Stereochemistry of disilanylene-containing cyclic compounds. Thermal reactions of cis - And trans -3,4-benzo-1,2-diisopropyl-1,2-dimethyl-1,2- disilacyclobut-3-ene

Article abstract of DOI:10.1021/om400824p

The thermal reaction of cis-3,4-benzo-1,2-diisopropyl-1,2-dimethyl-1,2- disilacyclobut-3-ene (1a) with tert-butyl alcohol at 300 C for 24 h proceeded with high stereospecificity to give erythro-1-(tert-butoxyisopropylmethylsilyl)- 2-(isopropylmethylsilyl)benzene (2a) in 71% yield, as a single stereoisomer. A similar reaction of trans-benzodisilacyclobutene (1b) with tert-butyl alcohol produced the threo isomer 2b in 81% yield, as the sole product. The photolysis of 1a,b in the presence of tert-butyl alcohol afforded a mixture of two adducts formed by the reaction of 1-[2-(isopropylmethylsilyl)phenyl]-1-isopropylsilene with tert-butyl alcohol, respectively. The reaction of 1a with ethylene in an autoclave at 300 C for 24 h gave 3,4,9,10-dibenzo(r-1)-trans-2,trans-5,cis-8- tetraisopropyl-1,2,5,8-tetrasilacyclododeca-3,9-diene (10a), as a single stereoisomer in 71% yield. The reaction of 1b with ethylene under the same conditions, however, produced poly[1,2-bis(isopropylmethylsilylene)phenylene- ethylene], whose molecular weight was determined to be 61000 (M _w/M_n = 2.32), relative to polystyrene standards. The reaction of 1a with 1-hexyne, tert-butylacetylene, phenylacetylene, and (trimethylsilyl)acetylene in benzene in a sealed degassed tube at 150 or 200 C proceeded stereospecifically to give the respective cis-5,6-benzo-1,4- diisopropyl-1,4-dimethyl-1,4-disilacyclohexa-2,5-diene derivatives (11a-14a) in high yields as the sole products. The reaction of 1b with the alkynes under the same conditions afforded the respective trans-benzodisilacyclohexadienes (11b-14b), as the sole stereoisomers in high yields. Theoretical treatment for the reaction of 3,4-benzo-1,1,2,2-tetramethyl-1,2-disilacyclobut-3-ene (1c) with tert-butyl alcohol and acetylene indicated that the substrates react directly with 1c to give the respective adducts. For the reaction of 1c with acetylene, it has been shown that the acetylene molecule inserts directly into a silicon-silicon bond in 1c to give the benzodisilacyclohexadiene system. Such direct reactions of the substrates with 1c were shown to be energetically more advantageous than the reaction of o-quinodisilane, 1,2-bis(dimethylsilylene) cyclohexa-3,5-diene, arising from a conrotatory ring-opening reaction of 1c with substrates.

Full text of DOI:10.1021/om400824p

Article
pubs.acs.org/Organometallics
Stereochemistry of Disilanylene-Containing Cyclic Compounds.
Thermal Reactions of cis- and trans-3,4-Benzo-1,2-diisopropyl-1,2-
dimethyl-1,2-disilacyclobut-3-ene
Akinobu Naka,* Junnai Ikadai, Jun Sakata, and Mitsuo Ishikawa*
Department of Life Science, Kurashiki University of Science and the Arts, 2640 Nishinoura, Tsurajima-cho, Kurashiki, Okayama
712-8505, Japan
Yoshihiro Hayashi, Liudmil Antonov,† and Susumu Kawauchi*
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552,
Japan
Tokio Yamabe
Institute for Innovative Science and Technology, Nagasaki Institute of Applied Science, 3-1 Shikumachi, Nagasaki 851-0121, Japan
S
* Supporting Information
ABSTRACT: The thermal reaction of cis-3,4-benzo-1,2-
diisopropyl-1,2-dimethyl-1,2-disilacyclobut-3-ene (1a) with
tert-butyl alcohol at 300 °C for 24 h proceeded with high
stereospecificity to give erythro-1-(tert-butoxyisopropyl-
methylsilyl)-2-(isopropylmethylsilyl)benzene (2a) in 71%
yield, as a single stereoisomer. A similar reaction of trans-
benzodisilacyclobutene (1b) with tert-butyl alcohol produced
the threo isomer 2b in 81% yield, as the sole product. The
photolysis of 1a,b in the presence of tert-butyl alcohol afforded
a mixture of two adducts formed by the reaction of 1-[2-
(isopropylmethylsilyl)phenyl]-1-isopropylsilene with tert-butyl
alcohol, respectively. The reaction of 1a with ethylene in an
autoclave at 300 °C for 24 h gave 3,4,9,10-dibenzo(r-1)-trans-
2,trans-5,cis-8-tetraisopropyl-1,2,5,8-tetrasilacyclododeca-3,9-diene (10a), as a single stereoisomer in 71% yield. The reaction of
1b with ethylene under the same conditions, however, produced poly[1,2-bis(isopropylmethylsilylene)phenylene−ethylene],
whose molecular weight was determined to be 61000 (Mw/Mn = 2.32), relative to polystyrene standards. The reaction of 1a with
1-hexyne, tert-butylacetylene, phenylacetylene, and (trimethylsilyl)acetylene in benzene in a sealed degassed tube at 150 or 200
°C proceeded stereospecifically to give the respective cis-5,6-benzo-1,4-diisopropyl-1,4-dimethyl-1,4-disilacyclohexa-2,5-diene
derivatives (11a−14a) in high yields as the sole products. The reaction of 1b with the alkynes under the same conditions
afforded the respective trans-benzodisilacyclohexadienes (11b−14b), as the sole stereoisomers in high yields. Theoretical
treatment for the reaction of 3,4-benzo-1,1,2,2-tetramethyl-1,2-disilacyclobut-3-ene (1c) with tert-butyl alcohol and acetylene
indicated that the substrates react directly with 1c to give the respective adducts. For the reaction of 1c with acetylene, it has been
shown that the acetylene molecule inserts directly into a silicon−silicon bond in 1c to give the benzodisilacyclohexadiene system.
Such direct reactions of the substrates with 1c were shown to be energetically more advantageous than the reaction of o-
quinodisilane, 1,2-bis(dimethylsilylene)cyclohexa-3,5-diene, arising from a conrotatory ring-opening reaction of 1c with
substrates.
tetramethyldisilanylene)phenylene] with high molecular
weight.7 The thermal reactions of 3,4-benzo-1,1,2,2-tetraethyl-
1,2-disilacyclobut-3-ene with tert-butyl alcohol, aldehydes, and
alkynes, however, afford the respective adducts.8,11 With tert-
INTRODUCTION
■
The chemistry of benzodisilacyclobutenes is interesting because
of their high strain energy, which results in high reactivity. They
show unique behavior in photochemical and thermal reactions
and also in transition-metal-catalyzed reactions.1−9 The thermal
reaction of 3,4-benzo-1,1,2,2-tetramethyl-1,2-disilacyclobut-3-
ene proceeds with ring opening to give poly[(o-
Received: August 15, 2013
Published: October 28, 2013
© 2013 American Chemical Society
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butyl alcohol, tetraethylbenzodisilacyclobutene produces an
alcohol adduct, while with aldehydes and alkynes, the
benzodisilacyclobutene gives products arising from insertion
of the C−O and C−C bonds into the silicon−silicon bond in
the benzodisilacyclobutene. In these reactions, we have
proposed the formation of o-quinodisilane as a reactive
intermediate.11
On the other hand, the photolysis of 3,4-benzo-1,1,2,2-
tetraethyl-1,2-disilacyclobut-3-ene proceeds by a route quite
different from that of the thermolysis, to afford a silene
intermediate, 1-ethyl-1-[2-(diethylsilyl)phenyl]-1-silaprop-1-
ene, derived from radical scission of a silicon−silicon bond in
the disilacyclobutenyl ring, followed by intramolecular dis-
proportionation of the resulting biradical species.10
of considerable interest to us to investigate whether or not the
stereochemistry in carbon compounds can be applied to the
thermal reactions of the benzodisilacyclobutene system. During
the course of our investigation concerning the stereochemistry
of cis- and trans-benzodisilacyclobutene with various substrates,
we found that the thermal reactions of cis- and trans-3,4-benzo-
1,2-diisopropyl-1,2-dimethyl-1,2-disilacyclobut-3-ene with al-
kynes proceed smoothly at relatively low temperature to give
the products, arising from insertion of a triple bond in alkyne
into a silicon−silicon bond of the benzodisilacyclobutenyl ring,
but these compounds do not react with alkenes, with the one
exception of ethylene, even at higher temperature. In this paper
we report the stereochemistry and reaction mechanism for the
thermal reaction of cis- and trans-3,4-benzo-1,2-diisopropyl-1,2-
dimethyl-1,2-disilacyclobut-3-ene with tert-butyl alcohol, ethyl-
ene, and monosubstituted acetylenes. We also report theoretical
calculations for the reactions of 3,4-benzo-1,1,2,2-tetramethyl-
1,2-disilacyclobut-3-ene with tert-butyl alcohol and acetylene.
