Gold-catalyzed direct hydrogenative coupling of nitroarenes to synthesize aromatic azo compounds

In-situ Oxidation and Coupling of Anilines towards Unsymmetric Azobenzenes Using Flow Chemistry

Molecular switches, especially azobenzenes, are used in numerous applications, such as molecular solar thermal storage (MOST) systems and photopharmacology. The Baeyer-Mills reaction of anilines and nitrosobenzenes has been established as an efficient synthetic method for non-symmetric azobenzenes. However, nitrosobenzenes are not stable, depending on their substitution pattern and pose a health risk. An in-situ oxidation of anilines with Oxone® was optimized under continuous flow conditions avoiding isolation and contact. The in-situ generated nitrosobenzene derivatives were subjected to a telescoped Baeyer-Mills reaction in flow. That way azobenzenes with a broad substituent spectrum were made accessible.

Synergy of Oxygen Vacancies and Base Sites for Transfer Hydrogenation of Nitroarenes on Ceria Nanorods

CeO2 nanorod based catalysts for the base-free synthesis of azoxy-aromatics via transfer hydrogenation of nitroarenes with ethanol as hydrogen donor have been synthesized and investigated. The oxygen vacancies (Ov ) and base sites are critical for their excellent catalytic properties. The Ov , i.e., undercoordinated Ce cations, serve as the sites to activate ethanol and nitroarenes by lowering the energy barrier to transfer hydrogen from α-Csp3 -H in ethanol to the nitro group coupling it to the redox reactions between Ce3+ and Ce4+ . At the same time, the base sites catalyze the condensation step to selectively produce azoxy-aromatics. The catalytic route opens a much improved way to use non-noble metal oxides without additives for the selective functional group reduction and coupling reactions.

Upgrading Waste Polylactide via Catalyst-Controlled Tandem Hydrolysis-Oxidation

As plastic waste pollution continues to pose significant challenges to our environment, it is crucial to develop eco-friendly processes that can transform plastic waste into valuable chemical products in line with the principles of green chemistry. One major challenge is breaking down plastic waste into economically valuable carbon resources. This however presents an opportunity for sustainable circular economies. In this regard, a flexible approach is presented that involves the use of supported-metal catalysts to selectively degrade polylactide waste using molecular oxygen. This protocol has several advantages, including its operation under organic solvent-free and mild conditions, simplicity of implementation, and high atom efficiency, resulting in minimal waste. This approach enables the chemical upcycling of polylactide waste into valuable chemicals such as pyruvic acid, acetic acid, or a mixture containing equimolar amounts of acetic acid and formaldehyde, providing a viable alternative for accessing key value-added feedstocks from waste and spent plastics.

Recent Advances in the Synthesis of Aromatic Azo Compounds

Aromatic azo compounds have -N=N- double bonds as well as a larger π electron conjugation system, which endows aromatic azo compounds with wide applications in the fields of functional materials. The properties of aromatic azo compounds are closely related to the substituents on their aromatic rings. However, traditional synthesis methods, such as the coupling of diazo salts, have a significant limitation with respect to the structural design of aromatic azo compounds. Therefore, many scientists have devoted their efforts to developing new synthetic methods. Moreover, recent advances in the synthesis of aromatic azo compounds have led to improvements in the design and preparation of light-response materials at the molecular level. This review summarizes the important synthetic progress of aromatic azo compounds in recent years, with an emphasis on the pioneering contribution of functional nanomaterials to the field.

Tailored photoenzymatic systems for selective reduction of aliphatic and aromatic nitro compounds fueled by light

The selective enzymatic reduction of nitroaliphatic and nitroaromatic compounds to aliphatic amines and amino-, azoxy- and azo-aromatics, respectively, remains a persisting challenge for biocatalysis. Here we demonstrate the light-powered, selective photoenzymatic synthesis of aliphatic amines and amino-, azoxy- and azo-aromatics from the corresponding nitro compounds. The nitroreductase from Bacillus amyloliquefaciens, in synergy with a photocatalytic system based on chlorophyll, promotes selective conversions of electronically-diverse nitroarenes into a series of aromatic amino, azoxy and azo products with excellent yield (up to 97%). The exploitation of an alternative nitroreductase from Enterobacter cloacae enables the tailoring of a photoenzymatic system for the challenging synthesis of aliphatic amines from nitroalkenes and nitroalkanes (up to 90% yield). This photoenzymatic reduction overcomes the competing bio-Nef reaction, typically hindering the complete enzymatic reduction of nitroaliphatics. The results highlight the usefulness of nitroreductases to create selective photoenzymatic systems for the synthesis of precious chemicals, and the effectiveness of chlorophyll as an innocuous photocatalyst, enabling the use of sunlight to drive the photobiocatalytic reactions.

