Studies on the rhodium- and ruthenium-catalyzed asymmetric hydrogenation of α-dehydroamino acids using a family of chiral dipyridylphosphine ligand (P-Phos)

Article abstract of DOI:10.1016/S0957-4166(03)00098-3

The applications of the chiral dipyridylphosphine ligand P-Phos and its derivatives Tol-P-Phos and Xyl-P-Phos in Ru- and Rh-catalyzed hydrogenations of the methyl esters of a variety of (Z)-2-acetamido-3-arylacrylic acids have been studied systematically.

Full text of DOI:10.1016/S0957-4166(03)00098-3

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 14 (2003) 987–992
Studies on the rhodium- and ruthenium-catalyzed asymmetric
hydrogenation of a-dehydroamino acids using a family of chiral
dipyridylphosphine ligand (P-Phos)
Jing Wu, Cheng Chao Pai, Wai Him Kwok, Rong Wei Guo, Terry T. L. Au-Yeung,
Chi Hung Yeung* and Albert S. C. Chan*
†
Open Laboratory of Chirotechnology of the Institute of Molecular Technology for Drug Discovery and Synthesis
and Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hong Kong
Received 8 October 2002; accepted 23 January 2003
Abstract—The applications of the chiral dipyridylphosphine ligand P-Phos and its derivatives Tol-P-Phos and Xyl-P-Phos in Ru-
and Rh-catalyzed hydrogenations of the methyl esters of a variety of (Z)-2-acetamido-3-arylacrylic acids have been studied
systematically. The results show that the electronic and steric properties of these ligands have significant influences on the
enantioselectivity of the reduction. Rh and Ru complexes of the same dipyridylphosphine ligand family exhibit different trends in
enantioselectivity toward the same substrate. © 2003 Elsevier Science Ltd. All rights reserved.
1
. Introduction
We have recently developed
a
highly effective
dipyridylphosphine ligand P-Phos (1, P-Phos=2,2%,6,6%-
tetramethoxy-4,4%-bis(diphenylphosphino)-3,3%-bipyri-
Asymmetric catalytic hydrogenation reactions have
been recognized as one of the most powerful tools for
obtaining a wide range of enantiomerically pure sub-
stances. Chiral amino acids are attractive targets for
enantioselective synthesis on account of their vital role
in the pharmaceutical industry both as nutritional sup-
plements and as synthetic intermediates. Asymmetric
catalytic hydrogenation of prochiral amidoacrylic acids
represents a convenient method for the synthesis of
such compounds. Rhodium catalysts containing chiral
phosphine ligands have been proved the most successful
5
dine) (Scheme 1) and a series of its derivatives Tol-P-
Phos (2, Tol-P-Phos=2,2%,6,6%-tetramethoxy-4,4%-bis-
[di(p-tolyl) phosphino]-3,3%-bipyridine) and Xyl-P-Phos
(3, Xyl-P-Phos=2,2%,6,6%-tetramethoxy-4,4%-bis[di(3,5-
dimethylphenyl)phosphino]-3,3%-bipyridine) for appli-
6
7
cations in the stereoselective Ru-catalyzed hydro-
5
genation of 2-(6%-methoxy-2%-naphthyl)propenoic acid,
5–7
8
b-ketoesters and aromatic ketones. The Ru-P-Phos
catalysts were found to be air-stable even in solution,
4
,5
indicating their potential for practical applications.
1
catalysts for this type of reaction. However, despite the
Herein, we present the results of our systematic investi-
gations on the performance of the ligands 1–3 in the
Ru- and Rh-catalyzed asymmetric hydrogenation of
acetamidoacrylic acids and esters
fact that rhodium catalyzed asymmetric hydrogenation
is a remarkably specific method for the production of
chiral amino acids and certain closely related com-
2
pounds, it is a reaction of limited scope. Unlike the
Rh-catalyzed hydrogenation of acetamidoacrylic acids
and esters, the corresponding Ru chemistry has not
3
,4
been studied extensively despite the wide application
of ruthenium catalysts in the asymmetric hydrogenation
if other types of substrates.
*
Corresponding author. Tel.: +852-27665646; fax: +852-23649932;
e-mail: bcachan@polyu.edu.hk
University Grants Committee Area of Excellence Scheme (Hong
Kong).
†
Scheme 1.
