Nucleophilic Thiols Reductively Cleave Ether Linkages in Lignin Model Polymers and Lignin

Abstract

Lignin may serve as a renewable feedstock for the production of chemicals and fuels if mild, scalable processes for its depolymerization can be devised. The use of small organic thiols represents a bioinspired strategy to cleave the β‐O‐4 bond, the most common linkage in lignin. In the present study, synthetic β‐O‐4 linked polymers were treated with organic thiols, yielding up to 90 % cleaved monomer products. Lignin extracted from poplar was also treated with organic thiols resulting in molecular weight reductions as high as 65 % (Mn) in oxidized lignin. Thiol‐based cleavage of other lignin linkages was also explored in small‐molecule model systems to uncover additional potential pathways by which thiols might depolymerize lignin. The success of thiol‐mediated cleavage on model dimers, polymers, and biomass‐derived lignin illustrates the potential utility of small redox‐active molecules to penetrate complex polymer matrices for depolymerization and subsequent valorization of lignin into fuels and chemicals.

Thio-assisted reductive electrolytic cleavage of lignin β-O-4 models and authentic lignin

“…Building on our previously reported thiolate-based reductive cleavage of -O-4 linkages via an SN2 pathway, 38,39 this study aimed to electrochemically reduce the disulfide byproduct to achieve an electrocatalytic cycle (Fig. 1).…”

“…S2) exhibited comparable rates of substrate consumption. The critical nucleophile that cleaved the ether bond in the previous studies, 38,39 2-hydroxyethanethiolate (here referred to as RS -), was initially presumed to be the active species in the electrochemically driven reaction. At the 5 mA current used in these experiments, two equivalents of electrons were calculated to pass through the cathode in 1 h, and the RSSR should ideally then be reduced to 2 equivalents of RS -.…”

“…We previously reported the successful reductive cleavage of lignin-relevant -keto aryl ether dimers by various small organic thiols. 38,39 This bio-inspired approach formed phenol and acetophenone quantitatively from 2-phenoxyacetophenone after 24 h treatment with 100 eq -mercaptoethanol (BME) in refluxing acetonitrile. The resulting disulfide byproduct could theoretically be reduced back to the active thiol by various reductants.…”

Metal Sulfide Photocatalysts for Lignocellulose Valorization

“…[57,58] Similar chemical processes using small organic thiols as nucleophilic reagents for the cleavage of the partially oxidized β-O-4 linkages have also been demonstrated. [59,60] A biomimetic S N 2 C β O-bond cleavage mechanism involving surface sulfur atoms with localized photogenerated electrons may occur over the metal sulfide catalyst. Further studies on the detailed functions of surface sulfur atoms for the cleavage of β-O-4 bond are needed in the future.…”

Tandem Triphosgene-Assisted Metal-Free One-pot Preparation of Nitriles and Amides from Aldehydes and Ketones

“…[20][21][22][23][24][25] Among the alternatives from biomass valorization, the production of 5-hydroxymethylfurfural (HMF) from hexoses is a versatile platform for the synthesis of furan-based chemicals such as furyl diamide, diamidine, diimidate, and diformylfuran. [26][27] Therefore, oxidative depolymerization of lignin and lignin model compounds can obtain aromatic aldehydes and ketones [28][29][30][31][32][33][34][35][36][37][38] such as vanillin veratraldehyde, acetovanillone, acetosyringone, and syringaldehyde. To further demonstrate the utility of our protocol, we used our one-pot strategy for the efficacious synthesis of 2,5-dicyanofuran from biomass-derived 2,5-diformylfuran.…”