Stereoselective catalytic oxidations of biomimetic copper complexes with a chiral trinucleating ligand derived from 1,1-binaphthalene.

The new octadentate ligand R-(−)-N,N-dimethyl-N,N-bis{3-[bis(1-methyl-2-benzimidazolyl)amino]propyl}1,1- binaphthalenyl-2,2-diamine (L) was employed for the synthesis of dinuclear and trinuclear copper(II) complexes. Two terminal binding sites with tridentate aminobis(benzimidazole) linkages (A sites) and one central binding site with the bidentate diamino-binaphthalenyl residue (B site) are used by the ligand to bind divalent metal centres in the trinuclear complex [Cu3L][ClO4]6. Spectroscopic measurements suggest that in the dinuclear complex [Cu2L][ClO4]4 the copper ions are five-coordinated, with ligation by the aminobis(benzimidazole) residues, one of the tertiary amine donors of the diamino-binaphthalenyl moiety, and one water molecule. The complexes bind azide in the -1,3 fashion at low concentration and in the terminal mode at high concentration. The copper(II) complexes derived from L are catalytically active in the oxidation of 3,5-di-tert-butylcatechol (DTBC) by dioxygen. The oxidations are biphasic, with a fast initial stoichiometric phase corresponding to reduction of a pair of copper(II) centres and oxidation of DTBC to quinone, followed by the catalytic reaction, that follows substrate saturation behaviour. The complexes act as stereoselective catalysts in the biomimetic oxidations of the optically active catechol derivatives l- and d-Dopa and their methyl esters. In all the cases, the preferred enantiomeric substrate has the L configuration. This preference is dictated by the chirality of the binaphthalenyl residue.