Effect of Transition Metal Compounds on the Cyclohexene Oxidation Catalyzed by N-Hydroxyphthalimide

Abstract

N-Hydroxyphthalimide (NHPI) is an efficient organic catalyst in the oxidation reactions of organic compounds occurring via a radical mechanism, often used together with redox-active ions or transition metal complexes. In this work the catalytic action of NHPI is studied together with Cu(II), Fe(III), and Mo(VI) compounds in the reaction of aerobic oxidation of cyclohexene in an acetonitrile solution at 60°C. It was found that iron(III) benzoate accelerates the reaction by rapidly generating the active form of the phthalimide-N-oxyl radical (PINO) catalyst, but does not cause decomposition of the hydroperoxide. The oxidation product is 2-cyclohexenyl hydroperoxide formed with a selectivity of 85% at a cyclohexene conversion of 50%. Copper(II) acetate initiates oxidation and is capable of catalyzing the radical decomposition of the hydroperoxide and secondary oxidation of allyl oxygenates. When reaching a cyclohexene conversion close to 80%, the overall selectivity to the main products, 2-cyclohexenyl hydroperoxide and 2-cyclohexen-1-on, was 70%. The addition of iron(III) and molybdenum(VI) compounds led to the intensive generation of hydroperoxide and its activation as an electrophilic reactant capable of cyclohexene epoxidation. As a result of the use of the multifunctional three-component NHPI–Mo(VI)–Fe(III) catalyst, cyclohexene oxidation by molecular oxygen occurred with the formation of epoxycyclohexane. The selectivity to the products of cyclohexene epoxidation was close to 50%, which is a value expected from theory.