Chemistry Reference
In-Depth Information
quantitative reactivity/selectivity of these species were not available. It was not
possible to assess whether the reactions were due to free ions, ion pairs, or
preassociation processes. Often concerted reactions could not be ruled out because
kinetics experiments had not been performed. Reduction products had been attrib-
uted to triplet ions,
60-64,72
but calculations suggested that triplet ions are not
accessible in thermal processes.
101
Singlet ions can also be reduced under certain
conditions.
18,86,91
Some reduction reactions of the precursors do not involve
nitrenium ions at all.
96
Therefore, the presence of reduction products is not a reliable
indicator of either singlet or triplet ions. Differences in product yields between
photochemical and thermal reactions were difficult to interpret since the experiments
were run at different temperatures.
67,72
Acid-base chemistry of nitrenium ions was
largely unexplored, hence it was unknown under what conditions they could be
protonated or deprotonated. It had not been demonstrated that nitrenium ions played
a role in the biological activity of mutagenic and carcinogenic esters of
N
-arylhy-
droxylamines or hydroxamic acids. Over the next two decades much progress was
made in sorting out these issues.
4.3.1 Lifetimes of Nitrenium Ions
Results of product and kinetic studies employing Cl
and I
had implicated an S
N
1
mechanism for the Bamberger rearrangement of
N
-arylhydroxylamines and the
hydrolysis of esters of
N
-arylhydroxylamines and
N
-arylhydroxamic
acids.
8,10,18,85-87,89-95
I
is an efficient nitrenium ion trap, but the mechanism
of reduction was not clear, while Cl
leads to products of nucleophilic trapping, but
is not a very efficient trap. Since the magnitudes of the rate constants for Cl
or I
trapping were unknown, the lifetimes of nitrenium ions in aqueous solution and,
therefore, the nature of the intermediates involved in the reactions were unknown.
Jencks and Richard had pioneered the use of the “azide clock” to determine the
lifetimes of carbenium ions generated under solvolysis conditions.
104
The method
uses the change in product yields as a function of [N
3
] to determine the ion's
N
3
/solvent selectivity, the ratio of the second-order rate constant for trapping of the
ion by N
3
and the pseudo-first-order rate constant for trapping of the ion by solvent:
k
az
/
k
s
. The assumption that
k
az
is diffusion limited at about 5
10
9
M
1
s
1
allows
1/
k
s
, the lifetime of the ion in water, to be estimated.
104
McClelland and Steenken
showed by direct measurement for diarylmethyl and triarylmethyl carbocations that
k
az
is approximately constant at (5-10)
10
5
s
1
.
105
The magnitude of the diffusion limited rate constant is slightly dependent on cation
structure, and solvent composition in CH
3
CN-H
2
O, but the assumption of a diffusion
limited reaction of carbocations with N
3
appears to be valid for reactive ions with
k
az
/
k
s
10
9
M
1
s
1
for ions with
k
s
10
4
M
1
.
105
The first application of the azide clock to nitrenium ions was made by Fishbein
and McClelland who showed that N
3
traps
5
67
(Scheme 4.18).
106
The selectivity,
k
az
/
k
s
, was 7.5M
1
.If
k
az
is diffusion limited, the
lifetime of the ion in H
2
O is about 1.5 ns.
106
The ion survives long enough to react
inefficiently with nonsolvent nucleophiles.
64m
, during the rearrangement of
