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Cyclopropenylium Compounds and Cyclopropenones.

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On the other hand, the addition of methylene to poiar
double bonds 1123,1241, which can formally be achieved
with trimethylsulfonium salts, as well as the formation
of ethylene from these compounds [124], must be regarded as a sulfonium ylide reaction. This is probably
also true of the similar reactions (w) of diphenylbenzylsulfonium salts (50) [ 1251.
8. Future Prospects
The results which have been referred to illustrate the
importance of cr-elimination in numerous substitution
and addition reactions. Investigation of these reactions
[1231 E. J. Corey and M . Chaykovsky, J. Amer. chem. SOC.84,
3782 (1962); Tetrahedron Letters 1963, 169.
11241 V. Franzen and H . E. Driessen, Tetrahedron Letters 1962,
661 ; Chem. Ber. 96, 1881 (1963).
“251 W. A . Johnson, V. J. Hruby, and J. L. Willimns, J. Amer.
chem. SOC.86, 918 (1964).
is made interesting, yet at the same time difficult, by
the various intermediates with similar chemical properties. According to present-day knowledge, both of
substituted alkylmetal compounds (or carbanions) and
carbenes behave as electrophiles. The conversion aolefins into cyclopropane derivatives, which originally
did much for the development of the carbene concept,
is becoming increasingly recognized as a reaction of
organometallic (carbanion) intermediates. Olefin addition has become completely useless as a means of
detecting carbenes. At present the only reliable criterion
for the occurrence of carbenes is the (intramolecular)
insertion into C-H bonds, which has permitted detection of alkyl- and dialkylcarbenes. It is not surprising
that these are the very compounds in which a-elimination proceeds to the formation of carbene intermediates.
With the transition from primary to secondary and
tertiary carbon, the tendency towards SN1 reactions
steadily increases, while the SN2 mechanism is suppressed. Alkyl groups should therefore also favor the
transition into the carbene, which is anatogous to the
SN1 mechanism, in the case of a-substituted alkylmetal
compounds (carbanions).
Received, J u l y 13th, 1964
[A 41 11190 IE]
G e r m a n version: Angew. Chem. 77, 1 (1965)
Translated by Express Translation Service, London
Cyclopropenylium Compounds and Cyclopropenones
The predictions of Hiickel’s rule gave the incentive to numerous investigations which led
ultimately to a new definition of aromatic character, and which have added greatly to our
knowledge o j the properties and reactivity of aromatic compounds. lncfeasing use was
made of physical measurements as criteria for the aromaticity of a compound. The first
member ( n = 0 )in the series of Hiickel’s (4n f 2) x systems is the cyclopropenylium cation.
The predictions regarding the stability of’ this system have been confirmed at least qualitatively by the synthesis of cyclopropenylium salts. Properties and reactions of cyclopropenylium compounds and cyclopropenones were investigated.
I. Introduction
Ever since the discovery of benzene by Faraday in 1825,
the theory of the cyclic unsaturated systems has been
of interest to organic chemists [I, 21. The formulation
of the (4n + 2)x-rule by Hiickel[3] in 1931 was a
milestone in this development. The (4n + 2)x-rule can
be expressed as follows:
[ I ] Concerning the historical development of the theory of
aromatic compounds, which is not discussed in detail in the
present paper, see e.g. C . K . Ingold: Structure and Mechanism
in Organic Chemistry. Cornell University Press, Ithaca 1953,
pp. 156-196 and [Z].
[2] W. Y . E. Doering and H . Krauch, Angew. Chem. 68,661 (1956).
[3j E. Hiickel, Z . Physik 70,204 (1931); 76, 628 (1932).
Pfanar monocyclic systems with trigonally hybridized
atoms containing (4n + 2) x-electrons possess a characteristic electronic stability.
Not only did this rule provide an explanation on the
basis of quantum theory of the special position of benzene, which was already known, but it also allowed predictions to be made regarding the stability of systems
which had not yet been synthesized.
Ths validity of Huckel’s rule for n = 1 was verified by the
preparation of the tropylium cation [2,4-5 b] and by
the agreement between the observed properties and
[4] P. L . Puuson, Chem. Reviews 55, 9 (1955).
[5a] T . Nozoe in D . Ginsburg: Non-Benzenoid Aromatic Compounds. Interscience, New York 1959, p. 339.
[5b] T. Nozoe: Progress in Organic Chemistry. Butterworths,
London 1961, Voi. 5, p. 132.
Angew. Chen?. internut. Edit.
Vol. 4 (196.5) Nu. 1
those predicted by theory [6].Roberts, Streitwiesev, and
Regan [7] were the first to point out the theoretically expected stability cf the simplest aromatic system, i. e. the
cyclopropenylism cation ( I ) , with n = 0. These predictions can be represented diagrammatically as shown in
Fig. 1 [2,8].
Table 2 . Delocalization energies D E of somc cyclopropenone derivatives as calculated from the Huckel theory, and the increase ADE o n
p h e n y l subs~itrrtion
D E “51
C yelopropenone
I - Phenylmethyienecyclo~ropene
I ,2-Diphenylmelhylenecyclopropene
++I +t
Fig. I . Graphical representation of the energy levels f or cyclopropenyl
The delocalization energy, DE, of the cyclopropenylium
cation ( I ) is found t o be the difference in the n-electron
energies of ( I ) and of ethylene; that is, DE = 4F - 2P =
29. Similarly, the delocalization energies of the cyclopropenyl radical (2) and the cyclopropenyl anion (3)
are found to be 19 and zero, respectively. Large delocalization energies were obtained in a similar manner for
cyclopropenone (4) and for methylenecyclopropene (5),
and for their derivatives [7, 9, lo].
SDE [$l
The predicted stability of the three-membered ring
cation ( I ) leads naturally to a comparison of it with the
tropylium cation, C T H ~In~fact,
. the development of
the chemistry of the cyclopropenyliurn compounds was
completely analogous to that of the tropylium compounds. In comparing ( I ) to C 7 H 7 ” , however, it should
be noted that the calculated delocalization energy DE of
an aromatic compound is not identical with the socalled empirical resonance energy RE which is experimentally determined from the heat of combustion or of
hydrogenation. The reason for this is that the M.O. calculations take into account only the contribution of the
delocalization of the welectrons towards the stability of
the molecules; no allowance is made for other factors
such as ring strain, change in state of hybridization or in
bond lengths, or, in certain cases, the electrostatic work
of charge separation.
T h e ring-strain energy o f t h e cyclopropenylium cation ( I ) ,
which can b e estimated f r o m t h e ring strain of cyclopropene
[11,12] an d t h e additional strain arising f r o m the change in
the hybridization in ( I ) [13], is ab o u t 74 kcal/mole, as compared with ab o u t 7 kcal/mole f o r the tropylium ion.
Table 1. Delocalization energies DE for cyclopropenyl derivatives as
calculated from the Huckel theory, and the increase A D € on phenyl
Phen ylcyclopropenyl
I .2.3-Triphenylcyclopropenyl
D E [$I
I .68
[6] In recent years, the validity of the Huckel rule has also been
confirmed for n = 2 by synthesis of the cyclooctatetraene dianion
and the cyclononatetraenyl anion: A. R . Ubbelohde, Chem. and
Ind. 1956, 153; T. J. Katz, J. Amer. chem. SOC-82, 3784, 3785
(1960); H . P. Fritz and H. Keller, 2. Naturforsch. 166, 231
(1961);T .J. Katz and P. J . Garratt, J . Amer. chem. SOC.85, 2852
(1963);E. A . La Lancette and R . E. Benswz, ibid. 85,2853 (1963).
