Chemistry Reference
In-Depth Information
+
H
H
H
H
2
O
H
H
H
H
H
N
N
N
N
N
N
+
+H
2
O
-H
2
O
H
+
+
1
2
3
OH
4
H
OH
2
+
FIGURE 6.2.
The Bamberger rearrangement.
The electronic configurations have also been shown to influence their reactivity.
2
Triplet nitrenium ions are known to provide products of radical reactions whereas
singlets tend to combine with nucleophiles or suffer unimolecular rearrangement.
Controlling the electronic configurations of nitrenium ions could potentially provide
useful new synthetic transformations. However, practical applications of triplet
reactivity have yet to be realized.
The first publication to postulate the existence of any nitrenium ion appears to be
from the work of Heller et al.,
7
who examined the kinetics of the Bamberger
rearrangement
(acid-catalyzed conversion of
N
-hydroxylaniline
1
to
para
-
aminophenol
) and concluded that this process occurs in a stepwise manner as
indicated in Figure 6.2. Dissociation of the O-protonated reactant
4
2
gives phenyl
nitrenium ion
, which subsequently combines with H
2
O at the position
para
to the
nitrogen to form, following deprotonation, the product
3
. Further support for this
mechanism was the trapping of the putative intermediate by other added nucleo-
philes (e.g., EtOH, Cl
).
Gassman et al.
8,9
established the existence of nitrenium ions as bona fide
intermediates (rather than transition structures) through analysis of solvolysis
kinetics of
N
-chloroaniline
4
, species that are presumed to be precursors to nitrenium
ions. It was shown that the dependence of the decomposition rate constants on the
nature of the aromatic ring substituents followed a Hammett linear free energy
relationship that was consistent with formation of a positively charged intermediate
6
5
(Fig. 6.3).
Aryl nitrenium ions have also been implicated in chemical carcinogenesis.
10
Miller et al. fed rats with aromatic amines
and were able to correlate tumor
formation with the formation of modified guanosine bases
8
. On the basis of indirect
evidence, they attributed this to metabolic formation of transient aryl nitrenium ion
11
Cl
H
H
H
H
H
H
+
N
N
N
N
Nuc
Nuc
ρ
= -6.35
Nuc-H
X
X
X
X
Nuc
5
6
FIGURE 6.3.
Solvolysis of
N
-chloroaniline derivatives
5
shows a dependence on substituents
(X) that is consistent with formation of an aryl nitrenium ion
6
.
