Chemistry Reference
In-Depth Information
the reaction proceeds through the well - known Weitz - Scheffer mechanism. The conju-
gate addition of hydrogen peroxide, which is a good nucleophile and leaving group, to
the
-carbon atom of the electrophilic iminium ion is reversible, and the attack on the
electrophilic peroxygen atom by the nucleophilic enamine is the step determining the
product ' s stereochemistry.
Asymmetric epoxidation of
β
-unsaturated aldehydes by the iminium/enamine
catalysis has also been investigated by MacMillan and Lee (Scheme 11.49) [84]. They
utilized chiral imidazolidinone salt
39
as a catalyst with not hydrogen peroxide but
iodosobenzene as a stoichiometric oxidant. A combination of [(nosylimino)iodo]benzene
(NsNIPh) and acetic acid slowly liberates the active iodosobenzene monomer. The reac-
tion of
α
,
β
-unsaturated aldehydes furnished the epoxide with high enan-
tioselectivity as single diastereomers, except for crotonaldehyde.
List and coworkers also reported secondary amine-catalyzed asymmetric epoxidation
β
- substituted,
α
,
β
of
-unsaturated aldehydes, and their strategy is very attractive, in which the authors
utilized a catalyst
40
consist of achiral dibenzylamine and BINOL-derived chiral phos-
phoric acid (Scheme 11.50) [85]. Together with TBHP as oxidant, the catalyst
40a
pro-
motes the epoxidation of both
α
,
β
- unsaturated aldehydes
to give the epoxides with high diastereo- and enantioselectivities. When the two
β
- mono - and disubstituted
α
,
β
-
substituents are identical, the enamine intermediate is achiral. However, the substrate
underwent epoxidation with high enantioselectivity. Thus, the authors proposed that the
chiral phosphoric acid must assist the ring-closing step.
List and coworkers also discovered that chiral ammonium salts pairing chiral diamines
and chiral/achiral acids catalyze the asymmetric epoxidation of cyclic enones using
β
39
(20 mol
%)
O
CHO
+ NsNIPh
CHO
R
R
CH
2
Cl
2
-AcOH, -30°C
O
85-97% ee
N
•HClO
4
H
Ph
O
O
O
O
CHO
CHO
CHO
CHO
39
Cy
Ph
p
-NO
2
-C
6
H
4
88%, 93% ee
1:7 dr
77%, 92% ee
92%, 92% ee
89%, 97% ee
Scheme 11.49.
i
Pr
R
2
R
2
CF
3
40a
(10 mol
%)
i
Pr
O
CHO
+ TBHP
CHO
R
1
R
1
F
3
C
70-96% ee
i
Pr
O
−
O
O
O
NH
2
+
P
O
O
i
Pr
CHO
CHO
O
O
CHO
F
3
C
CHO
C
6
H
13
i
Pr
CF
3
i
Pr
40a
75%, 91% ee
>99:1 dr
76%, 96% ee
>99:1 dr
67%, 70/92% ee
94:6 dr
83%, 94% ee
Scheme 11.50.
