Table Of ContentORGANIC 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.
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" 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.
