A new P-chiral bisphosphine, 1,1'-bis[(t- butyl)methylphosphino]ferrocene, as an effective ligand in catalytic asymmetric hydrosilylation of simple ketones

Article abstract of DOI:10.1016/S0957-4166(99)00042-7

The asymmetric hydrosilylation of simple ketones was catalyzed by a rhodium complex with a P-chiral bisphosphine, 1,1'-bis[(t- butyl)methylphosphino]ferrocene, to give optically active alcohols with enantiomeric excesses of up to 92%.

Full text of DOI:10.1016/S0957-4166(99)00042-7

Pergamon
Tetrahedron: Asymmetry 10 (1999) 877–882
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A new P-chiral bisphosphine, 1,1 -bis[(t-butyl)methyl-
phosphino]ferrocene, as an effective ligand in catalytic
asymmetric hydrosilylation of simple ketones
∗
Hideyuki Tsuruta and Tsuneo Imamoto
Department of Chemistry, Faculty of Science, Chiba University, Inage, Chiba 263-8522, Japan
Received 28 December 1998; accepted 15 January 1999
Abstract
The asymmetric hydrosilylation of simple ketones was catalyzed by a rhodium complex with a P-chiral
0
bisphosphine, 1,1 -bis[(t-butyl)methylphosphino]ferrocene, to give optically active alcohols with enantiomeric
excesses of up to 92%. © 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
The design and synthesis of new chiral ligands for transition metal complexes have played a significant
role in the development of catalytic asymmetric reactions, and there has been much effort devoted to
1
the preparation of efficient ligands. In particular, chiral diphosphine ligands with C -symmetry are of
2
great importance in the development of transition metal-catalyzed asymmetric reactions. Recently, we
2
synthesized electron-rich trialkylphosphine ligands BisP*, and proved their efficient utility in rhodium-
catalyzed asymmetric hydrogenation of α-(acylamino)acryl derivatives. Based on these results, we
planned to develop a new ligand having higher reactivity and enantioselectivity, and thus designed a
3
0
4,5
new P-chiral ligand, 1,1 -bis[(t-butyl)methylphosphino]ferrocene 1. In this paper we report on the
synthesis of the ligand and its use in the highly enantioselective hydrosilylation of simple ketones.
2. Results and discussion
The newly designed P-chiral ligand 1 was prepared by use of a phosphine borane as the interme-
6
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diate (Scheme 1). Thus, the 1,1 -dilithioferrocene–TMEDA complex was treated sequentially with t-
butyldichlorophosphine, methylmagnesium bromide, and borane–THF complexes to afford a mixture of
∗
Corresponding author. Tel: +81-43-290-2791; fax: +81-43-290-2791; e-mail: imamoto@scichem.c.chiba-u.ac.jp
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00042-7

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H. Tsuruta, T. Imamoto / Tetrahedron: Asymmetry 10 (1999) 877–882
both the dl-form (50%) and the meso-form (10%). The dl-form was resolved by HPLC using a preparative
chiral column (Daicel Chiralpak AD).
Scheme 1.
One enantiomer ([α]D26 −249 (c 0.5, CHCl )) was subjected to single-crystal X-ray analysis in order
3
to determine its absolute configuration. Fig. 1 shows its molecular structure.
Figure 1. An ORTEP drawing of the molecular structure of (S,S)-2
Compound (S,S)-2 was treated with excess pyrrolidine at 70°C for 90 min to furnish the phosphine
ligand (S,S)-1 in almost quantitative yield. Complexation of this ligand with [Rh(nbd) ]BF afforded the
2
4
desired chiral rhodium cation complex.
First, the rhodium complex was employed for the catalytic asymmetric hydrogenation of methyl
α-acetamidecinnamate and α-acetamideacrylic acid. The reaction proceeded smoothly in methanol

