Palladium-catalyzed intermolecular coupling of 2-silylaryl bromides with alkynes: Synthesis of benzosiloles and heteroarene-fused siloles by catalytic cleavage of the C(sp3)-Si bond

Article abstract of DOI:10.1002/anie.201108154

Unusual split: A wide variety of benzosiloles and derivatives are obtained by the Pd-catalyzed intermolecular coupling of 2-silylaryl bromides and alkynes and the accompanying selective cleavage of the C(sp³)-Si bonds as a key step (see scheme). The product spectrum includes benzosiloles, benzothiophene-fused siloles, ladder-type π-conjugated benzosiloles, and thiophene-bridged 2,5-bisbenzosiloles.

Full text of DOI:10.1002/anie.201108154

.
Angewandte
Communications
DOI: 10.1002/anie.201108154
3
À
C(sp ) Si Activation
Palladium-Catalyzed Intermolecular Coupling of 2-Silylaryl Bromides
with Alkynes: Synthesis of Benzosiloles and Heteroarene-Fused Siloles
3
À
by Catalytic Cleavage of the C(sp ) Si Bond**
Yun Liang, Weizhi Geng, Junnian Wei, and Zhenfeng Xi*
Siloles, including benzosiloles and heteroarene-fused siloles,
have become ever more attractive in recent years, because
they have practical applications as organic materials in
electronic and optoelectronic devices.[1] As a consequence,
much interest has been paid to the development of synthetic
methods toward siloles.[1–4] In the meantime, the cleavage of
catalyzed intermolecular coupling of 2-trimethylsilylphenyl-
3
À
boronic acids with alkynes by cleavage of the C(sp ) Si
bond.[13] So far, this latter example has been the only report of
the synthesis of siloles by a transition-metal-catalyzed inter-
molecular coupling accompanied by the cleavage of the
3
À
C(sp ) Si bonds. Recently, we reported a Pd-catalyzed
3
À
À
Si C bonds is an important process in synthetic chemistry for
selective cleavage of the C(sp ) Si bond in a trialkylsilyl
group and a subsequent intramolecular C(sp ) Si bond-
2
À
À
À
the construction of new C C or C X bonds (X = hetero-
atom).[5–7] The C(sp ) Si bond in trialkylsilyl groups, such as
SiMe3, is among the most frequently encountered C Si bonds,
because many compounds are substituted with trialkylsilyl
groups. Thus, the development of a transition-metal-catalyzed
forming process, which resulted in the efficient synthesis of
benzosilolo[2,3-b]indoles.[15] Remarkably, the addition of an
aldehyde promoted the efficiency of the catalytic process.
Herein, we report a facile Pd-catalyzed synthesis of siloles
2 from 2-silylaryl bromides 1 and alkynes by an intermolec-
3
À
À
coupling reaction that is accompanied by a selective cleavage
3
À
of the C(sp ) Si bonds would lead to a useful protocol for the
ular coupling reaction that is accompanied by the selective
3
À
synthesis of differently substituted siloles and derivatives.
However, in contrast to many reports on the cleavage of
cleavage of the three C(sp ) Si bonds. Compared to the
pioneering work by Chatani and co-workers,[13] our method
has the following features: 1) bromides 1 are used instead of
boronic acids as starting materials (the boronic acids used by
Chatani and co-workers were prepared from the correspond-
ing bromides 1 in low yields that ranged from 10% to 30%);
2) the transition-metal catalyst that is used is based on Pd
instead of Rh; 3) wider substitution patterns of 1 can be
applied; and 4) differently substituted siloles and derivatives
are obtained.
3
[8,9]
À
the C(sp ) Si bonds through stoichiometric reactions
or
through transition-metal-catalyzed reactions of activated
3
[3c,4b–d,10]
À
C(sp ) Si bonds,
metal-catalyzed cleavage of unactivated C(sp ) Si bonds.
little is known about the transition-
3
[11–15]
À
3
À
Rauf and Brown reported the cleavage of a C(sp ) Si bond in
an SiMe3 group by using the SiMe3 group as a methyl source in
a Pd-catalyzed oxidative methylation of olefins.[11] Nakao et
al. demonstrated that 2-(2-hydroxyprop-2-yl)phenyl-substi-
tuted alkylsilanes selectively transfer an alkyl group to
facilitate the alkylation of aryl halides by cleavage of the
Initially, we tested the reaction of 2-trimethylsilylphenyl
bromide 1a with diphenylacetylene as a model example
(Table 1). The expected benzosilole derivative 2a could be
generated under certain reaction conditions. When NaOtBu
and KOtBu were used as base, bromide 1a was completely
transformed to its corresponding 2-trimethylsilylphenyl tert-
butyl ether (entries 5 and 6). The base LiOtBu could mediate
the reaction to afford 2a in 57% yield (entry 8). In this
reaction, the addition of an aldehyde again promoted the
catalytic process, and the yield of 2a was increased from 57%
to 93% (entries 8 and 12, respectively). The yield of 2a was
increased from 35% to 65% (entries 4 and 7, respectively)
when K2CO3 was used as the base in the presence of one
equivalent of 4-nitrobenzaldehyde. Aromatic aldehydes with
electron-withdrawing groups gave better results than those
with electron-donating substituents. The addition of water
resulted in a slightly decreased yield of 2a, while the addition
of alcohols, such as tBuOH, slightly increased the yield. A
detailed investigation into the effect of additives is provided
in the Supporting Information. Optimized reaction conditions
include: [PdCl(p-allyl)]2 (2.5 mol%), PtBu3 (10 mol%),