Cyclodeca-f ,3,5,7,9-pentaene appears to be unstable in comparison with 9,lO-dihydronaphthaiene for steric reasons: E. E.
van Tamelen and B. Pappas, J. Amer. chem. SOC.85,3296 (1963);
E. Vogel and H . D. Roth, Angew. Chem. 76, 145 (1964);Angew.
Chem. internat. Edit. 3, 228 (1964).
On the question of the aromatic character of higher conjugated
cyclopolyolefins (n > 3), particularly the [l8]annulene, see:
W. Baker and J . F. W. McOmie in D . Ginsburg: Non-Benzenoid
Aromatic Compounds. Interscience Publishers, New York 1959,
p. 477; A . Streitwieser: Molecular Orbital Theory for Organic
Chemists. J. Wiley, NewYork 1962; L. M . Jackman, F. Sondheimer, Y. A m i d , D . A. Ben-Ejraim, Y . Gaoni, R . Wolovsky, and
A. A . Bother-By, J. Amer. chem. SOC.
84,4307 (19621.
[71 J. D. Roberts, A. Streitwieser, and C. M. Regan, J. Amer.
chem. SOC.74,4579 (1952).
Angew. Chem. internut. Edit. / Vol. 4 (1965) / No. I
0.00 “I
If we assume 7 - 3 2 kcai/moie 1141, the calculated delocalization energy 28 of the cyclopropenylium cation is
overcompensated by the ring-strain energy. It is, there-
fore, impossible to foretell whether the high delocalization energy makes the cydopropenylium system sufficiently stable to permit its isolation.
[8] A . A. Frost and B. Musulin, J. chem. Physics 21, 572 ( I 953).
[9] S.L. Manatt and J. D . Roberts, J. org- Chemistry 24, 1336
(1 959).
[lo] D . A. Bochvar, I. V . Stankevich, and A . L. Cliistyakov, Izv.
Akad. Nauk S.S.S.R.,Otd. Khim. Nauk 1958,793; Chem. Abstr.
52, 19424 (1958); Zhurn. fiz. Khim. 33, 2712 (1959); Chem.
Abstr. 55, 15104 (1961); Zhurn. fiz. Khim. 34, 2543 (1960);
Chem. Abstr. 55, 8324 (1961).
[I 11 H . A . Staab: Einfuhrung in die theoretische organische
Chemie. 3rd Edit., Verlag Chemie, Weinheim/Bergstr. 1962, pp.
[I21 K . B. Wiberg, W. J . Bartley, and F. P . Lossing, J. Amer.
chem. SOC.84, 3980 (1962).
[I31 R. Breslow and H . W. Chang, J. Amer. chem. SOC.83,2367
[14] For a discussion of the value of p in aromatic compounds,
see A . Streitwieser: Molecular Orbital ’Theory for Organic
Chemists. Wiley, New York 1961, and [13].
€I C7H7
11. Cyclopropenylium Compounds
A. Preparation of Cyclopropenylium Compounds
The starting materials for all syntheses were cyclopropenes which are generally obtained by addition of
carbenes to substituted acetylenes.
Thus, by reacting phenyldiazoacetonitrile (7) with diphenylacetylene (6), Bredow [15,16] obtained 1,2,3triphenylcyclopropenyl cyanide (8) which yielded the
mixed fluoroborate/hydroxofluoroborate of the 1,2,3triphenylcyclopropenylium cation (9) on treatment with
moist boron trifluoride etherate.
+ CH30H
1,2,3-Triphenylcyclopropenyl methyl ether (10) was
found to be a suitable starting material for the preparation of cyclopropenylium salts, giving 1,2,3-triphenylcyclopropenylium bromide on reaction with hydrogen
bromide [161.
A number of other triaryl-substituted cyclopropenylium
salts [13,17] and the diphenylcyclopropenylium cation
[18] were synthesized directly by addition of arylchlorocarbenes to diarylacetylenes or to phenylacetylene.
Another method of preparing disubstituted cyclopropenylium salts, e.g. (11) and (12), consists in the decarbonylation of 1,Zdisubstituted cyclopropenecarboxylic acids with strong acids in acetic anhydride [19
to 211.
B. Proof of Structure and Physical Properties of
Cyclopropenylium Compounds
Although many methods of preparation, such as the decarbonylation of the cyclopropenecarboxylic acids and hydride
abstraction with triphenylmethyl perchlorate clearly pointed
to the stability of the cyclopropenylium cation, final evidence
for the symmetry of the cation was only provided by physical
1. S o l u b i l i t y
The reaction of (12) with propyl-lithium gave 1,2,3-tripropylcyclopropene (13) which was then converted to
tripropylcyclopropenyliumperchlorate (14) by hydride
abstraction with the triphenylmethyl cation [21].
Amer. chem. SOC.79,
5318 (1957).
[I61 R. Breslow and C . Yuan, J. Amer. chem. SOC. 80, 5991 (1958).
[17] A . S. Kende, J. Amer. chem. SOC.85, 1882 (1963).
[18] R. Bredow, J. Lockhart, and H. W . Chang, J. Amer. chem.
SOC.83, 2375 (1961).
1191 D. G. Farnum and M. Burr, J. Amer. chem. Soc. 82, 2651
( 1960).
[20] R. Breslow and H. Hover, J. Amer. chem. Soc. 82, 2644 (1960).
[21] R. Breslow, H . Hover, and H . W. Chang, J. Amer. chem. SOC.
84,3168 (1962).
A method based on hydride abstraction with 2,3-dichloro5,6-dicyano-l,4-benzoquinone
in the presence of acids had
previously been used for the preparation of a number of
triphenylcyclopropenylium salts from 1,2,3-triphenylcyclopropene [22].
Attempts to prepare the unsubstituted cyclopropenylium
cation ( I ) by hydride abstraction from cyclopropene
with the triphenylmethyl cation were unsuccessful [12].
Attempts to detect ( I ) in tne gas phase and to determine
its heat of formation by mass spectrometry also failed
to yield any definite result, since it could not be decided
whether the observed C3HF ion had the structure (1)
or that of a propargyl cation [12,23]. On the other hand,
the trichlorocyclopropenylium (15) [24] and tribromocyclopropenylium cations [25] were obtained as the
tetrahalogenoaluminates by reaction of the corresponding tetrahalogenocyclopropenes with aluminum trihalides. Tetrachlorocyclopropene was reobtained on
careful hydrolysis of (15).
[15] R. Breslow, J.
( C S H ~ ) ~ C+H
All cyclopropenylium compounds, both aromatic and aliphatic, are soluble only in highly polar solvents such as alcohols, acetonitrile, dimethylformamide, or aqueous acids,
but insoluble in ether, chloroform, or benzene. On the other
hand, covalent cyclopropene derivatives, such as (S), are
soluble in benzene. In contrast to the cyclopropenylium chlorides and bromides, (8) does not give a precipitate with ethanolic silver nitrate [15,16].
2. X - R a y S t r u c t u r a l A n a l y s i s
An X-ray structural analysis carried out on triphenylcyclopropenylium perchlorate showed conclusively that
the compound consists of triphenylcyclopropenylium
[22] D. H . Reid, M. Fraser, B. B. Molloy, H. A . S. Payne, and R.
G . Sutherland, Tetrahedron Letters 1961, 530.
[23] G. P. Glass and G . B. Kistiakowsky, J. chem. Physics 40,1448
(1964); G . B.Kistiakowsky and J. Y.Michael, ibid. 40, 1447 (1964).
[24] S.W.Tobey and R . West, J. Amer. chem. SOC.86,1459 (1964).
1251 R . West, personal communication; S.W. Tobqv and R . West,
J. Amer. chem. SOC. 86, 4215 (1964).
Angew. Chem. internat. Edit.
