An introductory chapter deals with the general and theoretical aspects of the group.(b) Chapters discuss the characterization and characteristics of the functional groups,

i.e. qualitative and quantitative methods of determination including chemical and physicalmethods, MS, UV, IR, NMR, ESR and PES—as well as activating and directive effectsexerted by the group, and its basicity, acidity and complex-forming ability.

(c) One or more chapters deal with the formation of the functional group in question,either from other groups already present in the molecule or by introducing the new groupdirectly or indirectly. This is usually followed by a description of the synthetic uses ofthe group, including its reactions, transformations and rearrangements.

(d) Additional chapters deal with special topics such as electrochemistry, photochem-istry, radiation chemistry, thermochemistry, syntheses and uses of isotopically labeledcompounds, as well as with biochemistry, pharmacology and toxicology. Whenever appli-cable, unique chapters relevant only to single functional groups are also included (e.g.‘Polyethers’, ‘Tetraaminoethylenes’ or ‘Siloxanes’).

xiii

xiv Preface to the series

This plan entails that the breadth, depth and thought-provoking nature of each chapterwill differ with the views and inclinations of the authors and the presentation will neces-sarily be somewhat uneven. Moreover, a serious problem is caused by authors who delivertheir manuscript late or not at all. In order to overcome this problem at least to someextent, some volumes may be published without giving consideration to the originallyplanned logical order of the chapters.

Since the beginning of the Series in 1964, two main developments have occurred.The first of these is the publication of supplementary volumes which contain materialrelating to several kindred functional groups (Supplements A, B, C, D, E, F and S). Thesecond ramification is the publication of a series of ‘Updates’, which contain in eachvolume selected and related chapters, reprinted in the original form in which they werepublished, together with an extensive updating of the subjects, if possible, by the authorsof the original chapters. A complete list of all above mentioned volumes published todate will be found on the page opposite the inner title page of this book. Unfortunately,the publication of the ‘Updates’ has been discontinued for economic reasons.

Advice or criticism regarding the plan and execution of this series will be welcomedby the Editors.

The publication of this series would never have been started, let alone continued,without the support of many persons in Israel and overseas, including colleagues, friendsand family. The efficient and patient co-operation of staff-members of the publisher alsorendered us invaluable aid. Our sincere thanks are due to all of them.

The Hebrew University SAUL PATAIJerusalem, Israel ZVI RAPPOPORT

Sadly, Saul Patai who founded ‘The Chemistry of Functional Groups’ series died in1998, just after we started to work on the 100th volume of the series. As a long-termcollaborator and co-editor of many volumes of the series, I undertook the editorship andI plan to continue editing the series along the same lines that served for the preceedingvolumes. I hope that the continuing series will be a living memorial to its founder.

