Reaction of p-Hydroxycinnamyl Alcohols with Transition Metal Salts 3. Preparation and NMR Characterization of Improved DHPs

Abstract

Dehydropolymerization of p-hydroxycinnamyl alcohols with manganese(III) acetate in either aqueous acetic acid or pyridine resulted in dehydropolymers (DHPs) that more closely approximate the structure of natural lignins than do DHPs produced by enzymic techniques. The ¹³C NMR spectrum of a “biomimetic” guaiacyl-DHP (G-DHP) from coniferyl alcohol was very similar to that of a lignin isolated from spruce wood, unlike a corresponding spectrum of a G-DHP prepared by a conventional enzymic technique. A similar comparison between a biomimetic guaiacyl/syringyl DHP (GS-DHP), a conventional GS-DHP, and a natural lignin from elm wood also confirmed a closer resemblance between the biomimetic DHP and the natural lignin. Likewise, a biomimetic S-DHP prepared from sinapyl alcohol was remarkably similar to a high syringyl fraction of lignin isolated from birch wood. The flexibility of the biomimetic approach is illustrated by the preparation of a DHP that is enriched in coniferaldehyde entities and one that is almost devoid of free-phenolic entities.