Revisiting the mechanism of \u03b2-O-4 bond cleavage during acidolysis of lignin VII: acidolyses of non-phenolic C6-C2-type model compounds using HBr, HCl and H2SO4, and a proposal on the characteristic action of Br\u2212 and Cl\u2212

Qiaoqiao Ye,Tomoya Yokoyama

doi:10.1186/s10086-020-01928-6

Abstract

A non-phenolic C6-C2-type lignin model compound with the β-O-4 bond, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethanol (I), was acidolyzed in aqueous 82% 1,4-dioxane containing HBr, HCl, or H2SO4 with a concentration of 0.2 mol/L at 85 ℃ to examine the differences between these acidolyses. Compound I primarily converted to an enol ether compound, 1-(2-methoxyphenoxy)-2-(3,4-dimethoxyphenyl)ethene (II), via the benzyl cation followed by acidolytic β-O-4 bond cleavage regardless of the acid-type, although the disappearance rates of compound I were remarkably different (HBr > HCl >> H2SO4). Acidolyses of compound II using these acids under the same conditions showed a similar tendency, but the rate differences were much smaller than in the acidolyses of compound I. Acidolyses of the α-methyl-etherified derivative of compound I (I-α-OMe) using these acids under the same conditions suggested that the formation rates of the benzyl cation from compound I-α-OMe (also from compound I) are not largely different between the acidolyses using these acids, but those of compound II from the benzyl cation are remarkably different. Acidolysis of the α-bromo-substituting derivative of compound I (I-α-Br) using HBr under the same conditions showed a characteristic action of Br¯ in the acidolysis. Br¯ adds to the benzyl cation generated from compound I or I-α-OMe to afford unstable compound I-α-Br, resulting in acceleration of the formation of compound II and of the whole acidolysis reaction.

Highlights

Acidolysis is one of the most basic chemical treatments, and has recently been introduced as a pretreatment in the chemical conversion of woody biomass
The disappearance rates of all the compounds were in the order of the acidolyses using: HBr > HCl > H2SO4, those of compound I in the acidolyses using HBr and HCl were remarkably greater than that using H2SO4, which was different from those in the acidolyses of compound II or VI
The acidolyses of compound VI using these three acids indicated that benzyl cation IX, which is derived from compound I, mostly undergoes H2O addition to convert to compound I in the acidolysis using H2SO4 while in those using the other two acids benzyl cation IX progresses to β-proton abstraction to convert to compound II followed by the acidolytic βO-4 bond cleavage

Summary

Introduction

Acidolysis is one of the most basic chemical treatments, and has recently been introduced as a pretreatment in the chemical conversion of woody biomass. In our two primary reports [7, 8], we employed a common non-phenolic C6-C2-type lignin model compound with the β-O-4 bond, 2-(2-methoxyphenoxy)-1-(3,4dimethoxyphenyl)ethanol (veratrylglycol-β-guaiacyl ether, I, Fig. 2), which does not have the γ-hydroxymethyl group, because compound I seemed to react owing to the lack of the γ-hydroxymethyl group limiting the acidolysis reaction to the only remaining route. Since this limitation did not motivate us to focus on the type of applied acid, we used only HBr in these reports. The obtained results enabled us to propose a characteristic action of B r− and Cl−

Materials and methods

Results and discussion

Conclusions