Mathias

Publication date: 3 August 2024

Source: Tetrahedron, Volume 162

Author(s): Wei Wang, Ziying Liu, Miao Liu, Yong Ai, Zhengbing Fu, Caiqin Qin

A metal-free P, N co-doped carbon catalyst, derived from phytic acid and chitosan, was used to cleave α-O-4, β-O-4, and 4-O-5 lignin linkages in model compounds, using H₂O₂ and water.

Abstract

Developing efficient metal-free catalysts for lignin valorization is essential but challenging. In this study, a cost-effective strategy is employed to synthesize a P, N co-doped carbon catalyst through hydrothermal and carbonization processes. This catalyst effectively cleaved α-O-4, β-O-4, and 4-O-5 lignin linkages, as demonstrated with model compounds. Various catalysts were prepared at different carbonization temperatures and thoroughly characterized using techniques such as XRD, RAMAN, FTIR, XPS, NH₃-TPD, and HRTEM. Attributed to higher acidity, the P₅NC-500 catalyst exhibited the best catalytic activity, employing H₂O₂ as the oxidant in water. Additionally, this metal-free technique efficiently converted simulated lignin bio-oil, containing all three linkages, into valuable monomers. Density Functional Theory calculations provided insight into the reaction mechanism, suggesting substrate and oxidant activation by P−O−H sites in the P₅NC-500, and by N−C−O−H in the CN catalyst. Moreover, the catalyst′s recyclability and water utilization enhance its environmental compatibility, offering a highly sustainable approach to lignin valorization with potential applications in various industries.

A single catalytic system has been developed for the production of six products, namely nitroso, hydroxylamine, azoxy, azo, hydrazo and aniline compounds, from nitroaromatics. Furthermore, a detailed reaction network for the catalytic hydrogenation of nitroaromatics has been constructed based on systematic investigation into the controlled production of all products and their chemical reactivities via oxygen-isolated characterisation techniques.

Abstract

A full selectivity control over the catalytic hydrogenation of nitroaromatics leads to the production of six possible products, i.e., nitroso, hydroxylamine, azoxy, azo, hydrazo or aniline compounds, which has however not been achieved in the field of heterogeneous catalysis. Currently, there is no sufficient evidence to support that the catalytic hydrogenation of nitroaromatics with the use of heterogeneous metal catalysts would follow the Haber's mechanistic scheme based on electrochemical reduction. We now demonstrate in this work that it is possible to fully control the catalytic hydrogenation of nitroaromatics into their all six products using a single catalytic system under various conditions. Employing SnO₂-supported Pt nanoparticles facilitated by the surface coordination of ethylenediamine and vanadium species enabled this unprecedented selectivity control. Through systematic investigation into the controlled production of all products and their chemical reactivities, we have constructed a detailed reaction network for the catalytic hydrogenation of nitroaromatics. Crucially, using oxygen-isolated characterization techniques is essential for identifying unstable compounds such as nitroso, hydroxylamine, hydrazo compounds. The insights gained from this research offer invaluable guidance for selectively transforming nitroaromatics into a wide array of functional N-containing compounds, both advancing fundamental understanding and fostering practical applications in various fields.