LiOtBu (3 equiv), 4-nitrobenzaldehyde (1 equiv), toluene,
1208C, 24 hours.
3
[12]
À
C(sp ) Si bond. Chatani and co-workers developed a Rh-
[*] Dr. Y. Liang, W. Geng, J. Wei, Prof. Dr. Z. Xi
Beijing National Laboratory for Molecular Sciences (BNLMS)
Key Laboratory of Bioorganic Chemistry and Molecular
Engineering of Ministry of Education, College of Chemistry
Peking University, Beijing 100871 (China)
E-mail: zfxi@pku.edu.cn
Dr. Y. Liang
Key Laboratory of Resource Fine-Processing and Advanced
Materials of Hunan Province, Key Laboratory of Chemical
Biology and Traditional Chinese Medicine Research
Ministry of Education, Hunan Normal University
Changsha, Hunan 410081 (China)
Prof. Dr. Z. Xi
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Shanghai 200032 (China)
[**] This work was supported by the Natural Science Foundation of
China and the 973 Program (2012CB821603).
Supporting information for this article (including experimental
details and scanned NMR spectra of all new products) is available
on the WWW under http://dx.doi.org/10.1002/anie.201108154.
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Table 1: Optimization of reaction conditions for the reaction of 2-silylaryl
bromide 1a with diphenylacetylene.[a]
Entry
Pd source (2.5 mol%)
Ligand
Base
Yield [%][b]
1
2
3
4
5
6
7
8
Pd(OAc)2
[Pd2(dba)3]
[Pd(PPh3)4]
PtBu3
PtBu3
–
PtBu3
PtBu3
PtBu3
PtBu3
PtBu3
PCy3
P(2-tol)3
PtBu3
PtBu3
PtBu3
LiOtBu
LiOtBu
K2CO3
30[c]
32
10[c]
35
[PdCl(p-allyl)]2
[PdCl(p-allyl)]2
[PdCl(p-allyl)]2
[PdCl(p-allyl)]2
[PdCl(p-allyl)]2
[PdCl(p-allyl)]2
[PdCl(p-allyl)]2
[PdCl(p-allyl)]2
[PdCl(p-allyl)]2
[PdCl(p-allyl)]2
K2CO3
NaOtBu
KOtBu
K2CO3
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
0[d]
0[d]
65[e]
57
9
22
44
27
10
11
12
13
93 (90)[e]
76 (73)[f]
[a] Reaction conditions: 1a (0.36 mmol), PhCCPh (0.3 mmol), Pd
catalyst ([Pd]=2.5 mol%), ligand (10 mol%), base (0.9 mmol), toluene
(2 mL). [b] Yields determined by GC analysis. Yields of isolated products
given in parenthesis. [c] [Pd]=5 mmol%. [d] 2-Trimethylsilylphenyl tert-
butyl ether was obtained quantitatively. [e] 4-Nitrobenzaldehyde
(1 equiv). [f] 4-Nitrobenzaldehyde (0.5 equiv). Cy=cyclohexyl, dba=
trans,trans-dibenzylidene acetone, 2-tol=2-tolyl.
Scheme 1 shows the results obtained with the optimized
reaction conditions. Aromatic alkynes that were substituted
with electron-donating groups generally afforded the corre-
sponding products in high yields. Alkynes with electron-
withdrawing groups gave the corresponding products in
slightly lower yields. Alkynes that were unsymmetrically
substituted with a TMS group and an aromatic group afforded
the corresponding products in high yields and regioselectively
with the TMS group at the 2-position of the benzosiloles (2 f–
i). When 1-phenyl-1-propyne was used, the corresponding
product 2j was obtained as a 3:2 mixture of regioisomers.
When 1b (R’ = MeO) and 1c (R’ = CN) were treated with
diphenylacetylene, the corresponding products 2k and 2l
were also obtained in excellent yields.
Scheme 1. Formation of benzosiloles from 2-silylaryl bromides 1 and
various aromatic alkynes. Yields of isolated products given. TMS=tri-
methylsilyl.
Scheme 2. One-pot synthesis of compound 5 with an extended
p-conjugated system by using the reaction conditions shown in
Scheme 1.
In recent years, benzothiophene-fused siloles have been
the subject of growing interest as organic materials.[16] Our
synthetic protocol could also be applied to the coupling
reaction of benzothiophene derivative 3 with aromatic
3
of one of the C(sp ) Si bonds.[13,15] In SiMe2Et and SiMe2iPr,
À
À
alkynes. Benzothiophene-fused silole derivatives
4
mainly the C(Me) Si bond was cleaved, probably because of
(Scheme 1) were obtained in good yields, both from sym-
metrically substituted aromatic alkynes (4a,b) and from
alkynes that were unsymmetrically substituted with a TMS
group and an aromatic group (4c,d). Such benzothiophene-
fused silole skeletons are very rare.[3b] As a further demon-
stration of the application of the above-mentioned protocol,
the thiophene-bridged 2,5-bisbenzosilole 5 with its extended
p-conjugated system was obtained in 50% yield when the
thiophene-bridged diyne was treated with two equivalents of
1a under the optimized reaction conditions in a one-pot
synthesis (Scheme 2).[2b,17]
a steric effect (Scheme 3). The reaction of SiEt3 (1 f) afforded
2o in a much lower yield, again suggesting a steric effect.
Surprisingly, when aliphatic alkynes, such as 3-hexyne and
4-octyne, were subjected to the previously optimized reaction
conditions for aromatic alkynes (see Scheme 1), the expected
products 6a and 6b were formed in very low yields ( ꢀ 30–
40%), regardless of whether the aldehyde was added or not.
A mixture of other products was obtained, including desily-
lated and debrominated derivatives of starting materials 1.
The formation of 2-trimethylsilylphenyl tert-butyl ether was
also observed. The optimization of the reaction conditions for
the reaction of 1a with 4-octyne is provided in the Supporting
Information. Optimized reaction conditions for the reaction
We then investigated the effect of different substituents at
the silicon center, and expected the chemoselective cleavage
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1935

