| Epoxidation of olefins is one of the most important reactions in organic synthesis,because the epoxides can be easily converted to large variety of compounds.In terms of atom efficiency,after oxygen,aqueous hydrogen peroxide would undoubtedly be the ideal oxidant since it produces water as the sole by-product and can therefore be classified as the ultimate green oxidant.Moreover,for industrial applications,the development of epoxidation systems using cheap and nontoxic metal complexes as catalysts is significant.To achieve this goal,we designed and synthesized a series of hexadentate binding 8-quinolinolato manganese(â…¢)complexes [Q3Mnâ…¢],and investigated their catalytic efficiency in catalyzing the epoxidation of olefins with H2O2.Our works are summarized in details as follows:1)After a series of tests,we found the Q3Mnâ…¢complexes can highly efficiently catalyze the epoxidation of a scope of olefins with H2O2. Besides that,this epoxidation system has many advantages,such as utilization of more cost-effective and green water-acetone solvent, very high catalytic efficiency and stability,facile operation,capability of in situ generation of highly effective catalysts and etc..2)In situ cyclic voltammetry(CV)and quartz crystal microbalance (QCM)techniques are introduced to identify the structural transition of Q3Mnâ…¢.The results showed that the Jahn-Teller effect of Mn3+ions with d4 causes the distorted octahedral structure of the Q3Mnâ…¢with an axial Mn-O bond longer than its equatorial ones,and the axial Mn-O bond may be more easily cleaved to form a more soluble pentadentate structure with pendant hydroxyl groups.3)UV-vis spectra further showed that the Q3Mnâ…¢complexes have the feature of pH-regulated molecular switches which can be converted reversibly between hexadentate and pentadentate structures through unprotonated and protonated processes.4)Based on the stepwise overlay of UV-vis spectra for the reaction mixture,we proposed a reasonable mechanism for the catalytic cycle.
|
PDF Full Text Request