Asymmetric ring opening of meso-epoxides catalyzed by the chiral phosphine oxide BINAPO

Article abstract of DOI:10.1016/j.tetasy.2005.06.021

The enantioselective ring opening of meso-epoxides with tetrachlorosilane in the presence of diisopropylethylamine using chiral phosphine oxide BINAPO as the catalyst affords the corresponding chlorohydrins in high enantioselectivities of up to 90% ee.

Full text of DOI:10.1016/j.tetasy.2005.06.021

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 2391–2392
Asymmetric ring opening of meso-epoxides catalyzed by the
chiral phosphine oxide BINAPO
a
Eisuke Tokuoka, Shunsuke Kotani, Hirofumi Matsunaga, Tadao Ishizuka,
a
a
a
b
Shunichi Hashimoto and Makoto Nakajima
a,
*
a
Faculty of Medical and Pharmaceutical Sciences, Kumamoto University, Kumamoto 862-0973, Japan
b
Graduate School of Pharmaceutical Sciences, Hokkaido University, Sapporo 060-0812, Japan
Received 19 May 2005; accepted 21 June 2005
Abstract—The enantioselective ring opening of meso-epoxides with tetrachlorosilane in the presence of diisopropylethylamine using
chiral phosphine oxide BINAPO as the catalyst affords the corresponding chlorohydrins in high enantioselectivities of up to 90% ee.
ꢀ
2005 Elsevier Ltd. All rights reserved.
1. Introduction
catalysts for the ring opening of epoxides with
tetrachlorosilane.
Metal-free catalytic reactions, organocatalysis, have
recently attracted considerable attention because of their
1
environmentally benign nature. Although phosphine
oxides have strong electron-donor properties and form
complexes with various acceptors, less attention has
3
been paid to chiral phosphine oxides in the field of
2
. Results and discussion
2
We initially investigated the ring opening of meso-stil-
bene oxide with a solution of tetrachlorosilane in dichlo-
romethane using 10 mol % BINAPO at ꢀ78 ꢁC. The
reaction proceeded smoothly to afford the correspond-
ing chlorohydrin in high yield. However, the enantio-
selectivity of the reaction was not satisfactory (95%
yield, 59% ee). After considerable screening of the reac-
tion conditions, we found that the addition of diiso-
asymmetric synthesis except for their utility as synthetic
intermediates in the preparation of chiral phosphine
ligands. Quite recently, Kobayashi et al. reported on
the enantioselective allylation of imines promoted by
the chiral phosphine oxide BINAPO, which afforded
the corresponding homoallyl amines in high enantiose-
4
lectivity. We reported on the enantioselective allylation
of aldehydes promoted by substoichiometric amounts of
BINAPO, which represented the first example of the use
propylethylamine caused
a
dramatic increase in
1
1
enantioselectivity (94% yield, 90% ee). Sterically less
hindered amines gave products in less selectivity (trieth-
ylamine: 82% ee, pyridine: 11% ee), which may be due to
the coordination of nitrogen to silicon to provide a race-
5
of a chiral phosphine oxide as a catalyst. As part of our
ongoing program directed at the development of
6
asymmetric organocatalysis, we herein report on an
enantioselective ring opening reaction of meso-epoxides
catalyzed by BINAPO.
1
2–15
mic product in part.
Some selected data for the asymmetric ring opening of
epoxides are summarized in Table 1. Although the epox-
ides derived from butenediol, pyrroline and dihydro-
furan gave low selectivities (entries 2–4), epoxides
derived from six- and eight-membered cycloalkenes gave
moderate enantioselectivities (entries 5 and 6), the high-
est selectivities reported for the asymmetric synthesis of
cyclic chlorohydrins. The present reaction represents the
first example of the use of a chiral phosphine oxide as a
catalyst in the asymmetric opening of epoxides.
7
,8
The asymmetric ring opening of meso-epoxides is a
versatile method for preparing optically active chloro-
hydrins. Among the various catalysts reported for this
9
type of reaction, Lewis bases such as phosphoramides
d,10
6
and N-oxides
have been reported to be effective
*
Corresponding author. Tel.: +81 963714680; fax: +81 963627692;
e-mail: nakajima@gpo.kumamoto-u.ac.jp
0
957-4166/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2005.06.021

