Mechanistic Investigation of the Catalyzed Cleavage for the Lignin β-O-4 Linkage: Implications for Vanillin and Vanillic Acid Formation

The depolymerization of kraft lignin with a copper–vanadium hydrotalcite-like catalyst (HTc-Cu-V) and V(acac)3/Cu(NO3)2·3H2O mixtures to monomeric aromatic aldehydes and aromatic acids such as vanillin and vanillic acid using molecular oxygen as oxidant is reported. The obtained products correlate to model-based studies with erythro-1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol (1a). Kinetic investigations with 1a demonstrate that there is a combined effect of V(acac)3 together with Cu(NO3)2·3H2O that enhances the catalytic activity and increases the selectivity and yield for the cleavage products veratric acid and veratraldehyde. Veratric acid is formed through reaction pathways involving either the transient oxidation products 1-(3,4-dimethoxyphenyl)-3-hydroxy-2-(2-methoxyphenoxy)propan-1-one (2) and 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)prop-2-en-1-one (5) or through overoxidation of veratraldehyde. The formation of veratraldehyde likely proceeds through C–C bond cleavage involving either a retro-aldol or a single electron transfer mechanism.