Chemistry Reference
In-Depth Information
N
N
CH
N
1
2
3
N
N
92
91
CH
C
CH
CH
h
ν
N
N
C
H
1935 cm
-1
N
N
89
90
88
93
hν, Ar
h
ν
, Ar
CHN
2
CHN
2
C
CH
2
N
NC
N
94
SCHEME 8.16.
lead to phenyl nitrene (Scheme 8.16) as demonstrated impressively by matrix
isolation ESR and IR spectroscopy.
17,38
Phenyl nitrene
is the end product because
it is lower in energy than the isomeric carbenes, on both the triplet (T
o
) and the singlet
(S
1
) surfaces. As stated earlier, nitrenes are generally of lower energy than the
isomeric carbenes.
18,19
However, the normally expected seven-membered ring allene
91
1
connecting the 2- and 3-pyridyl carbenes
3
and
90
was not found. Instead, the
, was postulated,
38
but further work identified this species
as the open chain nitrile ylide
bond-shifted ylide,
92
.
39
It is formed by photochemical ring opening of
3-pyridyl carbene and absorbs strongly at 1935 cm
1
in the matrix IR spectrum
(Scheme 8.16). The same species is formed on matrix photolysis of 4- and
3-diazomethylpyridines, the precursors of 4- and 3-pyridyl carbenes
93
88
and
90
(Scheme 8.16). The allene
is probably formed as an unobserved intermediate en
route to 2-pyridyl carbene and phenyl nitrene, but ring opening to
91
93
is the dominant
process. Further photolysis of nitrile ylide
93
led to a new species identified as the
isocyanovinylallene
94
, formed by a 1,7-H shift (Scheme 8.16).
