Chemistry Reference
In-Depth Information
N
N
N
TFA
hν
R
R
R
N
N
N
N
N
3
N
N
N
256
257
258
R
260
(%)
N
N
TFA
H
Me
OMe
Cl
91
95
84
88
R
R
N
N
N
N
H
H
OCOCF
3
259
260
SCHEME 10.41.
in the presence of TFA provided 7-substituted 9
H
-pyrimido[4,5-
b
]indoles
in
excellent yields (Scheme 10.41).
93
Significantly, attempts to purposely access the
nitrene, via photolysis of
260
260
in significantly lower yields. Studies by McClelland suggest that, in light of the
basicity of singlet nitrenes, weaker acids than TFA and even water may serve as
proton donors for aryl nitrenes.
94
Recently, Driver
et al.
have reported a nonphotolytic, metal-mediated variant of
the transformation shown in Scheme 10.42, in which treatment of biaryl azides
259
in the absence of TFA or thermolysis, furnished
261
with substoichiometric quantities of Rh
2
(O
2
CC
3
F
7
)
4
or Rh
2
(O
2
CC
7
H
15
)
4
leads to the
formation of carbazoles
in excellent yields.
95
Although at first glance, this
transformation might be interpreted as involving the C
262
H insertion of a metal-
lonitrene intermediate, detailed Hammett and kinetic isotope effect studies by Driver
raise the intriguing possibility that the nitrenoid species generated upon loss of
nitrogen actually behaves in a manner more akin to an aryl nitrenium ion,
96
or more
precisely, a 4-iminocyclohexa-2,5-dienyl cation
.
6l
In addition to reaction with arenes, aryl nitrenium ions also undergo intra-
molecular electrophilic substitution with alkenes. Rodrigues
et al.
have found
that treatment of
trans-
2-azidostilbene (
266
Ph) with triflic or trifluoroacetic
acid mediates efficient cyclization to 2-phenyl-1
H
-indole (
270
,R
¼
) (Scheme 10.43).
97
A
276
lower yield was observed with 2-azidocinnamate (
CO
2
Et), a more electron-
deficient alkene is consistent with the assumption that substitution proceeds via
nitrenium ion
270
,R
¼
Ph), the authors noted lower yields
with the use of TfOH, in comparison to TFA, and suggested that this may reflect the
unproductive formation of dication
271
. In all substrates but
270
(R
¼
by the stronger acid.
94
Interestingly, cyclization of
cis
-2-azidostilbene (
274
277
) does not lead to indole
276
,
, the product of intramolecular
N
-arylation
(Scheme 10.44). This observation should be contrasted with the work of Smith,
98
who found that
thermolysis
of
but generates dibenz[
b
,
f
]azepine
278
277
generated
276
and, as a result, proposed that its
formation occurred not through C
H insertion, but rather via a nitrene-alkene
addition process.
