Methyl Iodide Accelerated Asymmetric Epoxidation of Alkenes Catalyzed by Chiral Salen-Mn(Ⅲ) Complexes with Tertiary Amine Units

摘要

A series of chiral salen-Mn(Ⅲ) complexes featuring two tertiary amine units were synthesized and employed in the enantioselective epoxidation of unfunctionalized alkenes in the presence of pyridine N-oxide and 2,6-dimethylpyridine N-oxide as proximal ligands, respectively. Moderate to high enantioselectivity and acceptable yields were achieved when NaClO was used as terminal oxidant under CH2Cl2/H2O biphasic media. Methyl iodide was found to be an effective additive to accelerate the epoxidation, possibly owing to the formation of quaternary ammonium units on catalysts, which may facilitate the reaction in an oil/water biphasic medium. The subsequent stimulation experiment was carried out, and the resulting ESI-HRMS analysis revealed the formation of a novel(salen)manganese(Ⅲ) intermediate featuring two quaternary ammonium units, and bearing a pyridine N-oxide and a molecule of water simultaneously axially-coordinated backbone.