Homogeneous Transition Metal Catalysis of Acceptorless Dehydrogenative Alcohol Oxidation: Applications in Hydrogen Storage and to Heterocycle Synthesis

Mechanistic Investigations of Ruthenium Catalyzed Dehydrogenative Thioester Synthesis and Thioester Hydrogenation

We have recently reported the previously unknown synthesis of thioesters by coupling thiols and alcohols (or aldehydes) with liberation of H₂, as well as the reverse hydrogenation of thioesters, catalyzed by a well-defined ruthenium acridine-9H based pincer complex. These reactions are highly selective and are not deactivated by the strongly coordinating thiols. Herein, the mechanism of this reversible transformation is investigated in detail by a combined experimental and computational (DFT) approach. We elucidate the likely pathway of the reactions, and demonstrate experimentally how hydrogen gas pressure governs selectivity toward hydrogenation or dehydrogenation. With respect to the dehydrogenative process, we discuss a competing mechanism for ester formation, which despite being thermodynamically preferable, it is kinetically inhibited due to the relatively high acidity of thiol compared to alcohol and, accordingly, the substantial difference in the relative stabilities of a ruthenium thiolate intermediate as opposed to a ruthenium alkoxide intermediate. Accordingly, various additional reaction pathways were considered and are discussed herein, including the dehydrogenative coupling of alcohol to ester and the Tischenko reaction coupling aldehyde to ester. This study should inform future green, (de)hydrogenative catalysis with thiols and other transformations catalyzed by related ruthenium pincer complexes.

Surface and Interface Engineering: Molybdenum Carbide-Based Nanomaterials for Electrochemical Energy Conversion

Molybdenum carbide (Mo_x C)-based nanomaterials have shown competitive performances for energy conversion applications based on their unique physicochemical properties. A large surface area and proper surface atomic configuration are essential to explore potentiality of Mo_x C in electrochemical applications. Although considerable efforts are made on the development of advanced Mo_x C-based catalysts for energy conversion with high efficiency and stability, some urgent issues, such as low electronic conductivity, low catalytic efficiency, and structural instability, have to be resolved in accordance with their application environments. Surface and interface engineering have shown bright prospects to construct highly efficient Mo_x C-based electrocatalysts for energy conversion including the hydrogen evolution reaction, oxygen evolution reaction, nitrogen reduction reaction, and carbon dioxide reduction reaction. In this Review, the recent progresses in terms of surface and interface engineering of Mo_x C-based electrocatalytic materials are summarized, including the increased number of active sites by decreasing the particle size or introducing porous or hierarchical structures and surface modification by introducing heteroatom(s), defects, carbon materials, and others electronic conductive species. Finally, the challenges and prospects for energy conversion on Mo_x C-based nanomaterials are discussed in terms of key performance parameters for the catalytic performance.

Homogeneous Reforming of Aqueous Ethylene Glycol to Glycolic Acid and Pure Hydrogen Catalyzed by Pincer-Ruthenium Complexes Capable of Metal-Ligand Cooperation

Glycolic acid is a useful and important α-hydroxy acid that has broad applications. Herein, the homogeneous ruthenium catalyzed reforming of aqueous ethylene glycol to generate glycolic acid as well as pure hydrogen gas, without concomitant CO₂ emission, is reported. This approach provides a clean and sustainable direction to glycolic acid and hydrogen, based on inexpensive, readily available, and renewable ethylene glycol using 0.5 mol % of catalyst. In-depth mechanistic experimental and computational studies highlight key aspects of the PNNH-ligand framework involved in this transformation.

NHC ligand-based half-sandwich iridium complexes: synthesis, structure and catalytic activity in acceptorless dehydrogenation and transfer hydrogenation

Half-sandwich iridium complexes exhibited high catalytic activity for acceptorless dehydrogenation of alcohols and transfer hydrogenation of ketones.

Low-temperature hydrogen production from methanol over a ruthenium catalyst in water

Efficient conversion of methanol to hydrogen gas and formate with an appreciably high TOF and TON is achieved over the in situ generated ruthenium catalyst in water at low temperature.