Vol. 4 (1965)
No. I
cations and perchlorate anions [26]. The three-membered
ring and the three phenyl carbon atoms attached to it lie
in the same plane. All the C-C distances in the threemembered ring are 1.40 & i. e. roughly the same as those
in benzene (1.39 A). The semicyclic C-C single bonds
are 1.45 A in length, and are thus shorter than the
“normal” C-C single bond (1.54 A), as is the central
C-C bond in butadiene (1.48 A) [27] or biphenyl (1.48
A) P81.
Contrary to earlier belief [iO,131, the triphenyicyclopropenyl-
relationship exists between the chemical shift of aromatic protons and the x-electron density per C-atom of
the aromatic system in question [32]. This rule, which
was established for the “classical” aromatic systems
(C7H7, C6H6, and CsH?), has frequently been used
as a test for aromatic character in a newly synthesized
compound [6].Fig. 3 shows that the value of -3.0 ppm
(in relation to benzene) found for the ring proton of the
dipropylcyclopropenylium cation [21] is in close agreement with the value expected from this rule.
ium cation is not planar in structure, the phenyl groups being
twisted out of the plane of the three-membered ring at an
average angle of 21 O owing t o steric hindrance of the hydrogen atoms in the ortho position. Therefore the shortening of
the semicyclic bonds can be attributed primarily t o the change
in hybridization, multiple-bond character resulting from
mesomerism being only a secondary factor [29].
Fig. 3. Chemical shift of aromatic protons 8 (in ppm in relation to
benzene as a function of the rr-electron density p, per C-atom) 1331.
Oridinate: 6 Ippml
Abscissa: pn
Fig. 2. Bond lengths in the triphenylcyclopropenylium cation
(A) 1261.
3. S p e c t r a
In contrast to cyclopropene [30], the cylcopropenylium
compounds with only alkyl substituents do not absorb
in the ultraviolet region above 185 m p [21]. This agrees
with the simple Hiickel theory which predicts an excitation energy of 3P for the x + x*-transition (see Fig. 1).
Since this excitation energy is much greater than that
calculated for ethylene (2p), the x +x*-transition in the
cyclopropenylium cation should occur at very short
wavelengths (< 180 mp).
With the exception of the triphenyl compound, the aryl-substituted cyclopropenylium compounds [13,16-191 also give
ultraviolet spectra which are quite different from those of the
covalent cyclopropenes.
The similarity and simplicity of the infrared spectra
of trichlorocyclopropenylium tetrachlaroaluminate (IS)
and the hexachloroantimonate were accepted as evidence
of the high symmetry of the C3C130 cation [24]. Only
one band in the IR spectrum, which occurs between
1400 and 1430 cm-1, depending on the substitutents, has
so far been assigned to the cyclopropenylium system itself [21,3 11.
The apparent reason for this agreement is that the effects of
the ring current, which is smaller than in benzene, and of the
changed hybridization in the three-membered ring o n the
position of the 1H-resonance absorption just balance each
other, so that only the influence of the n-electron density per
C-atom is of any importance.
However, interesting conclusions can also be drawn
from the N M R spectra of the substituents on the cyclopropenylium system. These show that all the propyl
groups in the tri- and dipropylcyclopropenylium cations
are equivalent, that all cyclopropenylium salts, irrespective of the nature of the substituents, have a similar
charge distribution, and that, even in the phenyl-substituted cations, not much of the positive charge is distributed over the phenyl groups [21].
4. C y c 1o p r o p e n y 1i u m - C y c l o p r o p e n o 1
The influence of substituents on the stability of cyclopropenylium ions has been studied mainly by measurement of the pK-values [34] for the equilibrium (16u)
The nuclear magnetic resonance spectra give more information on the bonding in the cyclopropenylium cations
[17,20,21] than the UV and IR spectra, since a linear
1261 M . Sundaralingam and L. H. Jensen, J. Amer. chem. SOC.85,
3302 (1963).
[27] A . Almenningen, 0. Bastiansen, and T . Munthe-Kans, Acta
chem. scand. 12, 1221 (1958).
[28] 0.Bastiansen and M.Traetteberg,Tetrahedron17, 147 (1962).
[29] Cf.: Epistologue on the Effect of Environment on the
Properties of Carbon Bonds. Organised by M . J. S. Dewar,
Tetrahedron 17, 123-266 (1962).
[30] K . B. Wiberg and B. J. Nist, J. Amer. chem. SOC.83, 1226
(311 J. Chatt and R . G. Guy, Chem. and Ind. 1963, 212.
Angew. Chem. internat. Edit.
1 Val. 4 (1965) 1 No. I
The pK-value of the cations is determined by two factors: 1. The cations are better stabilized by propyl
groups than by phenyl groups. 2. The covalent carbinols,
on the other hand, are better stabilized by phenyl groups
[32] T . Schaefer and W. G. Schneider, Canad. J. Chem. 41, 966
[33] H. Spiesecke and W. G . Schneider, Tetrahedron Letters 1961,
[34] The pK-value in this case is the pH-value at which 50q/, of
the carbinol is ionized to the cation.
Table 3. pl<-Values for some cyclopropenyliuni salts [13,18,20,2ll.
(4= 50 % aqueous acetonitrile; B = 23 % aqueous ethanol;
a = potentiometric titration; b = spectrophotometric titration).
R1, R', R3
of deter-
2 ' Propyl
3 x Propyl
2 r: Phenyl
1 x Propyl, 2 / Phenyl
3 / Phenyl
2 1 Phenyl, I x p-Anisyl
1 > Phenyl, 2 r p-Anisyl
3 'p-Anisyl
than by propyl groups. Since the pK-value depends on
the difference in the free energies of the cation and the
carbinol, both factors are involved.
The propyl groups stabilize the cation by an inductive effect,
just as a methyi group raises the p K value of the tropylium
cation [35]; however, the stabilization is much more pronounced in the case of the cyclopropenylium cation, owing t o
the higher charge density. Phenyl groups also stabilize the
cyclopropenylium cation, as shown by a comparison of the
pK-values of the di- and triphenylcyclopropenylium cations,
but by a mesomeric effect; an inductive effect, such as prevails e.g. in the phenyl [36] and heptaphenyltropylium cations
[37], would lower the p K value in this case. However, a comparison with the corresponding free radicals [38,391 and
anions [40,41] [*] shows that this resonance stabilization is
not the only factor determining the stability of the phenylsubstituted cations.
The pK-values of some cyclopropenylium compounds
were used as a test of the accuracy of the M.O. calculations. Good agreement was found between the calculated and experimentally determined pK-values within
a series of very similar compounds, using suitable parameters [131.
The pK-value of the triphenylcyclopropenylium cation
was related to that of other cations by hydride abstraction. Competitive experiments gave the following order
of stability:
tropylium > triphenylcyclopropenylium > perinaphthenylium > trianisylmethylium > triphenylmethylium
C . The Cyclopropenyl Radical and Anion
The enhanced resonance stabilization of the cyclopropenylium cation compared to the free radical and, in
particular, compared to the anion was the most im1351 K . Conrow, J. Amer. chem. SOC.83, 2343 (1961).
[36] C. Jufz and F. Voifhenleitner, Chem. Ber. 97, 29 (1964).
[37] M . A. Battiste, J. Amer. chem. SOC. 83, 4101 (1961).
[38] R. Breslow and P. Gal, J. Amer. chem. SOC.81, 4747 (19591.
1391 R. Breslow, W. Bahary, and W. Reinmrcth, J. Amer. chem.
SOC.83, 1763 (1961).
1 1 7 ~ ) :R = CsH,
(176): R = H
[38]. This qualitative finding was also supported by a
quantitative investigation, in which the difference in the
free energies of triarylcyclopropenylium cations and the
corresponding free radicals was determined polarographically from the reversible reduction potentials [39].
