Revisiting the mechanism of β-O-4 bond cleavage during acidolysis of lignin. Part 1: Kinetics of the formation of enol ether from non-phenolic C₆-C₂ type model compounds

Abstract

The reaction route of a dimeric non-phenolic C6-C2 type lignin model compound, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethanol (VIII), was kinetically examined under acidolysis conditions (0.2 mol l-1 HBr in 82% aqueous 1,4-dioxane at 85°C). The disappearance of (VIII) followed the pseudo-first-order rate law, and the rate constant k (VIII) was 0.00854. In the course of the reactions, the following compounds were produced quantitatively at any time: an enol ether, 1-(2-methoxyphenoxy)-2-(3,4-dimethoxyphenyl)ethylene (IX), 2-methoxyphenol (X), and a Hibbert's ketone, 3,4-dimethoxyphenylacetaldehyde (XI). The substances (X) and (XI) are the result of the β-O-4 bond cleavage and their amounts were always equal during the whole reaction. When (IX) was subjected to the acidolysis under the identical conditions, its disappearance followed the pseudo-first-order rate law, and the rate constant k (IX) was 0.00825. Furthermore, (VIII) was not observed at all, and (X) was produced quantitatively at any time. Based on these results, the formation rate of (IX) during the acidolysis of (VIII) is expressed by the equation: d[(IX)]/dt=A·k (VIII)[(VIII)]-k (IX)[(IX)], where A is the proportion of (VIII) that converted into (IX) when (VIII) degraded. It was confirmed by solving this differential equation that the formation and disappearance of (IX) is best simulated when A was assumed to be 1.00. Therefore, it was proven in this paper for the first time that (VIII) primarily converts into (IX), and subsequently the β-O-4 bond cleavage occurs and (X) and (XI) are yielded.

Analysis of products from the oxidation of technical lignins by oxygen and H3PMo12O40 in water and aqueous methanol by size-exclusion chromatography

“…This step is followed by elimination of the b-proton giving rise to an enol ether, which finally undergoes fast acidic hydrolysis. The formation of benzyl cation intermediates as an initial step is well accepted (Yokoyama and Matsumoto 2008) and common in many treatments of lignin in acidic media. However, treatment of kraft lignin under acidolysis conditions will not bring about a deep-going fragmentation, because most of the aryl-ether bonds have already been cleaved during kraft pulping (Gierer 1980).…”

Revisiting the Mechanism ofβ-O-4 Bond Cleavage during Acidolysis of Lignin. Part 2: Detailed Reaction Mechanism of a Non-Phenolic C6-C2Type Model Compound

“…[14] The enol ether compound EE was isolated from acidolysis mixtures of V G following the procedures described previously. [14] The isolated EE was a mixture of cis-and trans-isomers (60/40).…”

“…[3] However, because the hypothesized mechanism does not appear to explain the dependence of the acidolysis rate on the type of acid applied, [13,14] we reexamined the mechanism involved and published a paper on the reaction route of a C 6 -C 2 dimeric non-phenolic β-O-4 type lignin model compound, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethanol (veratrylglycol-β-guaiacyl ether, V G in Figure 1). [14] It has been proven kinetically that V G is primarily and irreversibly converted into an enol ether compound, 1-(2-methoxyphenoxy)-2-(3,4-dimethoxyphenyl)ethene (EE in Figure 1), and the successive β-O-4 bond cleavage affords 2-methoxyphenol (guaiacol, G in Figure 1) and a Hibbert's monomer, 3,4-dimethoxyphenyl acetaldehyde (HK in Figure 1). [14] However, the detailed reaction mechanism and route have not yet been clarified.…”

“…[14] It has been proven kinetically that V G is primarily and irreversibly converted into an enol ether compound, 1-(2-methoxyphenoxy)-2-(3,4-dimethoxyphenyl)ethene (EE in Figure 1), and the successive β-O-4 bond cleavage affords 2-methoxyphenol (guaiacol, G in Figure 1) and a Hibbert's monomer, 3,4-dimethoxyphenyl acetaldehyde (HK in Figure 1). [14] However, the detailed reaction mechanism and route have not yet been clarified.…”

“…For these purposes, the enol ether compound EE , a deuterated compound labeled at the β-position of V G, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)(2,2-2 H 2 )ethanol (V G-βD 2 in Figure 1), and a compound with a methyl ether at the α-position of V G, 1-methoxy-2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethane (V G-αOMe in Figure 1), were also acidolyzed under conditions identical to those described previously. [14] The chemical structures of the compounds referred to in this paper are shown in Figure 1.…”