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D-
ido
D-
xylo
D-
gluco
Me
Me
PPh
2
Ph
2
P
Ph
2
P
PPh
2
PPh
2
PPh
2
O
O
O
O
O
O
O
O
O
57
58
56
Figure 8.9
Diphosphines
56
-
58.
Table 8.5
Hydrogenation of unsaturated acids and esters with diphosphine ligands
56
-
58.
Entry
Product
Ligand
Conversion (%)
ee (%)
Ph
1
56
100
89
(S)
HO
2
C
*
2a
NHAc
2
57
100
98
(S)
Ph
MeO
2
C
*
2b
NHAc
3
58
100
53
(S)
4
56
100
92
(S)
5
MeO
2
C
*
2d
NHAc
57
100
98
(S)
6
58
100
53
(S)
CO
2
H
7
56
100
62
(R)
HO
2
C
*
4a
The difference between ligands
57
and
58
on one hand and ligand
56
on the
other hand is the presence of a new stereogenic center introduced at C5 in
57
and
58.
The results indicate that the methyl substituent at C5 significantly increased
activity (TOFs were approximately double for ligands
57
and
58
). The configuration
at C5 also influences strongly the enantioselectivity. The catalytic system Rh/
57,
with a
(R)
configuration at C5, proved to be the most efficient, providing an ee of
98% in the hydrogenation of methyl
-acetamidocinnamate (
1b
), and acetami-
doacrylate (
1d
) (entries 2 and 5, Table 8.5), while the system Rh/
58
only gave 53%
ee (entries 3 and 6, Table 8.5).
More recently, phosphorus functionalities have been incorporated into cyclodex-
trins (ligand
59,
Figure 8.10) to take advantage of the properties of cyclodextrins
as water-soluble chiral supports. Ligand
59
[35] contains phosphine in two of the
α
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