Revisiting the mechanism of β-O-4 bond cleavage during acidolysis of lignin. Part 9: Comprehensive results for guaiacyl-type compounds and the difference in participation mode of bromide and chloride anions between C6-C3-type and C6-C2-type compounds

Abstract

A phenolic β-O-4-type lignin model compound with a C6-C3-type side-chain and two guaiacyl nuclei and its derivatives were individually acidolyzed in aqueous 82 vol% 1,4-dioxane containing 0.2 mol/L HBr, HCl, or H2SO4 at 85 °C in this study. The obtained results, combined with those in the series of this study in which phenolic and non-phenolic C6-C3- and C6-C2-type analogues were used, generated the following results and proposals. The H2O addition reproducing the original compound is preponderant as a reaction mode of the benzyl cation intermediate (BC) over the other modes leading to the β-O-4 bond cleavage, especially in the acidolyses using H2SO4 and those of C6-C3-type compounds. A well-known phenomenon whether formaldehyde is liberated from C6-C3-type compounds is regulated by acid types appears less clearly in phenolic than in non-phenolic compounds. Br¯ as well as Cl¯ nucleophilically assists the H¯ transfer from the β- to α-carbon of BC, especially in C6-C3-types, which accelerates and regulates the acidolyses using HBr as well as HCl. Furthermore, Br¯ is suggested to nucleophilically attack the β-carbon of non-phenolic C6-C3-type BC to cleave the β-O-4 bond, which explains the characteristically peculiar observation in the acidolysis of the non-phenolic C6-C3-type compound using HBr described in our previous report.