Regioselectivity in lignin biosynthesis. The influence of dimerization and cross-coupling

We have studied the regioselectivity of oxidative phenol coupling in lignin formation using an oxidation system that distinguishes between dimerization reactions and cross-coupling reactions. We found that the regioselectivity of coupling was different in the two reactions. For instance, in coniferyl alcohol dimerization the formation of beta-5 coupling product has a slight prevalence over the formation of beta-O-4 product; in cross-coupling the beta-O-4 mode is favoured in a ratio of approximate to 10 : 1. This ratio is higher than that found in isolated softwood lignins. The degree of cross-coupling was influenced only to a small extent by changes in the rates of conventional addition of coniferyl alcohol (Zulauf versus Zutropf conditions). We found that diffusion through a dialysis membrane did effectively suppress the dimerization of coniferyl alcohol. Of the different oxidants investigated, manganese triacetate in acetic acid yielded the highest proportion of cross-coupling product.