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 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°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.