Synthesis of siloles via rhodium-catalyzed cyclization of alkynes and diynes with hexamethyldisilane

Article abstract of DOI:10.1055/s-0030-1259914

A rhodium-catalyzed silylative cyclization of alkynes and 1,6-diynes with hexamethyldisilane is described. These reactions enable the synthesis of densely substituted silole derivatives through the use of a rhodium(I)-norborna-2,5- diene complex as a cata

Full text of DOI:10.1055/s-0030-1259914

LETTER
813
Synthesis of Siloles via Rhodium-Catalyzed Cyclization of Alkynes and Diynes
with Hexamethyldisilane
R
hodium-Cataly
a
ze
d
Silyla
k
tive Cyclizat
a
ion of
A
lky
n
nes
a
nd 1,6-D
o
iynes ri Matsuda,* Yuya Suda, Yosuke Fujisaki
Department of Applied Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
Fax +81(3)52614631; E-mail: matta@rs.kagu.tus.ac.jp
Received 26 December 2010
in the formation of 3a in 39% yield (entry 5). The reaction
Abstract: A rhodium-catalyzed silylative cyclization of alkynes
and 1,6-diynes with hexamethyldisilane is described. These reac-
tions enable the synthesis of densely substituted silole derivatives
through the use of a rhodium(I)–norborna-2,5-diene complex as a
catalyst.
proceeded even at room temperature; however, it had low-
er efficiency (entry 6). Solvents also play an important
role in the formation of siloles. THF was the preferred sol-
vent in this case. The use of other solvents such as 1,1,2-
trichloroethane (TCE), toluene, and ethanol provided 3a
in 69%, 48%, and 35% yields, respectively (entries 7–9).
Key words: alkynes, cyclization, diynes, rhodium, siloles
Table 1 Optimization of Reaction Conditionsa
Heteroles and metalloles belong to a class of compounds
that have unique photophysical and electronic properties
because of the delocalization of their p-electrons.1 Among
them, siloles have received much attention because of
their low-lying LUMO.2 A series of catalytic reactions
that can be used to synthesize silole skeletons have been
reported recently.3–5 Herein, we report a silole-forming
reaction catalyzed by a rhodium(I) complex in which
alkynes and a disilane are used. 1,6-Diynes can be con-
verted to fused bicyclic siloles through similar reactions.
Ph
Ph
Ph
[RhCl(X)]2 (cat.)
4 h
+
Me3Si SiMe3
Ph
Ph
Si
2
Me2
Ph
1a
3a
Entry Equiv of Catalyst
NBD
Conditions
Yieldb
2
3
3
3
1
3
3
3
3
3
(X, mol%) (mol%)
1
2
3
4
5
6
7
8
9
nbd, 5
cod, 5
nbd, 5
nbd, 5
nbd, 2
nbd, 5
nbd, 5
nbd, 5
nbd, 5
–
THF, 50 °C
THF, 50 °C
THF, 50 °C
THF, 50 °C
THF, 50 °C
THF, r.t.
62%
trace
76%
53%
39%
35%
69%
48%
35%
Bis-silylation reactions of alkynes are generally catalyzed
by group 10 metal complexes.6 Recent reports on the suc-
cessful use of silylrhodium(I) species in catalytic
addition7 and substitution reactions8 prompted us to inves-
tigate the bis-silylation of alkynes catalyzed by rhodi-
um(I) complexes. In an attempt to carry out the addition
of silylrhodium(I) to alkynes, diphenylacetylene (1a) and
hexamethyldisilane (2, 3 equiv to 1a) were reacted in THF
in the presence of 5 mol% of [RhCl(nbd)]2 (10 mol% Rh,
nbd = norborna-2,5-diene) at 50 °C. However, instead of
the expected bis-silylation product, 1,1-dimethyl-2,3,4,5-
tetraphenylsilole (3a) was formed in 62% GC yield
(Table 1, entry 1). Unlike the corresponding nickel-cata-
lyzed silole-forming reaction, which utilizes sym-tetrame-
thyldisilane as the silicon source,5a this rhodium-catalyzed
2:1 coupling of 1a and 2 involves the cleavage of not only
the Si–Si bond but also the Si–C bond of 2. A rhodium(I)
complex ligated by cycloocta-1,5-diene (COD) exhibited
almost no catalytic activity, indicating that the nature of
bidentate diene ligands plays a prominent role in the out-
come of the reaction (entry 2). The yield of 3a improved
to 76% when 30 mol% of NBD was added as an additive
(entry 3). A slight decrease in yield was observed when
the reaction was carried out using one equivalent of 2 (en-
try 4). Reduction of catalyst loading (4 mol% Rh) resulted
–
30
30
10
30
30
30
30
TCE, 50 °C
toluene, 50 °C
EtOH, 50 °C
a All reactions were carried out at a concentration of 0.25 M of 1a in
solvent.
b Determined by GC analysis with dodecane as an internal standard.
A possible mechanism for the formation of silole 3a is
shown in Scheme 1. First, a rhodium(I) complex reacts
with disilane 2 to generate silylrhodium(I) species A.
Then, the silylrhodation of alkyne 1a occurs stereoselec-
tively to generate b-silylalkenylrhodium(I) B, which un-
dergoes further addition across 1a to form d-
silyldienylrhodium(I) C. Subsequent intramolecular cy-
clization accompanying the demethylation of the trimeth-
ylsilyl group furnishes silole 3a and methylrhodium(I) D.
Finally, silylrhodium(I) A is regenerated via a reaction
with 2 to complete the catalytic cycle; however, the de-
tailed mechanism of the final step (D → A) is currently
unclear.9 It is probable that the cleavage process of the sil-
SYNLETT 2011, No. 6, pp 0813–0816
x
x.
x
x.
2
0
1
1
Advanced online publication: 15.03.2011
DOI: 10.1055/s-0030-1259914; Art ID: U11410ST
© Georg Thieme Verlag Stuttgart · New York

