Radical characteristic reactions in lignin pyrolysis: Aryl migration on linkages and substituents

Lignin pyrolysis leads to the formation of diverse phenolic products that bear structural similarities to natural lignin, and the related mechanism has been widely explored based on the linkage cleavage-involved reactions. However, some unusual pyrolytic products exhibiting significant structure deviations from lignin, such as aldehydes, remain obscure in mechanism due to long-standing neglect of their formation pathways. The present work found the pivotal role of aryl migration, a special radical-mediated rearrangement process, in governing the formation of these atypical products for the first time. Herein, density functional theory calculations, electronic structure analyses, and pyrolysis experiments were combined to investigate rearrangement patterns and prerequisite structural characteristics of aryl migration by employing typical radicals derived from linkages and substituents of lignin as models. The results indicate that the radical with an unpaired electron located on the second atom of the aromatic side chain can undergo three-membered aryl 1,2-migration triggered by exo cyclization with the best superiority, determining the generation of aldehydes, alkenes, and other products through subsequent cleavage reactions. A clear correlation among the initial geometric and electronic structures of lignin, the patterns and types of aryl migration, the energy barriers, and the end products was established. This study contributes to systematically elucidating rearrangement mechanisms and constructing a more comprehensive lignin pyrolysis mechanism network.