Chemistry Reference
In-Depth Information
CO
2
H
H
O
OH
(0.5 mol %)
NaBH
4
Ar
+
+
ArNH
2
HCHO
H
DMSO, MW
Up to 98% ee
Scheme 1.48.
Chiral organocatalyzed aldol reaction, Michael-type reaction, and Diels-Alder reac-
tion have been applied to microwave-assisted reactions [94]. In all cases, the reaction
times are dramatically shortened. The most successful example in terms of enantiose-
lectivity was bipyrrolidine-catalyzed Michael-type reaction. Compared with conven-
tional heating (Conditions B), it is clear that microwave heating accelerates the reaction
without loss of diastereo - and enantioselectivities (Scheme 1.49 ).
O
O
Ph
Organocatalyst (15 mol %)
NO
2
NO
2
+
*
Ph
*
OH
OH
Conditions A: catalyst
L-34
, CHCl
3
, MW, 28°C, 4 h,
83% yield,
syn
/
anti =
11/89, 98% ee (
R
,
R
)
Conditions B: catalyst
L-35,
CHCl
3
, rt, 168 h,
79% yield,
syn
/
anti =
18/83, 98% ee (
S
,
S
)
H
N
H
N
L-34
L-35
Scheme 1.49.
1.5. CONCLUSION/PERSPECTIVE
Catalytic asymmetric synthesis in nonconventional media/conditions has been surveyed.
While chiral catalysts have been developed in conventional organic solvents, noncon-
ventional media/conditions are important not only from a scientifi c aspect but also from
an economical point of view. Since many elegant
in vitro
reactions are carried out in an
aqueous environment in our bodies, catalytic symmetric reactions in water under
in vitro
conditions are being studied to clarify and understand their mechanisms. Further, water
is inexpensive and the most environmentally friendly solvent. On the other hand, recov-
ery and reuse of catalysts in asymmetric reactions is extremely important especially in
industry, since most chiral catalysts are expensive. Immobilization of chiral catalysts
in nonconventional media such as fl uorous solvents, SCFs, and ILs can meet such
purposes.
