A novel reaction mode in the cycloaddition of thermally generated silylenes with conjugated dienes

Article abstract of DOI:10.1246/cl.2000.622

A thermally generated silylene bearing bulky substituents, Tbt(Mes)Si: (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl; Mes = 2,4,6-trimethylphenyl), reacted with isoprene at room temperature to give the corresponding 2-vinylsilirane, which thermally isomerized to the corresponding 3-silolene, while silylene underwent competitive [1+2] and [1+4] cycloadditions with 2,4-dimethyl-1,3-butadiene.

Full text of DOI:10.1246/cl.2000.622

622
Chemistry Letters 2000
A Novel Reaction Mode in the Cycloaddition of
Thermally Generated Silylenes with Conjugated Dienes
Nobuhiro Takeda,# Norihiro Tokitoh,*# and Renji Okazaki†‡
Institute for Fundamental Research of Organic Chemistry, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581
†Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received February 28, 2000; CL-000194)
A thermally generated silylene bearing bulky substituents,
Tbt(Mes)Si: (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl;
Mes = 2,4,6-trimethylphenyl), reacted with isoprene at room
temperature to give the corresponding 2-vinylsilirane, which
thermally isomerized to the corresponding 3-silolene, while
silylene underwent competitive [1+2] and [1+4] cycloadditions
with 2,4-dimethyl-1,3-butadiene.
In recent decades, silylenes (silicon analogues of carbenes)
have been extensively studied from the standpoints not only of
fundamental chemistry but also of applied chemistry such as
material science and organic syntheses.1 Reactions of photo-
chemically or thermally generated transient silylenes with 1,3-
dienes giving the corresponding 3-silolenes (silacyclopent-3-
enes) are known to be typical reactions for the detection of sily-
lenes.1 The mechanism of these reactions has been investigated
in detail,1,2 and it has been proposed that the reaction of silylenes
with 1,3-dienes proceeds via the initial [1+2] cycloaddition to
give 2-vinylsiliranes, followed by their isomerization into the
corresponding 3-silolenes (Equation 1). However, the ob-
servation and isolation of the intermediary 2-vinylsiliranes have
been limited to only a few reactions of photochemically gener-
ated silylenes with 1,3-dienes,2 and there has been no report on
the observation of 2-vinylsiliranes in the reactions of thermally
generated silylenes with 1,3-dienes. The lability of intermediary
2-vinylsiliranes in the reactions of silylenes with 1,3-dienes is
probably due to their ready isomerization into the corresponding
3-silolenes under the generation conditions of silylenes such as
ultraviolet irradiation or heating at high temperature.
When disilene 1 was allowed to react with isoprene at 70
°C in THF, the [1+2] adduct 4a6 of silylene 2 with the diene
was isolated as an air-stable colorless powder in 89% yield
(Scheme 2). Also, vinylsilirane 4a was isolated by the reaction
of silylene–isocyanide complex 3 with isoprene at room tem-
perature in 84% yield. These results indicate that these reac-
tions proceed via silylene 2 as an intermediate. The 29Si NMR
spectrum of 4a showed a characteristic signal (δSi = –88.5)
attributable to that of a silicon atom in a three-membered ring.2
The isopropenyl group in 4a is presumed to lie on the same side
as the Mes group for a steric reason, and the exclusive forma-
tion of 4a is most likely interpreted in terms of the severe steric
repulsion among bulky substituents on the silicon atom and the
methyl group of isoprene. Although 4a was stable in the solid
state even on exposure to the air, it underwent hydrolysis on sil-
ica gel to give the corresponding silanol 5a7 (Scheme 3). The
stability of 4a to air is in sharp contrast to that of less hindered
2-vinylsiliranes which readily react with methanol,8 indicating
the effective steric protection in the strained silirane ring of 4a
by the combination of bulky Tbt and Mes groups.