The benzotetraethyldisilacyclobutene readily reacts with
arenes, alkenes, and alkynes in the presence of a catalytic
amount of the transition-metal complexes, to give the
respective adducts in high yields. The types of products thus
formed depend highly on the nature of the metals in the
transition-metal complexes used as the catalysts. In these
reactions, we have proposed the transient formation of o-
quinodisilane−metal complexes and also 3,4-benzo-1-metalla-
2,5-disilacyclopent-3-ene, which play an important role in the
production of the adducts.11,12
The synthesis and some reactions of benzobis(disilacyclobut-
3-enes) have also been reported. Matsumoto et al.13a,b have
reported that the palladium-catalyzed reaction of benzo-
[1,2:4,5]bis(tetramethyl-1,2-disilacyclobut-3-ene) proceeds
with metathesis of the silicon−silicon bond to give
hexadecasilacyclophane and its open-chain homologues. We
have reported the palladium- and platinum-catalyzed reactions
of benzo[1,2,4,5]bis(tetraethyl-1,2-disilacyclobut-3-ene) with
alkynes, in which 1:2 adducts generated from insertion of
alkyne into the two silicon−silicon bonds in the starting
compound are produced.13c,d
RESULTS AND DISCUSSION
■
The starting compounds, cis- and trans-3,4-benzo-1,2-diiso-
propyl-1,2-dimethyl-1,2-disilacyclobut-3-ene (1a,b) were pre-
pared by the reaction of o-dibromobenzene with 2 equiv of
magnesium in the presence of 2 equiv of chloroisopropylme-
thylsilane in THF, followed by treatment of the resulting 1,2-
bis(hydrosilyl)benzene with chlorine gas in a carbon tetra-
chloride solution and a sodium condensation reaction of 1,2-
bis(chloroisopropylmethyl)benzene in refluxing toluene, as
reported previously (Scheme 1). Both 1a and 1b were
Scheme 1
Recently, we have investigated the stereochemistry of
benzodisilacyclobutenes and demonstrated that the palladium-
catalyzed reactions of cis- and trans-3,4-benzo-1,2-di-tert-butyl-
1,2-dimethyl-1,2-disilacyclobut-3-ene and cis- and trans-benzo-
1,2-diisopropyl-1,2-dimethyl-1,2-disilacyclobut-3-ene with al-
kynes, such as phenylacetylene, 1-hexyne, and diphenylacety-
lene, proceed stereospecifically to give the respective 5,6-benzo-
1,4-disilacyclohexa-2,5-dienes.12f−h
separated by fractional distillation with the use of a spinning
band column with 50 theoretical plates, as isomerically pure
forms.12h The cis configuration of 1a was determined by X-ray
crystallographic analysis of 5,6-benzo-1,4-diisopropyl-1,4-di-
methyl-2,3-diphenyl-1,4-disilacyclohexa-2,5-diene obtained
from the palladium-catalyzed reaction of 1a with diphenylace-
tylene, which proceeds with retention of the configuration.12h
According to the Woodward−Hoffmann rules14 in carbon
chemistry, cyclobutenes are smoothly transformed into
butadienes with a conrotatory fashion by heat treatment. It is
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Thermal Behavior of 1a,b in the Presence of tert-Butyl
Alcohol. We first examined the thermal reactions of cis- and
trans-benzodisilacyclobutenes 1a,b with tert-butyl alcohol to
elucidate an intermediate such as o-quinodisilane, which might
be involved in the reaction. When 1a,b were heated with tert-
butyl alcohol at 250 °C for 24 h, no adducts were formed; the
starting compounds 1a,b were recovered unchanged. The result
indicates that no reactive intermediates such as biradicals and o-
quinodisilane are produced at this temperature. However, at
temperatures higher than 250 °C, 1a,b reacted with tert-butyl
alcohol to give the respective adducts in high yields. Thus,
treatment of 1a with tert-butyl alcohol in a sealed glass tube at
300 °C for 24 h gave an alcohol adduct, 1-(tert-
butoxyisopropylmethylsilyl)-2-(isopropylmethylsilyl)benzene
(2a), which was tentatively assigned as an erythro isomer, in
71% yield as the sole product (Scheme 2).
erythro isomer, was obtained in 85% yield as a single
stereoisomer (Scheme 3). A similar reaction of 1b with n-
Scheme 3
Scheme 2
butyl alcohol afforded threo-1-(n-butoxyisopropylmethylsilyl)-2-
(isopropylmethylsilyl)benzene (4b) in 83% yield. Again, no
other isomers were detected in the reaction mixture. The
structures of 4a,b were confirmed by spectrometric analysis
(see the Experimental Section). These results strongly suggest
that n-butyl alcohol and also tert-butyl alcohol add directly to
1a,b to give the ring-opened products but not to the o-
quinodisilane intermediate.
Theoretical treatment for the ring-opening reaction of 3,4-
benzo-1,1,2,2-tetramethyl-1,2-disilacyclobut-3-ene (1c) in the
thermal reaction, as shown below, indicated that the energy for
the ring-opening reaction involving homolytic scission of a
silicon−silicon bond, leading to biradical species, is lower than
that of the conrotatory ring opening to produce o-
quinodisilane. Therefore, we thought that the thermolyses of
1a,b might involve biradical species as the reactive intermediate
and carried out their photolyses in the presence of tert-butyl
alcohol. As reported previously, the photolysis of 3,4-benzo-
1,1,2,2-tetraethyl-1,2-disilacyclobut-3-ene led to the formation
of the biradical intermediate, arising from homolytic scission of
the silicon−silicon bond, followed by intramolecular dispro-
portionation of the resulting biradical intermediate to produce
the corresponding silene.8,10
Irradiation of 1a in the presence of tert-butyl alcohol in
hexane with a low-pressure mercury lamp afforded a mixture of
erythro- and threo-1-(tert-butoxyisopropylmethylsilyl)-2-
(isopropylmethylsilyl)benzene (2a,b) in a ratio of 1:1.1, in
37% combined yield, and a mixture consisting of 1a,b (23%
combined yield) in a ratio of 2.8:1, indicating that the
photochemical cis and trans isomerization of 1a took place. In
this photolysis, two stereoisomers of cis- and trans-3,4-benzo-
2,5-diisopropyl-2,5-dimethyl-1-oxa-2,5-disilacyclopent-3-ene
(6a,b) were also obtained in 12% combined yield.15 All spectral
data for 6a,b were identical with those obtained by the reaction
of 1a,b with m-chloroperoxybenzoic acid, respectively.12i
The photolysis of 1a in the presence of tert-butyl alcohol-d1
afforded a mixture of two alcohol-d1 adducts, erythro- and threo-
1-[(tert-butoxy)(deuteriomethyl)isopropylsilyl]-2-
(isopropylmethylsilyl)benzene (5a,b), in 42% combined yield,
along with two stereoisomers of cis and trans cyclic siloxanes,
6a,b (16% combined yield) and a mixture of benzodisilacyclo-
butenes 1a,b (12%). The position of the deuterium atom in the
alcohol adducts was confirmed by spectrometric analysis and
No other isomers were detected by spectrometric analysis in
the reaction mixture, indicating that the reaction proceeded
with high stereospecificity. The structure for 2a was verified by
1
mass and H, 13C, and 29Si NMR spectrometric analysis (see
the Experimental Section). Similar treatment of 1b with tert-
butyl alcohol at 300 °C for 24 h gave a stereoisomer different
from 2a, the threo isomer 2b, in 81% yield. Again, no other
isomers were detected in the reaction mixture.
