Chemistry Reference
In-Depth Information
N
3
N
3
h
ν
N
3
N:
Ph
Ph
Ph
Ph
-N
2
Ph
Ph
77 K
Benzophenone
15
33
34
SCHEME 5.11.
*
O
O
O
O
Energy
transfer
*
h
ν
-N
2
N
3
N
3
N
3
N:
Ar
Ar
Ar
n
n
n
Ar
n
Triplet azide
Triplet nitrene
Triplet ketone
SCHEME 5.12.
5.5 INTRAMOLECULAR TRIPLET-SENSITIZED PHOTOLYSIS
OF AZIDOALKANES
The use of intermolecular triplet sensitization to form triplet alkyl nitrenes in
solutions makes it complicated to use spectroscopy to characterize these intermedi-
ates. To bypass this challenge, Gudmundsdottir and coworkers used intramolecular
sensitization to form triplet alkyl nitrenes by tethering azidoalkane moieties to
acetophenone derivatives (Scheme 5.12).
59,62-64
Acetophenone absorbs light and
forms the first excited singlet state of the ketone (S
1K
) chromophore, which under-
goes efficient intersystem crossing to the first excited triplet ketone (T
1K
).
65
More
specifically, acetophenone absorbs light well above 300 nm
66
to form its S
1K
, which
undergoes intersystem crossing (
10
11
s
1
)toT
1K
with a quantum efficiency
approximating unity.
65,67,68
In comparison, azidoalkanes have a weak band around
280 nm,
69
and thus, by irradiating above 300 nm, only the acetophenone chromo-
phore absorbs and forms its T
1K
, bypassing the singlet reactivity of the azido
chromophore. Thus, energy transfer from the T
1K
of acetophenone derivatives to
intramolecular azido moieties can be used to form triplet-excited azido chromo-
phores, which fall apart to give triplet alkyl nitrenes.
5.5.1 Characterization of Triplet Alkyl Nitrenes in Solution
5.5.1.1 Transient Absorption Spectroscopy.
Singh et al. demonstrated that triplet
alkyl nitrenes can be detected directly with transient absorption spectroscopy in
solution.
64
Laser flash photolysis of
35
in methanol resulted in the formation of a
triplet nitrene
36
(Scheme 5.13). The transient spectrum of the nitrene
36
has a
maximum absorbance at
325 nm. The calculated absorption spectrum (TD-DFT) for
36
0.0196) nm,
which are due to a mixed electron transition out of the lone pair on the nitrogen and the
in methanol contained major bands at 313 (
f
¼
0.0458) and 296 (
f
¼
