Successive Cleavage and Reconstruction of Lignin β-O-4 Models and Polymer to Access Quinoxalines

Catalytic conversion of chitin-based biomass to nitrogen-containing chemicals

The exploration of renewable alternatives to fossil fuels for chemical production is indispensable to achieve the ultimate goals of sustainable development. Chitin biomass is an abundant platform feedstock that naturally bears both nitrogen and carbon atoms to produce nitrogen-containing chemicals (including organonitrogen ones and inorganic ammonia). The expansion of biobased chemicals toward nitrogen-containing ones can elevate the economic competitiveness and benefit the biorefinery scheme. This review aims to provide an up-to-date summary on the overall advances of the chitin biorefinery for nitrogen-containing chemical production, with an emphasis on the design of the catalytic systems. Catalyst design, solvent selection, parametric effect, and reaction mechanisms have been scrutinized for different transformation strategies. Future prospectives on chitin biorefinery have also been outlined.

Sustainable production of triazoles from lignin major motifs

An efficiently catalyzed synthesis of pharmaceutically relevant 1,2,3-trazoles from renewable resources is highly desirable. However, due to incompatible catalysis conditions, this endeavor remained challenging so far. Herein, a practical access protocol to 1,2,3-triazoles, starting from lignin phenolic β-O-4 with γ-OH group utilizing a vanadium-based catalyst is presented. A broad substrate scope reaching up to 97 % yield of 1,2,3-triazoles are obtained. The reaction pathway includes selective cleavage of double C-O bonds, cycloaddition, and dehydrogenation. Mechanistic studies and density-functional theory (DFT) calculations suggest that the V-based complex acts as a bifunctional catalyst for both selective C-O bonds cleavage and dehydrogenation. This synthetic pathway has been applied for the synthesis of pharmacological and biological active carbohydrate derivatives starting from biomass components as feedstock, enabling a potential sustainable route to triazolyl carbohydrate derivatives, which paves the way for lignin-based heterocyclic aromatics in the pharmaceutical applications.

Aqueous amine enables sustainable monosaccharide, monophenol, and pyridine base coproduction in lignocellulosic biorefineries

Thought-out utilization of entire lignocellulose is of great importance to achieving sustainable and cost-effective biorefineries. However, there is a trade-off between efficient carbohydrate utilization and lignin-to-chemical conversion yield. Here, we fractionate corn stover into a carbohydrate fraction with high enzymatic digestibility and reactive lignin with satisfactory catalytic depolymerization activity using a mild high-solid process with aqueous diethylamine (DEA). During the fractionation, in situ amination of lignin achieves extensive delignification, effective lignin stabilization, and dramatically reduced nonproductive adsorption of cellulase on the substrate. Furthermore, by designing a tandem fractionation-hydrogenolysis strategy, the dissolved lignin is depolymerized and aminated simultaneously to co-produce monophenolics and pyridine bases. The process represents the viable scheme of transforming real lignin into pyridine bases in high yield, resulting from the reactions between cleaved lignin side chains and amines. This work opens a promising approach to the efficient valorization of lignocellulose.

Aerobic oxidative C-C bond cleavage and functionalization for the synthesis of value-added chemicals

The aerobic oxidative cleavage of C-C bonds is an attractive and sustainable route for constructing valuable molecules such as esters, nitriles, and amides. Traditionally homogeneous catalytic systems for C-C bond cleavage required harsh conditions, stoichiometric oxidants, and noble metal catalysts to overcome the thermodynamic and kinetic barriers of C-C bonds, imposing environmental concerns of the transformation. Therefore, developing efficient, low-cost, and environmentally benign methods for C-C bond cleavage is of great importance and a cutting-edge area in modern chemistry. This feature article summarizes the sustainable aerobic oxidative C-C bond cleavage method developed by our group in the past 5 years. Fundamental principles in catalyst design, substrate scope, and mechanism for C-C bond cleavage are also discussed.

Towards bioresource-based aggregation-induced emission luminogens from lignin β-O-4 motifs as renewable resources

One-pot synthesis of heterocyclic aromatics with good optical properties from phenolic β-O-4 lignin segments is of high importance to meet high value added biorefinery demands. However, executing this process remains a huge challenge due to the incompatible reaction conditions of the depolymerization of lignin β-O-4 segments containing γ-OH functionalities and bioresource-based aggregation-induced emission luminogens (BioAIEgens) formation with the desired properties. In this work, benzannulation reactions starting from lignin β-O-4 moieties with 3-alkenylated indoles catalyzed by vanadium-based complexes have been successfully developed, affording a wide range of functionalized carbazoles with up to 92% yield. Experiments and density functional theory calculations suggest that the reaction pathway involves the selective cleavage of double C-O bonds/Diels-Alder cycloaddition/dehydrogenative aromatization. Photophysical investigations show that these carbazole products represent a class of BioAIEgens with twisted intramolecular charge transfer. Distinctions of emission behavior were revealed based on unique acceptor-donor-acceptor-type molecular conformations as well as molecular packings. This work features lignin β-O-4 motifs with γ-OH functionalities as renewable substrates, without the need to apply external oxidant/reductant systems. Here, we show a concise and sustainable route to functional carbazoles with AIE properties, building a bridge between lignin and BioAIE materials.

Imidazo[1,2-a]pyridine derivatives synthesis from lignin β-O-4 segments via a one-pot multicomponent reaction

The catalytic conversion of lignin into N-containing chemicals is of great significance for the realization of value-added biorefinery concept. In this article, a one-pot strategy was designed for the transformation of lignin β-O-4 model compounds to imidazo[1,2-a]pyridines in yields up to 95% using 2-aminopyridine as a nitrogen source. This transformation involves highly coupled cleavage of C-O bonds, sp³C-H bond oxidative activation, and intramolecular dehydrative coupling reaction to construction of N-heterobicyclic ring. With this protocol, a wide range of functionalized imidazo[1,2-a]pyridines sharing the same structure skeleton as those commercial drug molecules, such as Zolimidine, Alpidem, Saripidem, etc., were synthesized from different lignin β-O-4 model compounds and one β-O-4 polymer, emphasizing the application feasibility of lignin derivatives in N-heterobicyclic pharmaceutical synthesis.

Degradation of three β-O-4 lignin model compounds via organic electrolysis and elucidation of the degradation mechanisms

Woody biomass comprising cellulose, hemicellulose, and lignin has been the focus of considerable attention as an alternative energy source to fossil fuel for various applications. However, lignin has a complex structure, which is difficult to degrade. Typically, lignin degradation is studied using β-O-4 lignin model compounds as lignin contains a large number of β-O-4 bonds. In this study, we investigated the degradation of the following lignin model compounds via organic electrolysis: 2-(2-methoxyphenoxy)-1-(4-methoxyphenyl)ethanol 1a, 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol 2a, and 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol 3a. The electrolysis was conducted for 2.5 h at a constant current of 0.2 A using a carbon electrode. Various degradation products such as 1-phenylethane-1,2-diol, vanillin, and guaiacol were identified upon separation via silica-gel column chromatography. The degradation reaction mechanisms were elucidated using electrochemical results as well as density functional theory calculations. The results suggest that the organic electrolytic reaction can be used for the degradation reaction of a lignin model with β-O-4 bonds.