An ionic manganeseporphyrin catalyst for alkene epoxidations: The significant roles of the cationic trimethylbenzylammonium functionalities and the anionic mercaptopropionate counterions

An ionic manganeseporphyrin, bearing cationic trimethylbenzylammonium functionalities and mercaptopropionate [HS(CH₂)₂CO ₂^-] counteranions, was synthesized and applied as a catalyst for alkene epoxidations using iodosylbenzene (PhIO) as an oxidant. The reactions proceeded with high activity and oxide selectivity, and the catalyst exhibited relatively good recyclability. It was found that the presence of thiol (SH) group and quaternary ammonium substituents contributed greatly to the improved activity and stability of this ionic manganeseporphyrin. This was due to the synergistic effects of HS(CH₂)₂CO₂ ^- as an axial ligand, and the suppression of oxidation degradation by the strongly electron-withdrawing nature of the quaternary ammonium cations.