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...Mn distance of ca. 5.1 A. 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.
Hirotsu
Masakazu
M
Department of Material Science, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan. mhiro@sci.osaka-cu.ac.jp
Ohno
Naoki
N
Nakajima
Takashi
T
Kushibe
Chie
C
Ueno
Keiji
K
Kinoshita
Isamu
I
D002384:Catalysis; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D017895:Manganese Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D010084:Oxidation-Reduction; D012545:Schiff Bases / Q000138:chemical synthesis / Q000737:chemistry*; D013440:Sulfides / Q000737:chemistry*; D014966:Xanthenes / Q000138:chemical synthesis / Q000737:chemistry*
D017895:Manganese Compounds; D012545:Schiff Bases; D013440:Sulfides; D014966:Xanthenes