Degradation mechanism of a lignin model compound during alkaline aerobic oxidation: formation of the vanillin precursor from the β-O-4 middle unit of softwood lignin

Vanillin (4-hydroxy-3-methoxybenzaldehyde), one of the platform chemicals in industry, has been industrially obtained by alkaline aerobic oxidation of softwood lignin, a major component of lignocellulosics. A major reaction pathway of vanillin formation from the lignin polymer involves oxidation of an end group carrying a glycerol moiety produced by alkaline hydrolysis of a β-O-4 linkage in a middle unit of the lignin polymer to a C1 aldehyde moiety. This study presents the oxidation of veratryl glycerol [threo-1-(3,4-dimethoxyphenyl)propane-1,2,3-triol, VGL] as a model compound of the end group with the glycerol moiety at 120 °C under air in NaOH aq., aiming for the elucidation of detailed mechanisms of the oxidation of the C3 side-chain of VGL. Our experimental observations were explained by the assumption that the initial stage of the aldehyde formation was oxidation of the C3 side-chain of VGL at the α-position to a CαO moiety, followed by isomerization of this intermediate to a CγO compound, which finally underwent a retro-aldol reaction to give the α-aldehyde vanillin precursor. This consideration was supported also by our computational investigation at the SCS-MP2//DFT(M06-2X) level of theory on the retro-aldol reactions.