Chemistry Reference
In-Depth Information
.
.
O
Ar
O
2
O
N
N:
N
O
Ar
Ar
O
N
O
Ar
O
O
ArNO
2
N O
Ar
SCHEME 5.20.
triplet phenyl nitrene derivatives, which are also trapped by oxygen to form nitro
aryl derivatives.
73,86,87
The mechanism for forming nitrobenzenes from triplet phenyl nitrenes has been
studied.
88-93
It has been theorized that triplet phenyl nitrene derivatives react with
oxygen to form triplet peroxy radicals, which form dioxaziridinyl derivatives either
photochemically or through rearrangements (Scheme 5.20). In comparison, the
triplet peroxy radicals dimerize at higher concentrations. Both the dimer and the
dioxaziridinyl presumably decay to form nitro products. It seems likely that triplet
alkyl nitrene would react similarly with oxygen to yield nitro products.
5.5.5 Photoreactions that Compete with Intramolecular Energy
Transfer in Alkyl Azides
Even though intramolecular sensitization can result in the efficient formation of
triplet alkyl nitrenes, tethering an azidoalkane moiety to a triplet sensitizer does
affect the photochemistry of the sensitizer in some instances, depending on the
structure. Thus, there are several photoreactions that compete with the energy
transfer to the azido chromophore. Below, three of these competing reactions are
reviewed:
-cleavage, azido cleavage, and intramolecular H-atom abstraction by the
triplet ketone sensitizer moiety.
a
5.5.5.1 a-Cleavage of Azidoalkanes.
Singh et al. demonstrated that laser flash
photolysis of
resulted in the formation of benzoyl radicals and triplet alkyl
nitrenes.
63
Thus, azides
37
-cleavage to form benzoyl radicals in compe-
tition with energy transfer to form triplet nitrenes (Scheme 5.21), which is consistent
with the fact
37
undergo
a
that
the radical-stabilizing effects of the azido group promote
.
62,63
Photolysis of
a
-cleavage in
37
37a
-
f
in toluene primarily yielded
53
, along
with smaller amounts of
(Scheme 5.21). Because benzoyl radicals do not
readily abstract hydrogen atoms from the solvent but are rapidly quenched by
molecular oxygen,
94,95
54
and
55
53
it was assumed that the major product
came from the
interception of benzoyl radicals by triplet alkyl nitrenes
(Scheme 5.22). Further-
more, formation of benzoyl radicals in the presence of azidoadamantane showed that
the benzoyl radicals do not react with ground state alkyl azides.
63
Therefore, these
38
