Imidazolium salts as an alternative for anti-Leishmania drugs: Oxidative and immunomodulatory activities

In this study we explored the previously established leishmanicidal activity of a complementary set of 24 imidazolium salts (IS), 1-hexadecylimidazole (C₁₆Im) and 1-hexadecylpyridinium chloride (C₁₆PyrCl) against Leishmania (Leishmania) amazonensis and Leishmania (Leishmania) infantum chagasi. Promastigotes of L. amazonensis and L. infantum chagasi were incubated with 0.1 to 100 μM of the compounds and eight of them demonstrated leishmanicidal activity after 48 h - C₁₀MImMeS (IC_{50 L. amazonensis} = 11.6), C₁₆MImPF₆(IC_{50 L. amazonensis} = 6.9), C₁₆MImBr (IC_{50 L. amazonensis} = 6), C₁₆M₂ImCl (IC_{50 L. amazonensis} = 4.1), C₁₆M₄ImCl (IC_{50 L. amazonensis} = 1.8), (C₁₀)₂MImCl (IC_{50 L. amazonensis} = 1.9), C₁₆Im (IC_{50 L. amazonensis} = 14.6), and C₁₆PyrCl (IC_{50 L. amazonensis} = 4).The effect of IS on reactive oxygen species production, mitochondrial membrane potential, membrane integrity and morphological alterations of promastigotes was determined, as well as on L. amazonensis-infected macrophages. Their cytotoxicity against macrophages and human erythrocytes was also evaluated. The IS C₁₀MImMeS, C₁₆MImPF₆, C₁₆MImBr, C₁₆M₂ImCl, C₁₆M₄ImCl and (C₁₀)₂MImCl, and the compounds C₁₆Im and C₁₆PyrCl killed and inhibited the growth of promastigote forms of L. amazonensis and L. infantum chagasi in a concentration-dependent manner, contributing to a better understanding of the structure-activity relationship of IS against Leishmania. These IS induced ROS production, mitochondrial dysfunction, membrane disruption and morphological alterations in infective forms of L. amazonensis and killed intracellular amastigote forms in very low concentrations (IC_{50 amastigotes} ≤ 0.3), being potential drug candidates against L. amazonensis.

Tunable and functional deep eutectic solvents for lignocellulose valorization

Stabilization of reactive intermediates is an enabling concept in biomass fractionation and depolymerization. Deep eutectic solvents (DES) are intriguing green reaction media for biomass processing; however undesired lignin condensation is a typical drawback for most acid-based DES fractionation processes. Here we describe ternary DES systems composed of choline chloride and oxalic acid, additionally incorporating ethylene glycol (or other diols) that provide the desired 'stabilization' function for efficient lignocellulose fractionation, preserving the quality of all lignocellulose constituents. The obtained ethylene-glycol protected lignin displays high β-O-4 content (up to 53 per 100 aromatic units) and can be readily depolymerized to distinct monophenolic products. The cellulose residues, free from condensed lignin particles, deliver up to 95.9 ± 2.12% glucose yield upon enzymatic digestion. The DES can be recovered with high yield and purity and re-used with good efficiency. Notably, we have shown that the reactivity of the β-O-4 linkage in model compounds can be steered towards either cleavage or stabilization, depending on DES composition, demonstrating the advantage of the modular DES composition.

Selective hydrogenolysis of aryl ethers over a nitrogen-doped porous carbon supported Ni–CeO2 catalyst at low temperature

The selective depolymerization of lignin into aromatics is a sustainable way to improve the economics of the overall biorefinery process.

An Introduction to Model Compounds of Lignin Linking Motifs; Synthesis and Selection Considerations for Reactivity Studies

The development of fundamentally new valorization strategies for lignin plays a vital role in unlocking the true potential of lignocellulosic biomass as sustainable and economically compatible renewable carbon feedstock. In particular, new catalytic modification and depolymerization strategies are required. Progress in this field, past and future, relies for a large part on the application of synthetic model compounds that reduce the complexity of working with the lignin biopolymer. This aids the development of catalytic methodologies and in-depth mechanistic studies and guides structural characterization studies in the lignin field. However, due to the volume of literature and the piecemeal publication of methodology, the choice of suitable lignin model compounds is far from straight forward, especially for those outside the field and lacking a background in organic synthesis. For example, in catalytic depolymerization studies, a balance between synthetic effort and fidelity compared to the actual lignin of interest needs to be found. In this Review, we provide a broad overview of the model compounds available to study the chemistry of the main native linking motifs typically found in lignins from woody biomass, the synthetic routes and effort required to access them, and discuss to what extent these represent actual lignin structures. This overview can aid researchers in their selection of the most suitable lignin model systems for the development of emerging lignin modification and depolymerization technologies, maximizing their chances of successfully developing novel lignin valorization strategies.

