Chemistry Reference
In-Depth Information
The reaction of cyclic vinylsilane
135
with methyl 4-bromophenyldiazoacetate
106
yields the allylic insertion product
136
in 64% yield, 88% de, and 95% ee when catalyzed
by Rh
2
(
S
- DOSP)
4
(Scheme 4.35) [209,214]. Although the enantioselectivity typically is
excellent for C-H insertion reactions, low to moderate diastereoselectivity is a major
problem. Another example of this is the insertion into 1-methylcyclohexene
138
(Scheme
4.36) with heteroaryldiazoacetate
137
, which again gives low diastereoselectivity (50%
de) but very high enantiomeric excess (94% ee). To achieve control of the diastereose-
lectivity, the two methylene substituents at the site of insertion must be sterically dif-
ferentiated [209,214]. This was effectively demonstrated for the acyclic silyl vinyl ether
140
(Scheme 4.37). The methylene site is electronically activated, but also has two steri-
cally distinct substituents, which leads to insertion product
141
in 65% yield with
>
90%
de and in 84% ee [208].
Ph
Ar
Ph
p
-(Br)Ph
CO
2
Me
Ph
H
Ph
Rh
2
(
S
-DOSP)
4
2,2-DMB, 23°C
Si
Si
CO
2
Me
N
2
106
135
64% yield
88% de
95% ee
136
Scheme 4.35.
Allylic C -H insertion into vinylsilane
135
.
S
S
Rh
2
(
S
-DOSP)
4
2,2-DMB, 23°C
H
CO
2
Me
N
2
CO
2
Me
137
138
40% yield
50% de
94% ee
139
Scheme 4.36.
Allylic C - H insertion into 1 - methylcyclohexene.
Me
OTBDPS
p
-(Br)Ph
CO
2
Me
Me
OTBDPS
Rh
2
(
S
-DOSP)
4
2,2-DMB, -30°C
MeO
2
C
Ph
N
2
106
Ph
Ar
1
65% yield
>90% de
84% ee
1
Scheme 4.37.
Diastereoselective C -H insertion into acyclic substrate
140
.
C -H insertion into doubly allylic sites is a very favorable process and typically pro-
ceeds in high yield and with enantiomeric excess >90% with aryldiazoesters when the
reaction is catalyzed by Rh
2
(
S
- DOSP)
4
(Scheme 4.38) [230,231]. For example, insertion
into 1,4-cyclohexadiene afforded product
143
in 80% yield and in 91% ee [230,231]. For
