A comparison of the asymmetric hydrogenation catalyzed by rhodium complexes containing chiral ligands with a binaphthyl unit and those with a 5,5′,6,6′,7,7′,8,8′-octahydro-binaphthyl unit

Article abstract of DOI:10.1016/S0957-4166(01)00399-8

The chiral ligands H₈-BINAPO and H₈-BDPAB were synthesized by reacting chlorodiphenylphosphine with H₈-BINOL and H₈-BINAM, respectively. Applications of these ligands in the Rh-catalyzed enantioselective hydrogenation of a variety of (Z)-acetamido-3-arylacrylic acid methyl esters provided chiral amino acid derivatives with good to excellent enantioselectivities (H₈-BINAPO: up to 84.0% e.e.; H₈-BDPAB: up to 97.1% e.e.). In the hydrogenation of acetamidoacrylic acid, 99% e.e. was obtained when a [Rh(H₈-BDPAB)]⁺ catalyst was used. The catalytic activities and enantioselectivities of [Rh(H₈-BINAPO)]⁺ and [Rh(H₈-BDPAB]⁺ are substantially better than those obtained with the corresponding rhodium catalysts containing BINAPO (up to 64% e.e.) and BDPAB (up to 92.6% e.e.).

Full text of DOI:10.1016/S0957-4166(01)00399-8

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 12 (2001) 2337–2342
A comparison of the asymmetric hydrogenation catalyzed by
rhodium complexes containing chiral ligands with a binaphthyl
unit and those with a 5,5%,6,6%,7,7%,8,8%-octahydro-binaphthyl unit
Fu-Yao Zhang, Wai Him Kwok and Albert S. C. Chan*
Open Laboratory of Chirotechnology and Department of Applied Biology and Chemical Technology,
The Hong Kong Polytechnic University, Hong Kong, China
Received 9 August 2001; accepted 11 September 2001
Abstract—The chiral ligands H8–BINAPO and H8–BDPAB were synthesized by reacting chlorodiphenylphosphine with H8–
BINOL and H8–BINAM, respectively. Applications of these ligands in the Rh-catalyzed enantioselective hydrogenation of a
variety of (Z)-acetamido-3-arylacrylic acid methyl esters provided chiral amino acid derivatives with good to excellent enantiose-
lectivities (H8–BINAPO: up to 84.0% e.e.; H8–BDPAB: up to 97.1% e.e.). In the hydrogenation of acetamidoacrylic acid, 99% e.e.
was obtained when a [Rh(H8–BDPAB)]+ catalyst was used. The catalytic activities and enantioselectivities of [Rh(H8–BINAPO)]+
and [Rh(H8–BDPAB)]+ are substantially better than those obtained with the corresponding rhodium catalysts containing
BINAPO (up to 64% e.e.) and BDPAB (up to 92.6% e.e.). © 2001 Published by Elsevier Science Ltd.
1. Introduction
with C2 symmetry were effective in asymmetric hydro-
genation reactions. For example, the aromatic BINAP
ligand, which possesses C2 axial chirality, has shown
great asymmetric induction potential. It has been sug-
gested that the highly skewed position of the naphthyl
rings in BINAP is the determining factor in its effective-
ness in asymmetric catalytic reactions.11
Asymmetric catalytic hydrogenation is one of the most
efficient and convenient methods for preparing a wide
range of enantiomerically pure compounds. Histori-
cally, the desire for practical routes to a-amino acids
ultimately led to the development of effective chiral
diphosphine ligands, such as DIPAMP,1 DIOP,2 Chi-
raphos,3 Norphos,4 BPPM,5 BDPP,6 BINAP,7
Duphos,8 BICP,9 and others,10 for the enantioselective
hydrogenation of a-amidoacrylates (a-enamides) (Eq.
(1)).
Takaya first prepared the atropisomeric ligand, H8–
BINAP,12 which possesses a unique structural feature
compared to the binaphthyl unit in BINAP. Recent
research showed that the chiral catalysts derived from
5,5%,6,6%,7,7%,8,8%-octahydro-1,1%-bi-2-naphthyl
ligands
COOH
COOH
H2
(e.g. H8–BINAP,13–15 H8–BINOL,16–18 H8–BINAM,18
H8–BDPAB,19 H8–binaphthoxy,20 H8–MAPs21) exhibit
higher efficiency and enantioselectivity for asymmetric
reactions than those prepared from their parent ligands,
due to the steric and electronic modulation in the
H8–binaphthyl backbone. To expand the scope of the
previous studies and to provide more support of the
rationale, it is of interest to examine the effectiveness of
chiral ligands H8–BINAPO 1 and H8–BDPAB 3 in
comparison with BINAPO 2 and BDPAB 4 in the
asymmetric hydrogenation of dehydroamino acid
derivatives and other substrates. Herein, we report the
results of this comparative study and provide more
evidence for the superior properties of the partially
hydrogenated binaphthyl species (Fig. 1).