Thermolysis of 4a, however, proceeded at 160 °C for 12
days in C6D6 to give a formal [1+4] cycloadduct 6a9 in 88%
yield, as monitored by 29Si NMR spectroscopy (Scheme 3).
On the other hand, we have succeeded in the synthesis of an
extremely hindered disilene 1 bearing an efficient steric protec-
tion group, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (denoted
as Tbt hereafter)3 and found that 1 is very stable with respect to
oligomerization but thermally labile to give the corresponding
silylene 2 under very mild conditions (Scheme 1).4 In addition,
we recently reported the synthesis of the first stable silylene– iso-
cyanide complex 35 by the reaction of 1 with a hindered iso-
cyanide and its facile dissociation of 3 into silylene 2 and the cor-
responding isocyanide at room temperature (Scheme 1). In this
paper, we describe the reaction of silylene 2, generated from disi-
lene 1 or complex 3 under very mild thermal conditions, with
dienes, leading to the isolation of the corresponding 2-vinylsili-
rane intermediate and the occurrence of unusual direct [1+4]
cycloaddition of silylene 2 to 2,3-dimethyl-1,3-butadiene.
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
623
The isolation of 4a and its thermal ring expansion leading to the
formation of 6a is the first experimental demonstration for the
stepwise formation of a 3-silolene via a 2-vinylsilirane intermedi-
ate in the reaction of a thermally generated silylene with a 1,3-di-
ene, although the existence of this type of intermediary [1+2]
cycloadducts has already been evidenced by NMR spectroscopy
for the cycloaddition reaction of photochemically generated sily-
lenes to 1,3-dienes.2a The isolation of 4a in the present study is
certainly due to the very mild conditions for the generation of 2.
The formation mechanism of 6a from 4a may be explained by the
direct [1+4] cycloaddition of silylene 2 regenerated from 4a to
isoprene because thermolysis of 4a in the presence of triethyl-
silane gave the silylene adduct 74c (20%) together with 6a (77%)
(Scheme 3),10 although other mechanistic possibilities can not be
ruled out.
formed 4b into 6b.
This work was partly supported by a Grant-in-Aid for
Scientific Research on Priority Areas (A) (No. 11133247) from
the Ministry of Education, Science, Sports and Culture, Japan.
We are grateful to Central Glass, Shin-Etsu Chemical, and
Tosoh Akzo Co., Ltds. for the generous gifts of tetrafluoro-
silane, chlorosilanes, and alkyllithiums, respectively.
References and Notes
#
Present address: Institute for Chemical Research, Kyoto University,
Gokasho, Uji, Kyoto 611-0011, Japan.
‡
Present address: Department of Chemical and Biological Sciences,
Faculty of Science, Japan Women’s University, 2-8-1 Mejirodai,
Bunkyo-ku, Tokyo 112-8681, Japan.
1
2
3
4
For recent reviews on silylenes, see: a) P. P. Gaspar and R. West, in
"The Chemistry of Organic Silicon Compounds," ed. by Z.
Rappoport and Y. Apeloig, John Wiley & Sons, New York (1998),
Vol. 2, Part 3, pp. 2463-2568. b) J. Hermanns and B. Schmidt, J.
Chem. Soc., Perkin Trans. 1, 1998, 2209.
a) S. Zhang and R. T. Conlin, J. Am. Chem. Soc., 113, 4272 (1991).
b) M. Weidenbruch, E. Kroke, H. Marsmann, S. Pohl, and W. Saak,
J. Chem. Soc., Chem. Commun., 1994, 1233. c) E. Kroke, S. Willms,
M. Weidenbruch, W. Saak, S. Pohl, and H. Marsmann, Tetrahedron
Lett., 37, 3675 (1996).
a) R. Okazaki, M. Unno, and N. Inamoto, Chem. Lett., 1987, 2293.
b) R. Okazaki, N. Tokitoh, and T. Matsumoto in "Synthetic Methods
of Organometallic and Inorganic Chemistry," ed. by W. A.
