Highly chemoselective reduction of aromatic nitro compounds by copper nanoparticles/ammonium formate

Waste-Derived Copper Flakes for Solvent-Free Reductive Acetamidation of Nitroarenes

Herein, waste-derived copper (Cu) flakes have been used as heterogeneous catalysts for the solvent-free and one-pot reductive acetamidation of nitroarenes. Metallic copper flakes (f-ZCu) were isolated from waste copper Cu scrap/flakes/turnings generated after the grinding and cutting (from the Cu industries). f-ZCu is being used to synthesize acetanilide with a considerable yield (∼82%) in one-step and solvent-free conditions within a reaction time of 6 h. Moreover, the same procedure is also being utilized for producing various substrates (9), including the gram-scale synthesis of the well-known important antipyretic drug, i.e., paracetamol. The plausible mechanism for the reaction was also proposed based on the spectroscopic analyses of spent f-ZCu.

Blue-LED activated photocatalytic hydrogenation of nitroarenes with Cu2O/CuO heterojunctions

This study describes how the optimization of Cu2O/CuO heterostructures can enhance their (photo)catalytic performance. More specifically, the evaluation of catalysts with different Cu2O/CuO molar ratios was used to optimize their performance for the hydrogenation of 4-nitrophenol under both blue-LED light and dark conditions. For the first time, we analyzed the effect of blue LED irradiation on this reaction and found that when blue LEDs are used as the light source, a Cu2O/CuO ratio of 0.15 results in rate constants 7 to 3 times higher than those of catalysts with either lower (0.01) or higher (0.42) ratios. Furthermore, this photocatalyst shows good stability, >70% after 5 cycles, and excellent chemoselectivity in the selective reduction of the nitro group in the presence of other functionalities, i.e. -COOH, -CONH2 and -OH.

Copper nanoparticles catalyzed carbon–heteroatom bond formation and synthesis of related heterocycles by greener procedures

A variety of procedures for the carbon–nitrogen, carbon–oxygen, carbon–sulfur and carbon–selenium bond formation using copper nanoparticles in greener conditions have been highlighted. The synthesis of several heterocyclic compounds of biological importance has also been reported using these protocols.

Polymer microgels for the stabilization of gold nanoparticles and their application in the catalytic reduction of nitroarenes in aqueous media

Polymer microgels containing a polystyrene core and poly(N-isopropylmethacrylamide) shell were synthesized in aqueous media following a free radical precipitation polymerization. Au nanoparticles were fabricated into the shell region of the core–shell microgels denoted as P(STY@NIPM) by the in situ reduction of chloroauric acid with sodium borohydride. Various characterization techniques such as transmission electron microscopy (TEM), ultraviolet–visible spectroscopy (UV-visible) and Fourier transform infrared spectroscopy (FTIR) were used for the characterization of Au–P(STY@NIPM). The catalytic potential of Au–P(STY@NIPM) toward the reductive reaction of 4-nitrophenol (4NP) under various reaction conditions was evaluated. The Arrhenius and Eyring parameters for the catalytic reduction of 4NP were determined to explore the process of catalysis. A variety of nitroarenes were converted successfully into their corresponding aminoarenes with good to excellent yields in the presence of the Au–P(STY@NIPM) system using NaBH₄ as a reductant. The Au–P(STY@NIPM) system was found to be an efficient and recyclable catalyst with no significant loss in its catalytic efficiency.

A core–shell microgel system was synthesized and used as a micro-reactor for the synthesis of gold nanoparticles. The resulting hybrid system has the ability to catalyze the reduction of various nitroarenes in aqueous media.

Transition-Metal-Free Transfer Hydrogenative Cascade Reaction of Nitroarenes with Amines/Alcohols: Redox-Economical Access to Benzimidazoles

This report describes an efficient transition-metal-free process toward the transfer hydrogenative cascade reaction between nitroarenes and amines or alcohols. The developed redox-economical approach was realized using a combination of KO^tBu and Et₃SiH as reagents, which allows the synthesis of benzimidazole derivatives via σ-bond metathesis. The reaction conditions hold well over a wide range of substrates embedded with diverse functional groups to deliver the desired products in good to excellent yields. The mechanistic proposal has been depicted on the basis of a series of control experiments, mass spectroscopic evidence which is well supported by density functional theory (DFT) calculations with a feasible energy profile.