Magnetically Separable Visible Light-Active Ag0.75Ni0.25 Binary Alloy Nanoparticles as a Highly Efficient Photocatalyst for the Selective Oxidative Coupling of Aniline to Azobenzene

Aniline wastes can be converted to useful pharmaceutical and industrial compounds like azobenzene. For this purpose, a bimetallic Ag_0.75Ni_0.25 alloy is designed in the nanoscale range resembling a fivefold twinned morphology using water as the solvent. These newly developed alloy nanoparticles (NPs) are employed for the first time as an efficient visible light-active photocatalyst for the oxidative homocoupling of aniline to azobenzene. Our catalytic protocol is highly sustainable for a large number of aniline substrates with a high yield of the product (up to 95%), which might be attributed to the combinational and superior properties achieved on alloy formation in comparison to the monometallic counterparts. High-electron density amines (p-anisidine) display greater photocatalytic proficiency than that of low-electron density amines (4-fluoroaniline). The developed photocatalyst is magnetically well-separable and can be reused for at least five catalytic cycles without appreciable loss in its activity.

Modifying electron injection kinetics for selective photoreduction of nitroarenes into cyclic and asymmetric azo compounds

Modifying the reactivity of substrates by encapsulation is essential for microenvironment catalysts. Herein, we report an alternative strategy that modifies the entry behaviour of reactants into the microenvironment and substrate inclusion thermodynamics related to the capsule to control the electron injection kinetics and the selectivity of products from the nitroarenes photoreduction. The strategy includes the orchestration of capsule openings to control the electron injection kinetics of electron donors, and the capsule’s pocket to encapsulate more than one nitroarene molecules, facilitating a condensation reaction between the in situ formed azanol and nitroso species to produce azo product. The conceptual microenvironment catalyst endows selective conversion of asymmetric azo products from different nitroarenes, wherein, the estimated diameter and inclusion Gibbs free energy of substrates are used to control and predict the selectivity of products. Inhibition experiments confirm a typical enzymatic conversion, paving a new avenue for rational design of photocatalysts toward green chemistry.

Polar hydrogen species mediated nitroarenes selective reduction to anilines over an [FeMo]Sx catalyst

We herein present an efficient approach for the chemoselective synthesis of arylamines from nitroarenes and hydrazine over an iron-molybdenum sulfide catalyst ([FeMo]S_x). The heterogeneous hydrogen transfer reduction can be efficiently carried out at 30 °C and provides anilines with 95-99% selectivities. The in situ gas product analysis demonstrates that [FeMo]S_x can catalyze the decomposition of N₂H₄ to H* species, not H₂. Combining with the kinetic analysis of the aniline generation rates from nitrobenzene and intermediates, the nitro group reduction to the nitroso group is confirmed to be the rate-determining step. The positive slope of Hammett's equation suggests that the critical intermediate in the rate-determining step is in the negative state, which suggests that the active H* should be in polar states (H^δ- and H^δ+). These findings will provide a novel route for the synthesis of substituted anilines and broaden the application of MoS_x catalysts under mild conditions.

1T-MoS2 catalysed reduction of nitroarenes and a one-pot synthesis of imines

An expedient synthesis of aromatic amines and imines via the reduction of nitroaromatics using 1T-MoS2 as a heterogeneous catalyst.

