Selective Transfer Semihydrogenation of Alkynes with Nanoporous Gold Catalysts

Boosting Energy-Transfer Processes via Dispersion Interactions

The generation of open-shell intermediates under mild conditions has opened broad synthetic opportunities during this century. However, these reactive species often require a case specific and tailored tuning of experimental parameters in order to efficiently convert substrates into products. We report a general approach that can overcome these ubiquitous limitations for several visible-light promoted energy-transfer processes. The use of either naphthalene (5-20 equiv.) or simple binaphthyl derivatives (10-30 mol %) greatly increases their efficiency, giving rise to a new strategy for catalysis. The trend is consistent among different media, photocatalysts, light sources and substrates, allowing one to improve existing methods, to more easily optimize conditions for new ones, and, moreover, to disclose otherwise inaccessible reaction pathways.

Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

H2O as the Hydrogen Donor: Stereo-Selective Synthesis of E- and Z-Alkenes by Palladium-Catalyzed Semihydrogenation of Alkynes

It is very desirable to develop a facile controllable method for selective semihydrogenation of alkynes to alkenes with a cheap and safe hydrogen donor but remains a big challenge. H₂O is one of the best choices of the transfer hydrogenation agent of the world, and the development of methods for synthesizing E- and Z-alkenes using H₂O as the hydrogen source is worthwhile. In this article, a palladium-catalyzed synthesis of E- and Z-alkenes from alkynes using H₂O as the hydrogenation agent was reported. The use of di-tert-butylphosphinous chloride (t-Bu₂PCl) and triethanolamine/sodium acetate (TEOA/NaOAc) was essential for the stereo-selective semihydrogenation of alkynes. The general applicability of this procedure was highlighted by the synthesis of more than 48 alkenes, with good yields and high stereoselectivities.

Nanoporous Gold Catalyst for the Oxidative N-dealkylation of Drug Molecules: A Method for Synthesis of N-dealkylated Metabolites

A novel method for the selective catalytic N‐dealkylation of drug molecules on a nanoporous gold (NPG) catalyst producing valuable N‐dealkylated metabolites and intermediates is described. Drug metabolites are important chemical entities at every stage of drug discovery and development, from exploratory discovery to clinical development, providing the safety profiles and the ADME (adsorption, distribution, metabolism, and elimination) of new drug candidates. Synthesis was carried out in aqueous solution at 80 °C using air (oxygen source) as oxidant, in single step with good isolated yields. Different examples examined in this study showed that aerobic catalytic N‐dealkylation of drug molecules on NPG has a broad scope supporting N‐deethylation, N‐deisopropylation and N‐demethylation, converting either 3° amines to 2° amines, or 2° amines to 1° amines.

CuCl2-catalyzed highly stereoselective and chemoselective reduction of alkynyl amides into α,β-unsaturated amides using silanes as hydrogen donors

A CuH-catalyzed Z-selective partial reduction of alkynyl amides to afford α,β-unsaturated amides using silane as the hydrogen donor is developed. This reaction is carried out under mild conditions and able to accommodate a broad scope of alkynyl amides including those bearing a terminal carbon-carbon double bond or triple bond, affording alkenyl amides with high stereoselectivity and excellent yields.

Noble metal nanoclusters-decorated NiFe layered double hydroxide superstructure as nanoreactors for selective hydrogenation catalysis

The ability to conduct the self-assembly of nanometer-scale building blocks is the core issue in achieving "bottom-up" fabrications of desired superstructures. However, the general fabrication strategy of a superstructure at an ultrathin nanoscale is not yet fully developed. Herein, we report a facile synthesis route for 3d transition metal layered double hydroxide (LDH) based laminar superstructures via a concise hydrothermal process. The LDH laminar superstructures are further decorated by noble metal clusters to form a new kind of 2-dimensional nanoreactor, which presents outstanding catalytic performance under mild conditions. It is suggested that dodecyl sulfate anions play a key role in the self-assembly of well-defined laminar superstructures, which provide a confinement effect and enhance the activity and selectivity of the catalytic process. The fabrication of an as-prepared nanoreactor reveals a new strategy to tailor the performance of catalysts with high selectivity.

