Polyoxometalate oxidation of non-phenolic lignin subunits in water: Effect of substrate structure on reaction kinetics

The effect of lignin-biopolymer structure on the mechanism of its oxidative depolymerization by polyoxometalates (ROMs) was Investigated by reacting an equilibrated POM ensemble with a series of ring-substituted benzyl alcohols. Under anaerobic conditions in mixed water/methanol, observed pseudo-first order reaction rates (150°C) of 8.96×10^-3 and 4.89×10 ^-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 kinetic-isotope effect of 1.6 and 1.7 was observed for the reactions of 1 and 2 deuterated at the benzylic-carbon atoms. This combined with the relatively large activation energies, indicate that C-H bond breaking (either H-atom or proton-coupled electron transfer) occurs late along the reaction coordinate of the rate-determining step. Replacing the 4-CH₃O- group in 2 by CH ₃CH₂O- to give 1-(4ethoxy-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.