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Attempts to Synthetize Tetramethylyclobutadiene.

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ANGEWANDTE CHEMIE
tion
VOLUME 1 * N U M B E R 10
OCTOBER 1 9 6 2
PAGE 5 1 9 - 5 6 2
Attempts to Synthetize Tetramethylcyclobutadiene
BY PROF. DR. R. CRIEGEE
INSTITUT F m ORGANISCHE CHEMIE DER TECHNISCHEN HOCHSCHULE, KARLSRUHE
(GERMANY)
Dedicated with friendship to Professor C. H. Nenitzescu on the occasion of his 60th birthday.
Attempts toprepare cyclobutadiene, which is primarily of theoretical interest, or one of its
derivatives have so far led only to products structurally closely related to teframethylcyclobutadiene. It has not been possible to prepare the compound itself nor to establish unequivocally that it occurs as a reaction intermediate.
Introduction
The following is a review of investigations begun by us
about eight years ago, which were aimed at achieving
the synthesis of a cyclobutadiene derivative. Originally,
a comprehensive study was planned, but we qoon found
out that other research teams were working on the same
problem. Partly as a result of agreement and partly on
our own initiative, we divided the work.
For example, C. D . Nenitzescu [l] in Bucharest studied
unsubstituted cyclobutadiene, M. C. Cava and his co-workers
[2] investigated benzocyclobutadiene, E. H. White worked on
diphenylcyclobutadiene [3], and H. H . Freedman [4] and
others, studied derivatives of tetraphenylcyclobutadiene. As
time went on, we came to concentrate all our efforts on the
tetramethyl compound.
According to the theorists, our chances of success were
doubtful. For example, in 1958, M . J. S. Dewar wrote
the following [ 5 ] : “In fact, cyclobutadiene would have
a negative resonance energy and I cannot encourage anyone to try to make it.” Nevertheless, we continued our
investigations in spite of this, firstly, because we, as experimenters, doubted the reliability of theoretical computations, which are necessarily based on approxima[l] Cf. M. Avram, E. Marica, and C. D . Nenitzescu, Chem. Ber.
92, 1088 (1959).
[2] M . P. Cava and D. R. Napier, J. Amer. chem. SOC. 79, 1701
(1957).
[3] E. H. White and H. C. Dunathan, Abstracts of Papers of the
ACS-Congress, Chicago, 1958, 41P.
[4] H. H. Freedman, J. Amer. chem. SOC. 83, 2194, 2195 (1961).
[5] W.Baker and J. F. W. McOmie, The Chemical Society Special
Publication 12, 49 (1958); M . J. S. Dewar, ibid., p. 108.
Angew. Chem. internal. Edit. Vol. I (1962) / N o . 10
tions and simplifications. Furthermore, we thought that
it would be interesting to see what actually happens in
reactions which, from a rational point of view, could
be expected to affoxd cyclobutadiene. Finally, it seemed
to us that it should be possible to demonstrate the existence of a cyclobutadiene derivative, at least as some
short-lived intermediate, and to learn something about
its properties from the course of its subsequent reactions.
It is not intended to discuss here the findings of other research
teams and the early work in this field, because both have
been adequately reviewed elsewhere [6]. However, we will
touch up on certain aspects of our own work which, although not directly related to the cyclobutadiene problem,
may still, in a manner of speaking, be considered as a byproduct.
The study of tetramethyl derivatives offered some advantages over investigations of the unsubstituted compound. First, tetramethylcyclobutadieneshould be more
stable than cyclobutadiene. If, as predicted by the theorists, the latter behaves as a biradical, then the substituted compound should be a ditertiary radical and hence
should have a lower energy content than the disecondary
radical represented by the parent compound. Second,
more favorable physical properties (higher boiling point
of the monomer, better crystallization of the possible
dimers) are to be expected of the methylated compound.
However, the decisive factor was the ready availability
of the starting material (see below).
On the other hand, some disadvantages were also apparent. These include steric hindrance caused by crowding of the methyl groups, which could prevent some “normal” reactions from occurring and interfere with certain
[a] Cf. E. Vogel, Angew. Chem. 72, 4 (1960), in particular p. 17.
519
structure determinations. More serious was the fact that
the presence of the side chain offered the possibility of
double bond migration from the ring to one of the side
chains, so that undesirable side reactions had to be considered. Finally, if cyclobutadiene were actually obtained, the presence of substituents on all four carbon
atoms would prevent determination of the compound's
ability t o undergo substitution, and thus no light would
be shed on at least this one facet of its aromatic character.
Starting Material
The starting material for all the substances described
below was dichlorotetramethylcyclobutene(11), which
was discovered in 1952 by Smirhov-Samkov 171. This
compound is obtained in a one-step reaction from 2butyne (I) [*] with either chlorine or sulfuryl chloride.
The yield obtained in this synthesis, the mechanism of
which has not as yet been elucidated, was increased by
Moschel[8] to 50 % by using boron trifluoride as catalyst. Attempts by both ourselves and the Russian worker
to apply this synthesis to other acetylenic compounds
failed. The dichloride 11 (m.p. 57 "C)is a highly reactive
hydrolysis than the acetone initially used, because the
latter tends to hold part of the diol in solution as an
acetone adduct 1111. Because of this and because the
diol readily rearranges in the presence of acids, into
another unsaturated diol (m. p. = I33 "C), the hydrochloric acid formed must be neutralized with bicarbonate. Judging from its reaction with potassium
triacetylosmate, and in view of the easy formation of
its adduct with acetone, the low-melting diol must be
assigned the cis- and the high-melting diol, the transconfiguration. In contrast to the trans-isomer, which
cannot be hydrogenated, the cis-diol is readily converted into the saturated cis-diol (IV, m.p. 60-61 "C).
