The design and properties of surface-active
ionic liquids that
are able to form stable microemulsions with heptane and water are
presented, and their promise as reaction media for thermomorphic palladium-catalyzed
cross-coupling reactions is demonstrated.
A thermomorphic ionic-liquid-based
microemulsion system was successfully
applied for the Ru-catalyzed asymmetric transfer hydrogenation of
ketones. On the basis of the temperature-dependent multiphase behavior
of the targeted microemulsion, simple product separation as well as
catalyst recycling could be realized. The use of water-soluble ligands
improved the immobilization of the catalyst in the microemulsion phase
and significantly decreased the catalyst leaching into the organic
layer upon extraction of the product. Eventually, the optimized microemulsion
system could be applied to a wide range of aromatic ketones that were
reduced with good isolated yields (up to 98%) and enantioselectivities
(up to 97%), while aliphatic ketones were less successful.
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