Chemistry Reference
In-Depth Information
a)
Pd
O
O
Mukaiyama aldol
reaction
COR
Pd enolate
G
O
OH
G
b)
Pd cat*
COR
*
Pd
Si
Si
OO
Aldol product
O
COR
G
Silatropic
+
O
G
R
c)
H
Pd
O
Si
O
G
OH
Pd cat*
Si
COR
O
COR
*
G
Ene reaction
Prototropic
Ene product
Scheme 8D.8.
8D.8a,b). Sodeoka, Lectka, and others reported the mechanistic difference
between Pd(II) enolate [30] and Pd(II) Lewis acid catalysis [31] in the asym-
metric aldol-type reaction with aldehydes or imines. The Pd enolate would be
generated via transmetallation of the silyl group with Pd(II) aqua complex,
which is generated from dicationic Pd(II) complex and H
2
O. The bulky and
stable silyl group would be resistant to transmetallation. In the silatropic pathway,
the sterically hindered silyl group should hardly involve in a pericyclic transition
state. Therefore, bulky silyl group such as triisopropyl silyl should be employed
to guide the prototropic ene reaction.
(2) The decomposition of silyl enol ether and overreaction, namely two-directional
reaction under the Lewis acid conditions.
Therefore, the reaction should be carried out at low temperature to keep the
one - directional product.
The Pd-complex-catalyzed keto ester-ene reaction, which constructs highly optically
active β-hydroxy silyl enol ether with quaternary carbon center, is reported [32]. Various
dicationic Pd catalysts are
in situ
generated from chiral PP * - PdCl
2
complex and 2.2 equiv
of AgSbF
6
in dichloromethane at room temperature. The reactions of silyl enol ethers
with keto esters by using 5 mol % of (
S
) - SEGPHOS - Pd
2+
5a
proceed smoothly to give
ene product in 94->99% ee without Mukaiyama aldol-type product (Scheme 8D.9). Even
with the smallest substrate/catalyst ratio (S/C 10,000), the high yield and enantioselectiv-
ity (85% yield, 90% ee) could be obtained. This is the fi rst example of ene reaction on
silyl enol ether with ketones to afford quaternary carbon centers. The chiral Pd com-
plexes are (1) air and moisture stable, (2) easily synthesized, and (3) catalytically active
(up to 0.01 mol %) with high yield and enantioselectivity.
