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

The cleavage mechanism of β-O-4 bond was compared 
between a C 6 -C 3 and a C 6 -C 2 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 85 o C). 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 H 2 O 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.