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Ph
Ph
hν
R
1
= H, Me
R
2
= H, Me
N
+
R
1
R
2
Ph
N
Ph
TFA
Ph
N
Ph
N
R
1
R
2
144
145
146
SCHEME 10.21.
10.5.3 Ring Expansion and Contraction of Alkyl Nitrenium Ions
(or Equivalents)
As might be anticipated from Stieglitz's original observations,
N
-cycloalkyl nitre-
nium ions, or their equivalents, are prone to undergo 1,2-alkyl shifts, leading to ring
expansion (Scheme 10.22). Gassman, for example, rationalized the formation of
152
from
147
, in terms of the generation and rearrangement of cyclobutylcarbinyl-like
,
56
while formation of
cation
148
151
was ascribed to the intersystem crossing of
148
and hydrogen abstraction by triplet species
. In the same study, Gassman also
presented evidence, albeit indirect, for the
ring contraction
of nitrenium ion.
Specifically, that Ag(I)-assisted solvolysis of
150
153
generated benzaldehyde
157
and amides
159
and
160
, following benzolylation, was posited as evidence for
the ring contraction for
(Scheme 10.23).
The silver(I)-mediated rearrangement of
N
-chloroamines has also been investi-
gated by Schell
et al.
as a route to bicyclic alkaloids.
57
Schell noted that when
8-chloro-8-azabicyclo[3.2.1]octane
153
to
158
was sequentially treated with silver nitrate
and sodium cyanoborohydride in methanol, pyrrolizidine
161
162
and parent tropane
163
were isolated (Scheme 10.24).
Ph
Ph
1,2-Alkyl
shift
Me
AgOCOCF
3
N
N
Ph
N
Me
Cl
Me
MeOH
147
148
149
ISC
NaBH
4
H-Atom
transfer
Ph
Ph
Me
N
N
Ph
N
Me
Me
H
150
151
(24%)
152
(36%)
SCHEME 10.22.
