Oxidative Coupling and Polymerization of Guaiacol, a Lignin Derivative

In order to better understand the chemical reactions involved in humus formation, guaiacol, a lignin derivative, was enzymatically oxidized. Under the conditions used, seven oligomers—five dimers and two trimers—were formed and comprehensive structural analyses were performed on these products. The oligomers were predominantly polyphenolic in nature, resulting from C‐C coupling of the guaiacol monomer. Two oligomers were formed by C‐O coupling and one dimer had a quinonoid structure. Based on the product structures, reaction pathways for the oxidative oligomerization of guaiacol are proposed. In this reaction pathway scheme, free‐radical coupling of the oxidized guaiacol leads to the formation of four dimeric intermediates. Nonenzymatic reactions of the four intermediates result in the formation of the seven stable guaiacol‐derived oligomers. Such reaction schemes may elucidate the mechanism of humus formation.