814
T. Matsuda et al.
LETTER
Table 2 Rhodium-Catalyzed Cyclization of Alkynes 1a–e with 2a
icon–methyl bond (C → D) is mechanistically related to
that observed in the rhodium-catalyzed coupling of 2-si-
lylphenylboronic acids with alkynes.4c,10 The GC–MS
analysis of the reaction mixture revealed the presence of a
small quantity of 1,2,3,4-tetraphenylpenta-1,3-diene,
which was derived from the successive insertion of 1a
into the Rh–Me bond of D and the subsequent protonation
of the resulting dienylrhodium(I) with an adventitious
proton source.
R
R
R
[RhCl(nbd)]2 (5 mol%)
NBD (30 mol%)
+
2
THF, 50 °C
R
R
Si
(3 equiv)
Me2
R
1
3
Entry
1 (R)
Time
4 h
3
Yieldb
35%
38%
34%
34%
26%
1
2
3
4
5
1a (Ph)
3a
3b
3c
3d
3e
1a
1b (4-MeC6H4)
1c (3,5-Me2C6H3)
1d (4-MeOC6H4)
1e (4-F3CC6H4)
4 h
4 h
Rh SiMe3
A
2
24 h
6 h
R
Rh Cl + 2
R
Rh Me
Rh
D
SiMe3
a Reaction conditions: alkyne 1, 2 (3 equiv), [RhCl(nbd)]2 (5 mol%,
10 mol% Rh), and NBD (30 mol%) were heated in THF (0.25 M) at
50 °C.
B
R
b Isolated yield after washing with EtOH.
R
R
R
3a
1a
SiMe3
Rh
ing silole was not formed; instead, silyldiene 4 was ob-
tained as the only identifiable product (Scheme 2). The
linear diene was produced as a result of the protonation of
the d-silyldienylrhodium(I) intermediate. The reaction in-
volving the use of phenylacetylene failed to give the prod-
uct.
C
Scheme 1 Proposed mechanism for silole formation
Rate constants for the reaction of alkyne 1a and disilane 2
were measured by GC using dodecane as an internal stan-
dard with the concentration of 2 varying from 0.25 M to
1.50 M. A plot of the initial reaction rate (D[3a]/Dt) versus
the initial [2] was linear (y = 0.0374x +0.00858, R2 =
0.9917), indicating a first-order dependence on the con-
centration of disilane 2. This result suggests that the final
step (D → A) may be the turnover-limiting.
CO2Et
CO2Et
[RhCl(nbd)]2 (5 mol%)
NBD (30 mol%)
+
2
EtO2C
THF, 50 °C
(3 equiv)
H
SiMe3
1g
4 31%
With the optimized conditions in hand, the scope of the re- Scheme 2
action with various alkynes was investigated. The results
obtained when the reaction was carried out using symmet-
Next, 1,6-diynes were subjected to the rhodium-catalyzed
silole-forming reaction (Table 3). The best results were
obtained in 1,4-dioxane, and not in THF, under otherwise
identical conditions. Dimethyl 2,2-bis(3-phenylprop-2-
ynyl)malonate (5a) underwent silole-forming cyclization
with disilane 2 to give bicyclic silole derivative 6a (entry
1).11 The preparative thin-layer chromatography of the
crude reaction mixture afforded 6a in 65% yield with a
small amount (3%) of a by-product. An analysis of the 1H
NMR spectra revealed that the by-product was exocyclic
diene 7a that was formed via the cyclization of 5a with the
methylrhodium(I) species instead of silylrhodium(I) (vide
supra). After recrystallization from ethanol, analytically
pure silole 6a was isolated in 49% yield (entry 1). The re-
action of diyne 5b (R = 4-MeC6H4) with 2 gave silole 6b
and diene 7b in 66% combined yield (6b/7b = 82:18) by
preparative thin-layer chromatography, and the pure prod-
uct 6b was isolated in 48% yield by recrystallization (en-
try 2). Alkyl-substituted siloles 6c and 6d were obtained
in poor yields owing to their higher solubility in ethanol
(entries 3 and 4). Sterically demanding o-tolyl and 1-
naphthyl derivatives 5e and 5f were good substrates for
the reaction, and they furnished the corresponding siloles
rical internal alkynes 1a–e are summarized in Table 2.
Unfortunately, we experienced difficulties while carrying
out the chromatographic purification of the products;
these difficulties could presumably be ascribed to the by-
products resulting from the oligomerization of alkynes.
For the reaction of diphenylacetylene (1a), preparative
thin-layer chromatography on silica gel was conducted,
and this was followed by washing with ethanol to afford
analytically pure silole 3a in 35% isolated yield (entry 1).
4-Tolyl and 3,5-xylyl derivatives (1b and 1c) afforded tet-
raarylsiloles (3b and 3c) in 38% and 34% yields, respec-
tively (entries 2 and 3). Diphenylacetylenes 1d and 1e,
which possess electron-donating methoxy and electron-
withdrawing trifluoromethyl groups, respectively, also re-
acted as desired (entries 4 and 5). However, attempts to
use aliphatic alkynes such as oct-4-yne and 1,4-bis(benzyl-
oxy)but-2-yne proved to be unsuccessful.
Silole-forming reactions with unsymmetrical alkynes
were then examined. The reaction involving 1-phenyl-1-
propyne provided an inseparable mixture of products in
which three possible isomers were found. When ethyl 2-
butynoate (1f) was used as the substrate, the correspond-
Synlett 2011, No. 6, 813–816 © Thieme Stuttgart · New York

