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limited so far to measuring their lifetimes. No bimolecular reactions with nonsolvent
species have been examined. Kung and Falvey have generated the
N
-methyl analog
of
in CH
3
CN using conventional LFP and confirmed that this ion reacts with H
2
O
about 1800-fold more rapidly than does
64c
.
125
This is consistent with expectations
64y
(Scheme 4.16) in water.
111
based on azide clock trapping of
64d
4.3.2 Acid-Base Chemistry
The apparent protonation of nitrenes during thermolysis and photolysis of azides had
been used by a number of investigators to generate nitrenium ions,
21,67-69,103
several
stable nitrenium dications had been characterized,
58,59,129
and evidence for the
involvement of transient nitrenium dications in certain reactions had been pre-
sented.
57,68
but no quantitative study of the acid-base chemistry of nitrenium ions
was reported until 1996.
22
Variation in yields of the nitrene-derived product
89
and the nitrenium-derived
products
, as a function of pH and CH
3
CN-H
2
O
ratio was analyzed in terms of the mechanism of Scheme 4.24.
22
Ring expansion is
much faster than intersystem crossing at 20
C for singlet phenyl nitrene,
90
,
91
, and products derived from
70
,
130
so
88a
triplet-derived products from
87n
were significant in 100%CH
3
CN. This result is in agreement with a subsequent study
by Platz and coworkers that shows that in pentane the temperature-independent
k
isc
is
enhanced for
87a
were negligible. Triplet-derived products from
88n
by a factor of 4 over
88a
, but the temperature-dependent
k
exp
is
.
131
A combination of ps-LFP observation of the
smaller for
88n
than for
88a
formation of
, and analysis of the
product yields as a function of pH, led to the determination of the p
K
a
of
64n
, ns-LFP measurements of the decay of
64n
64n
of 16.0
in water.
22
Because
is such a weak acid, and because (
k
H
p
[H
þ
]
k
w
p
)
64n
þ
>
(
k
isc
þ
k
exp
) for
88n
87n
, nitrenium ion-derived products predominate for photolysis of
even in 0.01M NaOH.
22
For the parent system, neither
88a
64a
or
was directly
SCHEME 4.24.
Kinetic scheme for protonation of aryl nitrenes.
