Chemistry Reference
In-Depth Information
AgNO
3
N
N
Cl
Me
H
Me
MeOH
Δ
138
139
H
H
H
Me
N
+
+
+
N
N
N
H
H
H
Me
Me
Me
OMe
O
2
NO
Cl
MeO
140
141
142
143
SCHEME 10.20.
shifts are believed to involve the intermediacy of an electron-deficient nitrogen
species. Unfortunately, Gassman's choice of substrate for his rebuttal was question-
able, since the solvolysis of 2-azabicyclo[2.2.1]heptane
involved a Wagner-
Meerwein rearrangement. In this case, critics were in agreement that a heterolytic
process, which involved the formation of an electron-deficient nitrogen was at play
and therefore, Gassman's data still did not eliminate the possibility that a radical
pathway was involved in the intramolecular addition reactions.
Although never raised during the debate between Edwards, Waegell, Hobson, and
Gassman, the work of Mokotoff actually adds support to Gassman's assertion that
nitrenium ions are intermediates in the
p
-route to heterocycles.
52
Seeking a route to
functionalized azabicyclo[3.3.1]nonanes, Mokotoff viewed the solvolysis of cyclo-
hexyl
N
-chloramines as an attractive alternative to existing methods for their
formation.
53
Upon treatment of a refluxing methanolic solution of chloramine
138
130
was formed in a
ratio of 1:75:1:3:23 (determined by GC) (Scheme 10.20). In rationalizing this
observation, the author favored a nitrenium ion intermediate over a radical chain
mechanism, citing Hobson's
46
work in support of his proposal. Mokotoff argued that
if a radical chain pathway was involved in the cyclization of
with AgNO
3
, a mixture of five products,
139
and
140
-
143
138
, that in the formation
of
,
cis
addition should also be observed, as was observed by
Hobson.
46
In the case of
140
,
141,
and
143
however, only products arising from
trans
addition were
isolated. The author also pointed out that if a radical chain mechanismwas involved, a
much larger amount of amine
138
should be formed, which was not the case.
In light of Mokotoff's work, it remains unclear as to whether the cyclization of
unsaturated
N
-chloramines proceeds via nitrenium ion, aminyl radical or, as more
recently suggested by Hoffman,
54
a protonated or metalated radical cation interme-
diate. In this regard, it is interesting to note that while Takeuchi has demonstrated that
alkyl and dialkyl nitrenium ions
139
can be generated unambiguously, through the
photolysis of 4,6-diphenylpyridinium tetrafluoroborates
145
,
55
this process has yet to
be employed to study the intramolecular addition of these
N
-electrophiles with
alkenes (Scheme 10.21).
144
