Synthesis and characterization of new chiral octadentate nitrogen ligands and related copper(II) complexes as catalysts for stereoselective oxidation of catechols

Three new octadentate ligands, namely (R)-N,N',-dimethyl-N,N'-bis{3-[bis(1-methyl-2-imidazolylmethyl)]aminopropyl}-1,1'-binaphthyl-2,2'-diamine, (R)-DABN-3Im(4), (R)-N,N'-dimethyl-N,N'-bis{4-[bis(1-methyl-2-benzimidazolylmethyl)laminobutyl}-1,1'-binaphthyl-2,2'-dia-mine, (R)-DABN-4BZ(4), and (S)-N2,N6-dimethyl-N2,N6-bis{2'-[bis(1-methyl-2-benzimidazolylmethyl)]aminomethyl}benzyl-2,6-diamino-1-exanol acetate, L-Lys-4BZ(4), were employed for the synthesis of dinuclear and trinuclear copper(II) complexes. The ligands contain two side arms of different nature and length which carry tridentate aminobis(benzimidazole) or aminobis(imidazole) residues as metal binding sites (A sites) connected to a central (R)-1,1'-binaphthyl-2,2'-diamine or L-lysine residue which can bind a third metal ion (B site). The chiroptical properties of the ligands and the complexes have been described. The complexes were tested as catalysts in the oxidation of 3,5-di-tert-butylcatechol, L-, D-Dopa and L-, D-Dopa methyl esters by dioxygen to give the corresponding quinones. The catalytic efficiency is moderate, but the complexes exhibit significant enantio-differentiating ability towards L-, D-Dopa methyl esters, albeit their enantio-differentiating ability towards L-, D-Dopa is lower. The (R)-1,1'-binaphthyl-2,2'-diamine spacer in the (R)-DABN complexes has much stronger recognition power than the aliphatic L-lysine spacer in the L-Lys complexes. In addition, the highest stereoselectivity in the catalytic oxidation is obtained with the (R)-DABN-3Im(4) complexes, containing carbon chains of three atoms between the (R)-1,1'-binaphthyl-2,2'-diamine groups and the tridentate donor units at the A metal binding sites. In all cases, the preferred enantiomeric substrate has the L configuration, which is dictated by the chirality of the spacer residue. (c) 2005 Elsevier B.V. All rights reserved.