H. Tsuruta, T. Imamoto / Tetrahedron: Asymmetry 10 (1999) 877–882
879
under ordinary conditions (2 atm, S/C=500, 50°C), but the enantiomeric excess of the products was
disappointingly low (ca. 20%).
We then examined the use of the rhodium complex as a catalyst for asymmetric hydrosilylation of
7
acetophenone (Scheme 2). To our delight, the reaction proceeded in THF at −20°C using 1 mol%
rhodium catalyst and 1.5 equiv. of diphenylsilane to give (S)-1-phenylethanol with 89% ee in 99%
yield. Furthermore, the enantiomeric excess improved to 92% when a more bulky silane agent, naphthyl-
phenylsilane, was used.
Scheme 2.
Other ketones were also subjected to hydrosilylation under the same conditions and the results are
summarized in Table 1. In the tolyl methyl ketones reaction, meta and para substituted ketones were
reduced with good enantioselectivity (entries 4, 5). However, reduction of the ortho derivative resulted
in lower enantioselectivity (entry 3). Other simple ketones were reduced with good enantioselectivity
(
entries 7–9). The only exception was the reduction of p-methoxyacetophenone, which led to a racemic
8
product (entry 6). This is probably ascribed to the so-called ‘p-methoxy effect’. It is also noted that an
aliphatic ketone was subjected to asymmetric hydrosilylation in good enantioselectivity (entry 10).
Table 1
Asymmetric hydrosilylation of ketone catalyzed by rhodium complexa
There have been many reports dealing with rhodium-catalyzed asymmetric hydrosilylation of ketones.
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In general, the ligands containing oxazoline units give rise to high-to-excellent enantioselectivity, while
diphosphine ligands lead to low-to-moderate selectivity except in the case of trans-chelating diphosphine
10
ligands (TRAP). The present results are the first example of highly enantioselective hydrosilylation
obtained by use of cis-chelating diphosphine.

