Cooperative chiral salen TiIV catalyst supported on ionic liquid-functionalized graphene oxide accelerates asymmetric sulfoxidation in water

Abstract

A cooperative chiral salen Ti^IV catalyst was prepared by covalently appending multiple chiral salen Ti^IV complexes on a graphene oxide (GO) surface through a flexible ionic liquid (IL) linker. Characterization results confirmed the decoration of intact Ti(salen) units on the edges and planes of exfoliated GO with the imidazolium-IL spacer. The IL-functionalized GO nanosheets endowed the chiral salen Ti^IV catalyst with high water-dispersion, diminished diffusion limitation, and particularly a reinforced intramolecular cooperative effect. The cooperative catalyst was thus highly efficient and universally applicable in the catalysis of asymmetric sulfoxidation in water, affording high yields of various chiral sulfoxides with excellent enantioselectivities, while traditional chiral salen Ti^IV complex was far less active. More importantly, the heterogeneous catalyst was perfectly stable and could be reused several times without significant loss of activity and selectivity.