methyl(1-naphthyl)phenylchlorosilane 1 (>99% enantiomer
excess (ee))7,8 was carried out in the presence of various
metal oxides (ZnO, Ag2O, Li2O, CuO, PbO, and MnO2). The
absolute configuration of 2 was determined on the reduced
product by LiAlH4. The reduction was reported to proceed
with retention of configuration.4,9 Among the above metal
oxides, the highest stereoselectivity ((R,R):(R,S):(S,S) ) 55:
37:8 determined by HPLC analysis)10 was observed in ethyl
acetate at refluxing temperature with PbO (Scheme 1),
although the reaction itself was slow. However, it was noticed
that considerable racemization of (S)-1 occurred under the
reaction condition to result in no stereoselectivity in many
cases.
and purified by fractional recrystallization from pentane at
-78 °C. The de of (S)-6 was found to be >99% as
1
determined by H NMR and HPLC.10 The halogenating
cleavage reaction of (S)-6 was carried out at -64 °C in
CHCl3 to give (S)-5a14 or (S)-5b (Table 1). As expected,
the optical yield of (S)-5a increased from 84.5% de (92%
inversion) to 91.3% de (96% inversion) (entries 1 and 3)
and that of (S)-5b from 73.2% de (87% inversion) to 90.5%
de (95% inversion) (entries 2 and 4) by the introduction of
a methoxy substituent.
Table 1. Halogenating Cleavage Reaction of (S)-3 and (S)-6
Scheme 1
One of the typical ways to functionalize organosilicon
compounds is to introduce a reactive functional group such
as Br, Cl, and OTf by the cleavage of the silicon-naphthyl
bond. We reported that the stereoselectivity of the cleavage
of the silicon-naphthyl bond of (R)-(+)-[(+)-menthyloxy]-
methyl(1-naphthyl)phenylsilane (>99% ee) by bromine was
significantly influenced by the reaction conditions, and the
best result (84.5% diastereomer excess (de), 92% inversion)11
was obtained by using CHCl3 as the solvent at -64 °C. Thus,
the development of a more stereoselective cleavage reaction
of the silicon-naphthyl bond by bromine is required to
prepare optically pure functionalized disiloxanes. However,
it was very difficult to carry out such reactions with high
stereoselectivity.12
It was reported that the brominating cleavage reaction of
the silicon-naphthyl bond proceeded with preference in
inversion of configuration of the silicon atom (Scheme 2).13
If the naphthyl group is modified so as to stabilize the
trigonal bipyramidal transition state 4, higher stereoselectivity
could be expected.
Thus, optically active (S,S)-1,3-di(4-methoxy-1-naphthyl)-
1,3-dimethyl-1,3-diphenyldisiloxane seems to be a suitable
starting material to functionalized disiloxanes. However, (S)-
(4-methoxy-1-naphthyl)methylphenylchlorosilane could not
be obtained by chlorination of easily accessible (R)-(4-
methoxy-1-naphthyl)methylphenylsilane15 because of some
side reactions. Therefore, (1S,3S)-1-(4-methoxy-1-naphthyl)-
1,3-dimethyl-3-(1-naphthyl)-1,3-diphenyldisiloxane 8, which
could be obtained similarly with compound 2 according to
the ref 5, was used as the starting material (Scheme 3). The
stereoisomer ratio and absolute configuration of (1S,3S)-8
were determined similarly with (R,R)-2. Since the ee10 of
(R)-(4-methoxy-1-naphthyl)methylphenylsilane 9 and (R)-
methyl(1-naphthyl)phenylsilane 10 were >99% and 90.4%,
respectively, the stereoisomer ratio of (1S,3S)-8 could be
determined as (1S,3S):(1S,3R):(1R,3S):(1R,3R) ) 95:5:0:0.
(9) Sommer, L. H.; Frye, C. L.; Parker, G. A. J. Am. Chem. Soc. 1964,
86, 3276
(10) HPLC analysis: Daicel Chem. Ind., CHIRALCEL OD, n-hexane
as an eluent, 0.4 mL/min, 254 nm.
(11) Kawakami, Y.; Takahashi, T.; Yada, Y.; Imae, I. Polym. J. 1998,
30, 1001.
Scheme 2
(12) Sommer, L. H.; Michael, K. W.; Korte, W. D. J. Am. Chem. Soc.
1967, 89, 868.
(13) Earbon, C.; Steward, O. W. Proc. Chem. Soc. 1963, 59.
(14) To a mixture of (S)-6 (0.4328 g, 1.0 mmol) in CHCl3 (8.0 mL) was
added a solution of bromine in CHCl3 (0.5 mol/L, 2.0 mL, 1.0 mmol)
dropwise during 30 min at -64 °C. The reaction mixture was stirred at the
same temperature for 45 min. The de of (S)-5a was determined to be 91.3%
by the integral ratio of the signals at 0.62 ppm and 0.79 ppm in 1H NMR.
(15) 1H NMR (300 MHz, CDCl3) δ 0.64 (d, J)3.8 Hz, 3H), 3.92 (s,
3H), 5.21 (q, J)3.8 Hz, 1H), 6.73 (d, J)7.7 Hz, 1H), 7.23-7.38 (m, 2H),
7.39-7.40 (m, 2H), 7.46-7.49 (m, 2H), 7.56 (d, J)7.7 Hz, 1H), 7.89 (d,
J)8.7 Hz, 1H), 8.23 (d, J)7.5 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ
-4.45, 55.4, 103.6, 122.7, 124.1, 125.2, 125.9, 126.1, 126.8, 127.9, 128.1,
129.5, 135.0, 136.1, 138.3, 157.7; IR (KBr, cm-1) 3067, 3049, 3014, 2958,
2935, 2124, 1586, 1417, 1130, 1088; EI-MS (m/e) 278 (M+), 263 ([M -
Me]+), 247 ([M - OMe]+), 185 ([M - Me - Ph]+), 121 ([M - NpOMe]+).
An optically active silicon compound having a methoxy
group as an electron-donating group at the 4-position of the
naphthyl group, (S)-(-)-[(-)-menthyloxy](4-methoxy-1-
naphthyl)methylphenylsilane 6, was designed, synthesized,
550
Org. Lett., Vol. 1, No. 4, 1999