*
R
NHCOCH3
R
NHCOCH3
chiral Rh catalyst
(1)
The design of such diphosphine ligands remains an
active area of research, and the homogeneous asymmet-
ric catalytic hydrogenation of prochiral C=X (X=C,
N, O, and so forth) double bonds is one of the most
important applications of these enantioselective cata-
lytic reactions. It has been found that many ligands
* Corresponding author. Fax: 852-23649932; e-mail: bcachan@
polyu.edu.hk
0957-4166/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0957-4166(01)00399-8

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F.-Y. Zhang et al. / Tetrahedron: Asymmetry 12 (2001) 2337–2342
OPPh2
OPPh2
OPPh2
OPPh2
NHPPh2
NHPPh2
NHPPh2
NHPPh2
(R)-H8-BDPAB, (R)-3
(S)-BINAPO, (S)-2
(R)-BDPAB, (R)-4
(S)-H8-BINAPO, (S)-1
Figure 1.
2. Results and discussion
observed for the desired products and the substituents
on the phenyl group of the methyl (Z)-2-acetamidocin-
namate had a small effect on the enantioselectivity
(entries 1–4). The electronic influence of the sub-
stituents on the enantioselectivity was not significant
(entries 1, 4, 7). A comparison of the hydrogenation of
(Z)-acetamido-3-arylacrylic acids or their esters with
catalyst A and B was carried out and the results are
summarized in Table 2. The experimental data clearly
shows that the rate and enantioselectivity of the hydro-
genation of either (Z)-acetamido-3-arylacrylic acids or
their methyl esters catalyzed by catalyst A (containing
(S)-H8–BINAPO ligand) were higher than those from
the same reaction catalyzed by catalyst B (containing
(S)-BINAPO ligand). From Table 2, it can also be seen
that the rate and enantioselectivities of the hydrogena-
tion of (Z)-acetamido-3-arylacrylic acids were lower
than those of the hydrogenation of their corresponding
methyl esters when using catalyst A in the asymmetric
hydrogenation reactions.
It has been reported that H8–BINOL22 and H8–
BINAM19,22b can be obtained by partial hydrogenation
of the corresponding BINOL and BINAM, respec-
tively, without loss of enantiomeric excess. Ligands
H8–BINAPO 1 and H8–BDPAB19 3 were conveniently
prepared by the reaction of chlorodiphenylphosphine in
the presence of an organic base with the corresponding
H8–BINOL and H8–BINAM in 94 and 84% yield,
respectively. For the purpose of comparison, BINAPO
and BDPAB23 ligands were also synthesized from
BINOL and BINAM in a similar method, respectively.
The cationic rhodium catalysts (A: [Rh(S)-
1(COD)]BF4, B: [Rh(S)-2(COD)]BF4, C: [Rh(R)-
3(COD)]BF4, D: [Rh(R)-4(COD)]BF4) were prepared in
situ by stirring [Rh(COD)Cl]2 and silver tetra-
fluoroborate with (S)-H8–BINAPO, (S)-BINAPO, (R)-
H8–BDPAB and (R)-BDPAB, respectively, in THF at
ambient temperature.
When catalyst A was used in the asymmetric hydro-
genation of methyl (Z)-2-acetamidocinnamate under
100 psi H2 in CH2Cl2 solvent at ambient temperature, it
gave the desired product with 84% e.e. The e.e.
obtained in the reaction with catalyst A was found to
be insensitive to variations in hydrogen pressure, reac-
tion temperature and the ratio of substrate to catalyst.