Recent advances in pincer-nickel catalyzed reactions

Organometallic catalysts have played a key role in accomplishing numerous synthetically valuable organic transformations that are either otherwise not possible or inefficient. The use of precious, sparse and toxic 4d and 5d metals are an apparent downside of several such catalytic systems despite their immense success over the last several decades. The use of complexes containing Earth-abundant, inexpensive and less hazardous 3d metals, such as nickel, as catalysts for organic transformations has been an emerging field in recent times. In particular, the versatile nature of the corresponding pincer-metal complexes, which offers great control of their reactivity via countless variations, has garnered great interest among organometallic chemists who are looking for greener and cheaper alternatives. In this context, the current review attempts to provide a glimpse of recent developments in the chemistry of pincer-nickel catalyzed reactions. Notably, there have been examples of pincer-nickel catalyzed reactions involving two electron changes via purely organometallic mechanisms that are strikingly similar to those observed with heavier Pd and Pt analogues. On the other hand, there have been distinct differences where the pincer-nickel complexes catalyze single-electron radical reactions. The applicability of pincer-nickel complexes in catalyzing cross-coupling reactions, oxidation reactions, (de)hydrogenation reactions, dehydrogenative coupling, hydrosilylation, hydroboration, C-H activation and carbon dioxide functionalization has been reviewed here from synthesis and mechanistic points of view. The flurry of global pincer-nickel related activities offer promising avenues in catalyzing synthetically valuable organic transformations.

Copper(i) pyrimidine-2-thiolate cluster-based polymers as bifunctional visible-light-photocatalysts for chemoselective transfer hydrogenation of α,β-unsaturated carbonyls

The photoinduced chemoselective transfer hydrogenation of unsaturated carbonyls to allylic alcohols has been accomplished using cluster-based MOFs as bifunctional visible photocatalysts.

Palladium(ii) N,N,O-pincer type complex-mediated dehydrogenative coupling of alcohols to quinazolines

A diverse range of quinazolines were synthesized in moderate to high yields using 0.5 mol% Pd(ii) catalyst loading under aerobic conditions.

NaOH-Mediated Direct Synthesis of Quinoxalines from o-Nitroanilines and Alcohols via a Hydrogen-Transfer Strategy

A NaOH-mediated sustainable synthesis of functionalized quinoxalines is disclosed via redox condensation of o-nitroamines with diols and α-hydroxy ketones. Under optimized conditions, various o-nitroamines and alcohols are well tolerated to generate the desired products in 44-99% yields without transition metals and external redox additives.

Recent advances in sustainable synthesis of N-heterocycles following acceptorless dehydrogenative coupling protocol using alcohols

In this review, we have summarized various aspects of homogeneous and heterogeneously catalyzed recent advancements in the synthesis of heterocycles following the ADC approach.

Alcohols as Fluoroalkyl Synthons: Ni-catalyzed Dehydrogenative Approach to Access Polyfluoroalkyl Bis-indoles

An acceptorless dehydrogenative strategy for the synthesis of polyfluoroalkylated bis-indoles is described by employing an earth-abundant nickel-based catalytic system under air. The notable feature of the present transformation is the use of bench stable and easily affordable polyfluorinated alcohols without any pre-functionalization for the introduction of precious polyfluoroalkyl groups. The developed straightforward protocol accomplished biologically relevant fluoroalkyl bis-indoles in a sustainable fashion. Extensive DFT study predicts the unique role of indole molecules which stabilizes the transition states during the dehydrogenation process of polyfluorinated alcohols, presumably through non-covalent π⋅⋅⋅π and H-bonding interactions.

C-C coupling formation using nitron complexes

A series of RuII (1), RhIII (2), IrIII (3, 4), IrI (5) and PdII (6-9) complexes of the 'instant carbene' nitron were prepared and characterized by 1H- and 13C-NMR, FT-IR and elemental analysis. The molecular structures of complexes 1-4 and 6 were determined by X-ray diffraction studies. The catalytic activity of the complexes (1-9) was evaluated in alpha(α)-alkylation reactions of ketones with alcohol via the borrowing hydrogen strategy under mild conditions. These complexes were able to perform this catalytic transformation in a short time with low catalyst and base amounts under an air atmosphere. Also, the PdII-nitron complexes (6-9) were applied in the Suzuki-Miyaura C-C coupling reaction and these complexes successfully initiated this reaction in a short time (30 minutes) using the H2O/2-propanol (1.5 : 0.5) solvent system. The DFT calculations revealed that the Pd0/II/0 pathway was more preferable for the mechanism.