Table 4. Reversible one-electron reduction potentials of
cyclopropenylhn cations and of the triphenylmethyl cation.
reduction potential
Triphenylcyclopro penylium-CIO,O
Diphen yl-p-anisylcyclopropen ylium-ClO~
Triphenylmeth yliurn-Cl0~
The difference of 1.04 eV (= 24 kcal/mole) between the
triphenylcyclopropenyl and triphenylmethyl compounds
roughly agrees with the result required by the simple
Hiickel theory, and evidently reflects the loss of resonance in the cyclopropenyl radical.
According t o the predictions of the simple Hiickel theory, the
cyclopropenyl anion (3), like cyclobutadiene, should possess
no delocalization energy, and should exist as a triplet in the
ground state; even the anion with three phenyl substituents
has a delocalization energy which is lower by 1 than that of
the cation (see Introduction).
Attempts to prepare the 1,2,3-triphenylcyclopropenyl
anion from 1,2,3-triphenylcyclopropenewith alkali metal amides [41] or from 1,2,3-triphenyl-3-ethoxycyclopropene with potassium (451 were unsuccessful; undesired reactions occurred in every case.
When [3-3H]-1,2,3-triphenylcyclopropenewas treated with
potassium t-butoxide in boiling t-butanol, there was no exchange of tritium for hydrogen, whereas [~-3H]-triphenylmethane had lost 80 % of its activity after 8 hours under the
same conditions [41]. All these results demonstrate the instability of the triphenylcyclopropenyl anion. It is possible,
however, that cyclopropenyl anions can be stabilized by substituents in certain cases. For example, 1,2-diphenyl-3-tbutoxycarbonylcyclopropene (19) exchanges 5
of the
Section 1I.C.
[40] R . Breslow and M . Batliste, Chem. and Ind. 1958, 1143.
1411 R . Breslow and P . Dowd, J. Amer. chem. SOC.85,2729 (1963).
1421 H . J. Dauben and L . M . McDonough, Abstracts 142nd
Meeting of the American Chemical Society, Atlantic City, September 1962. p. 55 Q.
portant of the M.O. predictions regarding the unsaturated three-membered ring systems (see Introduction)
[7,9,10,41,43,44]. Since ring strain is comparable in
these three systems, it should be possible to check the
M.O. calculations on this point.
The properties of bis(triphenylcyclopropeny1) (17a)
were as expected: even at high temperatures (17a)
shows no tendency to dissociate into the radicals (18)
[43] A . Streitwieser, J. Amer. chem. SOC.82, 4123 (1960).
[44] A. Streitwieser, Tetrahedron Letters 1960, No. 6, 23.
[45] P. L. Dowd, Ph. D. Thesis, Columbia University, 1962;
Ph. D. Thesis Abstracts 24, 509 (1963).
Angew. Chem. internat. Edit. Vol. 4 (1965) / No. 1
hydrogen in t h e three-membered ring for deuterium on treatment with potassium t-butoxide in boiling deuterio-t-butanol ;
nevertheless, this exchange proceeds m u ch more slowly t h an
i n t h e case of a n ester o f a cyclopropanecarboxylic acid, a
fact which indicates th at t h e double bond has a destabilizing
influence [40].
react with a further cation to form the ethers (24). Consequently, the carbinols, unlike the ethers, cannot be isolated [13,15,16,19,21].
D. Homocyclopropenylium Cations
According to the concept of homoaromaticity [46,47],
the non-classical carbonium ions (20), (21), and (22)
[and their derivatives, e.g. (23)],which have been postulated as intermediates in solvolytic reactions, were regarded as mono- [48], bis- [49-521, and trishomocyclopropenylium ions [47,53,54], and their relative stabilities
were explained by their relationship to the cyclopropenylium system.
In strongly alkaline solution (10 % NaOH), on the
other hand, the ring is irreversibly opened ; triphenylcyclopropenylium bromide (2Sa) gives 1,2,3-triphenylpropenone (26a) [16] via the ether as an intermediate,
while the diphenyl derivative (2Sb) gives 2,3-diphenylpropen-1-a1 (26b) [18,19].
However, the evidence for the existence of these cations
is the subject of the more general controversy regarding
classical [55,56] and non-classical [52] carbonium ions.
(25a): R = C&
(2Sb): R = H
1260): R = C&5
126b): R = H
E. Chemical Reactions of the Cyclopropenylium Salts
Owing to their integral positive charge, the cyclopropenylium compounds readily react with nucleophilic
reagents. These reactions often constitute the first stage
of a rearrangement, leading either to ethylene derivatives or to less strained five- or six-membered ring compounds.
1. R e a c t i o n s with Bases
An equilibrium is set up between the cyclopropenylium
cations (16b) and the covalent carbinols (16a) in
aqueous or aqueous-alcoholic solution ; the position of
the equilibrium depends on the substitutents on the
cation and on the pH of the solution (cf. Section II.B.4).
However, the carbinols (16a) can also act as bases and
[46] S . Winstein, J. Amer. chern. SOC. 81, 6524 (1959)
[47] S. Winstein and J. Sonnenberg, J. Amer. chem. SOC.83, 3244
[48] E. F. Kiefer and J. D. Roberts, J. Amer. chem. SOC.84, 784
[49] S . Winstein, M. Shatavsky, C . Norton, and R. B. Woodward,
J. Amer. chern. SOC. 77, 4183 (1955).
[50] S. Winstein and M . Shatavsky, J. Amer. chem. SOC.78, 592
[51] W . G . Woods, R. A. Cnrboni, and J . D . Roberts, J. Amer.
chem. SOC. 78, 5653 (1956).
[52] S. Winstein, A. H . Lewin, and K . C. Pande, J. Amer. chem.
SOC. 8S,2325 (1963).
[53] S. Winstein and J . Sonnenberg, J. Amer. chem. SOC.83, 3235
(1961); T . Norin, Tetrahedron Letters 1964, 37.
[54] R . R. Sauers, Tetrahedron Letters 1962, 1015.
1551 H. C . Brown, J. Amer. chem. SOC. 85, 2324 (1963).
1561 H. C . Brown, F. J. Chhloupek, and M . H . Rei, J. Amer. chem.
SOC.86, 1246, 1247, 1248 (1964).
Angew. Chem. internat. Edit.
VoI. 4 (1965) 1 No. I
The cyclopropenylium cation is also attacked by the
anion of organometallic reagents (cf. Section 1I.A) ;thus,
for example, the reaction of (2Sa) with methylmagnesium bromide yields 3-methyl-l,2,3-triphenylcyclopropene [41].
2. R e d u c t i o n of t h e C a t i o n s
The reduction of (25a)with lithium aluminum hydride
gave triphenylcyclopropene (27) in high yield [41]. This
reaction is, therefore, formally the reverse of the preparation of cyclopropenylium salts by hydride abstraction
The reaction of triphenyl- or diphenylcyclopropeny1ium salts with zinc dust, like polarographic reduction
(see Section II.C), gave the dimers (17a) and (176) of
the corresponding free radicals. These dimers rearrange
at 130°C to form benzene derivatives; 1,2,4,5- and
1,2,3,4-tetraphenylbenzeneshave been isolated as products of the thermal rearrangement of bis-3,3’-(diphenylcyclopropenyl) (17b),which may proceed via a prismane
intermediate [*I [38,57,58].
[*I For the reaction scheme, see D . Seebach, Angew. Chem.,
in the press.
[57] R. Bres/ow in P. de Mayo: Moiecular Rearrangements. IntersciencsPublishers,NewYork-London
1963,Vol. 1, pp. 243-245.
1581The photochemical dimerization of cyclopropenes to form tricyclohexanes is known: H . H . Stechl, Angew. Chem. 75, 1176
(1963); Angew. Chem. internat. Edit. 2, 743 (1963).
