Different pyrolytic cleavage mechanisms of β-ether bond depending on the side-chain structure of lignin dimers

Influence of Cγ–OH on pyrolytic cleavage mechanism of β-ether-linkage in lignin dimer was studied in N2 with Cγ-deoxy-type dimers, which have various p-substituents (–H, –Cl, –OCH3) at their Cβ-phenoxy groups. Reactivities at 400 °C and the influence of radical scavenger (tetralin) indicated that Cγ-deoxy types, especially in phenolic forms, are degraded through radical chain mechanism. Furthermore, substituent effects on their reactivities under excess amount of tetralin as discussed with Hammett's substituent constant (σp) and bond dissociation energy (BDE)-reducing parameter (ΔBDE) revealed that this chain reaction is initiated with the radical species formed through Cβ–O bond homolysis. These chain reactions were only effective in a closed-type reactor and not effective in an open-type reactor. Contrary to this, Cγ–OH type in phenolic form was degraded at 250 °C, which is much lower than that (400 °C) of the Cγ-deoxy types, and the reactivity was not influenced by tetralin. These results indicate that introducing OH at Cγ changes the β-ether cleavage mechanism in phenolic form from radical chain to quinone methide mechanism.