Heterogeneous manganese-oxide-catalyzed successive cleavage and functionalization of alcohols to access amides and nitriles

Manganese Carbodiimide (MnNCN): A New Heterogeneous Mn Catalyst for the Selective Synthesis of Nitriles from Alcohols

Earth-abundant manganese oxides (MnOx) were competitive candidates when screening catalysts for ammoxidation of alcohols into nitriles due to their redox property. However, over-oxidation and possible acid-catalyzed hydrolysis of nitriles into amides still limited the application of MnOx in nitrile synthesis. In this work, manganese carbodiimide (MnNCN) was first reported to be robust for the ammoxidation of alcohols into nitriles, avoiding over-oxidation and the hydrolysis. Besides the high activity and selectivity, MnNCN demonstrated wide substrate scope including the ammoxidation of primary alcohols into nitriles, the oxidative C-C bonds cleavage and ammoxidation of secondary alcohols, phenyl substituted aliphatic alcohols, and diols into nitriles. Controlled experiments and DFT calculation results revealed that the excellent catalytic performance of MnNCN originated from its high ability in the activation of O2 molecules, and favorable oxidative dehydrogenation of C=N bonds in the aldimine intermediates (RCH=NH) into nitriles, inhibiting the competitive side reaction of the oxidation of aldehydes into carboxylic acids, followed to amide byproducts. Moreover, the hydrolysis of nitriles was also inhibited over MnNCN for its weak acidity as compared with MnOx. This study provided new insights into Mn-catalyzed aerobic oxidations as a highly important complement to manganese oxides.

Enhanced electro-catalysis for methanol oxidation reaction performance by edge defects of ordered mesoporous carbon

Heteroatom-doped carbon materials are widely used to improve the electrocatalytic oxidation of methanol; however, the underlying mechanisms driving this enhancement remain poorly understood. A major challenge lies in developing non-doped carbon supports with tunable intrinsic defect types tailored for metal-based catalysts. In this study, we synthesize a series of ordered mesoporous carbon (OMC) supports with adjustable edge defect densities by varying roasting temperatures and employing a zinc (Zn) evaporation strategy to systematically investigate the impact of edge defects on methanol oxidation reaction (MOR) performance. Theoretical calculations and structural characterizations confirm that the electron metal-support interaction (EMSI) between OMC edge defects and palladium nanoparticles (Pd NPs) effectively modulates the electronic structure of Pd NPs. This modulation not only enhances overall reaction activity and selectivity for the non-CO pathway but also strengthens the anchoring of Pd NPs, leading to superior activity and stability of the Pd/OMC-Zn0.55 catalyst in methanol electrocatalytic oxidation. Notably, after rigorously excluding the influence of various physicochemical properties of the carbon supports, the crucial role of edge defects in improving MOR performance is established. This work provides essential insights into the controlled synthesis of carbon-based catalysts with edge defects and introduces promising strategies for the development of high-performance anode catalysts for direct methanol fuel cells.

Conversion of aromatic methyl ketones to esters and carboxylic acids using o-phthalaldehyde as an oxidant

Herein we describe a two-step conversion of aromatic methyl ketones to esters and carboxylic acids employing o-phthalaldehyde as an oxidant. In the first step, o-phthalaldehyde oxidizes the methyl group to 1-indanone, which acts as a leaving group in a subsequent regioselective retro-Claisen condensation to form esters and carboxylic acids. The mild oxidation conditions ensure the method is applicable to a broad range of substrates. Additionally, the two-step method is operationally simple and scalable, and can also be performed in a single pot.

Accessing monomers from lignin through carbon-carbon bond cleavage

Lignin, the heterogeneous aromatic macromolecule found in the cell walls of vascular plants, is an abundant feedstock for the production of biochemicals and biofuels. Many valorization schemes rely on lignin depolymerization, with decades of research focused on accessing monomers through C-O bond cleavage, given the abundance of β-O-4 bonds in lignin and the large number of available C-O bond cleavage strategies. Monomer yields are, however, invariably lower than desired, owing to the presence of recalcitrant C-C bonds whose selective cleavage remains a major challenge in catalysis. In this Review, we highlight lignin C-C cleavage reactions, including those of linkages arising from biosynthesis (β-1, β-5, β-β and 5-5) and industrial processing (5-CH2-5 and α-5). We examine multiple approaches to C-C cleavage, including homogeneous and heterogeneous catalysis, photocatalysis and biocatalysis, to identify promising strategies for further research and provide guidelines for definitive measurements of lignin C-C bond cleavage.

