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

Dalton Transactions Pub Date: 2009-11-09 DOI: 10.1039/B909491A

Abstract

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?Mn distance of ca. 5.1 ?. 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.

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