Development of a New Generation of Gold Catalysts for Amine Oxidation

Ammonolytic transfer dehydrogenation of amines and amides: a versatile method to valorize nitrogen compounds to nitriles

The dehydrogenation of amines has been identified as an efficient method for nitrile synthesis. At present, this approach is restricted to (oxidative) dehydrogenations of primary amines, most often with specialized homogeneous catalysts. In this work, amines were transfer dehydrogenated to nitriles using simple and cheap alkenes (e.g. ethylene or propene) as hydrogen scavengers. The scope was expanded to secondary amines, tertiary amines and even aldehydes. Additional nitrogen is built in from NH3. The versatility of the process was proven by coupling it to the ammonolysis of secondary amides. This enabled us to recycle long-chain polyamides (LCPA) into monomeric compounds, i.e. α,ω-amidonitriles and dinitriles. Reactions were performed with a recyclable heterogeneous Pt catalyst, at 200 °C and with limited addition of NH3 and ethylene. High yields of up to 94% were obtained for the corresponding nitriles.

Texture or Linker? Competitive Patterning of Receptor Assembly toward Ultra-Sensitive Impedimetric Detection of Viral Species at Gold-Nanotextured Titanium Surfaces

In this work, we study the electrodes with a periodic matrix of gold particles pattered by titanium dimples and modified by 3-mercaptopropionic acid (MPA) followed by CD147 receptor grafting for specific impedimetric detection of SARS-CoV-2 viral spike proteins. The synergistic DFT and MM/MD modeling revealed that MPA adsorption geometries on the Au-Ti surface have preferential and stronger binding patterns through the carboxyl bond inducing an enhanced surface coverage with CD147. Control of bonding at the surface is essential for oriented receptor assembling and boosted sensitivity. The complex Au-Ti electrode texture along with optimized MPA concentration is a crucial parameter, enabling to reach the detection limit of ca. 3 ng mL-1. Scanning electrochemical microscopy imaging and quantum molecular modeling were performed to understand the electrochemical performance and specific assembly of MPA displaying a free stereo orientation and not disturbed by direct interactions with closely adjacent receptors. This significantly limits nonspecific interceptor reactions, strongly decreasing the detection of receptor-binding domain proteins by saturation of binding groups. This method has been demonstrated for detecting the SARS virus but can generally be applied to a variety of protein-antigen systems. Moreover, the raster of the pattern can be tuned using various anodizing processes at the titania surfaces.

Trends in Sustainable Synthesis of Organics by Gold Nanoparticles Embedded in Polymer Matrices

Gold nanoparticles (AuNPs) have emerged in recent decades as attractive and selective catalysts for sustainable organic synthesis. Nanostructured gold is indeed environmentally friendly and benign for human health; at the same time, it is active, under different morphologies, in a large variety of oxidation and reduction reactions of interest for the chemical industry. To stabilize the AuNPs and optimize the chemical environment of the catalytic sites, a wide library of natural and synthetic polymers has been proposed. This review describes the main routes for the preparation of AuNPs supported/embedded in synthetic organic polymers and compares the performances of these catalysts with those of the most popular AuNPs supported onto inorganic materials applied in hydrogenation and oxidation reactions. Some examples of cascade coupling reactions are also discussed where the polymer-supported AuNPs allow for the attainment of remarkable activity and selectivity.

Anion-cation synergistic metal-free catalytic oxidative homocoupling of benzylamines by triazolium iodide salts

Triazolium iodide salts are excellent catalysts for the selective oxidative coupling of benzylamines to yield imines. This metal-free reaction proceeds in quantitative spectroscopic yields when run in refluxing 1,2-dichlorobenzene and open to the air. No catalytic activity was observed with related triazolium tetrafluoroborate salts. Variation of catalyst and reaction atmosphere provides mechanistic insights, and revealed dioxygen as the terminal oxidant and the iodine/iodide couple as key redox component in the catalytic dehydrogenation pathway. While molecular iodine is competent as a catalyst in its own right, the triazolium cation triples the reaction rate and reaches turnover frequencies up to 30 h-1, presumably through beneficial interactions of the electron-poor azolium π system and I2, which facilitate the electron transfer from the substrate to iodine and concomitant formation of I-. This acceleration is specific for triazolium cations and represents a hybrid anion/cation catalytic process as a simple and straightforward route towards imine products, with economic advantages over previously reported metal-based catalytic systems.

Efficient photocatalytic oxidative deamination of imine and amine to aldehyde over nitrogen-doped KTi3NbO9 under purple light

Nitrogen doped titanoniobate (N-KTi₃NbO₉) exhibits excellent performance in photocatalytic oxidative deamination of imine and amine to aldehyde.

A hierarchical Nb2O5@NiFe-MMO rod array, fabricated and used as a structured photocatalyst

Recently, using sunlight as a driving force with transitional metal oxides as photocatalysts, due to their unique optical and catalytic properties for organic reactions, has been considered to be a promising strategy in synthetic chemistry. Here, a hierarchically structured photocatalyst, a NiFe mixed metal oxide coated Nb2O5 (denoted as Nb2O5@NiFe-MMO) rod array has been successfully fabricated using Nb foil as a substrate. The Nb2O5 rod array was synthesized by the oxidative etching of Nb metal on the surface of the a substrate. The coating NiFe-MMO was obtained by the calcination of a NiFe layered double hydroxide (NiFe-LDH) precursor via the in situ epitaxial growing technique. The Nb2O5@NiFe-MMO rod array extended the photoresponse light region from ultraviolet light around 400 nm to visible light around 600 nm. With the well-designed architecture and highly dispersed NiO and Fe2O3, the as-prepared photocatalyst exhibited excellent activity and recyclability toward the reaction of aerobic coupling under relatively green conditions, with catalytic efficiency of 228 μmol cm-2 (the area is that of the Ni foil substrate) at 30 °C for 5 h. The present work provides a new strategy for the exploration of excellent structured photocatalysts based on transition metal oxide materials for selective aerobic oxidation of benzylamine to imine.

