Supramolecular assembly of chiral polyoxometalate complexes for asymmetric catalytic oxidation of thioethers

Abstract

In this paper, a chiral amphiphilic cation with two stereocenters has been employed to encapsulate a well-known catalytically activated sandwich-type polyoxometalate, Na₁₂[WZn₃(H₂O)₂(ZnW₉O₃₄)₂], through electrostatic interaction. The prepared chiral organic cation-encapsulated polyoxometalate complexes were found to self-aggregate into spherical supramolecular assemblies with a diameter of ca. 100 nm in the reaction solution. These assemblies, serving as microreactors, exhibited an efficient asymmetric catalytic activity for the oxidation of sulfide with up to 72% enantiomeric excess. The coverage density of chiral organic cations enwrapped on the polyoxometalate surface was proved to have an important influence on the enantioselectivity. Detailed kinetic study of the catalytic process showed that the enantioselectivity of sulfide oxidation was derived from the combination of an ineffective asymmetric sulfoxidation and an effective kinetic resolution of the sulfoxide. The present research strategically offers an understanding for the direct and efficient construction of polyoxometalate based supramolecular catalysts for asymmetric reactions through controlling the surface microenvironment.