Chemistry Reference
In-Depth Information
10.7 HETEROATOM-STABILIZED ACYL NITRENIUM IONS
10.7.1 Generation and Intramolecular Aromatic Substitution
Although rarely cited, Cooley and Jacob's 1975 report of the oxidative ring closure
of 1-benzyloxy-3-arylureas marks the earliest documentation of a reaction involv-
ing, what can be formulated as a heteroatom-stabilized acyl nitrenium ion
(Scheme 10.46).
100,101
Treatment of 1-benzyloxy-3-arylureas
295
with a slight
excess of lead tetraacetate was found to mediate rapid annulation, generating
1-benzyloxybenzimidazolones
in excellent yield. From a mechanistic stand-
point, Cooley proposed that decomposition of lead(IV) amide
298
ledtothe
formation of the observed product but did not invoke the involvement of
296
.The
supposition that this process proceeds through the intermediacy of a nitrenium
ion is supported by the observed inhibitory effects of electron-withdrawing aryl
substituents (
o
-nitro) upon ring closure: in these cases, unstable nitrogen-to-
nitrogen coupling products were formed.
102
In retrospect, Cooley's findings pointed the way forward for nitrenium ions as
they pertain to synthetic organic chemistry: whereas alkyl nitrenium ions are
difficult to generate and prone to intersystem crossing and rearrangement,
N-
acyl-
N
-alkoxy nitrenium ions are readily accessible and, as a result of the adjoining
heteroatom donor, singlet in nature (see below); while aryl nitrenium are ambi-
philic electrophiles and undergo reaction at one of three possible sites, acyl
nitrenium ions react only at the nitrogen center and, as a consequence, are
synthetically useful
N
-electrophiles, in contrast to their aryl congeners. In light
of these points, it is understandable why, in 1984, independent reports made by
Glover and coworkers
13,103
298
and Kikugawa and coworkers
104
concerning the
H
N
O
R
2
N
OBn
N
O
Pb(OAc)
4
L
R
1
O
O
HN
Me
Me
Pb
CHCl
3
OBn
O
O
295
O
O
296
Me
L = AcOH
R
1
R
2
298
(%)
R
2
p
-MeO
p
-Me
H
95
R
2
N
O
H
100
N
R
1
R
1
O
p
-Cl
H
94
N
N
OBn
H
Ph
99
OBn
297
298
SCHEME 10.46.