A variety of methyl esters of (Z)-acetamido-3-aryl-
acrylic acids were hydrogenated with catalyst A and
the results were summarized in Table 1. It was found
that moderate to good enantioselectivities were
The asymmetric hydrogenation of methyl (Z)-2-
acetamidocinnamate using catalyst C (containing (R)-
H8–BDPAB ligands) gave the desired product with
higher e.e. (92.0% e.e.) than that obtained when using
catalyst A (containing (R)-H8–BINAPO ligand, 84.0%
e.e.) in CH2Cl2. After optimizing the reaction condi-
tions, the asymmetric hydrogenation of methyl (Z)-2-
acetamidocinnamate using catalyst C gave the desired
product with 96.1% e.e. under 1 atm. pressure of H2 at
ambient temperature in THF solvent. The rate of the
reaction was very fast and the completion time for the
Table 1. The asymmetric hydrogenation of (Z)-acetamido-3-arylacrylic acid methyl esters using catalyst Aa
COOCH3
NHCOR2
COOCH3
NHCOR2
H2
R1
R1
catalyst A
Entry
R1
R2
Time (min)
Conv. (%)
E.e.b (%)
1
2
3
4
5
6
7
8
Phenyl
CH3
CH3
CH3
CH3
Phenyl
CH3
CH3
10
10
10
10
10
10
10
10
100
100
100
100
100
49.0
100
95.7
84.0
85.0
78.3
80.8
63.9
81.0
83.5
80.0
o-Chloro-phenyl
m-Chloro-phenyl
p-Chloro-phenyl
2-Furyl
H
p-Methyl-phenyl
p-Methyl-phenyl
Phenyl
a Sub./cat.=500; pressure of H2=100 psi; ambient temperature; solvent: dichloromethane. The conversion and e.e. values were determined by
GLC using a Chrompack Chirasil-L-Val column.
b S configuration was obtained for all products.

F.-Y. Zhang et al. / Tetrahedron: Asymmetry 12 (2001) 2337–2342
2339
Table 2. A comparison of the hydrogenation of substituted dehydroalanines with catalysts A and Ba
COOR2
COOR2
H2
R1
R1
NHCOCH3
NHCOCH3
catalyst A or B
Entry
R1
R2
Catalyst
Time (min)
Conv. (%)
E.e.b (%)
1
2
3
4
5
6
7
8
Phenyl
Phenyl
Phenyl
Phenyl
m-Chloro-phenyl
m-Chloro-phenyl
m-Chloro-phenyl
m-Chloro-phenyl
CH3
CH3
H
A
B
A
B
A
B
A
B
10
10
30
30
10
10
30
30
100
85.5
81.9
71.2
84.0
64.0
74.2
18.0
78.3
54.7
37.0
10.5
H
CH3
CH3
H
100
70.3
87.9
56.3
H
a Sub./cat.=500; pressure of H2=100 psi; ambient temperature. The hydrogenation of methyl esters of (Z)-acetamido-3-arylacrylic acids were
carried out in dichloromethane. The hydrogenation of (Z)-acetamido-3-arylacrylic acids were carried out in methanol. The conversion and e.e.
values were determined by GLC using a Chrompack Chirasil-L-Val column.
b S configuration was obtained for all products.
reaction was only 10 min. In contrast to other rhodium/
phosphine catalyst systems,24 the addition of an organic
base, such as triethylamine, decreased the enantioselec-
tivity of the hydrogenation of methyl (Z)-2-acetamido-
cinnamate. The enantioselectivities of catalyst C in the
hydrogenation of a variety of (Z)-acetamido-3-aryl-
acrylic acid methyl esters were found to be high (>90%
e.e.) in all cases (Table 3). These results clearly show
the high potential for the application of catalyst C in
the asymmetric synthesis of chiral amino acids. Table 3
also shows the comparative catalytic activity of catalyst
C and D in the asymmetric hydrogenation of methyl
esters of (Z)-acetamido-3-arylacrylic acids. It is obvious
that the enantioselectivity of catalyst C was higher than
that of catalyst D in the same reaction (entries 1–5, 12,
15). Catalysts C and D were also tested in the hydro-
genation of a variety of (Z)-acetamido-3-arylacrylic
acids e.e.s of over 90% were observed with catalyst C in
all cases (Table 4). It was found that polar or protic
reaction media gave higher enantioselectivity and the
enantioselectivity decreased with increasing hydrogen
pressure. Similar to the hydrogenation of their methyl
esters, the e.e. values of the hydrogenation products
