Revisiting the Mechanism of β-O-4 Bond Cleavage during Acidolysis of Lignin. Part 2: Detailed Reaction Mechanism of a Non-Phenolic C6-C2 Type Model Compound

Abstract

The detailed reaction mechanism of a C6-C2 dimeric non-phenolic β-O-4 type lignin model compound, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethanol (V′G), was examined under acidolysis conditions (mainly 0.2 mol/l HBr in 82% aqueous 1,4-dioxane at 85°C), and was suggested to be as follows. The initial elementary reaction step is protonation of the α-hydroxyl group, followed by the release of water to afford a benzyl cation intermediate (BC′). The latter step is relatively slow but reversible. The β-proton abstraction from BC′ by the solvents affords an enol ether compound, 1-(2-methoxyphenoxy)-2-(3,4-dimethoxyphenyl)ethene (EE′). This step is practically irreversible, and is the rate-determining step in the disappearance of V′G. The stereoisomers of EE′ are rapidly converted into each other, accompanied by protonation of the double bond. Complete protonation affords a β-oxymethylene cation intermediate (OMC′), which is also formed via hydride transfer from the β- to α-position of BC′ as a minor route. OMC′ preferentially undergoes the addition of water at the β-position, and the consequent β-O-4 bond cleavage affords 2-methoxyphenol and a Hibbert's monomer, 3,4-dimethoxyphenylacetaldehyde.