Mechanistic studies on the reaction of monopersulfate with a manganese(III) porphyrin in the presence of phenates and benzoates as axial ligands and subsequent epoxidation of olefins in homogeneous solution

Abstract

The oxygen transfer reaction from Ph 4PHSO 5 to manganese(III) porphyrins in the presence of a series of tetrabutylammonium phenates and benzoates leading to a manganese-oxo species and the subsequent epoxidation of olefin has been studied in dichloroethane. When benzoates act as axial ligands of the catalyst, the Hammett plots obtained by changing the substituents on the phenyl ring of the ligand give negative rho values for both oxene formation and epoxidation. On the contrary, when phenates are employed the Hammett plot for olefin epoxidation provides a positive rho value. This is rationalized by assuming that the variation of the nature of the ligand produces a change of the rate determining step of the catalytic process. In addition, the oxo species formed in the presence of phenates behaves as an electrophilic oxidant whereas that formed in the presence of benzoates has a definite radical character.