Herrmann, Vol. ed. by N. Auner and U. Klingebiel, Thieme, New
York (1996), Vol. 2, pp. 260-269.
a) N. Tokitoh, H. Suzuki, R. Okazaki, and K. Ogawa, J. Am. Chem.
Soc., 115, 10428 (1993). b) H. Suzuki, N. Tokitoh, R. Okazaki, J.
Harada, K. Ogawa, S. Tomoda, and M. Goto, Organometallics, 14,
1016 (1995). c) H. Suzuki, N. Tokitoh, and R. Okazaki, Bull. Chem.
Soc. Jpn., 68, 2471 (1995).
N. Takeda, H. Suzuki, N. Tokitoh, R. Okazaki, and S. Nagase, J.
Am. Chem. Soc., 119, 1456 (1997).
When complex 3 was allowed to react with 2,3-dimethyl-
1,3-butadiene in C6D6 in a sealed tube at room temperature for
2.5 h (Scheme 4), the original deep blue solution of 3 turned
greenish yellow. The 29Si NMR spectrum of the mixture showed
three peaks (–76.3, –72.9, and –5.3 ppm with the peak height
ratio of ca. 2 : 6 : 3) besides the peaks for the trimethylsilyl
groups of Tbt group. The former three peaks are most reasonably
assignable to that for [1+4] cycloadduct 6b4c (δSi = –5.3) and
those for the geometric isomers of [1+2] cycloadducts 4b (δSi
=
–76.3, –72.9). Since heating the reaction mixture at 50 °C for 7 h
resulted in no change as judged by NMR spectroscopy, it is con-
sidered that silylene 2 undergoes both [1+2] and [1+4] cyclo-
addition reactions with the 1,3-diene competitively. This result is
very interesting because this is the first unambiguous evidence
for the occurrence of unusual direct [1+4] cycloaddition of a sily-
lene to a 1,3-diene moiety as far as we know. Since it is theoreti-
cally predicted that [1+4] cycloaddition of silylenes to dienes
more easily occurs in the reaction of triplet silylenes than in that
of singlet silylenes, the experimental results obtained here may
imply some triplet character for silylene 2. However, this is
probably not the case; the occurrence of the direct [1+4] cycload-
dition in this reaction is better explained as below. The [1+2]
cycloaddition may be suppressed by the steric repulsion among
the methyl groups of the diene and the bulky substituents on the
silicon atom of 2, while the effect of the steric repulsion in the
[1+4] cycloaddition is smaller than that in the [1+2] cy-
cloaddition. Therefore, the [1+4] cycloaddition, which is nor-
mally considered to be much slower reaction than the [1+2] cy-
cloaddition, would occur competitively with the [1+2] cycload-
dition. This interpretation is in keeping with the absence of [1+4]
cycloadduct 6a in the reaction with isoprene. Thus, the results
here obtained indicate that silylenes bearing very bulky sub-
stituents may undergo direct [1+4] cycloaddition to dienes.
5
6
Spectral data for 4a: colorless powder, mp 172–175 °C (dec); 1H
NMR (500 MHz, C6D6, 75 °C) δ 0.01 (s, 18H), 0.14 (s, 9H), 0.15 (s,
9H), 0.28 (s, 18H), 1.41–1.49 (m, 3H), 1.86 (s, 3H), 2.04 (s, 3H),
2.44–2.47 (m, 1H), 2.67 (s, 6H), 2.89 (br s, 2H), 4.32 (s, 1H), 4.43
(s, 1H), 6.57 (s, 2H), 6.70 (s, 2H); 13C NMR (125 MHz, C6D6, 75
°C) δ 1.0 (q), 1.2( q), 2.1 (q), 11.9 (t), 21.0 (q), 25.0 (q), 26.5 (q),
29.3 (d), 29.9 (d), 31.3 (d), 103.7 (t), 125.8 (s), 128.3 (d), 128.6 (s),
128.7 (d), 139.6 (s), 145.4 (s), 145.6 (s), 145.7 (s), 153.5 (s); 29Si
NMR (99 MHz, C6D6, 75 °C) δ –88.5, 2.6. Anal. Calcd for
C41H78Si7·H2O: C, 62.68; H, 10.26%. Found: C, 62.59; H, 10.07%.
