Chemistry Reference
In-Depth Information
Ph
Ph
Rh
2
(cap)
4
Rh
2
(pfb)
4
N
2
O
O
O
8.499
8.500
8.501
Scheme 8.134
t
-Bu
N
O
N
2
MeO
8.502
Rh
2
(cap)
4
Rh
2
(pfb)
4
t
-Bu
N
O
MeO
N
t
-Bu
O
MeO
8
.
3
8
5
Scheme 8.135
be obtained by structure variation within this small group (Scheme 8.134). For instance, the diazoketone
8.500
gives the product of intramolecular cyclopropanation
8.499
using a rhodium-caprolactam complex, but
the product of C-H insertion
8.501
with a perfluorobutyrate complex.
158
Similarly, with a substrate bearing
an electron-rich aryl ring
8.502
, the caprolactam complex favoured CH insertion, while the perfluorobutyrate
complex led to predominant attack on the aryl ring in a Buchner reaction (Scheme 8.135).
8.5.1 Nucleophilic Trapping
Trapping by nucleophiles such as alcohols, which can also be drawn as insertion into the O-H bond, is a
useful process. Cyclic ethers can be formed under very mild conditions (Scheme 8.136).
159
Similarly, amines
and other nitrogen derivatives can be employed to trap the carbene, most notably in the commercial synthesis
of the antibiotic thienamycin in which a five-membered ring is annulated onto an existing
-lactam
8.507
(Scheme 8.137).
160
The carbenes may also be trapped by nucleophilic groups that lack a hydrogen atom. The use of
carbonyl groups, in an intramolecular fashion, in this way generates unstable, reactive oxonium ylids
(Scheme 8.138). These ylids
8.511
are 1,3-dipoles and can participate in a cycloaddition reaction with
an added dipolarophile.
161,162
Many tandem processes can be designed around this concept.
163
N
2
O
R
Rh
2
(OAc)
4
CO
2
t
-Bu
OH
O
O
R
CO
2
t
-Bu
8.506
8.505
Scheme 8.136
