Asymmetric Catalysis, 118
FULL PAPER
(l ϭ 25 m, ⊘ ϭ 0.25 mm), integrator: Varian 4290, retention
times (160°C): 8.6Ϫ8.8 min [(R)-1-(2,5-dimethoxyphenyl)ethanol]
and 9.2Ϫ9.4 min [(S)-1-(2,5-dimethoxyphenyl)ethanol].
All these results show that the inhibition of the catalyst
by CCl4 is only possible during its formation. Once the
catalyst is formed, CCl4 cannot inhibit its activity. Instead,
it leads to a lower amount of silylenol ether and a higher
enantioselectivity compared to the hydrosilylation without
CCl4 (Table 3). The ЉinertnessЉ of the prepared Rh(I)-com-
plex 12 towards CCl4 was also proven by recording 1H
NMR spectra of 12 in CDCl3 before and after adding 25
vol% of CCl4. Even 24 h after the addition no changes of
the original spectrum were evident.
Two other Rh(I)-complexes 13 and 14 (Scheme 3) were
tested in the hydrosilylation of acetophenone (Table 3). Al-
though they only differ in the nature of their anion, the
BF4-complex 13 gave a much higher optical yield than the
PF6-complex 14. The enantioselectivities of these complexes
(Rh/ligand ratio: 1:1) are quite low compared to the in situ
catalysts (Rh/ligand ratio: 1:5). In line with this, addition
of a fourfold excess of 7e to 14 resulted in similar chemical
and optical yields as the in situ catalyst of [Rh(cod)Cl]2 and
7e (Tables 3 and 2).
1-(2,5-Dimethoxyphenyl)ethyl Diphenylsilyl Ether (2b). Ϫ 1H
3
NMR (250 MHz, CDCl3): δ ϭ 1.45 (d, J ϭ 6.2 Hz, 3 H, CH3),
3
3.65 (s, 3 H, OCH3), 3.73 (s, 3 H, OCH3Ј), 5.36 (q, J ϭ 6.2 Hz, 1
H, CHCH3), 5.43 (s, 1 H, SiH), 6.71 / 6.72 / 7.19 (3 m, 3 H, H-3,
H-4, H-6), 7.29Ϫ7.45 (m, 6 H, Ph-H), 7.56Ϫ7.65 (m, 4 H, Ph-H).
1-(2,5-Dimethoxyphenyl)ethenyl Diphenylsilyl Ether (3b)[18]. Ϫ
1H NMR (250 MHz, CDCl3): δ ϭ 3.69 (s, 3 H, OCH3), 3.72 (s, 3
H, OCH3Ј), 4.77 (d, 2 J ϭ 1.2 Hz, 1 H, CϭCHHЈ), 5.10 (d, 2 J ϭ
1.2 Hz, 1 H, CϭCHHЈ), 5.64 (s, 1 H, SiH), 6.79Ϫ6.81 (m, 2 H,
Ar-H), 7.15 (m, 1 H, Ar-H), 7.27Ϫ7.49 and 7.52Ϫ7.71 (m, 10 H,
Ph-H).
rac-1-(2,5-Dimethoxyphenyl)ethanol (4b): 2,5-Dimethoxyaceto-
phenone (1.5 ml, 9.5 mmol) and KBH4 (800 mg, 14.8 mmol) in dry
methanol (10 ml) were stirred for 15 h. After acidifying with 2
HCl (10 ml) the solution was extracted with ether (3ϫ 10 ml). The
combined organic layers were washed with water (10 ml) and brine
(10 ml), and dried over MgSO4. Evaporation of the solvent yielded
1.73 g (100%) of the oily alcohol 4b. Ϫ 1H NMR (250 MHz,
CDCl3): δ ϭ 1.48 (d, 3J ϭ 6.4 Hz, 3 H, CH3), 2.76 (d, 3J ϭ 4.8
Hz, 1 H, OH), 3.76 (s, 3 H, OCH3), 3.81 (s, 3 H, OCH3Ј), 5.05 (dq,
3J ϭ 6.4 Hz, 3J ϭ 4.8 Hz, 1 H, CH3CHOH), 6.74 (dd, 3J ϭ 8.7
We thank the Fonds der Chemischen Industrie for providing a
´
Kekule scholarship for R.S. and for financial support.
4
3
5
Hz, J ϭ 2.8 Hz, 1 H, H-4), 6.79 (dd, J ϭ 8.7 Hz, J ϭ 0.8 Hz, 1
H, H-3), 6.94 (dd, 4J ϭ 2.8 Hz, 5J ϭ 0.8 Hz, 1 H, H-6). Ϫ 13C
NMR (62.9 MHz, CDCl3): δ ϭ 23.1 (CH3), 55.8 and 55.9 (OCH3),
Experimental Section
Chromatography: Merck silica gel 60 (63Ϫ200 mesh). Ϫ Elemen-
tal analysis: Microanalytical Laboratory, University of Regensburg. 66.4 (CHOH), 111.6 / 112.4 / 112.5 (C-3, C-4, C-6), 134.9 (C-1),
Ϫ H / 13C NMR: Bruker AW-80 (80 MHz, T ϭ 31°C) and AC-
250 (250 / 62.9 MHz, T ϭ 24°C), TMS as internal standard. Ϫ 167 (95) [Mϩ Ϫ CH3], 152 (24), 139 (88), 137 (32), 124 (33), 43
MS: Finnigan MAT 311 A (EI, 70 eV). Ϫ IR: Beckman IR 4240
(29). Ϫ IR (film): ν (cmϪ1) ϭ 3400 s (OϪH), 2960/2945 s (CϪH),
150.7 (C-2), 154.0 (C-5). Ϫ MS (EI): m/z (%) ϭ 182 (100) [Mϩ],
1
˜
(film between NaCl plates), only characteristic bands are listed. Ϫ 2830 s (OCϪH), 1275/1210/1070/1045/1020 s (CϪO). Ϫ C10H14O3
UV/Vis: Kontron Instruments Spectrophotometer UVIKON 922. (182.22): calcd. C 65.91, H 7.74; found C 65.64, H 7.81.
