Pentacoordinated manganese(III) dihydrosalen complexes as biomimetic oxidation catalysts

A series of seven chiral pentadentate dihydrosalen ligands: carrying an imidazole group as a fifth, axial donor was synthesized in racemic and enantiomerically pure form. All of these ligands afforded mononuclear manganese(III) complexes in good yields. In two cases, the pentacoordination of the manganese ion could be confirmed by X-ray crystallography. The complexes catalyzed the epoxidation 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 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 peroxide as oxidant and a salen-type complex as catalyst. Control experiments 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 enantiomeric excess of the product epoxide(s) is basically constant during the whole reaction time. Therefore, the high enantioselectivity of the oxygen transfer process must be ascribed solely to an efficient enantioface selection by the chiral catalysts, and not to secondary transformations of the product epoxide(s).