Azobenzene Functionalized Chiral Salen Ti(Ⅳ)Catalysts For Light-controllable Cooperative Catalysis Of Asymmetric Sulfoxidation

The asymmetric oxidation of sulfides to sulfoxides with aqueous H2O2 was one of the most important reactions in organic synthesis,because the obtained chiral sulfoxidesare valuable intermediates for the synthesis of fine chemicals and many pharmaceuticals.Azobenzene has received much attention in recent years due to its well known reversible photoisomerization from trans-to cis-conguration.Herein,we introduced the azobenzene moiety into chiral salen Ti(Ⅳ)complex,and expected that the conguration of azobenzene in catalysts would controlled by UV irradiation.It should thus adjust the intramolecular,cooperative effect of chiral salen Ti(Ⅳ)complex in asymmetric sulfoxidation.Self-assembled metallomicelles with ultraviolet(UV)-controlled morphologies were constructed from a synthesized azobenzene-containing amphiphilic chiral salen Ti(Ⅳ)catalyst.The morphologies of the metallomicelles could be well adjusted by changing the UV irradiation time,and this was confirmed by TEM analyses.The UV-induced change in morphology potentially adjusted the catalyst concentration and/or accessibility in real-time,allowing photocontrollable catalysis of asymmetric sulfoxidation in water.UV-responsive catalytic activities with excellent selectivities were observed over the metallomicelles for a wide range of alkyl aryl sulfides.Moreover,a thermo-responsive NIPAAm block in the catalyst enables it to be easily recovered for steady reuse by thermo-controlled separation.A novel light-controllable cooperative chiral salen Ti(Ⅳ)catalyst was prepared by bridging double chiral salen Ti(Ⅳ)units with azobenzene.The azobenzene-bridged chiral salen Ti(Ⅳ)catalyst exhibited reversible isomerization from trans-to cis-conguration upon the alternate irradiation of UV and visible light.It allowed to adjust the relative location of bridged chiral salen Ti(Ⅳ)complexes,which thus real-time regulate the intramolecular,cooperative effect of chiral salen Ti(Ⅳ)complex in asymmetric sulfoxidation.Furthermore,the catalyst could be separated from reaction system through solvent precipitation,and be reused for several times.Ionic liquid(IL)functionalized azobenzene was synthesized and used as a linker to bridge the chiral salen Ti(Ⅳ)complexes.The chiral salen Ti(Ⅳ)catalyst containing IL-functionalized azobenzene bridge exhibited a light-controllable cooperative catalysis in asymmetric sulfoxidation due to the photoinduced trans-cis isomerization of azobenzene unit.More importantly,the imidazolium-based IL moiety with special ’ionophilicity’ and polarity has a positive effect on stabilizing the chiral salen Ti(Ⅳ)complex and further enhancing the catalytic efficiency.