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 H 2 SO 4 at 85ºC. 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 H 2 SO 4 , indicating the participation of Br¯ or Cl¯, respectively, in the acidolysis. It was suggested that Br¯ (as well as Cl¯) 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 .