Oxidative dehydrogenation of hydrazines and diarylamines using a polyoxomolybdate-based iron catalyst

We report an efficient method for the oxidative dehydrogenation of hydrazines and diarylamines in aqueous ethanol using Anderson-type polyoxomolybdate-based iron(iii) as a catalyst and hydrogen peroxide as an oxidant. A series of azo compounds and tetraarylhydrazines were obtained in moderate to excellent yields. The reaction conditions and substrate scopes are complementary or superior to those of more established protocols. In addition, the catalyst shows good stability and reusability in water. The preliminary mechanistic studies suggest that a radical process is involved in the reaction.

Invisible Silver Guests Boost Order in a Framework That Cyclizes and Deposits Ag3Sb Nanodots

The infusion of metal guests into (i.e., metalating) the porous medium of metal-organic frameworks (MOFs) is a topical approach to wide-ranging functionalization purposes. We report the notable interactions of AgSbF₆ guests with the designer MOF host ZrL1 [Zr₆O₄(OH)₇(L1)_4.5(H₂O)₄]. (1) The heavy-atom guests of AgSbF₆ induce order in the MOF host to allow the movable alkyne side arm to be fully located by X-ray diffraction, but they themselves curiously remain highly disordered and absent in the strucutral model. The enhanced order of the framework can be generally ascribed to interaction of the silver guests with the host alkyne and thioether functions, while the invisible heavy-atom guest represents a new phenomenon in the metalation of open framework materials. (2) The AgSbF₆ guests also participate in the thermocyclization of the vicinal alkyne units of the L1 linker (at 450 °C) and form the rare nanoparticle of Ag₃Sb supported on the concomitantly formed nanographene network. The resulted composite exhibits high electrical conductivity (1.0 S/cm) as well as useful, mitigated catalytic activity for selectively converting nitroarenes into the industrially important azo compounds, i.e., without overshooting to form the amine side products. The heterogeneous/cyclable catalysis entails only the cheap reducing reagents of NaBH₄, ethanol, and water, with yields being generally close to 90%.

Direct Oxidative Azo Coupling of Anilines Using a Self-Assembled Flower-like CuCo2O4 Material as a Catalyst under Aerobic Conditions

Herein, we report the synthesis of a self-assembled flower-like CuCo₂O₄ material by the oxalate decomposition method. The crystalline structure and morphology of the material have been analyzed by powder X-ray diffraction, Raman spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray measurement techniques. The self-assembled flower-like CuCo₂O₄ material showed remarkable catalytic activity in the direct aerobic oxidative azo coupling of anilines under oxidant and other additive-free reaction conditions. The mechanistic insight of CuCo₂O₄ in the oxidative azo coupling reaction has been established by density functional theory calculations, which disclosed that the absorption and dissociation of areal oxygen preferentially take place at the Cu site and dissociation of aniline takes place at the Co site. Thus, the Cu and Co sites of CuCo₂O₄ exert a cooperative effect on the direct oxidative azo coupling reactions through the selective activation of anilines and aerobic oxygen. The CuCo₂O₄ material was recovered from the reaction mixture and reused for at least eight runs without appreciable loss of catalytic activity.

A Self-Polymerized Nitro-Substituted Conjugated Carbonyl Compound as High-Performance Cathode for Lithium-Organic Batteries

Conjugated carbonyl compounds have received much attention as cathode materials for developing green lithium-ion batteries (LIBs). However, their high dissolution and poor electronic conductivity in organic electrolyte restrict their further application. Herein, a self-polymerized nitro-substituted conjugated carbonyl compound (2,7-dinitropyrene-4,5,9,10-tetraone, PT-2 NO₂ ) is applied as a high-performance cathode material for LIBs. PT-2 NO₂ exhibits a high reversible capacity of 153.9 mAh g^-1 at 50 mA g^-1 after 120 cycles, which is higher than that of other substituted compounds. Detailed characterization and theoretical calculations have testified that PT-2 NO₂ is transformed into an azo polymer through an irreversible reductive coupling reaction in the first discharge process, and then carbonyl and azo groups reversibly react with Li ions in subsequent cycles. In addition, this azo polymer is also synthesized and applied as the electrode material, which shows similar electrochemical performance to PT-2 NO₂ but with higher initial coulombic efficiency. Thus, this work provides a simple but effectively way to construct organic cathode materials with multiple redox sites for green and high-performance LIBs.