The Hebrew University ZVI RAPPOPORTJerusalem, IsraelMay 2000

Contents1 Cyclobutane—physical properties and theoretical studies 1

Kenneth B. Wiberg

2 Antiaromaticity and aromaticity in carbocyclic four-membered rings 17M. Eckert-Maksić and Z. B. Maksić

3 Stereochemical aspects—conformation and configuration 83Ulf Berg

4 Thermochemistry of cyclobutane and its derivatives 133Joel F. Liebman and Suzanne W. Slayden

5 Acidity and basicity of cyclobutanes 177Esther Quintanilla, Juan Z. Dávalos, José Luis M. Abboudand Ibon Alkorta

6 NMR spectroscopy of cyclobutanes 213Peter Rudolf Seidl and Jacques Fernandes Dias

7 Mass spectrometry and gas-phase ion chemistry of cyclobutanes 257Dietmar Kuck

8 Synthesis of cyclobutanes 281E. Lee-Ruff

9 The application of cyclobutane derivatives in organic synthesis 357Nan-Yan Fu, Siu-Hin Chan and Henry N. C. Wong

10 Structural effects of the cyclobutyl group on reactivity and properties 441Marvin Charton

11 Rearrangements of cyclobutanes 497J. M. Tanko

12 Cyclobutyl, cyclobutyl-substituted and related carbocations 521Hans-Ullrich Siehl

13 Cation radicals in the synthesis and reactions of cyclobutanes 549Nathan L. Bauld

14 Highly unsaturated cyclobutane derivatives 589Richard P. Johnson

xv

xvi Contents

15 Cyclobutarenes 617Amnon Stanger

16 Organometallic derivatives 655Holger Butenschön

17 Photochemistry of cyclobutanes: Synthesis and reactivity 715William M. Horspool

18 Solvent-free photosynthesis of cyclobutanes: Photodimerizationof crystalline olefins 807

Arunkumar Natarajan and V. Ramamurthy

19 Chemistry of cubane and other prismanes 873A. Bashir-Hashemi and Hiroyuki Higuchi

20 Bicyclo[2.1.0]pentanes and bicyclo[2.2.0]hexanes 923Barry K. Carpenter

21 Fluorinated cyclobutanes and their derivatives 955David M. Lemal and Xudong Chen

22 Cyclobutane pyrimidine dimers as UV-induced DNA lesions 1031Marcus G. Friedel, Johannes Gierlich and Thomas Carell

23 Cubanes, fenestranes, ladderanes, prismanes, staffanesand other oligocyclobutanoids 1061

Henning Hopf, Joel F. Liebman and H. Mark Perks

Author index 1111

Subject index 1179

List of abbreviations usedAc acetyl (MeCO)acac acetylacetoneAd adamantylAIBN azoisobutyronitrileAlk alkylAll allylAn anisylAr aryl

Bn benzylBu butyl (C4H9)Bz benzoyl (C6H5CO)

CD circular dichroismCI chemical ionizationCIDNP chemically induced dynamic nuclear polarizationCNDO complete neglect of differential overlapCp η5-cyclopentadienylCp∗ η5-pentamethylcyclopentadienyl

DABCO 1,4-diazabicyclo[2.2.2]octaneDBN 1,5-diazabicyclo[4.3.0]non-5-eneDBU 1,8-diazabicyclo[5.4.0]undec-7-eneDIBAH diisobutylaluminium hydrideDME 1,2-dimethoxyethaneDMF N,N-dimethylformamideDMSO dimethyl sulphoxide

ee enantiomeric excessEI electron impactESCA electron spectroscopy for chemical analysisESR electron spin resonanceEt ethyleV electron volt

xvii

xviii List of abbreviations used

Fc ferrocenylFD field desorptionFI field ionizationFT Fourier transformFu furyl(OC4H3)

GLC gas liquid chromatography

Hex hexyl(C6H13)c-Hex cyclohexyl(c-C6H11)HMPA hexamethylphosphortriamideHOMO highest occupied molecular orbitalHPLC high performance liquid chromatography

i- isoICR ion cyclotron resonanceIp ionization potentialIR infrared

LAH lithium aluminium hydrideLCAO linear combination of atomic orbitalsLDA lithium diisopropylamideLUMO lowest unoccupied molecular orbital

M metalM parent moleculeMCPBA m-chloroperbenzoic acidMe methylMNDO modified neglect of diatomic overlapMS mass spectrum

n normalNaph naphthylNBS N-bromosuccinimideNCS N-chlorosuccinimideNMR nuclear magnetic resonance

Pen pentyl(C5H11)Ph phenylPip piperidyl(C5H10N)ppm parts per millionPr propyl (C3H7)PTC phase transfer catalysis or phase transfer conditionsPy, Pyr pyridyl (C5H4N)

List of abbreviations used xix

R any radicalRT room temperature

s- secondarySET single electron transferSOMO singly occupied molecular orbital

t- tertiaryTCNE tetracyanoethyleneTFA trifluoroacetic acidTHF tetrahydrofuranThi thienyl(SC4H3)TLC thin layer chromatographyTMEDA tetramethylethylene diamineTMS trimethylsilyl or tetramethylsilaneTol tolyl(MeC6H4)Tos or Ts tosyl(p-toluenesulphonyl)Trityl triphenylmethyl(Ph3C)

Xyl xylyl(Me2C6H3)

In addition, entries in the ‘List of Radical Names’ in IUPAC Nomenclature of OrganicChemistry, 1979 Edition, Pergamon Press, Oxford, 1979, p. 305–322, will also be usedin their unabbreviated forms, both in the text and in formulae instead of explicitly drawnstructures.

