Influence of vinyl ether intermediate over formation of Cβ = O and Cα = Cβ monomers during pyrolysis of lignin model dimers

Radical chain reactions of a beta-ether-type lignin model dimer, 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1-propanol, were reinvestigated at 350 degrees C in closed ampoule reactors under the influence of wood (Japanese cedar) pyrolysis. The formation of vinyl ether dimers and C-beta = O monomers was observed in addition to previously reported C-alpha = C-beta and C-alpha = O monomers and veratraldehyde. The vinyl ether dimer was readily hydrolyzed to the C-beta = O monomer, which suggests that the formation of the C-beta = O-type monomers, such as homovanillin from natural lignin pyrolysis, originates from the vinyl ether intermediates. Unlike the non-phenolic dimer, the corresponding phenolic dimer did not form the vinyl ether dimer and C-beta = O monomer. The results, when using C-beta-deuterium-labeled dimers, suggest that a portion of the C-alpha = C-beta monomer from the phenolic dimer forms via the vinyl ether intermediate, probably through conversion into quinone methide, in which the bond dissociation energy of C-beta-O is reduced (19.1 kcal mol(-1) by density functional theory calculations). This leads to the simultaneous homolytic cleavage of the C-beta-O bond to form the C-alpha = C-beta monomer. Such reactions did not proceed in the pyrolysis of the nonphenolic dimer, which is not able to form the stable quinone methide intermediate. This study can provide insight into the pyrolytic conversion of lignin, particularly the formation of C-beta = O- and C-alpha = C-beta-types monomer.