Revisiting the mechanism of β-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− and Cl

A non-phenolic C 6 -C 2 -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 H 2 SO 4 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 >> H 2 SO 4 ). 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.