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Frontispiece: Continuous flow systems are of growing interests in diverse fields attributing to their prominent ability to uniform the system and facilitate the operation process. Immobilizing catalysts in flow reactors allows high-efficiency transformations and excludes the subsequent separation procedures. In this Concept article, G. Lin and H. Qiu outline the approaches to incorporate catalysts within various flow reactors, with particular focus on the employment of additional supports with a diverse array of constitutions and structures. Notably, the synergy between the supports and catalysts are discerned, as well as the specific designs. For more information, see the Review by Lin and Qiu (DOI: 10.1002/chem.202200069)

Nature, Published online: 31 May 2022; doi:10.1038/d41586-022-01375-x

A readily available and mild CuCl catalyst system has been developed for converting CO₂ with aryl boronic acids to aryl carboxylate esters. The system has a good functional group tolerance, and substrates containing electron-withdrawing and electron-donating groups could be converted into corresponding products with good yields.

Abstract

The CuCl-catalyzed reaction of aryl boronic acid with carbon dioxide to form carboxylate ester after treatment with CH₃I has been developed. The procedure featured mild conditions and good functional group tolerances. A diverse range of aryl boronic acids were effectively converted into carboxylate esters. Even those bearing sensitive groups such as carbonyl, ester, and amide could produce the desired products in good yields.

Abstract

Aryltriazenes can hardly take part in productive organic transformations unless stoichiometric Brönsted or Lewis acid activators are used. We report here for the first time a palladium-catalyzed Suzuki coupling of aryltriazenes activated by a sulfonyl group at N3 atom under the common basic conditions. Benefiting from elimination of stoichiometric acid activators, activated aryltriazenes could efficiently couple with arylboronic acids to afford diaryls in modest to excellent yields by using a simple catalyst at low loading, 0.3 mol% Pd(PPh₃)₂Cl₂. Scope and limitation of the coupling are demonstrated with 26 examples.

Lignin breakdown: The valorization of lignin is key to generate value-added compounds. For this, high-yield and green strategies to convert lignin to fine chemicals are intensively investigated. A reductive electrochemical route provides a feasible and sustainable way to obtain lignin-derived aromatic monomers and dimers from the solubilization and further depolymerization of lignin in levulinic acid, a biomass-based solvent.

Abstract

Breaking down lignin into smaller units is the key to generate high value-added products. Nevertheless, dissolving this complex plant polyphenol in an environment-friendly way is often a challenge. Levulinic acid, which is formed during the hydrothermal processing of lignocellulosic biomass, has been shown to efficiently dissolve lignin. Herein, levulinic acid was evaluated as a medium for the reductive electrochemical depolymerization of the lignin macromolecule. Copper was chosen as the electrocatalyst due to the economic feasibility and low activity towards the hydrogen evolution reaction. After depolymerization, high-resolution mass spectrometry and nuclear magnetic resonance spectroscopy revealed lignin-derived monomers and dimers. A predominance of aryl ether and phenolic groups was observed. Depolymerized lignin was further evaluated as an anti-corrosion coating, revealing enhancements on the electrochemical stability of the metal. Via a simple depolymerization process of biomass waste in a biomass-based solvent, a straightforward approach to produce high value-added compounds or tailored biobased materials was demonstrated.

HMF upgrading: 5-Hydroxymethylfurfual (HMF) is regarded as the bridge between biomass and value-added chemicals, attracting widespread attention to develop novel catalytic technologies over the past decade. This Review is focused on selective activation of C−O, C=O, and C=C bonds of HMF in metal-catalyzed hydrogenation, hydrogenolysis, decarbonylation, and oxidation.

Abstract

With increasing concern for reducing CO₂ emission and alleviating fossil resource dependence, catalytic transformation of 5-hydroxymethylfurfural (HMF), a vital platform compound derived from C₆ sugars, holds great promise for producing value-added chemicals. Among several well-established catalytic systems, hydrogenation and oxidation processes have been efficiently adopted for upgrading HMF. This Review covers recent advances in the development of thermocatalytic conversion of HMF into value-added chemicals. The advances of metal-catalyzed hydrogenation, hydrogenolysis, ring-opening, decarbonylation, and oxidation involving selective activation of C−O, C=O, and C=C groups are described. The roles played by nature of metals, supports, additives, synergy of metal–acid sites, and metal–support interaction are also discussed at the molecular level. Finally, an outlook is provided to highlight major challenges associated with this huge research area.