CHAPTER 1

Cyclobutane—physical propertiesand theoretical studies

KENNETH B. WIBERG

Department of Chemistry, Yale University, New Haven, CT 06520-8107, USAFax: +1 203 432 5161; e-mail: kenneth.wiberg@yale.edu

I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1II. CYCLOALKANE STRUCTURES AND BONDING . . . . . . . . . . . . . . . 1

III. BOND STRENGTHS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3IV. ENERGIES OF CYCLOALKANES . . . . . . . . . . . . . . . . . . . . . . . . . . 4V. NMR SPECTRA OF CYCLOALKANES . . . . . . . . . . . . . . . . . . . . . . 5

VI. CYCLOPROPYL AND CYCLOBUTYL CATIONS . . . . . . . . . . . . . . . 7VII. INTERACTION OF CYCLOPROPANE AND CYCLOBUTANE RINGS

WITH ELECTRON-DEFICIENT CENTERS . . . . . . . . . . . . . . . . . . . . 8VIII. PROTONATED CYCLOPROPANES AND CYCLOBUTANES . . . . . . . 9

IX. THERMAL FORMATION OF CYCLOBUTANES BY CYCLOADDITIONAND THERMAL CLEAVAGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

X. ANTIAROMATICITY IN CYCLOBUTADIENE . . . . . . . . . . . . . . . . . 12XI. SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

XII. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

I. INTRODUCTIONCyclobutane is interesting because it provides a bridge between the very reactive (for ahydrocarbon) cyclopropane and the ‘normal’ cycloalkanes from cyclopentane to the largercycloalkanes. Cyclopropane reacts readily with bromine to form 1,3-dibromopropane1

and reacts with sulfuric acid to give 1-propylsulfuric acid2. Cyclobutane does not reactwith either of these reagents, but some cyclobutanes undergo C−C bond cleavage withtransition metal species3. It is very difficult to cleave the C−C bonds of cyclopentane andthe higher cycloalkanes.

II. CYCLOALKANE STRUCTURES AND BONDINGIn order to understand these differences, it is helpful to examine the structures andenergies of these compounds. Some data are given in Table 1. Cyclopentane undergoes

The chemistry of cyclobutanesEdited by Z. Rappoport and J. F. Liebman 2005 John Wiley & Sons, Ltd

1

2 Kenneth B. Wiberg

TABLE 1. Structural data for some cycloalkanes

Compound Observed Calculated

r(C−C) r(C−H) H−C−H r(C−C) r(C−H) H−C−HCyclopropane a 1.512(3) 1.083(3) 114.0(7) 1.509 1.083 115.1Cyclobutane b 1.556(1) 1.091(1) 1.552 1.094 ax 109.3

1.093 eqCyclopentane c 1.546(1) 1.114 1.540 1.095Cyclohexane d 1.536(1) 1.097(2) ax 1.530 1.099 ax 106.9

1.085(6) eq 1.096 eqCyclopropene e 1.505(1) 1.085(1) 1.513 1.089 114.5

1.293(1) 1.072(1) 1.304 1.077Cyclobutene f 1.566(3) 1.094(5) 1.568 1.094 109.2

1.517(3) 1.5171.342(4) 1.083(5) 1.351 1.086

a Reference 5.b Reference 6.c Reference 4.d Reference 9.e R. J. Berry and M. D. Harmony, Struct. Chem., 1, 49 (1990).f B. Bak, J. J. Led, L. Nygaard, J. Rastrup-Andersen and G. O. Sørensen, J. Mol. Struct., 3, 369 (1969).

pseudorotation in which the carbons undergo a motion perpendicular to the average planewithout significant change in energy4. The average C−C bond length is only 0.013 Ågreater than that of n-alkanes. In contrast, cyclopropane has markedly shorter C−C bondlengths5 and cyclobutane has markedly longer C−C bond lengths6.

