Electronic Effects in (salen)Mn-Based Epoxidation Catalysts

Abstract

Presented are density functional calculations on various Mn(salen) systems that are active catalysts in the epoxidation of olefins. Correlation of various structural properties such as MnO bond strengths, atomic charges, and C−O distances of evolving bonds in transition state geometries with modified Hammett constants reveal a mechanistic picture of the epoxidation reaction, supporting previous experimental results. Enantioselectivity is tied to the position of a transition state along the reaction coordinate for the first C−O bond formation step, when an olefin is approaching the epoxidation catalyst. Electronic effects exhibited by the 5,5‘ substituents of the salen ligand manifest themselves in a tuning of the MnO bond strength, which in turn influences the C−O distance of the forming bond in the transition state geometry.