Chemistry Reference
In-Depth Information
CH
3
hν
336
n
m
or FVT
800
o
CH
3
CH
3
475 nm
336 nm
N
N
N
3
23
24
28
FVT 500
o
FVT
>385 nm
CH
2
>385
n
m
NH
CH
3
NH
>286 nm
N
27
25
29
CHN
2
CH
544
n
m
NH
2
N
NH
2
26
30
SCHEME 8.5.
up to 80 K in Xe matrix, and it was not seen to transform directly to
25
. In contrast,
photolysis of nitrene
24
at
l
>
475 nm caused ring expansion to the azacyclohepta-
(1898 cm
1
), which at
tetraene
28
l
>
385 nm rearranged to
27
(perhaps via the
nitrene) (Scheme 8.5).
FVT of
o
-tolyl azide
with matrix isolation of the products generated not only
the iminoquinone methide
23
30
(Scheme 8.5).
15
The explanation for the formation of this product involves the
nitrene-carbene rearrangement, that is, the interconversion of phenyl nitrene
25
but also a small amount of 2-vinylpyridine
,
1-azacyclohepta-1,2,4,6-tetraene
2
, and 2-pyridyl carbene
3
(Scheme 8.1), which
takes place both thermally (FVT)
16
and photochemically (Scheme 8.6).
17
Thus, FVT
of 2-methyl- and 2,6-dimethylphenyl azides
1
(about
8%), the majority of the product being nitrene-derived (azobenzenes and anilines)
(Scheme 8.6).
16a,18
The carbene-nitrene interconversion is in favor of the nitrene, which is thermo-
dynamically more stable than the carbene.
18,19
FVT of triazolo[1,5-
a
]pyridine/
2-diazomethylpyridine
31
affords 2-vinylpyridines
32
, which
can be directly observed by ESR, IR, and UV spectroscopy by trapping the
thermolyzate in an Ar matrix (Scheme 8.6).
20
There are several other examples
of the direct observation of phenyl nitrenes generated by FVT of pyridyl carbene
precursors. The rearrangements of 2-quinolyl- and 1-isoquinolyl carbenes to the 1-
and 2-naphthyl nitrenes, respectively,
21
of phenanthridyl carbene to phenanthryl
nitrene,
22
and of 4-quinazolyl carbene to 3-quinolyl nitrene,
23
are described later.
The phenyl nitrenes can, of course, also be observed by matrix spectroscopy
following either photolysis
24
or FVT of the azides.
17,20,22,23,25
The pyridyl carbenes,
in contrast, do not generally survive FVT, as they rearrange to the nitrenes; therefore,
33T/33D
affords a high yield of phenyl nitrene
1