The elevated rate at which this compound is cleaved by
lead tetraacetate (k20, in 99.5 % glacial acetic acid, is
350,000; time in minutes) [12] confirms the cis-configuration of its hydroxyl groups. Preparative scale
oxidation of this diol with lead tetraacetate affords the
diketone V, from which the pyrrole derivative VI is
obtained.
I
VI
IV
I1
I
I
I1 [**I
and versatile compound. It owes its reactivity to the two
tertiary, and at the same time, allylic chlorine atoms, to
the double bond, and to the strained unsaturated ring
system.
Structure and Configuration of
Dichlorotetramethykyclobutene
The structure of the dichloride was established on the
basis of two degradation sequences both yielding the
same aliphatic diketone (V). This compound was characterized as the crystalline pyrrole derivative VI obtained by reaction with p-phenylenediarnine.
Degradation R o u t e 1 [7,91
The dichloride is very readily hydrolyzed to the unsaturated diol (111), which may be isolated in two
polymorphous forms (m.p. 92 and 126 "C) [lo]. Tetrahydrofuran was found to be a better solvent for this
[7] I. V. Smirnov-Samkov, Doklady Akad. Nauk SSSR 83, 869
(1952); Chem. Zbi. 1939 (1954).
[*I The 2-butyne used was in part a product furnished by Chemische Werke Hiils and in part a product repeatedly prepared
from methyl ethyl ketone on a 5-kg scale in facilities kindly provided by Farbwerke Hoechst.
(81 R . Criegee and A. Moschel, Chem. Ber. 92, 2181 (1959).
[**I Here and hereinafter, the lines denote methyl groups
[9] R. Criegee and G. Louis, Chem. Ber. 90,417 (1957).
[lo] C. Jahn, Diploma Thesis, Technische Hochschule, Karlsruhe, 1960.
520
\
VJI
VIII
Degradation R o u t e 2 [13]
Hydrogenolysisof the dichloride with lithium aluminum
hydride affords a mixture of two stereoisomeric tetramethylcyclobutenes (VII), which are readily separated
by distillation. On ozonolysis, both give relatively stable,
isolable ozonides (VIII). On hydrogenation, these yield
the two possible stereoisomeric diketones V, both of
which were converted into the same pyrrole derivative
VI. The fact that the opening of the cyclobutene ring on
either side leads to the same end-product proves the
high symmetry of the ring system.
The dichloride I1 most probably has the cis-configuration, as
indicated by its high dipole moment of 2.38 D. Despite
considerable effort, we have been unable to prepare the
stereoisomeric dichloride. Thus, with hydrogen chloride, the
cis- and trans-diol 111 give the same dichloride 11; with
hydrogen bromide, too, only one dibromide (m. p. 61-62 "C)
is formed [13,14]. Its dipole moment of 2.35 D indicates that
the latter must also be the cis compound. The same is true for
the di-iodide (m. p. 79-80 "C), which is readily prepared
from 111 and hydrogen iodide 111,141. The fact that mild
hydrolysis of the dichloride yields exclusively cis-diol cannot
be considered as confirming evidenca for the configuration
of the dichloride, because the detailed mechanism of the
reaction is not known.
[l 11 H. Kristinsson, Diploma Thesis, Technische Hochschule,
Karlsruhe, 1962.
1121 H. Fiedler, Ph. D. Thesis, Technische Hochschule, Karlsruhe, 1959.
[13] R. Criegee and K. No& Liebigs Ann. Chem. 627, 1 (1959).
[14] R. Riemschneider and D. Becker, Mh. Chem. 90,524(1959).
Angew. Chem. internat. Mil. ] Vol. I (1962) / No. 10
Other Simple Reactions of the Dichloride
Like all cyclobutene derivatives, the dichloride 11 is
thermally unstable. At about 190 “C,it is converted,
with bond isomerization, in 61 % yield into a 2,5dichloro-3,4-dimethyl-2,4-hexadiene(IX) of unknown
steric configuration at the double bonds. Its structure
was determined from its reaction with concentrated sulfuric acid, which affords the known tetramethylfuran X.
Another simple route, leading from the dichloride I1 to
heterocycles such as tetramethylpyrrole (XII) and its
N-alkyl derivatives [16], is the reaction at room temperature of compound 11 with ammonia or primary
aliphatic amines. The assumption follows readily that
this reaction proceeds through an intermediate of the
ethylenimine type (e.g., XI). The bond isomerization
XI + M I must occur particularly readily here, because
the double bond in the four-membered ring would tend
to weaken further the already very strained bond between the two rings and because the transition state is
favored by the low energy level of the aromatic system
being formed.
XI
On heating in quinohe, I1 is converted into a mixture
of the chlorodiene XI11 and the triene XIV, with loss
of hydrogen chloride [17]. Hydrogenolysis of chlorodiene MI1 with lithium aluminum hydride affords the
diene XVI, which may also be obtained by partial
hydrogenation of triene XIV. As Blomquist found
/ T C \I
t
t
i
l
/ n
XI11
XIV
tetracyanoethylene
11
-+
XVI
\-k
I1 I
\-/
+
XVII
XVIIJ
CIyp
CHzCl
xIx
c
-\/l
previously for simpler compounds [l8], the diene and
the triene give a 1,2.1,2-addition with tetracyanoethylene, to yield derivatives of spiro[3.3]heptane, e.g.
compound XV. The inability of triene XIV to undergo
a normal diene synthesis again indicates the high
energy content of the cyclobutadiene system.
