COMPARISON OF THE β-O-4 BOND CLEAVAGE MECHANISM OF C6-C3 AND C6-C2 LIGNIN MODEL COMPOUNDS DURING ACIDOLYSIS

Paper in proceeding, 2009

The cleavage mechanism of β-O-4 bond was compared between a C6-C3 and a C6-C2 type lignin model compounds, veratrylglycerol-β-guaiacyl ether (VG), and veratrylglycol-β-guaiacyl ether (V’G), under acidolysis conditions (82% aqueous 1,4-dioxane solution containing HBr or HCl at 85oC). The disappearance rate of V’G was faster than VG. It was confirmed that V’G primarily converts into an enol ether compound, 1-(2- methoxyphenoxy)-2-(3,4-dimethoxyphenyl)ethylene, via the H2O elimination, and subsequently, the β-O-4 bond cleaves affording guaiacol and a Hibber’s ketone, 3,4- dimethoxyphenylacetaldehyde. On the other hand, the mechanism for the VG disappearance was unclear mainly because of the extreme lability of 2-(2-methoxyphenoxy)- 3-(3,4-dimethoxyphenyl)prop-2-en-1-ol, another enol ether compound. The proton activity in two acidolysis systems, 0.20 mol/l HBr and 0.25 mol/l HCl, were assumed to be the same based on the observation that the hydrolysis rates of a carbohydrate model compound, methyl α-D-glycopyranoside, were the same in both systems. The disappearance rates of V’G in these systems were almost the same, while VG disappeared faster in the HBr system. These results suggest that the mechanism of the disappearances of VG and V’G is not the same and the counter anions of the acids influence the disappearance rate of VG more significantly than V’G.