0
957-4166/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0957-4166(03)00098-3

9
88
J. Wu et al. / Tetrahedron: Asymmetry 14 (2003) 987–992
2
. Results and discussion
decrease of reaction rate (entry 10 versus entry 9). At
high initial hydrogen pressures (entries 5–8), lower
2
.1. Synthesis of ruthenium and rhodium complexes of
enantioselectivities were observed.
(
R)-P-Phos 1, (R)-Tol-P-Phos 2 and (R)-Xyl-P-Phos 3
Having established the preferred conditions, the effec-
tiveness of the catalysts Cat. 1a–Cat. 1c in the asym-
metric hydrogenation of methyl (Z)-2-acetamido-
cinnamate and the free acid was evaluated (Table 2).
P-Phos 1 showed better enantioselectivity over Tol-P-
Phos 2 and Xyl-P-Phos 3 in this catalytic system. For
example, the reaction was completed in 3 h affording a
product with 90% ee when Ru-P-Phos complex Cat. 1a
was employed (entry 2, Table 2), whereas the catalytic
activity and enantioselectivity were substantially lower
(64% conv., 73% ee) in the case of Ru-Xyl-P-Phos (Cat.
1c, entry 4, Table 2).
Ruthenium complexes, RuL*(C H )Cl [L*=(R)-P-
6
6
2
6
Phos (Cat. 1a), (R)-Tol-P-Phos (Cat. 1b), (R)-Xyl-P-
7
Phos (Cat. 1c) ] were prepared by mixing
[
RuCl (C H )] with the corresponding dipyridylphos-
2 6 6 2
phine ligands in an 8:1 mixture of ethanol–benzene at
0–60°C for 1 h according to the method of Mashima
5
9
et al. The structures of these complexes were character-
1
31
ized by H and P NMR. The rhodium complexes,
RhL*(COD)BF [L*=(R)-P-Phos (Cat. 2a), (R)-Tol-P-
4
Phos (Cat. 2b), (R)-Xyl-P-Phos (Cat. 2c)] were pre-
pared in situ by mixing Rh(COD) BF with 1.05
2
4
equivalents of the corresponding dipyridylphosphine
ligand in methanol under nitrogen and were character-
Further studies on the hydrogenation of a variety of
other acetoamidoacrylic esters confirmed the consis-
tently better enantioselectivity of Cat. 1a. In all cases,
the desired products were found to have ee values of
over 90% for Cat. 1a, with the best result being 97% ee
(entry 1, Table 3). The details are summarized in Table
3
1
ized by P NMR.
2
.2. Asymmetric hydrogenation of methyl-(Z)-2-
acetamidocinnamate using cationic Ru-complexes (Cat.
a–Cat. 1c) as catalysts
1
3
. In general, for Cat. 1a–c, the substrate with an
In the initial study, (Z)-acetamidocinnamic acid methyl
ester was chosen to be a model substrate. The
RuL*(C H )Cl complex was found to be an effective
electron-withdrawing ortho-Cl-substitution on the
phenyl ring reacted favorably to give product with
higher enantiopurity when compared with substrates
bearing para- or meta-Cl-substituents (entries 1–3 ver-
sus 4–9). On the other hand, electron-donating para-
substituents led to lower levels of enantioselection
(entries 10–15). More specifically, the replacement of a
s-electron-releasing methyl group with a p-electron-
releasing methoxy group at the para-position did not
show any significant changes on the eventual enan-
tiomeric excess of the product.
6
6
2
catalyst for the asymmetric hydrogenation of (Z)-
acetamidocinnamic acid methyl ester (Table 1). Cat. 1c
showed almost no catalytic activity in aprotic solvents
such as acetone, CH Cl or THF (entries 1–3) while
2
2
protic solvents such as methanol appear to be the
solvent of choice (entry 4). Lowering of the reaction
temperature from room temperature to 0°C did not
enhance the enantioselectivity but resulted in the
Table 1. The effect of hydrogen pressure and substrate concentration on the hydrogenation of methyl (Z)-2-acetamidocinna-
a
mate catalyzed by Cat. 1b
Entry
Catalyst
Solvent
Temp. (°C)
PH2 (atm)
Time (h)
Conv. (%)b
Ee (%)b
1
2
3
4
5
6
7
8
9
Cat. 1c
Cat. 1c
Cat. 1c
Cat. 1c
Cat. 1b
Cat. 1b
Cat. 1b
Cat. 1b
Cat. 1a
Cat. 1a
Acetone
CH Cl
THF
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
rt
rt
rt
rt
rt
rt
rt
rt
rt
0
4
4
4
4
1
12
12
12
12
3
3
3
3
3
B1
B1
B2
\99.9
\99.9
\99.9
\99.9
\99.9
\99.9
53
–c
–
–
77
85
83
78
77
90
89
2
2
4
15
35
1
1
0
1
18
a
Reaction conditions: substrate (4 mg); substrate/catalyst=100 (M/M); substrate concentration=0.05–0.09 M.
