Chemistry Reference
In-Depth Information
with HPO
4
2
and H
2
PO
4
.
149
Data for reactivity with HPO
4
2
and H
2
PO
4
are also
provided in Table 4.2. The reactivity of these cations with phosphate is reminiscent
of the reaction of
with phosphate.
143
Based on analogies to carbocyclic nitrenium
ions,
k
az
should be at or near the diffusion limit for all ions with
k
az
/
k
s
<
95
10
5
M
1
5
[log(
k
az
/
k
s
)
5.7], so
k
az
/
k
s
can be used to evaluate the kinetic stability of these ions
within those limits. Comparisons of
k
az
/
k
s
for
<
, and the 4-biphenylyl ion
64n
show that a 3-pyridyl-
N
and 2-pyridyl-
N
kinetically destabilize the cation by
about 35- and 300-fold, respectively, as expected based on differences in the
electronegativities of N and C and the resulting differences in their abilities to
stabilize a positive charge. The effect of a heterocyclic N can be quite different in
cases in which the lone pair on the N can participate in resonance stabilization. This
is true for the cations
103a
,
103c
103d-i
, and
103l-n
and it leads to considerable kinetic
stabilization in each case. The comparison of
103m
and
103n
with the 2-fluorenyl ion
64g
is beyond the range that
k
az
can be confidently predicted to be near the diffusion limit, these ions have
k
az
/
k
s
similar to that of 4
0
-methoxy-4-biphenylyl ion
is enlightening. Although
k
az
/
k
s
for
103d
and
103e
. That ion has
k
az
within a factor of
2 of the apparent diffusion limit.
23
The measured
k
az
for the highly selective ions
103m
64p
10
9
M
1
s
1
.
153
and
103n
is also close to the diffusion limit at about 3
are summarized in Scheme 4.30.
149
The
Reaction products for
103d
and
103e
solvent- and phosphate-derived products
104
and
106
are analogous to those reported
.
142,143
These products appear to be generated
by nucleophilic attack on an initial addition product such as
by McClelland and coworkers for
95a
.TheN
3
-adducts
105
are similar to those observed for carbocyclic esters.
19,27,109,110
The
107
and
108d
SCHEME 4.30.
Reaction products from
103d
and
103e
.
