Mass-Spectrometric and Kinetic Studies on the Mechanism and Degradation Pathways of Titanium Salalen Catalysts for Asymmetric Epoxidation with Aqueous Hydrogen Peroxide

Abstract

The composition and degradation of a highly active and enantioselective titanium salalen in situ catalyst for the asymmetric epoxidation of olefins with aqueous hydrogen peroxide was investigated. Kinetic data and ESI-MS studies point to a mononuclear titanium salalen as the catalytically active species. By means of ESI-MS and selective monodeuteration of the salalen ligand, the oxidative degradation was studied. Upon exposure to aqueous hydrogen peroxide, the amine functionality of the salalen ligand is converted to the hydroxylamine, followed by loss of water and generation of the inactive titanium-salen complex. This transformation limits the activity of the catalyst in the epoxidation of less electron-rich olefins, such as 1-octene.