Synthesis
DOI: 10.1055/s-0042-1751582

This review highlights the use of DACH as a versatile ligand in catalytic asymmetric transformations providing mechanistic rationales and relevant comments presented in chronological order for each of the 21 reaction types with references up to December 25, 2023. Intended to be as practically comprehensive as possible, this review assembles useful examples of using DACH as a ligand in organocatalytic or as metal complexes in asymmetric transformations. The resulting enantiomerically enriched, if not pure, chiral non-racemic small molecules are of great utility as value added intermediates in the total synthesis of natural products, in the design and synthesis of medicinally important compounds, and in other areas in organic and bioorganic chemistry where chirality plays a role. The graphic image depicts Spartacus with his arms folded in the same sense of chirality as (R,R)-DACH.1 Introduction2 DACH: A Brief Historical Narrative3 Catalytic Asymmetric Hydrogenation of Alkenes4 Catalytic Asymmetric Dihydroxylation of Alkenes5 Catalytic Asymmetric Sulfoxidation and Sulfimidation6 Catalytic Asymmetric 1,4-Conjugate Addition6.1 Using Jacobsen’s DACH Metal–salen Complexes as Catalysts6.2 Using Takemoto’s Bifunctional H-Bonding DACH Thiourea Organocatalyst6.3 Using DACH Ni(II) Complexes as Catalysts6.4 Using DACH H-Bonding Catalysis7 Catalytic Asymmetric Epoxidation of Alkenes8 Catalytic Asymmetric Claisen Rearrangement9 Catalytic Asymmetric 1,2-Nucleophilic Addition to Carbonyl Compounds9.1 Catalytic Asymmetric Addition of Dialkylzinc to Aldehydes and Ketones9.2 Catalytic Asymmetric Alkynylation of Aldehydes and Ketones9.3 Catalytic Asymmetric Addition of Cyanide to Aldehydes and Ketones10 Catalytic Asymmetric Allylic Alkylation11 Catalytic Asymmetric Cyclopropanation of Alkenes12 Catalytic Asymmetric Cycloaddition Reactions13 Catalytic Asymmetric Aziridination of Alkenes14 Catalytic Asymmetric Hydrogenation of Prochiral Ketones and Imines15 Catalytic Asymmetric Aldol Reactions16 Catalytic Asymmetric Opening of Small Ring Systems16.1 Desymmetrization of meso-Epoxides and meso-Aziridines16.2 Kinetic Resolution of Racemic Epoxides16.3 Enantioselective Addition of CO2 to Epoxides16.4 Enantioselective Ring Opening of Oxetanes17 Catalytic Asymmetric Strecker Reactions18 Catalytic Asymmetric Mannich Reactions19 Catalytic Asymmetric Henry and Aza-Henry Reactions20 Catalytic Asymmetric Morita–Baylis–Hillman and Rauhut–Currier Reactions21 Catalytic Asymmetric Petasis Reactions22 Organocatalytic Asymmetric Cascade Reactions23 Miscellaneous Catalytic Reactions24 Conclusion and Outlook25 DACH Catalysts and Ligands List
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

Article in Thieme eJournals:
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For the first time, “cathodic hydrogenolysis of C _β −O−4 linkage” and “anodic C−H/N−H cross-coupling reaction” are paired in an undivided cell, thus the lignin (models) depolymerization and the synthesis of valuable triarylamine derivatives could be simultaneously achieved in a selective and energy-effective manner.

Abstract

The cleavage of C−O bonds is one of the most promising strategies for lignin-to-chemicals conversion, which has attracted considerable attention in recent years. However, current catalytic system capable of selectively breaking C−O bonds in lignin often requires a precious metal catalyst and/or harsh conditions such as high-pressure H₂ and elevated temperatures. Herein, we report a novel protocol of paired electrolysis to effectively cleave the C _β −O−4 bond of lignin model compounds and real lignin at room temperature and ambient pressure. For the first time, “cathodic hydrogenolysis of C _β −O−4 linkage” and “anodic C−H/N−H cross-coupling reaction” are paired in an undivided cell, thus the cleavage of C−O bonds and the synthesis of valuable triarylamine derivatives could be simultaneously achieved in an energy-effective manner. This protocol features mild reaction conditions, high atom economy, remarkable yield with excellent chemoselectivity, and feasibility for large-scale synthesis. Mechanistic studies indicate that indirect H* (chemical absorbed hydrogen) reduction instead of direct electron transfer might be the pathway for the cathodic hydrogenolysis of C _β −O−4 linkage.

By purely thermolytic treatment of different industrially produced lignins in highly concentrated aqueous KOH either vanillic acid or protocatechuic acid can be accessed in attractive yields and good selectivity. By employing a work-up of the reaction mixture with ion-exchange resins, the products could be obtained, and the media directly reused in further reactions.

Abstract

A new and practical method for the thermal degradation of technically relevant bio-based lignin is presented. By heating a solution of lignin in highly concentrated caustic potash, vanillic acid is almost exclusively obtained in yields up to 10.6 wt %. By altering the reaction parameters, the selectivity of the reaction can be shifted towards the demethylation product, protocatechuic acid, which is obtained in a yield of 6.9 wt %. Furthermore, the procedure was applicable to different types of Kraft and organosolv lignin. To create an economically feasible process, ion exchange resins were used for the work-up of the highly caustic reaction media without neutralizing the complete mixture. By the selective removal of the desired vanillic acid from the caustic potash, this alkaline media could directly be reused for at least 5 further lignin degradations without significant loss of yield.