The reaction of 1a with tert-butyl alcohol-d1 under the same
conditions produced erythro-1-(tert-butoxyisopropylmethylsil-
yl)-2-(deuterioisopropylmethylsilyl)benzene (3a) in 35% yield,
together with 48% of the unchanged starting compound 1a. As
expected, no other stereoisomers were detected in the reaction
mixture. The 2H NMR spectrum for 3a showed a signal at 4.66
ppm, indicating that a D−Si bond was formed in the thermal
reaction. A similar reaction of 1b with tert-butyl alcohol-d1 again
proceeded stereospecifically to give a threo isomer (3b) as a
single stereoisomer in 39% yield, along with 52% of the
unchanged starting compound 1b. Benzodisilacyclobutenes
1a,b have a bulky isopropyl group on each silicon atom and,
therefore, are very sensitive to the steric interaction in their
reactions. tert-Butyl alcohol-d1, bearing the larger deuterium
atom instead of hydrogen, seems to be much more prone to
steric repulsion than tert-butyl alcohol, consequently resulting
in lower yields of the products 3a,b.
As expected, the thermal reaction of 1a,b with the less
hindered alcohol n-butyl alcohol proceeded readily at low
temperature to give alcohol adducts in high yields. When a
mixture of 1a and n-butyl alcohol was heated in a sealed tube at
250 °C for 24 h, 1-(n-butoxyisopropylmethylsilyl)-2-
(isopropylmethylsilyl)benzene (4a), tentatively assigned as an
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verified to be attached on the methyl carbon bonded to the
silicon atom, but not on the silicon atom, as shown in Scheme
4. In fact, the 13C NMR spectrum indicated triplet signals at
Scheme 5
Scheme 4
Thermal Behavior of 1a,b in the Absence of a
Trapping Agent. We have previously reported that the
thermolysis of 3,4-benzo-1,1,2,2-tetraethyl-1,2-disilacyclobut-3-
ene in the absence of a trapping agent at 250 °C produced 4,5-
benzo-1,1,2,2,3,3-hexaethyl-1,2,3-trisilacyclopent-4-ene and
2,3,5,6-dibenzo-1,1,4,4-tetraethyl-1,4-disilacyclohexa-2,5-diene,
derived from dimerization of the o-quinodisilane intermedi-
ate.11 Since compounds 1a,b did not undergo thermolysis at
250 °C, we carried out the reaction at 300 °C and found that
the reaction proceeded to give the products in low yields. Thus,
heating 1a in a degassed sealed glass tube at 300 °C for 24 h
gave two stereoisomers (7a and 8a) of 4,5-benzo-1,2,3-
triisopropyl-1,2,3-trimethyl-1,2,3-trisilacyclopent-4-ene in a
ratio of 1:1, in 6% combined yield, and trans-2,3,5,6-dibenzo-
1,4-diisopropyl-1,4-dimethyl-1,4-disilacyclohexa-2,5-diene (9a)
in 1% yield, along with the unchanged starting compound 1a
(Scheme 6).
The products 7a−9a were isolated by the use of recycling
preparative HPLC. Unfortunately, all attempts to separate 7a
from 8a were unsuccessful. Therefore, the structures of 7a and
8a were confirmed by spectrometric analysis, using a 1:1
mixture (see Experimental Section), although we could not
determine their configurations. The structure of 9a was also
verified by spectrometric analysis and tentatively assigned to
have the trans configuration.
On the basis of the theoretical treatment shown below (see
Figure 2), the energies of the biradical intermediates, singlet
and triplet biradicals, arising from scission of the silicon−silicon
bond are shown to be lower than that of o-quinodisilane by
approximately 12 kcal/mol. The energy difference between
singlet and triplet states is very small, only 1.44 kcal/mol, but
the singlet state is more stable than the triplet. Moreover, the
thermal reaction involves no abstraction of a hydrogen atom;
therefore, the singlet biradical intermediate seems to play an
important role in the formation of the products.
The formation of the products 7a−9a may be best explained
by addition of a singlet biradical (A) formed thermally from 1a
to the starting compound 1a, followed by splitting to 7a and 8a
and the benzosilacyclopropene intermediate (C) as reported in
the thermolysis of benzotetraethyldisilacyclobutene.11 The
intermediate C thus formed would undergo σ dimerization4,16
to produce dibenzodisilacyclohexadiene 9a. The formation of
trans isomer 9a by a σ-dimerization reaction4,16 of the 1,2-
benzo-3-silacyclopropene intermediate C seems to be sterically
more favorable than that of the cis isomer.
−1.79 and −1.33 ppm due to deuterio-substituted methyl
2
carbons in 5a,b, respectively. The H NMR spectrum for 5a,b
also showed a signal at 0.57 ppm attributable to the
deuteriomethyl group.
Irradiation of 1b with tert-butyl alcohol under the same
conditions also afforded a mixture of 2a and 2b in 40%
combined yield, in a ratio of 0.9:1, together with the cis and
trans cyclic siloxanes 6a,b in 32% combined yield and 14% of a
mixture of 1a and 1b. With tert-butyl alcohol-d1, 1b again gave a
mixture of 5a and 5b in a ratio of 1:1, in 38% combined yield,
and a mixture of the cis and trans cyclic siloxanes 6a,b in 43%
combined yield, along with 18% of a mixture of benzodisila-
cyclobutenes 1a,b.
These results clearly show that the photolysis of 1a,b
proceeds with homolytic scission of the silicon−silicon bond to
give the biradical with a triplet state, which undergoes
intramolecular hydrogen abstraction to afford the 1-[2-
(isopropylmethylsilyl)phenyl]-1-isopropylsilene. The silene
thus formed reacts with tert-butyl alcohol to give the two
stereoisomers 2a,b, respectively.
In contrast with the photolysis, the thermal reaction of 1a,b
with tert-butyl alcohol proceeds stereospecifically to give the
respective adducts. Therefore, we thought that the thermal
reactions might involve o-quinodisilane as the reactive
intermediate, but not biradical species. The theoretical
calculations for the reaction of benzotetramethyldisilacyclobu-
tene 1c with tert-butyl alcohol, however, indicated that the
direct addition of tert-butyl alcohol to the benzodisilacyclobu-
tene is more advantageous than the reaction involving o-
quinodisilane. According to the theoretical calculations shown
below, addition of tert-butyl alcohol to 1a,b occurs in the plane
involving the benzodisilacyclobutenyl ring and the H−O bond
in tert-butyl alcohol, as shown in Scheme 5. Thus, the oxygen
atom of the hydroxyl group in tert-butyl alcohol attacks directly
one of the two silicon atoms in 1a,b, and then the hydrogen
atom is transferred to another silicon atom, to give 2a,b,
respectively.
A similar thermolysis of 1b at 300 °C for 24 h gave a single
stereoisomer, whose configuration was different from those of
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Scheme 6
Scheme 7
7a and 8a, in 1% yield, together with the unchanged starting
compound 1b. We tentatively assigned this compound as 4,5-
benzo(r-1)-trans-2,trans-3-triisopropyl-1,2,3-trimethyl-1,2,3-tri-
silacyclopent-4-ene (7b). Again, the formation of 7b may be
understood by the reaction of singlet biradical (B) with 1b,
analogously to the reaction of A with 1a.
Thermal Reaction of 1a,b with Ethylene. The thermal
reaction of 1a,b with alkenes such as 1-hexene and propylene at
300 °C afforded no adducts at all. However, with ethylene, 1a,b
reacted to give the respective adducts under the same
conditions in high yields. Thus, treatment of 1a with ethylene
in benzene in an autoclave at 300 °C for 24 h proceeded cleanly
to give a crystalline product, consisting of two molecules of 1a
and one ethylene molecule, 3,4,9,10-dibenzo(r-1)-trans-2,trans-
5,cis-8-tetraisopropyl-1,2,5,8-tetramethyl-1,2,5,8-tetrasilacyclo-
dodeca-3,9-diene (10a), in 71% yield (Scheme 7). In this
reaction, a small amount of cis-3,4-benzo-2,5-diisopropyl-2,5-
dimethyl-1-oxa-2,5-disilacyclopent-3-ene, whose retention time
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on GLC was identical with that of the authentic sample,12i was
formed, but no 1:1 adduct, cis-2,3-benzo-1,4-diisopropyl-1,4-
dimethyl-1,4-disilacyclohex-2-ene was detected in the resulting
mixture. All spectral data for the product 10a were identical
with those obtained by the palladium-catalyzed reaction of 1a
with ethylene,12i indicating that the reaction proceeds with high
stereospecificity.
A similar reaction of 1b with ethylene in benzene proceeded
to afford a quite different type of product. Treatment of 1b with
ethylene in benzene at 300 °C for 24 h gave white polymeric
substances. Neither a 1:1 adduct nor a 1:2 adduct such as 10a
was detected in the resulting mixture. Reprecipitation of the
polymeric substances from benzene−methanol afforded poly-
[1,2-bis(isopropylmethylsilylene)phenylene−ethylene] (8b),
whose molecular weight was determined to be 61000 (Mw/
Mn = 2.32) relative to polystyrene standards, in 75% yield. The
1H NMR spectrum for the polymer 8b reveals four broad
signals at 0.20, 0.59, 0.78, and 1.08 ppm, due to the methylsilyl
protons, ethylene protons, and methyl protons and methine
protons of the isopropyl group, respectively, as well as broad
signals attributable to the phenylene ring protons at 7.14 and
7.42 ppm. Its 29Si NMR spectrum shows four signals at 3.6, 3.7,
3.8, and 3.9 ppm.
