Aerobic Oxidation of Alcohols under Mild Conditions Catalyzed by Novel Polymer‐Incarcerated,Carbon‐Stabilized Gold Nanoclusters

Polymer‐incarcerated, carbon‐stabilized gold nanoclusters were found to be highly active in heterogeneous catalysis for the oxidation of secondary alcohols using molecular oxygen in aqueous medium. After optimization of catalyst preparation methods, highly loaded and highly effective catalysts were obtained, and a broad range of secondary alcohols could be oxidized by using these catalysts under mild conditions. The catalysts could be recovered and reused several times without significant loss of activity. Moreover, kinetics of the oxidation reaction with (±)‐1‐phenylethanol was investigated.     

Thiazolium‐Based Catalysts for the Etherification of Benzylic Alcohols under Solvent‐Free Conditions

Thiazolium and imidazolium hybrid materials were prepared by radical reactions between a mercaptopropyl‐modified SBA‐15 mesoporous silica and bis‐vinylthiazolium or bis‐vinylimidazolium dibromide salts. These hybrid materials were characterized by several techniques and were employed in the etherification reaction of 1‐phenylethanol. Solvent‐free conditions at 160 °C under different gas phases (oxygen, air, nitrogen and argon) were used. The thiazolium‐based material displayed excellent performances. Further studies were carried out using unsupported thiazolium salts, with or without a methyl group at the C‐2 position of the thiazolium moiety. These studies allowed us to propose a reaction mechanism. The supported thiazolium‐based material was successfully used in the etherification reaction of two other benzylic alcohols and also in seven consecutive cycles. This work represents the first use of thiazolium‐based compounds as catalysts for the etherification reaction of alcohols.

     

Ligand‐Free Buchwald–Hartwig Aromatic Aminations of Aryl Halides Catalyzed by Low‐Leaching and Highly Recyclable Sulfur‐Modified Gold‐Supported Palladium Material

A stable heterogeneous catalyst precursor, sulfur‐modified gold‐supported palladium material (SAPd), has proved to be an excellent source of leached, ligand‐free, Pd for the amination of aryl bromides and chlorides. The reaction‐enabling catalyst is provided in situ as leached Pd in low catalyst loading (0.21±0.02 mol%). This allows the precatalyst (SAPd) to be filtered off and used for a minimum of ten reaction cycles without loss of catalytic activity. SAPd released only trace amounts, less than 0.6 ppm, of highly active Pd during the reaction without any aggregation.     

Iron‐Catalyzed Direct Synthesis of Amides from Methylarenes

An efficient, green and first catalytic process has been developed for the direct synthesis of amides from readily available petroleum by‐products (methylarenes) and amines using an iron catalyst. In this new catalytic reaction, the methyl group of the methylarene is oxidized to the corresponding aldehyde through non‐directed C H oxidation followed by its oxidative amidation with N‐chloroamine, yielding the carboxylic amide. Oxidation with an iron catalyst, tert‐butyl hydroperoxide (TBHP) as sole oxidant, the synthesis of amides under mild reaction conditions and the utilization of methylarenes as starting material make this methodology novel and environmentally benign.

     

Magnetite (Fe3O4) Nanoparticles‐Catalyzed Sonogashira– Hagihara Reactions in Ethylene Glycol under Ligand‐Free Conditions

A novel application of nanoparticles of paramagnetic magnetite (Fe₃O₄) as an efficient catalyst for carbon‐carbon bond formation via the Sonogashira–Hagihara reaction under heterogeneous ligand‐free conditions in ethylene glycol (EG) is described. By using this catalyst, arylalkynes are produced from the reaction of aryl iodides and activated heteroaryl bromides with alkynes. The results are reproducible using the catalyst, which was prepared from different sources. The catalyst is easily separated by an external magnetic field from the reaction mixture. The separated catalyst can be recycled for several consecutive runs without appreciable loss of its catalytic activity.     

Solid‐Supported Rhodium(0) Nano‐/Microparticles: An Efficient Ligand‐Free Heterogeneous Catalyst for Microwave‐Assisted Suzuki–Miyaura Cross‐Coupling Reaction

Solid‐supported nano‐ and microparticles of rhodium(0) (SS‐Rh) were prepared and applied as a ligand free heterogeneous catalyst for Suzuki–Miyaura cross‐coupling reaction with wide range of substrate scope. A hitherto unknown Rh‐catalyzed Suzuki cross‐coupling reaction of aldehyde and cyanohaloarenes was observed rather than the usual nucleophilic arylation.The catalyst can be removed by simple filtration and recycled upto twelve runs without any deterioration of activity.     

