Synthesis and characterization of xanthene-bridged Schiff-base dimanganese(III) complexes: bimetallic catalysts for asymmetric oxidation of sulfides

Dimanganese(III) complexes of salen-type ligands anchored by 9,9-dimethylxanthene-4,5-diyl spacers were synthesized. Two types of structures, cyclic and acyclic forms, are presented. The 2 + 2 Schiff-base condensation of 5,5'-(9,9-dimethylxanthene-4,5-diyl)bis(salicylaldehyde) and 1,2-diaminobenzene gave a macrocyclic ligand, from which a cyclic dimanganese(III) complex was synthesized. A similar dimanganese(III) complex with stereogenic centers was synthesized by the metal-assisted condensation reaction using (1R,2R)-1,2-diaminocyclohexane. Acyclic dimanganese(III) complexes anchored by one xanthene spacer were obtained by metal-assisted stepwise condensation reactions using the xanthene-bridged bis(salicylaldehyde), (1R, 2R)-1,2-diaminocyclohexane monohydrochloride, and salicylaldehydes. X-Ray crystal structure analysis confirmed that both cyclic and acyclic complexes have two cofacially oriented salen-type manganese(III) units in a complex cation, showing an Mn center dot center dot center dot Mn distance of ca. 5.1 angstrom. Asymmetric oxidation of sulfides was performed by using the chiral dimeric complexes as catalysts, which showed enantiomeric excesses ranging from 5 to 19%. The addition of 4-(dimethylamino)pyridine to the reaction system improved the enantioselectivity up to 39% ee. This effect was not observed for the corresponding mononuclear catalyst.