Chemistry Reference
In-Depth Information
Shortly after Anderson and Falvey reported the observation of short-lived
nitrenium ions in CH
3
CN, Novak and McClelland and coworkers demonstrated
that
(Scheme 4.19) could be detected by UV spectros-
copy in H
2
O after LFP of esters or
N
-chloro derivatives of the corresponding
hydroxamic acids.
20
The transients were identified as nitrenium ions by their
second-order reactions with N
3
, their insensitivity to O
2
, and by the equivalence
of
k
az
/
k
s
measured by direct observation and by the azide clock method.
20
Subse-
quently, McClelland and coworkers demonstrated that LFP of aryl azides was a
general method for producing transient 1
aryl nitrenium ions in aqueous solution,
apparently by rapid protonation of the singlet nitrene.
21-23,28,113,114
This allowed for
an investigation of a wider range of cations and made comparisons to data obtained
by the azide clock procedure possible.
Azide/solvent selectivity data in water for selected ions determined either by the
azide clock method or by direct measurement of
k
az
and
k
s
on ions generated by LFP
are presented in Table 4.1. A more extensive table has been published elsewhere.
54
Where they have been measured,
k
az
and
k
s
are given along with the logarithm of the
azide/solvent selectivity, log
S
, where
S
is defined as the observed azide adduct/hy-
dration product concentration ratio extrapolated to 1M N
3
.
110,115
For log
S
64h
(Scheme 4.16) and
64o
2.0,
2.0, N
3
trapping includes
contributions from ion pair and preassociation processes.
110
The deviation between
log
S
and log (
k
az
/
k
s
) is small until log
S
approaches 0.
110
Ions included in Table 4.1
are described in Scheme 4.21. The selectivity data for ions with log
S
log
S
is equivalent to log (
k
az
/
k
s
), but for ions with log
S
<
<
2.0 were
collected by the azide clock method, whereas data for ions with log
S
2.0 were
obtained by both methods.
The values of
k
az
cluster around 5
10
9
M
1
s
1
at ionic strength 0.5 and around
10
10
M
1
s
1
at ionic strength 0.0. Only the most selective ions such as
64p
1
deviate from this trend and even these ions have
k
az
within a factor of 2-3 of
the apparent diffusion limits. It appears that
k
az
is diffusion limited as long as
k
s
>
10
4
s
1
or
k
az
/
k
s
<
5
10
5
M
1
. The assumption that
k
az
is diffusion limited
for the more reactive ions for which direct observations have not been made appears
to be valid.
Effects of 4-aryl and 4-vinyl substituents on kinetic stabilities are striking. The
biphenylyl ions
64w
and
have about 10
3
-fold longer lifetimes in water than do
64n
and
64o
their 4-tolyl counterparts,
have
lifetimes that are about 300-fold longer than corresponding 4-tolyl ions. The
4-phenyl substituent has about the same effect on stability of the cation as a 4-alkoxy
substituent (compare
64r
and
64s
. The 4-stilbenyl ions
64k
and
64l
), and 4-NMeAc stabilizes the
cation about 10
4
-fold compared to a 4-alkoxy substituent (compare
64n
to
64t
or
64u
, and
64o
to
64v
64t
and
64u
to
64w
) The effect of 4-substituents on aryl nitrenium ion stability does not correlate
with
s
รพ
, but does correlate well with the relative thermodynamic driving force for
hydration (
E
of the isodesmic reaction of Scheme 4.22) calculated at the RHF/6-
31G
//3-21G level of theory.
109,114,115
Substituent effects for 4
0
-substituents in the 4-
biphenylylnitrenum ions are much more carbocation like (see
D
)
and, like 1-arylethyl and triarylmethyl carbocations, are correlated well by the
Yukawa-Tsuno equation.
23
Effects of
N
-substituents are modest compared to
64n
,
64p
, and
64q