2392
E. Tokuoka et al. / Tetrahedron: Asymmetry 16 (2005) 2391–2392
Table 1. Asymmetric ring opening of meso-epoxide catalyzed by chiral phosphine oxide (S)-BINAPO
a
Yield (%)
b
ee (%) (confgn)
c
25
½aꢁD
+19.7 (c 1.0, EtOH)
Entry
Epoxide
R = Ph
Time (h)
1
2
3
4
12
1
94
97
95
73
81
81
90 (S,S)
31 (S,S)
34
R = CH
2
OCH
2
Ph
NTsCH
OCH
+1.0 (c 1.0, EtOH)
R, R = –CH
R, R = –CH
2
2
2
–
+1.2 (c 1.0, CHCl
+43.3 (c 1.3, CHCl
+67.3 (c 1.0, CHCl
+42.9 (c 1.0, CHCl
3
)
3
)
3
)
3
)
d
4
2
–
1
39
d
5
R, R = –(CH
R, R = –(CH
2
)
)
4
–
–
1
72
71
50
d
6
2
6
a
b
c
Isolated yields.
Determined by chiral HPLC analysis.
9
Assigned by comparison to the literature values of optical rotations.
Isolated as the 4-nitrobenzoate.
d
In summary, the effectiveness of BINAPO as a catalyst
for the enantioselective ring opening of meso-epoxide
with tetrachlorosilane has been demonstrated. Studies
on the design of chiral phosphine oxides to further
enhance enantioselectivity are currently in progress.
Leighton, J. L.; Carsten, D. H.; Jacobsen, E. N. J. Am.
Chem. Soc. 1995, 117, 5897–5898; (c) Cole, B. M.;
Shimizu, K. D.; Krueger, C. A.; Harrity, J. P. A.;
Snapper, M. L.; Hoveyda, A. H. Angew. Chem. Int. Ed.
Engl. 1996, 35, 1668–1671; (d) Iida, T.; Yamamoto, N.;
Sasai, H.; Shibasaki, M. J. Am. Chem. Soc. 1997, 119,
4
dron Lett. 1998, 39, 9023–9026.
. Denmark, S. E.; Barsanti, P. A.; Wong, K.-T.; Stavenger,
R. A. J. Org. Chem. 1998, 63, 2428–2429.
783–4784; (e) Oguni, N.; Miyagi, Y.; Itoh, K. Tetrahe-
Acknowledgements
9
This work was partly supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Sci-
ence, Sports, and Culture of Japan and the SUNBOR
GRANT.
1
0. Tao, B.; Lo, M. M.-C.; Fu, G. C. J. Am. Chem. Soc. 2001,
23, 353–354.
1
1
1. Typical procedure: To a stirred solution of (S)-BINAPO
(16.6 mg, 10 mol %), cis-stilbene oxide (50 mg) and diiso-
propylethylamine (0.065 mL, 1.5 equiv) in dichlorometh-
ane (1 mL) was added l M tetrachlorosilane in
dichloromethane (0.38 mL, 1.5 equiv) at ꢀ78 ꢁC under
an argon atmosphere. The mixture was stirred at the same
temperature for 4 h. After quenching the reaction with
aqueous satd sodium bicarbonate (2 mL), the aqueous
layer was treated with aqueous potassium fluoride/potas-
sium dihydrogenphosphate and extracted with ethyl ace-
tate (30 mL · 3). The organic layer was washed with brine
(20 mL), dried over sodium sulfate and concentrated. The
crude material was purified by column chromatography
(silica gel, 5 g, hexane–AcOEt = 10:1) to give 2-chloro-1,2-
diphenylethanol (56 mg, 94%). HPLC (AD-H, hexane–2-
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achieving optimum chemical and optical yields. It is
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hydrogen chloride produced through the adventitious
hydrolysis of tetrachlorosilane which reacts directly with
the epoxide to form a chlorohydrin in a nonstereoselective
process.
3
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gave the racemic chlorohydrin in 84% yield, while chlo-
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(propionitrile: 83% yield, 74% ee; toluene: 70% yield, 25%
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BINAPO itself (p-Tol-BINAPO: 96% yield, 82% ee;
m-Xyl-BINAPO: 98% yield, 30% ee).