1,4-diisopropyl-1,4-dimethyl-1,4-disilacyclohexa-2,5-diene
(11a), arising from formal insertion of a triple bond of alkyne
into the silicon−silicon bond in 1a, was obtained in 62% yield,
along with 34% of the unchanged starting compound 1a. At
higher temperature, 1a gave 11a in higher yield. Thus, the
reaction of 1a with 1-hexyne at 200 °C for 24 h afforded 11a in
76% yield as the sole product. No other stereoisomers were
detected in the reaction mixture by spectrometric analysis.
All spectral data for the product 11a were identical with
those obtained by the palladium-catalyzed reaction of 1a with
1-hexyne.12h The fact that no stereoisomers were detected in
the reaction mixture shows that the reaction proceeded with
high stereospecificity. The reaction of trans isomer 1b with 1-
hexyne at 200 °C for 24 h again proceeded stereospecifically to
produce trans-5,6-benzo-2-butyl-1,4-diisopropyl-1,4-dimethyl-
1,4-disilacyclohexa-2,5-diene (11b), whose spectral data were
identical with those of the authentic sample,12h in 67% yield as
the sole stereoisomer (Scheme 8).
Scheme 8
In contrast to the direct reactions of 1a,b with tert-butyl
alcohol and monosubstituted acetylene discussed below, in the
reactions of 1a,b with ethylene, a singlet biradical intermediate,
as in the thermolyses of 1a,b in the absence of a trapping agent,
seems to play a role in the formation of 1:2 adduct 10a and
alternative copolymer 8b. Thus, the formation of the 1:2 adduct
10a may be understood in terms of the reactions shown in
Scheme 7: addition of the singlet biradical A to 1a, giving the
dimeric biradical, and the reaction of the resulting dimeric
biradical with ethylene. That the reaction proceeds with
retention of configuration may also be explained as follows.
In the benzodisilacyclobutene 1a, both of the less bulky methyl
groups on the disilanylene moiety are located on one side of the
plane consisting of the benzodisilacyclobutenyl ring, and
therefore, one of two silyl radicals in the intermediate A can
add stereospecifically to one of the two silicon atoms in 1a from
the less hindered side. On the other hand, for the formation of
alternative copolymer 8b, the sterically bulky isopropyl groups
in compound 1b are located on both sides, above and below
the benzodisilacyclobutenyl ring, and would prevent the
approach of a silyl radical in B to the silicon atom of 1b.
However, ethylene, which is a smaller molecule, can react with
B to give the alternative copolymer 8b. The result is consistent
with the production of 7b in extremely low yield in the
thermolysis of 1b in the absence of a trapping agent.
The formation of a silicon−silicon bond in the dimerization
of some silenes has been reported.17 Retention of the
configuration of the silyl center in the radical reaction has
also been reported.12f For example, the reaction of cis- and
trans-3,4-benzo-1,2-di-tert-butyl-1,2-dimethyl-1,2-disilacyclobut-
3-ene with oxygen in the presence of azobis(isobutyro)nitrile in
refluxing benzene proceeds stereospecifically to produce cis-
and trans-3,4-benzo-2,5-di-tert-butyl-2,5-dimethyl-1-oxa-2,5-dis-
ilacyclopent-3-ene, respectively.12f
Thermal Reactions of 1a,b with Monosubstituted
Alkynes. In marked contrast to the reaction of 1a,b with tert-
butyl alcohol and ethylene, 1a,b readily react with mono-
substituted alkynes to give the respective adducts at much lower
temperature in high yields. When 1a was heated with 1-hexyne
in a sealed glass tube at 150 °C for 24 h, cis-5,6-benzo-2-butyl-
Surprisingly, the reaction of cis isomer 1a with tert-
butylacetylene, bearing a bulky substituent on the sp carbon
atom, took place readily at 200 °C for 24 h to give the single
stereoisomer cis-benzodisilacyclohexadiene (12a) in 91% yield
as the sole product. The reaction of 1b with tert-butylacetylene
under the same conditions readily produced trans-benzodisila-
cyclohexadiene (12b) in 90% yield. Again, no other stereo-
isomers were detected in the product. Similar treatment of 1a,b
with phenylacetylene at 150 °C for 24 h gave cis- and trans-
benzodisilacyclohexadiene (13a,b) in 77% yields, respectively.
Both 1a and 1b also reacted with silyl-substituted acetylene to
give the addition products. Thus, treatment of 1a with
(trimethylsilyl)acetylene in a sealed glass tube at 200 °C for
24 h again proceeded stereospecifically to produce cis-
benzodisilacyclohexadiene (14a) in 77% yield, while 1b yielded
the trans isomer 14b in 55% yield as the sole product. All
spectral data for 12a−14a and 12b−14b were identical with
those obtained by the palladium-catalyzed reactions of 1a,b
with the corresponding alkynes.12h
With monosubstituted acetylene, 1a,b readily react to give
the corresponding benzodisilacyclohexadienes at 150 or 200
°C; however, 1a,b do not react with disubstituted acetylenes,
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such as 3-hexyne and diphenylacetylene, even at higher
temperature. For example, when a mixture of 1a and
diphenylacetylene was heated in a sealed glass tube at 300 °C
for 24 h, no adduct was formed. No 1:1 adduct was detected in
the reaction mixture by mass spectrometric analysis.
In the thermal reactions of 1a,b with monosubstituted
acetylene at 150 or 200 °C, it seems unlikely that the o-
quinodisilane intermediate or singlet biradical intermediate
plays a role in the formation of the benzodisilacyclohexadienes.
In fact, the theoretical calculations shown below indicated that
a new type of addition reaction, including direct interaction
between a terminal carbon atom in alkynes and a silicon−
silicon σ bond in 1a,b, may be involved in the formation of the
benzodisilacyclohexadienes.
Theoretical Calculations. To clarify the mechanism for
the reactions of the cis- and trans-benzodisilacyclobutenes 1a,b
with tert-butyl alcohol and monosubstituted acetylenes, we have
carried out density functional calculations, using the ωB97X-D
method18 combined with the 6-311G(d,p) basis set.19 3,4-
Benzo-1,1,2,2-tetramethyl-1,2-disilacyclobut-3-ene (1c) was
used as the starting compound for the reactions. The optimized
structures were verified by vibrational analysis: i.e., the
equilibrium structures have no imaginary frequencies, and the
transition state structures have only one imaginary frequency.
We first investigated the reaction mechanism via conrotatory
ring-opening reactions involving scission of the silicon−silicon
bond of 1c, leading to a closed-shell ring-opened product, o-
quinodisilane (15). The optimized structures of 15 and the
transition state (TS-15) from 1c to 15 are illustrated in Figure
1. The energy profile for this reaction is shown in Figure 2. The
structure of TS-15 is close to that of the ring-opened product o-
quinodisilane (15). Furthermore, the energy difference between
the transition state TS-15 (70.90 kcal/mol) and 15 (70.87
kcal/mol) is quite small (0.03 kcal/mol). This is consistent
with the B3LYP/6-31G(d,p) calculations previously reported
by Yoshizawa and co-workers.20 It is interesting to note that the
silicon atoms in 15 are nonplanar, a trigonal pyramid with a
pyramidalization angle of 20.4°, and the wave function of 15 is
shown to be unstable. Therefore, the electronic state of 15
should not be the closed-shell singlet but the open-shell
biradical at the ground state. The vertical energy of the open-
shell singlet biradical state is 5.21 kcal/mol lower than that of
the closed-shell singlet state of 15. Then, we optimized the
structures of the open-shell singlet biradical (116; see Figure 1)
and the triplet biradical (316), respectively. The singlet biradical
116 is 12.97 kcal/mol more stable than that of 15, and the
triplet biradical 316 is 1.44 kcal/mol less stable than that of 116.
Figure 1. Optimized structures of the closed-shell ring-opening
transition state TS-15, closed-shell ring-opened state 15 and open-
shell singlet biradical ring-opened state 116 for 3,4-benzo-1,1,2,2-
tetramethyl-1,2-disilacyclobut-3-ene (1c) at the ωB97X-D/6-311G-
(d,p) level. Selected bond lengths are given in Å and dihedral angles
(Φ) in deg. The blue arrows in TS-15 indicate the vibrational mode of
the reaction coordinate.