decreased significantly when an organic base was added
Table 3. The asymmetric hydrogenation of (Z)-acetamido-3-arylacrylic acid methyl esters using catalyst C or Da
COOCH3
NHCOR2
COOCH3
NHCOR2
H2
R1
R1
catalyst C or D
Entry
R1
R2
Cat. C, e.e.b (%)
Cat. D, e.e.b (%)
1
2
3
4
5
6
7
8
Phenyl
H
CH3
CH3
CH3
CH3
CH3
CH3
Phenyl
CH3
Phenyl
CH3
Phenyl
CH3
Phenyl
CH3
CH3
CH3
CH3
Phenyl
Phenyl
95.8
97.1
97.0
94.0
94.0
95.5
96.4
92.9
94.2
93.1
95.2
94.4
94.6
90.7
92.6
90.2
90.5
93.2
92.5
90.3
92.6
90.0
90.2
88.0
–
–
–
–
–
–
89.5
–
–
80.3
–
–
–
–
o-Chloro-phenyl
m-Chloro-phenyl
p-Chloro-phenyl
4-Bromo-phenyl
4-Bromo-phenyl
4-Fluoro-phenyl
4-Fluoro-phenyl
4-Methoxy-phenyl
4-Methoxy-phenyl
p-Methyl-phenyl
p-Methyl-phenyl
4-Nitro-phenyl
3,4-Methylene-dioxyphenyl
trans-Cinnamyl
2-Furyl
2-Furyl
N-Aceto-3-indole
9
10
11
12
13
14
15
16
17
18
19
a Sub./cat.=100; pressure of H2=30 psi; ambient temperature; time=10 min.; solvent: THF; quantitative conversion for all cases. The conversion
and e.e. values were determined by GLC using a Chrompack Chirasil-L-Val column.
b R configuration was obtained for all products.

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F.-Y. Zhang et al. / Tetrahedron: Asymmetry 12 (2001) 2337–2342
Table 4. The asymmetric hydrogenation of a variety of dehydroamino acids using catalyst C or Da
COOH
COOH
H2
R1
R1
NHCOR2
NHCOR2
catalyst C or D
Entry
R1
R2
Cat. C, e.e.b (%)
Cat. D, e.e.b (%)
1
2
3
4
5
6
7
8
9
Phenyl
H
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
Phenyl
94.2
99.0
94.1
92.8
93.1
92.5
90.0
91.1
93.9
90.3
93.5
89.6
88.2
86.0
–
–
76.8
–
o-Chloro-phenyl
m-Chloro-phenyl
p-Chloro-phenyl
2-Methoxy-phenyl
4-Nitro-phenyl
3,4-Methylene-dioxyphenyl
2-Furyl
a Sub./cat.=100; pressure of H2=30 psi; ambient temperature; time=10 min; solvent: ethanol; quantitative conversion for all cases. The
conversion and e.e. values were determined by GLC using a Chrompack Chirasil-L-Val column.
b R configuration was obtained for all products.
4. Experimental
4.1. General considerations
to the reaction. It can also be seen from Table 4 that
the enantioselectivities in the hydrogenation of (Z)-
acetamido-3-arylacrylic acids catalyzed by catalyst C
were higher than those from the same reactions with
catalyst D (entries 1–5, 8). These results further
exhibited the generality of using the more sterically
demanding H8–binaphthyl unit in the C2-symmetric
chiral ligands. It was reported that the dihedral angle
All experiments were carried out under nitrogen
atmosphere. Unless otherwise stated, commercial
reagents were used as received without further purifi-
cation. All solvents used were dried using standard,
published methods and were distilled before use. The
preparation of samples and the setup of reactions
were either performed in a nitrogen-filled MBRAUN
Lab Master 130 glovebox or using standard Schlenk-
type techniques. The hydrogenation reactions were
performed in a 50 mL stainless-steel autoclave from
Parr company. NMR spectra were obtained on a
BRUKER Model ADVANCE DPX 400 spectrometer
(1H: 400 MHz, 13C: 101 MHz, 31P: 162 MHz). 13C
and 31P spectra were referenced from Me4Si and
external 85% H3PO4, respectively. Melting points were
determined using an Electrothermal 9100 apparatus in
capillaries sealed under nitrogen. Mass analyses were
performed on a V.G. MICROMASS, Fisons VG plat-
form and a Finnigan Model Mat 95 ST spectrometer.
Optical rotations were measured on a Perkin–Elmer
Model 341 polarimeter. GLC and HPLC analyses
were performed using a Hewlett–Packard Model HP
5890 Series II GC and a Hewlett–Packard Series 1050
HPLC, respectively.
