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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. [ABSTRACT FROM AUTHOR]

The purpose of this study was to examine how the type of solvent among aqueous 1,4-dioxane, tetrahydrofuran, ethanol, iso-propyl alcohol, or ethylene glycol and its content (mol%) affect the formation rate of benzyl cation intermediate (BC) in the acidolysis of lignin, using a simple model compound, 1,2-dimethoxy-4-methoxymethylbenzene. Because the BC forms from the model compound via two steps, i.e., protonation of the benzyl methoxymethyl group as the pre-equilibrium step and liberation of the methanol as the rate-determining step, the observed variation of the formation rate with type of solvent and solvent content originates from the effects on both steps undistinguishably. The formation rate of BC decreased with increasing mol% of any of the organic solvents for a range of relatively low mol%, but increased with it for relatively high mol%. The formation rate varied more in the ether than in the alcohol systems. These results seem to be regulated by the effect of changing the mol% on the pre-equilibrium step, i.e., on the proton activity, rather than on the rate-determining step. Two reaction products, 4-alkoxymethyl-1,2-dimethoxybenzene and 4-hydroxymethyl-1,2-dimethoxybenzene, exclusively formed in the aqueous alcohol systems. The former compound was confirmed to be thermodynamically more stable and kinetically the more favorable product. [ABSTRACT FROM AUTHOR]

A phenolic C6-C2-type lignin model compound with the β-O-4 bond, 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)ethanol (I), was acidolyzed in aqueous 82 vol% 1,4-dioxane containing 0.2 mol/L HBr, HCl, or H2SO4 at 85 °C. It was confirmed that about 15%, 15%, or 60%, respectively, of molecules of compound I do not primarily convert to the enol-ether compound, 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)ethene (II), but undergo direct β-O-4 bond cleavage. This contrasts with our previous result that most molecules of the corresponding non-phenolic compound primarily convert to the corresponding non-phenolic enol ether compound under the same conditions. The disappearance rates of compound I were markedly different between the acidolyses using three acids (HBr > HCl ≫ H2SO4), while those of compound II or the α-methyl-etherified derivative of compound I (III) were not largely different in its acidolysis under the same conditions. These results suggest that the marked difference in the acidolysis rates of compound I originates from the participation of Br− or Cl−. Acidolysis of the α-bromo-substituting derivative of compound I (IV) using HBr showed that Br− adds to the benzyl cation generated from compound I or III to afford compound IV, which consecutively undergoes the HBr elimination to convert to compound II, bypassing direct conversion of the benzyl cation to compound II and resulting in the fast acidolysis. [ABSTRACT FROM AUTHOR]