Enantioselective epoxidation of non-functionalized olefins
(
(
media V) and catalyst [both homogeneous and heterogeneous
salen)Mn(III)] changed from brown to deep green then to yellow
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under pre-stirring before catalysis, but the color of the mixture
containing BMImBF4 (media IV) and catalyst remained brown
duringthestirring.Possibly,akindof(salen)Mn(III)PF6 typecatalyst,
which was the productof anion exchanging procedure, could offer
better ee values than (salen)Mn(III)Cl type catalyst.
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−
Cl and PF6, the complex of (salen) Mn (III) PF6 was synthesized
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and dispersed in BMImPF6 under stirring, and only the yellow
mixing solution was obtained. It was proved that the speculation
was right by the experiment. Ionic liquid in our manipulation
was only reaction medium without catalytic function. The colored
5
[
13] N. S. Finney, P. J. Pospisil, S. Chang, M. Palucki, R. G. Konsler,
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−
solutions owning to anion exchange between Cl and PF6 could
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catalyzeepoxidation, but(salen)Mn(III)complexwasseparatedby
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that (salen) Mn (III) PF6 was the more effective catalyst.
Tetrahedron Asymm. 2001, 12, 433.
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998, 136, 47.
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1
Conclusions
T. Ramani,
H. Maheswaran,
L. Prashant,
In summary, three goals of this work were achieved to some
extent. At first, the recycling potential of BMImBF4 and BMImPF6
for the homogeneous or heterogeneous (salen)Mn(III) catalyzed
asymmetric epoxidation of unfunctionalized alkenes was revealed
in a stepwise manner: accelerated reactions, acceptable ee values
and yields were achieved. Secondly, the presence of NH4OAc
playedanimportantroleintheimprovementofenantioselectivity.
At last, the two silica-supported chiral (salen)Mn(III) catalysts
used in conjunction with BMImPF6 afforded not only high yields
4
[
[
20] H. Zhang, S. Xiang, C. Li, Chem. Commun. 2005, 5, 1209.
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fact, this work emphasized an effective approach for amplifying
the chiral induction capability and reactivity of Jacobsen-type
catalyst. This new approach included the initial immobilization
of Jacobsen-type catalyst on silica support through the linkage
anchored on the 5,5 -positions of salen ring and the subsequent
dispersion of supported catalyst into ionic liquid, which brought
about excellent catalytic results.
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Acknowledgments
The authors are grateful for the financial support of Xi’an Jiaotong
University.
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