(80.3°) of the two tetralin rings in [Rh((S)-H8–
12b
BINAP)(COD)]ClO4
is wider than those of the two
naphthalene rings in the BINAP–Rh complexes
reported (71.0–75.5°).25 This difference is a reflection
of the greater steric hindrance of hydrogen atoms
attached to sp3 carbon atoms in tetralin rings than
that of hydrogen atoms on sp2 carbon atoms in
naphthalene rings. Unfortunately, no X-ray structures
of the H8–BINAPO–Rh and H8–BDPAB–Rh com-
plexes are available.
3. Conclusion
We have studied the effects of chiral bisphosphinite
(H8–BINAPO) and chiral bisaminophosphine (H8–
BDPAB) ligands prepared from H8–BINOL and H8–
BINAM, respectively. Both ligands were found to be
effective in the Rh-catalyzed hydrogenation of (Z)-
acetamido-3-arylacrylic acids and their esters. The
enantioselectivities of the catalyst containing chiral
bisaminophosphine ligand in the hydrogenation of
(Z)-acetamido-3-arylacrylic acids and their esters were
found to be higher than those from the same reaction
using a catalyst containing the chiral bisphosphinite
ligand. Furthermore, both the H8–binaphthyl chiral
ligands exhibited higher catalytic activities and enan-
tioselectivities than those prepared from their parents
ligands. These findings show the excellent opportuni-
ties for the improvement of many existing catalyst
systems.
4.2. Synthesis of (S)-2,2%-bis(diphenylphosphino)-
5,5%,6,6%,7,7%,8,8%-octahydro-1,1%-binaphthyl [(S)-H8-
BINAPO], (S)-1
A solution of (S)-H8–BINOL22 (200 mg, 0.7 mmol) in
diethyl ether (20 mL) containing dried pyridine (0.13
mL, 1.6 mmol) was charged to a 50 mL Schlenk flask
under a nitrogen atmosphere. This flask was cooled to
0°C and a solution of chlorodiphenylphosphine (0.32
mL, 1.8 mmol) in THF (5 mL) was added dropwise.

F.-Y. Zhang et al. / Tetrahedron: Asymmetry 12 (2001) 2337–2342
2341
The system was allowed to stir for 5 h at 0°C and the
4.6. Synthesis of [Rh(R)-3(COD)]BF4 (Catalyst C)
temperature was raised to room temperature. The solu-
tion was filtered to remove the solid. The solvent was
evaporated in vacuo to give crude product (450 mg),
which was dissolved in CH2Cl2 (2 mL) and then diethyl
ether (10 mL) was added to the solution. The final
solution was kept at −30°C for 24 h to allow the growth
of pure crystals of (S)-H8–BINAPO. After filtration
and drying in vacuo, white, needle-like crystals were
obtained (436 mg, 94% yield). Mp 130–132°C; [h]D
The procedure is the same as the synthesis of [Rh(S)-
1(COD)]BF4. 31P NMR (162 MHz, THF): l 63.45 (d,
JRh-P=155.1 Hz).
4.7. Synthesis of [Rh(R)-4(COD)]BF4 (Catalyst D)
The procedure is the same as the synthesis of [Rh(S)-
1(COD)]BF4. 31P NMR (162 MHz, THF): l 69.06 (d,
JRh-P=158.0 Hz).
1
−82.0 (c=1.0, THF); H NMR (400 MHz, CDCl3): l
7.38 (m, 20H), 6.99 (m, 4H), 2.72 (m, 4H), 2.23 (m,
4H), 1.52 (m, 4H), 1.30 (m, 4H). 13C NMR (101 MHz,
CDCl3): l 152.9, 152.8, 142.6, 142.5, 142.4, 137.3,
132.0, 129.9, 129.8, 129.3, 129.2, 129.1, 128.9, 128.8,
128.6, 128.4, 115.8, 115.6, 29.8, 28.0, 23.2. 31P NMR
(162 MHz, CDCl3): l 103.89. HRMS: calcd for
C44H40O2P2: 663.2582; found: 663.4310 [M+H]+.
4.8. A typical procedure for the catalytic asymmetric
hydrogenation of methyl (Z)-2-acetamidocinnamate
A THF solution of Catalyst C (600 mL, 0.0024 mmol)
and methyl (Z)-2-acetamidocinnamate (0.053 g, 0.24
mmol) in THF (10 mL) were charged to a 50 mL
autoclave. The hydrogenation was carried out under
200 kPa of hydrogen pressure at room temperature for
10 min. A portion of the reaction mixture was analyzed
by GLC to determine the product composition. Quanti-
tative conversion of the starting material to the hydro-
genation product, methyl (R)-2-acetamido-3-phenyl-
propanoate, with 95.8% e.e. was observed. (The e.e.