Lignin Precipitation and Fractionation from OrganoCat Pulping to Obtain Lignin with Different Sizes and Chemical Composition

Fractionation of lignocellulose into its three main components, lignin, hemicelluloses, and cellulose, is a common approach in modern biorefinery concepts. Whereas the valorization of hemicelluloses and cellulose sugars has been widely discussed in literature, lignin utilization is still challenging. Due to its high heterogeneity and complexity, as well as impurities from pulping, it is a challenging feedstock. However, being the most abundant source of renewable aromatics, it remains a promising resource. This work describes a fractionation procedure that aims at stepwise precipitating beech wood (Fagus sp.) lignin obtained with OrganoCat technology from a 2-methyltetrahydrofuran solution, using n-hexane and n-pentane as antisolvents. By consecutive antisolvent precipitation and filtration, lignin is fractionated and then characterized to elucidate the structure of the different fractions. This way, more defined and purified lignin fractions can be obtained. Narrowing down the complexity of lignin and separately valorizing the fractions might further increase the economic viability of biorefineries.

Stabilization strategies in biomass depolymerization using chemical functionalization

A central feature of most lignocellulosic-biomass-valorization strategies is the depolymerization of all its three major constituents: cellulose and hemicellulose to simple sugars, and lignin to phenolic monomers. However, reactive intermediates, generally resulting from dehydration reactions, can participate in undesirable condensation pathways during biomass deconstruction, which have posed fundamental challenges to commercial biomass valorization. Thus, new strategies specifically aim to suppress condensations of reactive intermediates, either avoiding their formation by functionalizing the native structure or intermediates or selectively transforming these intermediates into stable derivatives. These strategies have provided unforeseen upgrading pathways, products and process solutions. In this Review, we outline the molecular driving forces that shape the deconstruction landscape and describe the strategies for chemical functionalization. We then offer an outlook on further developments and the potential of these strategies to sustainably produce renewable-platform chemicals.

Tandem Acid/Pd-Catalyzed Reductive Rearrangement of Glycol Derivatives

Herein, we describe the acid/Pd-tandem-catalyzed transformation of glycol derivatives into terminal formic esters. Mechanistic investigations show that the substrate undergoes rearrangement to an aldehyde under [1,2] hydrogen migration and cleavage of an oxygen-based leaving group. The leaving group is trapped as its formic ester, and the aldehyde is reduced and subsequently esterified to a formate. Whereas the rearrangement to the aldehyde is catalyzed by sulfonic acids, the reduction step requires a unique catalyst system comprising a Pd^II or Pd⁰ precursor in loadings as low as 0.75 mol % and α,α'-bis(di-tert-butylphosphino)-o-xylene as ligand. The reduction step makes use of formic acid as an easy-to-handle transfer reductant. The substrate scope of the transformation encompasses both aromatic and aliphatic substrates and a variety of leaving groups.

Brønsted Acid Catalyzed Tandem Defunctionalization of Biorenewable Ferulic acid and Derivates into Bio-Catechol

An efficient conversion of biorenewable ferulic acid into bio-catechol has been developed. The transformation comprises two consecutive defunctionalizations of the substrate, that is, C-O (demethylation) and C-C (de-2-carboxyvinylation) bond cleavage, occurring in one step. The process only requires heating of ferulic acid with HCl (or H₂ SO₄ ) as catalyst in pressurized hot water (250 °C, 50 bar N₂ ). The versatility is shown on a variety of other (biorenewable) substrates yielding up to 84 % di- (catechol, resorcinol, hydroquinone) and trihydroxybenzenes (pyrogallol, hydroxyquinol), in most cases just requiring simple extraction as work-up.

