Aniline Derivatives from Lignin under Mild Conditions Enabled by Electrochemistry

The development of environmentally friendly methods for the valorization of important phenolic platform chemicals originating directly from lignin-first depolymerization into value-added N-chemicals, such as aniline derivatives, is of high industrial interest. In this work, we tackle this challenging transformation by the judicious combination of electrochemical conversion and chemical functionalization steps. In the first step, lignin-derived para-substituted guaiacols and syringols undergo an atom-efficient, room-temperature anodic oxidation using methanol both as solvent and reagent towards the formation of the corresponding cyclohexadienone derivatives, which are subsequently converted to synthetically challenging ortho-methoxy substituted anilines by reaction with ethyl glycinate hydrochloride under mild conditions. The developed method was applied to crude lignin depolymerization bio-oils, derived from reductive catalytic fractionation (RCF) mediated either by copper-doped porous metal oxide (Cu20PMO) or Ru/C, allowing the selective production of 4-propanol-2-methoxyaniline (1Gb) and 4-propyl-2-methoxyaniline (2Gb), respectively, from pine lignocellulose. Finally, the application of 2Gb was further studied in the synthesis of carbazole 2Gc, a lignin-derived analogue of biologically active alkaloid murrayafoline A. Electrochemical oxidation of lignin-derived guaiacols & syringols leads to cyclohexadienone intermediates in moderate to good yields. These react with ethyl glycinate hydrochloride under mild conditions to afford lignin-based anilines. This methodology can be incorporated to reductive catalytic fractionation processes towards selective obtention of 4-propanol-2-methoxyaniline (1Gb) and 4-propyl-2-methoxyaniline (2Gb) in up to 4.6 wt% yield.image

Automated summary

In this study, an environmentally friendly method for extracting important phenolic chemicals from lignin was developed using electrochemical conversion and chemical functionalization steps. This method enables the transformation of lignocellulose into nitrogen-containing chemicals, such as aniline derivatives, to meet industrial demands.