Sc3+-Promoted Selective Oxygenation of Olefins with H2O2 by a Ligand-Free Iron Catalyst: Insights into the Nature of Active Oxidant.

Abstract

Developing catalytic methods for the selective oxygenation of olefins remains an important objective in oxidation chemistry. Many iron-based catalysts have been reported in that pursuit. However, most catalytic systems rely on supporting ligands and often display nonselective oxidation via free radical pathways. Herein, we report a catalytic system combining a simple iron salt and Lewis acid (Sc(OTf)3) in acetonitrile for the selective epoxidation of a series of olefins with hydrogen peroxide. An iron(II)-scandium(III) triflate complex, formed in the reaction mixture, activates hydrogen peroxide, leading to a metal-based oxidant and not any radical species, thereby suggesting a mechanism different from the Fenton chemistry. An electrophilic oxidant is involved in the Sc3+-promoted epoxidation reactions. Quantum chemical analysis reveals the identity of the active oxidant responsible for the epoxidation reaction and elucidates the role of Sc3+ ions in the mechanistic pathway. The proximity of Sc(OTf)3 to the iron center helps in the O-O bond cleavage and generates the active oxidant required for the epoxidation reaction. This work demonstrates that the catalytic activity of simple iron salts in combination with Sc(OTf)3 in acetonitrile without the requirement of any supporting ligand does not involve free radicals, while affecting the selective oxygenation of olefins.