PENTACOORDINATED MANGANESE(III) DIHYDROSALEN COMPLEXES AS BIOMIMETIC OXIDATION CATALYSTS

A series of seven chiral pentadentate dihydrosalen ligands: carrying a n imidazole group as a fifth, axial donor was synthesized in racemic a nd enantiomerically pure form. All of these ligands afforded mononucle ar manganese(III) complexes in good yields. In two cases, the pentacoo rdination of the manganese ion could be confirmed by X-ray crystallogr aphy. The complexes catalyzed the epoxidation of olefins with a variet y of terminal oxidants, but most importantly, with dilute (1%) aqueous hydrogen peroxide and without any added co-ligands. With 1,2-dihydron aphthalene as substrate and 10 mol% of catalyst enantiomeric excesses up to 66% were achieved. This value is the highest so far reported for an asymmetric epoxidation of 1,2-dihydronaphthalene, using hydrogen p eroxide as oxidant and a salen-type complex as catalyst. Control exper iments using a tetradentate chelate lacking the axial imidazole donor showed that the pentacoordination of the manganese ion is crucial for the peroxidase activity. Furthermore, it was shown that the enantiomer ic excess of the product epoxide(s) is basically constant during the w hole reaction time. Therefore, the high enantioselectivity of the oxyg en transfer process must be ascribed solely to an efficient enantiofac e selection by the chiral catalysts, and not to secondary transformati ons of the product epoxide(s).