Remarkable effects of metal, solvent, and oxidant on metalloporphyrin-catalyzed enantioselective epoxidation of olefins

Three metal complexes of one particular homochiral porphyrin were investigated as catalysts for enantioselective epoxidation of unfunctionalized olefins under various reaction conditions. Much better results were obtained with the iron and ruthenium complexes than with the manganese derivative. The absence of any effect of amines on the iron porphyrin-catalyzed reaction in benzene, as well as the superior results in aromatic as opposed to both more and less polar nonaromatic solvents, suggest that specific association of aromatic molecules to the metalloporphyrin affects its solution structure. Strong evidence for the involvement of active oxidants that are more selective than trans-dioxoruthenium(VI) porphyrin is provided by the significant effect of primary oxidants on the ruthenium porphyrin-catalyzed reactions. Preliminary results with the iron complex of an only slightly modified porphyrin wider the optimized reaction conditions found in this study resulted in epoxidation of styrene and 4-chlorostyrene to their epoxides with enantiomeric excesses identical to the best ever reported. These results were obtained with an unprecedented large number of catalytic turnovers, requiring only 0.01 mol % of catalyst.