Chiral Salen Ti(Ⅳ) Catalysts Containing Double/Multiple Active Centers Cooperatively Catalyzed Asymmetric Sulfoxidation

Chiral sulfoxide which is a kind of important chiral synthesis and chiral auxiliary reagents, has extensive applications in the field of biological medicine and chemical industry. Based on the idea of cooperative catalysis, double or multiple chiral salen Ti（Ⅳ） centers were introduced into a molecular structure. This kind of catalyst exhibiting higher catalytic activity and selectivity than tranditional neat salen Ti（Ⅳ） complex with H2O2（30 wt.%） as oxidant. In addition, the catalyst can effectively recycle and reuse, has the characteristics of green catalysis.（1） Double chiral salen Ti（Ⅳ） units were covalently introduced into the double imidazole ionic liquids as the ends of the connection groups, giving ionic liquid function dimeric chiral salen Ti（Ⅳ） catalyst（denoted as Catalyst A-n（n=1, 2））. This kind of catalyst with double catalytic active center cooperatively catalyze the asymmetric sulfoxidation, exhibiting excellent catalytic activity and selectivity in asymmetric sulfoxidation. In addition, the catalysts can recycle and reuse effectively.（2） A series of cooperative chiral salen Ti（Ⅳ） catalysts（denoted as Catalyst B-n（n=4,8,13）） 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 well suggested the presence of polyether-based IL spacer 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. Meanwhile, this catalyst can be realized and separated from reaction system only through appropriate solvent.（3） Multiple chiral salen Ti（Ⅳ） complexes were covalently anchored on the surface of ionic liquid functionalized SiO2@Fe₃O₄ nanoparticles, providing the magnetic chiral salen Ti（Ⅳ）-IL-SiO2@Fe₃O₄ catalysts（denoted as Catalyst C）. Multiple catalytic active centers in catalyst cooperatively catalyzed the asymmetric sulfoxidation, exhibiting high catalytic activity and selectivity in asymmetric sulfoxidation in water. In addition, the catalyst could be easily separated for reuse by using a permanent magnet externally.