Chemistry Reference
In-Depth Information
benzene ring will be the lowest energy isomer in this case. Both
69
and
72
can be
observed.
Two types of ring opening of (hetero)aryl nitrenes have been identified: ylidic and
diradicaloid/nitrenoid ring opening (Schemes 8.15 and 8.46).
37,58
The ring-opening
reactions may take place directly from the nitrenes or from their seven-membered
ring cumulene counterparts. In some cases, the ring-opening reactions take over to
the extent of almost completely obscuring the traditional ring expansion to the cyclic
cumulenes. Ylidic, diradicaloid, and nitrenoid ring-opened species have been
identified by direct observation in several cases, and the reactions appear to be
ubiquitous.
There are undoubtedly many more ring-opening reactions to be identified. For
example, the isomeric 4-quinolyl nitrene (
277
,Y,W
¼
N; X, Z
¼
CH; Scheme 8.46)
and 2-quinoxalinyl carbene (
279
,Y,W
¼
N; X, Z
¼
CH) undergo photolyses to the
(1902/1909 cm
1
) as well as
expected cyclic 1,5-diazabenzocycloheptatetraene
278
ring opening to a nitrile ylide
CH),
which was observed by ESR spectroscopy (Scheme 8.46).
58,70
In compounds where
W
281
and another nitrene
282 (
Y, W
¼
N; X, Z
¼
may
be observed instead. These reactions illustrate the different ring-opening reactions—
to ylides, diradicals, carbenes, or nitrenes—that have been identified (Scheme 8.46).
Similarly, photochemical generation of 2-quinolyl nitrene
6¼
N, nitrile ylides or diazo compounds
281
cannot form, and diradicals
280
(Scheme 8.35)
from tetrazolo[1,5-
a
]quinoline in Ar matrix gives rise to strong ESR signals, not only
of the nitrene but also of the two prominent diradicals due to ring opening, as well as
a carbene. Matrix photolysis of tetrazolophenanthridine
217
233
generates not only the
two nitrenes
234
and
238
(Scheme 8.37) but also a carbene. Both 1- and 2-naphthyl
nitrenes
and/or their ring-expanded cumulene equivalents undergo
secondary matrix photochemistry, probably involving ring opening.
34
Most aryl- and heteroaryl nitrenes undergo ring contraction to five-membered ring
nitriles (Scheme 8.14) as the thermodynamically most stable end products of
thermolysis, and in many cases they are also formed on photolysis. Although
56
and
67
W, X, Y, Z = CR or N
Y
Y
Y
X
X
Z
X
Z
Z
W
W
W
277
278
279
Type II
Type I
W=N
X
Y
Y
Y
C
C
C
X
X
Z
Z
N
W
W
Z
280
281
282
SCHEME 8.46.
