PECULIARITY OF HYDROGEN BROMIDE AS A CATALYST OF THE β-O-4 BOND CLEAVAGE DURING ACIDOLYSIS OF LIGNIN

摘要

The dependence of acidolysis reaction of a C6-C3 dimeric non-phenolic β-O-4 type lignin model compound, 2-(2methoxyphenoxy)-1 -(3,4-dimethoxyphenyl)propane-1, 3-diol (veratrylglycerol-β-guaiacyl ether, VG), on the type of acid applied was examined using three different acids (0.2 mol/l HCl, HBr, and 0.1 mol/l (0.2 N) H2SO4 in 82% aqueous 1,4-dioxane at 85 °C). In the HCl system, the major reaction modes of the corresponding benzyl cation-type intermediate (BC), which is produced by protonation of the α-hydroxyl group of VG and successive release of the water molecule, are the abstraction of the β-proton and hydride transfer from the β- to αposition. The liberation of formaldehyde from the γ-hydroxymethyl group of BC is the predominant mode in the H2SO4 system. Apparently, an unknown reaction mode(s) is operative in the early stage of the HBr system, which 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 those in the HCl system after the early stage.