Elite cliques: Palladium clusters with three and four atoms were found to be the catalytically active species for ligand‐free palladium‐catalyzed CC bond‐forming reactions (see picture). These ...palladium cluster species could be stabilized in water and stored for long periods of time for use on demand with no loss of activity. High yields of products and turnover frequencies (TOFs) of up to 105 h−1 were observed.
HCl in solution accepts electron density from Au NPs and partially reduces at room temperature, as occurs with other simple diatomic molecules, such as O2 and H2. The activation can be run ...catalytically in the presence of alkynes to give exclusively E‐vinyl chlorides, after the regio‐ and stereoselective transfer of HCl. Based also on this method, vinyl chloride monomer (VCM) can be produced in a milder and greener way than current industrial processes.
A helping hand from gold: Hydrogen chloride is partially reduced by supported gold nanoparticles at room temperature. It is catalytically added to alkynes to give, regio‐ and stereoselectively, E‐α‐vinyl chlorides. This approach enabled the synthesis of vinyl chloride monomer from acetylene in continuous mode.
The supporting of pre‐formed soluble metal catalysts on solids is a typical methodology to transform soluble but unrecoverable metal complexes into recoverable and reusable solid catalysts. However, ...this methodology has been barely implemented for ligand‐less, bare single metal atoms (SMAs) and nanoclusters (NCs) in organic synthesis, despite these ultrasmall species can be formed in‐situ during reaction and be the truly catalytic species. The aim of this review is to explore how to speciate active single metal atoms and clusters during homogeneous catalysis (in solution), without ligands, and to prepare them independently, to be transferred to solid supports and catalyze organic reactions. In many cases, the translation to solids gives single atom catalysts (SACs). Supporting of ultrasmall metal aggregates gives more stable, reusable and, sometimes, chemoselective catalysts for representative organic reactions.
Single Atom Catalysts: Metal salts, complexes and nanoparticles can evolve in solution to catalytically active single atoms and clusters, which can be incorporated onto solids (such as single atom catalysts, SACs) to perform key organic reactions in a more sustainable and, many times, efficient way.
The engagement in tandem of well‐known organic reactions such as the Pd‐catalyzed Sonogashira cross‐coupling reaction, nucleophilic substitution and elimination reactions, enables the synthesis of ...otherwise difficult to obtain linear dienynes, in moderate to high yields. This retrosynthetic approach opens new ways to prepare highly conjugated alkenes and alkynes. Furthermore, ionic liquids are suitable solvents to perform the cascade reaction and recycle the metal catalysts.
A Sonogashira cross‐coupling, nucleophilic substitution and elimination cascade reaction allows the synthesis of otherwise difficult to obtain linear dienynes, in moderate to high yields. Ionic liquids are suitable recyclable solvents for the reaction. This synthetic strategy opens new ways to prepare highly conjugated alkenes and alkynes.
2,5‐Dimethyl‐2,4‐hexadiene is a readily available and easily managable compound, whose symmetric and polymethylated dienic structure should be prone to engage in cross‐metathesis reactions with other ...alkenes, but this has not been apparently exploited so far. Here we show that this reactant enables the easy synthesis of tri‐ and tetra‐susbtituted alkenes (i. e. isobutylenyl and prenyl groups) from simple alkenes under mild reaction conditions, not only with the conventional 2nd generation Grubbs catalyst but also with other Grela‐type catalyts such as StickyCat,TM AquaMetTM and GreenCatTM. The use of liquid and low volatile 2,5‐dimethyl‐2,4‐hexadiene avoids the use of gaseous alkene reactants and, besides, showcases the reactivity of polyisoprene (rubber), thus allowing to optimize the reaction conditions for rubber upcycling, after metathesis reaction of the pristine or used polymer with simple alkenes. These results bring low volatile isoprene‐type compounds as privileged poly‐substituted reactants for alkene cross‐metathesis reactions.
2,5‐Dimethyl‐2,4‐hexadiene is presented here as a suitable and selective reactant for delivering either prenyl or isobutylenyl groups to alkenes through a cross‐metathesis reaction. The process smoothly operates at room temperature, give high yields of products, includes a late‐stage transformation, and enables the valorization of real samples of recycled rubber.
