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The oxidation of benzyl alcohols with the enzyme laccase, under mediation by appropriate mediator compounds, yields carbonylic products, whereas laccase can not oxidise these non-phenolic substrates directly. The oxidation step is performed by the oxidised form of the mediator (Med(ox)), generated on its interaction with laccase. The Med(ox) can follow either an electron transfer (ET) or a radical hydrogen atom transfer (HAT) route of oxidation of the substrates. Experimental evidence is reported that enables unambiguous assessment of the occurrence of either one the oxidation routes with each of the investigated mediators, namely, ABTS, HBT, HPI and VLA. Support to the conclusions is provided by (i) investigating the intermolecular selectivity of oxidation with appropriate substrates, (ii) attempting Hammett correlations for the oxidation of a series of 4-X-substituted benzyl alcohols, (iii) measuring the kinetic isotope effect, (iv) investigating the product pattern with suitable probe precursors. Based on these points, a HAT mechanism results to be followed by the laccase-HBT, laccase-HPI and laccase-VLA systems, whereas an ET route appears feasible in the case of the laccase-ABTS system.

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The oxidation of alcohols and ethers by O2 with the enzyme laccase, mediated by the stable N‐oxyl radical TEMPO, affords carbonylic products. An ionic mechanism is proposed, where a nucleophilic attack of the oxygen lone‐pair of the alcohol (or ether) onto the oxoammonium form of TEMPO (generated by laccase on oxidation) takes place leading to a transient adduct. Subsequent deprotonation of this adduct α to the C−O bond leads to the carbonylic product. Additional mechanistic considerations for the laccase‐mediated oxidation of ethers and thioethers are offered. The proposed mechanism is supported by: (i) investigating the inter‐ and intramolecular selectivity of oxidation with appropriate substrates, (ii) thermochemical considerations, and (iii) attempting a Hammett correlation for the oxidation of a series of 4‐X‐substituted benzyl alcohols, wherein a shift of the rate‐determining step as a function of the 4‐X‐substituent results. Based on the above points, the lack of mediation efficiency of another stable N‐oxyl radical (viz., IND‐O·) can be explained. (© Wiley‐VCH Verlag GmbH & Co KGaA, 69451 Weinheim, Germany, 2002)

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