Chemistry Reference
In-Depth Information
OH
66
R'
+ H
2
O
2
R'
R
R
CH
3
CN
OH
N
N
Fe
2+
N
N
OH
OH
OH
OH
O
OEt
(CF
3
SO
-
)
2
66
OH
OH
OH
OH
97% ee
96% ee
76% ee
78% ee
Scheme 11.78.
While the process has not yet reached a level for practical use in terms of the require-
ment for limiting oxidant, future work will provide improvements that overcome this
drawback. Recently, Chen and White disclosed the remarkable oxidation catalysis of
the related iron catalyst for predictable stereoselective hydroxylation of unactivated
C - H bonds [140] .
[141]
As described above, the toxicity of osmium reagents is a serious problem for the
Sharpless asymmetric dihydroxylation, and the development of alternative methods
using nontoxic, environmentally benign catalysts is desirable. Que's iron catalysts still
fail to stand up to practical use, but further improvements will lead to truly powerful
asymmetric dihydroxylation methods.
11.7. ASYMMETRIC AZIRIDINATION
Aziridines, the nitrogen counterpart of epoxides, are versatile building blocks in organic
synthesis and an important class of biologically active compounds. Signifi cant progress
has been made in the fi eld of catalytic asymmetric aziridination of olefi ns [141] . Evans
and coworkers fi rst identifi ed chiral copper/bisoxazoline complexes as catalysts for
nitrene-transfer aziridination and enabled the highly enantioselective synthesis of chiral
aziridines [142]. Simultaneously, Jacobsen and coworkers demonstrated the utility of
chiral diimine ligands for copper-catalyzed aziridination [143]. Subsequent to the pub-
lications, a large number of copper-based variants have been reported, and
manganese(porphyrin) and -(salen) complexes were also found to be effective catalysts
for the reaction [144,145]. These reactions were proposed to proceed via active nitrenoid
species and most of the known methods employ a hypervalent iodine reagent such as
PhI=NTs as nitrenoid precursor. While chiral aziridines were yielded as the
N
- sulfonyl
protected forms, the deprotection of
N
-sulfonyl groups usually requires harsh conditions.
Thus, the development of catalytic asymmetric synthesis of optically active aziridines
without a protecting group or with a readily removable group has attracted much atten-
tion in recent years. On the other hand, with respect to atom economy, the use of
PhI=NTs as nitrene precursor is not favorable due to the generation of a copious amount
of iodobenzene as a by-product. Azide compounds are an ideal nitrene precursor in terms
of atom effi ciency since they liberate only nitrogen gas along with the nitrene formation.
p -
Toluenesulfonyl azide (TsN
3
) is known to decompose, giving a free nitrene intermedi-
ate under ultraviolet irradiation or heating [146]. Jacobsen and coworkers employed TsN
3
in their copper/diimine-catalyzed asymmetric aziridination under irradiation [147].
Mueller and coworkers also reported asymmetric aziridination using
p
NsN
3
in the