The short bond lengths in cyclopropane are in part explained using the Coulson–Moffittbonding model7. With nominal 60◦ C−C−C bond angles, it is not possible to formcoaxial C−C bonds since the smallest interorbital angle for first row elements is 90◦,corresponding to pure p-orbitals. The angle must be somewhat larger since a bond formedwith just p-orbitals will be quite weak. They estimated an interorbital angle of 104◦,corresponding to 80% p-character in the C−C bonds vs. the normal value of about 75%p-character. Thus, the bonds in cyclopropane are bent, and a better representation of thebond length would be given by the path of maximum electron density between the carbons(the bond path)8 and it has been estimated to be 1.528 Å. It is approximately 0.008 Åshorter than the C−C bonds in cyclohexane9.

The bent bonds in cyclopropane derivatives are readily observed in the results of X-ray crystallographic studies10. The output of such a study is an electron density map,and the maximum in electron density between two cyclopropane carbons lies outsidethe line of centers of the atoms. Bond angle bending based on ab initio calculationsmay be described in terms of the angle between the C−C bond paths at the C nucleus.With cyclopropane, the angle deviates from the conventional angle by 18.8◦ whereas thedeviation for cyclobutane is only 6.7◦11.

The structure of cyclobutane presents some interesting questions. The C−C−C bondangle is 88◦, indicating that it adopts a puckered structure6. This is probably due to atorsional interaction between two adjacent methylene groups. Ethane is known to prefera staggered arrangement and the eclipsed arrangement is 3 kcal mol−1 higher in energy12.Planar cyclobutane, with a 90◦ C−C−C bond angle, has eclipsed methylene groups,resulting in considerable torsional strain. Puckering the ring leads to a reduction of thisstrain term, but at the same time the C−C−C bond angle is reduced, leading to increasedbond angle strain. The equilibrium geometry is a result of the tension between these two

1. Cyclobutane—physical properties and theoretical studies 3

strain terms. The C−C−C bond angle (α) is related to the ring puckering angle (τ ) bytan(α/2) = cos(τ/2).

a

180-t

Another feature of the cyclobutane geometry is that the methylene groups are rotatedinwards13, whereas one might expect them to rotate outwards in order to reduce H · · · Hnon-bonded interactions. Bartell and Anderson have proposed that the methylene groupsprefer a local C2v geometry, and with bent C−C bonds this would result in the inward bend.

The most puzzling feature of the cyclobutane geometry is the long C−C bond length.This has been observed in a variety of cyclobutane derivatives, and C−C bond lengthscover a range of 1.521–1.606 Å depending on the substitution pattern, with an averageof 1.554 Å14. With cyclobutane itself, the bond length is 1.556 Å6.

The short C−C bond length in cyclopropane and the long length in cyclobutane maybe explained by invoking a 1–3 C · · · C non-bonded repulsion15. It might be noted thatthis is contained in the Urey–Bradley force field16. Cyclopropane does not have such aninteraction because all of the carbons are bonded to each other. Cyclobutane, on the otherhand, has two 1–3 C · · · C non-bonded interactions with a relative small distance betweenthe carbons. This repulsion will lead to a lengthening of the C−C bonds.

One might wonder if it would also lead to flattening of the ring in order to minimizethis interaction. An ab initio calculation for cyclobutane gives a bond length of 1.555 Åand a CCC bond angle of 88◦. If the C−C length is forced to be 1.536 Å (the cyclohexanebond length) and the geometry is again optimized, the CCC bond angle changes very littleand the energy increases by only 0.4 kcal mol−1 17. Near their equilibrium values, bondscan initially be stretched with little increase in energy, but further extension become costlybecause of the quadratic nature of the bond stretching potential.

