Chemistry Reference
In-Depth Information
N
N
N
N
N
N
276
38
N
37
N
N
N
N
N
216
215a
215b
SCHEME 8.44.
shallow energy minima, and their formation may in fact be bypassed when vibra-
tionally excited singlet nitrenes are involved. The (singlet) nitrenes undergo ring
expansion to seven-membered ring cumulenes (aza-, diaza-, triazacycloheptate-
traenes, etc.), which are directly observable in most cases (e.g.,
, Scheme 8.44).
These reactions take place both thermally (FVT) and photochemically.
The ring expansion of heteroaryl nitrenes usually takes place in the direction away
from a neighboring nitrogen atom, for example,
38
(Scheme 8.44). The
alternative ring expansion toward the neighboring ring nitrogen atom, for example,
37!216!215
37!276!38
(Scheme 8.44) is also in principle possible but is calculated to be of
higher energy. However, the relative energies can be modified by suitable annelation.
There is evidence for this mode of ring expansion in a few cases (see Schemes 8.39,
8.40, 8.41, and 8.42).
is a formally antiaromatic 1
H
-diazirene, which provides a
rationale for its higher energy compared with
216
276
. A resonance structure of the cyclic
N
-amino ketenimine (ketene hydrazone)
215a
is a cyclic nitrile imine
215b
, which
69
may also be stabilized by annelation, just as the cyclic nitrile ylide
.
69
Cyclic nitrile ylides such as
are zwitterionic “bond-shift” valence isomers of
70
“normal” ketenimines such as
(Scheme 8.45). They can be observed in cases
where they become energetically favorable due to benzo-annelation (Schemes 8.12,
8.13, and 8.45). The isomeric ketenimine
72
(Scheme 8.45) retaining an intact
H
N
:
N
N
72
69
67
N
70
SCHEME 8.45.