Synthesis of N-aryl amines enabled by photocatalytic dehydrogenation

Catalytic dehydrogenation (CD) via visible-light photoredox catalysis provides an efficient route for the synthesis of aromatic compounds. However, access to N-aryl amines, which are widely utilized synthetic moieties, via visible-light-induced CD remains a significant challenge, because of the difficulty in controlling the reactivity of amines under photocatalytic conditions. Here, the visible-light-induced photocatalytic synthesis of N-aryl amines was achieved by the CD of allylic amines. The unusual strategy using C6F5I as an hydrogen-atom acceptor enables the mild and controlled CD of amines bearing various functional groups and activated C-H bonds, suppressing side-reaction of the reactive N-aryl amine products. Thorough mechanistic studies suggest the involvement of single-electron and hydrogen-atom transfers in a well-defined order to provide a synergistic effect in the control of the reactivity. Notably, the back-electron transfer process prevents the desired product from further reacting under oxidative conditions.

Homogeneous Nickel-Catalyzed Sustainable Synthesis of Quinoline and Quinoxaline under Aerobic Conditions

Dehydrogenative coupling-based reactions have emerged as an efficient route toward the synthesis of a plethora of heterocyclic rings. Herein, we report an efficacious, nickel-catalyzed synthesis of two important heterocycles such as quinoline and quinoxaline. The catalyst is molecularly defined, is phosphine-free, and can operate at a mild reaction temperature of 80 °C. Both the heterocycles can be easily assembled via double dehydrogenative coupling, starting from 2-aminobenzyl alcohol/1-phenylethanol and diamine/diol, respectively, in a shorter span of reaction time. This environmentally benign synthetic protocol employing an inexpensive catalyst can rival many other transition-metal systems that have been developed for the fabrication of two putative heterocycles. Mechanistically, the dehydrogenation of secondary alcohol follows clean pseudo-first-order kinetics and exhibits a sizable kinetic isotope effect. Intriguingly, this catalyst provides an example of storing the trapped hydrogen in the ligand backbone, avoiding metal-hydride formation. Easy regeneration of the oxidized form of the catalyst under aerobic/O₂ oxidation makes this protocol eco-friendly and easy to handle.

α-Alkylation of Nitriles with Primary Alcohols by a Well-Defined Molecular Cobalt Catalyst

The α-alkylation of nitriles with primary alcohols to selectively synthesize nitriles by a well-defined molecular homogeneous cobalt catalyst is presented. Thirty-two examples with up to 95% yield are reported. Remarkably, this transformation is environmentally friendly and atom economical with water as the only byproduct.

Nickel-catalysed chemoselective C-3 alkylation of indoles with alcohols through a borrowing hydrogen method

An inexpensive, air-stable, isolable nickel catalyst is reported that can perform chemoselective C3-alkylation of indoles with a variety of alcohols following "borrowing hydrogen". A one-pot, cascade C3-alkylation starting from 2-aminophenyl ethyl alcohols, and thus obviating the need for pre-synthesized indoles, further adds to the broad scope of this method. The reaction is radical-mediated, and is significantly different from other examples, often dictated by metal-ligand bifunctionality.

Electrochemical alcohols oxidation mediated by N-hydroxyphthalimide on nickel foam surface

Alcohol to aldehyde conversion is a critical reaction in the industry. Herein, a new electrochemical method is introduced that converts 1 mmol of alcohols to aldehydes and ketones in the presence of N-hydroxyphthalimide (NHPI, 20 mol%) as a mediator; this conversion is achieved after 8.5 h at room temperature using a piece of Ni foam (1.0 cm²) and without adding an extra-base or a need for high temperature. Using this method, 10 mmol (1.08 g) of benzyl alcohol was also successfully oxidized to benzaldehyde (91%) without any by-products. This method was also used to oxidize other alcohols with high yield and selectivity. In the absence of a mediator, the surface of the nickel foam provided oxidation products at the lower yield. After the reaction was complete, nickel foam (anode) was characterized by a combination of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and spectroelectrochemistry, which pointed to the formation of nickel oxide on the surface of the electrode. On the other hand, using other electrodes such as Pt, Cu, Fe, and graphite resulted in a low yield for the alcohol to aldehyde conversion.

