Cyclobutadiene and Related Compounds PDF
Preview Cyclobutadiene and Related Compounds
ORGANIC CHEMISTRY A SERIES OF MONOGRAPHS Edited by ALFRED T. BLOMQUIST Department of Chemistry, Cornell University, Ithaca, New York 1. Wolfgang Kirmse. CARBENE CHEMISTRY, 1964 2. Brandes H. Smith. BRIDGED AROMATIC COMPOUNDS, 1964 3. Michael Hanack. CONFORMATION THEORY, 1965 4. Donald J. Cram. FUNDAMENTALS OF CARBANION CHEMISTRY, 1965 5. Kenneth B. Wiberg (Editor). OXIDATION IN ORGANIC CHEMISTRY, PART A, 1965; PART B. In preparation. 6. R. F. Hudson. STRUCTURE AND MECHANISM IN ORGANO-PHOSPHORUS CHEMISTRY, 1965 7. A. William Johnson. YLID CHEMISTRY, 1966 8. Jan Hamer (Editor). 1 ,4-CYCLOADDITION REACTIONS, 1967 9. Henri Ulrich. CYCLOADDITION REACTIONS OF HETEROCUMULENES, 1967 10. M. P. Cava and M. J. Mitchell. CYCLOBUTADIENE AND RELATED COM- POUNDS, 1967 IN PREPARATION Reinhard W. Hoffmann. Dehydrobenzene and Cycloalkynes CYCLOBUTADIENE and Related Compounds M. P. CAVA and M. J. MITCHELL Chemistry Department, Wayne State University, Detroit, Michigan WITH A CHAPTER ON THEORY BY Η. E. SIMMONS AND A. G. ANASTASSIOU Central Research Laboratories, Ε. I. du Pont de Nemours & Co., Inc. Wilmington, Delaware 1967 ACADEMIC PRESS New York and London COPYRIGHT © 1967, BY ACADEMIC PRESS INC. ALL RIGHTS RESERVED. NO PART OF THIS BOOK MAY BE REPRODUCED IN ANY FORM, BY PHOTOSTAT, MICROFILM, OR ANY OTHER MEANS, WITHOUT WRITTEN PERMISSION FROM THE PUBLISHERS. ACADEMIC PRESS INC. Ill Fifth Avenue, New York, New York 10003 United Kingdom Edition published by ACADEMIC PRESS INC. (LONDON) LTD. Berkeley Square House, London W.l LIBRARY OF CONGRESS CATALOGUE CARD NUMBER: 66-16434 PRINTED IN THE UNITED STATES OF AMERICA " We see a little, presume a great deal, and so jump to a conclusion" JOHN LOCKE Preface The aim of this monograph is to present in readily accessible form all of the information available on four-membered carbocyclic compounds having only trigonally hybridized carbon atoms in the ring. Cyclobutadiene is the most elusive and the most celebrated compound of this type, but biphenylene, benzocyclobutadiene, dimethylenecyclobutene, and the rest are in many ways of equal interest and importance. All of these compounds have been investigated experimentally in recent years and all are treated fully in the present volume. The guidelines which we followed were: (1) the text should be as comprehen- sive as possible; (2) the organization should be highly formal in order to serve the specialized needs of workers in the field; and (3) critical commentary should be introduced wherever necessary to resolve conflicting reports in the literature. In attempting to make the monograph as comprehensive as possible, we have made an exhaustive survey of all of the usual reference sources, including Chemical Abstracts, Beilsteins Handbuch der Organischen Chemie, and the Zentralblatt, and, in addition, we have made a reference-by-reference inves- tigation of all previous bibliographies on the subject and have solicited per- tinent preprints and reprints from workers in the field. Each of the cyclobutadienoid ring systems cited above has been treated in a separate chapter which has been subdivided into standard sections dealing with specific topics, e.g., methods of preparation, chemistry, physical properties. While it is true that a rigid organizational scheme of this type presents a frag- mented view of the subject to the uninitiated reader, it has the overriding ad- vantage in our opinion of better serving the needs of current workers in the field, most of whom require an easily consulted comprehensive reference work rather than a qualitative review. The text on cyclobutadiene has been divided into three chapters, of which the first deals with cyclobutadiene and sub- stituted cyclobutadienes, the second with cyclobutadiene-metal complexes, and the third with the divalent ions of cyclobutadiene. In addition, cyclo- butene-3,4-dione and its analogs and 3, 4-dimethylenecyclobutene and its analogs have been termed "cyclobutadienequinones" and "methylene analogs of cyclobutadienequinone," respectively, and have been assigned to separate vii viii PREFACE chapters. The text on benzocyclobutadiene has been apportioned among four chapters (Benzocyclobutadiene, 1,2-Benzocyclobutadienequinone, Methylene Analogs of 1,2-Benzocyclobutadienequinone, and Higher Aromatic Analogs of Benzocyclobutadiene) and the text on biphenylene has been divided into two chapters (Biphenylene