Chemistry Reference
In-Depth Information
O
OTIPS
5a
(5 mol %)
+
R
2
COR
1
CH
2
Cl
2
-78°C, 2-48 h
R
2
OH
COR
1
R
2
OH
COR
1
TIPSO
O
+
Not observed
R
1
= OEt, R
2
= Me
R
1
= OEt, R
2
= Ph
R
1
= Me, R
2
= Me
R
1
= Me, R
2
= Et
96% ee (93%)
98% ee (72%)
94% ee (81%)
>99% ee (84%)
R
1
= OEt, R
2
= Me
90% ee (85%)
(0.01 mol %,
S
/
C
10,000)
Scheme 8D.9.
OTIPS
O
+
GCO
2
Et
(
S
)-SEGPHOS-Pd
2+
5a
O
Si
H
P
P
P
P
Pd
Pd
O
O
O
O
Et
Et
O
O
H
G
G
Si
O
TIPSO
G
OH
TIPSO
G
OH
CO
2
Et
CO
2
Et
(
R
)-product
(
S
)-product
Figure 8D.9.
The sense of enantioselectivity can be explained as follows (Fig. 8D.9). The keto
ester-ene reaction should proceed through the six-membered transition state. Compared
with the right transition state, the left transition state to give the (
S
) - product should be
less favorable by steric repulsion between the equatorial phenyl group of SEGPHOS
and silyl enol ether substrate. Therefore, the reaction via the right transition state should
proceed to give the (
R
) - product.