Spectral data for 5a: colorless powder, mp 175–177 °C (dec); 1H
NMR (500 MHz, CDCl3, 60 °C) δ -0.12 (s, 18H), 0.06 (s, 18H), 0.09
(s, 18H), 1.32 (s, 1H), 1.37 (s, 3H), 1.60 (s, 3H), 1.94 (s, 1H), 1.97
(dd, 1H, 2J = 14 Hz, 3J = 8 Hz), 2.19 (dd, 1H, 2J = 14 Hz, 3J = 9 Hz),
2.21 (s, 3H), 2.23 (br s, 2H), 2.36 (br s, 6H), 4.95 (dd, 1H, 3J = 8 and
9 Hz), 6.28 (br s, 1H), 6.37 (br s, 1H), 6.68 (s, 2H); 13C NMR (125
MHz, CDCl3, 60 °C) δ 0.9 (q), 1.0 (q), 1.9 (q), 17.7 (q), 20.9 (q),
24.5 (q), 25.7 (q), 26.3 (t), 27.4 (d×2), 30.6 (d), 119.9 (d), 123.1 (d),
128.1 (d), 129.3 (d), 130.6 (s), 131.7 (s), 134.0 (s), 138.5 (s), 143.3
(s), 144.3 (s), 151.1 (s×2); 29Si NMR (99 MHz, CDCl3, 60 °C) δ
–6.5, 1.6, 1.7, 2.35, 2.41. Anal. Calcd for C41H80OSi7: C, 62.68; H,
10.26%. Found: C, 62.65; H, 9.97%.
7
8
9
a) M. Ishikawa, F. Ohi, and M. Kumada, J. Organomet. Chem., 86,
C23 (1975). b) M. Ishikawa, K. Nakagawa, and M. Kumada, J.
Organomet. Chem., 178, 105 (1979).
Spectral data for 6a: colorless powder, mp 159−161 °C; 1H NMR
(500 MHz, CDCl3) δ –0.09 (s, 9H), –0.08 (s, 9H), –0.02 (s, 9H),
–0.01 (s, 9H), 0.04 (s, 18H), 1.28 (s, 1H), 1.78 (s, 3H), 1.78–1.84 (m,
Further heating the reaction mixture containing 4b and 6b
at 100 °C for 5 h led to the complete isomerization of initially
2
2
1H, CH2CH), 1.88 (d, 1H, J = 19 Hz, CH2CMe), 2.02 (d, 1H, J =
19 Hz, CH2CMe), 2.06 (s, 1H), 2.09–2.15 (m, 1H, CH2CH), 2.16 (s,
1H), 2.21 (s, 3H), 2.37 (s, 6H), 5.56 (m, 1H), 6.24 (s, 1H), 6.37 (s,
1H), 6.74 (s, 2H); 13C NMR (125 MHz, CDCl3) δ 0.89 (q), 0.93 (q),
1.1 (q), 1.2 (q), 1.4 (q), 1.5 (q), 20.8 (q), 22.4 (q), 25.6 (q), 26.0 (t),
27.6 (d), 27.9 (d), 29.4 (t), 30.3 (d), 122.8 (d), 125.4 (d), 128.0 (d),
128.9 (s), 129.4 (d), 137.1 (s), 138.2 (s), 140.0 (s), 143.2 (s), 143.5
(s), 151.6 (s),151.8 (s); 29Si NMR (99 MHz, CDCl3) δ 1.7, 2.2, 2.4,
2.6. Anal. Calcd for C41H78Si7: C, 64.15; H, 10.24%. Found: C,
64.14; H, 10.15%.
10 No change was observed in thermolysis of 6a at this temperature in
the presence of triethylsilane.