Dendritic Chiral Salen Titanium(IV) Catalysts Enforce the Cooperative Catalysis of Asymmetric Sulfoxidation

A series of new dendritic chiral salen Ti-IV catalysts was prepared by the covalent confinement of multiple chiral salen Ti-IV complexes on the periphery of a polyamidoamine (PAMAM) dendrimer through a flexible ionic liquid (IL) linker. The characterization results suggested the presence of a dendritic PAMAM scaffold, IL spacer, and intact peripheral active sites in the catalysts. The unique dendritic structure of catalysts together with a flexible IL spacer enforced the intramolecular, cooperative catalysis of asymmetric sulfoxidation to result in improved catalytic efficiency and, especially, a remarkably high selectivity. A chemoselectivity of 90% with an enantioselectivity of 85% was obtained in the asymmetric sulfoxidation of methyl phenyl sulfide catalyzed by the dendritic catalyst, which was significantly higher than that observed for the neat complex (64% chemoselectivity with 68% enantioselectivity). Furthermore, the catalysts could be recovered easily by solvent precipitation for efficient reuse.