Selective demethylation reactions of biomass-derived aromatic ether polymers for bio-based lignin chemicals

Abstract

The selective demethylation of aryl methyl ethers is a reaction of high relevance both in the context of deprotection in organic syntheses as well as in the modification of lignin and lignin-derived chemicals. The resulting vicinal diol or triol configurations originating from guaiacyl or synapyl lignin moieties are particularly relevant for adhesive properties, while other positions are applicable for condensation polymerization. Throughout the years, a plethora of methods have been developed to efficiently cleave the particularly stable aryl methyl bond. This review provides, on the one hand, a comprehensive overview of conventional approaches associated with the field of organic chemistry, with a focus on recently developed methods with a focus on aspects of green chemistry. On the other hand, acid- and base-catalyzed as well as (hydro)thermal methods are discussed as well as the occurrence of demethylation reactions under metal-catalyzed hydrotreatment conditions, which, however, proved to be the most environmentally friendly principle. While the different chemical approaches are first presented for the conversion of selected model compounds, a subsequent section presents the application of these methods to lignin. The selective cleavage of aryl methyl ether moieties of lignin to increase the amount of phenolic hydroxyl groups has recently gained considerable attention, and research can benefit from the emerging knowledge gained from the new approaches on model compound demethylation. In sum, we provide a multi-faceted and critical overview of demethylation strategies to obtain phenol and catechol moieties from the renewable lignin and related compounds.