PENTACOORDINATED MANGANESE COMPLEXES AS BIOMIMETIC CATALYSTS FOR ASYMMETRIC EPOXIDATIONS WITH HYDROGEN-PEROXIDE

Chiral pentadentate dihydrosalen ligands, carrying an imidazole group as a fifth, axial donor have been synthesized in racemic and enantiome rically pure form. All of these ligands afforded mononuclear manganese (III) complexes in good yields. The complexes catalyzed the epoxidatio n of olefins with a variety of terminal oxidants, but most importantly , with dilute (1%) aqueous hydrogen peroxide and without any added co- ligands. With 1,2-dihydronaphthalene as substrate and 10 mol% of catal yst, enantiomeric excesses up to 64% were achieved. Control experiment s using a tetradentate chelate lacking the axial imidazole donor showe d that the pentacoordination of the manganese ion is crucial for the p eroxidase activity.