Highly Selective Synthesis of Hydrazoarenes from Nitroarenes via Polystyrene-Supported Au-Nanoparticle-Catalyzed Reduction: Application to Azoarenes, Aminoarenes, and 4,4'-Diaminobiaryls

A selective synthesis of hydrazoarene from nitroarene and its application are reported. Using polystyrene (PS) resins as solid supports for Au nanoparticles (AuNPs), polystyrene-supported Au nanoparticles (AuNPs@PS) were synthesized and characterized. In the presence of AuNPs@PS (1.0 mol %) as a catalyst, nitroarenes afforded corresponding hydrazoarenes (up to 99%) with high selectivity (up to 100%) under mild reaction conditions (NaBH₄, 50% aq. EtOH, and room temperature). Depending on the reaction conditions (the amount of NaBH₄, the substituent of nitroarenes, and the sequential addition of HCl), nitroarenes were converted to corresponding azoarenes (up to 95%), aminoarenes (up to 99%), and 4,4'-diaminobiaryls (up to 99%). Our easily recyclable catalytic system using a solid-phase reaction vessel provides an attractive synthetic method in an eco-friendly and sustainable manner.

Effects of Preparation Parameters of NiAl Oxide-Supported Au Catalysts on Nitro Compounds Chemoselective Hydrogenation

Effects of preparation parameters of NiAl oxide-supported Au nanocatalysts on their performance in the chemoselective hydrogenation of nitro compounds were investigated. The deposition-precipitation method, low Au loading, and low Ni/Al molar ratio of the support contributed to the generation of small-sized Au nanoparticles. High catalytic properties were related to the small sizes of Au particles and appropriate basicity of supports. Accordingly, the 0.43% Au/NiAlO-2-500 (the Ni/Al molar ratio of the support = 2) showed high activity and excellent selectivity for nitro hydrogenation. It also showed good versatility for other nitro compounds and good recyclability. Interestingly, for the first time, this Au catalyst switched its selectivity to vinyl hydrogenation by mere regulation of the composition of the support (the Ni/Al molar ratio of the support = 4). The observed shift in selectivity was ascribed to the different adsorption behaviors of the nitro and vinyl group on Au nanocatalysts. It provides a novel and facile strategy to construct Au nanocatalysts with high activity and switchable selectivity for hydrogenation of nitro compounds by the fine tuning of preparation parameters.

Versatile Applications of Metal Single-Atom @ 2D Material Nanoplatforms

Recently, emerging 2D material-supported metal single-atom catalysts (SACs) are receiving enormous attention in heterogeneous catalysis. Due to their well-defined, precisely located metal centers, unique metal-support interaction and identical coordination environment, these catalysts serve as excellent models for understanding the fundamental issues in catalysis as well as exhibiting intriguing practical applications. Understanding the correlations between metal-support combinations and the catalytic performance at the atomic level can be achieved on the SACs@2D materials nanoplatforms. Herein, recent advances of metal SACs on various types of 2D materials are reviewed, especially their exciting applications in the fields of chemicals, energy, and the environment. Based on the summary and perspectives, this work should contribute to the rational design of perfect metal SACs with versatile properties.

Interface-tuned selective reductive coupling of nitroarenes to aromatic azo and azoxy: a first-principles-based microkinetics study

It is of great importance to regulate a catalyst to control its selectivity. In general, the Pt catalyzed hydrogenation of nitrobenzene (PhNO₂) would produce aniline.

Superfine CoNi alloy embedded in Al2O3 nanosheets for efficient tandem catalytic reduction of nitroaromatic compounds by ammonia borane

Tunable Co_xNi_1−x/Al₂O₃ nanocatalysts have been prepared and used for the efficient tandem catalytic dehydrogenation of ammonia borane and hydrogenation of nitroaromatics.

