Revisiting the Mechanism of beta-O-4 Bond Cleavage during Acidolysis of Lignin. Part 2: Detailed Reaction Mechanism of a Non-Phenolic C-6-C-2 Type Model Compound

The detailed reaction mechanism of a C6-C2 dimeric non-phenolic -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 degrees C), and was suggested to be as follows. The initial elementary reaction step is protonation of the -hydroxyl group, followed by the release of water to afford a benzyl cation intermediate (BC'). The latter step is relatively slow but reversible. The -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 -oxymethylene cation intermediate (OMC'), which is also formed via hydride transfer from the - to -position of BC' as a minor route. OMC' preferentially undergoes the addition of water at the -position, and the consequent -O-4 bond cleavage affords 2-methoxyphenol and a Hibbert's monomer, 3,4-dimethoxyphenylacetaldehyde.