Elimination mechanism of vanillin from non-phenolic β-O-4-type terminals formed in guaiacyl lignin: a combined kinetic and theoretical study

The alkaline aerobic oxidation of lignin to vanillin (4-hydroxy-3-methoxybenzaldehyde) has been employed as an industrial method for producing bio-based low-molecular-weight aromatic compounds. To deepen the molecular-level understanding of this reaction, we have been investigating the vanillin formation mechanism from native softwood lignin. One of the major reaction pathways involves oxidative degradation of β-O-4-type internal units, followed by alkaline-induced elimination of vanillin from the resulting vanillin end group. This study examined the reaction mechanism of a model compound, 4-[2-(3-ethoxy-4-methoxyphenyl)-2-hydroxy-1-(hydroxymethyl)ethoxy]-3-methoxybenzaldehyde, VE_β, which mimics the vanillin end group, in 4.0 mol L^-1 aqueous NaOH, with particular focus on the formation pathways of vanillin and byproducts. VE_β rapidly formed an equilibrium mixture comprising various rearranged compounds, in which the vanillin residue had migrated to the α- and γ-positions of the side-chain <i>via</i> an acetal-type intermediate. Kinetic analysis based on a pseudo-first-order competitive reaction model revealed that this equilibrium mixture was consumed through two distinct pathways: vanillin elimination and side reaction accompanied by polymerization. The activation energy (<i>E</i> _a) for vanillin elimination was determined to be 17.0 kcal mol^-1, which agreed moderately with the <i>E</i> _a value of 20.7 kcal mol^-1 calculated by DFT(M06-2X) for the α-oxyanion-assisted elimination process. Although the details of the side reaction pathway remain unclear, the overall reaction followed pseudo-first-order kinetics despite the involvement of bimolecular steps, suggesting that the rate-determining step of the side reaction proceeds <i>via</i> a unimolecular process.