LETTER
Rhodium-Catalyzed Silylative Cyclization of Alkynes and 1,6-Diynes
815
6e and 6f in 52% and 60% isolated yields, respectively,
without any noticeable formation of 7 (entries 5 and 6).
Although siloles 6g and 6h, which have methoxy and bro-
mo groups on the phenyl groups, were obtained in 42%
and 46% yields, respectively (entries 7 and 8), the reaction
was less efficient when conducted with 4-acetylphenyl
derivative 5i, which gave a mixture of silole 6i and diene
7i in favor of 7i (6i/7i = 45:55 by 1H NMR; entry 9). No
silole formation was observed with 2-pyridyl- and 2-thie-
nyl-substituted diynes.
MeO
OMe
MeO
OMe
[RhCl(nbd)]2 (5 mol%)
NBD (30 mol%)
+
2
1,4-dioxane
50 °C, 24 h
(3 equiv)
Ph
Ph
Si
Me2
Ph
Ph
5j
6j 44%
Scheme 3
reaction affords various fully substituted siloles from
readily available starting materials under mild conditions.
Table 3 Rhodium-Catalyzed Cyclization of Diynes 5a–i with 2a
MeO2C CO2Me
[RhCl(nbd)]2 (5 mol%)
NBD (30 mol%)
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
+
2
1,4-dioxane, 50 °C, 24 h
(3 equiv)
R
R
References and Notes
5
MeO2C CO2Me
MeO2C CO2Me
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+
R
R
R
R
Si
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H
Me
Me2
6
7
Entry 5 (R)
Yieldb (6 + 7), ratioc (6/7) 6 (Yieldd)
1
2
3
4
5
6
7
8
9
5a (Ph)
65%, 97:3
66%, 82:18
55%, 87:13
–, –
6a (49%)
6b (48%)
6c (30%)
6d (29%)
6e (52%)
6f (60%)
6g (42%)
6h (46%)
6i (–e)
5b (4-MeC6H4)
5c (4-t-BuC6H4)
5d (3,5-Me2C6H3)
5e (2-MeC6H4)
5f (1-naphthyl)
5g (4-MeOC6H4)
5h (4-BrC6H4)
5i (4-AcC6H4)
–, >99:<1
–, –
–, –
49%, 94:6
33%, 45:55
(5) For earlier studies on catalytic construction of silole
skeletons, see: (a) Okinoshima, H.; Yamamoto, K.;
Kumada, M. J. Am. Chem. Soc. 1972, 94, 9263.
a Reaction conditions: 1,6-diyne 5, 2 (3 equiv), [RhCl(nbd)]2 (5
mol%, 10 mol% Rh), and NBD (30 mol%) were heated in 1,4-dioxane
(0.1 M) at 50 °C for 24 h.
(b) Sakurai, H.; Kamiyama, Y.; Nakadaira, Y. J. Am. Chem.
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M. L.; Lim, T. F. O.; Duncan, D. P. J. Organomet. Chem.
1977, 135, C37. (d) Schäfer, A.; Weidenbruch, M.; Pohl, S.
J. Organomet. Chem. 1985, 282, 305. (e) Ikenaga, K.;
Hiramatsu, K.; Nasaka, N.; Matsumoto, S. J. Org. Chem.
1993, 58, 5045. (f) Ojima, I.; Fracchiolla, D. A.; Donovan,
R. J.; Banerji, P. J. Org. Chem. 1994, 59, 7594. (g) Palmer,
W. S.; Woerpel, K. A. Organometallics 1997, 16, 1097.
(6) For reviews, see: (a) Han, L.-B.; Tanaka, M. Chem.
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Rev. 2006, 106, 2320. (c) Suginome, M.; Matsuda, T.;
Ohmura, T.; Seki, A.; Murakami, M. In Comprehensive
Organometallic Chemistry III, Vol. 10; Crabtree, R.;
Mingos, M.; Ojima, I., Eds.; Elsevier: Oxford, 2007, 725.
(7) (a) Walter, C.; Auer, G.; Oestreich, M. Angew. Chem. Int.
Ed. 2006, 45, 5675. (b) Nakao, Y.; Chen, J.; Imanaka, H.;
Hiyama, T.; Ichikawa, Y.; Duan, W.-L.; Shintani, R.;
Hayashi, T. J. Am. Chem. Soc. 2007, 129, 9137. (c) Walter,
C.; Fröhlich, R.; Oestreich, M. Tetrahedron 2009, 65, 5513.
b Combined yield of 6 and 7 after preparative thin-layer chromatogra-
phy.
c Determined by 1H NMR.
d Isolated yield by recrystallization.
e Not isolated as a pure compound.
Diyne 5j tethered by a C(CH2OMe)2 group reacted simi-
larly with 2 to give silole 6j in 44% isolated yield by
recrystallization (66% yield after thin-layer chromatogra-
phy, 6j/7j = 84:16; Scheme 3). On the other hand, the re-
action of 1,6-diynes that were tethered by CH2, O, and
NTs groups was sluggish.12
In summary, we have developed a silole-forming cycliza-
tion reaction of alkynes and 1,6-diynes with hexamethyl-
disilane catalyzed by [RhCl(nbd)]2. This cyclization
Synlett 2011, No. 6, 813–816 © Thieme Stuttgart · New York

