High Turnover Numbers for the Catalytic Selective Epoxidation of Alkenes with 1 atm of Molecular Oxygen

摘要

Catalytic epoxidation of alkenes has attracted much attention both in industrial processes and in organic syntheses, because epoxides are among the most useful synthetic intermediates. Epoxidation of alkenes can be carried out by various techniques with various oxidants. However, stoichiometric (noncatalytic) epoxidation is still widely used, and large amounts of byproducts, particularly salts, are formed. For example, propylene oxide has been produced commercially by alkaline dehydrochlorination of propylene chlorohydrin with formation of large amounts of metal salts. The utilization of molecular oxygen for a catalytic epoxidation without reducing reagents or radical initiators is a rewarding goal, because, molecular oxygen has the highest content of active oxygen, and no byproducts are formed from it, in contrast to various other oxidants. However, only a few ideal homogeneous epoxidations of alkenes with molecular oxygen at 1 atm without reducing reagents or radical initiators have been achieved, because organic ligands of the catalysts are degraded or the catalysts act as stoichiometric oxidants.