Ni-catalysed acceptorless dehydrogenative aromatisation of cyclohexanones enabled by concerted catalysis specific to supported nanoparticles

The dehydrogenative aromatisation of cyclohexanone derivatives has had a transformative influence on the synthesis of aromatic compounds because functional groups can be easily introduced at desired positions via classic organic reactions without being limited by ortho-, meta- or para-orientations. However, research is still limited on acceptorless dehydrogenative aromatisation, especially with regard to nonprecious-metal catalysts. Ni is a promising candidate catalyst as a congener of Pd, but thermally Ni-catalysed dehydrogenative aromatisation has not been reported even in an oxidative manner because of the difficulty of β-hydride elimination and the fast re-insertion of Ni-H species. Here, we report a CeO2-supported Ni(0) nanoparticle catalyst for acceptorless dehydrogenative aromatisation of cyclohexanone derivatives. This catalyst is widely applicable to various compounds such as cyclohexanols, cyclohexylamines, N-heterocycles, enamines and β-heteroatom-substituted ketones. Through various experiments, we demonstrate that the present reaction was achieved by the concerted catalysis utilizing metal ensembles unique to supported metal nanoparticle catalysts.

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.

Sustainable production of active pharmaceutical ingredients from lignin-based benzoic acid derivatives via “demand orientation”

Catalytic production of several representative active pharmaceutical ingredients (APIs) from lignin.

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.