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H. Tsuruta, T. Imamoto / Tetrahedron: Asymmetry 10 (1999) 877–882
3. Conclusion
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We have synthesized optically active 1,1 -bis(t-butylmethylphosphino)ferrocene, and demonstrated its
utility in rhodium-catalyzed asymmetric hydrosilylation of simple ketones.
4. Experimental
4.1. General
Optical rotations were measured with a JASCO DIP-370 polarimeter. NMR spectra were measured
with a JEOL JMN-GSX-500 (500 MHz) spectrometer or a JEOL JMN-LA-400 (400 MHz) spectrometer
in CDCl . Chemical shifts are reported in δ ppm. HRMS (FAB) spectra were measured with a JEOL HX-
3
110 spectrometer at the Chemical Analysis Center, Chiba University. HPLC analyses were performed on
a Hitachi L-6000 pump and L-4000 UV detector with a chiral column.
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4
.2. Synthesis of 1,1 -bis[boranato(t-butyl)methylphosphino]ferrocene
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A hexane slurry of 1,1 -dilithioferrocene–TMEDA (20 mmol), prepared from ferrocene (3.72 g), n-
BuLi (46 mmol), and TMEDA (25 mmol) in hexane, was added dropwise over a period of 1 h into a
solution of t-butyldichlorophosphine (6.7 g, 42 mmol) in dry THF (60 mL) with vigorous stirring at
−78°C under argon. After the addition, the cooling bath was removed, and stirring was continued for 1
h. The mixture was cooled at 0°C, then methylmagnesium bromide (70 mL, 1.0 M THF solution) added,
and the mixture warmed to 50°C. After stirring for 1 h, borane−THF complex (100 mL, 1.0 M) was
added at 0°C. The reaction mixture was carefully poured into a mixture of ice-water containing HCl.
The organic layer was separated, and the aqueous layer was extracted with ethyl acetate. The combined
extracts were washed with brine, dried (Na SO ), and concentrated in vacuo. The product was isolated
2
4
by column chromatography (silica gel, toluene:ethyl acetate, 50:1) to afford 4.15 g of the dl-form (50%)
and 0.84 g of the meso-form (10%). A small amount (ca. 200 mg) of the dl-form was separated to each
enantiomer by HPLC using a semi-preparative chiral column [Daicel Chiralpak AD, EtOH:hexane, 5:95,
(
S,S) t =7.5 min, (R,R) t =13.4 min].
1 2
0
.2.1. (S,S)-1,1 -Bis[boranato(t-butyl)methylphosphino]ferrocene
4
26
Orange crystals: mp 203°C (dec.); [α]D −249 (c 0.49, CHCl ); R =0.29 (toluene:ethyl acetate=50:1);
3
f
1
3
2
H NMR (CDCl ) δ 1.00 (d, J =14.1 Hz, 9H), 1.51 (d, J =9.8 Hz, 3H), 4.24–4.25 (m, 1H),
3
HP
HP
13
2
4
.67–4.68 (m, 2H), 4.72–4.73 (m, 1H); C NMR (CDCl ) δ 5.91 (d, J =38.0 Hz), 25.08 (d, J =2.5
3
CP
CP
2
2
Hz), 28.80 (d, J =34.7 Hz), 70.81 (s), 70.83 (s), 73.92 (d, J =8.3 Hz), 73.98 (d, J =5.0 Hz), 75.51
CP
CP
CP
1
1
31
(
d, J =15.7 Hz); B NMR (CDCl ) δ −62.5 (d, J =58.6 Hz); P NMR (CDCl ) δ 24.2 (m); IR (KBr)
CP
3
BP
3
−1
+
3
085, 2980, 2370, 1475, 1295, 1065, 830 cm ; HRMS calcd for C H B FeKP (M+K ) 457.1630,
20 38 2 2
found 457.1618.
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.2.2. (R,R)-1,1 -Bis[boranato(t-butyl)methylphosphino]ferrocene
4
27
Orange crystals: mp 205°C (dec.); [α]D +250 (c 0.54, CHCl ); R =0.29 (toluene:ethyl acetate=50:1);
3
f
1
3
2
H NMR (CDCl ) δ 1.00 (d, J =13.9 Hz, 9H), 1.51 (d, J =10.0 Hz, 3H), 4.23–4.24 (m, 1H),
3
HP
HP
13
2
4
.67–4.68 (m, 2H), 4.72–4.73 (m, 1H); C NMR (CDCl ) δ 5.90 (d, J =38.0 Hz), 25.07 (d, J =2.5
3
CP
CP
2
2
Hz), 28.79 (d, J =34.7 Hz), 70.82 (br s), 73.91 (d, J =8.3 Hz), 73.98 (d, J =5.8 Hz), 75.50 (d,
CP
CP
CP

