Revisiting the mechanism of β-O-4 bond cleavage during acidolysis of lignin. Part 8: Comparison between phenolic and non-phenolic C6-C2-type model compounds

Abstract

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.