Polyoxometalate oxidation of nonphenolic lignin subunits in water: Effect of substrate structure on reaction kinetics

The effect of ligninbiopolymer structure on the mechanism of its oxidative depolymerization by polyoxometalates (POMs) was investigated by reacting an equilibrated POM ensemble with a series of ringsubstituted benzyl alcohols. Under anaerobic conditions in mixed water/ methanol, observed pseudofirst order reaction rates (150degreesC) of 8.96x10(-3) and 4.89x10(-3) sec(-1) were obtained for oxidations of 1-(3,4-dimethoxyphenyl) ethanol (1) and 1-(3,4,5-trimethoxyphenyl)ethanol (2), respectively. Organic products from the oxidation of 2 (95% recovery) indicate the reaction proceeds via successive oxidations of the benzylic carbon atom; aromatic ring cleavage was not observed. A primary kineticisotope effect of 1.6 and 1.7 was observed for the reactions of 1 and 2 deuterated at the benzyliccarbon atoms. This combined with the relatively large activation energies, indicates that C-H bond breaking (either H-atom or protoncoupled electron transfer) occurs late along the reaction coordinate of the ratedetermining step. Replacing the 4-CH3O group in 2 by CH3CH2O to give 1-(4- ethoxy-3,5-dimethoxyphenyl)ethanol (3), had a substantial effect on the temperature dependence of the observed reaction rate, suggestive of a possible change in mechanism.