Chemistry Letters 2002
415
2
3
Y. Ishii, S. Sakaguchi, andT. Iwahama, J. Synth. Org. Chem. Jpn., 57, 24
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1,1-Diphenylethylene was successfully oxidized to benzo-
phenone in 66% yield (entry 7). In the cases of tri- and tetra-
substituted alkenes, not only carbon-carbon double bond cleaved
products, but also epoxides were formed. That is, benzophenone
and 1,1-diphenyl-1,2-epoxypropane were obtained in 31% and
22% yields, respectively in the case of 1,1-diphenyl-1-propene,
trisubstituted alkene (entry 8), while the reaction of 1,1-diphenyl-
2-methyl-1-propene, tetrasubstituted alkene, was slow, affording
benzophenone and 1,1-diphenyl-1,2-epoxy-2-methylpropane in
8% and 13% yields, respectively (entry 9). Low reactivity of the
tetrasubstituted alkene would be attributed to the steric reason.
Styrene is not good substrate for this oxidation by molecular
oxygen because several unidentified products were obtained with
less than a 5% yield of benzaldehyde.
The present reaction also proceeded in the dark, indicating
that it is not photochemically activated. The reaction was
completely suppressed in the presence of a radical scavenger
such as 2,6-di-tert-butyl-p-cresol, implying that a radical species
is involved. It was also observed that electron rich alkenes are
more reactive than electron poor alkenes in this reaction. Though
more experiments are necessary to clarify the reaction mechan-
ism, the results obtained so far are in good agreement with the
radical chain mechanism proposed by several researchers for the
oxidation of alkenes.15 In the radical generation step would be
involved a charge-transfer complex proposed by Sakuragi et al. in
the oxidation of cis ꢁ-alkoxystyrenes.10 That is, a charge transfer
complex between an electron rich alkene and oxygen was formed,
from which an electron transfer occurs to generate a radical
cation. The radical thus generated reacts with the triplet ground
state oxygen molecule to yield an oxygenated radical cation. As
this radical cation reacts with alkene to generate a dioxetane and
another radical cation, radical cation chain reaction continues.9;16
While radical generator or photo-activation is usually employed
to accelerate the initiation step, heat (90 ꢁC) is enough for the
initiation in the present reaction owing to the facile electron
transfer from alkenes to oxygen because of the high HOMO level
of alkenes. Neat reaction condition would be appropriate for the
efficient radical chain reaction.
4
5
6
7
8
9
M. Fujita, A. Shindo, A. Ishida, T. Majima, S. Takamuku, and S.
Fukuzumi, Bull. Chem. Soc. Jpn., 69, 743 (1996), and references cited
therein.
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10 T. Kanno, M. Hisaoka, H. Sakuragi, and T. Tokumaru, Bull. Chem. Soc.
Jpn., 54, 2330 (1981).
11 W. Suprun, K. Blau, and K. Reinker, J. Prakt. Chem., 337, 496 (1995);
W. Suprun, J. Prakt. Chem., 338, 231 (1996);W. Suprun, J. Prakt.
Chem., 339, 664 (1997);W. Suprun, J. Prakt. Chem., 340, 247 (1998);
W. Suprun, J. Prakt. Chem., 341, 52 (1999).
12 The yields for other reaction times are as follows;39% (4 h), 43% (8 h),
45% (10 h), 48% (14 h).
13 Semiempirical molecular orbital calculation at AM1 level was
performed with the program package MACSPARTANPRO 1.0.2 of
Shiina of Science University of Tokyo for the help of the calculation.
14 While HOMO of 2-(1-naphthyl)propene is composed of orbitals of
naphthyl moiety, HOMO(-2) is mostly composed of ꢂ-orbital of
isopropenyl moiety.
15 S. F. Nelsen, D. L. Kapp, F. Gerson, and J. Lopez, J. Am. Chem. Soc.,
108, 1027 (1986);E. L. Clennan, W. Simmons, and C. W. Almgren, J.
Am. Chem. Soc., 103, 2098 (1981);S. F. Nelsen and R. Akaba, J. Am.
Chem. Soc., 103, 2096 (1981).
16 Epoxide would be formed according to the Mayo’s mechanism
proposed for oxidation of styrene and related olefins with oxygen. That
is, a radical reacts with alkenes and molecular oxygen to afford
copolymeric radicals, which finally either afford polymeric peroxides or
undergo scission to ketone, formaldehyde and epoxide. Ratio of the
formation of ketone and epoxide is dependent on the substrates. F. R.
Mayo, A. A. Miller, and G. A. Russell, J. Am. Chem. Soc., 80, 2500
(1958);F. R. Mayo, Acc. Chem. Res., 1, 193 (1968).
17 Typical experimental procedure: To a glass tube of about 14 cm3
volume was added 2-(4-methoxyphenyl)propene 93 mg and a stirring
bar. The tube was filled with pure oxygen and sealed tightly, and then
was deeply immersed in an oil bath at 90 ꢁC, and the reaction mixture
was stirred vigorously. After 12 h, the tube was taken out of the bath.
After allowing the reaction mixture to cool to room temperature,
saturated Na2SO3 solution was added and the organic materials were
extracted 3 times with chloroform. The extracts were dried with Na2SO4
then evaporated, and the residue was purified by thin layer chromato-
graphy (ethyl acetate : hexane ¼ 1 : 3) to give the ketone in 48% yield
with recovery of the staring material in 3% yield.
Scheme 2.
In summary it has been found that the carbon-carbon double
bond of styrene derivatives is oxidatively cleaved by molecular
oxygen alone, without metal catalysts, photo-activation, or high
oxygen pressure, to give the corresponding ketone in moderate
yields. As only molecular oxygen and heat are employed to
convert alkenes to ketones, this is an example of an ideal
ecological oxidation.17
References and Notes
1
T. Mukaiyama, Aldrich. Acta, 29, 59 (1996);T. Yamada, T. Takai, and
T. Mukaiyama, J. Synth. Org. Chem. Jpn., 51, 995 (1993).