Closed-loop recyclable polymers: from monomer and polymer design to the polymerization-depolymerization cycle

The extensive utilization of plastic, as a symbol of modern technological society, has consumed enormous amounts of finite and non-renewable fossil resources and produced huge amounts of plastic wastes in the land or ocean, and thus recycling and reuse of the plastic wastes have great ecological and economic benefits. Closed-loop recyclable polymers with inherent recyclability can be readily depolymerized into monomers with high selectivity and purity and repolymerized into polymers with the same performance. They are deemed to be the next generation of recyclable polymers and have captured great and increasing attention from academia and industry. Herein, we provide an overview of readily closed-loop recyclable polymers based on monomer and polymer design and no-other-reactant-involved reversible ring-opening and addition polymerization reactions. The state-of-the-art of circular polymers is separately summarized and discussed based on different monomers, including lactones, thiolactones, cyclic carbonates, hindered olefins, cycloolefins, thermally labile olefin comonomers, cyclic disulfides, cyclic (dithio) acetals, lactams, Diels-Alder addition monomers, Michael addition monomers, anhydride-secondary amide monomers, and cyclic anhydride-aldehyde monomers, and polymers with activatable end groups. The polymerization and depolymerization mechanisms are clearly disclosed, and the evolution of the monomer structure, the polymerization and depolymerization conditions, the corresponding polymerization yield, molecular weight, performance of the polymers, monomer recovery, and depolymerization equipment are also systematically summarized and discussed. Furthermore, the challenges and future prospects are also highlighted.

A Metallomicellar Catalyst for Controlled Oxidation of Alcohols and Lignin Mimics in Water using Open Air as Oxidant

Alcohol groups and β-O-4 (C-C) linkages are widespread in biomass feedstock that are abundant renewable resource for value-added chemicals. The development of sustainable protocols for direct oxidation or oxidative cleavage of feedstock materials in a controlled fashion, using open air as an oxidant is an intellectually stimulating task to produce industrially important value-added carbonyls. Further, the oxidative depolymerization of lignin into fine chemicals has evoked interest in recent times. Herein, we report the first example of a catalyst system that could activate molecular oxygen from atmospheric air for controlled oxidation and oxidative cleavage/depolymerization of feedstock materials such as alcohols, β-O-4 (C-C) linkages and real lignin in water under open air conditions. The selectivity of carbonyl products is controlled by altering the pH between ~7.0 and ~12.0. The current strategy highlights the non-involvement of any external co-catalyst, oxidant, radical additives, and/or destructive organic solvents. The catalyst shows a wide substrate scope and eminent functional group tolerance. The upscaled multigram synthesis using an inexpensive catalyst and easily available oxidant evidences the practical utility of the developed protocol. A plausible mechanism has been proposed with the help of a few controlled experiments, and kinetic and computational studies.

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.

Cobalt nanoparticles-catalyzed aerobic oxygenation and esterification of alkynes via C≡C bonds cleavage

An unprecedented efficient protocol is developed for the oxidative cleavage of C≡C bonds in alkynes to produce structure-diverse esters using heterogeneous cobalt nanoparticles as catalyst with molecular oxygen as the oxidant. A diverse set of mono- and multisubstituted aromatic and aliphatic alkynes can be effectively cleaved and converted into the corresponding esters. Characterization analysis and control experiments indicate high surface area and pore volume, as well as nanostructured nitrogen-doped graphene-layer coated cobalt nanoparticles are possibly responsible for excellent catalytic activity. Mechanistic studies reveal that ketones derived from alkynes under oxidative conditions are formed as intermediates, which subsequently are converted to esters through a tandem sequential process. The catalyst can be recycled up to five times without significant loss of activity.