With lithium amalgam, the chlorodiene XI11 undergoes a
Wurtz reaction to form the dimeric hydrocarbon XVII. This
highly reactive compound [17], also obtainable from I1 by
dehalogenation of a pentachlorocyclobutane (presumably
XIX), gives with acids, even in the cold, crystalline derivatives
of the tricyclic system XVIII, which will not be discussed
further here.
Dehalogenation of Dichloride I1
Dehalogenation of the dichloride I1 should yield tetramethylcyclobutadiene. Actually, however, it proved to
be very difficult to split off the two chlorine atoms. The
Finkelstein reaction with sodium iodide [19], as well as
many other applicable reactions, failed. Only the use of
lithium amalgam, which had been employed shortly
before by G. Wittig for the preparation of dehydrobenzene, led to partial success of some significance: Louis
191 obtained a hydrocarbon, m. p. 196°C (decomp.),
in yields exceeding 90 %. Analysis and molecular weight
determination led to the formula c16H24, indicating that
the compound was a dimer of tetramethylcyclobutadiene csHI2.
Since the compound could not be hydrogenated because
of the strong steric hindrance caused by the crowding of
the methyl groups mentioned above, and since this hindrance caused other reactions to take an anomalous
course, the structure of the compound was determined
first by a combination of physical methods. Nuclear
magnetic resonance was particularly useful. The NMR
spectrum of the substance showed only two sharp, unsplit signals of equal intensity, indicating by their position the presence of protons from paraffinic and allylic
methyl groups. However, the presence of four allylic
methyl groups requires (at least) two double bonds.
This and the empirical formula indicate a tricyclic
system. The infrared spectrum showed a band at 1680
cm-1, which i s typical for the disubstituted cyclobutene
double bond. This indicates that, as in the starting material, the double bonds are lacated in the four-membered ring. The absence of absorption in the ultraviolet
in the region ’above210 mp proves the absence of a conjugated system. All this suggests formula XX for Louis’
hydrocarbon. At first, only the high stability of the new
compound seemed incompatible with the structure assumed, which might be expected to be highly strained.
However, heating the compound above its melting point
brought about a strongly exothermic reaction, with
quantitative formation of a “liquid isomer”, the structure of which will be discussed later.
\
[15] W. EngeI, Ph. D. Thesis, Technische Hochschule, Karls-
ruhe, 1962.
[la] M, Krieger, Diploma Thesis, TechnischeHochschuie, Karlsruhe, 1962
[17] J. Dekker, Ph. D. Thesis, Technische Hochschule, Karlsruhe, 1961.
Angew. Chem. inrernaf. Edit. I Yol. 1 (1962)/ No M
xx
I
\
/
/I I II II\
/--
‘--!-CI
c1- I
/
I L L - II II\
/ I
xxl
[18] A . T. Blomquist and Y.C. Meinwald, J. h e r . chem. S O ~79,
.
5316 (1957).
1191 H. Finkelstein, Chem. Ber. 92, xxxvii (1959).
521
It would seem obvious to explain the formation of XX by
spontaneous dimerization of tetramethylcyclobutadiene.
However, a twofold Wurtz reaction, presumably involving
the dichloride XXI as an intermediate, is also quite possible.
Although the preparation of compound XXI has not yet been
accomplished, the reaction cited above of the chlorodiene
XI11 with lithium amalgam suggests that the corresponding
reaction with I1 should also be possible. By dehaIogenation,
the second step of such a reaction sequence shodd then lead
to the formation of the middle four-membered ring. Normally, the reaction of 1,4-dihalides with metals, if it takes
place at all, gives cyclobutane only in moderate amounts [20].
In the case of XXI, however, the otherwise common sidereaction, i.e., fragmentation with formation of two olefin
molecules, is probably not favored, since it would have to
lead to the cyclobutadiene system.
best evidence for the formula assumed and, in addition,
showed that the nickel atom was situated above the
center of the four-membered ring. It is noteworthy that
the crystals consist of double molecules held together by
chlorine atoms. Obviously, the 16-electron shell of the
nickel (dspz structure ) as it occurs in the single molecules is completed to form an 18-electronshell by addition
of an electron pair from the chlorine atom of an adjacent molecule.
In solution, solvent molecules seem to play the role of the
third chlorine ligand. Strong electron donors such as triphenylphosphine or o-phenanthroline give new complexes
of composition C B H ~ Z N ~ C ~ ~ ~ . and
P ( CCsHl2NiCllz.phe~H&
nanthroline, respectively, in which, however, the bond with
the new ligands is relatively weak.
Nickel Chloride Complex
Decomposition of the Compbx
In 1956, Longuet-Higgins and Orgel [21] predicted the
possibility of the existence of a C&-NiC12 complex
of cyclobutadiene. Later, Schroder [22] treated the dichloride I1 with nickel carbonyl in benzene, acetone, and
ether, and obtained an almost quantitative yield of a
purple nickel complex of empirical formula CsH1zNiCl2.
