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Abstract

Based on the concept of “asymmetric counteraion-directed catalysts (ACDC)”, a series of ion-pair catalysts containing an achiral metallosalen cation and a chiral anion was designed and prepared for the asymmetric epoxidation of unfunctional alkenes and the oxidation of sulfides.
With the optimal ion-pair catalyst that consits of an achiral Mn-salen cation and a chiral BINOL-derived phosphate anion, high yields and enantioselectivities were obtained in the epoxidation of various alkenes like chromenes and cinnamates. With this type of ion-pair catalysts, a level of enantioselectivity similar to Jacobsen’s Mn-salen catalyst was observed in the asymmetric oxidation of sulfides. The corresponding iron complex was found much more efficient for sulfoxidation, and the high enantioselectivities observed in the cases of electron-poor sulfides are unprecedented in Mn- or Fe-based metallosalen systems.
The combination of chiral salen ligands and chiral counteranions has also been investigated. In the epoxidation of alkenes with the configuration-matched ion pairs the enantioselectivity of the Mn-salen complexes consisting of only chiral salen ligands can be further improved.
The application of the ACDC concept to porphyrin chemistry was also attempted. Substantial albeit low enantioselectivity was observed in epoxidation reactions. These results further demonstrate that ACDC can be employed as a general strategy in the arena of asymmetric transition metal catalysis.