Catalytic, Transition-Metal-Free Semireduction of Propiolamide Derivatives: Scope and Mechanistic Investigation

We report a transition-metal-free trans-selective semireduction of alkynes with pinacolborane and catalytic potassium tert-butoxide. A variety of 3-substituted primary and secondary propiolamides, including an analog of FK866, a potent nicotinamide mononucleotide adenyltransferase (NMNAT) inhibitor, are reduced to the corresponding (E)-3-substituted acrylamide derivatives in up to 99% yield with >99:1 E/Z selectivity. Mechanistic studies suggest that an activated Lewis acid-base complex transfers a hydride to the α-carbon followed by rapid protonation in a trans fashion.

Ligand-Promoted Iridium-Catalyzed Transfer Hydrogenation of Terminal Alkynes with Ethanol and Its Application

A ligand-promoted iridium-catalyzed transfer hydrogenation of terminal alkynes with ethanol and its application has been developed. Highly chemical selectivity control is achieved based on ligand regulation. 1,2-Bis(diphenylphosphino)ethane was found to be critical for the transfer hydrogenation of alkynes. The general applicability of this procedure is highlighted by the synthesis of 30 terminal alkenes with a good yield. In addition, we conducted drug effect studies of phenelzine using zebrafish as the vertebrate model. Phenelzine shows a significant effect on promoting vascular proliferation and inhibiting nerve growth. The results of these studies have an important reference value for promoting drug research in cerebrovascular diseases, epilepsy, mania, and psychosis.

Synthesis of nickel/gallium nanoalloys using a dual-source approach in 1-alkyl-3-methylimidazole ionic liquids

NiGa is a catalyst for the semihydrogenation of alkynes. Here we show the influence of different dispersion times before microwave-induced decomposition of the precursors on the phase purity, as well as the influence of the time of microwave-induced decomposition on the crystallinity of the NiGa nanoparticles. Microwave-induced co-decomposition of all-hydrocarbon precursors [Ni(COD)2] (COD = 1,5-cyclooctadiene) and GaCp* (Cp* = pentamethylcyclopentadienyl) in the ionic liquid [BMIm][NTf2] selectively yields small intermetallic Ni/Ga nanocrystals of 5 ± 1 nm as derived from transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and supported by energy-dispersive X-ray spectrometry (EDX), selected-area energy diffraction (SAED) and X-ray photoelectron spectroscopy (XPS). NiGa@[BMIm][NTf2] catalyze the semihydrogenation of 4-octyne to 4-octene with 100% selectivity towards (E)-4-octene over five runs, but with poor conversion values. IL-free, precipitated NiGa nanoparticles achieve conversion values of over 90% and selectivity of 100% towards alkene over three runs.

Nickel-Catalyzed Stereodivergent Synthesis of E- and Z-Alkenes by Hydrogenation of Alkynes

A convenient protocol for stereodivergent hydrogenation of alkynes to E- and Z-alkenes by using nickel catalysts was developed. Simple Ni(NO3 )2 ⋅6 H2 O as a catalyst precursor formed active nanoparticles, which were effective for the semihydrogenation of several alkynes with high selectivity for the Z-alkene (Z/E>99:1). Upon addition of specific multidentate ligands (triphos, tetraphos), the resulting molecular catalysts were highly selective for the E-alkene products (E/Z>99:1). Mechanistic studies revealed that the Z-alkene-selective catalyst was heterogeneous whereas the E-alkene-selective catalyst was homogeneous. In the latter case, the alkyne was first hydrogenated to a Z-alkene, which was subsequently isomerized to the E-alkene. This proposal was supported by density functional theory calculations. This synthetic methodology was shown to be generally applicable in >40 examples and scalable to multigram-scale experiments.

