Incomplete Cleavage of β–O–4 Linkages Involving p -Hydroxyphenyl Units by Thioacidolysis and Elucidation of the Underlying Mechanism

Thioacidolysis is a classical structural analysis method widely used to elucidate the structure of lignin, a potential sustainable source of aromatic compounds. Although thioacidolysis selectively cleaves β–O–4 linkages, its interpretation relies on the unverified assumption that all β–O–4 linkages connecting different aromatic units undergo quantitative cleavage. In this study, 14 lignin model dimers containing p -hydroxyphenyl (H), guaiacyl (G), and/or syringyl (S) units were subjected to thioacidolysis, revealing that β–O–4 linkages involving the 4-hydroxy group of H units did not undergo quantitative cleavage. Density functional theory (DFT) calculations indicated that the methoxy group on the B-ring restricted the torsional motion of the aromatic ring and created a sterically favorable environment for BF 3 coordination, thereby accelerating β–O–4 cleavage. These findings reveal that the presence or absence of the methoxy group on the B-ring significantly impacts thioacidolysis efficiency. Overall, this study provides a new perspective on the long-standing issue of H-unit underestimation in lignin analysis by thioacidolysis and contributes to a deeper understanding of the structural features of lignin.