Angewandte
Chemie
DOI: 10.1002/anie.201409733
Asymmetric Catalysis
Rhodium-Catalyzed Asymmetric Synthesis of Silicon-Stereogenic
Dibenzosiloles by Enantioselective [2+2+2] Cycloaddition**
Ryo Shintani,* Chihiro Takagi, Tomoaki Ito, Masanobu Naito, and Kyoko Nozaki*
Abstract:
silicon-stereogenic dibenzosiloles has been developed through
[2+2+2] cycloaddition of silicon-containing prochiral
triynes with internal alkynes. High yields and enantioselectiv-
ities have been achieved by employing an axially chiral
monophosphine ligand, and the present catalysis is also
applicable to the asymmetric synthesis of a germanium-
stereogenic dibenzogermole. Preliminary studies on the optical
properties of these compounds are also described.
A
rhodium-catalyzed asymmetric synthesis of
transition-metal-catalyzed enantioselective synthesis of
dibenzosiloles.[7a,b] In this context, herein we describe the
first intermolecular asymmetric synthesis of silicon-stereo-
genic dibenzosiloles through a rhodium-catalyzed [2+2+2]
cycloaddition of silicon-containing prochiral triynes with
internal alkynes.[12,13]
Initially, we employed prochiral triyne 1a, which has two
identical alkynyl groups on the silicon atom, as a model
substrate for the rhodium-catalyzed [2+2+2] cycloaddition
with 1,4-dimethoxy-2-butyne (2a) in the presence of (R)-
Binap[14] as the ligand (Table 1, entry 1). The reaction
a
D
ibenzosiloles are widely found as a structural motif in
various useful materials,[1] such as light-emitting diodes,[2]
thin-film transistors,[3] solar cells,[4] and detectors for explo-
sives,[5] because of their optoelectronic properties derived
from the p-conjugated system. Several efficient synthetic
methods for dibenzosiloles using transition-metal catalysts
have therefore been actively developed in recent years.[6] In
contrast, the preparation of enantioenriched chiral dibenzo-
siloles has been scarcely investigated[7] despite their potential
future applications, for example as materials for circularly
polarized luminescence (CPL).[8] This lack of available
methods is presumably due to the planar nature of these
compounds except at the silicon atom. However, the intro-
duction of chirality at the silicon center, particularly in
a catalytic asymmetric manner, is not a trivial task compared
with the construction of carbon stereocenters.[9–11] In fact, only
two intramolecular approaches have been reported for the
Table 1: The effect of the ligand in the rhodium-catalyzed asymmetric
[2+2+2] cycloaddition of 1a with 2a.
Entry
Ligand
Yield [%][a]
ee[b]
1
2
3
4
5
(R)-Binap
74
83
95
63
98
18
9
73
80
88
(R)-H8-Binap
(R)-Segphos
(R)-MeO-Mop
(R)-L
[a] Yield of isolated product. [b] Determined by chiral HPLC on
a Chiralpak IF-3 column with hexane/2-propanol=500:1 v/v.
[*] Dr. R. Shintani, C. Takagi, Prof. Dr. K. Nozaki
Department of Chemistry and Biotechnology
Graduate School of Engineering, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: shintani@chembio.t.u-tokyo.ac.jp
T. Ito
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo, Kyoto 606-8502 (Japan)
Dr. M. Naito
National Institute of Materials Science
1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan)
[**] Support has been provided in part by a Grant-in-Aid for Young
Scientists (B), the Ministry of Education, Culture, Sports, Science,
and Technology (Japan) and in part by the Asahi Glass Foundation.
We thank Prof. Takuzo Aida at the University of Tokyo for the
measurement of fluorescence and CD spectra. We thank Prof.
Yoshiaki Nishibayashi and Dr. Kazunari Nakajima at the University
of Tokyo for the measurement of optical rotations. We thank
Masahiro Miyamoto at Daicel Corporation for the help of the chiral
HPLC analysis for compound 3ac. We thank Dr. Shingo Ito and Dr.
Shuhei Kusumoto for X-ray crystallographic analysis.
proceeded at 258C to give the desired silicon-stereogenic
dibenzosilole 3aa in 74% yield, but the enantiomeric excess
was only 18%.[15] The change of ligand to (R)-H8-Binap[16]
resulted in even lower enantioselectivity (9% ee; entry 2),
whereas the use of (R)-Segphos[17] improved the enantio-
selectivity to 73% ee (Table 1, entry 3). Compared to these
axially chiral bisphosphine ligands that are typically used for
rhodium-catalyzed asymmetric [2+2+2] cycloaddition reac-
tions,[12] (R)-MeO-mop,[18] an axially chiral monophosphine
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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