1
The result indicates that the singlet biradical 16 is the ground
1
state for the ring-opened product. In the structure of 16, the
silicon atom is highly pyramidalized with a pyramidalization
angle of 32.8° and the spin density is strongly localized on the
silicon atom (0.817e). The result indicates that the
destabilization energy by losing aromaticity of the benzene
ring is larger than the stabilization energy gained by extension
of the π conjugation. We could not optimize the structure of
1
the transition state TS-116 leading to 16, but the vertical
energy of the open-shell singlet biradical state with the
optimized structure of TS-15 could be calculated to be −6.54
kcal/mol. Therefore, the activation energy for the formation of
116 would be estimated to be less than 64.36 kcal/mol. Thus,
the activation energy for the ring-opening process to the
formation of the biradical seems to still be large for the
thermally allowed reactions observed experimentally.
Figure 2. Energy profile for the ring-opening reaction of 3,4-benzo-
1,1,2,2-tetramethyl-1,2-disilacyclobut-3-ene (1c) at the ωB97X-D/6-
311G(d,p) level.
Therefore, we have extensively investigated the alternative
reaction routes leading to the addition products observed
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experimentally. As the result, we found novel reaction pathways
which involve no ring-opening process: that is, 1c reacts with
the substrates directly to give the adducts. This mechanism
involves the attacks of tert-butyl alcohol and acetylene onto a
silicon atom or a silicon−silicon bond of 1c, respectively. In the
case of tert-butyl alcohol, the oxygen atom attacks one of the
two silicon atoms of the starting compound 1c, and then the
hydrogen atom in the hydroxyl group is transferred to another
silicon atom to finally afford the product 2c. The optimized
structure of the transition state TS-2c from 1c to 2c is
illustrated in Figure 3. The energy profile for this reaction is
Figure 4. Energy profile for the reaction between 1c and tert-butyl
alcohol without a ring-opening process at the ωB97X-D/6-311G(d,p)
level.
sufficiently small to produce the acetylene insertion product in
the thermal reaction. Moreover, the reaction definitely proceeds
with retention of the configuration, as observed experimentally.
Further detailed theoretical investigations into the thermal
reaction of 1a,b are now in progress and will be published
elsewhere.
In conclusion, all thermal reactions reported here proceeded
with high stereospecificity to give the respective adducts. The
thermolysis of 1a,b in the presence of tert-butyl alcohol or tert-
butyl alcohol-d1 at 300 °C proceeded stereospecifically to give
the respective adducts, arising from direct addition of the
alcohol to 1a,b. In contrast to the thermal reactions, the
photochemical reactions of 1a,b afforded silene intermediates
derived from radical scission of the silicon−silicon bond in the
disilacyclobutenyl ring, followed by disproportionation of the
resulting triplet biradical, and the silenes thus formed reacted
with tert-butyl alcohol to give the adducts. The reaction of 1a
with ethylene at 300 °C afforded the product 10a, consisting of
two molecules of 1a and one molecule of ethylene, as the sole
stereoisomer. A similar reaction of 1b with ethylene, however,
gave poly[1,2-bis(isopropylmethylsilylene)phenylene−ethyl-
ene] (8b) with a 1:1 ratio of 1b and ethylene, whose molecular
weight was determined to be 61000, as the sole product. The
reaction of 1a,b with alkynes, however, proceeded at lower
temperature (150 or 200 °C) with high stereospecificity to give
the respective cis- and trans-5,6-benzo-1,4-diisopropyl-1,4-
dimethyl-1,4-disilacyclohexa-2,5-dienes. Theoretical investiga-
tions for the reaction of 3,4-benzo-1,1,2,2-tetramethyl-1,2-
disilacyclobut-3-ene (1c) with tert-butyl alcohol indicated that
the alcohol adds directly to 1c to give the adduct, while with
acetylene 1c reacts to afford 1,4-disilacyclohexa-2,5-diene
derived from direct insertion of acetylene into the silicon−
silicon σ bond in 1c.
Figure 3. Optimized structures of transition states TS-2c and TS1-17
in a new reaction path without a ring-opened state for the thermal
reaction of 3,4-benzo-1,1,2,2-tetramethyl-1,2-disilacyclobut-3-ene (1c)
with tert-butyl alcohol and acetylene at the ωB97X-D/6-311G(d,p)
level. Selected bond lengths are given in Å and dihedral angles (Φ) in
deg. The blue arrows in each transition state indicate the vibrational
mode of the reaction coordinate.
shown in Figure 4. The activation energy calculated for this
reaction is 23.39 kcal/mol, and this value is sufficiently small to
allow the thermal reaction. Furthermore, this reaction would
proceed with high stereospecificity to produce the alcohol
adduct, which is also consistent with the experimental results.
In the reaction of 1c with acetylene, we found that acetylene
inserts directly into the silicon−silicon bond in the starting
compound 1c, to give the six-membered intermediate IM-17
through the two transition states TS1-17 and TS2-17.
Interestingly, the structure of the transition state TS1-17
indicates the presence of an appreciable interaction between
one of the ethynyl carbons in acetylene and the silicon−silicon
σ bond of 1c. As shown in Figure 3, the charge transfer at the
transition state TS1-17 plays an important role in this
mechanism. Then, TS1-17 leads to the five-membered-ring
structure of the transition state TS2-17. The intermediate IM-
17 thus formed is readily transformed into the final product
(17), via the biradical transition state TS3-17, as displayed in
Figure 5. In this mechanism, the activation energy for 1c with
acetylene is calculated to be 32.16 kcal/mol, and this value is
EXPERIMENTAL SECTION
■
The thermal reactions of 1a,b with tert-butyl alcohol and alkynes were
carried out in a degassed sealed glass tube (1.0 cm × 15 cm). Similar
reactions with ethylene were performed in a 10 mL autoclave. NMR
spectra were measured on JNM-LA300 and JNM-LA500 spectrom-
eters. Infrared spectra were recorded on a JEOL Model JIR-
DIAMOND 20 infrared spectrophotometer. Mass spectra were
measured on a JEOL Model JMS-700 instrument. cis- and trans-3,4-
benzo-1,2-diisopropyl-1,2-dimethyl-1,2-disilacyclobut-3-ene (1a,b)
were separated by a TSA-SB2 spinning band type distillation column
(Taika Kogyo). Melting points were measured with a Yanaco MP S3
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Figure 5. Energy profile for the reaction between 1c and acetylene without a ring-opening process at the ωB97X-D/6-311G(d,p) level.
apparatus. Column chromatography was performed by using Wakogel
C-300 (WAKO). Tetrahydrofuran used as a solvent was dried over
sodium/benzophenone under a nitrogen atmosphere and carbon
tetrachloride was dried over P4O10 and distilled before use. Benzene
and toluene were dried over sodium and distilled before use.
127.61, 127.69, 134.73, 135.11, 142.85, 146.17 (phenylene ring
carbons); H NMR δ (CDCl3) 4.66.
2
Thermal Reaction of 1b with tert-Butyl Alcohol-d1. A mixture
of 0.130 g (0.52 mmol) of 1b and 0.21 g (2.80 mmol) of tert-butyl
alcohol-d1 was heated in a sealed tube at 300 °C for 24 h. The mixture
was analyzed by GLC as being 3b (39% yield) and 52% of the
unchanged starting compound 1b. The product 3b (colorless liquid)
was isolated by column chromatography. Data for 3b: MS m/z 280
Thermal Reaction of 1a with tert-Butyl Alcohol. A mixture of
0.140 g (0.56 mmol) of 1a and 0.231 g (3.12 mmol) of tert-butyl
alcohol was heated in a sealed glass tube at 300 °C for 24 h. The
mixture was analyzed by GLC as being 2a (71% yield). The product 2a
(colorless liquid) was isolated by column chromatography: MS m/z
279 (M+ − i-Pr); IR 3045, 2956, 2941, 2864, 2162, 1464, 1363, 1250,
1
(M+ − i-Pr); H NMR δ (CDCl3) 0.26 (s, 3H, MeSi), 0.48 (s, 3H,
MeSi), 0.89 (d, 3H, MeCH, J = 7.3 Hz), 0.97 (d, 3H, MeCH, J = 7.3
Hz), 0.99 (d, 3H, MeCH, J = 7.3 Hz), 1.06 (d, 3H, MeCH, J = 7.3
Hz), 1.16 (sep, 2H, HC, J = 7.3 Hz), 1.27 (s, 9H, t-Bu), 7.32−7.34 (m,
2H, phenylene ring protons), 7.53−7.55 (m, 1H, phenylene ring
proton), 7.75−7.77 (m, 1H, phenylene ring proton); 13C NMR δ
(CDCl3) −6.00, −1.10 (MeSi), 12.63, 16.09, 17.18, 17.32, 18.01, 18.99
(Me2CH), 32.15 (Me3C), 72.60 (CMe3) 127.62, 127.71, 134.67,
135.23, 142.71, 146.14 (phenylene ring carbons); 2H NMR δ (CDCl3)
4.61.
