Mechanism for Aerobic Oxidation of 3,5-Di-tert-butylcatechol to 3,5-Di-tert-butyl-o-benzoquinone Catalyzed by Di-[mu]-hydroxo-dicopper(II) Complexes of Peralkylated Ethylelnediamine Ligands

The aerobic oxidation of 3,5-di-tert-butylcatechol (H2dbc) to the corresponding quinone (dbq), catalyzed by di-μ-hydroxo-dicopper(II) complexes [Cu2(OH)2L2](ClO4)2 [L = N,N'-tetramethylethylenediamine (tmeda) (1) and N,N'-tetraethylethylenediamine (teeda) (2)], has been spectroscopically and kinetically studied to gain some insight into the reaction mechanisms. The ESR spectra show that catecholato complexes [Cu(dbc)L] [L = tmeda (3) or teeda (4)] are formed upon reactions of 1 or 2 with H2dbc, respectively. The UV-vis and ESI mass spectra show that 3 is converted to a semiquinonato complex [Cu(dbsq)tmeda]+ (5) in the aerobic oxidation, while [Cu(dbsq)teeda]+ (6) is only a minor species. In the presence of acid, 3 and 4 are aerobically oxidized to dbq quantitatively. Kinetic studies show that rate constants for the catalytic aerobic oxidation of H2dbc are proportional to the second or the first order with 1 or 2, respectively, and to the first-order with the O2-partial pressure. Based on these data, different reaction mechanisms are proposed for 1 and 2, and may be caused by the steric hindrances around the copper center of 1 and 2.