Ligand dependence in the copper-catalyzed oxidation of hydroquinones.
Transition metal-mediated oxidation of hydroquinones is an important physiologic reaction, and copper(II) effectively catalyzes the reaction in phosphate-buffered saline (PBS). Studies reported herein in phosphate buffer alone demonstrate that copper(II) is an ineffective catalyst in the absence of coordinating ligands, but that 1,10-phenanthroline and histamine facilitate the copper(II)-mediated oxidation of hydroquinone and its 2,5- and 2,6-di-tert-butyl analogs to the corresponding benzoquinones. The high concentration of chloride in PBS is the key element that allows copper(II) to work in this system. Although the bis-bathocuproine disulfonate complex of Cu(II), (BC)(2)Cu(II), is a strong stoichiometric oxidant, stoichiometric amounts of copper(II) in the presence of ligands other than BC oxidize hydroquinones very slowly under anaerobic conditions. Thus, the rapid copper(II)-catalyzed reaction operating aerobically does not involve a simple ping-pong reduction of copper(II) to copper(I) by hydroquinone and reoxidation of copper(I) by O(2).
Mandal
Subrata
S
Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106, USA.
Kazmi
Najam H
NH
Sayre
Lawrence M
LM
D016227:Benzoquinones / Q000138:chemical synthesis*; D002384:Catalysis; D003300:Copper / Q000737:chemistry*; D006632:Histamine / Q000737:chemistry*; D006873:Hydroquinones / Q000737:chemistry*; D008024:Ligands; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D010618:Phenanthrolines / Q000737:chemistry*
D016227:Benzoquinones; D006873:Hydroquinones; D008024:Ligands; D010618:Phenanthrolines; D003300:Copper; D006632:Histamine; D010100:Oxygen