and The Benzobiphenylenes). An alphabetical bibliography has been provided at the end of each chapter in which related works, e.g., preliminary communications and their matching papers, have been brought together into groups bearing collective reference numbers. Such works are designated individually by the letters a, b, c, etc., and are cited in the text either by a number alone (which indicates that the infor- mation in question is to be found in all references of the group bearing that number) or by a number and one or more letters (which indicates that the information occurs only in the particular reference or references cited). In this way, the reader can determine at a glance whether he should consult the preliminary communication, the full paper, or both in order to obtain further information on a particular subject. Recent years have seen a remarkable increase in the number of papers published in the field of cyclobutadiene chemistry. While this increase has been gratifying to us as researchers, it has also been a source of considerable difficulty to us as authors; on more than one occasion we have had to make substantial changes in the manuscript in order to incorporate a newly published paper. Fortunately, many of our colleagues were able to send us preprints and personal communications prior to publication, and this privileged information helped us to keep the number of revisions to a minimum. But even so, we found it necessary in the end to set an arbitrary cut-off date (January 1, 1964) beyond which no new material was to be incorporated into the manuscript. Of course, all papers that reached us in preprint form before that date were incorporated, even though the published papers did not appear until later. Finally, in an effort to update the contents, we added an appendix containing abstracts of papers published in 1964 and 1965. Because of the haste with which the appen- dix was compiled, a few papers have probably escaped detection, but we believe that the preponderant majority of papers have been included and have been adequately abstracted. The information contained in the appendix, though informally organized, is retrievable through the index. We are indebted to more than two hundred of our colleagues for sending us reprints, preprints, and personal communications during the time when the manuscript was in progress. Their help was of inestimable value and saved us many hours of library work. We owe a debt of gratitude to Professor G. W. Griffin for critically reading the entire manuscript and to Dr. E. R. Atkinson, Dr. Margareta Avram, Professor J. Ε. H. Hancock, Professor P. M. Maitlis, Dr. K. W. Ratts, and Dr. R. P. Stein for reading selected portions of it and offering advice. We also wish to express our thanks to Dr. Leonard T. Capell, PREFACE ix Mr. John F. Stone, and Mr. Donald F. Walker of the Chemical Abstracts Service for their advice on nomenclature; to our former colleagues of the Chemistry Department of The Ohio State University for many helpful dis- cussions ; and to the librarians of the Chemical Abstracts Service, the Batelle Memorial Institute, and the McPherson Chemistry Library of The Ohio State University for their unstinting aid. M. P. CAVA February, 1967 M. J. MITCHELL CHAPTER 1 Cyclobutadiene 4 1 3 2 Cyclobutadiene is the simplest member of the series of fully conjugated cyclic polyenes having the general composition (C H)„, of which benzene has long 2 2 been the best-known example. In the era before the development of molecular orbital theory, organic chemists quite naturally assumed that cyclobutadiene would be a stable aromatic system with a chemistry paralleling that of benzene. As a result, several attempts were made around the turn of the century to prepare cyclobutadiene and substituted cyclobutadienes, and a number of compounds were assigned cyclobutadiene structures on the basis of rather limited experimental evidence. These attempts were entirely unsuccessful and none of the proposed cyclobutadiene structures has withstood the test of reinvestigation; to this day, neither cyclobutadiene nor a substituted cyclo- butadiene has beei) isolated and characterized directly. Furthermore, modern molecular orbital theory contradicts the earlier ingenuous assumption of stability and predicts that, unlike the An+ 2 7r-electron systems, cyclobutadiene, which is a An 7r-electron system, should be a highly unstable species having zero derealization energy and exhibiting the properties of a triplet diradical.