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Angewandte
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similar mechanism to the one we proposed previously might
be operating.[15] The presence of an aldehyde in the reaction
system would change the intermediate from the MePdBr
species to the MePdH species.[19] Since reductive elimination
of MePdH is more facile than that of MePdBr,[19,20] the yields
of products are remarkably increased by the addition of an
aldehyde. A discussion of this proposed reaction mechanism
is provided in the Supporting Information. For the coupling
reaction involving aliphatic alkynes, it should be mentioned
that addition of PtBu3, which is an electron-rich ligand,
dramatically decreased the yield of products 6 (see the
Supporting Information). This observation indicated that the
Scheme 3. Investigation into the reactivity of the substituents on the
silicon center.
À
coupling reaction of the C Si bond might be suppressed when
electron-rich ligand PtBu3 is coordinated to the palladium
center. Furthermore, the addition of aldehydes also decreased
the yields of the products 6, which again suggested that the
reaction pathway for the coupling reaction with aromatic
alkynes might be different from that with aliphatic alkynes.
Further investigations into the reaction mechanism,
including the reason for the different optimized coupling
conditions for aromatic and aliphatic alkynes, and into further
applications of this synthetic method are in progress.
of 1 with aliphatic alkynes include: [Pd(PPh3)4] (5 mol%),
K2CO3 (3 equiv), toluene, 1208C, 20 hours (Scheme 4). 2,3-
Bisalkyl-substituted benzosiloles 6 could be obtained in high
yields.
In summary, we developed an intermolecular coupling
reaction of 2-silylaryl bromides with alkynes accompanied by
3
À
a Pd-catalyzed cleavage of the C(sp ) Si bond to obtain
benzosiloles and heteroarene-fused siloles. This methodology
conveniently affords silole-containing p-conjugated systems,
which have great potentials as organic materials in electronic
and optoelectronic devices.
Received: November 21, 2011
Published online: January 17, 2012
Scheme 4. Formation of benzosiloles 6 from 1 and various aliphatic
alkynes.
À
Keywords: cleavage reactions · C Si activation ·
intermolecular coupling · palladium · trialkylsilyl group
.
Ladder-type p-conjugated molecules have received con-
siderable attention because of the effective conjugation that is
achieved by their rigid coplanar structures.[2c,4f,18] Fortunately,
the above-mentioned Pd-catalyzed intermolecular coupling
reaction of aliphatic alkynes could be successfully applied to
the preparation of the ladder-type p-conjugated molecules 8,
which were obtained in good yields from the doubly
functionalized starting material 7 and two equivalents of an
aliphatic alkyne (Scheme 5).
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