This proposal also explains why cyclopentane has C−C bonds a little longer than thosein cyclohexane. The 1,3-C · · · C non-bonded distances are shorter in cyclopentane than incyclohexane18, leading to greater repulsion in the former. It also explains the observed111◦ C−C−C bond angles in n-alkanes.

III. BOND STRENGTHSThe high p-character in the C−C bonds of cyclopropane must lead to high s-character inits C−H bonds. It is known that increasing s-character leads to shorter and stronger C−Hbonds19, and this is found with cyclopropane (Table 2). The force constant for stretchingthe C−H bond is significantly greater than for cyclobutane, the bond length is shorter,and the bond dissociation energy is greater than found with other cycloalkanes or openchain alkanes. The effect is further increased with cyclopropene. Here, the olefinic C−Hbond would have an s-character approaching that of acetylene, and it is one of the fewunsubstituted hydrocarbons that will undergo base catalyzed exchange of the vinylic C−Hbonds with ROD to give C−D bonds20.

The properties of the C−H bonds in cyclobutane are much closer to those of the othercycloalkanes, although there is an indication of somewhat increased s-character. The C−Hbond lengths are somewhat shortened, and the bond dissociation energy is calculated tobe 1.5 kcal mol−1 greater than in cyclohexane. Further information may be gained fromthe 13C−H NMR coupling constants (see below).

4 Kenneth B. Wiberg

TABLE 2. Cycloalkane C−H force constantsand bond dissociation energies

Compound k(C−H) a BDE bCyclopropane 6.3 108.4Cyclobutane 5.1 c 99.8 c

Cyclopentane 4.2 c 95.5 c

Cyclohexane 5.3 c 98.4 c

a Calculated at the B3LYP/6-311+G* level of theory.b In kcal mol−1; calculated at the G3B3 level of the-ory17.c Equatorial hydrogens.

IV. ENERGIES OF CYCLOALKANESThe heats of formation of a number of small cycloalkanes and related compounds havebeen determined via combustion calorimetry, and additional data have been obtained bymeasuring heats of hydrogenation. Some representative data are summarized in Table 3.

One item of interest with these compounds is the strain energy. This is defined as thedifference in heat of formation between the compound of interest and that of an ‘unstrainedmodel’. The choice of this model has been the subject of some controversy, but almostany choice would be satisfactory as long as it is applied consistently. The values of thestrain energies may differ, but the only quantities of importance are the relative values.The Franklin group equivalents21 (Table 4) are frequently used for this purpose.

TABLE 3. Heats of formation and strain energies of cycloalkanes, gas phase, 25 ◦C, kcal mol−1

Compound �Hf Strain energy Reference

Cyclopropane 12.7 ± 0.1 27.5 aCyclobutane 6.6 ± 0.3 26.3 bCyclopentane −18.3 ± 0.2 6.3 bCyclohexane −29.5 ± 0.2 0.0 bCyclopropene 66.2 ± 0.6 52.2 cCyclobutene 37.4 ± 0.4 28.4 dCyclopentene 8.1 ± 0.3 4.0 eCyclohexene −1.2 ± 0.1 0.4 f1-Methylcyclopropene 58.6 ± 0.3 53.5 dMethylenecyclopropane 29.1 ± 0.2 32.7 dBicyclo[1.1.0]butane 51.9 ± 0.2 63.9 dBicyclo[2.1.0]pentane 37.8 ± 0.3 54.8 gBicyclo[2.2.0]hexane 29.8 ± 0.3 51.7 gBicyclo[1.1.1]pentane 50.4 71.0 hCubane 148.7 ± 0.9 157.4 iBis(1,1′-bicyclo[1.1.1] pentane) 96.8 ± 1.2 126.9 ja J. W. Knowlton and F. D. Rossini, J. Res. Natl. Bur. Stand., 43, 113 (1949).b S. Kaarsaemaker and J. Coops, Recl. Trav. Chim. Pays-Bas, 71, 261 (1952).c K. B. Wiberg, W. J. Bartley and F. D. Lossing, J. Am. Chem. Soc., 84, 3980 (1962).d K. B. Wiberg and R. A. Fenoglio, J. Am. Chem. Soc., 90, 3395 (1968).e M. A. Dolliver, T. L. Gresham, G. B. Kistiakowsky and W. E. Vaughan, J. Am. Chem. Soc., 59, 831 (1937).f A. Labbauf and F. D. Rossini, J. Phys. Chem., 65, 476 (1961).g W. R. Roth, F.-G. Klärner and H.-W. Lennartz, Chem. Ber., 113, 1818 (1980).h Calculated energy: K. B. Wiberg, J. Comput. Chem., 5, 197 (1984).i B. D. Kybett, S. Carroll, P. Natalis, D. W. Bonnell, J. L. Margrave and J. L. Franklin, J. Am. Chem. Soc., 88,626 (1966). However, see V. V. Diky, M. Frenkel and L. S. Karpushenkava, Thermochim. Acta 408, 115 (2003).j V. A. Luk’yanova, V. P. Kolesov and V. P. Vorob’eva, Russ. J. Phys. Chem. (Engl. Transl.), 69, 1908 (1995).

