Revisiting the Mechanism of [Beta]-O-4 Bond Cleavage During Acidolysis of Lignin. Part 3: Search for the Rate-Determining Step of a Non-Phenolic C^sub 6^-C^sub 3^ Type Model Compound

• Published in: Journal of wood chemistry and technology 2011-04, Vol.31 (2), p.172
• Main Authors: Ito, Hiroaki, Imai, Takaaki, Lundquist, Knut, Yokoyama, Tomoya, Matsumoto, Yuji
• Format: Article
• Language: English
• Subjects: Chemical bonds; Chemical compounds; Isotopes; Lignin; Reaction kinetics
• ISSN: 0277-3813; 1532-2319

The rate-determining step of a C6-C3 dimeric non-phenolic β-O-4 type lignin model compound, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)propane-1,3-diol (veratrylglycerol-β-guaiacyl ether, VG), was evaluated under acidolysis conditions (0.2 mol/l HBr in 82% aqueous 1,4-dioxane at 857deg;C) by comparing the disappearances between VG and the corresponding compound labeled at the β-position of VG, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)(2-2H)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 β-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 β-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 β- to the α-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. [PUBLICATION ABSTRACT]