Chemistry Reference
In-Depth Information
N
3
NH
NH
N
H
2
O
h
ν
-N
2
NEt
2
NEt
2
NEt
2
NEt
2
Singlet
nitrene
Nitrenium
ion
Quinodiiminium
ion
SCHEME 3.6.
reported by Baetzold and Tong in 1971,
37
but was largely unrecognized by the
photochemical community for the next several decades. These workers observed that
p
-aminophenyl azides afforded the corresponding quinodiimines on irradiation in
buffered weakly acidic aqueous solutions, Scheme 3.6. While they did not
unequivocally establish the structure of the unstable nitrenium ions formed in these
reactions, or the products of their hydrolysis, they did select an optimal system for
switching the aryl nitrene chemistry from its usual ring-expanding, ketenimine mode
to its nitrenium ion mode.
Ten years later, Takeuchi and Koyama investigated the photochemistry of phenyl
azide in the presence of acetic acid and ethanol.
38,39
They found a new line of
products,
, in these reactions, Scheme 3.7. They argued that these products
arose from a branch of nitrene chemistry that was in competition with the ketenimine
branch described in Scheme 3.5. They speculated that these new ring-substituted
products arose from collapse of a nitrenium ion pair which in turn arose from
protonation of the nitrene by the acidic solvent.
Only in 1995 did it become possible to directly observe the nitrenium ions formed
in these reactions.
40
McClelland systematically characterized aryl azide photo-
chemistry to the limits of the capabilities of his time when he observed aryl
nitrenium ions in aqueous acetonitrile and water alone, and found that nitrenium
ions were formed within the laser pump pulse width of 20 ns via the rapid protonation
16
and
17
N
Ketenimine
branch
O
N
3
N
h
ν
-N
2
1.4
1
NHAc
NHAc
AcOH
OR
Nitrenium
ion branch
17
16
R = H, Ac
OR
SCHEME 3.7.
