R NHCOCH
3
COOH H
2
R NHCOCH
3
COOH
chiral Rh catalyst
*
TETRAHEDRON:
ASYMMETRY
Tetrahedron: Asymmetry 12 (2001) 2337–2342 Pergamon
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 H
8
–BINAPO and H
8
–BDPAB were synthesized by reacting chlorodiphenylphosphine with H
8
–
BINOL and H
8
–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 (H
8
–BINAPO: up to 84.0% e.e.; H
8
–BDPAB: up to 97.1% e.e.). In the hydrogenation of acetamidoacrylic acid, 99% e.e.
was obtained when a [Rh(H
8
–BDPAB)]
+
catalyst was used. The catalytic activities and enantioselectivities of [Rh(H
8
–BINAPO)]
+
and [Rh(H
8
–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
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 -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 -amidoacrylates (-enamides) (Eq.
(1)).
(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
with C
2
symmetry were effective in asymmetric hydro-
genation reactions. For example, the aromatic BINAP
ligand, which possesses C
2
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
Takaya first prepared the atropisomeric ligand, H
8
–
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
(e.g. H
8
–BINAP,
13–15
H
8
–BINOL,
16–18
H
8
–BINAM,
18
H
8
–BDPAB,
19
H
8
–binaphthoxy,
20
H
8
–MAPs
21
) exhibit
higher efficiency and enantioselectivity for asymmetric
reactions than those prepared from their parent ligands,
due to the steric and electronic modulation in the
H
8
–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 H
8
–BINAPO 1 and H
8
–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).
* 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