Synthesis
DOI: 10.1055/a-2326-6363

The reduction of nitro compounds into the highly valuable anilines is reported using a Cu catalyst and B2Pin2. The reactions proceed under very mild conditions and showcase excellent functional group tolerance. This method is applied to a large panel of nitro derivatives, including biorelevant molecules and important synthetic intermediates, toward the synthesis of active pharmaceutical ingredients (APIs). This novel reaction manifold intends to provide a complementary approach to the existing portfolio of nitro-reduction methods.
[...]

Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

Article in Thieme eJournals:
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Synlett
DOI: 10.1055/a-2322-0816

Nitroaromatic compounds, as hazardous industrial pollutants, have long been extensively studied for their conversion into high-value aromatic amines. However, most of these transformation reactions require either transition-metal catalysts or high-temperature conditions. Therefore, we report an electrochemical approach utilizing pinacolborane as the reducing agent for the efficient reduction of nitroaromatic compounds. The reaction is characterized by its mild conditions and simplicity of operation, and it demonstrates excellent substrate adaptability and functional group compatibility.
[...]

Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

Article in Thieme eJournals:
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A dual nickel/photoredox catalyzed selective radical-radical cross-coupling reaction has been developed with readily available carboxylic acids and their NHPI ester derivatives as coupling partners. This synergistic catalysis enables the mild and efficient building of C(sp²)−C(sp³) bonds or C(sp³)−C(sp³) bonds, affording structurally complex ketones and congested products with all-carbon quaternary centers.

Abstract

Controlling the cross-coupling reaction between two different radicals is a long-standing challenge due to the process occurring statistically, which would lead to three products, including two homocoupling products and one cross-coupling product. Generally, the cross-coupling selectivity is achieved by the persistent radical effect (PRE) that requires the presence of a persistent radical and a transient radical, thus resulting in limited radical precursors. In this paper, a highly selective cross-coupling of alkyl radicals with acyl radicals to construct C(sp²)−C(sp³) bonds, or with alkyl radicals to construct C(sp³)−C(sp³) bonds have been achieved with the readily available carboxylic acids and their derivatives (NHPI ester) as coupling partners. The success originates from the use of tridentate ligand (2,2′ : 6′,2′′-terpyridine) to enable radical cross-coupling process to Ni-mediated organometallic mechanism. This protocol offers a facile and flexible access to structurally diverse ketones (up to 90 % yield), and also a new solution for the challenging double decarboxylative C(sp³)−C(sp³) coupling. The broad utility and functional group tolerance are further illustrated by the late-stage functionalization of natural-occurring carboxylic acids and drugs.

Synthesis
DOI: 10.1055/a-2307-8257

In the past few decades, transition-metal-catalyzed direct amination of aryl halides with ammonia has attracted significant attention from chemists because of its broad substrate scope, good functional group compatibility, and high reaction selectivity. Herein, recent examples of transition-metal-catalyzed syntheses of aniline derivatives starting from aryl halides are reviewed.1 Introduction2 Heat-Driven Transition-Metal-Catalyzed Amination of Aryl Electrophiles2.1 Palladium-Catalyzed Amination2.2 Copper-Catalyzed Amination2.3 Nickel-Catalyzed Amination3 Light-Driven Transition-Metal-Catalyzed Amination of Aryl Electrophiles4 Conclusion and Outlook
[...]

Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

Article in Thieme eJournals:
Table of contents  |  Abstract  |  Full text

Synthesis
DOI: 10.1055/a-2288-6944

This paper outlines a novel strategy for the preparation of seven-membered-ring lactams from simple nitroarenes. The approach is based on a photochemical dearomative ring expansion starting with the conversion of the nitro group into a singlet nitrene. This process is mediated by blue light, occurs at room temperature and overall enables the insertion of the nitro N-atom into the benzenoid framework. This step transforms the aromatic starting material into a seven-membered ring azepine that, following hydrogenation and hydrolysis, is converted into the desired caprolactams in just three steps.
[...]

Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

Article in Thieme eJournals:
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The Cover Feature illustrates an acceleration effect of novel anionic N-heterocyclic carbene (NHC) ligands in Pd-catalyzed cross-coupling reactions with unactivated aryl chlorides. In their Research Article, V. M. Chernyshev, V. P. Ananikov et al. report on the synthesis and catalytic activity studies of new base-ionizable NHCs and their Pd complexes that deprotonate in basic reaction media and, due to the negative charge on the NHC, significantly facilitate the oxidative addition of aryl chlorides compared to conventional NHCs of close steric bulkiness.More information can be found in the Research Article by V. M. Chernyshev, V. P. Ananikov et al.

Nature Chemistry, Published online: 01 March 2024; doi:10.1038/s41557-024-01472-6

Paracetamol can be produced from p-hydroxybenzamide at >95 % purity in 90 % yield. Aqueous ammonia treatment of poplar or oil palm empty fruit bunches forms p-hydroxybenzamide. Under continuous processing conditions, a Hofmann rearrangement converts p-hydroxybenz-amide to p-aminophenol, then liquid/liquid extraction combined with acid/base chemistry and acetylation purifies, forms, and isolates paracetamol.

Abstract

As we work to transition the modern society that is based on non-renewable chemical feedstocks to a post-modern society built around renewable sources of energy, fuels, and chemicals, there is a need to identify the renewable resources and processes for converting them to platform chemicals. Herein, we explore a strategy for utilizing the p-hydroxybenzoate in biomass feedstocks (e. g., poplar and palm trees) and converting it into a portfolio of commodity chemicals. The targeted bio-derived product in the first processing stage is p-hydroxybenzamide produced from p-hydroxybenzoate esters found in the plant. In the second stage a continuous reaction process converts the p-hydroxybenzamide to p-aminophenol via the Hofmann rearrangement and recovers the unreacted p-hydroxybenzamide. In the third stage the p-aminophenol can be acetylated to form paracetamol, which is readily isolated by liquid/liquid extraction at >95 % purity and an overall p-hydroxybenzamide-to-paracetamol process yield of ~90 %. We explore how utilization of protecting groups alters the challenges in this process and expands the portfolio of possible products to include p-(methoxymethoxy)aniline and N-acetyl-p-(methoxymethoxy)aniline. These target compounds could become value-added renewably-sourced platform chemicals that could be used to produce biodegradable plastics, pigments, and pharmaceuticals.

Building on the pioneering works using imines to form metallacycle with transition metals, recent developments have realized the potential of using transient directing group (TDG) to direct C−H functionalization. This article discusses factors defining a transient directing group and strategies to achieve catalytic processes with low directing group loading by highlighting recent advancements in this field.

Abstract

‘Transient’ C−H functionalization has emerged in recent years to describe the use of a dynamic linkage, often an imine, to direct cyclometallation and subsequent functionalization. As the field continues to grow in popularity, we consider the features that make an imine directing group transient. A transient imine should be i) formed dynamically in situ, ii) avoid discrete introduction or cleavage steps, and iii) offer the potential for catalysis in both the directing group and metal. This concept article contrasts transient imines with pioneering early studies of imines as directing groups for the formation of metallacycles and the use of preformed imines in C−H functionalization. Leading developments in the use of catalytic additives to form transient directing groups (as aldehyde or amine) are covered including selected highlights of the most recent examples of catalytic imine directed C−H functionalization with transition metals.

Table of content shows the reduction of nitroarenes to respective amines using defect-rich CuO. Hydrazine is decomposed and in situ hydrogen is utilized for the concomitant reduction of nitroarenes. Reproduced from ref. [Adapted from 39] Copyright (2021), with permission from Elsevier.

Abstract

Oxygen vacancies are one of the prominent types of defects in metal oxides. In addition to their role in modifying the electronic properties in solids, these sites have been shown to activate chemical bonds. As a result, the use of defect-rich metal oxides in the organic transformations is promising. The article gives an overview of various chemical processes that are in use for the chemical reduction of aromatic nitro compounds to amines. Recently, application of oxygen vacancies in transition metal oxide for the selective reduction of aromatic nitro compounds to the corresponding amines has emerged as a new method of synthesis of amines. Oxygen vacancies in reducible oxide such as CuO have been shown to decompose hydrazine hydrate to H₂ and in situ cause the reduction of nitro arenes to corresponding amines. This concept article discusses the literature pertaining to the synthesis of nitro reduction reaction, various strategies employed thus far and the place the defect-mediated synthesis holds. The article also discusses the perspectives and challenges that remain to be addressed to gain more insights into the mechanistic understanding of the role of oxygen vacancies in nitro reduction and beyond.

