Chiral model complexes for methane monooxygenase and their asymmetric catalysis of styrene epoxidation

Four chiral π-phenolato dinuclear (VO)2, Cr2, Mn2, and Fe2 complexes with a phenolic ligand bearing two L-phenylanalines as complexing arms have been synthesized and characterized. Asymmetric epoxidations of styrene with these complexes as chiral models of methane monooxygenase (MMO) and iodosylbenzene as an oxygen atom donor have been studied. The results reveal that the Mn2, complex catalyzes asymmetric epoxidation of styrene with the formation of R-(+)-styrene oxide in 42.8% e. e. Contrarily, the (VO)2 analogue complex gives S-(-)-styrene oxide in 19.2% e.e. EPR investigations suggest that the catalyzed epoxidations with the Mn2, Cr2 and Fe2 complexes proceed via high-valent oxometal intermediates; while in the case of the (VO)2 complex as catalyst, the epoxidation proceeds by a pathway that involves a complex of metal-iodine ligand. The catalytic activities of the complexes decrease in a sequence of Mn2>Cr2>(VO)2> Fe2.