Catalytic Performance of Nanoporous Metal Skeleton Catalysts for Molecular Transformations

Nanoporous metal (MNPore) skeleton catalysts have attracted increasing attention in the field of green and sustainable heterogeneous catalysis owing to their unique three-dimensional nanopore structural features. In general, MNPores are fabricated through chemical or electrochemical corrosive dealloying of monolithic alloys. The dealloying process produces various MNPores with an open nanoporous network structure by formation of concave and convex hyperboloid-like ligaments. The large surface-to-volume ratio compared to bulk metals and high density of steps and kinks on ligaments of the unsupported MNPores make them promising heterogeneous catalyst candidates for highly active and selective molecular transformations. In this context, a variety of heterogeneous catalytic reactions using MNPores as nanocatalysts under gas- and liquid-phase conditions were developed over the last decade. In addition, the bulk metallic shape and mechanistic rigidity of the MNPore catalysts make the processes of catalyst recovery and reuse more facile and greener. This Minireview mainly focuses on the catalytic performance of nanoporous Au, Pd, Cu, and AuPd with respect to the achievements on catalytic applications in various molecular transformations.

Highly stereoselective heterogeneous palladium-catalyzed transfer semihydrogenation of internal alkynes to access cis-alkenes

A highly efficient heterogeneous palladium-catalyzed transfer semihydrogenation of internal alkynes was achieved in dimethylformamide at 145 ºC by using an MCM-41-immobilized bidentate nitrogen palladium(II) complex [MCM-41-2N-Pd(OAc)2] as catalyst and dimethylformamide/KOH as hydrogen source, yielding a variety of cis-alkenes in good to high yields with excellent stereoselectivity. This supported palladium catalyst can be easily recovered by filtration of the reaction solution and recycled up to seven times with almost consistent activity.

Transfer Hydrogenation of Alkenes Using Ethanol Catalyzed by a NCP Pincer Iridium Complex: Scope and Mechanism

The first general catalytic approach to effecting transfer hydrogenation (TH) of unactivated alkenes using ethanol as the hydrogen source is described. A new NCP-type pincer iridium complex (^BQ-NC^OP)IrHCl containing a rigid benzoquinoline backbone has been developed for efficient, mild TH of unactivated C-C multiple bonds with ethanol, forming ethyl acetate as the sole byproduct. A wide variety of alkenes, including multisubstituted alkyl alkenes, aryl alkenes, and heteroatom-substituted alkenes, as well as O- or N-containing heteroarenes and internal alkynes, are suitable substrates. Importantly, the (^BQ-NC^OP)Ir/EtOH system exhibits high chemoselectivity for alkene hydrogenation in the presence of reactive functional groups, such as ketones and carboxylic acids. Furthermore, the reaction with C₂D₅OD provides a convenient route to deuterium-labeled compounds. Detailed kinetic and mechanistic studies have revealed that monosubstituted alkenes (e.g., 1-octene, styrene) and multisubstituted alkenes (e.g., cyclooctene (COE)) exhibit fundamental mechanistic difference. The OH group of ethanol displays a normal kinetic isotope effect (KIE) in the reaction of styrene, but a substantial inverse KIE in the case of COE. The catalysis of styrene or 1-octene with relatively strong binding affinity to the Ir(I) center has (^BQ-NC^OP)Ir^I(alkene) adduct as an off-cycle catalyst resting state, and the rate law shows a positive order in EtOH, inverse first-order in styrene, and first-order in the catalyst. In contrast, the catalysis of COE has an off-cycle catalyst resting state of (^BQ-NC^OP)Ir^III(H)[O(Et)···HO(Et)···HOEt] that features a six-membered iridacycle consisting of two hydrogen-bonds between one EtO ligand and two EtOH molecules, one of which is coordinated to the Ir(III) center. The rate law shows a negative order in EtOH, zeroth-order in COE, and first-order in the catalyst. The observed inverse KIE corresponds to an inverse equilibrium isotope effect for the pre-equilibrium formation of (^BQ-NC^OP)Ir^III(H)(OEt) from the catalyst resting state via ethanol dissociation. Regardless of the substrate, ethanol dehydrogenation is the slow segment of the catalytic cycle, while alkene hydrogenation occurs readily following the rate-determining step, that is, β-hydride elimination of (^BQ-NC^OP)Ir(H)(OEt) to form (^BQ-NC^OP)Ir(H)₂ and acetaldehyde. The latter is effectively converted to innocent ethyl acetate under the catalytic conditions, thus avoiding the catalyst poisoning via iridium-mediated decarbonylation of acetaldehyde.