This compound is stable up to 180 “C,even if exposed
to air; its solution, for example, in chloroform or
methylene chloride, is darkpurple. It dissolves very
readily in water to give a blood-red solution. Evaporation of aqueous solutions, in which the compound is
apparently present in a hydrated form, again yields the
purple salt. With silver nitrate, instantaneous precipitation of silver chloride takes place, but with dimethylglyoxime, separation of the nickel salt is slow, beginning
only after about an hour.
Cryosopic measurements indicated strong association
but did not resolve the question whether, in the complex,
tetramethylcyclobutadieneis bound to one nickel atom,
or whether one of its dimers is bound to two nickel
atoms. However, chemical degradation reactions unequivocally pointed to the monomeric structure. Catalytic hydrogenation in aqueous solution quickly destroyed the complex and, along with nickel, tetramethylcyclobutane was formed, 90 % of which was in the allcis form. The hydrocarbon was identical with the product of catalytic hydrogenation of cis-tetramethylcyclobutene (MI) 1131. This sompound was also prepared by
treating the complex with zinc and hydrochloric acid.
Finally, reaction of an aqueous solution of the complex
with sodium nitrite in the cold gaye a good yield of cistetramethylcyclobutenediol(111). This indicates unequivocally that the monomeric four-membered ring is present in the complex. The NMR spectrum is also in
agreement with structure XXII as it shows a single
signal, proving the equivalenceof all four methyl groups.
X-ray structural investigations by Dunitz [23] offered the
[20] W. B. Smith, J. Org. Chemistry 23, 509 (1958).
[21] H. C. Longuet-Higgins and L. E. Orgel, J. chem. SOC.(London) 1969 (1956).
1221 R. Criegee and G. Schrdder, Liebigs Ann. Chem. 633, 1
(1959).
[23] J. D. Dunitzet al., Angew. Chem. 72, 755 (1960); Helv. chim.
Acta 45, 647 (1962).
522
Since the nickel chloride complex was proved to be a
structural derivative of tetramethylcyclobutadiene, it
seemed promising to attempt to obtain the desired
hydrocarbon from its complex with the metal. First,
thermal decomposition in vucuo was studied [22]. This
reaction took piace at about 190°Cand gave three volatile
products, A, B, and C along with nickel chloride. Small
amounts of A were deposited in the distillation bridge in
the form of very long, colorless needles,m.p. 113°C.Its
empirical formula, c16H24, showed it to be a new dimer
of tetramethylcyclobutadiene. Its NMR spectrum exhibited only a single, sharp signal. A satisfactory explanation of both the equivalence of all the methyl groups
and the unsaturated character of the compound was
obtained only on assumption of the octamethylcyclooctatetraene structure (XXIII). Its ultraviolet absorption
spectrum resembles that of the unsubstituted tetraene.
The main product, B, was liquid and was found to be
identical with the “liquid dimer” obtained by thermal
decomposition of hydrocarbon XX. The cold trap contained a small amount of the very volatile compound C.
This material was found to be a hydrocarbon, C8H14,
identified as tetramethylbutadiene (XXIV) by its spec-
I*
XXII
Hz0
I
I
Ic
XXIII
;c
H
XXIV
I
t
trum and reactions. It was probably formed by thermolysis of tetramethylcyclobutene (VII) ,which in turn was
formed by thermal hydrogenation of tetramethylcyclobutadiene or of the nickel complex. In fact, thermal
cleavage of the cyclobutene derivative VII to give XXIV
was readily accomplished [13].
Augew. Chem. internat. Edit. ] VoI. I (1962)
I NO.10
According to G. Schroder, the behavior of the nickel
complex, on prolonged refluxing of its aqueous solution,
was entirely different. Gradually, colorless crystals
(m.p. 127 “C) formed in the reflw condenser, a 50 %
yield being obtained. This yield was increased to 70-80 %
by using ethylene glycol as solvent. The new hydrocarbon is also a dimer of tetramethylcyclobutadiene; it
is, however, different from XX, XXIII, and the “liquid
dimer”. Spectral data for this compound and XX are
surprisingly similar, indicating that the compound is
possibily a stereoisomer of Louis’ hydrocarbon. This
supposition was confirmed 1241.
syn- and anti-Tricyclooctadiene
For a compound of the type of Louis’ hydrocarbon
(XX), theory predicts two steric forms, i.e. the syn(XXa) and the anti-form (XXb). It was possible to show
that the isomer melting at 196 “C possesses the syn-, and
that melting at 127 “C the anti-configuration.
This assignment is supported by their dipole moments :
viz. 0 and 0.63 k 0.2 D, respectively. Decisive evidence
was obtained from oxidative degradation, which is relatively easy to effect via isolable diozonides. Hydrogenation of the two diozonides (XXVa and XXVb) gave
two products of formula C16H2404. Only one product
(that from XXVb) proved to be a tetraketone (XXVIb);
the latter was oxidized further with hypobromite to a
tetracarboxylic acid (XXVIII), whose anhydride was
identical with a compound obtained by photodimerization from dimethylmaleic anhydride [25, 261.
By contrast, the compound obtained from XXVa contained no carbonyl, hydroxyl, or perhydroxyl groups.
Hence, the four oxygen atoms necessarily formed ether
fl
XX a
, H C O C + - )
CHJIC (OCHi COCHa
XXVI a
XXV a
XXVII
+
X X bi
Q-
XXVI b
XXVIIl
1241 R. Criegee, G . Schr6der, G. Maier, and H , G. Fischer, Chem.
Ber. 93, 1553 (1960).