Ligand controlled switchable selectivity in ruthenium catalyzed aerobic oxidation of primary amines

A ligand controlled catalytic system for the aerobic oxidation of 1° amines to nitriles and imines has been developed where the varying π-acidic feature of BIAN versus phen in the frameworks of ruthenium catalysts facilitates switchable selectivity.

Carbon nanotube-supported Au–Pd alloy with cooperative effect of metal nanoparticles and organic ketone/quinone groups as a highly efficient catalyst for aerobic oxidation of amines

Au–Pd alloy nanoparticles loaded on CNTs with high surface concentrations of ketone/quinone groups are efficient for aerobic oxidation of amines.

Ruthenium-catalyzed selective imine synthesis from nitriles and secondary alcohols under hydrogen acceptor- and base-free conditions

Ruthenium-catalyzed selective imine synthesis from nitriles and secondary alcohols under hydrogen acceptor- and base-free conditions was achieved.

Factors influencing the regioselectivity of the oxidation of asymmetric secondary amines with singlet oxygen

Aerobic amine oxidation is an attractive and elegant process for the α functionalization of amines. However, there are still several mechanistic uncertainties, particularly the factors governing the regioselectivity of the oxidation of asymmetric secondary amines and the oxidation rates of mixed primary amines. Herein, it is reported that singlet-oxygen-mediated oxidation of 1° and 2° amines is sensitive to the strength of the α-C-H bond and steric factors. Estimation of the relative bond dissociation energy by natural bond order analysis or by means of one-bond C-H coupling constants allowed the regioselectivity of secondary amine oxidations to be explained and predicted. In addition, the findings were utilized to synthesize highly regioselective substrates and perform selective amine cross-couplings to produce imines.

Solventless oxidative coupling of amines to imines by using transition-metal-free metal-organic frameworks

A highly efficient, simple, and versatile transition-metal-free metal-organic framework catalytic system is proposed for the oxidative coupling of amines to imines. The catalytic protocol features high activities and selectivities to target products; compatibility with a variety of substrates, including aliphatic amines and secondary amines; and the possibility to efficiently and selectively promote amine cross-coupling reactions. A high stability and recyclability of the catalyst is also observed under the investigated conditions. Insights into the reaction mechanism indicate the formation of a superoxide species able to efficiently promote oxidative couplings.

Highly efficient aerobic oxidation of various amines using Pd3Pb intermetallic compounds as catalysts

Various amines including primary, secondary, benzylic, aliphatic and cyclic ones are converted into imines with high yields using the Pd₃Pb/Al₂O₃ catalyst.

New multicomponent catalysts for the selective aerobic oxidative condensation of benzylamine to N-benzylidenebenzylamine

Nano-oxide domains partially embedded inside the UVM-7 silica walls act as excellent support for Au in the oxidative condensation of benzylamine.

Structural evaluation and catalytic performance of nano-Au supported on nanocrystalline Ce0.9Fe0.1O2−δ solid solution for oxidation of carbon monoxide and benzylamine

Nano-Au supported on nanocrystalline Ce_0.9Fe_0.1O_2−δ solid solution was found to show excellent catalytic performance for both CO oxidation and benzylamine oxidation.

Oxidation of primary amines to oximes with molecular oxygen using 1,1-diphenyl-2-picrylhydrazyl and WO3/Al2O3 as catalysts

The oxidative transformation of primary amines to their corresponding oximes proceeds with high efficiency under molecular oxygen diluted with molecular nitrogen (O2/N2 = 7/93 v/v, 5 MPa) in the presence of the catalysts 1,1-diphenyl-2-picrylhydrazyl (DPPH) and tungusten oxide/alumina (WO3/Al2O3). The method is environmentally benign, because the reaction requires only molecular oxygen as the terminal oxidant and gives water as a side product. Various alicyclic amines and aliphatic amines can be converted to their corresponding oximes in excellent yields. It is noteworthy that the oxidative transformation of primary amines proceeds chemoselectively in the presence of other functional groups. The key step of the present oxidation is a fast electron transfer from the primary amine to DPPH followed by proton transfer to give the α-aminoalkyl radical intermediate, which undergoes reaction with molecular oxygen and hydrogen abstraction to give α-aminoalkyl hydroperoxide. Subsequent reaction of the peroxide with WO3/Al2O3 gives oximes. The aerobic oxidation of secondary amines gives the corresponding nitrones. Aerobic oxidative transformation of cyclohexylamines to cyclohexanone oximes is important as a method for industrial production of ε-caprolactam, a raw material for Nylon 6.

Potential of Gold Nanoparticles for Oxidation in Fine Chemical Synthesis

In recent years supported gold nanoparticles have emerged as efficient catalysts with considerable synthetic potential for liquid-phase oxidation reactions based on molecular oxygen as oxidant. Here we critically review the most attractive applications related to the selective oxidation of functional groups containing O, N, or Si heteroatoms. The reactions include the oxidation of alcohols, aldehydes, and organosilanes; the diverse transformations of amines; benzylic oxidations; and some one-pot multistep reactions starting with alcohol or amine oxidation. In complex liquid-phase transformations relying on bifunctional catalysis, appropriate choice of the support is frequently more important than the size of the gold particles. In some oxidation reactions gold nanoparticles outperform the traditional platinum-group metal catalysts, but the latest results indicate the superiority of bimetallic particles containing gold and platinum, palladium, or rhodium. The environmentally benign nature of the transformations is discussed.