Studies on the condensation of lignins in alkaline media. Part II. The formation of stilbene and arylcoumaran structures through neighbouring group participation reactions

Abstract

The alkaline condensation of p-hydroxyarylglycerol-β-aryl ether units in lignin with other phenolic structures and the subsequent reactions of the condensation products have been elucidated using appropriate model compounds. It is shown that the primary condensation of simple phenols (2,6-xylenol or resorcinol) into the α-position of phenolic compounds of the β-aryl ether type is followed by an aryl participation (A1-3) reaction resulting in the cleavage of the β-aryl ether bond and formation of the corresponding stilbene derivative. When resorcinol is used as reaction partner, the condensation is also followed by the cleavage of the β-aryl ether linkage through another type of neighbouring group participation reaction involving the phenolate anion ortho to the site of condensation and giving rise to the corresponding aryl-coumaran derivative.The consequences of these findings for the course of alkaline delignification processes are briefly discussed.