Metal-based Heterogeneous Catalysts for One-Pot Synthesis of Secondary Anilines from Nitroarenes and Aldehydes

Recently, N-substituted anilines have been the object of increasing research interest in the field of organic chemistry due to their role as key intermediates for the synthesis of important compounds such as polymers, dyes, drugs, agrochemicals and pharmaceutical products. Among the various methods reported in literature for the formation of C-N bonds to access secondary anilines, the one-pot reductive amination of aldehydes with nitroarenes is the most interesting procedure, because it allows to obtain diverse N-substituted aryl amines by simple reduction of nitro compounds followed by condensation with aldehydes and subsequent reduction of the imine intermediates. These kinds of tandem reactions are generally catalyzed by transition metal-based catalysts, mainly potentially reusable metal nanoparticles. The rapid growth in the last years in the field of metal-based heterogeneous catalysts for the one-pot reductive amination of aldehydes with nitroarenes demands for a review on the state of the art with a special emphasis on the different kinds of metals used as catalysts and their recyclability features.

Novel g-C3N4/Fe-ZnO/RGO nanocomposites with boosting visible light photocatalytic activity for MB, Cr (VI), and outstanding catalytic activity toward para-nitrophenol reduction

A novel g-C₃N₄/Fe-ZnO/RGO nanocomposite has been synthesized using the facile solvothermal method to boost the catalytic efficiency of ZnO. The structure and morphology of nanocomposites were examined by a wide range of characterization methods. The obtained g-C₃N₄/Fe-ZnO/RGO nanocomposite (Z-scheme heterostructure) exhibits improved photocatalytic activity toward the photodegradation of MB, Cr (VI) under visible-light irradiation and 4-nitrophenol reduction. The enhancement in activity of nanocomposite is ascribed to a unique heterostructure system, which facilitates excellent transport and separation of the photogenerated charge carrier, resulting in prolonged lifetime leading to continuous generation of reactive species. Moreover, the synergistic effect on the interface of ZnO and g-C₃N₄ and the introduction of reduced graphene oxide (RGO) serve as a booster for charge separation at the Z-scheme, which ultimately speeds up the degradation of pollutants. The present study provides a novel and facile approach for establishing an efficient nanocomposite for environmental remediation.

Glycerol: An Optimal Hydrogen Source for Microwave-Promoted Cu-Catalyzed Transfer Hydrogenation of Nitrobenzene to Aniline

The search for sustainable alternatives for use in chemical synthesis and catalysis has found an ally in non-conventional energy sources and widely available green solvents. The use of glycerol, an abundant natural solvent, as an excellent “sacrificial” hydrogen source for the copper-catalyzed microwave (MW)-promoted transfer hydrogenation of nitrobenzene to aniline has been investigated in this work. Copper nanoparticles (CuNPs) were prepared in glycerol and the efficacy of the glycerol layer in mediating the interaction between the metal active sites has been examined using HRTEM analyses. Its high polarity, low vapor pressure, long relaxation time, and high acoustic impedance mean that excellent results were also obtained when the reaction media was subjected to ultrasound (US) and MW irradiation. US has been shown to play an important role in the process via its ability to enhance CuNPs dispersion, favor mechanical depassivation and increase catalytic active surface area, while MW irradiation shortened the reaction time from some hours to a few minutes. These synergistic combinations promoted the exhaustive reduction of nitrobenzene to aniline and facilitated the scale-up of the protocol for its optimized use in industrial MW reactors.

Reduction of nitroarenes catalyzed by microgel-stabilized silver nanoparticles

Poly(N-isopropylacrylamide-co-acrylamide) (PNA-BIS-2) microgels were synthesized by free radical precipitation polymerization in aqueous medium. Spherical Ag nanoparticles with diameter of 10-20 nm were fabricated inside the PNA-BIS-2 microgels by in-situ reduction of silver nitrate using sodium borohydride as reducing agent. The Ag nanoparticles- loaded hybrid microgels were characterized by Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Energy dispersive X-ray (EDX), Scanning transmission electron microscopy (STEM), Ultraviolet visible spectroscopy (UV Visible), Thermogravimetric analysis (TGA) and X-ray diffraction (XRD). Ag contents in the hybrid system were determined by inductively coupled plasma - optical emission spectrometry (ICP-OES). Various nitroarenes were successfully converted into their respective aromatic amines with good to excellent yields (ranging from 75% to 97%) under mild reaction conditions. The catalyst has ability to successfully convert substituted nitroarenes into desired products keeping many functionalities intact. The catalyst can be stored for long time without any sign of aggregation and can be used multiple times without any significant loss in its catalytic activity.