b
The conversion and ee values were determined by chiral GC with a 25 m×0.25 mm Chrompack Chirasil-
obtained for all products. The absolute configuration was determined by comparing the retention time with that reported in the literature (Ref.
0).
L-Val column. R configuration was
1
c
The ee value could not be determined accurately due to the low conversion.

J. Wu et al. / Tetrahedron: Asymmetry 14 (2003) 987–992
989
Table 2. Ruthenium-catalyzed asymmetric hydrogenation of (Z)-2-acetamidocinnamic acid and its methyl ester using Cat.
a
1a–Cat. 1c
Entry
Substrate, R=
Catalyst
Time (h)
Conv.(%)b
Ee (%)b
Config.c
1
2
3
4
5
6
7
CH3
CH3
CH3
CH3
H
Cat. 1a
Cat. 1a
Cat. 1b
Cat. 1c
Cat. 1a
Cat. 1b
Cat. 1c
2
3
3
96
88
90
85
73
90
87
85
R
R
R
R
R
R
R
\99.9
\99.9
64
\99.9
\99.9
\99.9
4
10
10
10
H
H
a
Reaction conditions: H (1 atm); ambient temperature; substrate (4 mg); substrate concentration=0.05–0.09 M in MeOH; substrate/catalyst=100
2
(
M/M).
b
c
Conversion and ee values were determined by chiral GC with a 25 m×0.25 mm Chrompack Chirasil-
to the corresponding methyl esters with methyl iodide/KHCO3 before GC analysis.
The absolute configuration was determined by comparing the retention time with that reported in the literature (Ref. 10).
L-Val column. The acids were converted
Table 3. Ruthenium-catalyzed asymmetric hydrogenation
of the derivatives of methyl (Z)-2-acetamidocinnamate
using Cat. 1a–Cat. 1c
hydrogenation of methyl (Z)-2-acetamidocinnamate,
Rh-catalyzed (Cat. 2c) asymmetric hydrogenation could
be carried out in a variety of common organic solvents
a
(
entries 1–5, Table 4) although MeOH was again found
to be the best solvent.
Thus, the reaction proceeded smoothly in methanol at
ambient temperature with 1 atm of initial hydrogen
pressure for 2 h to give a quantitative yield of the
product with 90% ee (entry 5, Table 4). This result is in
sharp contrast with the Ru case where the conversion
and the product enantiopurity were only 64 and 73%,
respectively, using the same ligand (i.e. 3) under similar
reaction conditions (entry 4, Table 2). Hydrogen pres-
sure and temperature effects were also studied. Higher
Entry
Substrate, R= Catalyst
Time (h)
Ee (%)b
1
2
3
4
5
6
7
8
9
2-Cl-C H4
Cat. 1a
Cat. 1b
Cat. 1c
Cat. 1a
Cat. 1b
Cat. 1c
Cat. 1a
Cat. 1b
Cat. 1c
Cat. 1a
Cat. 1b
Cat. 1c
Cat. 1a
Cat. 1b
Cat. 1c
4
4
4
4
4
4
4
4
4
5
4
4
5
10
10
97
95
92
91
86
82
94
84
81
91
83
74
90
81
76
6
6
2-Cl-C H4
2-Cl-C H4
6
3-Cl-C H4
6
3-Cl-C H4
6
3-Cl-C H4
6
4-Cl-C H4
6
4-Cl-C H4
H pressure led to a slight decrease in the enantiomeric
2
6
4-Cl-C H4
purity of the product (entry 5 versus entries 7 and 8)
whilst lower reaction temperature (0°C) facilitated an
enhancement in the enantioselectivity but at the
expense of reaction rate (entry 6).