* This prediction is in line with what is known about the chemistry of cyclobutadiene and it leaves little hope that a stable cyclobutadiene can be synthesized. Despite this unencouraging prospect (or perhaps because of it) chemists have sought in recent years to synthesize cyclobutadienoid systems in which the *See, however, the Addendum to Chapter 12 for a recent prediction by Dewar of a singlet ground state for cyclobutadiene. 1 2 1. CYCLOBUTADIENE instability of the four-membered system would be diminished through some special structural or electronic feature. This approach has met with a certain degree of success, inasmuch as it has resulted in the preparation of biphenylene (7), tetramethylcyclobutadienenickel chloride (2), and 1,2-diphenylnaphtho- [o]cyclobutadiene (5), all of which are isolable and may be viewed as cyclo- butadienoid systems. It must be noted, however, that the major contributors to the resonance hybrid of both biphenylene and diphenylnaphthocyclobutadiene are dimethylenecyclobutene structures rather than cyclobutadiene structures and that the carbocycle of the metal complex has little or no cyclobutadienoid character. However, it is not impossible that a simple cyclobutadiene, substituted with carefully chosen groups, might prove to have a singlet ground state and hence be isolable. A particularly interesting idea along these lines has been proposed by Roberts,128 who has suggested that the cyclobutadiene system might be appreciably stabilized by a pair of neighboring conjugative substituents, one of which would be electron-releasing and the other of which would be electron- attracting. In the last decade the problem of synthesizing a stable cyclobutadiene derivative has come to be recognized as a challenge of classical proportions. It has also become increasingly apparent that we are only now beginning to understand the chemistry of cyclobutadiene and that this understanding might well portend the development of a new and more sophisticated phase of cyclo- butadiene studies. Perhaps one of the most significant pieces of research in the field was published only recently by Watts, Fitzpatrick, and Pettit, in which the formation of free cyclobutadiene is described unambiguously for the first time.*'148 * Pettit's work demonstrates that, although cyclobutadiene is an extremely unstable substance, it has, nevertheless, a detectable lifetime. This work, which was published after the manuscript of Chapter 1 had been prepared, is described in ref. 148 and in the Appendix. A. HISTORY 3 Various aspects of cyclobutadiene chemistry have been reviewed by Avram et #/.,13 Baker and McOmie,15 Criegee,45 Gelin,76 Murata,115 Nenitzescu et a/.,117 Vogel,144 and Vol'pin.146 A. History The first attempt to synthesize compounds of the cyclobutadiene series appears to have been made by W. H. Perkin, Jr., who tried to prepare "tetrene- dicarboxylic acid" [cyclobutadiene- 1,2-dicarboxylic acid (7)], which he likened to phthalic acid, and "tetrenecarboxylic acid" [cyclobutadienecarboxylic acid, (6)], the formal cyclobutadienoid analog of benzoic acid.*'120 Treatment of 1,2-dibromocyclobutane- 1,2-dicarboxylic acid (2) with various bases gave only 2-bromocyclobutenecarboxylic acid (5), while treatment of the dimethyl ester (5) with the same reagents gave only cyclobutene-1,2-dicarboxylic acid (4), and not the expected cyclobutadiene diacid (7). Similarly, 2-bromocyclobutene- carboxylic acid (3) failed to give cyclobutadienecarboxylic acid (6) on treat- ment with aqueous or alcoholic solutions of potassium hydroxide. Br ,COOH ,Br -COOH -4- -Br NCOOH COOH COOH (7) (2) (i) Br XOOH -C0 CH 2 3 « sCOOH -Br2 -Br XOOH CO2CH3 (4) (6) Some ten years later (1905) an attempt was made by Willstatter and Schmadel to synthesize the parent hydrocarbon itself.156 Thus, 1,2-dibromo- cyclobutane (#), prepared from cyclobutene (7), was dehydrobrominated by treatment (a) with quinoline at high temperatures, which gave only a little butadiene and a red nitrogen-containing high molecular weight material, and (b) with potassium hydroxide at 100°-105°, which gave 1-bromocyclobutene * For several incorrect cyclobutadiene structure assignments antedating Perkin's work, see p. 6 ff. An earlier attempt to prepare cyclobutadiene from crotonaldehyde [A. Kekule, Ann., 162,77 (1872)] failed almost at the outset and is not included in the present discussion.
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