1251 D. Erickson, unpublished data.
[26] G. 0.Schenck et al., Chem. Ber. 95 (1962), in press.
Angew. Chem. infernat. Edit. Vol. I (1962)/ N o . 10
linkages. The supposition that the hypothetical all-cis
tetraketone XXVI a underwent spontaneous cyclization
to a tetraketal (XXVII) was verified by the NMR spectrum of the new compound. This spectrum showed only
two proton signals of equal intensity. The formation of
this tetraketal proves that both double bonds in M a
are in the syn-form.
Valence Isomerism of the C,,H,, Hydrocarbons
Theoretically,five bond isomers are possible for a hydrocarbon ( 0 8 , the methyl-free parent compound of
hydrocarbons XXa, XXb, and XXIII. These are: monocyclic cyclooctatetraene (XXIX), bicyclo[4.2.01 octatriene (XXX), containing a four-membered and a sixmembered ring, the syn- and anti-isomers of tricyclooctadiene (XXXI), and pentacyclooctane (“cubane”,
XXXII). Of these compounds, M I X is well known and
seems to be related by bond tautomerism to XXX; at
any rate all efforts to prepare XXX have so far failed.
One of the tricyclic compounds (XXXI) seems to have
been found among the decomposition products of the
silver complex C4H4eAgN03, prepared by Nenitzescu
[U]. Cubane is still unknown [*I.
XXXI
m
fg
XXXll
The compounds missing in the series of permethylated
compounds were octamethylcubane(attempts to prepare
this compound by sensitized irradiation of XXa were unsuccessful) and the bicyciic isomer XXX. Wirtli [28]
succeeded in preparing the latter by subjecting Louis’
hydrocarbon (XXa) to thermal decomposition in the
presence of a small amount of sodium ethoxide. In
the absence of base, the “liquid isomer” C16H24
mentioned above is formed. This compound is not a
bond isomer, because it is certain that it contains a semicyclic methylene group formed by a proton shift. Isomerization of XXa into the “liquid isomer” may be accomplished not only thermally, but also at 0°C using
conc. sulfuric acid. Hence, the thermal isomerization
could logically also be ascribed to the presence of traces
of acids or acid sites on the reactor wall. As already
mentioned, the isomerization does in fact take a different course in the absence of acids and yields exclusively a hydrocarbon (XXXIII), m.p. 23 “C Under identical conditions, both the anti-compound XXb and,
interestingly, also the cyclooctatetraenederivativeXXIII
[27] M. Avram, G. Mateescu, J. G. Dinulescu, E. Marica, and
C . D . Nenitzescu, Tetrahedron Letters 21 (1961).
[*I Addition in proof: In a paper just published H. H. Freedman
and D . R . Petersen ascribe the structure of octaphenylcubane to a
hydrocarbon C56H40: J. Amer. chem. SOC. 2837 (1962).
[28] W. D. Wirfh,Ph. D. Thesis, Technische Hochschule, Karlsruhe, 1960.
9,
523
are isomerized to the bicyclic compound XXXIII. Of
the four isomers, XXXIII is obviously the most stable
the?modynamically.
The structure o f the new isomer was determined from its
physical properties and chemical behavior. Its NMRspectrum
reveals an 18:6 value for the ratio of allylic to paraffinic
protons, indicating the presence of three double bonds and,
hence, two rings. The strong ultraviolet absorption between
225 and 250 mp (log E = 3.6 to 3.7) indicates that two of
these double bonds are conjugated. The absence of the
expected maximum at 270-280 m p might be due to the fact
that a planar arrangement of the six-membered ring is
impossible owing to crowding of the methyl groups. A
transannular interaction of the double bond in the fourmembered ring with the conjugated system could also lead
t o this anomalous behavior.
gJ7-J
I
XXXLV
P
/
XXa .
XXbXXlll
I
xxxv
XXXlll
The isolation of XXXIII thus indicates that the permethylated compounds are capable of forming such bond
isomers (these even possess relativelyhighstability),which
are unknown so far for the unsubstituted compounds. It
would be worthwhile to determine whether in other
cases, too, an accumulation of methyl groups can stabilize the structure of bond isomers.
The behavior of XXXIII towards oxygen is unusual [28].
The hydrocarbon autoxidizes so readily that its preparation is possible only with complete exclusion of oxygen. On exposure to air for a few hours, it is quantitatively transformed into a crystalline peroxide. On shaking its methanolic solution with oxygen, exactly one
mole of oxygen is absorbed in 20 minutes. The rate of
peroxide formation is independent of illumination; it
occurs equally rapidly in the dark and is also independent of the presence of inhibitors. Since physical and
chemical analysis of the compound indicates the absence
of -0OH groups, the compound must be a transannular peroxide of structure XXXIV. Its infrared,
ultraviolet, and NMR spectra are compatible with this
structure. It is known that cyclohexadienes can form
endoperoxides; this, however, seems to be the first example where such a peroxide is formed in the dark. The
presence of methyl groups on the unsaturated system
cannot be the only reason for such behavior, considering
that hexamethylcyciohexadiene, synthetized to check
this point [29], is not appreciably oxidized in the dark
Rather, it appears that the unsaturated four-membered
ring of the condensed system is required for this unusual behavior.
Compound XXXIII also acts as a diene with tetracyanoethylene [15] and azodicarboxylic esters [30],
1291 W. Trieself, Ph. D. Thesis, Technische Hochschule, Karlsruhe, 1961.
[30] A. Ludwig, unpublished data.