H. Tsuruta, T. Imamoto / Tetrahedron: Asymmetry 10 (1999) 877–882
881
JCP=15.7 Hz); 11B NMR (CDCl ) δ −62.5 (d, J =58.6 Hz); P NMR (CDCl ) δ 24.2 (m); IR (KBr)
31
3
BP
3
−1
+
3
085, 2980, 2370, 1475, 1295, 1065, 830 cm ; HRMS calcd for C H B FeKP (M+K ) 457.1630,
20 38 2 2
found 457.1620.
0
4
.2.3. meso-1,1 -Bis[boranato(t-butyl)methylphosphino]ferrocene
1
Orange crystals: mp 160–162°C; R =0.35 (toluene:ethyl acetate=50:1); H NMR (CDCl ) δ 1.00 (d,
f
3
3J =13.9 Hz, 9H), 1.56 (d, J =10.0 Hz, 3H), 4.37–4.39 (m, 1H), 4.61–4.63 (m, 2H), 4.65–4.68 (m,
2
HP
HP
1
3
2
1
7
(
1
H); C NMR (CDCl ) δ 5.87 (d, J =38.9 Hz), 25.08 (d, J =2.5 Hz), 28.78 (d, J =34.7 Hz),
1.70 (s), 71.72 (s), 73.35 (d, J =5.0 Hz), 73.59 (d, J =8.3 Hz), 75.23 (d, J =15.7 Hz); B NMR
CDCl ) δ −57.9 (d, J =60.9 Hz); P NMR (CDCl ) δ 24.7 (m); IR (KBr) 3085, 2975, 2380, 1475,
295, 1070, 835 cm ; HRMS calcd for C H B FeKP (M+K ) 457.1630, found 457.1631.
3
CP
CP
CP
2
2
2
11
CP
CP
CP
31
3
BP
3
−1
+
20 38 2 2
0
4
.2.4. Crystal data of (S,S)-1,1 -bis[boranato(t-butyl)methylphosphino]ferrocene
An orange, thin plate crystal of dimensions 0.2×0.06×0.7 mm was mounted on a glass fiber. All
measurements were made using a Rigaku AFC7S with graphite monochromated Cu-Kα radiation
(λ=1.54178 Å) at 25°C. Cell constants and an orientation matrix for data collection were obtained
from a least-squares refinement using the setting angles of 20 carefully centered reflections in the
range 38.55<2θ<41.38°. The data were collected using a ω–2θ scan technique to a maximum 2θ
value of 138.2°. Scans of (1.31+0.3tan θ)° were made at a speed of 8.0°/min (in omega). Crystal data:
C H B FeP , Mw=417.93, monoclinic, space group P2 (#4), a=7.267(2), b=12.441(2), c=13.314(4)
2
0
38
2
2
1
3
3
−1
Å, β=93.56(2)°, V=1201.3(4) Å , Z=2, D
=1.15 g/cm , F(000)=448.0, µ(Cu-Kα)=62.7 cm . The
calcd
structure was solved by direct methods and expanded using Fourier techniques. The non-hydrogen atoms
were refined anisotropically, and the hydrogen atoms were included but not refined. The final cycle of full
matrix least-squares refinement was based on 4433 observed reflections (I>1.00σ(I)) and 227 variable
parameters and converged with unweighted and weighted agreement factors of: R=0.074, R =0.089. The
w
absolute configuration was determined by the Flack parameter method, value=0.005(6).
4.3. Removal of the boranato group of (S,S)-2
A solution of bisphosphine-borane (S,S)-2 (54 mg, 0.13 mmol) in pyrrolidine (1 mL) was stirred at
70°C under argon. After 90 min, pyrrolidine was removed in vacuo, and the residue was passed through
a column of silica using degassed toluene. The eluent was evaporated in vacuo to leave practically pure
bisphosphine in almost quantitative yield.
4.4. Preparation of the rhodium complex of (S,S)-1
The phosphine prepared above was dissolved in THF (6 mL), and the solution added to a stirred
suspension of [Rh(nbd) ]BF (42 mg, 0.11 mmol) in THF (6 mL) under argon. After 90 min, the
2
4
resulting, almost clear solution, was filtered under argon and the filtrate evaporated in vacuo. The residual
pasty oil was washed with ether to give orange powder which was dried in vacuo.
4.5. A typical procedure for rhodium-catalyzed asymmetric hydrosilylation
In a 20 mL Schlenk tube was placed the rhodium catalyst (3.2 mg, 0.005 mmol) under argon. Freshly
distilled THF (1 mL) and acetophenone (60 µL, 0.5 mmol) were added, and the solution was cooled
to −20°C. To the solution was added 1-naphthylphenylsilane (160 µL, 0.75 mmol), stirred at −20°C

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H. Tsuruta, T. Imamoto / Tetrahedron: Asymmetry 10 (1999) 877–882
until the ketone disappeared. The reaction mixture was quenched with 1 M HCl (3 mL) and stirred at rt
for 2 h. The mixture was extracted with ether, washed with brine, dried over Na SO , and evaporated.
2
4
The residue was purified by TLC to give (S)-1-phenylethanol (59 mg, 96%). The ee value (92%) was
determined by HPLC analysis using a chiral column (Daicel Chiralcel OJ, 2-propanol:hexane=1:9).
Acknowledgements
We thank Dr. Yoshinori Matsumoto, Fuji Chemical Industries, Ltd, for the resolution of compound
2. This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education,
Sciences, Sports and Culture, Japan.
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0
4
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0
5
. 1,1 -Diphosphine substituted ferrocene ligand has a large P–M–P bite angle at the transition metal complex, see: (a)
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8
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1
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1