Autoxidation of Catechols and Related Compounds

In non-aqueous solution, the copper catalyzed autoxidation of catechol was interpreted in terms of a Cu(I)/Cu(II) redox cycle (34). It was assumed that the formation of a dinuclear copper(II)-catecholate intermediate is followed by an intramolecular two-electron step. The product Cu(I) is quickly reoxidized by dioxygen to Cu(II). A somewhat different model postulated the reversible formation of a substrate-catalyst-dioxy-gen ternary complex for the Mn(II) and Co(II) catalyzed autoxidations in protic media (35). 

In a thorough study by Balia and coworkers, it was shown that in slightly acidic aqueous solution (25 °C, /jl = 1.0 KN03), the mechanism is very similar to that followed in aprotic media (36). Parallel measurement of the 02 consumption and quinone formation confirmed the following stoichiometry up to 35% conversion  

At longer reaction times, the formation of an acidic product, probably cfs,cfs-muconic acid, and a copper containing precipitate were observed. This latter could be a polynuclear product of quinone and semiquinone fragments (37,38). In agreement with this stoichiometry, about 50% of 02 was regenerated by adding a small amount of catalase to the reaction mixture at relatively short reaction times. 

The stability constants of the mono- and bis-complexes between Cu(II) and catecholate were determined under anaerobic conditions and were found to be the same as reported earlier, i.e. log p1 = 13.64 (CuC) and log p2 24.92 (CuC2+) (36,39). A comparison of the speciation and oxidation rate as a function of pH clearly indicated that the mono-catechol complex is the main catalytic species, though the effect of other complexes could not be fully excluded. The rate of the oxidation reaction was half-order in [02] and showed rather complex concentration dependencies in [H2C]0, [Cu(II)]0 and pH. The experimental data were consistent with the following rate equation  

The following non-radical chain mechanism was proposed for the reaction in aqueous solution (for sake of simplicity, fast protolytic and complex-formation reactions are not shown) (36)  

---