3. R e a c t i o n s w i t h T r a n s i t i o n - M e t a l
C o in p lexe s
Cobalt tetracarbonylate reacts with (25a), with carbonylation of the cation, to form a n-complex which has been
formulated as (28) ; the desired symmetrical x-complex
of the triphenylcyclopropenylium cation is not obtained
Sodium azide reacts with (25a) to form 1,2,3-triphenylcyclopropenyl azide (34), which slowly rearranges, even
at room temperature, to 4,5,6-triphenyltriazine (35)
[60b, ~ O C ] ;this stabilization with ring expansion is
typical of many similar cyclopropene derivatives [60 d].
Attempts to obtain this complex by reaction of (25a)
with potassium trichloro(ethy1ene)platinate (II), followed by thermal elimination of ethylene from the resulting
product, were also unsuccessful [31]. On the other hand,
f25a) reacted with tetracarbonylnickel with evolution
of CO to form a complex which was stable in the solid
state, and which was assigned the structure (29) [60].
Compound (29) is regarded as a complex of formally
zerovalent nickel with the triphenylcyclopropenylium
cation. Each atom is tetrahedrallynickel surrounded by
four pairs of electrons, two of which are supplied by the
bromide anions, one by the carbon monoxide group, and
one by the cyclopropenylium group. Recently a diphenylcyclopropenone-nickel carbonyl complex has
also been prepared [65].
4. R e a c t i o n s w i t h D i a z o C o m p o u n d s
a n d Azides
Compound (25a) reacts with phenyldiazomethane to
form 1,2,3-triphenylazulene (33). It is assumed that the
diazonium compound (30) is first formed, and that this
i 30I
1591 C. E. Co.fej,,3 . Aser. chem. SOC.84, 118 (1962).
[60] E. W. Cowling and S. F. A . Kettle, Inorg. Chem. 3,604 (1964).
loses nitrogen, with ring expansion, to form the cyclobutenyl cation (31). The latter undergoes rearrangement,
via the phenonium ion (32), to form (33). In the presence of excess phenyldiazomethane, the principal product is pentaphenylcyclopentadiene,formed by ring expansion of (31) [60a].
111. Cyclopropenones
Whereas in the case of the seven-membered ring compounds
tropone was synthesized before the tropylium cation 121, the
first cyclopropenone, namely diphenylcyclopropenone (37)
[61,62] was only described after the first cyclopropenylium
However, cyclopropenones had previously been postulated
(although with no conclusive evidence) as intermediates 1631,
particularly in the catalytic carbonylation of acetylenes (36),
which, in the presence of water or alcohols, leads t o acrylic
acids or acrylates 1641.
This assumption has been tested in recent years in the case of
diphenylcyclopropenone (37). The results indicate that no
cyclopropenone intermediate occurs in the carbonylation of
acetylenes 1651.
Interest in the cyclopropenones was rekindled by the predictions of the M.O. calculations (see Introduction) and the preparation of cyclopropenylium salts. The validity of the M.O.
calculations was clearly shown by the isolation of the stable
cyclopropenones. According to classical concepts, the more
highly strained cyclopropenones should be much less stable
[60a] See 1571, p. 276.
[60b] R . Breslow, R. Boikess, and M. Battiste, Tetrahedron Letters 1960, No. 26, 42.
[~OC]E. Chandross and E. Smolinsky,Tetrahedron Letters 1960,
No. 13, 19.
[60d] See [57], pp. 240--241.
1611 R . Breslow, R . Haynie, and J. Mirra, J. Amer. chem. SOC.81,
247 (1960).
[62] D . N. Kursanov, M . E. Volpin, and Yu. D . Koreshkov, Izv. Acad.
Nauk S.S.S.R., Otd. Khim. Nauk 1959, 560; Chem. Abstr. 53,
21 799 (1959); Zh. Obshch. Khim. (English translation) 30, 2855
[63] L. Wolft;Liebigs Ann. Chem. 260, 79 (1890).
[64] W. Reppe, Liebigs Ann. Chem. 582, 1 (1953).
[65] C. W. Bird and J. Hudec, Chem. and Ind. 1959, 570,
C. W. Birdand E. M . Hollins, ibid. 1964, 1362.
Angew. Chem. internat. Edit./ Vol. 4 (1965) / No. 1
than the cyclopropanones, as is the case with the corresponding four-membered ring compounds [66]. However,
cyclopropanones have so far only been detected as intermediatesin a fewreactions, but have not been isolated [67-711.
of hydrogen bromide to form cyclopropenones on treatment with triethylamine or other bases [74-771.
A. Preparation of Cyclopropenones
Of the four known syntheses of cyclopropenones, three
proceed via substituted cyclopropenes which can be converted to cyclopropenones by hydrolysis; the fourth
method is similar to the Favorskii reaction, the threemembered ring being formed only in the last step.
Diphenylcyclopropenone (37) was first prepared by the
addition of phenylchlorocarbene to phenylketene acetal
(38), followed by elimination of HCl to form cyclopropenone ketal (37a) which is readily hydrolysed to
(37) 1611.
+ H5C6-CHC1z
The unsubstituted cyclopropenone ( 4 ) has not yet been
synthesized; however, this is due to its instability rather
than to failure of the methods of preparation [62,74].
B. Physical and Chemical Properties of the
The high stability of the cyclopropenylium cation suggests that, as in the case of tropone (40),the zwitterionic
form of (4) makes a considerable contribution to the
ground state of the cyclopropenones; this should also be
reflected in their properties.
A simpler synthesis of cyclopropenones is based on the
addition of dichloro- or dibromo-carbenes to disubstituted acetylenes (36a) or (366) and hydrolysis of the
resulting product [62,72,73].
Compound (37) is also formed from benzene and the
trichlorocyclopropenylium cation (15) via a FriedelCrafts reaction followed by hydrolysis 1251.
2 C&
1 . Basicity
Cyclopropenones are much more basic than other u,punsaturated ketones, with the exception of tropone
[78-801. Hammett's Ho-values (at which half of the ketone is protonated in aqueous solution) are -2.5 f 0.3
(ca. 5.5 N HC104) for (37) and -0.4
0.2 (ca. 1.3 N
HC104) for (40) [SO]. Dipropylcyclopropenone is more
basic than diphenylcyclopropenone (37) [72,73]; the
substitutent effect in this case is therefore the same as in
the case of the cyclopropenylium cations.
1 3 6 ~ ) :R = C6H5;X = C1, B r
(366): R = n-C3H7
The high basicity of the cyclopropenones is often used in their
separation and purification [62,72-741. The salts (41) of (37)
can be obtained in crystalline form, and can be decomposed
by heat or by weak bases t o give cyclopropenone again 1621.
A fundamentally different method uses u,a'-dibromoketones (39) as the starting materials; these lose 2 moles
[66] Cf. E. Vogel and K. Hasse, Liebigs Ann. Chem. 615, 22
[671 P. Lipp, J. Buchkremer, and H . Seeles, Liebigs Ann. Chem.
499, 1 (1 932).
[68] R . B. Loftfield, J. Amer. chem. SOC.72, 632 (1950); 73, 4707
[69] D . A . Semenow, E. F. Fox, and J. D. Roberts, J. Amer. chem.
SOC.78, 3221 (1956); earlier work is alqo found there.
[70] W . B. De More, H. 0. Pritchard, and N. Dovidson, J. Amer.
chem. SOC.81, 5874 (1959).
[71] A . W. Fort, J. Amer. chem. SOC.84,4979 (1962).
[72] R . Breslow and R. Peterson, J. Amer. chem. SOC. 82, 4426
( 1960).
[73] R. A. Peterson, Ph. D. Thesis, Columbia University, 1962.