6
1
1
1
1
1
1
0
1
2
3
4
5
4-CH -C H
3
6
4
4
4
4-CH -C H
3
6
4-CH -C H
3
6
4-CH O-C H
3
6
4
4
4
4-CH O-C H
3
6
(
Z)-2-Acetamidocinnamic acid and a number of its
4-CH O-C H
3
6
derivatives were also hydrogenated with Cat. 2a–Cat.
2c. When (R)-P-Phos 1 or (R)-Tol-P-Phos 2 was used
as ligand, the enantioselectivity of the Rh-catalyzed
hydrogenation of methyl (Z)-2-acetamidocinnamate
decreased dramatically in comparison with that
observed when using (R)-Xyl-P-Phos 3 as a ligand
a
Reaction conditions: H2 (1 atm); ambient temperature; substrate (4
mg); substrate concentration=0.05–0.09 M in MeOH; substrate/cat-
alyst=100 (M/M); >99% conversion was observed in all cases.
The conversion and ee values were determined by chiral GC with a
b
2
5 m×0.25 mm Chrompack Chirasil-L-Val column. R configuration
was obtained for all products. The absolute configuration was
determined by comparing the retention time with that reported in
the literature (Ref. 10).
(
entries 2 and 3 versus entry 1, Table 5). Apparently,
steric hindrance effects of the ligand enhanced the
enantioselectivity. This trend was consistently observed
in the hydrogenation of (Z)-2-acetamidocinnamic acid
(entries 4–6). These results are completely opposite to
the corresponding Ru-catalyzed hydrogenations. Cat.
2c was also effective for hydrogenating a variety of
methyl (Z)-2-acetamidocinnamate derivatives leading
to good enantioselectivities (92–94% ee, entries 7–11).
2
.3. Asymmetric hydrogenation of methyl-(Z)-2-
acetamidocinnamate using cationic Rh-complexes (Cat.
2
a–Cat. 2c) as catalysts
For broader investigations, it was of interest to com-
pare the effects of the more conventional Rh complexes
versus Ru-complexes in the same hydrogenation reac-
tions (Table 4). Unlike the Ru-catalyzed asymmetric
It has been reported that the hydrogenation of the
enamide 4 catalyzed by (R)-BINAP-Ru(II) complexes

9
90
J. Wu et al. / Tetrahedron: Asymmetry 14 (2003) 987–992
Table 4. The effect of solvent and hydrogen pressure on the Rh-catalyzed hydrogenation of methyl (Z)-2-acetamidocinna-
a
mate catalyzed with Cat. 2c
Entry
Temp. (°C)
PH2 (atm)
Solvent
Time (h)
Ee (%)b
Config.c
1
2
3
4
5
6
7
8
rt
rt
rt
rt
rt
0
1
1
1
1
1
1
35
15
Acetone
CH Cl
Toluene
THF
MeOH
MeOH
MeOH
MeOH
2
2
2
2
2
18
2
2
88
87
86
87
90
93
85
88
R
R
R
R
R
R
R
2
2
rt
rt
a
Reaction conditions: ambient temperature; 4 h; substrate (4 mg); substrate concentration=0.09 M; substrate/catalyst=100 (M/M); >99%
conversion was observed in all cases.
The conversion yield and ee value were determined by chiral GC with a 25 m×0.25 mm Chrompack Chirasil-
The absolute configuration was determined by comparing the retention time with that reported in the literature (Ref. 10).
b
c
L-Val column.
Table 5. Rhodium-catalyzed asymmetric hydrogenation of the derivatives of methyl (Z)-2-acetamidocinnamate using Cat.
a
2a–Cat. 2c
1
Conv. (%)b
Ee (%)b
Entry
R=
R =
Temp. (°C)
Catalyst
Time (h)
1
2
3
4
5
6
7
8
9
C H -
CH3
CH3
CH3
H
H
H
CH3
CH3
CH3
CH3
CH3
rt
rt
rt
rt
rt
rt
0
0
0
0
0
Cat. 2a
Cat. 2b
Cat. 2c
Cat. 2a
Cat. 2b
Cat. 2c
Cat. 2c
Cat. 2c
Cat. 2c
Cat. 2c
Cat. 2c
2
2
2
\99.9
70
38
46
90
64
69
91
92
93
93
94
94
6
5
C H -
6
5
C H -
\99.9
\99.9
\99.9
\99.9
\99.9
\99.9
\99.9
\99.9
\99.9
6
5
C H -
10
10
10
18
18
18
18
18
6
5
C H -
6
5
C H -
6
5
2-Cl-C H
3-Cl-C H
4-Cl-C H
6
4
4
4
6
6
1
0
1
4-CH -C H
3
6
4
1
4-CH O-C H
4
3
6
a
Reaction conditions: 1 atm H ; 4 mg substrate; substrate concentration=0.05 M in MeOH; substrate/catalyst=100 (M/M).
