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

Abstract

Radical chain reactions of a β-ether-type lignin model dimer, 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1-propanol, were reinvestigated at 350 °C in closed ampoule reactors under the influence of wood (Japanese cedar) pyrolysis. The formation of vinyl ether dimers and Cβ=O monomers was observed in addition to previously reported Cα=Cβ and Cα=O monomers and veratraldehyde. The vinyl ether dimer was readily hydrolyzed to the Cβ=O monomer, which suggests that the formation of the Cβ=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 Cβ=O monomer. The results, when using Cβ-deuterium-labeled dimers, suggest that a portion of the Cα=Cβ monomer from the phenolic dimer forms via the vinyl ether intermediate, probably through conversion into quinone methide, in which the bond dissociation energy of Cβ‒O is reduced (19.1 kcal mol−1 by density functional theory calculations). This leads to the simultaneous homolytic cleavage of the Cβ‒O bond to form the Cα=Cβ monomer. Such reactions did not proceed in the pyrolysis of the non-phenolic 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 Cβ=O- and Cα=Cβ-types monomer.