Chemistry Reference
In-Depth Information
Triplet sensitizer
Toluene
1,2-Alkyl shift
N
N
N
3
N
3
Triplet alkylnitrene
29
Biradical
Thermal
dimerization
Nitrene dimerization
Toluene radical
N
N
Ph
N
N
NH
2
H
N
Ph
25
26
27
28
24
SCHEME 5.9.
*
O
OH
Ph
Ph
- N
2
Ph
N
3
H
Ph
Ph
Ph
N
3
N
Ph
30
32
31
SCHEME 5.10.
10
6
M
1
s
1
, respectively. Thus, as expected, the
rate of quenching is slower for triplet benzophenone, which has its first excited triplet
state at a lower energy (66 kcal/mol) than triplet acetophenone (74 kcal/mol).
60
In comparison, triplet-sensitized photolysis of azidomethylbenzene (
10
8
and 5
determined to be 1
30
) with benzo-
phenone yielded only
31
. The authors theorized that benzophenone did not decay by
energy transfer to
30
, but rather abstracted a H-atom to form the radical
32
, which
resulted in
is favorable because the
resulting radical is resonance stabilized by both the phenyl ring and the azido moiety.
The resonance stabilization energy of an azidoalkane group has been estimated to be
similar to a phenyl substituent.
61
The rate of the chemical quenching of benzophenone
with
31
(Scheme 5.10). H-atom abstraction from
30
10
7
M
1
s
1
, which is somewhat faster than when
benzophenone is quenched via energy transfer to
30
was measured to be 5
24
. This further supports the theory
30
that
is not formed via energy transfer from benzophenone.
Yager et al. studied the triplet-sensitized photolysis of diazidoalkane
in solvent
matrices at 77K with benzophenone as the sensitizer. They observed the formation of
triplet nitrene
15
with EPR spectroscopy (Scheme 5.11). Interestingly, EPR spectros-
copy showed that prolonged irradiation resulted in the formation of triplet carbene
33
.
Thus, simple intermolecular triplet sensitization of azidoalkane can be used to
form various triplet alkyl nitrenes, whereas quintet alkyl dinitrenes have not yet been
reported. However, it should be highlighted that the hydrogen atom abstraction by
the triplet sensitizer can compete with energy transfer, especially for azidoalkanes
with a hydrogen atom adjacent to the azido group.
34