524
the adducts XXXV and XXXVI being readily prepared. By contrast, it is unusual that no reaction takes
place with maleic anhydride and acetylenedicarboxylic
esters evcn at 140°C [15]. At higher temperatures, the
compound simply rearranges to the “liquid isomer”.
This inertness towards maleic anhydride is even more
unusual in view of the recent finding of Cookson [31]
that Louis’ hydrocarbon (XXa) reacts with acetylenedicarboxylic esters at 125 “C to give an adduct which
seems to have structure XXXVII. Hence, either this
structure must be revised or XXXVII cannot be obtained via XXXIII.
Most unusual, however, is the behavior of XXXIII
toward dehydrogenating agents. On shaking its solution
in ether with aqueous silver nitrate, silver is deposited,
and a hydrocarbon, C16H22 (m.p. 55 “C),can be isolated
from the ether solution [28]. The same reaction may be
achieved with chloranil in the cold or with benzoquinone
on heating.
The two detached hydrogen atoms must originate from two
methyl groups since no other possibility exists. In agreement
with this, the infrared and NMR spectra of the new compound, which was first prepared by Muier [32], indicate the
presence of two semicyclic methylene groups. These cannot
be directly conjugated with each other, since the new hydrocarbon does not undergo diene syntheses with dienophiles.
However, a conjugated system is present, as indicated by the
ultraviolet absorption spectrum showing maxima at 226 mp
(log E = 4.33) and 255 mp (log E = 4.29). Since, judging from
the NMR spectrum two paraffinic methyl groups must be
xxxv I I I
xxxlx
present, the only possible structure remaining for Muier’s
hydrocarbon is XXXVIII 1151. This structure also explains
why diol XXXIX, formed by reduction of peroxide XXXIV,
also yields XXXVIII on elimination of water. Energetically,
the hydrocarbon must be very much favored, as indicated by
th: fact that it is formed as the main product of thermal
decomposition, with loss of water, of the epoxides of XXa
and XXIII [32].
Structure of the “Liquid Isomer” 19,15,281
The “liquid isomer” is the product of stabilization of all four
C16H24 bond isomers, viz. XXa, XXb, XXIII, and XXXIII;
it is formed from these either thermally or by the action of
acids. The “compound” has a semicyclic double bond and a
= 241 mp; log E =
system of conjugated double bonds (A,
4.26). This conjugation is either absent in the other isomers
or, because of their nonplanar structure, is not fully effective
and must be the reason for the greater stability. On hydrogenation, the “compound” adds on two atoms of hydrogen
and loses its semicyclic double bond, giving rise to a mixture
of C16H26 hydrocarbons. These can be separated by gas
[3 11 C.E. Berkoff, R . C. Cookson, J. Hudec, and R . 0. Williams,
Proc. chern. SOC.(London) 312 (1961).
[32] G. Muier, Ph. D. Thesis; Technische Hochschule, Karlsruhe, 1959.
Angew. Chem. internat. Edit. I Vol. I (1962) I Nu.I0
chromatography into five very similar components, two
of which are crystalline at room temperature. A mixture of
the same products in different proportions is obtained on
hydrogenation of Muier’s hydrocarbon. Adding four hydrogen atoms, this compound also loses its two semicyclic
double bonds, while the cyciobutene double bond, which can
be easily identified, by its characteristic infrared spectrum,
remains unaltered. This certainly indicates that the “liquid
isomer” and XXXVIIl are closely related. Thus, we can
assume that the former has the structure XL, and that the
common hydrogenation product has the structure XLII.
However, if the four- and six-membered rings are assumed
to be joined in a cis-configuration, then only three stereoisomers are possible for XLII. whereas five were found.
Hence, it is probable that the “liquid isomer” is a nonseparable mixture of the two similar hydrocarbons XL and
XLI, and that the dihydro compounds are mixtures of the
stereoisomers XLII and XLIII. If this is so, it must be
assumed that a partial 1,4-addition of hydrogen takes place
in the conversion of XXXVIII into XLIII. Both XL and XLI
could have been easily formed from XXXIII as a result of a
prototropy, and their formation from the other three isomeric
C&24 hydrocarbons can also be readily explained. Thus,
the preliminary structure of the “liquid isomer” previously
proposed, i. e., a condensed three- and seven-membered ring
system [9,22], is not tenable.
bond shift to the side chain, had taken place. Possibly,
this shift is similar to the acid-catalyzed double bond
migration observed in the case of the dimers. This migration might be caused by the small amount of hydrogen chloride which is formed in all thermolyses of nickel
chloride complexes and the formation of which could
so far not be prevented. The occurrence of the side reaction indicates that methylenecyclobutene is more
stable than rnethylcyclobutadiene, whereas, normally,
in a four-membered ring, an endocyclic double bond is
favored over an exocyclic one [34].
Reaction of the Nickel Chloride Complex with
Cyclopentadienylsodium t35 J
The chlorine atoms in the nickel chloride complex XXII
can be replaced easily by other ligands [36]. For example,
it was possible to prepare the bromide, iodide, periodide,
azide, nitrate, sulfate, trichloroacetate, and oxylate. Reaction of XXII in aqueous solution with the sodium salt
of dimercaptodicyanoethylene,described by G. B2hr [37],
gave a crystalline precipitate, which may be used for the
quantitative determination of the CsHrzNi2+cation.