The first semi-hydrogenation/reduction strategy for photocatalytic depolymerization of oxidized native lignin was realized by the synergistic effect between CsPbBr₃ quantum dots and Hantzsch ester. Mechanism analysis reveals this approach generates a C_α radical intermediate with 4 times lower bond dissociation energy of C_β−O−Ar bond from the substrate's carbonyl group, resulting in the excellent reactivity in a wide substrate scope.

Abstract

Due to the demanding depolymerization conditions and limited catalytic efficiency, enhancing lignin valorization remains challenging. Therefore, lowering the bond dissociation energy (BDE) has emerged as a viable strategy for achieving mild yet highly effective cleavage of bonds. In this study, a photocatalytic semi-hydrogenation/reduction strategy utilizing CsPbBr₃ quantum dots (CPB-QDs) and Hantzsch ester (HEH₂) as a synergistic catalytic system was introduced to reduce the BDE of C_β−O−Ar, achieving effective cleavage of the C_β−O−Ar bond. This strategy offers a wide substrate scope encompassing various β-O-4 model lignin dimers, preoxidized β-O-4 polymers, and native oxidized lignin, resulting in the production of corresponding ketones and phenols. Notably, this approach attained a turnover frequency (TOF) that is 17 times higher than that of the reported Ir-catalytic system in the photocatalytic depolymerization of the lignin model dimers. It has been observed via meticulous experimentation that HEH₂ can be activated by CPB-QDs via single electron transfer (SET), generating HEH₂⋅⁺ as a hydrogen donor while also serving as a hole quencher. Moreover, HEH₂⋅⁺ readily forms an active transition state with the substrates via hydrogen bonding. Subsequently, the proton-coupled electron transfer (PCET) from HEH₂⋅⁺ to the carbonyl group of the substrate generates a C_α⋅ intermediate.

Oxygen-rich biomass-derived substrates as reducing agents with a dioxomolybdenum catalyst. Stepwise transformation of nitrobenzene to aniline through three consecutive Mo(VI)/Mo(IV) catalytic cycles. In-situ produced water provides the protons needed for nitro to amine reduction.

Abstract

Density Functional Theory is used to unravel the mechanism of the nitrobenzene to aniline reduction, catalyzed by dioxomolybdenum (VI) dichloride. The use of pinacol as an oxoaccepting reagent and the production of only acetone and water as byproducts, signals a novel and environmentally friendly way to add value to the oxygen-rich biomass-derived polyols. The reaction proceeds through three consecutive cycles, each one responsible for one of the three reductive steps needed to yield aniline from nitrobenzene, with nitrosobenzene and hydroxylamine as intermediates. Each cycle regenerates the catalyst and releases one water and two acetone molecules. The mechanism involves singlet/triplet state crossings, a crucial feature in polyoxomolibdate catalyzed processes. The role of the Mo-coordinated water as the provider of the mysterious protons needed to reduce the nitro group, was revealed. The disclosure of this challenging mechanism and its rate limiting step can contribute to the design of more effective Mo(VI) catalysts.

An effective strategy of C-O bonds cleavage in lignin using metal triflate as the catalyst was developed, and the carboxylic acid or alcohol could be used as the nucleophile to stabilize the reactive intermediates formed during the depolymerization of lignin to prepare corresponding ester/ether compounds.

Abstract

The effective cleavage of C−O bonds in linkages of lignin was one of the significant strategies promoting lignin valorization. Herein, the strategy of C−O bonds cleavage of lignin using metal triflate as the catalyst was developed. The carboxylic acid or alcohol could be used as the nucleophile to stabilize the reactive intermediates formed during the depolymerization of lignin, and the corresponding ester/ether compounds could be obtained. This catalytic system was suitable for the C−O bond cleavage in α-O-4 and β-O-4 linkages with excellent efficiency. Additionally, reaction conditions were optimized. The reaction mixture was detected by ¹H NMR, and no other byproducts were found. As for treated lignin samples, the cleavage of C−O bonds in linkages was determined by 2D HSQC NMR, the increased content of the phenol hydroxyl group was proved by FT-IR, and the reduced molecular weight was investigated by GPC. Furthermore, multiple phenolic compounds were detected by GC-MS in the reaction mixtures.