Versatile routes for synthesis of diarylamines through acceptorless dehydrogenative aromatization catalysis over supported gold-palladium bimetallic nanoparticles

Dual role of nitroarenes as electrophiles and arylamine surrogates in Buchwald-Hartwig-type coupling for C-N bond construction

One of the most widely utilized methods for the construction of C(sp2)-N bonds is the transition-metal-catalyzed cross-coupling of aryl halides/boronic acids with amines, known as Ullmann condensation, Buchwald-Hartwig amination, and Chan-Lam coupling. However, aryl halides/boronic acids often require multi-step preparation while generating a large amount of corrosive and toxic waste, making the reaction less attractive. Herein, we present an unprecedented method for the C(sp2)-N formation via Buchwald-Hartwig-type reactions using synthetically upstream nitroarenes as the sole starting materials, thus eliminating the need for arylhalides and pre-formed arylamines. A diverse range of symmetrical di- and triarylamines were obtained in a single step from nitroarenes, and more importantly, various unsymmetrical di- and triarylamines were also highly selectively synthesized in a one-pot/two-step process. Furthermore, the success of the scale-up experiments, the late-stage functionalization of a drug intermediate, and the rapid preparation of hole-transporting material TCTA showcased the utility and practicality of this protocol in synthetic chemistry. Mechanistic studies indicate that this transformation may proceed via an arylamine intermediate generated in situ from the reduction of nitroarenes, which is followed by a denitrative Buchwald-Hartwig-type reaction with another nitroarene to form a C-N bond.

Heterogeneously Catalyzed Selective Acceptorless Dehydrogenative Aromatization to Primary Anilines from Ammonia via Concerted Catalysis and Adsorption Control

Although catalytic dehydrogenative aromatization from cyclohexanones and NH₃ is an attractive synthetic method for primary anilines, using a hydrogen acceptor was indispensable to achieve satisfactory levels of selectivity in liquid-phase organic synthetic systems without photoirradiation. In this study, we developed a highly selective synthesis of primary anilines from cyclohexanones and NH₃ via efficient acceptorless dehydrogenative aromatization heterogeneously catalyzed by an Mg(OH)₂-supported Pd nanoparticle catalyst in which Mg(OH)₂ species are also deposited on the Pd surface. The basic sites of the Mg(OH)₂ support effectively accelerate the acceptorless dehydrogenative aromatization via concerted catalysis, suppressing the formation of secondary amine byproducts. In addition, the deposition of Mg(OH)₂ species inhibits the adsorption of cyclohexanones on the Pd nanoparticles to suppress phenol formation, achieving the desired primary anilines with high selectivity.

Green Energy by Hydrogen Production from Water Splitting, Water Oxidation Catalysis and Acceptorless Dehydrogenative Coupling

In this review, we want to explain how the burning of fossil fuels is pushing us towards green energy. Actually, for a long time, we have believed that everything is profitable, that resources are unlimited and there are no consequences. However, the reality is often disappointing. The use of non-renewable resources, the excessive waste production and the abandonment of the task of recycling has created a fragile thread that, once broken, may never restore itself. Metaphors aside, we are talking about our planet, the Earth, and its unique ability to host life, including ourselves. Our world has its balance; when the wind erodes a mountain, a beach appears, or when a fire devastates an area, eventually new life emerges from the ashes. However, humans have been distorting this balance for decades. Our evolving way of living has increased the number of resources that each person consumes, whether food, shelter, or energy; we have overworked everything to exhaustion. Scientists worldwide have already said actively and passively that we are facing one of the biggest problems ever: climate change. This is unsustainable and we must try to revert it, or, if we are too late, slow it down as much as possible. To make this happen, there are many possible methods. In this review, we investigate catalysts for using water as an energy source, or, instead of water, alcohols. On the other hand, the recycling of gases such as CO2 and N2O is also addressed, but we also observe non-catalytic means of generating energy through solar cell production.

Reductive amination of phenol over Pd-based catalysts: elucidating the role of the support and metal nanoparticle size

The Pd-catalyzed reductive amination of phenol is sensitive to the support's nature, and to the atoms' coordination in palladium clusters.