The conversion and ee values were determined by chiral GC with a 25 m×0.25 mm Chrompack Chirasil-L-Val column. The acids were converted
2
b
to the corresponding methyl esters with methyl iodide/KHCO before GC analysis. R configuration was obtained for all products. The absolute
3
configuration was determined by comparing the retention time with that reported in the literature (Ref. 10).
3
b,11
gave
5
in 79–92% ee
while the [Rh-(R)-
derivatives has been examined. The results demon-
strated that the electronic and steric properties of these
ligands have significant influences on their enantioselec-
tivities. In addition, these ligands exhibited different
catalytic properties and trends when different transition
metal ions were used in the asymmetric hydrogenation.
BINAP(CH OH) ]ClO catalyst exhibited an opposite
sense of asymmetric induction to give enantiomeric 6
with ees of 92–100% (Scheme 2). In this study, it was
of interest to note that the products of hydrogenations
catalyzed by Rh and Ru complexes of the same chiral
dipyridylphosphine ligand, such as (R)-Cat. 1c and
3
2
4
1
2
(
(
R)-Cat. 2c, were of the same absolute configurations
Table 2, entry 4 and Table 4, entry 5).
3. Conclusions
The effectiveness of chiral dipyridylphosphine ligands
–3 in the Ru- and Rh-catalyzed asymmetric hydro-
genation of methyl (Z)-2-acetamidocinnamate and their
1
Scheme 2.

J. Wu et al. / Tetrahedron: Asymmetry 14 (2003) 987–992
991
4. Experimental
4.5. Preparation of a stock solution of [Rh(R-1)(COD)]-
BF , Cat. 2a
4
4
.1. General and materials
Under a nitrogen atmosphere, (R)-1 (3.4 mg, 0.0053
mmol) was dissolved in methanol (1 mL). A solution of
All manipulations with air-sensitive reagents were car-
ried out under a dry nitrogen atmosphere using stan-
dard Schlenk techniques or in
MBRAUN Lab Master 130 glovebox. The hydrogena-
tion reactions were performed in a 50 mL stainless-steel
autoclave from Parr company. H NMR and P NMR
[
Rh(COD) ]BF (2.1 mg, 0.005 mmol) in methanol (1
2 4
mL) was added dropwise to the above solution with
stirring. The reaction mixture was stirred overnight to
a nitrogen-filled
give a methanolic solution of [Rh(R-1)(COD)]BF (Cat.
4
−
1
31
1
31
2a, 0.0025 mol L ). P NMR (MeOH, 202 MHz): l
2
1.1 (d, JRh–P=144.8 Hz).
were recorded in CDCl on a Varian AS 500 at room
3
temperature, and the chemical shifts were expressed in
ppm. Gas chromatographic analyses were conducted on
a HP 4890A or HP 5890 series II system. Optical
rotations were measured on a Perkin–Elmer model 341
polarimeter. Commercial reagents were used as received
without further purification unless otherwise stated. All
solvents used were dried using standard, published
methods and were distilled before use. Optically pure
P-Phos (1), Tol-P-Phos (2) and Xyl-P-Phos (3) were
synthesized according to our previously reported
4
.6. Preparation of a stock solution of [Rh(R-2)(COD)]-
4
BF , Cat. 2b
A stock solution of Cat. 2b was prepared in a similar
fashion as that of Cat. 2a. P NMR (MeOH, 202
MHz): l 19.7 (d, JRh–P=145.0 Hz).
31
4
.7. Preparation of a stock solution of [Rh(R-3)(COD)]-
BF , Cat. 2c
4
5
–7
procedures and their optical purities were determined
by HPLC analysis using a Hewlett–Packard model HP
A stock solution of Cat. 2c was prepared in a similar
fashion as that of Cat. 2a. P NMR (MeOH, 202
MHz): l 21.2 (d, JRh–P=144.6 Hz).