The reaction of the nickel complex with cyclopentadienylsodium was also investigated in connection with
these reactions A new, red, beautifully crystalline
nickel complex was obtained in tetrahydrofuran according to the equation:
CsHlzNiClz
+ 2 CsHsNa +
2 NaCl 4- CisH2~Ni
\\+4H,P
XXXVIIl
Thermolysis of the C8H,,.NiCl,. Phenanthroline
Complex 1331
Having discussed the dimers of tetramethylcyclobutadiene and their various reactions, let us again consider
the monomer. Since thermolysis of the simple nickel
chloride complex did not give the desired result, and
attempts to displace the four-membered ring system
from the nickel by the use of other ligands failed, thermal decomposition of adducts of the nickel complex
with electron donors was investigated. The triphenylphosphine complex revealed no new information. By
contrast, decomposition of the phenanthroline complex
proceeded somewhat differently. In this case, too, the
“liquid isomer” XL plus XLI was the main reaction
product. However, in place of octamethylcyclooctatetraene (XXIII) the anti-form of the tricyclic hydrocarbon XX b was obtained. A relatively large amount of
a monomer (yields of up to 30 %) was found in the cold
trap. However, this material was neither tetramethylcyclobutadiene nor tetramethylbutadiene (XXIV) but
methylenetrimethylcyclobutene (XVI), an isomer of the
cyclobutadiene derivative. This indicated that the undesirable side reaction discussed earlier, viz. the double
I331 P. Ludwig and K. Noll, unpublished data.
Angew. Chem. internat. Edit
1 Vol. I
(1962) / No. 10
This complex is soluble in hydrocarbons, insoluble in
water, and sublimes undecomposed in vacuo; in the
solid state, it is fairly resistent to atmospheric oxygen
but is quite sensitive solution.
On hydrogenation, the compound adds on four atoms of
hydrogen without undergoing any appreciable color change.
A tetrahydro complex, ClsHz6Ni, very similar to the unreduced complex, may thus be isolated. Nickel is split off
only oa further vigorous hydrogenation, and a saturated
hydrocarbon, C13H24, is obtained, along with cyclopentane.
An unsaturated C13 hydrocarbon is obtained on treatment
of the unhydrogenated compIex with two moles of acid,
with simultaneous formation of one mole of cyclopentadiene,
as shown in the equation:
CISHZZNi f 2 HX
+
+ Ct3Hl8
NIX2 Jr CJHS
The C13H18 hydrocarbon contains three double bonds
and, on hydrogenation, affords the saturated bicyclic
C ~ ~ Hhydrocarbon.
M
Two double bonds are conjugated, as indicated by the fact that the compound forms
an adduct with acetylenedicarboxylicesters. At 300 OC,
this adduct undergoes decomposition into cyclopentadiene and tetramethylphthalicester XLVIII. This is best
explained by assuming structure XLVII. The hydrocarbon may then be represented by formula XLVI,
[34] F. F. Caserio, S. H. Parker, R. Piccolini, and .I
D.
.Roberts,
3. Amer. chem. SOC.80, 5507 (1958); E. Gil-Av and J. Herfing,
Tetrahedron Letters 27 (1961).
[35] R. Criegee and P. Ludwig, Chem. Ber. 94, 2038 (1961).
[36] I . F. Pfromrner, Ph. D. Thesis, Technische Hochschule,
Karlsruhe, 1961.
[37] G. Bahr and G. Schleitzer, Chem. Ber. 90, 441 (1957).
525
whereas the complex must be of the x-allyl-x-cyclopentadienyl type, as indicated by formula XLV. As can
be seen, the two double bonds in the six-membered ring
are not necessary to hold the complex together. Thus,
they may be hydrogenated without ill effects.
former would loose its halogen under milder conditions
than those required ,for the dichloride. Whereas, with
zinc, Louis' hydrocarbon (=a) was obtained nearly
quantitatively, shaking with mercury or silver produced
In the formation of complex XLV from the original
nickel complex XXII, tetramethylcyclobutadienemust
have added onto one of the cyclopentadienyl residues
bound to the nickel. The tricyclic intermediate XLIV
then rearranged to XLV with bond isomerization. The
synthesis of the indane skeleton is reminiscent of the
synthesis of tetrametyhlnaphthalene involving a chromium complex, as carried out by Zeiss [381.
t
I1
Zn
2-Butyne
IL
I1
XXII
XLIV
XLV
XLVI
XLVII
XLVllI
Dehalogenationof Diiodotetramethylcyclobutene
colorless solutions, which deposited dark resins on exposure to air. Additions of cyclopentadiene or furan had
no effect on this phenomenon. However, when the reaction was conducted in the presence of maleic or dimethylmaleic anhydride, the adducts LII and LIII were
obtained, sometimes in exceIlent yields. In contrast to
IL and L, these adducts are stable, since simple bond isomerization cannot produce a low-energy, aromatic
system. The bicyclic compound is cleaved only upon
thermal treatment. In the case of LII, this simultaneously causes dehydrogenation to give tetramethylphthalic anhydride (LIV), whereas LIII affords the
cyclohexadiene derivative LV, which does not undergo
dehydrogenation. This derivative had been previously
synthesized by Trieselt [29] from tetramethylbutadiene
(XXIV) via its adduct with dimethylmaleic anhydride
(LVI).
