Chemistry Reference
In-Depth Information
presence of a rhodium catalyst [148]. However, the enantioselectivities were only
moderate.
Katsuki and coworkers reported that chiral ruthenium(salen) complex
67
promotes
the aziridination of olefi ns with TsN
3
without irradiation at room temperature, and opti-
cally active aziridines were yielded in high enantiomeric excesses [149,150]. The authors
further investigated azide compounds as nitrene precursors and found that
p
- and
o
-
nitrobenzenesulfonyl azide (
p -
and
o
- NsN
3
) and 2 - (trimethylsilyl)ethanesulfonyl azide
(SESN
3
) also underwent aziridination enantioselectively [151]. Especially, the reaction
with SESN
3
effi ciently proceeded to give aziridines with high enantioselectivity. It is
noteworthy that less nucleophilic,
-unsaturated esters also underwent aziridination
with complete enantioselectivity and that the SES-protecting group was readily removed
by tris(dimethylamino)sulfonium difl uorotrimethylsilicate (TASF) without the loss of
the enantioselectivity (Scheme 11.79 ) [152] .
α
,
β
O
O
O
67
(2 mol
%)
CH
2
Cl
2
, RT
TASF
+ SESN
3
OBn
OBn
OBn
SESN
HN
81%, >99% ee
>99% ee
N
N
Ru
L
O
O
Ar
Ar
L = CO
Ar = 3,5-Cl
2
-4-Me
3
SiC
6
H
2
67
Scheme 11.79.
An aminimide that is produced by deprotonation of the corresponding aminimine
has been reported to undergo aziridination of chalcone by the following sequence: con-
jugate addition and ring closure via N-N bond cleavage [153]. Shi and coworkers discov-
ered that
N
-methylmorpholine-derived aminimide could be catalytically generated in
the presence of
O
- mesitylenesulfonylhydroxylamine and CsOH · H
2
O. The reaction also
works with a catalytic amount of a chiral tertiary amine, (+)-Tröger's base
68
, giving
enantio-enriched aziridines (Scheme 11.80) [154]. Although the reaction is substoichio-
metric and the enantioselectivity is moderate, this method could produce nonprotected
aziridines in a nonracemic form. Subsequent to this publication, Armstrong and cowork-
ers also reported aminimide-mediated asymmetric aziridination of chalcone using quini-
clidine and
O
- (diphenylphosphinyl)hydroxylamine (Scheme 11.81 ) [155] . Although the
method needs a stoichiometric amount of quiniclidine
69
, the enantio - enriched
α
- keto
aziridine was yielded with 56% ee.
The iminium/enamine catalysis, which has been successfully demonstrated in the
secondary amine-catalyzed asymmetric epoxidation of
- unsaturated aldehydes, was
also applied to asymmetric aziridination by using acetyl hydroxylcarbamate as nitrene
equivalent by Córdova and coworkers. The reaction proceeds through a quite similar
α
,
β