Atomic engineering of high-density isolated Co atoms on graphene with proximal-atom controlled reaction selectivity

Controllable synthesis of single atom catalysts (SACs) with high loading remains challenging due to the aggregation tendency of metal atoms as the surface coverage increases. Here we report the synthesis of graphene supported cobalt SACs (Co₁/G) with a tuneable high loading by atomic layer deposition. Ozone treatment of the graphene support not only eliminates the undesirable ligands of the pre-deposited metal precursors, but also regenerates active sites for the precise tuning of the density of Co atoms. The Co₁/G SACs also demonstrate exceptional activity and high selectivity for the hydrogenation of nitroarenes to produce azoxy aromatic compounds, attributable to the formation of a coordinatively unsaturated and positively charged catalytically active center (Co–O–C) arising from the proximal-atom induced partial depletion of the 3d Co orbitals. Our findings pave the way for the precise engineering of the metal loading in a variety of SACs for superior catalytic activities.

Controllable synthesis of single atom catalysts with sufficiently high metal loading remains challenging due to the tendency of agglomeration. Here the authors synthesize a series of stable atomically dispersed cobalt atoms on graphene with high Co loadings via the regeneration of active sites by atomic layer deposition.

One-pot synthesis of Schiff base compounds via photocatalytic reaction in the coupled system of aromatic alcohols and nitrobenzene using CdIn2S4 photocatalyst

The use of solar energy to drive organic reactions under mild conditions provides a sustainable pathway for green synthesis and has been one of the primary goals pursued by scientists. In this research, the cadmium indium sulfide (CdIn2S4) photocatalyst was prepared using a simple solvothermal method and was thoroughly characterized using X-ray powder diffraction, UV-visible absorption spectra, nitrogen adsorption-desorption isotherms, scanning electron microscopy, transmission electron microscopy and X-ray spectroscopy measurements. The photocatalytic performance of the CdIn2S4 photocatalyst was evaluated using photocatalytic synthesis of Schiff base compounds in a coupled system of aromatic alcohols and nitrobenzene under visible light irradiation. The yield of N-benzylideneaniline reached up to 32% in the coupled system of benzyl alcohol and nitrobenzene under visible light illumination for 8 h. Furthermore, the changes for the amounts of aromatic aldehydes and AL as intermediate products during the photocatalytic process were also investigated. Using isotopic tracing, a possible reaction mechanism for the photocatalytic synthesis of N-benzylideneaniline and the redox reactions in the coupled system of benzyl alcohol and nitrobenzene was proposed. It is hoped that this strategy can provide an effective pathway for the traditional organic synthesis and transformation using photocatalytic technology under mild conditions.

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results obtained from different systems and try to give a picture on how different types of metal species work in different reactions and give perspectives on the future directions toward better understanding of the catalytic behavior of different metal entities (single atoms, nanoclusters, and nanoparticles) in a unifying manner.

Single-site catalyst promoters accelerate metal-catalyzed nitroarene hydrogenation

Atomically dispersed supported metal catalysts are drawing wide attention because of the opportunities they offer for new catalytic properties combined with efficient use of the metals. We extend this class of materials to catalysts that incorporate atomically dispersed metal atoms as promoters. The catalysts are used for the challenging nitroarene hydrogenation and found to have both high activity and selectivity. The promoters are single-site Sn on TiO₂ supports that incorporate metal nanoparticle catalysts. Represented as M/Sn-TiO₂ (M = Au, Ru, Pt, Ni), these catalysts decidedly outperform the unpromoted supported metals, even for hydrogenation of nitroarenes substituted with various reducible groups. The high activity and selectivity of these catalysts result from the creation of oxygen vacancies on the TiO₂ surface by single-site Sn, which leads to efficient, selective activation of the nitro group coupled with a reaction involving hydrogen atoms activated on metal nanoparticles.

Understanding of the structures and roles of catalyst promoters markedly lags behind the understanding of the structures and roles of catalytic sites. Here, the authors address this challenge by incorporating a single-site promoter—tin—on a TiO₂ surface to enhance the catalytic activity of various metals on the TiO₂ in selective hydrogenation of nitroarenes.

Highly efficient aqueous phase reduction of nitroarenes catalyzed by phosphine-decorated polymer immobilized ionic liquid stabilized PdNPs

Phosphino-decorated polymer immobilised ionic liquid-stabilised PdNPs are highly efficient catalysts for the aqueous phase hydrogenation and transfer hydrogenation of aromatic nitro compounds in batch and continuous flow.