Highly selective olefin epoxidation with manganese triazacyclononane complexes: Impact of ligand substitution

Manganese complexes of 1,4,7-triazacyclononane with different substituents catalyze the selective epoxidation of a large number of olefins to epoxides with H₂O₂. The activities of complexes with methyl (L₁), 2-hydroxybutyl (L₂) and acetato (L₃) substituents are compared. The effects of solvent and temperature on the epoxide yield are very different for the three complexes. It is proposed that these differences are related to the binding of the pendant arms in Mn---L₂ and Mn---L₃ complexes. In general, acetone or methanol are preferred solvents. Variations of stereoretention are also observed: with Mn---L₁ in acetone, isomer scrambling occurs, while with Mn---L₁ in methanol, the epoxidation is almost stereospecific. UV-visible and electron spin resonance spectroscopy are used to characterize the state of manganese under oxidizing conditions.