Revisiting the Mechanism of β-O-4 Bond Cleavage During Acidolysis of Lignin. Part 3: Search for the Rate-Determining Step of a Non-Phenolic C6-C3 Type Model Compound

The rate-determining step of a C-6-C-3 dimeric non-phenolic beta-O-4 type lignin model compound, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)propane-1,3-diol (veratrylglycerol-beta-guaiacyl ether, VG), was evaluated under acidolysis conditions (0.2mol/l HBr in 82% aqueous 1,4-dioxane at 85 degrees C) by comparing the disappearances between VG and the corresponding compound labeled at the beta-position of VG, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)(2-H-2)propane-1,3-diol. The disappearance of VG occurred more rapidly than that of the latter compound, and a primary kinetic isotope effect was clearly observed. This result indicates that the C-H bond at the beta-position of VG is broken in the rate-determining step. Two possible mechanisms are presented as the rate-determining step: (1) A base abstracts the beta-proton of a benzyl cation-type intermediate produced from VG affording an enol ether compound, 2-(2-methoxyphenoxy)-3-(3,4-dimethoxyphenyl)prop-2-en-1-ol; (2) The hydride transfers from the beta- to the alpha-position of the benzyl cation. It was confirmed that both mechanisms certainly exist and that the latter seems to contribute more than has generally been considered.