Chemistry Reference
In-Depth Information
such as
5
can work well in water under the conditions, and that this is the fi rst demon-
stration of a combination of silica gel-supported metal catalysts and ILs to create effi -
cient hydrophobic environments for organic reactions in water.
Although there is room to improve the results, an asymmetric catalysis can be real-
ized using Silica-Sc-IL combined with a ligand
L - 3
(Scheme 1.18 ).
L-3
(6-6.5 mol %)
Silica-Sc
4
(0.24-0.28 mmol/g, 5 mol %)
IL (50 wt %)
O
OSiMe
3
aq. HCHO
+
HO
Ph
(5 equiv)
H
2
O, 35°C, 20-24 h
*
Ph
N
N
SbF
6
47% yield, 49% ee
N
N
NTf
2
28% yield, 66% ee
Scheme 1.18.
1.2.2. Chiral Organocatalysis in Water
In contrast to metal-based catalysts, organocatalyses are easy to handle, since they are,
in general, stable in water. Amino acids are representative of organocatalysts and their
derivatives have been widely investigated. Tert-butyldimethylsilyl (TBS)-protected
hydroxyproline
L - 5
catalyzes a direct-type aldol reaction, leading to the aldol adduct in
good yield with high diastereo- and enantioselectivities. And a Michael reaction of a
ketone or an aldehyde with
-nitrostyrene in brine can be catalyzed by a proline deriva-
tive
L - 6
to afford the corresponding product in good yield with good diastereo- and
enantioselectivities (Scheme 1.19). Moreover, a threonine derivative
L - 7
catalyzes
Mannich reaction to produce the product in good yield with high diastereo- and enan-
tioselectivities (Scheme 1.20 ) [22] .
The Diels-Alder reaction is a powerful transformation method in organic chemistry.
Chiral imidazolidinone can successfully catalyze the reaction of
β
- unsaturated ketones
with dienes, where chiral Lewis acid catalysts showed lower enantioselectivities (Scheme
1.21 ) [23,24] .
α
,
β
1.2.3. Others
1.2.3.1. Asymmetric Reactions Using Alkynes
[25]
Optically active propargylamines
are important synthetic intermediates for various nitrogen-containing compounds, a
structural feature of many biologically active compounds and natural products. The
most reliable and effi cient methods for the preparation of optically active propargylic
amines are still dependent on the addition of appropriate organometallic reagents to
chiral imine derivatives. While methods for catalyzed preparation of optically active
propargylic amines are still limited, highly enantioselective direct-type alkyne-imine
addition was recently reported using a chiral Cu(I)•
L - 9
complex. The process is simple
and provides a diverse range of propargylic amines in high enantioselectivity (Scheme
1.22 ) [26] .
