Aerobic coupling of aqueous phenols catalyzed by binuclear copper: Ring substituent effect and the kinetics of the coupling of o-methylphenol



The substituent effect on the aerobic and catalytic coupling of aqueous phenols conforms to a Hammett correlation in which the reaction constant is −2.16; the results are very similar to that of the enzymatic coupling reaction, in which the reaction constant is −2.4. Deviations from the Hammett correlation are attributed to steric factor, product inhibition, and the blocking of coupling sites on phenoxy radicals. Available evidence indicates that catalytic and enzymatic coupling reactions are strikingly similar in many respects. The striking resemblance suggests a common reaction mechanism in which binuclear metal (copper or iron) plays a key role. The common mechanistic features of the coupling reactions are suggested, and the implications of the findings are discussed with reference to (1) the design of a novel dephenolization scheme, (2) the elucidation of the widely occurring but poorly understood enzymatic coupling reactions, and (3) the synthesis of active polymer-bound coupling catalysts.