Ph. D. Thesis Abstracts 23, 1517 (1962).
Angew. Chem. internat. Edit.
VoI. 4 (1965) 1 No. I
The structure of the salts (41) is shown by the disappearance
of the two typical cyclopropenone bands (see below) and the
appearance of a n OH band and of the cyclopropenylium band
1741 R . Breslow, J. Posner, and A. Krebs, J. Amer. chem. SOC.85,
234 (1963).
[75] R. Breslow and J . Posner, unpublished work.
I761 R. Breslow and Th. Eicher, unpublished work.
[77] R. Breslow and A. Krebs, unpublished work.
[78] Yu. G. Borodka and Ya. K . Syrkin, Dokl. Akad. Nauk
S.S.S.R., I36, 1335 (1961); Chem. Abstr. 55, 19743 (1961).
[79] B. E. Zaitsev, Yu. D . Koreshkov, M. E. VoIpin, and Yu. N .
Sheinker, Dokl. Akad. Nauk S.S.S.R.,139, 1107 (1961); Chem.
Abstr. 56, 344 (1962).
1801 A. S. Kende, personal communication.
(see Section II.B.3) in the I R spectrum. This change in the I R
spectrum has been used for quantitative determination of the
temperature dependence of the equilibrium (37)
and in this way the thermodynamic data were determined
2. S p e c t r a
The IR spectra of all cyclopropenones are characterized
by two very intense bands at 1845-1865 and 1620 to
1645 cm-1 [61,62,72-791. Theauthors are not in agreement as to the assignment of these bands; however, according to the solvent dependence of the positions of the
bands, the band at 1850 cm-1 arises from the cyclopropene system with two substituents on the double
bond, while that at 1640 cm-1 is due to the C=O stretching vibration [8l]. The occurrence of the carbonyl band
at this wave number, which is low for small-ring ketones
[82], points to the high single-bond character of the C=O
bond, and hence to the considerable contribution of the
zwitterionic form of (4) to the ground state of the cyclopropenones. This is further supported by the high intensity of the carbonyl band and by the large dipole moments of 5.08 D and 5.3 D for diphenyl- (37) and cycloheptenocyclopropenone (42), respectively (cf. benzophenone 3.0 D; tropone 4.3 D; trimethylamine oxide
5.03 D)[62,81,83-851.
The nuclear magnetic resonance spectra of the cyclopropenones also agree with this structure. Thus the
signals of the a-methylene protons occur at lower fields
than those of the corresponding covalent cyclopropenes,
but are not so strongly shifted as those of the cyclopropenylium salts [73,81]. In the case of the cyclopropenones, the differences A in the chemical shifts of the
signals of the a - and $-CH2 groups (which are proportional, in many cases, to the electronegativity of
the substituents [21,86]) also lie between the values
found for cyclopropenes and for cyclopropenylium salts
C. Chemical Reactions of the Cyclopropenones
The reactions of the cyclopropenones can be subdivided
into decarbonylations and additions. In the addition reactions, the reactant adds on either to the C=O or to the
C=C double bond; this is often accompanied by rearrangements with opening of the three-membered ring
1. Decarbonylation
The thermal decomposition of the cyclopropenones at
130 - 250 "C leads to the formatian of the corresponding acetylenes, with loss of carbon monoxide; the decarbonylation of dialkylcyclopropenones requires higher
temperatures than that of diphenylcyclopropenones (37)
[61,72-74,771. Photolysis of (37) also yields diphenylacetylene and carbon monoxide [65,88]. Triscycloheptenobenzene (43) is formed on heating cycloheptenocyclopropenone (42) [74]. If (42) is decomposed in the
L-/ 143)
presence of tetraphenylcyclopentadienone, a small
quantity of (43) (about 3 74) is formed, together with
(44) (23 %)
and (45) (21 %) as the principal products [77].
Compounds (43) and (44) are probably formed via cycloheptyne as an intermediate [ 8 9 ] ; however, addition of the
strained C=C double bond of (42) t o the tetraphenylcyclopentadienone, followed by elimination of 2 moles of carbon
monoxide, cannot be excluded.
It is not yet certain whether the two C-C bonds are broken
successively or simultaneously in the decarbonylation of
cyclopropenones (which can be regarded as the reversal of a
carbene addition). The dimer, which probably has the structure (46), may be formed from (37) via an intermediate, since
diphenylacetylene is the principal product of thermal decomposition of(37) only at high temperatures(> 160"C). Between
145 and 15OoC, particularly in the presence of catalytic
amounts of base, the main product is (46), which cannot be
thermally decomposed t o give diphenylacetylene [77].
[81] A . Krebs, unpublished work.
[82] Cydopropanone has a band at 1825 cm-1 [701.
[83] Yu. G. Borodko and Ya. K. Syrkin, Dokl. Akad. Nauk
S.S.S.R. 134, 1127 (1960); Chem. Abstr. 55, 12039 (1961).
[84] B. E. Zaitsev, Yu. N . Sheinker, and Yu. D. Koreshkov, Dokl.
Akad. Nauk S.S.S.R. 136, 1090 (1961); Chem. Abstr. 55, 19480
(1 96 1).
[ 8 5 ] A . N. Shidlovskaja and Ya. K. Syrkin, Dokl. Akad. Nauk
S.S.S.R. 139, 418 (1961).
[86] J. R . Cavanaugh and B. P. Dailey, J . chem. Physics 34, 1094
(1961); B. P. Dailey and J. N . Shoolery, J. Amer. chem. SOC.77,
3977 (1955).
I871 None of the investigations carried out so far makes possible
a decision as to whether, in a reaction of a cyclopropenone, the
ring-opening takes place before, simultaneously with, or after the
addition of a reactant.
2. A d d i t i o n R e a c t i o n s
Additions to the carbonyl group can occur by electrophilic attack on the electron-rich oxygen atom, or by the
attack of nucleophiles on the carbon atom which carries
1881 G . Quinkert, K. Opifz, W. W. Wiersdorfl, and J . Weidich, Tetrahedron Lettors 1963, 1836.
[89] Cf. G . Witrig and A . Krebs, Chem. Ber. 94, 3260 (1961); G .
Wittig and R. PohNce, ibid. 94, 3276 (1961); G. Wittig, Angew.
Chem. 74,479 (1962); Angew. Chem. internat. Edit. I , 415 (1962);
F. G. Wilier, Angew. Chem. 76, 144 (1964); Angew. Chem. internat. Edit. 3, 138 (1964).
Angew. Cheni. internat. Edit./ VoI. 4 (1965) / No. 1
a partial positive charge. However, nucleophilic addition
to the carbonyl carbon atom is more difficult than in
Gther cc,$-unsaturated ketones, owing to the charge delocalization in the zwitterionic form of (4).
x) Electrophilic Additions t o the Carbonyl G r o u p
This group of reactions includes protonation of the
cyclopropenones with strong acids (see Section 1II.B. 1)
and alkylation with triethyloxonium fluoroborate, which
in the case of (37) leads [76,89a] to ethoxydiphenylcyclopropenylium fluoroborate (47). The enhanced reactivity of (47) as compared with (37) is shown in the
reaction with dimethylamine; the product is dimethylaminodiphenylcyclopropenylium fluoroborate (48)
whose staDility towards boiling water can be explained
by resonance [76].
[( 4 H J @ I BFI
H3C-N- CH3
C6H5 H5C6
Grignard compounds also add in some cases to the
carbonyl group of the cyclopropenones, whereas in the
case of the tropones, this reaction occurs only to a very
small extent, the principal reaction being 1 $addition
[91]. The different course taken by the reaction of (37)
with phenylmagnesium bromide [73] is due either to
steric factors or to the fact that the formation of a cyclopropanone is so difficult that the 1,3-addition is suppressed in favor of the formation of the ether (24a).