Ϫ All liquids were distilled and stored under argon. Ϫ η4-1,5-
Cyclooctadiene ϭ cod.
Asymmetric Hydrosilylation of Acetophenone: 10 mg (0.02 mmol)
of [Rh(cod)Cl]2 (0.04 mmol Rh) and ligand (0.2 mmol, if not other-
wise stated) were dissolved in acetophenone (1.0 ml, 8.5 mmol)
under argon. Usually, 2.0 ml of CCl4 were added and the solution
was stirred at r. t. for 30 min. After cooling to 0°C for at least 30
min diphenylsilane (1.6 ml, 8.6 mmol) was added and stirring in the
ice bath, which was warming-up, continued for the period quoted.
Asymmetric Hydrosilylation of 2,5-Dimethoxyacetophenone: 8 mg
(0.016 mmol) of [Rh(cod)Cl]2 (0.032 mmol Rh) and ligand (0.16
mmol, if not otherwise stated) were dissolved in 2,5-dimethoxyace-
tophenone (1.1 ml, 7.0 mmol) under argon. Eventually, 1.7 ml of
CCl4 were added and the solution was stirred at r. t. for 30 min.
After cooling to 0°C for at least 30 min diphenylsilane (1.3 ml, 7.0
mmol) was added and stirring in the ice bath, which was warming-
up, continued for the period quoted.
To determine the amount of silylenol ether, the degree of hydrosi-
lylation and the chemical yield, a sample was taken and a 1H NMR
(CDCl3, 80 MHz) recorded. The following integrals were used: δ ϭ
5.70 ppm (s, SiH, silylenol ether IE), δ ϭ 5.40 (s, SiH, silylalkyl
ether IA), and δ ϭ 2.50 (s, CH3, acetophenone IAP). Calculations:
To determine the amount of silylenol ether, the degree of hydrosi-
lylation and the chemical yield, a sample was taken and a 1H NMR
spectrum (CDCl3, 80 MHz) recorded. The following integrals were
used: δ ϭ 5.43 (s, SiH) and 5.36 (q, CH) together (silylalkyl ether
IA), δ ϭ 5.10 and 4.77 (2 d, CH2) together (silylenol ether IE), and
δ ϭ 2.60 (s, CH3, 2,5-dimethoxyacetophenone IDMAP). Calcu-
lations: silylenol ether [%] ϭ [IE /(IA ϩ IE)]·100, conversion [%] ϭ
[(3 IE ϩ 3 IA)/(3 IA ϩ 3 IE ϩ 2 IDMAP)]·100, and chemical yield
[%] ϭ [3IA/(3IA ϩ 3IE ϩ 2IDMAP) ]·100.
silylenol ether [%] ϭ [IE /(IA ϩ IE)]·100, conversion [%] ϭ [(3 IE
ϩ
3 IA)/(3 IA ϩ 3 IE ϩ IAP)]·100, and chemical yield [%] ϭ [3 IA/(3
IA ϩ 3 IE ϩ IAP)]·100.
Hydrolysis was performed by adding methanol (10 ml) and a few
crystals of p-TosOH. After stirring at r. t. for 30 min the solvents
were evaporated and the residue was distilled in a kugelrohr appa-
ratus at 100Ϫ120°C / ca. 1 Torr. The enantiomeric excess was deter-
mined by injecting 0.4 µl of a solution of the distillate (3Ϫ4 drops)
in 1 ml of CH2Cl2 (Merck Uvasol ) into a Fisons 8130 gas chroma-
tograph. Column: Chrompack Chirasil-DEX CB (l ϭ 25 m, &
osol; ϭ 0.25 mm), integrator: Varian 4290, retention times (118°C):
7.3Ϫ7.7 min [(R)-1-phenylethanol] and 8.0Ϫ8.3 min [(S)-1-phenyl-
ethanol].
Hydrolysis was performed by adding methanol (8 ml) and a few
crystals of p-TosOH. After stirring at r. t. for 30 min the solvents
were evaporated. Ca. 0.1 ml of the residue were purified by chroma-
tography (4Ϫ5 cm SiO2 in a Pasteur pipette) with CH2Cl2 as eluent
(Rf ϭ 0.12). The first fraction (ca. 3 ml) was discarded, the follow-
ing 10 ml were collected and evaporated. The enantiomeric excess
was determined by injecting 0.4 µl of a solution of the resulting oil
(1Ϫ2 drops) in 1 ml of CH2Cl2 (Merck Uvasol ) into a Fisons
8130 gas chromatograph. Column: Chrompack Chirasil-DEX CB
Ƞ
Dedicated to Professor H. Nöth on the occasion of his 70th
birthday.
Eur. J. Inorg. Chem. 1998, 783Ϫ788
787