4.3. Synthesis of (R)-2,2%-bis(diphenylphosphinoamino)-
5,5%,6,6%,7,7%,8,8%-octahydro-1,1%-binaphthyl [(R)-H8–
BDPAB], (R)-3
Chlorodiphenylphosphine (0.76 mL, 4.28 mmol) and
NEt3 (1.0 mL, 7.04 mmol) were added in the solution of
(R)-H8–BINAM19 (0.5 g, 1.71 mmol) in anhydrous
CH2Cl2 (15 mL), and the mixture was stirred for 24 h
under reflux. The solution was cooled, concentrated
under reduced pressure and anhydrous diethyl ether (10
mL) was added. The precipitate was filtered and
washed with anhydrous diethyl ether (10 mL). The
solvent was removed in vacuo and the crude residue
was purified by recrystallization from anhydrous
diethyl ether (2 times) at −20°C for 24 h (85% yield).
was determined by chiral capillary GC using
Chrompack Chirasil-L-Val Column.)
a
4.9. A typical procedure for the catalytic asymmetric
hydrogenation of N-acetamidoacrylic acid
A THF solution of Catalyst C (600 mL, 0.0024 mmol)
and N-acetamidoacrylic acid (0.031 g, 0.24 mmol) in
ethanol (10 mL) were charged to a 50 mL autoclave.
The hydrogenation was carried out under 200 kPa of
hydrogen pressure at room temperature for 10 min. A
portion of the reaction mixture was analyzed by GLC
to determine the product composition. Quantitative
conversion of the starting material to the hydrogena-
tion product, (R)-acetamidopropanoic acid, with 99.0%
e.e. was observed. (The enantiomeric excess was deter-
mined by chiral capillary GC using a Chrompack Chi-
rasil-L-Val Column after converting the product to
methyl ester.)
1
Mp 137–139°C; [h]D: −47.0 (c=1.0, CH2Cl2); H NMR
(400 MHz, CDCl3): l 7.24 (m, 22H), 6.98 (d, JH-H
=
8.34 Hz, 2H), 4.27 (d, JP-H=7.0 Hz, 2H), 2.67 (m, 4H),
2.10 (m, 4H), 1.58 (m, 8H). 13C NMR (101 MHz,
CDCl3): l 141.9, 141.7, 141.2, 141.0, 140.6, 140.5,
136.1, 130.8, 130.4, 130.2, 129.6, 128.9, 128.7, 128.4,
128.3, 128.2, 123.2, 112.7, 112.5, 29.3, 27.3, 23.1, 23.0.
31P NMR (162 MHz, CDCl3): l 27.25. HRMS: calcd
for C44H42N2P2: 661.2901; found: 661.4382 [M+H]+.
Anal. calcd for C44H42N2P2: C, 79.97; H, 6.41; N, 4.24.
Found: C, 79.78; H, 6.40; N, 4.24%.
4.4. Synthesis of [Rh(S)-1(COD)]BF4 (catalyst A)
Acknowledgements
[Rh(COD)Cl]2 (5.0 mg, 0.01 mmol) and AgBF4 (4.0 mg,
0.02 mmol) in THF (2 mL) were stirred at room
temperature for 30 min under a nitrogen atmosphere.
The solution was filtered to remove the solid AgCl.
After the addition of (S)-H8–BINAPO (13 mg, 0.02
mmol) in THF (3 mL) to the solution, [Rh(S)-
1(COD)]BF4 (Catalyst A) in THF was obtained in situ
(4×10−6 mol/mL). 31P NMR (162 MHz, THF): l 28.4
(d, JRh-P=168.0 Hz).
We thank the Hong Kong Research Grants Council
(project c PolyU5152/98P) and The Hong Kong Poly-
technic University ASD for financial support of this
study.
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4.5. Synthesis of [Rh(S)-2(COD)]BF4 (Catalyst B)
The procedure is the same as the synthesis of [Rh(S)-
1(COD)]BF4. 31P NMR (162 MHz, THF): l 125.5 (d,
JRh-P=170.0 Hz).

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F.-Y. Zhang et al. / Tetrahedron: Asymmetry 12 (2001) 2337–2342
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