Cooperative chiral salen TiIV catalysts with built-in phase-transfer capability accelerate asymmetric sulfoxidation in water

Abstract

A series of cooperative chiral salen Ti^IV catalysts with inherent phase-transfer capability were prepared by covalently bridging double chiral salen Ti^IV units with various polyethylene glycol (PEG)-based dicationic ionic liquid (IL) linkers. Characterization results suggested the presence of polyether-based IL spacers and intact active sites in the catalysts. The polyether-based dicationic IL spacer not only enforced an intramolecular, cooperative reaction pathway favored for the asymmetric suloxidation, but also endowed the bimetallic catalysts with built-in phase transfer capability. High yields of chiral sulfoxides (in the range of 74–90%) with excellent ee values (in the range of 85–91%) were achieved within 45 min when the asymmetric oxidation of methyl phenyl sulfide, methyl p-methoxyphenyl sulfide, and methyl o-methoxyphenyl sulfide were performed in water. The catalytic efficiency was significantly higher than that over neat complex (yields, 10–16% and ee values, 77–79%). More attractively, the phase transfer catalysts could be facilely recovered by solvent precipitation for efficient reuse.