Nucleophilic Cleavage of Lignin Model Compounds under Acidic Conditions in an Ionic Liquid: A Mechanistic Study

A range of lignin model compounds were examined for their reactivity with hydrogen bromide in the ionic liquid N-butylpyridinium triflate. It was found that the ionic liquid enabled rapid reaction at both the hydroxy and methyl ether sites of the model compounds at room temperature. Reactions at the phenyl ether moieties were more complex; rather than facilitating cleavage at these sites, alternate breakdown products that had not been seen in previous studies were observed; these products are consistent with functionalisation of the aromatic components of the model compounds.

An ionic liquid gel: a heterogeneous catalyst for Erlenmeyer–Plochl and Henry reactions

Agar gel entrapped [Bmim]OH has been prepared and employed as an efficient heterogeneous catalyst for the synthesis of β-nitro alcohols and azlactones.

Alcoholysis: A Promising Technology for Conversion of Lignocellulose and Platform Chemicals

In the catalytic conversion of lignocellulose to valuable products, the first entry point is to break down these biopolymers to sugar units or aromatic monomers, which is conventionally achieved by hydrolysis in water medium. Recent years have seen tremendous progress in the alcoholysis process, which has remarkable advantages, such as the avoidance of treating waste water, suppression of humins or chars, and enhancement of reaction rate and product yield. Advances have been focused on the alcoholysis of cellulose, hemicellulose, and lignin to alkyl glucosides, xylosides, and aromatic monomers, respectively. Alcoholysis of the platform molecule furfuryl alcohol (FAL) to alkyl levulinate (AL) and integrated alcoholysis of cellulose and furfural into AL are also summarized. This Minireview highlights the comparisons between alcoholysis and hydrolysis, the reaction mechanism of alcoholysis, and future challenges for industrial applications.

Cleavage of ethers in an ionic liquid. Enhancement, selectivity and potential application

An ionic liquid is used to facilitate ether cleavage without the presence of side reactions; the microscopic origins of the solvent effects are examined. Controlled cleavage of this kind of ether is relevant in the fractionation of lignin.

Catalytic Transformation of Lignocellulose into Chemicals and Fuel Products in Ionic Liquids

Innovative valorization of naturally abundant and renewable lignocellulosic biomass is of great importance in the pursuit of a sustainable future and biobased economy. Ionic liquids (ILs) as an important kind of green solvents and functional fluids have attracted significant attention for the catalytic transformation of lignocellulosic feedstocks into a diverse range of products. Taking advantage of some unique properties of ILs with different functions, the catalytic transformation processes can be carried out more efficiently and potentially with lower environmental impacts. Also, a new product portfolio may be derived from catalytic systems with ILs as media. This review focuses on the catalytic chemical conversion of lignocellulose and its primary ingredients (i.e., cellulose, hemicellulose, and lignin) into value-added chemicals and fuel products using ILs as the reaction media. An outlook is provided at the end of this review to highlight the challenges and opportunities associated with this interesting and important area.

Metal Triflates for the Production of Aromatics from Lignin

The depolymerization of lignin into valuable aromatic chemicals is one of the key goals towards establishing economically viable biorefineries. In this contribution we present a simple approach for converting lignin to aromatic monomers in high yields under mild reaction conditions. The methodology relies on the use of catalytic amounts of easy-to-handle metal triflates (M(OTf)_x ). Initially, we evaluated the reactivity of a broad range of metal triflates using simple lignin model compounds. More advanced lignin model compounds were also used to study the reactivity of different lignin linkages. The product aromatic monomers were either phenolic C2-acetals obtained by stabilization of the aldehyde cleavage products by reaction with ethylene glycol or methyl aromatics obtained by catalytic decarbonylation. Notably, when the method was ultimately tested on lignin, especially Fe(OTf)₃ proved very effective and the phenolic C2-acetal products were obtained in an excellent, 19.3±3.2 wt % yield.

A comparison of the oxidation of lignin model compounds in conventional and ionic liquid solvents and application to the oxidation of lignin

The oxidation of lignin model compounds in ionic liquid solvents was investigated as a prelude to the oxidation of lignin in these solvents where the polymer is appreciably soluble.