Chemistry Reference
In-Depth Information
A major problem in the formation of cyclopentanes via intramolecular C- H inser-
tions of
- elimination to form
cis
- alkenes [160] . Taber
and Joshi reported that the ratio of C-H insertion versus
α
-diazoesters is the competing
β
- elimination in such systems
is infl uenced by the electronic nature of the catalyst as well as the steric bulk of the
ligands [161]. Based on this work, signifi cant progress in the area was reported by Hashi-
moto and coworkers (Scheme 4.6 ) [160] . Diazoesters
24
were reported to undergo
exclusive insertion to form the corresponding
cis
- cyclopentanes
25
in 81-85% yield with
92-95% ee when catalyzed by Rh
2
(
S
- PTTL)
4
. Further exploration revealed that simple
alkyl diazoester
26
(Scheme 4.7) also gave insertion products exclusively with the
trans
-
diastereomer as major product in 68% yield and 94% ee. A signifi cant amount of the
cis
-diastereomer was also formed (7%). Reaction of the phenyl system
28
(Scheme 4.8 )
afforded the corresponding disubstituted
cis
- indane product
29
in 85% yield and 92%
ee. A remarkable feature of these examples is that no trace of competing
β
- elimination
was observed. These examples underscore the importance of choice of conditions and
catalyst for carbenoid transformations [160] .
β
CO
2
Me
CO
2
Me
N
2
Rh
2
(
S
-PTTL)
4
R =
H
OMe
Cl
Toluene, -78°C
R
81-85% yield
92-95% ee
R
24
25
Scheme 4.6.
Enantioselective cyclopentane formation.
CO
2
Me
CO
2
Me
CO
2
Me
N
2
Rh
2
(
S
-PTTL)
4
Toluene, -78°C
26
cis-
27
7% yield
trans
-27
68% yield
94% ee
Scheme 4.7.
Intramolecular reaction of alkyldiazoacetate
26
.
CO
2
Me
Rh
2
(
S
-PTTL)
4
CO
2
Me
N
2
Toluene, -78°C
Ph
Ph
85% yield
92% ee
28
29
Scheme 4.8.
Formation of substituted indane
29
.