Metal-ligand-Lewis acid multi-cooperative catalysis: a step forward in the Conia-ene reaction

An original multi-cooperative catalytic approach was developed by combining metal–ligand cooperation and Lewis acid activation. The [(SCS)Pd]₂ complex featuring a non-innocent indenediide-based ligand was found to be a very efficient and versatile catalyst for the Conia-ene reaction, when associated with Mg(OTf)₂. The reaction operates at low catalytic loadings under mild conditions with HFIP as a co-solvent. It works with a variety of substrates, including those bearing internal alkynes. It displays complete 5-exo vs. 6-endo regio-selectivity. In addition, except for the highly congested ^tBu-substituent, the reaction occurs with high Z vs. E stereo-selectivity, making it synthetically useful and complementary to known catalysts.

An original multi-cooperative catalytic approach was developed by combining metal–ligand cooperation and Lewis acid activation.

Synthesis of Tetrahydroquinolines via Borrowing Hydrogen Methodology Using a Manganese PN3 Pincer Catalyst

A straightforward and selective synthesis of 1,2,3,4-tetrahydroquinolines starting from 2-aminobenzyl alcohols and simple secondary alcohols is reported. This one-pot cascade reaction is based on the borrowing hydrogen methodology promoted by a manganese(I) PN³ pincer complex. The reaction selectively leads to 1,2,3,4-tetrahydroquinolines thanks to a targeted choice of base. This strategy provides an atom-efficient pathway with water as the only byproduct. In addition, no further reducing agents are required.

Selective oxidation of alcohols and sulfides via O2 using a Co(ii) salen complex catalyst immobilized on KCC-1: synthesis and kinetic study

The aim of this study was to immobilize a Co(ii) salen complex on KCC-1 as a catalyst that can be recovered (Co(ii) salen complex@KCC-1). Field-emission transmission electron microscopy, FT-IR spectroscopy, thermogravimetric analysis, elemental analysis, atomic absorption spectroscopy, and XRD were used to confirm the structure and chemical nature of Co(ii) salen complex@KCC-1. The oxidation efficiency was obtained for an extensive range of sulfides and alcohols using this sustainable catalyst, alongside O₂ as an oxygen source and isobutyraldehyde (IBA) as an oxygen acceptor, with superior selectivity and conversion for the relevant oxidation products (sulfoxides and ketones or aldehydes) under moderate conditions. The μ-oxo and peroxo groups on the ligands of the Co complex appeared to be responsible for the superior activity of the catalyst. Essential factors behind the oxidation of alcohol and sulfoxides were investigated, including the catalyst, solvent, and temperature. In this paper, molecular oxygen (O₂) was used as a green oxidant. Furthermore, kinetic studies were conducted, revealing a first-order reaction for the oxidation of both benzyl alcohol and sulfide. The reaction progressed at mild temperature, and the catalyst could be easily recovered and reused for numerous consecutive runs under the reaction conditions, without any substantial reduction in the functionality of the catalytic system.

(N),C,N-Coordinated Heavier Group 13-15 Compounds: Synthesis, Structure and Applications

The aim of this review is to summarize recent achievements in the field of (N),C,N-coordinated group 13-15 compounds not only regarding their synthesis and structure, but mainly focusing on their potential applications. Relevant compounds contain various types of N-coordinating ligands built up on an ortho-(di)substituted phenyl platform. Thus, group 13 and 14 derivatives were used as single-source precursors for the deposition of semiconducting thin films, as building blocks for the preparation of high-molecular polymers with remarkable optical and chemical properties or as compounds with interesting reactivity in hydrometallation processes. Group 15 derivatives function as catalysts in the Mannich reaction, in the allylation of aldehydes or activation of CO₂ . They were used as transmetallation reagents in transition metal catalysed coupling reactions. The univalent species serve as ligands for transition metals, activate alkynes or alkenes and are utilized as catalysts in the transfer hydrogenation of azo-compounds.

Direct Catalytic Symmetrical, Unsymmetrical N,N-Dialkylation and Cyclization of Acylhydrazides Using Alcohols

Herein, direct N,N-dialkylation of acylhydrazides using alcohols is reported. This catalytic protocol provides one-pot synthesis of both symmetrical and unsymmetrical N,N-disubstituted acylhydrazides using an assortment of primary and secondary alcohols with remarkable selectivity and excellent yields. Interestingly, the use of diols resulted in intermolecular cyclization of acylhydrazides, and such products are privileged structures in biologically active compounds. Water is the only byproduct, which makes this catalytic protocol sustainable and environmentally benign.

