Pyrolytlc cleavage mechanisms of lignin-ether linkages: A study on p-substituted dimers and trimers

摘要

Pyrolytic cleavage mechanisms of lignin-ether linkages were studied with some dimers and trimers which have various p-substituted C_α-phenoxy groups (-H, -OCH_3, -Cl or -COCH_3). Pyrolysis of these model compounds provides phenols and isoeugenol type products. To determine whether the reactions mechanisms are hetero-lytic or homolytic, the reactivities were compared based on Hammett's substituent constant (σ_p) and the ΔBDE parameter, namely the bond dissociation energy (BDE) reduction. The α-ether-linkages in phenolic forms are cleaved in a heterolytic mechanism, while in non-phe-nolic forms the α-ether linkages are cleaved homolyti-cally. Cleavage of these a-ether linkages is the rate-determining step for the scission of the C_β-O bond in trimers. The p-ether-linkages in the non-phenolic trimers are cleaved through the β-scission type reaction from the benzyl radical intermediates. On the other hand, quinone methide formation through heterolytic cleavage of the α-ether linkages is the key step for following homolysis of the C_β-O bonds in the phenolic trimers. Electron attracting character of the quinone methide structure reduces the BDE of the C_β-O bond.