Chemistry Reference
In-Depth Information
H
H
hν
N
2
hν
N
N
N
N
N
or FVT
500-600ºC
N
N
N
186
187
Z+E
188
h
ν
H
N
2
190
189
h
ν
or
Δ
H
H
H
+
+
192
193
191
SCHEME 8.30.
for isomerization of carbene
are only 5.12 and 2.43 kcal/mol,
respectively (B3LYP/6-31G
) (Scheme 8.30). A further rearrangement of 3-pyridazinyl
carbene
190
to
191
and
192
is described below (Scheme 8.34).
57
The benzo derivative 1-azidophthalazine
188
203
to 2-pyridyl nitrene
(2121 cm
1
) can be generated in small
amount by mild FVTof tetrazole
195
(Scheme 8.31).
49
Ar matrix photolysis at 254 nm
generates two conformers of diazo compound
194
(2075 and 2230 cm
1
), followed by
197
and finally the known
50
cyano(phenyl)carbene
(
o
-cyanophenyl)carbene
198
199
both
detected by ESR spectroscopy (
D
/
hc
¼
0.5078,
E
/
hc
¼
0.0236 and
D
/
hc
¼
0.6488,
0.0195 cm
1
, respectively; Scheme 8.31).
49
The rearrangement of
E
/
hc
¼
198
to
199
is a carbene-carbene rearrangement.
18,51
Interestingly, FVT of
at 600
C affords
194
the products of ring contraction of
o
-cyanophenyl carbene
,
in a process analogous to the ring contraction of phenyl carbene to fulvenallene and
ethynylcyclopentadiene.
18,52
198
, namely,
200
and
201
8.4.2 Type II Ring Opening to Nitrenes or Diradicals
The Type I ring-opening reactions (Scheme 8.15) described above can only take
place where there is a 1,3-relationship between a ring nitrogen and the carbene or
nitrene substituent. When this is not the case, nitrenes can still undergo ring opening
via diradicals or vinyl nitrenes. This is termed Type II ring opening. Type II ring
openings tend to have higher activation barriers.
37,58
