Revisiting the mechanism of β-O-4 bond cleavage during acidolysis of lignin. Part 1: Kinetics of the formation of enol ether from non-phenolic C₆-C₂ type model compounds

Abstract

The reaction route of a dimeric non-phenolic C₆-C₂ type lignin model compound, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethanol (VIII), was kinetically examined under acidolysis conditions (0.2 mol l^-1 HBr in 82% aqueous 1,4-dioxane at 85°C). The disappearance of (VIII) followed the pseudo-first-order rate law, and the rate constant k_(VIII) was 0.00854. In the course of the reactions, the following compounds were produced quantitatively at any time: an enol ether, 1-(2-methoxyphenoxy)-2-(3,4-dimethoxyphenyl)ethylene (IX), 2-methoxyphenol (X), and a Hibbert's ketone, 3,4-dimethoxyphenylacetaldehyde (XI). The substances (X) and (XI) are the result of the β-O-4 bond cleavage and their amounts were always equal during the whole reaction. When (IX) was subjected to the acidolysis under the identical conditions, its disappearance followed the pseudo-first-order rate law, and the rate constant k_(IX) was 0.00825. Furthermore, (VIII) was not observed at all, and (X) was produced quantitatively at any time. Based on these results, the formation rate of (IX) during the acidolysis of (VIII) is expressed by the equation: d[(IX)]/dt=A·k_(VIII)[(VIII)]-k_(IX)[(IX)], where A is the proportion of (VIII) that converted into (IX) when (VIII) degraded. It was confirmed by solving this differential equation that the formation and disappearance of (IX) is best simulated when A was assumed to be 1.00. Therefore, it was proven in this paper for the first time that (VIII) primarily converts into (IX), and subsequently the β-O-4 bond cleavage occurs and (X) and (XI) are yielded.