Journal
HOLZFORSCHUNG
Volume 58, Issue 2, Pages 116-121Publisher
WALTER DE GRUYTER GMBH
DOI: 10.1515/HF.2004.016
Keywords
etherified benzyl alcohols; kinetic isotope effect; oxidation; kinetics; polyoxometalate (POM)
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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.
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