Chemistry Reference
In-Depth Information
10
4
s
1
solvent with
k
s
¼
3.8
to generate the expected product
70bb
that
is
.
133
The reactivity of
hydrolyzed to produce
72bb
64bb
with H
2
O lies between
.
23
The
that of 4
0
-methoxy-4-biphenylyl ion
and 4
0
-methyl-4-biphenylyl ion
64p
64q
ion
is a typical nitrenium ion.
The 4
0
-acetylamino ion
64bb
64cc
has a UV spectrum similar to that of
64bb
, but it
reacts quite differently. In acid solution,
decays into a long-lived transient with
l
max
430 nm; whereas in base solution,
64cc
generates a different long-lived transient
with
64cc
l
max
410 nm. These transients are in a pH-dependent equilibrium with p
K
a
7.6.
133
2.3 s
1
) predominately
The intermediates slowly decay (
k
s
(apparent)
¼
.
133
The long-lived intermediates
(about 70% yield) into
70cc
and subsequently
72cc
were identified as the tautomeric ion
64dd
and its bis-imine conjugate base
93dd
.
The product
70cc
must be formed from the more reactive tautomer
64cc
. An estimate
10
4
was based on the assumptions that
k
s
for
for
K
T
of 2
64cc
is equivalent to that of
.
133
The stability
64bb
and that 70% of the hydrolysis product comes from
64cc
difference of the tautomers
appears to be due to the greater basicity of
NH
2
compared to NHAc that allows it to more strongly conjugate with the positive
charge of the cation.
133
There was no direct evidence for a dication, but the observed acid catalysis of the
decay of
64cc
and
64dd
could occur through such a species.
133
This study provided
an example of a tautomeric nitrenium ion equilibrium in which the species exist
primarily in the more stable, less reactive tautomer, but react through the less stable,
significantly more reactive tautomer. This tautomerism appears to play a role in the
in vivo
chemistry of the carcinogen.
135,136
McClelland and coworkers photolyzed the azides
64cc
into
64dd
(Scheme 4.27) to
investigate the equilibrium that governed protonation of the nitrenium ions
87ee
and
87ff
64ee
and
64ff
to generate the corresponding dications
92ee
and
92ff
and the deprotonation of
.
134
The dications were also obtained by
oxidation of the corresponding amines under acidic conditions. Both procedures
resulted in identical spectra at all pH. The p
K
a
s and rate constants, shown in
Scheme 4.27, were determined in the study, and show that these are remarkably
long-lived species in water. The cations
64ee
93ee
to generate the bis-imine
are at least two orders of
magnitude more stable than predicted based on the Yukana-Tsuno correlation for
other 4
0
-substituted-4-biphenylyl nitrenium ions.
23
This stabilization is not unique to
nitrenium ions. Similar stabilizing effects are observed for the 4-dimethylamino
substituent in trityl cations.
105
The 4
0
-NH
2
substituent has a considerable effect on
the acid-base chemistry of
64ee
and
64ff
64ee
compared to the parent ion
64n
. The amino
(p
K
a
0
5.1 for
substituent significantly increases the basicity of
64ee
92ee
vs. 0.1
for
92n
) and considerably strengthens the acidity of the nitrenium ion (p
K
a
9.0 for
is on the 4
0
-NH
2
substituent rather
64ee
vs. 16.0 for
64n
). The acidic proton in
64ee
than on the nitrenium N, and the bis-imine
93ee
is considerably more stable than the
singlet nitrene
is more comparable to an iminium ion.
The benzidine dications are less reactive toward solvent than the monocations.
134
This was not observed for the 2-fluorenyl and 4-biphenylyl nitrenium ions
88n
, so the p
K
a
of
64ee
64g
and
.
22
The reason for this may be related to the
64n
and their conjugate acids
92g
and
92n
fact that the two positive charges in
92ee
and
92ff
can be localized onto the two
