REVISITING THE MECHANISM OF beta-O-4 BOND CLEAVAGE DURING ACIDOLYSIS OF LIGNIN. PART 6: A REVIEW

This paper reviews our results on the acidolysis of dimeric non-phenolic beta-O-4-type lignin model compounds in aqueous 82% 1,4-dioxane containing an acid at 85 degrees C. It was shown that the mechanism of a C-6-C-2-type model compound, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl) ethanol IX, is fairly different from that of a C-6-C-3 analogue, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl) propane-1,3-diol XX, suggesting the significance of the presence of the gamma-hydroxymethyl group. It was confirmed that the hydride transfer mechanism exists as a reaction route of the benzyl-cation-type intermediates derived from both compounds, and the contribution of this mechanism is greater than expected in the acidolysis of compound XX. An enol ether compound, 2-(2-methoxyphenoxy)-3-(3,4-dimethoxyphenyl) prop-2-en-1-ol, was first detected in the acidolysis of compound XX using the DBr/D2O/1,4-dioxane system. It was confirmed in the acidolysis of compound XX using HBr that the mechanism is different from that in the system using either HCl or H2SO4 and an unknown mechanism contributes to the reaction. This unknown mechanism surprisingly contributed more with decreasing concentration of HBr or Br-, provided that Br- still existed in the system.