31
1
050 LC interfaced to an HP 1050 series computer
5–7
workstation.
4
.8. Typical procedure for the asymmetric hydrogena-
4
.2. Synthesis of Ru(R-1)(C H )Cl , Cat. 1a
6 6 2
tion of methyl (Z)-2-acetamidocinnamate
[
RuCl (C H )] (100 mg, 0.2 mmol) and (R)-P-Phos
(R)-1, 264 mg, 0.42 mmol) were placed in a 100 mL
2 6 6 2
−3
−1
A solution of 1.73×10 mol L Cat. 1a in methanol
(
−
4
−1
(
106 mL, 1.83×10 mmol) and a 0.183 mol L methyl
Z)-2-acetamidocinnamate solution in methanol (100
round-bottomed Schlenk flask. After the air in the flask
(
was replaced by N , dried and degassed ethanol (48
mL) and benzene (6 mL) were added respectively by
syringe. The mixture was stirred under N at 50–60°C
for 1 h to form a clear brownish yellow brown solution.
After the solution was cooled to room temperature, the
insoluble solid was filtered off and the solvent was
evaporated under vacuum to give a yellowish green
solid Cat. 1a (257 mg, 72%). H NMR (CDCl , 500
MHz): l 3.56 (s, 3H, OCH ), 3.66 (s, 3H, OCH ), 3.67
(
5
2
mL, 0.0183 mmol) were charged to a 25 mL round-bot-
tomed flask equipped with a magnetic stirring bar
under a nitrogen atmosphere. A stream of H2 was
bubbled through the solution while it was magnetically
stirred at ambient temperature for 3 h. The resulting
solution was then submitted to analysis to determine
the conversion and enantiomeric excess. Quantitative
conversion of the starting material to the hydrogena-
tion product, (R)-2-acetamido-3-phenyl-propanoste,
with 90% ee was observed by chiral GC analysis
2
1
3
3
3
s, 3H, OCH ), 3.80 (s, 3H, OCH ), 5.85 (s, 6H, C H ),
3 3 6 6
.99 (d, J=10 Hz, 1H, PyH), 6.49 (d, J=10.5 Hz, 1H,
(
column, Chrompack Chirasil-
L
-Val, 25 m×0.25 mm,
3
1
PyH), 7.21–7.83 (m, 20H, PhH). P NMR (CDCl , 202
MHz): l 32.36 (d, J=62.82 Hz), 39.49 (d, J=62.82
3
carrier gas, N2).
Hz).
Acknowledgements
4
.3. Synthesis of Ru(R-2)(C H )Cl , Cat. 1b
6 6 2
Cat. 1b was synthesized in 74% yield and characterized
We thank the Hong Kong Research Grants Council
Project number PolyU 5177/99P), The University
6
according to our previously reported procedures.
(
Grants Committee Areas of Excellence Scheme in Hong
Kong (AoE P/10-01) and The Hong Kong Polytechnic
University ASD Fund for financial support of this
study.
4
.4. Synthesis of Ru(R-3)(C H )Cl , Cat. 1c
6 6 2
Cat. 1c was synthesized in 69% yield according to the
1
same procedure as in the preparation of Cat. 1a. H
NMR (CDCl , 500 MHz): l 2.24–2.39 (m, 24H,
3
PhCH ), 3.48 (s, 3H, OCH ), 3.59 (s, 3H, OCH ), 3.64
References
3
3
3
(
s, 3H, OCH ), 3.75 (s, 3H, OCH ), 5.65 (s, 6H, C H ),
3 3 6 6
5
.96 (d, J=11 Hz, 1H, PyH), 6.46 (d, J=10.5 Hz, 1H,
1. (a) Noyori, R. Asymmetric Catalysis in Organic Synthe-
sis; Wiley: New York, 1994; Chapter 2; (b) Ojima, I.
Catalytic Asymmetric Synthesis; 2nd ed.; Wiley: New
York, 2000; pp. 9–17; (c) Nacobsen, E. N. Pfaltz, A.;
3
1
PyH), 6.74–7.29 (m, 12H, PhH). P NMR (CDCl , 202
MHz): l 33.49 (d, J=62.9 1 Hz), 39.96 (d, J=62.51
Hz).
3

9
92
J. Wu et al. / Tetrahedron: Asymmetry 14 (2003) 987–992
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