Hydrolytic Metalloenzyme Models. Metal Ion Dependent Site-Selective Acylation of Hydroxyl Groups of Bis-Imidazole Ligands Catalyzed by Zn2+ and Cu2+ in the Reaction with p-Nitrophenyl 2-Pyridinecarboxylate in a Cationic Surfactant Micelle

Abstract

Co-micelles of hexadecyltrimethylammonium bromide and a lipophilic bis-imidazole ligand, 1-dodecyl-4-hydroxymethyl-α-(1-dodecyl-2-imidazolyl)-2-imidazolemethanol (4) having two primary and secondary hydroxyl groups, together with the co-micelles of the related lipophilic ligands, were investigated for their catalytic activities in the hydrolysis of p-nitrophenyl 2-pyridinecarboxylate (PNPP) in the presence of Zn²⁺ or Cu²⁺. The kinetic and product analyses indicated that the reaction proceeds through the transacylation from PNPP to the hydroxyl group of ligand : metal ion complexes and that the active complexes undergoing transacylation are a 2 : 1 and a 1 : 1 complex for Zn²⁺ and a 1 : 1 complex for Cu²⁺, respectively. As for the ligand 4, the transacylation was found to be highly site-selective, i.e. it occurred predominantly on the secondary hydroxyl group with Zn²⁺, while on the primary hydroxyl group with Cu²⁺, respectively. Such selectivity was discussed in terms of the coordination structures of the active complexes.