Extremely high electrophilic metal complexes, composed by a metal cation and very electron poor σ‐donor ancillary ligands, are expected to be privileged catalysts for oxidation reactions in organic ...chemistry. However, their low lifetime prevents any use in catalysis. Here we show the synthesis of fluorinated pyridine‐Pd2+ coordinate cages within the channels of an anionic tridimensional metal‐organic framework (MOF), and their use as efficient metal catalysts for the aerobic oxidation of aliphatic alcohols to carboxylic acids without any additive. Mechanistic studies strongly support that the MOF‐stabilized coordination cage with perfluorinated ligands unleashes the full electrophilic potential of Pd2+ to dehydrogenate primary alcohols, without any base, and also to activate O2 for the radical oxidation to the aldehyde intermediate. This study opens the door to design catalytic perfluorinated complexes for challenging organic transformations, where an extremely high electrophilic metal site is required.
Fluorinated pyridine‐Pd2+ coordinate cages have been synthesized and characterized with atomic resolution within the channels of a metal‐organic framework (MOF), and used as catalysts during the aerobic oxidation of aliphatic alcohols to carboxylic acids without any additive. These results illustrate a rare example of stable per‐fluorinated cationic metal complex, with potential implications in the future design of catalytic organometallic complexes.
Heterogeneous catalysts are complex materials with multiple interfaces. A critical proposition in exploiting bifunctionality in alloy catalysts is to achieve surface migration across interfaces ...separating functionally dissimilar regions. Herein, we demonstrate the enhancement of more than 10
in the rate of molecular hydrogen reduction of a silver surface oxide in the presence of palladium oxide compared to pure silver oxide resulting from the transfer of atomic hydrogen from palladium oxide islands onto the surrounding surface formed from oxidation of a palladium-silver alloy. The palladium-silver interface also dynamically restructures during reduction, resulting in silver-palladium intermixing. This study clearly demonstrates the migration of reaction intermediates and catalyst material across surface interfacial boundaries in alloys with a significant effect on surface reactivity, having broad implications for the catalytic function of bimetallic materials.
Clusters get the gold: Atomic gold clusters Aun (n=3–6) are very active species (sub‐mol %) for gold‐catalyzed carbon–carbon and carbon–heteroatom bond‐forming reactions of interest in organic ...synthesis. The gold clusters can be formed in situ or generated ex situ and introduced into the reaction media. Salts, complexes, and nanoparticles (NPs) can be used as a starting source of gold. Ts=para‐toluenesulfonyl.
The selective ortho–alkylation of 1–naphthol with methanol is carried out over various commercially available acid solid catalysts under relatively mild reaction conditions (<300 ºC), in batch, and ...anhydrous zeolite HY shows the best catalytic activity. Removal of the strongly adsorbed water in the zeolite is key for the alkylation reaction. Mechanistic studies based on isotopically labelled experiments reveal the transformation of O–methylated 1–naphthol into the desired ortho–C–methylation product after intramolecular rearrangement of the methyl group. These results open the way to design a new synthesis of ortho–methyl 1–naphthol and, consequently, of vitamin K3, based on a commercially available, inexpensive and non–toxic solid catalyst such as HY zeolite.
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Metal single-atom catalysts (SACs) promise great rewards in terms of metal atom efficiency. However, the requirement of particular conditions and supports for their synthesis, together with the need ...of solvents and additives for catalytic implementation, often precludes their use under industrially viable conditions. Here, we show that palladium single atoms are spontaneously formed after dissolving tiny amounts of palladium salts in neat benzyl alcohols, to catalyze their direct aerobic oxidation to benzoic acids without ligands, additives, or solvents. With this result in hand, the gram-scale preparation and stabilization of Pd SACs within the functional channels of a novel methyl-cysteine-based metal–organic framework (MOF) was accomplished, to give a robust and crystalline solid catalyst fully characterized with the help of single-crystal X-ray diffraction (SCXRD). These results illustrate the advantages of metal speciation in ligand-free homogeneous organic reactions and the translation into solid catalysts for potential industrial implementation.
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