1
1196, 1119, 1045, 926, 881, 829, 781, 742, 710 cm−1; H NMR δ
(CDCl3) 0.28 (d, 3H, MeSi, J = 3.4 Hz), 0.48 (s, 3H, MeSi), 0.82 (d,
3H, MeCH, J = 7.3 Hz), 0.98 (d, 6H, MeCH, J = 7.3 Hz), 1.04 (d, 3H,
MeCH, J = 7.3 Hz), 1.07−1.20 (m, 2H, HC), 1.24 (s, 9H, t-Bu), 4.56
(quint, 1H, HSi, J = 3.4 Hz), 7.31−7.34 (m, 2H, phenylene ring
protons), 7.52−7.53 (m, 1H, phenylene ring proton), 7.70−7.72 (m,
1H, phenylene ring proton); 13C NMR δ (CDCl3) −6.10, −1.51
(MeSi), 13.10, 16.12, 17.05, 17.31, 17.99, 18.94 (Me2CH), 32.15
(Me3C), 72.62 (CMe3) 127.60, 127.67, 134.73, 135.12, 142.89, 146.19
(phenylene ring carbons); 29Si NMR δ (CDCl3) −10.3, −0.5. Anal.
Calcd for C18H34OSi2: C, 67.01; H, 10.62. Found: C, 66.82; H, 10.74.
Thermal Reaction of 1b with tert-Butyl Alcohol. A mixture of
0.119 g (0.48 mmol) of 1b and 0.201 g (2.71 mmol) of tert-butyl
alcohol was heated in a sealed tube at 300 °C for 24 h. The resulting
mixture was analyzed by GLC as being 2b (81% yield). The product
2b (colorless liquid) was isolated by column chromatography: MS m/z
279 (M+ − i-Pr); IR 3043, 2956, 2864, 2154, 1464, 1363, 1250, 1196,
Thermal Reaction of 1a with n-Butyl Alcohol. A mixture of
0.176 g (0.71 mmol) of 1a and 0.269 g (3.63 mmol) of n-butyl alcohol
was heated in a sealed glass tube at 250 °C for 24 h. The mixture was
analyzed by GLC as being 4a (85% yield). The product 4a (colorless
liquid) was isolated by column chromatography: exact mass calcd for
C18H34OSi2 [M+] 322.2149, found 322.2133; MS m/z 332 (M+); IR
3046, 2956, 2865, 2150, 1463, 1383, 1252, 1119, 1095, 909, 882, 832,
1
790, 738 cm−1; H NMR δ (CDCl3) 0.29 (d, 3H, MeSi, J = 3.7 Hz),
0.43 (s, 3H, MeSi), 0.94 (t, 3H, CH3, J = 7.3 Hz), 0.98 (d, 3H, MeCH,
J = 7.3 Hz), 1.01 (d, 3H, MeCH, J = 7.3 Hz), 1.03 (d, 3H, MeCH, J =
7.3 Hz), 1.05 (d, 3H, MeCH, J = 7.3 Hz), 1.15−1.27 (m, 2H, HC),
1.40 (sex, 2H, CH2, J = 7.3 Hz), 1.54−1.62 (m, 2H, CH2), 3.68 (dt,
1H, OCH, J = 16.8 Hz, 7.3 Hz), 3.70 (dt, 1H, OCH, J = 16.8 Hz, 7.3
Hz), 4.46 (quint, 1H, HSi, J = 3.7 Hz), 7.34−7.38 (m, 2H, phenylene
ring protons), 7.56−7.59 (m, 1H, phenylene ring proton), 7.66−7.69
(m, 1H, phenylene ring proton); 13C NMR δ (CDCl3) −6.18, −4.32
(MeSi), 12.60, 13.93, 15.45, 17.32, 17.38, 17.87, 18.90, 19.10 (Me2CH,
CH2CH3), 34.93 (CH2), 63.04 (OCH2), 127.76, 128.05, 134.87,
134.96, 143.10, 144.17 (phenylene ring carbons); 29Si NMR δ
(CDCl3) −9.2, 7.5.
1
1119, 1059, 1045, 924, 881, 833, 781, 742, 719 cm−1; H NMR δ
(CDCl3) 0.26 (d, 3H, MeSi, J = 3.7 Hz), 0.48 (s, 3H, MeSi), 0.88 (d,
3H, MeCH, J = 7.3 Hz), 0.96 (d, 3H, MeCH, J = 7.3 Hz), 0.99 (d, 3H,
MeCH, J = 7.3 Hz), 1.06 (d, 3H, MeCH, J = 7.3 Hz), 1.11−1.20 (m,
2H, HC), 1.27 (s, 9H, t-Bu), 4.52 (quint, 1H, HSi, J = 3.4 Hz), 7.31−
7.35 (m, 2H, phenylene ring protons), 7.52−7.54 (m, 1H, phenylene
ring proton), 7.74−7.76 (m, 1H, phenylene ring proton); 13C NMR δ
(CDCl3) −5.91, −1.07 (MeSi), 12.70, 16.11, 17.17, 17.31, 18.01, 18.99
(Me2CH), 32.15 (Me3C), 72.60 (CMe3) 127.61, 127.69, 134.66,
135.24, 142.75, 146.17 (phenylene ring carbons); 29Si NMR δ
(CDCl3) −10.0, −0.6. Anal. Calcd for C18H34OSi2: C, 67.01; H, 10.62.
Found: C, 67.13; H, 10.62.
Thermal Reaction of 1b with n-Butyl Alcohol. A mixture of
0.147 g (0.59 mmol) of 1b and 0.225 g (3.04 mmol) of n-butyl alcohol
was heated in a sealed tube at 250 °C for 24 h. The resulting mixture
was analyzed by GLC as being 4b (83% yield). The product 4b
(colorless liquid) was isolated by column chromatography: exact mass
calcd for C18H34OSi2 [M+] 322.2149, found 322.2152; MS m/z 332
(M+); IR 3044, 2956, 2864, 2151, 1462, 1383, 1251, 1096, 882, 830,
Thermal Reaction of 1a with tert-Butyl Alcohol-d1. A mixture
of 0.144 g (0.58 mmol) of 1a and 0.241 g (3.21 mmol) of tert-butyl
alcohol-d1 was heated in a sealed tube at 300 °C for 24 h. The mixture
was analyzed by GLC as being 3a (35% yield) and 48% of the
unchanged starting compound 1a. The product 3a (colorless liquid)
was isolated by column chromatography. Data for 3a: MS m/z 280
1
788, 738 cm−1; H NMR δ (CDCl3) 0.29 (d, 3H, MeSi, J = 3.7 Hz),
1
(M+ − i-Pr); H NMR δ (CDCl3) 0.27 (s, 3H, MeSi), 0.48 (s, 3H,
0.42 (s, 3H, MeSi), 0.93 (t, 3H, CH3, J = 7.3 Hz), 0.95 (d, 3H, MeCH,
J = 7.3 Hz), 1.00 (d, 3H, MeCH, J = 7.3 Hz), 1.02 (d, 3H, MeCH, J =
7.3 Hz), 1.05 (d, 3H, MeCH, J = 7.3 Hz), 1.13−1.25 (m, 2H, HC),
1.39 (sex, 2H, CH2, J = 7.3 Hz), 1.54−1.61 (m, 2H, CH2), 3.66 (dt,
1H, OCH, J = 16.5 Hz, 7.3 Hz), 3.68 (dt, 1H, OCH, J = 16.5 Hz, 7.3
Hz), 4.45 (quint, 1H, HSi, J = 3.7 Hz), 7.34−7.37 (m, 2H, phenylene
ring protons), 7.55−7.59 (m, 1H, phenylene ring proton), 7.65−7.68
MeSi), 0.82 (d, 3H, MeCH, J = 7.3 Hz), 0.97 (d, 6H, MeCH, J = 7.3
Hz), 1.04 (d, 3H, MeCH, J = 7.3 Hz), 1.07−1.20 (m, 2H, HC), 1.24
(s, 9H, t-Bu), 7.31−7.34 (m, 2H, phenylene ring protons), 7.51−7.53
(m, 1H, phenylene ring proton), 7.69−7.71 (m, 1H, phenylene ring
proton); 13C NMR δ (CDCl3) −6.18, −1.54 (MeSi), 13.04, 16.11,
17.06, 17.32, 17.99, 18.94 (Me2CH), 32.15 (Me3C), 72.62 (CMe3)
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(m, 1H, phenylene ring proton); 13C NMR δ (CDCl3) −6.17, −4.40
(MeSi), 12.81, 13.93, 15.46, 17.32 (2C), 17.92, 18.91, 19.08 (Me2CH,
CH2CH3), 34.90 (CH2), 63.05 (OCH2), 127.76, 128.05, 134.87,
135.01, 143.09, 144.13 (phenylene ring carbons); 29Si NMR δ
(CDCl3) −9.0, 7.7.
