Chemistry Reference
In-Depth Information
strongly exothermic. The further isomerization of
215
to diazirene
216
requires a
barrier of about 13 kcal/mol. The ring opening of
has undoubtedly a very
small barrier, and the reaction to the triplet ground state (
3
A
00
)of
216
to
203
is exothermic
by about 34 kcal/mol. Open-shell singlet phenyl nitrene is about 15 kcal/mol higher
in energy than the triplet;
59
203
this value is close to the singlet-triplet splitting of
calculated at the CASSCF level.
57
Thus, the overall reaction,
188
(
1
A
0
)
!203
(
1
A
00
), is exothermic by about 39-43 kcal/mol. A weak signal at
1988 cm
1
in the photolyses of
15.7 kcal/mol for
203
. The complete scheme
linking 2-pyridyl nitrene and 3-pyridazinyl carbene is shown in Scheme 8.34.
57
186/187
is possibly due to
204
8.4.2.2 2-Quinolyl Nitrene and 1-Isoquinolyl Nitrene. 2-Quinolyl- and 1-isoqui-
nolyl nitrenes
217
and
219
interconvert via the seven-membered ring carbodiimide
(Scheme 8.35).
60
The nitrenes have been detected by ESR spectroscopy
20,58
and
the ring expanded carbodiimide by IR spectroscopy under both FVT and photolysis
conditions.
60
Under FVT conditions, both
218
217
and
219
afford the nitriles
221
and
222
as end products. On matrix photolysis,
(2041,
2238, 3305 cm
1
). These three products are readily explained in terms of initial ring
opening of isoquinolyl nitrene to the vinyl nitrene
217
affords the NH-ketenimine
223
(Scheme 8.35).
60
220
8.4.2.3 3-Isoquinolyl Nitrene. This species has a rich and fascinating chemistry,
revealing many details of the nitrene rearrangements. It is generated by matrix
photolysis or FVT of
the tetrazoloisoquinoline/3-azidoisoquinoline mixture
(Scheme 8.36); it was directly observed by ESR spectroscopy.
58
The
224T/224A
FVT
or hν
FVT
or hν
N
N
N
N
N
N
217
218
219
HN
Δ
CN
+
N
CN
CN
CN
221
220
222
Matrix
NH
CN
223
SCHEME 8.35.