A series of chromium complexes bearing a carbene- and phosphine-based PCP-type pincer ligand has been newly prepared, and some of them are found to work as effective catalysts to reduce dinitrogen into ammonia and hydrazine under atmospheric pressure. This reaction can be expressed as nitrogen molecules (raindrops) being converted into ammonia and hydrazine on the chromium complex (umbrella). More information can be found in the Research Article by K. Yoshizawa, Y. Nishibayashi et al. (DOI: 10.1002/chem.202200557).

Sustainable organic afterglow room-temperature phosphorescence (RTP) with long lifetime is a particularly attractive phenomenon but remains difficult to achieve. Here, we prepared sustainable afterglow RTP materials (GA@SA) with a lifetime up to 934.7 ms by embedding gallic acid (GA) within a Ca²⁺-crosslinked sodium alginate (SA) matrix.

Abstract

Long-lived afterglow room-temperature phosphorescence (RTP) from natural phenolics has seldom been reported yet this is essential for the development of sustainable afterglow RTP materials. With this research, we have prepared sustainable afterglow RTP materials (GA@SA) with a lifetime of up to ≈934.7 ms by embedding gallic acid (GA) within a Ca²⁺-crosslinked sodium alginate (SA) matrix. Theoretical simulations indicate that the restricted carbonyl moieties of the GA and H-type aggregates of GA in a SA matrix promoted the spin orbit coupling (SOC) of GA and induced afterglow emission. Moreover, afterglow RTP emission could be produced by embedding different types of natural phenolics such as, tannic acid, caffeic acid and chlorogenic acid into Ca²⁺-crosslinked networks of SA. As an illustration of potential applications, GA@SA was used to prepare anti-counterfeit afterglow clothing and paper. This work provides an innovative method for the activation of long-lived afterglow RTP from sustainable phenolics.

Abstract

A rhodium(I)-catalyzed decarboxylative hydroacylation of readily available vinylethylene carbonates with salicylaldehydes for regioselective preparation of esters was developed. Reaction optimization revealed that methacrylamide might promote the hydroacylation by bidentate chelation assistance to the cationic rhodium. Mechanistic findings suggested that this one-pot coupling reaction proceeds via Markovnikov hydrorhodation-initiated site-selective β-C−O bond cleavage with concurrent release of CO₂.

Hydrodeoxygenation and reductive etherification of biomass-derived aldehydes were realized on the same Pd/TiO₂ photocatalyst via photocatalytic transfer hydrogenation using aliphatic alcohols as hydrogen donor, and the selectivity was easily switched by the substrate concentration.

Abstract

The development of selectivity control strategies based on external operational conditions without modification of catalyst is an attractive but challenging issue in photocatalysis. Herein we reported the substrate concentration-switched selectivity in photocatalytic transfer hydrogenation of biomass-derived aromatic aldehydes. With the same catalyst, hydrodeoxygenation was realized at low aldehyde concentration and reductive etherification was realized at high aldehyde concentration. Mechanistic studies revealed that the substrate concentration affected the electron density on catalyst, thus controlled the initial reduction of formyl group and types of intermediates. Competitive adsorption of these intermediates on the catalyst affected hydrogenolysis of C−O bonds in the later stage. This method is easy to handle, and achieved selective hydrogenation of diverse bio-based aldehydes to corresponding deoxygenated products and unsymmetric ether products as potential fuel additives under mild conditions. This work sheds light on the effect of substrate concentration on the selectivity in photocatalysis.

Provided by Nature: Selective biocatalytic defunctionalization reactions enable the utilization of the diverse and rich molecular structure of biobased raw materials made in a renewable way by the biosphere. The selective removal of specific oxygen- and nitrogen-containing functional groups in biobased raw materials by biocatalytic reactions provides new sustainable routes for target compounds.

