Reavisitng 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

The reaction route of a dimeric non-phenolic C-6-C-2 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 degrees 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 (W, 2-methoxyphenol (X), and a Hibbert's ketone, 3,4-dimethoxyphenylacetaldehyde (XI). The substances (X) and (XI) are the result of the beta-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(, 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 center dot 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 (VII) primarily converts into (IX), and subsequently the 8-0-4 bond cleavage occurs and (X) and (XI) are yielded.