Photochemical Reaction of 1a with tert-Butyl Alcohol. A
solution of 0.312 g (1.26 mmol) of 1a and 0.9360 g (12.6 mmol) of
tert-butyl alcohol in 65 mL of hexane was placed in a 70 mL reaction
vessel fitted internally with a low-pressure mercury lamp bearing a
Vycor filter. The mixture was irradiated at ambient temperature for 40
min with a slow stream of argon bubbling through the mixture. After
the solvent was evaporated off, the residue was chromatographed on a
silica gel column, with hexane as the eluent, to give 0.148 g (0.46
mmol) of a mixture of 2a and 2b in a ratio of 1:1.1 (37% combined
yield), 0.040 g (0.15 mmol) of cis and trans cyclic siloxanes 6a,b (12%
combined yield), and 0.070 g (0.28 mmol) of a mixture of 1a,b (23%).
All spectral data for 2a,b and 6a,b are identical with those of the
authentic samples.
give 0.299 g (94% yield) of 6a: MS m/z 221 (M+ − i-Pr); IR (cm−1)
1
2942, 2865, 1462, 1252, 1111, 997, 923, 883, 795, 740; H NMR δ
(CDCl3) 0.29 (s, 6H, Me), 0.98−1.07 (m, 14H, i-Pr), 7.38 (dd, 2H,
phenylene ring protons, J = 5.3 Hz, J = 3.1 Hz), 7.57 (dd, 2H,
phenylene ring protons, J = 5.3 Hz, J = 3.1 Hz); 13C NMR δ (CDCl3)
−3.08 (Me), 15.42 (CHMe2), 16.77, 16.91 (Me2C), 128.76, 131,74,
147.19 (phenylene ring carbons); 29Si NMR δ (CDCl3) 16.1. Anal.
Calcd for C14H24OSi2: C, 63.57; H, 9.15. Found: C, 63.18; H, 9.15.
Similar treatment of 1b (0.315 g, 1.268 mmol) and m-chloroperox-
ybenzoic acid (0.304 g, 1.761 mmol) in hexane afforded 6b (colorless
liquid, 0.341 g, 94% yield): MS m/z 264 (M+); IR (cm−1) 2954, 2863,
1
1463, 1251, 1112, 997, 925, 881, 829, 792, 746, 728; H NMR δ
(CDCl3) 0.32 (s, 6H, Me), 0.95−1.05 (m, 14H, i-Pr), 7.37 (dd, 2H,
phenylene ring protons, J = 5.3 Hz, J = 3.1 Hz), 7,55 (dd, phenylene
ring protons, J = 5.3 Hz, J = 3.1 Hz); 13C NMR δ (CDCl3) −2.88
(Me), 15.37 (CHMe and MeC), 16.74 (MeC), 128.71, 131.62, 146.92
(phenylene ring carbons); 29Si NMR δ (CDCl3) 16.6. Anal. Calcd for
C14H24OSi2: C, 63.57; H, 9.15. Found: C, 63.50; H, 9.03.
Thermolysis of 1a at 300 °C. A 0.1860 g portion (0.748 mmol)
of compound 1a was heated in a degassed sealed tube at 300 °C for 24
h. The resulting mixture was analyzed by GLC as being a mixture
consisting of 7a and 8a (6%) and product 9a (1%). Unfortunately,
compounds 7a and 8a could not be isolated, but compound 9a
(colorless liquid) was separated by column chromatography. The ratio
Photochemical Reaction of 1b with tert-Butyl Alcohol. A
mixture of 0.312 g (1.25 mmol) of 1b and 0.948 g (12.8 mmol) of tert-
butyl alcohol in 65 mL of hexane was irradiated with a low-pressure
mercury lamp at ambient temperature for 50 min. After evaporation of
the solvent, the residue was chromatographed on a silica gel column,
with hexane as the eluent, to give 0.160 g (0.50 mmol) of the mixture
of 2a,b in a ratio of 0.9:1 (40% combined yield), 0.105 g (0.40 mmol)
of 6a,b (32% combined yield), and 0.043 g (0.17 mmol) of the
mixture of 1a,b (14%). All spectral data for 2a,b and 6a,b are identical
with those of the authentic samples.
1
of 7a and 8a was determined to be 1:1 by H NMR spectrometric
analysis. Data for 7a and 8a: MS m/z 334 (M+); 1H NMR δ (CDCl3)
0.10 (s, 3H, MeSi), 0.29 (s, 6H, MeSi), 0.33 (s, 6H, MeSi), 0.35 (s,
3H, MeSi), 0.98−1.18 (m, 35H, MeCH, CHSi), 1.21 (d, 6H, MeCH, J
= 7.1 Hz), 1.36 (sep, 1H, CHSi, J = 7.1 Hz), 7.29−7.33 (m, 4H,
phenylene ring protons), 7.52−7.57 (m, 4H, phenylene ring protons);
13C NMR δ (CDCl3) −12.32, −10.43, −6.43, −4.81 (MeSi), 11.25,
11.50, 14.61, 14.71 (CHSi), 18.48, 18.99, 19.03, 19.18, 20.67, 21.19
(MeCH), 127.62, 127.70, 133.72, 134.13, 149.35, 150.01 (phenylene
ring carbons); 29Si NMR δ (CDCl3) −40.3, −37.7, −7.0, −6.5. Anal.
Calcd for C18H34Si3: C, 64.59; H, 10.24. Found: C, 64.50; H, 10.48.
Data for 9a: HRMS calcd for C20H28Si2 324.1722, found 324.1730; MS
m/z 324 (M+); 1H NMR δ (CDCl3) 0.51 (s, 3H, MeSi), 0.87 (d, 12H,
MeCH, J = 7.4 Hz), 1.08 (sep, 2H, CHSi, J = 7.4 Hz), 7.39−7.41 (m,
4H, phenylene ring protons), 7.63−7.65 (m, 4H, phenylene ring
protons); 13C NMR δ (CDCl3) −3.75 (MeSi), 15.81 (CHSi), 17.80
(MeCH), 127.91, 133.69, 143.25 (phenylene ring carbons); 29Si NMR
δ (CDCl3) −14.5.
Photochemical Reaction of 1a with tert-Butyl Alcohol-d1. A
mixture of 0.316 g (1.27 mmol) of 1a and 0.958 g (12.8 mmol) of tert-
butyl alcohol-d1 in 65 mL of hexane was irradiated with a low-pressure
mercury lamp for 50 min at ambient temperature. After evaporation of
the solvent, the residue was chromatographed on a silica gel column,
with hexane as the eluent, to give 0.172 g (0.53 mmol) of the mixture
of 5a,b in a ratio of 1:1 (42% combined yield), 0.055 g (0.21 mmol) of
6a,b (16% combined), and 0.038 g (0.15 mmol) of the mixture of 1a,b
(12%). Data for 5a,b: MS m/z 280 (M+ − i-Pr); IR 3043, 2956, 2864,
2160, 1464, 1363, 1250, 1196, 1119, 1059, 1045, 881, 831, 742 cm−1;
1H NMR δ (CDCl3) 0.25 (d, 3H, MeSi, J = 3.7 Hz), 0.28 (d, 3H,
MeSi, J = 3.7 Hz), 0.46 (br s, 4H, CH2D), 0.82 (d, 3H, MeCH, J = 7.3
Hz), 0.88 (d, 3H, MeCH, J = 7.3 Hz), 0.95 (d, 3H, MeCH, J = 7.3
Hz), 0.975 (d, 6H, MeCH, J = 7.3 Hz), 0.983 (d, 3H, MeCH, J = 7.3
Hz), 1.04 (d, 3H, MeCH, J = 7.3 Hz), 1.06 (d, 3H, MeCH, J = 7.3
Hz), 1.08−1.20 (m, 4H, HC), 1.24 (s, 9H, t-Bu), 1.26 (s, 9H, t-Bu),
4.52 (quint, 1H, SiH, J = 3.7 Hz), 4.56 (quint, 1H, SiH, J = 3.7 Hz),
7.30−7.34 (m, 4H, phenyl ring protons), 7.51−7.54 (m, 2H, phenyl
ring protons), 7.69−7.77 (m, 2H, phenyl ring protons); 13C NMR δ
(CDCl3) −6.05, −5.86 (MeSi), −1.79 (t, CH2D, J = 18.1 Hz), −1.33
(t, CH2D, J = 18.1 Hz), 12.74, 13.14, 16.14 (2C), 17.07, 17.20, 17.33
(2C), 18.02, 18.04, 18.96, 19.02 (i-Pr), 32.12 (Me3C, 2C), 72.60,
72.62 (CMe3), 127.64, 127.65, 127.71, 127.74, 134.68, 134.74, 135.13,
135.26, 142.75, 142.88, 146.17, 146.19 (phenyl ring carbons); 2H
NMR δ (CDCl3) 0.57.
