because of the faster generation of a reactive silicon
intermediate. The cyclization of 1 (R ) Ph) bearing a variety
of R′ substituents on the alkyne moiety (entries 5-12)
showed that the reaction is very general. A sterically
demanding 1-naphthyl group did not hinder the reaction at
all to give 2d in quantitative yield (entry 5). Both electron-
poor and electron-rich substrates reacted well, as exemplified
by the synthesis of 4-CF3- and 4-MeO-phenylbenzosiloles
2e and 2f in excellent yields (entries 6 and 7). Benzosiloles
bearing a 2-pyridyl (2g) and a 2-thienyl (2h) group could
also be synthesized (entries 8 and 9). An enyne substrate
reacted with retention of the double-bond geometry to give
a conjugated benzosilole 2i in excellent yield (entry 10). The
substrate possessing a trimethylsilyl group also cyclized to
obtain 2j in excellent yield (entry 11), whereas a substrate
possessing an aliphatic group gave 2k in moderate yield,
together with recovered starting material (entry 12).
The 2-substituted benzosiloles can be further functional-
ized. For example, 2-trimethylsilylbenzosilole (2j) was
converted first to a 2-bromobenzosilole (3) and then coupled
with 3-zincioindole1b in the presence of Pd(0) catalyst12 to
obtain a mixed heterocyclic compound 4 in high yield (eq
2).
Scheme 1
.
Cyclization of (2-Alkynylphenyl)silane 1 to
2-Substituted Benzosilole 2
A variety of conditions that would generate a dimethylsilyl
anion from 1 (R ) Me) failed to give 2 in a synthetically
significant yield.8 Following a report by Corriu, who sug-
gested the formation of the desired silyl anion by the reaction
of a hydrosilane with KH,9 we found that 0.2-1.5 equiv of
KH in 1,2-dimethoxyethane (DME) promotes formation of
2a in up to 70% yield (Table 1, entries 1 and 2). Consistently
Table 1. KH-Promoted Cyclization of (2-Alkynylphenyl)silane 1
to 2-Substituted Benzosilole 2a
An Fe-catalyzed C-H bond activation reaction that we
recently reported13 can introduce an aryl group in the C3
position of the 2-pyridylbenzosilole (2g, eq 3). Thus, 2g was
coupled with a phenylzinc reagent in the presence of
Fe(acac)3 (20 mol %), 1,10-phenanthroline (20 mol %), and
1,2-dichloroisobutane (2.0 equiv) to obtain the expected
2-phenyl-3-pyridylbenzosilole (5) in excellent yield.14
entry
R′
R
2
conditions
yieldb (%)
1
2
3
4
5
6
7
8
phenyl
phenyl
4-biphenyl
phenyl
1-naphthyl
4-CF3C6H4
4-MeOC6H4
2-pyridyl
2-thienyl
(E)-styryl
SiMe3
Me
Me
Me
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
2a
2a
2b
2c
2d
2e
2f
2g
2h
2i
25 °C, 3 h
25 °C, 4 hc
25 °C, 3 h
0 °C, 0.5 h
0 °C, 2 h
0 °C, 0.5 h
25 °C, 1 h
0 °C, 1 h
70
61d
71
90
99
93
97
81
60
92
98
46d
9
0 °C, 0.5 h
0 °C, 1 h
0 °C, 2 h
10
11
12
2j
2k
butyl
25 °C, 16 h
a Reaction of 1 with ca. 1 equiv of KH in DME under the indicated
conditions. See the Supporting Information for details. b Isolated yield. c 0.2
equiv of KH was used. d Determined by 1H NMR in the presence of 1,1,2,2-
tetrachloroethane as an internal standard.
high yields were obtained when reagent-grade DME without
further purification was used as a solvent, or when the
reaction was carried out in the open air or in the presence of
excess or a subequimolar amount of KH.10 However, ethereal
solvents other than DME dramatically decreased the yield.11
The reaction of the diphenylsilyl compound 1 (R ) Ph)
was much faster and higher yielding (entry 4), presumably
The crystal structure of 2b (Figure 1) indicated that the
2-substituent is tilted out of the benzosilole plane by 39.4(2)°,
(7) (a) Yu, G.; Yin, S.; Liu, Y.; Chen, J.; Xu, X.; Sun, X.; Ma, D.;
Zhan, X.; Peng, Q.; Shuai, Z.; Tang, B.; Zhu, D.; Fang, W.; Luo, Y. J. Am.
Chem. Soc. 2005, 127, 6335–6346. (c) Lee, S. H.; Jang, B.-B.; Kafafi, Z. H.
J. Am. Chem. Soc. 2005, 127, 9071–9078. (d) Shimizu, M.; Tatsumi, H.;
Mochida, K.; Oda, K.; Hiyama, T. Chem. Asian J. 2008, 3, 1238–1247.
(8) Organic bases such as n-BuLi, t-BuLi, Et2Zn, and t-BuOK did not
effect the cyclization of 1a.
(5) (a) Dubac, J.; Laporterie, A.; Manuel, G. Chem. ReV. 1990, 90, 215–
263, and references cited therein. (b) Xu, C.; Wakamiya, A.; Yamaguchi,
S. Org. Lett. 2004, 6, 3707–3710. (c) Sudo, T.; Asao, N.; Yamamoto, Y. J.
Org. Chem. 2000, 65, 8919–8923. (d) Ma¨rkl, G.; Berr, H.-P. Tetrahedron
Lett. 1992, 33, 1601–1604. (e) Kunai, A.; Yuzuriha, Y.; Naka, A.; Ishikawa,
M. J. Organomet. Chem. 1993, 455, 77–81. (f) Matsuda, T.; Kadowaki, S.;
Yamaguchi, Y.; Murakami, M. Chem. Commun. 2008, 2744–2746. (g)
Matsuda, T.; Yamaguchi, Y.; Murakami, M. Synlett 2008, 561–564. (h)
Tobisu, M.; Onoe, M.; Kita, Y.; Chatani, N. J. Am. Chem. Soc. 2009, 131,
7506–7507.
(9) (a) Corriu, R. J. P.; Gue´rin, C. J. Chem. Soc., Chem. Commun. 1980,
168–169. (b) Corriu, R. J. P.; Gue´rin, C.; Kolani, B. Bull. Soc. Chim. Fr.
1985, 5, 973–979.
(10) Pretreatment of KH to remove impurities was not necessary.
(11) THF: 25% yield. Et2O: 0% yield.
(12) Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 2719–2724.
(13) Norinder, J.; Matsumoto, A.; Yoshikai, N.; Nakamura, E. J. Am.
Chem. Soc. 2008, 130, 5858–5859.
(6) (a) Tamao, K.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Yamagu-
chi, S. J. Am. Chem. Soc. 1996, 118, 11974–11975. (b) Murata, H.;
Malliaras, G. G.; Uchida, M.; Shen, Y.; Kafafi, Z. H. Chem. Phys. Lett.
2001, 339, 161–166.
(14) Compound 5 was slightly sensitive to silica gel chromatography,
leading to lower isolated yield.
Org. Lett., Vol. 11, No. 17, 2009
3967