Chapter 6 - Selective Aerobic Radical Epoxidation of α-Olefins Catalyzed by N-Hydroxyphthalimide

Abstract

This chapter discusses the reaction mechanism in order to explain the significant differences in selectivity with respect to the epoxidation by peracids and provides preliminary successful results in the synthesis of propylene oxide. The combination of N-hydroxyphthalimide (NHPI) and peracids is employed for the free-radical epoxidation of alkenes with high yields. A quite different selectivity is obtained from that of the well-known epoxidation by peracids, which is attributed to a different reaction mechanism and which evidences the molecule-induced homolysis of NHPI by peracids and dioxiranes. Electron paramagnetic resonance (EPR) and thermodynamic studies, as well as the marked solvent effect and the high selectivity in the a-position, strongly support the mechanistic interpretation. The in situ generation of acyl peroxyl radicals from acetaldehyde and molecular oxygen and the optimization of the reaction conditions allow the development of an innovative process for the convenient metal-free synthesis of several epoxides, suggesting an intriguing alternative route to the production of propylene oxide.