Efforts to prepare tetramethylcyclobutadienehave recently received new stimuli from two sides. First, Cookson [31] investigated the dehalogenation of dichlorotetramethylcyclobutene (11) in the presence of acetylene
derivatives. Whereas with lithium amalgam, independent
of such additives, he always obtained Louis' hydrocarbon m a ) , with zinc and 2-butyne, he obtained
hexamethylbenzene as well. The interpretation of this
reaction as an addition of tetramethylcyclobutadiene
onto the butyne to give the bond isomer of hexamethylbenzene (IL) with simultaneous, spontaneous isomerization, found support in the corresponding reaction
carried out in the presence of an acetylenedicarboxylic
ester, which led to a tetramethylphthalic ester (XLVIII)
via the bicyclic compound L.
Independently, Kristinsson [l 11 carried out some preliminary experiments on the dehalogenation of the diiodide
(LI) corresponding to the dichloride 11, hoping that the
[38] W . Herwig, W. Merlesics, and H. Zeiss, J. Amer. chem. SOC.
81, 6203 (1959).
526
LVII
LVI
While at first sight these experiments Seem to prove the
formation of tetramethylcyclobutadiene as an intermediate
a corresponding experiment with addition of 3-methylpentenone suggests caution. This experiment afforded a .10 %
Angew. Chem. internat. Edit. 1 VoI. I (1962)I No. 10
yield of the iodine-containing compound LIX, which must
have been formed by the action of the monomercury compound LVIII on the ketone. Whether an orginometal synthesis or a free-radical addition on to the c - C double bond
has taken place in this case still remains to be seen.
Shaking the diiodide with degassed Raney nickel smoothly
afforded tetramethylcyclobutadiene-nickeliodide (LVII).
LVIII
LIX
Conclusions
The experiments described dealt with compounds which
are structurally closely related to tetramethylcyclobutadiene; these compounds can be arranged into the three
following groups:
1. Compounds of the CS series:
Tetramethylbutadiene (XXIV)
Methylenetrimethylcyclobutene(XVI)
Nickel complexes CsH12NiHaI2 (XXII, LVII)
2. Dimeric, C1&24 hydrocarbons:
syn-Tricyclooctadiene (XXa)
anti-Tricyclooctadiene (XXb)
Octamethylcyclooctatetraene (XXIII)
“Liquid isomer” (XL plus XLI)
3. Condensation products:
Nickel complex C18H22Ni (XLIV)
Hexamethylbenzene
Tetramethylphthalic ester (XLVIII)
Tetramethylbicyclohexenedicarboxylicanhydride (LII)
Hexamethylbicyclohcxenedicarboxylic anhydride (LIII)
In particular, the followingshould also be noted. In most
instances, the type of product formed is highly specific.
For example, the reaction of the dichIoride with lithium
amalgam affords only the syn-dimer(XXa),the decomposition of the nickel complex with water only the antidimer (XXb). Thermal decomposition of the nickel
chloride complex gives, apart from the “liquid dimer”,
only octamethylcyclooctatetraene,while the apparently
very similar decomposition of the phenanthroline complex yields, apart from the “liquid dimer” only the tricyclic anti-compound XX b. The dichloride yields the
tricyclic syn-compound(XXa) not only when it is reacted
with lithium amalgam, but also (although in markedly
lower yields) in the reaction with zinc. In the first instance, the reaction cannot be influenced by addition of
an acetylenic compound, in the second, it can. With
Angew. Chem. internat. Edit. 1 Val. I (19621 [No.10
zinc, the diiodide LI yields the tricyclic syn-compound
XXa in nearly quantitative yield, whereas, with mercury or silver, not more than traces are formed. With
nickel, only a nickel iodide complex is formed, but no
dimer.
Do these experiments, or at least some of them, constitute evidence for the occurrenceof tetramethylcyclobutadiene as an intermediate? We should like to answer this
question in the negative. Certainly the formation of all
the products cited can be postulated to involve a cyclobutadiene derivative as an intermediate, which then
undergoes stabilization by dimerization, complex formation, reaction with dienophiles, double bond migration to a side chain, or addition of two hydrogen atoms
supplied by other molecules. Some postulates of this
type even appear quite plausible. The specificity mentioned above, however, suggests caution. If the same
intermediate were in fact involved in all the reactions,
the products obtained would form a more uniform
picture. Moreover, all the reactions can be interpreted
by assuming a mechanism involving products similar to
cyclobutadiene, but not this compound itself.
The use of labelled compounds would hardly afford the
desired proof, either. For example, if it were possible to
label the two methyl groups adjacent to the chlorine
atoms in dichloride 11, then it could be expected that, in
reactions proceeding via free tetramethylcyclobutadiene,
the labelled isotope in Louis’ hydrocarbon XXa for
example would be distributed over all four methyl
groups. However, since other possible mechanisms
involve the occurrence of ions or free radicals (or at
least cryptoions or cryptoradicals), interaction of these
with the allylic cyclobutene double bond could be
expected, and this, too, would lead to a uniform
distribution of the labelled carbon. Because of the high
reactivity of dichloride 11, it is even doubtful that
specific labelling of this compound is possible, since
tautomerism of the following type certainly takes place
very rapidly.
Hence, in future, other routes will have to be taken to
demonstrate the existence of cyclobutadiene. Our studies
certainly prove that tetramethylcyclobutadiene, if it
exists at all, not only has nonaromatic properties but
must also be a compound of high energy content.
Received, March 9th. 1962
[A 222/48 IEJ
527
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