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not the initial adduct
9
, which ultimately loses a proton to form the observed
product
11
.
3.2.1 Properties of Aryl Azides that Serve as Nitrenium Ion Precursors
As indicated by McClelland's studies, aryl azides would seem to provide a
particularly versatile photochemical source of nitrenium ions.
24,25,27-29
In recent
years, the photochemical behavior of aryl azides has been studied in detail by Platz
et al.
30
However, nitrenium ion formation was not observed in most of the systems
that they studied. The typical photochemical behavior of most simple phenyl azides
is to initially undergo loss of nitrogen from the singlet excited state of the azide. This
process is very rapid occurring within a few hundred femtoseconds of excitation
(Scheme 3.5).
31,32
The singlet nitrene, then, rapidly cyclizes to azirines
, which is
also very short-lived, and rapidly undergoes ring expansion to the ketenimine
12
.In
the case of 1-azidonaphthalene, the singlet excited state of the azide loses nitrogen
with a time constant of
13
t ¼
730 fs, and the resulting nitrene cyclizes to the azirine
12 ps.
32
The phenyl nitrene closes to the azirine
with a time constant of
t ¼
12
then
ring expands to the ketenimine
13
much more rapidly with a time constant of
0.1-1.0 ns.
33,34
Azirines related to
can be trapped with ethanthiol
35
and
t ¼
12
.
30,36
While both
dimethylamine to form
14
12
and
13
react with nucleophiles to form
14
, respectively, and thus, might be considered viable PAL agents, they do not
seem to have been widely used for this purpose. Since most of the aforementioned
mechanistic studies have been conducted in nonaqueous media, the suitability of
and
15
12
and
as cross-linking species in aqueous media is questionable. Furthermore, even
if these species were applied as cross-linking species under biological conditions, the
cross-linked molecules would be susceptible to hydrolysis, and probably would not
survive the analysis processes. Therefore, this chemistry had to be evolved further to
be useful in PAL applications.
When the aforementioned studies were expanded to acidic and aqueous media, a
new branch of aryl azide photochemistry was observed. This branch was first
13
N
N
N
h
ν
N
N
N
-N
2
Singlet
nitrene
Azirine
Ketenimine
12
13
Nuc
N
Nuc
Nuc
Nuc
N
H
2
N
14
15
SCHEME 3.5.
