Chemistry Reference
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O
O
CF
3
COO
NPhth
PIFA
CHCl
3
Me
N
O
N
N
R
+
R
H
reflux
0.5 h
Me
O
477
478
479
480
(%)
R
O
H
2-Cl
3-Cl
4-Me
4-CO
2
Me
90
92
74
80
68
OH
Ac
CF
3
O
Ac
NH
2
NH
2
EtOH
N
N
NPhth
NH
2
R
R
reflux
0.5 h
480
(see table)
481
(R = H); (75%)
SCHEME 10.78.
Most recently, in an extension of his studies of the diazenium ions generated from
N
-aceylaminophthalimides (
), Kikugawa has reported a useful method for the
intermolecular hydrazidohydroxylation of styrenes (Scheme 10.78).
182
In this case,
treatment of
N
-acetylaminophthalimide (
477
477
) with phenyliodine(III) bis(trifluoroa-
cetate) in the presence of styrenes
with excellent
regiocontrol and in high yield. Removal of the trifluoroacetyl and phthalimido group,
using hydrazine hydrate, then provides
479
yields addition products
480
, the product of alkene hydrazidohy-
droxylation. In contrast to Urry and Cauquis, Kikugawa did not observe an
aziridinium ion intermediate in this case, and in fact proposes that this transformation
proceeds through a
carbocation
intermediate generated by the regioselective attack
of
N-
stabilized nitrenium ion
481
,
the iodine(III)-mediated reaction of
N-
aminophthalimide with alkenes leads to the
formation of aziridines by way of an amino nitrene or nitrene-type intermediate,
rather than a nitrenium ion.
183
479
. In notable contrast to the addition reaction of
474
10.8.2 Alkene Addition of O-Stabilized and Aryl Nitrenium Ions
Oxygen-stabilized nitrenium ions, which, given the diminished ability of oxygen to
stabilize an adjacent electron-deficient center, are more reactive than diazenium ions,
have played a predominant role in the development of nitrenium-alkene addition
chemistry. In 1988, Hoffman reported that oxidation of
O
-benzylhydroxylamine (
)
with [
m
-(trifluoromethyl)phenyl]sulfonylperoxide (
m
-TFBSP) in the presence of
tetramethylethylene (
482
,albeitin
low yield (Scheme 10.79).
184
This product was proposed to arise from hyponitrite
ester
485
) generated
O
-allyl-
N
-benzylhydroxylamine
487
, through a sequence of ionization, nitrenium ion addition, and eliminative
ring opening of the resulting
N-
alkoxyaziridinium ion
483
. The inefficiency of this
transformation appears to stem largely from competitive cleavage of the reagent N-O
486