, 4SI
Nucleophilic Additions t o the Carbonyl G r o u p
The hydrolysis of cyclopropenones with sodium hydroxide to yield cc,P-unsaturated acids (49) [62,72-741
probably begins with an addition of the OHe ion to the
C-atom of the carbonyl group; thus it proceeds in a
manner similar to the ring-opening of the cyclopropanone intermediate in the Favorskii rearrangement [90].
T h e same gradation of reactivity is observed in this reaction
as in the decarbonylation: the alkyl-substituted cyclopropenones are more stable than diphenylcyclopropenone 172-741.
This may be due either to the greater stabilizing action of the
alkyl groups on the ketone (as suggested by basicity measurements) or t o an increase in the stabilization of the transition state by the phenyl groups during ring-opening.
On the other hand, dipropylcyclopropenone reacts with
other bases, such as sodium methoxide in methanol, to
give the 2-propyl-3-propylidenecyclopropenolateanion
(50), as has been demonstrated by deuterium-exchange
[89a] B. Fohlisch, personal communication.
[90] A . Kende, Organic Reactions 11, 261 (1960).
[90a] For a discussion of the mechanism of the reaction of (37)
with hydroxylamine, see [73], pp. 42-44.
Angew. Chem. internat. Edit. 1 Vol. 4 (1965) 1 No. I
When it was attempted to convert (37) into the thioketone ( 4 9 4 by reaction with phosphorus pentasulfi,le,
only 4,5-diphenyltrithione (496) was obtained [89a].
However, (49a) could be prepared from 3,3-dichloro1,2-diphenylcyclopropene (49c) and thioacetic acid
Lithium aluminum hydride attacks the C=O and C=C
double bonds of (37) to form 2,3-diphenylcyclopropan1-01[62], the structure of which, however, does not appear to be quite certain.
Even in the polarographic reduction of (37), it could not
be decided which of the two double bonds was first reduced [92].
Compound (37) forms a hydrazone with 2,4-dinitrophenylhydrazine [62]; the p-toluenesulfonylhydrazone,on the other
hand, could only be obtained by the reaction of 3,3-dichloro1,2-diphenylcyclopropene with p-toluenesulfonylhydrazine
[93]. Hydrazine [93], semicarbazide, and hydroxylamine [62]
did not yield the normal carbonyl derivatives with (37). This
inertia is evidently a result of the delocalization of the positive
charge in the zwitterionic form of (4).
I911 G. L. Closs and L. E. Closs, J. Amer. chern. SOC.83, 599
(1961); T. Nozoe, T. Mukai, and T.Tezuka, Bull. chem. SOC.
Japan 34,619 (1961).
[91a] Th. Eicher, personal communication.
[92] S. I. Zhdanov and M . K . Polievktov, Zh. obshch. Khim.
(English translation) 31, 3607 (1961); Chem. Abstr. 57, 10935
1931 W. M . Jones, personal communication.
y) A d d i t i o n s t o t h e C = C D o u b l e B o n d
However, of these compounds only (57) has been
synthesized; the reaction of 1 , I ,3,3-tetrachloro-2phenylpropene (60) with potassium t-butoxide, which
probably proceeds via the vinylcarbene (61a), led to
(57), phenylpyruvic acid (61b), and phenylpropiolic
acid (61c). In accord with the predictions, (57) has
pK, m I [97].
The reaction of (37) with hydroxylamine yields 3,4diphenylisoxazolone (51) and desoxybenzoin oxime
(52) 1731.
The first step is assumed to be a I ,4-or I ,2-addition (to the
C = C double bond) of the hydroxylamine. The intermediates
then undergo oxidation, or oxidation and decarboxylation, to
form (5I) or (52) [90a].
Diazomethane does not attack the carbonyl group, as it
does in the case of saturated cyclic ketones, but adds as
a 1,3-dipole (as in the case of normal a$-unsaturated ketones) to the strained cyclopropene double bond of (37)
[76,94] or dipropylcyclopropenone [94]. The cyclopropanone intermediate (53) isomerizes to 3,5-diphenyl-4pyridazone (54), the structure of which has been proved
by reduction to (55) [76,94].
Whereas the “aromatic” 0x0-anions C40420, C50520, and
C60$@ are known, the first member of this series, namely the
dianion of (58) C30320, has not yet been described [96]. Attempts to obtain (58) by hydrolysis of tetrachlorocyclopropene always led to ring-opening 1981.
2. M e t h y l e n e c y c l o p r o p e n e s (Triafulvenes)
The catalytic hydrogenation (Pt/H2) of (37) gave dibenzyl ketone in one case [73]; other authors isolated
2,3-diphenylcyclopropan1-01as the sole product [62].
Methylenecyclopropene (5) is the simplest cross-conjugated
cyclic system. For this reason, and owing to its relationship
with cyclopropenone on the one hand and 1-methylenecycloheptatriene (heptafulvene) (62) on the other hand, methylenecyclopropene ( 5 ) and its derivatives became the objects of
theoretical studies at an early stage [7,9,99]. Despite the large
calculated delocalization energy (see Table 2), the M. 0.
theory requires that ( 5 ) owing to the high free-valence index
at the exocyclic methylene group should be readily polymerizable and should be highly reactive towards free radicals
[”I [7]. In agreement with this, no derivatives of (5) without
substituents on the exocyclic C-atom have as yet been iso-
D. Cyclopropenone Derivatives
1. Hydroxycyclopropenones
The hydroxycyclopropenones are of interest on account of the analogy to the tropolone series and the high
calculated delocalization energies of the anions of hydroxycyclopropenone (56) [95] and of 2-hydroxy-3phenylcyclopropenone (57), and of the dianion of dihydroxycyclopropenone (58) [96]. Thus (56), (57), and
(58) should be very strong acids (pK, m 0), as is
4-hydroxy-3-phenylcyclobutene1,2-dione (59) [95].
i 56)
The first synthesis of a stable methylenecyclopropene,
namely 1,2-diphenyl- 3,4‘- (2’,6’- dibromoquino)cyclopropene (64),was achieved by thermal cleavage of the
ether linkage in the cyclopropenylium salt (63), bromination of the resulting diphenyl-(phydroxypheny1)-
g g H x
i 58j
[94] P . T . Izzo and A . S. Kende, Chem. and Ind. 1964, 839.
[951 E. J . Smutny, M . C. Caserio, and J. D. Roberts, J. Amer.
chem. SOC.82, 1793 (1960).
[96] R. West and D . L . PoweII, J.Amer.chem. SOC.85,2577 (1963).
lated. On the other hand, substituents which can stabilize
a negative charge on the exocyclic C-atom by inductive or
mesomeric effects should increase the contribution of the
zwitterionic form of ( 5 ) to the ground state, and so cause
an increase in the chemical stability of the methylenecyclopropene.
[97] D . G . Farnum and P . E.Thirrston, J. Amer. chem. SOC.86,
4206 (1964).
[98] S. W. Tobey and R. Wesf,Tetrahedron Letters 1963, 1179.
[99] J. Svrkin and M . Dyatkina, Acta physicochem. (U.S.S.R.)
21, 641 (1946); Chem. Abstr. 41, 1648 (1947); G . Berthier and B.
Pullman, Bull. SOC.chim. France 16 D , 457 (1949); A . Julg and
P. Francois, J. Chirn. Physique 59, 339 (1962); 0 . Chalvet, R .
Daudel, and J . J. Kaufman, J. physic. Chem. 68, 490 (1964).
[*I For a definition of the free-valence index, see e.g. A . Streitwieser jr. : Molecular Orbital Theory. Wiley, New York-London
1961, p. 56.