A versatile, immobilized gold catalyst for the reductive amination of aldehydes in batch and flow

A process for the reductive amination of aldehydes in continuous flow catalyzed by gold nanoparticles immobilized onto mesoporous silica has been developed.

BF3·OEt2-mediated syn-selective Meyer-Schuster rearrangement of phenoxy propargyl alcohols for Z-β-aryl-α,β-unsaturated esters

Synthesis of Z-β-aryl-α,β-unsaturated esters from readily available 1-aryl-3-phenoxy propargyl alcohols is achieved via a BF3-mediated syn-selective Meyer-Schuster rearrangement under ambient conditions. The reaction mechanism is postulated to involve an electrophilic borylation of an allene intermediate as the key step to kinetically control the stereoselectivity.

Coinage Metal Hydrides: Synthesis, Characterization, and Reactivity

Hydride complexes of copper, silver, and gold encompass a broad array of structures, and their distinctive reactivity has enabled dramatic recent advances in synthesis and catalysis. This Review summarizes the synthesis, characterization, and key stoichiometric reactions of isolable or observable coinage metal hydrides. It discusses catalytic processes in which coinage metal hydrides are known or probable intermediates, and presents mechanistic studies of selected catalytic reactions. The purpose of this Review is to convey how developments in coinage metal hydride chemistry have led to new organic transformations, and how developments in catalysis have in turn inspired the synthesis of reactive new complexes.

Au/TiO2 catalyzed reductive amination of aldehydes and ketones using formic acid as reductant

The combination of commercial easily available Au/TiO₂ as catalyst and cost-effective formic acid as reductant was able to render reductive amination of various carbonyl compounds.

Combining visible light catalysis and transfer hydrogenation for in situ efficient and selective semihydrogenation of alkynes under ambient conditions

By combining visible light catalysis and transfer hydrogenation, we are able to convert a series of alkynes to their corresponding alkenes in high chemical yields.

Supported gold nanoparticles catalyzed cis-selective semihydrogenation of alkynes using ammonium formate as the reductant

TiO₂ supported gold nanoparticles with a low loading (0.5 mol%) are able to semihydrogenate non-fluorinated and gem-difluorinated alkynes to cis-alkenes with high selectivity, using cost-effective and easy-to-handle ammonium formate as the reductant.

Nanocatalysts Fabricated by a Dealloying Method

Two types of nanomaterials with different morphologies are described in this article: nanoporous metals and titanate nanowires. Both materials are fabricated by a dealloying method. In the former case, the catalytic properties of nanoporous gold and palladium are exemplified by many chemical transformations. The reactions proceed without any support, stabilizer, or ligands. The catalyst can be easily recovered by a simple separation process and reused many times without significant loss of catalytic activity. In the latter case, the dealloying of Ti-Al alloy is described as a new fabrication method for producing ultrafine titanate nanowires. This method does not require high-temperature conditions, which is advantageous for the construction of fine structures. The key to this process is achieving a fine dispersion of intermetallic TiAl3 nanocrystals in the Al matrix in the mother alloy. The resulting nanowires exhibit remarkable Sr(2+) ion-exchange properties.