Abstract

Biobased raw materials, such as carbohydrates, amino acids, nucleotides, or lipids contain valuable functional groups with oxygen and nitrogen atoms. An abundance of many functional groups of the same type, such as primary or secondary hydroxy groups in carbohydrates, however, limits the synthetic usefulness if similar reactivities cannot be differentiated. Therefore, selective defunctionalization of highly functionalized biobased starting materials to differentially functionalized compounds can provide a sustainable access to chiral synthons, even in case of products with fewer functional groups. Selective defunctionalization reactions, without affecting other functional groups of the same type, are of fundamental interest for biocatalytic reactions. Controlled biocatalytic defunctionalizations of biobased raw materials are attractive for obtaining valuable platform chemicals and building blocks. The biocatalytic removal of functional groups, an important feature of natural metabolic pathways, can also be utilized in a systemic strategy for sustainable metabolite synthesis.

Homogeneous catalysis: This Review describes advances in the homogeneously catalyzed conversion of 5-hydroxymethylfurfural (HMF) to several other bio-based building blocks. State of the art in oxidation, reduction, reductive amination, decarbonylation, and Friedel-Craft arylation by metal salts with co-catalysts or molecular complexes as catalytic systems is critically analyzed to draw a perspective from both sustainability and possible development to real application point of view.

Abstract

5-Hydroxymethylfufural (HMF) is an intriguing platform molecule that can be obtained from biomasses and that can lead to the production of a wide range of products, intermediates, or monomers. The presence of different moieties in HMF (hydroxy, aldehyde, furan ring) allows to carry out different transformations such as selective oxidations and hydrogenations, reductive aminations, etherifications, decarbonylations, and acetalizations. This is a great chance in a biorefinery perspective but requires the development of active and highly selective catalysts. In this view, homogeneous catalysis can lead to efficient conversion of HMF at mild reaction conditions. This Review discussed the recent achievements in homogeneous catalysts development and application to HMF transformations. The effects of metal nature, ligands, solvents, and reaction conditions were reported and critically reviewed. Current issues and future chances have been presented to drive future studies toward more efficient and scalable processes.

Ni-catalyzed ligand-controlled tunable cyclization/cross-couplings was developed for the divergent synthesis of pharmacologically important 2-benzazepine frameworks. The bidentate ligand facilitates the formation of 2-benzazepin-5-ones and benzo[c]pyrano[2,3-e]azepines. The tridentate ligand promotes the formation of 2-benzazepin-3-ones.

Abstract

Ligand-directed divergent synthesis can transform common starting materials into distinct molecular scaffolds by simple tuning different ligands. This strategy enables the rapid construction of structurally rich collection of small molecules for biological evaluation and reveals novel modes of catalytic transformation, representing one of the most sought-after challenges in synthetic chemistry. We herein report a Ni-catalyzed ligand-controlled tunable cyclization/cross-couplings for the divergent synthesis of pharmacologically important 2-benzazepine frameworks. The bidentate ligand facilitates the nucleophilic addition of the aryl halides to the amide carbonyl, followed by 1,4-acyl transfer and cross-coupling to obtain 2-benzazepin-5-ones and benzo[c]pyrano[2,3-e]azepines. The tridentate ligand promotes the selective 7-endo cyclization/cross-coupling to access to 2-benzazepin-3-ones. The protocol operates under mild reaction conditions with divergent cyclization patterns that can be easily modulated through the ligand backbone.

Nature Communications, Published online: 07 April 2022; doi:10.1038/s41467-022-29554-4

The Heck reaction for carbon-carbon bond formation is one of the most studied coupling reactions in synthetic organic chemistry. These reactions have been widely used for the synthesis of a diverse range of heterocycles that find application in pharmaceutical and agrochemical industries. This review comprehends the developments in low-cost transition metal-catalyzed Heck couplings covering the literature from 2016 to today.

Abstract

Heck coupling has emerged as an effective and powerful tool for C−C bond formations, with applications in academic and industrial grounds. The conventional Mizoroki-Heck reaction employed expensive Pd based catalysts, however, developments to meet low-cost replacements to Pd is relevant from a sustainable point of view. Heck-type couplings involving cost-effective transition metals such as Fe, Co, Ni, and Cu have been established in the recent decades. Herein, we have comprehended the developments in low-cost transition metal-catalyzed Heck couplings covering the literature from 2016.