Chemistry Reference
In-Depth Information
O
OMe
Fragment.
R
N
O
+
-
CF
3
CO
2
OMe
O
O
R
N
Ph-I
F
3
C
312
(
1
A
1
)
OMe
H
R
N
O
310
I
O
I
O
H
+
Al
AL
1
TFA
F
3
C
F
3
C
Ligand
coupling
O
O
O
PIFA (
309
)
311
N
R
sp
3
OTFA
Ph-I
OMe
313
SCHEME 10.48.
group,
110
Glover's theoretical studies suggest this not to be the case. In this regard,
the predicted absence of effect on the energy of
upon rotation of the carbonyl-
nitrogen bond suggests that the acyl substituent plays little or no role in the
stability of alkoxy nitrenium ions.
103
These findings are also supported by Ford,
who has found that in aryl nitrenium ions, there is little
308
-electron transfer between
the divalent nitrogen and an
N-
acetyl group, which are predicted to be
p
s
-electron
donors.
111
In light of the practical limitations of using
N
-chlorohydroxamate esters, which
include the need to use stoichiometric quantities of silver salts and
tert
-butyl
hypochlorite, which is potentially explosive,
112
Kikugawa
et al.
developed a more
direct route to stabilized nitrenium ions, based on the oxidation of
O-
alkyl
hydroxamates
310
with iodine(III) reagents, including phenyliodine bis(trifluor-
,PIFA)
113
and Koser's reagent
114
([hydroxy(tosyloxy)iodo]ben-
zene; HTIB) (Scheme 10.48).
115
Nitrenium ion formation in the case of PIFA (
oacetate) (
309
309
)
is believed to be instigated by ligand exchange at the iodine center
116
to form
.
117,118
Decomposition of what might be considered an
“anomeric amide”, through nitrogen-iodine bond heterolysis, then leads to nitre-
nium ion
3
-iodane
amido-
l
311
312
. Alternatively, ligand coupling of
311
would generate
N
,
N
-bisheter-
oatom-substituted amide
, which upon protonation of the acyloxy group could
also serve as a source of nitrenium ions,
313
through ionization, via an A
AL
1
.
119
In 2003, Kikugawa
et al.
reported that the oxidation of
N
-acylaminophthalimides with hypervalent iodine
reagents generated
N
-stabilized nitrenium ions which, in the presence of trifluor-
oethanol (TFEA), underwent benzannulation and spriocyclization with pendant
arene group (see below).
120
At this stage, it is instructive to reiterate the critical role that the
N-
alkoxy and
N-
phthalimido substituents play in facilitating the formation and stabilization of the
autocatalytic process, to form nitrenium ion
312