1. Cyclobutane—physical properties and theoretical studies 5

TABLE 4. Franklin’s group equiv-alents, �Hf kcal mol−1 (25 ◦C) a

Group Value

CH3 −10.12CH2 −4.926CH −1.09C 0.80=CH2 6.25cis-CH=CH 18.88C=CH 20.19a Reference 21.

The strain energy of cyclobutane is then the heat of formation of cyclobutane less fourtimes the CH2 equivalent, or 26 kcal mol−1. The strain energies of some compounds ofinterest are given in Table 3. Cyclohexane has essentially no strain energy; cyclopen-tane has a small strain energy which results from the partial eclipsing of adjacent C−Hbonds plus some bond angle strain. Cyclopropane and cyclobutane have essentially thesame strain energy, which at first appears surprising in view of the large difference inC−C−C bond angles, and the difference in hybridization. One factor that may contributeto the strain energy of cyclobutane is the cross-ring 1–3 repulsion between the methylenecarbons15. This is not present in cyclopropane.

There is another important factor that contributes to the lack of difference in strain ener-gies. The C−H bonds in cyclopropane are considerably stronger than those in cyclobutane.If the normal C−H bond dissociation energy (cyclohexane) is 98 kcal mol−1, a C−H bondin cyclopropane is 10 kcal mol−1 stronger. With six C−H bonds, this could lead to a netstabilization that may approach 60 kcal mol−1. The strain in the carbon skeleton of cyclo-propane may approach 88 kcal mol−1, and for cyclobutane, with 8 C−H bonds that are1.5 kcal mol−1 stronger than those in cyclohexane, the strain may approach 38 kcal mol−1.This is, of course, only a very rough approximation, but it does indicate that the strainin the skeleton of cyclopropane is significantly greater than that for cyclobutane, and thatfor the latter is still considerable.

The heats of hydrogenation of cyclopropene, methylenecyclopropane and cyclobuteneare interesting. The heat of hydrogenation of cyclohexene (assumed to be unstrained)is just the difference in heat of formation between cyclohexene and cyclohexane, or28 kcal mol−1. Cyclobutene has a heat of hydrogenation of 31 kcal mol−1, only a littlelarger than for cyclohexene, indicating that the introduction of a C=C bond does not leadto much of an increase in strain energy.

The value for cyclopentene is 26 kcal mol−1, indicating that cyclopentene is less strainedthan cyclopentane because some of the methylene eclipsing strain in cyclopentane isrelieved on going to cyclopentene.

Cyclopropene is remarkable, giving a heat of hydrogenation of 54 kcal mol−1, 26 kcalmol−1 greater than that for cyclohexene. This effect is reduced somewhat in methylenecy-clopropane and can be seen by comparing its heat of formation with the isomeric 1-methylcyclopropene. The origin of the high heat of hydrogenation has been attributed tothe strong C−H bonds in cyclopropane that are lost on going to cyclopropene22. The effectis smaller with methylenecyclopropane since it has only one trigonal center in the ring.

