2 Frustrated Lewis Pair Catalyzed Asymmetric Reactions
47
Table 2.11 Asymmetric hydrogenation of 1,4-benzoxazines 30
N
O
Ar
N
H
O
Ar
HB(C 6 F 5 ) 2 (5 mol %)
H 2 (20 bar), CH 2 Cl 2
rt, 12 h
chiral diene 9i (2.5 mol %)
30
31
Entry product
yield (%) ee (%)
1
31a
N
H
O
Ph
95
33
2
31b
N
H
O
OMe
90
40
3
31c
N
H
O
Ph
90
42
4
31d
N
H
O
F
92
31
5
31k
N
H
O
OMe
92
33
6
31l
N
H
O
Cl
97
30
2.2.4 Asymmetric Hydrogenations of Ketones and Enones
Generally, the asymmetric induction for the FLP catalysis comes from the chiral
Lewis acid component. As a two-component catalyst, chiral Lewis base is also likely
to control the asymmetric process. However, very limited intermolecular FLPs with
chiral Lewis bases have been developed. Chiral Lewis bases have been widely utilized
as ligands or catalysts in the asymmetric catalysis. To combine chiral Lewis bases
with achiral Lewis acids as FLP catalysts, would provide a novel and convenient
direction toward the FLP construction.
The asymmetric hydrogenation of ketones and electron-deficient unsaturated
compounds have seldom been reported [50, 51]. Very recently, a novel type of
chiral FLPs was developed by a combination of chiral oxazoline Lewis bases with