Aerobic Epoxidation of Olefins Catalyzed by Electronegative Vanadyl Salen Complexes

Vanadyl salen complexes bearing electron-withdrawing substituants have been prepared and characterized. Systematic substitutions on the ancillary ligand have allowed V⁵⁺/V⁴⁺ reduction potentials to be tuned over a range of approximately 500 mV. The complexes are catalysts for the aerobic epoxidation of cyclohexene; catalytic activity roughly increases with increasing V⁵⁺/V⁴⁺ reduction potential. The mechanism likely involves oxygen transfer from intermediate hydroperoxides that are formed by radical-chain autoxidation.