V. NMR SPECTRA OF CYCLOALKANESThere are interesting differences between the NMR chemical shifts of cyclopropane,cyclobutane and the higher cycloalkanes (Table 5). The 1H shift for cyclopropane is

6 Kenneth B. Wiberg

TABLE 5. NMR chemical shifts (ppm) a

Compound 1H 13C

CH2 =CH CH2 =CHCyclopropane 0.22 −2.6Cyclobutane 1.96 23.3Cyclopentane 1.51 26.5Cyclohexane 1.54 27.8Cyclopropene 0.93 7.06 2.3 108.9Cyclobutene 2.57 6.03 31.4 137.2Cyclopentene 2.28 b 5.60 32.3 c , 22.7 130.2Cyclohexene 1.96 b 5.59 25.1 c , 22.6 126.9

a Reference 28.b Protons adjacent to the double bond.c Methylene carbons adjacent to the double bond.

TABLE 6. NMR 13C−H coupling constants a

Compound J 13C−H (Hz) %sMethane 125 25Cyclopropane 161 32Cyclobutane 134 27Cyclohexane 123 25Bicyclo[1.1.0]butane 153 (equatorial) 31

169 (axial) 34205 (bridgehead) 41

Bicyclo[1.1.1]pentane 144 (methylene) 29164 (bridgehead) 33

Cubane 154 31Cyclopropene b 228.2 46Cyclobutene b 170 34Cyclopentene b 162 32Cyclohexene b 158 31

a Data were taken from Reference 28.b Vinylic hydrogens.

found to be remarkably upfield, and this has been used as a diagnostic for the presence ofa cyclopropane ring23. Cyclobutane, on the other hand, has its 1H band downfield fromthat in cyclohexane. The same trend is found with the 13C shifts.

The upfield shift for cyclopropane has been attributed to a ring current associated withσ -aromaticity, and the downfield shift for the cyclobutane protons has been attributedto σ -antiaromaticity. The subject of σ -aromaticity has been the object of many studies.Recent work suggests that it is not a viable proposal24. Nevertheless, it is clear thatcyclopropane has a higher than normal magnetic susceptibility25. In addition, the nucleusindependent chemical shifts (NICS) at the center of the ring for cyclopropane is positive26

and that for cyclobutane is negative26,27. This quantity has been suggested as a test foraromaticity and antiaromaticity respectively, although the detailed origin of these shiftsis not as yet understood.

The 13C−H NMR coupling constants can be used to gain information on hybridization28and the empirical relationship %s = J 13C−H/5 has been proposed. The values of thesecoupling constants are given in Table 6 for cyclobutane and a number of other related

1. Cyclobutane—physical properties and theoretical studies 7

compounds, along with the empirically derived %s values. Again, the cyclobutane C−Hbonds appear to have increased s character, but not as much as is found with cyclopropane.

Large long-range 1H–1H coupling constants are observed with cyclobutyl derivatives.One of the largest, 18 Hz, is found for the bridgehead hydrogens of bicyclo[1.1.1]pentane(1)29. With bicyclo[2.1.1]hexane (2), there is a 6 Hz coupling between the endo protonsof the cyclobutane methylene groups30. When the distance is further increased as inbicyclo[2.2.1]heptane (3), the coupling between the bridgehead hydrogens is less than1 Hz31. The coupling presumably involves the overlap of the backsides of the C−H bondorbitals which increases rapidly as the distance is decreased.

