Chemistry Reference
In-Depth Information
Progress in the fi eld of asymmetric organocatalysis of chiral amines has increased
remarkably in recent years. The amine catalysis can be divided into two classes: iminium
ion formation, which leads to lowering the LUMO energy, and generation of nucleo-
philic enamine intermediates [67,80]. The iminium/enamine catalysis has been success-
fully applied to asymmetric epoxidation. Jørgensen and coworkers identifi ed chiral
pyrrolidine
38
as a catalyst for asymmetric epoxidation of
α
,
β
- unsaturated aldehydes
with aqueous hydrogen peroxide (Scheme 11.47 ) [81 - 83] .
α
,
β
- Unsaturated aldehydes
having an aromatic substituent at the
-position are good substrates for the reaction,
and the epoxides were obtained with high diastereo- and enantioselectivity. High stere-
oselectivity was also observed in the reaction of the alkyl-substituted,
β
α
,
β
- unsaturated
aldehydes. While the reaction of
- unsaturated aldehydes smoothly
proceeded, the enantioselectivity was slightly decreased. As described in Scheme 11.48,
β
- disubstituted,
α
,
β
R
2
R
2
38
(10 mol
%)
F
3
C
O
+ 35% H
2
O
2
CHO
CHO
CF
3
R
1
R
1
CH
2
Cl
2
, RT, 4 h
OTMS
H
O
O
O
O
CHO
CHO
CHO
CHO
Ph
Et
BnOH
2
C
CF
3
F
3
C
80%, 96% ee
93:7 dr
>90%, 96% ee
97:3 dr
84%, 94% ee
96:4 dr
65%, 75% ee
38
Scheme 11.47.
O
O
H
H
Ar
O
OTMS
R
R'
H
R
R'
Ar
38
Ar
OTMS
Ar
OTMS
N
+
N
+
Catalytic cycle
Ar
Ar
O
OH
−
OH
−
R
R'
R
R'
H
2
O
2
Ar
OTMS
N
Ar
H
2
O
O
R'
HO
R
Scheme 11.48.