Angew. Chem. internat. Edit. 1 Vol. 4 (1965) No. I
cyclopropenylium bromide with N-bromosuccinirnide,
and finally reversible elimination of HBr with a tertiary
amine to yield (64) [17]. Compound (64) is much more
- co,
basic than diphenylcyclopropenone (37). 10-(Diphenylcyclopropy1idene)anthrone resembles compound (64)
and was obtained [99a] by reacting diphenylcyclopropenylium perchlorate (11) with anthrone.
However, the simplest route to the methylenecyclopropenes is the Wittig olefination of cyclopropenones.
Thus, 1,2 - diphenyl - 4 - ethoxycarbonylmethylenecyclopropene (66) is obtained by the reaction of (37) with
triphenylphosphine-ethoxycarbonylmethylene(65) [1001.
( 701
The NMR spectra of (66) [loo] and of (66b) [100b], the
high dipole moment of 7.9 D for (66a) [100a], the position of the carbonyl band in (64a) [99a], and also the
strong solvent dependence of the ultraviolet spectra of
- H,O
Condensation of (37) [100a] and dipropylcyclopropenone [100bI with malonodinitrile yields the correcponding 1,l-dicyanomethylenecyclopropenes (66a) and
(66 b) .
1660) R = C&
(66b) R = C3H7
Another methylenecyclopropene, namely (70), has been
obtained during attempts to trap 1,2-diphenylcycIopropenylidene (68) with dimethyl fumarate (69) ; (68) was
formed on decomposition of the urea derivate (67)
Attempts to prepare tetraphenylmethylenecyclopropene
(71) by elimination of water from (72) led to rearrangement with formation of 1,2,4-triphenylnaphthaleneand
1,2,3-triphenyIazulene (33) [102].
[99a] E. Fohlisch and P . Burgle, Angew. Chem. 76, 784 (1964);
Angew. Chem. internat. Edit. 3, 699 (1964).
[I001 M. A. Eattiste, J. Amer. chem. SOC. 86, 942 (1964).
[IOOa] E. D. Eergmann and I . Agranat, J. Amer. chem. SOC. 86,
3587 (1964).
[IOOb] A . S . Kende and P.T. Izzo, J. Amer. chem. SOC.86, 3587
[ l o l l W. M . Jones and 3. M . Denham, J. Amer. chem. S O C . 86,
944 (1 964).
[I021 R. Breslow and M . Baffiste,J. Amer. chem. SOC. 82, 3626
Angew. Chem. internat. Edit. 1 Val. 4 (1965) No. I
(66) [loo] and of (66a) [100a] prove how considerable
the contribution is of the zwitterion form to the
ground state of the methylenecyclopropene compounds :
it was estimated to be almost 50 % in the case of (66b).
This also agrees with the fact that protons add preferentially to the exo-methylene carbon atom of (66) and
(70) to form the cyclopropenylium system.
The formal combination of the cyclopropenylium cation
with the cyclopentadienyl anion gives two interesting
derivatives of methylenecyclopropene, namely calicene
(73) and bicyclo[3,1,O]hexatriene (75) ; these are the
three-membered ring analogs of sesquifulvalene (74)
and of azulene (76).
i 73)
Although the considerably large values of 2.94 and
2.39 p have been obtained for the calculated delocalization energies of (73) and (75) [7], attempted syn-
theses of these systems have so far been unsuccessful
Compound (73), like (74),m ay lack the postulated resonance
stabilization, since t h e properties of (74) a n d its derivatives
can be satisfactorily explained by th e structure with substantially localized double bonds [105,106]. I n the case of
(75), t h e resonance energy is probably consumed by t h e
enormous ring-strain energy; this is shown by th e fact that
(75) c a n also be described a s a valence isomer of a m-dehydrobenzene.
IV. Conclusion
The chemistry of the cyclopropenylium compounds is
only eight years old. In this short period, however, the
important questions have already been answered, so that
only a few problems of synthesis remain unsolved, viz.
[103j H. Prinzbach and W . Rosswog, Angew. Chem. 73, 543
(1961); H. Prinzbach, personal communication.
[lo41 B. Fohlisch, Chem. Ber. 97, 88 (1964).
[I051 H. Prinzbach, Angew. Chern. 76,235 (1964); Angew. Chem.
internat. Edit. 3, 319 (1964).
[lo61 T. Nakujima and S . Katagiri, Bull. Chem. SOC.Japan 35,
910 (1962).
the unsubstituted cyclopropenylium cation ( I ) , the
benzocyclopropenylium cation, and bicyclo[3,1 ,O]hexatriene (75).
The preparation of the substituted cyclopropenylium
salts, cyclopropenones, and methylenecyclopropenes has
extended the number of the “Hiickel aromatic” systems.
The study of their properties and reactivities has impressively demonstrated the expected relationship with
the “Hiickel homologous” seven-membered ring compounds. If equality of the bond lengths and the existence
of a ring current are accepted as criteria of aromatic
character, the cyclopropenylium salts deserve to be
called aromatic compounds.
I am indebted to Prof. R. Breslow for discussions and suggestions and also to Dr. Th. Eicher, Dr B. Fiihlisch, Prof:
W. M . Jones, Dr. A. S. Kende, Dr. H . Prinzbach, and
Prof: R. West for making available unpublished results.
Thanks are also due to Prof. H . A. Staab for suggestions
made on perusal of the manuscript.
Received, July loth, 1964
[A 410/195 IE]
German version: Angew. Chem. 77, 10 (1965)
Translated by Express Translation Service, London
Syntheses of Oxide Halides
Knowledge of the oxide halides has made significant advances in recent years as a result
of new methods ofpreparation. Moreover, the systemaric study ojolder methods has led to
versatile syntheses which permit the preparation of many new oxide halides. A brief outline
Gf the scope, limitations, and difficulties of the various processes is given together with a
number of exampies.
I. Partial Hydrolysis of Halides
Although the method of partial hydrolysis of halides
has been known for a very long time it can only be
used in special cases to prepare definite anhydrous
oxide halides. The reaction proceeds according to the
General Equation (a).
+ H 2 0 = MOX3 + 2 HX
However, the course of such hydrolyses is greatly
complicated by the fact that other reactions, such as the
formation of stable hydrates, oxyacids of the halides,
hydroxo compounds, oxides, etc. are also formed in
many cases. The method can, therefore, be succesful
only if the reaction product is somehow stabilized
or removed, e.g. by making use of its volatility. Examples are found in the preparations of POCI3 [ I ]
[ll C. A . Wurtz, Ann. Chirn. physique 20, 472 (1847).
from PCl5, of P20jC14 [2] from POCl,, or of the silicon
oxide halides of general formula SinOn-lC12n+Zwhich
can be obtained by partial hydrolysis of SiC14, and of
which the representatives up to n=10 have been isolated
Oxide halides can also be stabilized in the crystal lattice.
In such cases partial hydrolysis of the halide can even
be carried out with excess water. This group includes
certain oxide halides of mercury, e.g. 2HgClz.HgO [8].
In addition to the homopolar character of the Hg-X
[2] E. Fluck, Angew. Chem. 72, 752 (1960).
[3] R. Rheinboldt and W. Wi&?/d, Liebigs Ann. Chem. 517, 197
( 1935).
[4] W. C. Schuinb and D . F. Holioway, J. Amer. chem. SOC. 63,
2753 (1941).
[ 5 ] W. C. Schumb and A . J. Stevens, J. Amer. chem. SOC. 69, 726
(1947); 72,3178 (1950); 75, 1513 (1953).
[6] J. Goubeau and R . Warncke, Z. anorg. allg. Chem. 259, 109
( 1949).
[7] F. K. Scholl, Ph. D. Thesis, Technische Hochschule Stuttgart,
Angew. Chem. internat. Edit.1 Vol. 4 (1965)
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