Iron-Catalyzed Direct Julia-Type Olefination of Alcohols

Herein, we report an iron-catalyzed, convenient, and expedient strategy for the synthesis of styrene and naphthalene derivatives with the liberation of dihydrogen. The use of a catalyst derived from an earth-abundant metal provides a sustainable strategy to olefins. This method exhibits wide substrate scope (primary and secondary alcohols) functional group tolerance (amino, nitro, halo, alkoxy, thiomethoxy, and S- and N-heterocyclic compounds) that can be scaled up. The unprecedented synthesis of 1-methyl naphthalenes proceeds via tandem methenylation/double dehydrogenation. Mechanistic study shows that the cleavage of the C-H bond of alcohol is the rate-determining step.

On the importance of the crystalline surface structure on the catalytic activity and stability of tailored unsupported cobalt nanoparticles for the solvent-free acceptor-less alcohol dehydrogenation

Unsupported nanoparticles are now recognized as model catalysts to evaluate the intrinsic activity of metal particles, irrespectively of that of the support. Co nanoparticles with different morphologies, rods, diabolos and cubes have been prepared by the polyol process and tested for the acceptorless catalytic dehydrogenation of alcohols under solvent-free conditions. Rods crystallize with the pure hcp structure, diabolos with a mixture of hcp and fcc phases, while the cubes crystallize in a complex mixture of hcp, fcc and ε-Co phases. All the cobalt particles are found to be highly selective towards the oxidation of a model secondary alcohol, octan-2-ol, into the corresponding ketone while no significant activity is found with octan-1-ol. Our results show the strong influence of particle shape on the activity and catalytic stability of the catalysts: Co nanorods display the highest conversion (85%), selectivity (95%) and recyclability compared to Co diabolos and Co cubes. We correlate the nanorods excellent stability with a strong binding of carboxylate ligands on their {1 1 2¯ 0} facets, preserving their crystalline superficial structure, as evidenced by phase modulation infrared reflection absorption spectroscopy.

Hydrogenation/dehydrogenation of N-heterocycles catalyzed by ruthenium complexes based on multimodal proton-responsive CNN(H) pincer ligands

Ru complexes based on lutidine-derived pincer CNN(H) ligands having secondary amine side donors are efficient precatalysts in the hydrogenation and dehydrogenation of N-heterocycles. Reaction of a Ru-CNN(H) complex with an excess of base produces the formation of a Ru(0) derivative, which is observed under catalytic conditions.

Copper-Catalyzed Modular Assembly of Polyheterocycles

Easy operation, readily accessible starting materials, and short syntheses of the privileged scaffold indeno[1,2-c]isoquinolinone were achieved by an multicomponent reaction (MCR)-based protocol via an ammonia–Ugi-four component reaction (4CR)/copper-catalyzed annulation sequence. The optimization and scope and limitations of this short and general sequence are described. The methodology allows an efficient construction of a wide variety of indenoisoquinolinones in just two steps.

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.

Acceptorless dehydrogenation and hydrogenation of N- and O-containing compounds on Pd3Au1(111) facets

Catalytic dehydrogenation and hydrogenation of amines and alcohols are important in the synthesis of fine chemicals. Despite several efficient homogeneous catalysts having been identified, highly active heterogeneous catalysts remain elusive, although they would meet an unmet need. Here, we show that bimetallic Pd-Au nanoparticles with Pd-to-Au molar ratios of 3:1 immobilized on multiwall carbon nanotubes (Pd3Au1/CNT) display high catalytic activity in the oxidant-free and acceptorless dehydrogenation and hydrogenation of N- and O-containing heterocyclic compounds, amines/imines, and alcohols/ketones. Transmission electron microscopy analysis demonstrates the preferential exposure of Pd3Au1(111) facets on the Pd3Au1/CNT catalyst. Mechanistic insights combining experimental data with density functional theory calculations reveal that the Pd3Au1(111) surface enhances both dehydrogenation and hydrogenation reactions and provides a rationale for the observed enhancements.