Revisiting the mechanism of β-O-4 bond cleavage during acidolysis of lignin IV: dependence of acidolysis reaction on the type of acid

The dependence of the acidolysis reaction of a C(6)-C(3) dimeric nonphenolic beta-O-4 type lignin model compound, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl) propane-1,3-diol (veratrylglycerol-beta-guaiacyl ether, VG), on the type of acid applied was examined using three different acids [0.2 mol/l HCl, 0.2 mol/l HBr, and 0.1 mol/l (0.2 N) H(2)SO(4) in 82% aqueous 1,4-dioxane at 85A degrees C]. In the HCl system, the major reaction modes of the corresponding benzyl cation-type intermediate (BC), which is produced by protonation of the alpha-hydroxyl group of VG and successive release of the water molecule, are the abstraction of the beta-proton and hydride transfer from the beta-to the alpha-position. The liberation of formaldehyde from the gamma-hydroxymethyl group of BC is the predominant reaction mode in the H(2)SO(4) system. Apparently, an unknown reaction mode or modes is operative in the early stage of the HBr system that causes rapid disappearance of VG accompanied by the quantitative formation of 2-methoxyphenol without affording the common counterpart of a Hibbert's ketone, 1-hydroxy-3-(3,4-dimethoxyphenyl) propan-2-one. The reaction mode in the HBr system changes with the progress of the reaction and is the same as that in the HCl system after the early stage.