Experimental protocol for the study of One-pot amination of Cyclohexanone-to-secondary amines over Carbon-supported Pd

The validation of protocols for carrying out the experimental analysis of amination reactions is of paramount importance to enhance the scientific knowledge and reproducibility of results. Accordingly, in the present paper, a protocol has been proposed for the study of the amination of cyclohexanone-to-secondary amines (Diphenylamine and N-Cyclohexylaniline) over heterogeneous catalysts. The results of activity and selectivity, and the elucidation of a plausible reaction pathway were described in a parent paper. Therefore, the purpose of this document is to inform about the details of the experimental setups, the methods, and the analytical techniques to identify and quantify the reaction products. Finally, some practical and safety considerations are also included.•

One-pot catalytic amination of cyclohexanone with aniline was performed efficiently in liquid phase on Pd/C.

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Stirring, He atmosphere and temperature control were critical to achieve reproducible activity results.

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Ultra-High Performance Liquid Chromatography allows identifying products and reaction intermediates, while nonane performed well as internal standard for GC-FID quantification.

Coupling without Coupling Reactions: En Route to Developing Phenols as Sustainable Coupling Partners via Dearomatization-Rearomatization Processes

Transition-metal-catalyzed cross-coupling reactions represent one of the most straightforward and efficient protocols to assemble two different molecular motifs for the construction of carbon-carbon or carbon-heteroatom bonds. Because of their importance and wide applications in pharmaceuticals, agrochemicals, materials, etc., cross-coupling reactions have been well recognized in the 2010 Nobel Prize in chemistry. However, in the classical transition-metal-catalyzed cross-coupling reactions (e.g., the Suzuki-Miyaura, the Buchwald-Hartwig, and the Ullmann cross-coupling reactions), organohalides, which mainly stem from the nonrenewable fossil resources, are often utilized as coupling partners with halide wastes being generated after the reactions. To make cross-coupling reactions more sustainable, we initiated a general research program by employing phenols and cyclohexa(e)nones (the reduced forms of phenols) as pivotal feedstocks (coupling partners), instead of the commonly used fossil-derived organohalides, for cross-coupling reactions to build C-O, C-N, and C-C bonds. Phenols (cyclohexa(e)nones) are widely available and can be obtained from lignin biomass, highlighting their renewable and sustainable features. Moreover, water is expected to be the only stoichiometric byproduct, thus avoiding halide wastes.Notably, the cross-coupling reactions utilizing phenols/cyclohexa(e)nones are not based on the traditional transition-metal-catalyzed "oxidative-addition and reductive-elimination" mechanism, but via a novel "phenol-cyclohexanone" redox couple. This new working mechanism opens up new horizons of designing cross-coupling reactions via simple nucleophilic addition of cyclohexanones along with aromatization processes, thereby simplifying the design and avoiding laborious optimization of transition-metal precursors (e.g., Pd, Ni, Cu, etc.), as well as ligands in classical transition-metal-catalyzed cross-coupling reactions. Specifically, in this Account, we will summarize and discuss our related research work in the following three categories: "formal oxidative couplings of cyclohexa(e)nones", "formal reductive couplings of phenols", and "formal redox-neutral couplings of phenols". The successes of these research projects clearly demonstrated our initial inspirations and rational designs to develop cross-coupling reactions without the "conventional cross-coupling conditions" by pushing the reaction frontiers from initial cyclohexanones, ultimately, to the sustainable phenol targets.

Arylation of indoles using cyclohexanones dually-catalyzed by niobic acid and palladium-on-carbons

3-Arylindoles were easily constructed from indoles and cyclohexanone derivatives using a combination of catalytic niobic acid-on-carbon (Nb₂O₅/C) and palladium-on-carbon (Pd/C) under heating conditions without any oxidants. The Lewis acidic Nb₂O₅/C promoted the nucleophilic addition of indoles to the cyclohexanones, and the subsequent dehydration and Pd/C-catalyzed dehydrogenation produced the 3-arylindoles. The additive 2,3-dimethyl-1,3-butadiene worked as a hydrogen acceptor to facilitate the dehydrogenation step.

