Epoxidation of olefins catalysed by vanadium–salan complexes: a theoretical mechanistic study

Abstract

Plausible mechanisms of olefin epoxidation catalysed by a V–salan model complex [V^IV(O)(L)(H₂O)] (1, L = (CH₂NHCH₂CHCHO⁻)₂) in the presence of H₂O₂ are investigated and compared by theoretical methods using density functional theory. Three main routes, i.e. the Mimoun, Sharpless and biradical mechanisms, were examined in detail, and the Sharpless pathway was found to be the most favourable one. The reaction starts from the formation of an active catalytic species [V^V(O)(OO)(LH)] (3c) upon interaction of 1 with H₂O₂, then concerted, highly synchronous attack of the olefin to 3c occurs yielding the epoxide and catalyst [V^V(O)₂(LH)], the latter being oxidized by H₂O₂ to 3c. The activation barrier strongly depends on the proton location in the catalyst molecule and is the lowest when one of the oxygen atoms of the salan ligand is protonated and the vanadium atom is penta-coordinated with one vacant coordination position (complex 3c). The olefin in this reaction acts as an electron donor (nucleophile) rather than as an electron acceptor (electrophile).