Revisiting the mechanism of β-O-4 bond cleavage during acidolysis of lignin part 10: reactions of C6-C2-type non-phenolic syringyl model compounds and comparison of the reactions with those of the guaiacyl analogues

A C-6-C-2-type non-phenolic syringyl lignin model compound (I, 2-(2-methoxyphenoxy)-1-(3,4,5-trimethoxyphenyl)ethanol) as well as its derivatives was acidolyzed in aqueous 82vol% 1,4-dioxane containing 0.2 mol/L HBr, HCl, or H2SO4 at 85 degrees C. The results were compared with those on the guaiacyl analogues (V as well as its derivatives) obtained in our previous reports. The acidolysis of compound I was slower than that of compound V, which is in accordance with our previous result on the formation rates of the benzyl cation intermediates (BC) from a syringyl compound and its guaiacyl analogue. The enol ether-type compound was primarily produced in the acidolysis of compound I, similar to that of compound V. The acidolysis using HBr or HCl was faster than that using H2SO4, indicating the participation of Br<overline> or Cl<overline>, respectively, in the acidolysis. It was suggested that Br<overline> (as well as Cl<overline>) adds to the cation center of the BC to afford the adduct in the acidolysis of compound I, similar to that of compound V, and hence, this adduct formation is a major bypass of the common route. The bypass activity in the acidolysis of compound I was about half in that of compound V.