Synthesis of unsymmetrically substituted triarylamines via acceptorless dehydrogenative aromatization using a Pd/C and p-toluenesulfonic acid hybrid relay catalyst

An efficient and convenient procedure for synthesizing triarylamines based on a dehydrogenative aromatization strategy has been developed. A hybrid relay catalyst comprising carbon-supported Pd (Pd/C) and p-toluenesulfonic acid (TsOH) was found to be effective for synthesizing a variety of triarylamines bearing different aryl groups starting from arylamines (diarylamines or anilines), using cyclohexanones as the arylation sources under acceptorless conditions with the release of gaseous H₂. The proposed reaction comprises the following relay steps: condensation of arylamines and cyclohexanones to produce imines or enamines, dehydrogenative aromatization of the imines or enamines over Pd nanoparticles (NPs), and elimination of H₂ from the Pd NPs. In this study, an interesting finding was obtained indicating that TsOH may promote the dehydrogenation.

An efficient and convenient procedure for synthesizing triarylamines based on a dehydrogenative aromatization strategy has been developed.

Tunable bimetallic Au-Pd@CeO2 for semihydrogenation of phenylacetylene by ammonia borane

The fabrication of a bimetallic core and ceria shell nanostructure is considered a promising way to promote catalytic performance and stability. Here, we report an Au-Pd@CeO2 core-shell structure with a tunable Au/Pd ratio through a self-assembly autoredox reaction approach. This process involves the sequence reduction of Au and Pd precursors and then self-assembly of CeO2 nanoparticles to encapsulate the noble metal core. The as-obtained samples exhibit excellent activity and selectivity towards the ammonia borane initiated hydrogenation of phenylacetylene with an enhanced stability owing to the protection from outside CeO2 nanoparticles. Through the construction of an Au-Pd bimetallic structure, an electron modification of Pd due to charge transfer between Au and Pd results in an enhanced catalytic performance. Such a strategy is promising for the synthesis of other bimetallic noble core and ceria shell structures for further applications.

Transition metal-catalyzed dehydrogenation of amines

This review focuses on the use of homogeneous transition metal complexes for the catalytic dehydrogenation of amines for synthetic purposes, and for hydrogen storage applications. The catalytic dehydrogenation of primary, secondary and cyclic amines is reviewed looking at reaction conditions, different catalysts and common side reactions. Recent developments in this active field of research showcase how cooperative ligands and photocatalysts can overcome the need for noble metals or harsh reaction conditions.

A galvanic exchange process visualized on single silver nanoparticles via dark-field microscopy imaging

The study of the galvanic exchange (GE) mechanism is beneficial for designing and developing new bimetallic nanocrystal structures with excellent bifunctional catalytic properties. Herein, we have visually demonstrated a GE process by real-time monitoring of the reaction between silver nanoparticles (AgNPs) and Au3+ at the single nanoparticle level using light scattering dark-field microscopy imaging. The localized surface plasmon resonance (LSPR) scattering spectral shifts of the AgNPs which reveal the Ag removal rate and Au deposition rate on the surface of the AgNPs can be observed. Furthermore, a pixel meta three color channel method has been introduced for analyzing the scattering light color changes of plasmonic particles to reveal the kinetics of the atomic deposition process on a single AgNP during GE, thus making the reaction kinetics of the GE process directly observable. Therefore, this study provides an efficient and promising approach for understanding the GE mechanism and exploiting its reaction kinetics.

Direct liquid-phase phenol-to-aniline amination using Pd/C

Here, we report the first direct amination of phenol with ammonia into primary anilines in the liquid phase using Pd/C.

Formal arylation of NH3 to produce diphenylamines over supported Pd catalysts

In the presence of Pd/Al₂O₃, various diphenylamines could be synthesized through acceptorless formal arylation using urea as a nitrogen source and cyclohexanones as arylation sources.

A Ni–Mg–Al layered triple hydroxide-supported Pd catalyst for heterogeneous acceptorless dehydrogenative aromatization

In the presence of a Ni–Mg–Al layered triple hydroxide-supported Pd catalyst, the acceptorless dehydrogenative aromatization of a wide range of substrates efficiently proceeded with the liberation of molecular hydrogen.