Photochemical Reaction of 1b with tert-Butyl Alcohol-d1. A
mixture of 0.315 g (1.27 mmol) of 1b and 0.949 g (12.6 mmol) of tert-
butyl alcohol-d1 in 65 mL of hexane was irradiated for 60 min at
ambient temperature. After evaporation of the solvent, the residue was
chromatographed on a silica gel column, with hexane as the eluent, to
give 0.154 g (0.48 mmol) of the mixture of 5a,b in a ratio of 1:1 (38%
combined yield), 0.146 g (0.55 mmol) of 6a,b (43% combined yield),
and 0.057 g (0.23 mmol) of the mixture of 1a,b (18%). All spectral
data for 5a,b and 6a,b are identical with those of the authentic samples.
Reaction of 1a,b with m-Chloroperoxybenzoic Acid. In a 50
mL two-necked flask fitted with a stirrer and condenser were placed
0.300 g (1.21 mmol) of 1a and 0.304 g (1.76 mmol) of m-
chloroperoxybenzoic acid in 25 mL of benzene. The mixture was
stirred at room temperature for 4.5 h. At this point, GLC analysis of
the mixture showed that 1a was completely transformed into 6a
(colorless liquid). The solvent was evaporated off, and the residue was
chromatographed on a silica gel column, with hexane as the eluent, to
Thermolysis of 1b at 300 °C. A 0.1352 g portion (0.544 mmol)
of compound 1b was heated in a degassed sealed tube at 300 °C for 24
h. The resulting mixture was analyzed by GLC as being product 7b
(1%). Compound 7b (colorless liquid) was separated by column
chromatography: MS m/z 334 (M+); 1H NMR δ (CDCl3) 0.25 (s, 3H,
MeSi), 0.30 (s, 3H, MeSi), 0.33 (s, 3H, MeSi), 0.98−1.20 (m, 3H,
CHSi), 0.97 (d, 3H, MeCH, J = 7.0 Hz), 1.035 (d, 3H, MeCH, J = 7.0
Hz), 1.042 (d, 3H, MeCH, J = 7.0 Hz), 1.13 (d, 3H, MeCH, J = 7.0
Hz), 1.17 (d, 3H, MeCH, J = 7.0 Hz), 1.19 (d, 3H, MeCH, J = 7.0
Hz), 7.31 (d, 1H, phenylene ring protons, J = 3.3 Hz), 7.33 (d, 1H,
phenylene ring protons, J = 3.3 Hz), 7.56 (d, 1H, phenylene ring
protons, J = 3.3 Hz), 7.58 (d, 1H, phenylene ring protons, J = 3.3 Hz);
13C NMR δ (CDCl3) −11.15, −5.30, −4.02 (MeSi), 11.18, 14.73,
14.93 (CHSi), 18.78, 19.06 (2C), 19.25, 21.00, 21.24 (MeCH),
127.67, 127.81, 133.55, 133.83, 149.52, 149.97 (phenylene ring
carbons); 29Si NMR δ (CDCl3) −38.8, −6.4, −6.2. Anal. Calcd for
C18H34Si3: C, 64.59; H, 10.24. Found: C, 64.19; H, 10.49.
Thermal Reaction of 1a with Ethylene. In a 10 mL autoclave
was placed 0.294 g (1.18 mmol) of 1a in 0.5 mL of dry benzene. To
this was introduced ethylene until the pressure reached 50 kg/cm2.
The contents of the autoclave were heated at 300 °C for 24 h. The
resulting white crystals were recrystallized from ethanol to give 0.220 g
(71% yield) of 10a: all spectral data for 10a are identical with those of
the authentic sample.12i
Thermal Reaction of 1b with Ethylene. In a 10 mL autoclave
was placed 0.515 g (1.07 mmol) of 1b in 0.5 mL of benzene under a
pressure of ethylene at 50 kg/cm2. The contents of the autoclave were
heated at 300 °C for 24 h. The resulting white solids were
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reprecipitated from benzene−methanol to give 0.430 g (75% yield) of
the polymer 8b, including an ethylene and disilacyclobutene unit in a
ratio of 1:1: mp 104−105 °C; Mw 61000 (Mw/Mn = 2.32); H NMR
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Tables giving Cartesian coordinates for all compounds derived
from theoretical treatment. This material is available free of
charge via the Internet at http://pubs.acs.org.
δ(CDCl3) 0.20 (br s, 6H, MeSi), 0.59 (br s, 4H, CH2Si), 0.78 (br s,
12H, MeCH), 1.08 (br s, 2H, CHMe), 7.14 (br s, 2H, phenylene ring
protons), 7.42 (br s, 2H, phenylene ring protons); 13C NMR
δ(CDCl3) −4.9, −4.8 (MeSi), 6.7 (CH2Si), 13.8 (CHMe), 17.9, 18.2
(MeCH), 127.1, 136.3, 143.8 (phenylene ring carbons); 29Si NMR
δ(CDCl3) 3.6, 3.7, 3.8, 3.9.
Thermal Reaction of 1a with 1-Hexyne. A mixture of 0.153 g
(0.62 mmol) of 1a and 0.206 g (2.51 mmol) of 1-hexyne was heated in
a degassed sealed tube at 200 °C for 24 h. The resulting mixture was
chromatographed on a silica gel column with hexane as eluent, to give
0.088 g (76% yield) of 11a. No other isomers were detected by GLC
and spectrometric analyses. All spectral data for 11a were identical
with those of the authentic sample.
Thermal Reaction of 1b with 1-Hexyne. A mixture of 0.154 g
(0.62 mmol) of 1b and 0.216 g (2.63 mmol) of 1-hexyne was heated in
a sealed tube at 200 °C for 24 h. The mixture was chromatographed
on a silica gel column with hexane as eluent, to give 0.136 g (67%
yield) of 11b. All spectral data for 11b were identical with those of the
authentic sample.
Thermal Reaction of 1a with tert-Butylacetylene. A mixture of
0.154 g (0.62 mmol) of 1a and 0.233 g (2.84 mmol) of tert-
butylacetylene was heated in a degassed sealed tube at 200 °C for 24 h.
The resulting mixture was chromatographed on a silica gel column
with hexane as eluent to give 0.186 g (91% yield) of 12a. No other
stereoisomers were detected in the reaction mixture. All spectral data
for 12a are identical with those of the authentic sample.12h
Thermal Reaction of 1b with tert-Butylacetylene. Treatment
of a mixture consisting of 0.148 g (0.60 mmol) of 1b and 0.165 g (2.01
mmol) of tert-butylacetylene at 200 °C for 24 h gave 0.178 g (90%
yield) of 12b. All spectral data for 12b were identical with those of the
authentic sample.12h
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail for M.I.: ishikawa-m@zeus.eonet.ne.jp.
*E-mail for S.K.: skawauch@polymer.titech.ac.jp.
Notes
The authors declare no competing financial interest.
†On leave from the Institute of Organic Chemistry with Centre
of Phytochemistry, Bulgarian Academy of Sciences, Sofia,
Bulgaria.
ACKNOWLEDGMENTS
■
We thank Sumitomo Chemical Co., Ltd. and Hokko Chemical
Industry Co., Ltd. for financial support. L.A. is indebted to the
JSPS for the fellowship provided. Numerical calculations were
carried out on the TSUBAME2.0 supercomputer at the Tokyo
Institute of Technology, Tokyo, Japan, and on the super-
computer at the Research Center for Computational Science,
Okazaki, Japan.
REFERENCES
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