COMMUNICATION
DOI: 10.1002/chem.200901048
Highly Enantioselective Epoxidation of cis-Alkenylsilanes
Kazuhiro Matsumoto, Takuya Kubo, and Tsutomu Katsuki*[a]
Since the development of the titanium/tartrate-catalyzed
asymmetric epoxidation of cis-alkenylsilanes, in which the
cis-epoxysilanes are obtained with complete enantioelectivi-
ty using 0.5–2 mol% of the catalyst.
asymmetric epoxidation of allyl alcohols in 1980, significant
progress has been made in the field of asymmetric epoxida-
tion of olefins, and now most olefins can be transformed to
epoxides with high optical purity.[1–3] However, there are no
satisfactory methods available thus far for the asymmetric
epoxidation of terminal olefins such as styrene.[4] While ep-
oxysilanes are a synthetic equivalent of epoxides, a highly
enantioselective epoxidation of simple alkenylsilanes is also
quite rare in the literature.[5,6] Although Shi and co-workers
applied their sugar-derived ketone catalyst to the epoxida-
tion of 2,2-disubstituted alkenylsilanes and obtained the ep-
oxysilanes with high enantioselectivity up to 94% ee, the
method has a major drawback that it requires a substoichio-
metric amount of the catalyst.[7,8] While the introduction of
a sterically more demanding silyl group at the vinyl position
is expected to further facilitate the enantioface-differentia-
tion, there is a concern that the steric hindrance strongly in-
hibits the reaction progress.[5] Thus, in order to implement
the catalytic asymmetric epoxidation of alkenylsilanes, the
utilization of a more refined epoxidation catalyst in terms of
catalytic activity, asymmetric induction and durability is nec-
essary.
Figure 1. Di-m-oxo-TiACTHNUTRGNEUGN(salalen) complex 1.
cis-Alkenylsilanes were readily prepared by simple reduc-
tion of the corresponding alkynylsilanes with diisobutylalu-
minum hydride.[10] We first examined the scope of alkenylsi-
lanes in the presence of TiACTHNUTRGENU(GN salalen) 1 (Table 1). The complex
1 effectively promoted the epoxidation of cis-alkenylsilanes
bearing aromatic groups. Silicon substituents have little
effect on the enantioselectivity (entries 1 and 2). The reac-
tions furnished both the trimethylsilyl- and dimethylphenyl-
silyl epoxides with ee values of >99%. Substituted alkenyl-
silanes with electron-donating methoxy and electron-with-
drawing bromide groups on the aromatic ring also under-
went the epoxidation with complete enantioselectivity (en-
tries 3–7). However, the substitution at the ortho-position
decreased the reaction rate (entries 3 and 5). The reaction
with methoxy group at this position required a longer reac-
tion time, and an ortho-bromide group significantly impeded
the reaction progress. Alkenylsilanes with other groups, such
as biphenyl and naphthyl groups, were also good substrates
(entries 8–10). Noteworthy the epoxysilanes were effectively
desilylated by TBAF to give the corresponding styrene
oxide derivatives without erosion of the enantioselectivity.[11]
We have reported that the di-m-oxo-TiACTHNUTRGNEUNG(salalen) complex 1
(Figure 1) is an effective catalyst for the asymmetric epoxi-
dation of olefins with aqueous hydrogen peroxide as the oxi-
dant.[9] Not only conjugate olefins but also non-conjugate
olefins, which still are the most challenging substrate for
asymmetric epoxidation with regard to both reactivity and
enantioselectivity, underwent the epoxidation to give the ep-
oxides with high enantioselectivity. Thus, we expected that
TiACHTUNGTRENNUNG(salalen) 1 would best meet the requirements for the ep-
oxidation of alkenylsilanes. Herein, we report the catalytic
[a] Dr. K. Matsumoto, T. Kubo, Prof. T. Katsuki
Department of Chemistry, Faculty of Science
Graduate School, Kyushu University, Hakozaki
Higashi-ku, Fukuoka 812-8581 (Japan)
Fax : (+81)92-642-2607
A combination of Ti
effective for the epoxidation.[12] In the reaction of
dimethylphenyl(styryl)silane, the epoxysilane was obtained
ACHTUNGERTN(NUNG OiPr)4 and salan ligand 2 was also
AHCTUNGTRENNUNG
Supporting information for this article is available on the WWW
in 59% with >99% ee (Scheme 1). Although the ee value is
Chem. Eur. J. 2009, 15, 6573 – 6575
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6573