Chemistry Reference
In-Depth Information
The ESR transitions of these two nitrenes overlap but are sufficiently different that it
is possible to detect both in the same matrix. While the nitrenes observed by ESR
spectroscopy are necessarily in their triplet states, their interconversion described
below may take place on the singlet energy surface.
Broadband matrix photolysis with IR detection revealed that the two azides/
tetrazoles give rise to the same intermediates. A species absorbing at 2005 cm
1
is
formed rapidly, reaching maximum intensity after 20 min. All the IR bands belong-
ing to this species were identical in the two photolyses and agreed well with the
calculated spectrum of the cyclic carbodiimide
152
. Continued photolysis caused
disappearance of
152
and formation of the two end products
154
(3423, 2157,
2131 cm
1
) and
(Scheme 8.25).
45
Due to its partly
o
-quinoidal nature, the open
155
chain nitrile ylide
by a calculated 22.4 kcal/mol, and
it was not observed. This is reasonable because it has only a small calculated barrier
toward the isomerization to
153
is higher in energy than
152
154
(4.5 kcal/mol). However, it was observed in the
C
-phenyl derivative
172
described below. The reaction of nitrile ylide
153
to 1-
cyanobenzimidazole
155
(only observed on FVT) has a calculated barrier of
N
N
N
N:
N
N
3
N
N
2
40
o
C
hν
N
N
N
150A
151
150T
N
N
N
C
C
~1,7-H
NH
N
N
NC
N
N
C
152
153
154
H
Δ
Δ
N:
CN
N
N
CN
N
N
N
NC
159
156
155
FVT
hν
N
3
N
N
N
N
260
o
C
CN
N
N
N
N
NC
158A
158T
157
SCHEME 8.25.
