Lignin is an abundant biopolymer with an aromatic backbone and offers a bio-based source of aromatic commodity chemicals upon depolymerization.[i] Oxidation methods are widely used for the deconstruction of lignin; successful examples include oxidative delignification in the pulp and paper industry and breakdown of lignin by oxidative fungal enzymes. The hydroxyl functional groups of lignin are particularly susceptible to oxidation. Building on these concepts, our group recently began exploring the controlled oxidation of lignin hydroxyl groups as a means to facilitate lignin depolymerization.[ii] We previously reported an aerobic oxidation method that employs aminoxyl radicals, such as TEMPO, as the catalyst. Acid treatment of the resulting oxidized lignin lead to depolymerization, affording up to 60 wt% yield of low-molecular-weight aromatic compounds.[iii]

Aminoxyl radicals undergo facile electron transfer at the electrode surface to generate an oxoammonium species that is reactive towards hydroxyl oxidation (Figure 1a). This facile electron transfer provides a foundation for selective oxidation of hydroxyl groups under electrochemical conditions.[iv] This talk will highlight some of our advances in the use of aminoxyl radicals as mediators for chemoselective oxidation of hydroxyl groups in the structure of lignin. The beta-O-4 linkage (Figure 1b) is the most prevalent linkage in the structure of lignin that includes hydroxyl functional groups and was used as the model for this study.

Nitroxyl-mediated electrochemical oxidation methods enable selective and quantitative conversion of the primary hydroxyl group of lignin and beta-O-4 model compounds to the corresponding carboxylate under mildly basic conditions. The modified lignin has a high loading of carboxylic acids, which enhance its water solubility relative to native lignin. The sodium salt of the modified lignin behaves as a polyacid polyelectrolyte in aqueous solution. Acid treatment of the oxidized lignin model affords approximately 30 wt% of monomeric aromatic compounds.

[i]. Upton, B. M.; Kasko, A. M. Chem. Rev., 2016, 116, 2275.

[ii]. Rahimi, A.; Azarpira, A.; Kim, H.; Ralph, J.; Stahl, S. S. J. Am. Chem. Soc. 2013, 135, 6415.

[iii]. Rahimi, A.; Ulbrich, A.; Coon, J. J.; Stahl S. S. Nature 2014, 515, 249.

[iv]. Rafiee, M; Miles, K. C.; Stahl, S. S. J. Am. Chem. Soc. 2015, 137, 14751.