Oxidative cleavage and ammoxidation of organosulfur compounds via synergistic Co-Nx sites and Co nanoparticles catalysis

The cleavage and functionalization of C-S bonds have become a rapidly growing field for the design or discovery of new transformations. However, it is usually difficult to achieve in a direct and selective fashion due to the intrinsic inertness and catalyst-poisonous character. Herein, for the first time, we report a novel and efficient protocol that enables direct oxidative cleavage and cyanation of organosulfur compounds by heterogeneous nonprecious-metal Co-N-C catalyst comprising graphene encapsulated Co nanoparticles and Co-Nx sites using oxygen as environmentally benign oxidant and ammonia as nitrogen source. A wide variety of thiols, sulfides, sulfoxides, sulfones, sulfonamides, and sulfonyl chlorides are viable in this reaction, enabling access to diverse nitriles under cyanide-free conditions. Moreover, modifying the reaction conditions also allows for the cleavage and amidation of organosulfur compounds to deliver amides. This protocol features excellent functional group tolerance, facile scalability, cost-effective and recyclable catalyst, and broad substrate scope. Characterization and mechanistic studies reveal that the remarkable effectiveness of the synergistic catalysis of Co nanoparticles and Co-Nx sites is crucial for achieving outstanding catalytic performance.

Oxidative Cleavage and Ammoxidation of Unsaturated Hydrocarbons via Heterogeneous Auto-Tandem Catalysis

The oxidative cleavage and functionalization of unsaturated C-C bonds are important processes for synthesis of carbonyl compounds from hydrocarbon feedstocks, yet there has been no report of direct amidation of unsaturated hydrocarbons via an oxidative cleavage of unsaturated C-C bonds with molecular oxygen as an environmentally benign oxidant. Herein, for the first time, we describe a manganese oxide-catalyzed auto-tandem catalysis strategy that enables direct synthesis of amides from unsaturated hydrocarbons by coupling oxidative cleavage with amidation. With oxygen as an oxidant and ammonia as a nitrogen source, a wide range of structurally diverse mono- and multisubstituted activated and unactivated alkenes or alkynes can smoothly undergo unsaturated C-C bond cleavage to deliver one- or multiple-carbon shorter amides. Moreover, a slight modification of the reaction conditions also allows for the direct synthesis of sterically hindered nitriles from alkenes or alkynes. This protocol features excellent functional group tolerance, a broad substrate scope, flexible late-stage functionalization, facile scalability, and a cost-effective and recyclable catalyst. Detailed characterizations reveal that the high activity and selectivity of the manganese oxides are attributed to the large specific surface area, abundant oxygen vacancies, better reducibility, and moderate acid sites. Mechanistic studies and density functional theory calculations indicate that the reaction proceeds through divergent pathways depending on the structure of substrates.

Protocol for the preparation of amorphous manganese oxide and its application as heterogeneous catalyst in the direct synthesis of amides and nitriles

The cleavage and functionalization of carbon-carbon (C–C) bonds has emerged as a powerful tool for preparing value-added chemicals. In this protocol, we describe the preparation of amorphous manganese oxide and its application as a heterogeneous catalyst in the direct synthesis of amides via successive cleavage and amidation of C–C bonds in alcohols. Furthermore, we describe how a slight modification of reaction conditions allows for the cleavage and cyanation of alcohols to access sterically hindered nitriles.

For complete details on the use and execution of this protocol, please refer to He et al. (2022).

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Heterogeneous MnO_x-catalyzed cleavage and amidation/cyanation of alcohols

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Convenient preparation of MnO_x in large scale

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Recycling and reusing of MnO_x

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Wide substrate scope and gram-scale synthesis allowable

Publisher’s note: Undertaking any experimental protocol requires adherence to local institutional guidelines for laboratory safety and ethics.

Copper-Catalyzed Oxidative C-C Bond Cleavage of Alkyl-(Hetero)arenes Enabling Direct Access to Nitriles

The cleavage and functionalization of C-C bonds has emerged as a powerful tool for discovery of new transformations. Herein, we report a protocol that enables direct synthesis of nitriles via copper-catalyzed oxidative cleavage and cyanation of C-C bonds in a wide variety of multicarbon alkyl-substituted (hetero)arenes. Detailed mechanistic studies reveal that a tandem oxidative process is involved in this transformation.