OXIDATION REACTIONS OF A MACROCYCLIC DINUCLEAR COPPER(I) DIOXYGEN COMPLEX AND A DINUCLEAR COPPER(II) COMPLEX

Recently a solution phase copper(I) dinuclear complex of the ligand 3,6,9,16,19,22-hexaazatricyclo-[22.2.1.111,14]-octacosa-1(26),2,9,11,13,15,22,24-octaene, (FD)2(DIEN)2 (1), and its dioxygen adduct were spectroscopically characterized. In this report, the use of the dinuclear complex in performing oxidation reactions with hydroquinones, phenols, catechols, ascorbic acid, and 3,4-dimethylaniline in a 3:1 mixture of methanol and acetonitrile is described. It has been determined that the copper(I) dioxygen complex of (FD) 2(DIEN)2 (3) catalytically converts hydroquinones to the corresponding benzoquinones, ascorbic acid to dehydroascorbic acid, and phenols to their 1,4-benzoquinones and diphenoquinones, in the presence of excess dioxygen. Turnover numbers range between 5 and 20 h-1. Under the same conditions 4-tert-butylcatechol (4-TBC) was transformed to the gamma-lactone of the corresponding muconic acid ester, 3,5-di-tert-butylcatechol (3,5-DTBC) was oxidized to the corresponding 1,2-benzoquinone, and 3,4-dimethylaniline (3,4-DMA) was converted to 3,4-nitrosobenzene, all with turnover numbers of less than 1. These reactions were found to occur under stoichiometric conditions as well. Hydroquinones, phenols and ascorbic acid, but not catechols and 3,4-DMA, were also found to be oxidized, under stoichiometric conditions, by a dinuclear copper(II) complex 5, prepared from the 2:1 stoichiometric reaction between CuCl2 and 1 in methanol. The rates for the stoichiometric oxidations were determined to be 5 to 100 times greater for copper(I)-dioxygen oxidations than for corresponding copper(II) oxidations. On the basis of these observations, a catalytic scheme based on the copper(I) complex as the initial active species is proposed. The following steps are suggested: (i) formation of a dinuclear copper(I)-dioxygen adduct, (ii) oxidation of the substrate by the dioxygen adduct with simultaneous formation of a Cu(II) dinuclear complex, and (iii) oxidation of the substrate by the copper(II) complex to produce a copper(I) dinuclear complex and the oxidation product of the substrate.