ZnNb2O6 fibre surface as an efficiently product-selective controller for the near-UV-light-induced nitrobenzene reduction reaction

A high aniline yield was achieved by the combination of near-UV light as the driving force of nitrobenzene reduction and ZnNb₂O₆ surface as the product-selective controller.

Reduction of Nitro Compounds Using 3d-Non-Noble Metal Catalysts

The reduction of nitro compounds to the corresponding amines is one of the most utilized catalytic processes in the fine and bulk chemical industry. The latest development of catalysts with cheap metals like Fe, Co, Ni, and Cu has led to their tremendous achievements over the last years prompting their greater application as "standard" catalysts. In this review, we will comprehensively discuss the use of homogeneous and heterogeneous catalysts based on non-noble 3d-metals for the reduction of nitro compounds using various reductants. The different systems will be revised considering both the catalytic performances and synthetic aspects highlighting also their advantages and disadvantages.

Metal-Free Reduction of Nitrobenzene to Aniline in Subcritical Water

An environmentally friendly approach for the reduction of nitrobenzene to aniline promoted by carbonaceous bio-based materials was successfully achieved under subcritical water conditions. The proposed methodology features a metal-free process, no-hydrogen input as reductor, the use of commercial bio-based carbon materials having low cost and availability, and water as green solvent under subcritical conditions. Using optimized conditions, reduction of nitrobenzene in the presence of commercial NORIT GAC 12-40 or DACARB PC1000 was accomplished at 310 °C for 6 h and quantitatively furnished the target aniline. Treatment of NORIT GAC 12-40 with KOH allowed to decrease charcoal loading (6 g vs 40 g) and increase aniline yields (80% vs 66%).

An alkaline-resistant Ag(i)-anchored pyrazolate-based metal–organic framework for chemical fixation of CO2

An alkaline-resistant Ag(i)-anchored metal–organic framework has been achieved via postsynthetic modification of pyrazolate-based linkers.

Nitrogen-doped graphene-activated metallic nanoparticle-incorporated ordered mesoporous carbon nanocomposites for the hydrogenation of nitroarenes

Herein, nanoscale metallic nanoparticle-incorporated ordered mesoporous carbon catalysts activated by nitrogen-doped graphene (NGr) were fabricated via an efficient multi-component co-assembly of a phenolic resin, nitrate, acetylacetone, the nitrogen-containing compound 1,10-phenanthroline, and Pluronic F127, followed by carbonization. The obtained well-dispersed nitrogen-doped graphene-activated transition metal nanocatalysts possess a 2-D hexagonally arranged pore structure with a high surface area (∼500 m² g⁻¹) and uniform pore size (∼4.0 nm) and show excellent activity for the selective hydrogenation–reduction of substituted nitroarenes to anilines in an environmentally friendly aqueous solution. The high catalytic performance and durability is attributed to the synergistic effects among the components, the unique structure of the nitrogen-doped graphene layer-coated metallic nanoparticles, and electronic activation of the doped nitrogen.

The non-noble M/NGr@OMC-T catalyst exhibits excellent catalytic activity and chemoselectivity for the hydrogenation of nitroarenes to anilines in water with hydrogen.

Surprisingly high sensitivity of copper nanoparticles toward coordinating ligands: consequences for the hydride reduction of benzaldehyde

Depending on the ligand, ligand-induced leaching of copper nanoparticles may produce catalytically active species for the reduction of benzaldehyde.

Large-scale preparation of B/N co-doped graphene-like carbon as an efficient metal-free catalyst for the reduction of nitroarenes

Boron and nitrogen co-doped graphene-like carbon catalysts were fabricated by mechanochemistry and demonstrated outstanding catalytic activity for the reduction of nitroarenes.