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

Abstract

Pyrolytic cleavage mechanisms of lignin-ether linkages were studied with some dimers and trimers which have various p-substituted C_α-phenoxy groups (-H, -OCH₃, -Cl or -COCH₃). Pyrolysis of these model compounds provides phenols and isoeugenol type products. To determine whether the reactions mechanisms are heterolytic 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-phenolic forms the α-ether linkages are cleaved homolytically. Cleavage of these α-ether linkages is the rate-determining step for the scission of the C_β-O bond in trimers. The β-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.