Chemistry Reference
In-Depth Information
( )
n
AlCl
3
N
2
δ
+
N
3
AlCl
3
N
A
r
N
2
δ+
N
Me
AlCl
3
( )
n
Ar = 4-MeC
6
H
5
Me
509
510
511
Cl
Ar
N
Ar
N
or
AlCl
3
( )
n
( )
n
( )
n
NHAr
512
513
(81%,
n
= 1)
514
(59%,
n
= 2)
515
(93%,
n
= 3)
SCHEME 10.84.
formation of
was rationalized in terms of the formation and ring-opening
of
N
-phenylaziridinium ion
504
and
505
501
:S
N
2 attack by trifluoroacetate would lead to
503
(pathway a), while proton abstraction by this anion would yield olefin
505
(pathway b).
198
Substituted aniline
508
, on the other hand, was envisioned as
506
; generated by attack of cyclohexene at the
ortho
arising from phenylium ion
position of
.
Intriguingly, the outcome of the reaction between cyclic alkenes and aryl azides in
the presence of aluminum trichloride is highly dependent on substrate ring size
(Scheme 10.84).
197
In contrast to activation with TFA, where the intermediacy of a
free nitrenium ion is implied, the reaction of aryl azides with aluminum chloride is
thought to generate an azide-AlCl
3
complex
500
, which undergoes induced decom-
position with alkenes to form an aziridinium-AlCl
3
complex
510
512
. In the case of
cyclohexene, 1,3-diaxial strain in aziridinium ion
512
(
n
¼
1) promotes concerted
ring opening by Cl
to yield
trans
-
, while the cycloheptene and cyclooctene
complexes are inert to ring-opening and yield aziridines
513
upon treatment
with aqueous Na
2
CO
4
. Unfortunately, both methodologies developed by Takeuchi
are limited by the need to employ both TFA as a co-solvent, and a large excess of
alkene, as well as by the incompatibility of styrene and 1,1-disubstituted ethylene
derivatives: in the presence of TFA, these substrates undergo competitive proto-
nation and reaction with phenyl azide to form
N
-substituted anilines.
In addition to azirdination, aryl nitrenium ions also undergo annulation with
alkenes to generate 1
H
-indole,
199
dihydroindole and 4,5-dihydrobenzooxazepine
derivatives. In this regard, Swenton
et al.
have reported that treatment of quinone
imine ketal
514
and
515
516
with TsOH, in the presence of electron-rich styrene derivatives
518
,
in reasonable overall yield (Scheme 10.85).
200
This unusually
mild entry to nitrenium ions is believed to proceed via selective protonation and
ionization of the acetal moiety in
generates
520
and
521
516
517
at the
ortho
. Reaction of the styrene with
519
position then generates benzylic cation
, which, upon ring closure, leads to the