H

H

H

H

H

H

(1) (2) (3)

VI. CYCLOPROPYL AND CYCLOBUTYL CATIONSIn contrast to most reactions in which cyclopropane derivatives are more reactive thancyclobutanes, the opposite is true for solvolytic reactions. Cyclopropyl tosylate is relativelyunreactive32, and its lack of reactivity has been attributed to two factors. First, an SN 1solvolytic reaction would normally lead to an increase in C−C−C bond angle at thereaction site as a carbocation is formed, and this is not possible with a cyclopropane ring33.As a result, there is an increase in strain energy. Second, the hybridization of the carbonsin cyclopropane is close to that of ethylene, and vinyl halides are resistant to solvolyticreactions34. Despite its low reactivity, it is important to note that it is considerably morereactive than 7-norbornyl tosylate that has a 94◦ C−C−C bond angle35. It appears that thesolvolysis of cyclopropyl tosylate is assisted by the development of allyl cation characterin the transition state36.

Cyclobutyl tosylate (4) would be expected to have reduced reactivity because it, again,will suffer an increase in strain on going to a carbocation due to the constrained C−C−Cbond angles. However, it has a reactivity comparable to cyclopentyl tosylate35b. Thereis now much evidence that cyclobutyl cations are stabilized by an interaction with thecross-ring carbon, leading to a species that might be described as a ‘bicyclobutoniumion’ (5)37 in which the cationic center is stabilized by an interaction with the cross-ringmethylene group.

OTs+

(4) (5)

The cross-ring distance is important for such an interaction, and it increases inimportance as the distance is decreased. Thus, 1-chlorobicyclo[1.1.1]pentane (6) is quitereactive38.

5-Substituted bicyclo[2.1.1]hexane derivatives are interesting in that the endo-tosylate(7) is 106 times as reactive in solvolysis as the exo-tosylate (8)39. This indicates the need

8 Kenneth B. Wiberg

Cl(6)

OTs

H

H

OTs

(7) (8)

for the remote carbon of the cyclobutane ring to be anti to the leaving group in orderto have an assisted solvolysis. This appears to be a general feature of the solvolysis ofbridged cyclobutyl derivatives40.

In these solvolytic reactions, cyclopropylcarbinyl and cyclobutyl cations frequentlyare interconverted. B3LYP/6-311+G* calculations for the parent ions find both to beminima on the potential energy surface, with the cyclobutyl cation slightly lower inenergy (1 kcal mol−1). These ions are in rapid equilibrium, and substitution can easilyshift the equilibrium composition41.

VII. INTERACTION OF CYCLOPROPANE AND CYCLOBUTANE RINGSWITH ELECTRON-DEFICIENT CENTERS

The interaction of cyclopropane rings with a cationic site has been well studied. Withdimethylcyclopropylcarbinyl cation, the ‘bisected’ conformer, in which the cationic p-orbital is aligned to interact with the bent C−C bonds of the cyclopropane ring, hasa 14 kcal mol−1 lower energy than the ‘perpendicular’ conformer, with the latter beinga transition state42. The ion can be observed by NMR spectroscopy. Methyl substitu-tion at the cationic center is important since cyclopropylcarbinyl cation rearranges to abridged cyclobutyl cation38. The interaction of the cyclopropane ring with an electron-deficient center is also seen with cyclopropylcarboxaldehyde where the rotational barrieris 6 kcal mol−1 43. The minimum energy conformers correspond to the ‘bisected’ arrange-ment and the transition state has the ‘perpendicular’ arrangement.

The interaction with a cationic site is much weaker with cyclobutane. The rotationalbarrier for cyclobutanecarboxaldehyde has not been measured, but calculations indicate itis only 0.8 kcal mol−1 44. There are two low energy conformers where the carbonyl groupis eclipsed with either the adjacent hydrogen or one of the adjacent carbons. A rotamercorresponding to the perpendicular conformer is neither a minimum nor a transition state.

Dimethylcyclobutylcarbinyl derivatives (9) on solvolysis rearrange to cyclopentylcations. Relief of strain energy is an important driving force, but this is reduced bythe conversion of a tertiary cation to the usually less stable secondary cation45. In orderto stabilize a cyclobutylcarbinyl cation enough to allow it to be observed by NMR, it wasnecessary to have two cyclopropane rings attached to the cationic center46.

OTs

Me MeMe

Me+

(9)