816
T. Matsuda et al.
LETTER
monoxide reportedly gave a silole via a mechanism
involving a Si–Me bond cleavage. See ref. 5f.
(12) When 1,6-enyne 8 was employed as the substrate, analogous
silylative cyclization occurred to afford bicyclic dihydro-
silole 9 in 44% isolated yield (Scheme 4).
(8) (a) Tobisu, M.; Kita, Y.; Chatani, N. J. Am. Chem. Soc.
2006, 128, 8152. (b) Tobisu, M.; Ano, Y.; Chatani, N.
Chem. Asian J. 2008, 3, 1585. (c) Tobisu, M.; Kita, Y.;
Ano, Y.; Chatani, N. J. Am. Chem. Soc. 2008, 130, 15982.
(d) Kita, Y.; Tobisu, M.; Chatani, N. Org. Lett. 2010, 12,
1864.
(9) The attempted detection of tetramethylsilane formation was
unsuccessful when the reaction was performed in C6D6 at 30
°C in a sealed NMR tube.
(10) For Si–Me bond cleavage observed in palladium-catalyzed
reactions, see: (a) Rauf, W.; Brown, J. M. Angew. Chem. Int.
Ed. 2008, 47, 4228. (b) Nakao, Y.; Takeda, M.; Matsumoto,
T.; Hiyama, T. Angew. Chem. Int. Ed. 2010, 49, 4447.
(11) The reaction of a diyne with t-BuMe2SiH catalyzed by a
rhodium(I) complex under an ambient pressure of carbon
Ts
N
Ts
N
[RhCl(nbd)]2 (5 mol%)
NBD (30 mol%)
+
2
1,4-dioxane
50 °C, 24 h
(3 equiv)
Ar
Si
Me2
Ar
8 (Ar = 4-MeC6H4)
9 44%
Scheme 4
Synlett 2011, No. 6, 813–816 © Thieme Stuttgart · New York

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