Water‐Soluble Gold–N‐Heterocyclic Carbene Complexes for the Catalytic Homogeneous Acid‐ and Silver‐Free Hydration of Hydrophilic Alkynes

Water‐soluble gold(III/I) N‐heterocylic carbene complexes behave as efficient catalysts for the hydration of terminal alkynes in neat water. The transformation proceeds in the absence of Brønsted acids or halide scavengers and is suitable for sensitive substrates. Kinetic profiles and DFT studies provide a clear picture of intermediates present during catalysis.

     

Insights into Supported Copper(II)‐Catalyzed Azide‐Alkyne Cycloaddition in Water

Cross‐linked polymeric ionic liquid material‐supported copper (Cu‐CPSIL), imidazolium‐loaded Merrifield resin‐supported copper (Cu‐PSIL) and silica dispersed CuO (CuO/SiO₂), were prepared and proved to be efficient catalysts for the one‐pot synthesis of 1,4‐disubsituted‐1,2,3‐triazoles by the reaction of alkyl halides with sodium azide and terminal alkynes in water at room temperature. Moreover, these supported copper catalysts were recovered quantitatively from the reaction mixture by simple filtration and reused for five consecutive recycles without significant loss of catalytic activity. Among the three immobilized copper catalysts, Cu‐CPSIL exhibited excellent catalytic activity for the reaction of aliphatic bromides, sodium azide and terminal alkynes. The differences in the catalytic performances of the catalysts could be ascribed to the copper dispersion and the interaction between copper and the supports. In addition, water was used as the reaction media and the proton provider, the latter was found to be very important for the reaction. The XPS results suggested that the supported Cu(II) catalysts were reduced to catalytic Cu(I) species via alkynes homocoupling reaction. By means of IR and ESI‐MS studies, a possible mechanism of cycloaddition based on the reduction of Cu(II) to Cu(I) species was proposed.     

An Ion‐Pair Immobilization Strategy in Rhodium‐Catalyzed Asymmetric Transfer Hydrogenation of Aromatic Ketones

A chiral diamine‐based homogeneous cationic rhodium catalyst was developed and two heterogeneous cationic rhodium catalysts were obtained via the encapsulation of the homogeneous cationic rhodium catalyst within Me‐SBA‐15 and Me‐SBA‐16. All these catalysts presented excellent catalytic activities and high enantioselectivities in ultrasound‐promoted asymmetric transfer hydrogenation of aromatic ketones and represent a successful use of the ion‐pair immobilization strategy. More importantly, the encapsulation of the cationic rhodium functionality within Me‐SBA‐16 had an obvious high recyclability, in which the recycled catalyst could be reused nine times without significantly affecting its enantioselectivity, showing good potential in industrial application.     

Silica‐Supported Vanadium‐Catalyzed N‐Oxidation of Tertiary Amines with Aqueous Hydrogen Peroxide

A recyclable silica supported vanadium 1 catalyzes the oxidation of tertiray amines to the corresponding N‐oxides with 30% H₂O₂ in high yield.     

Gold Nanoparticles Supported on the Periodic Mesoporous Organosilicas as Efficient and Reusable Catalyst for Room Temperature Aerobic Oxidation of Alcohols

Gold nanoparticles supported on the channels of a bifunctional periodic mesoporous organosilica, were found to be a highly efficient catalyst system for the aerobic oxidation of various types of alcohols into their corresponding aldehydes and ketones at room temperature. The catalyst showed no significant loss of efficiency for the aerobic oxidation of benzyl alcohol to give benzaldehyde after 7 reaction cycles.     

Oxidation of N,N‐Disubstituted Hydroxylamines to Nitrones with Hydrogen Peroxide Catalyzed by Polymer‐Supported Methylrhenium Trioxide Systems

Poly(4‐vinylpyridine)/methylrhenium trioxide (MTO) compounds I III and microencapsulated polystyrene/MTO systems IV V are efficient catalysts for the oxidation of secondary hydroxylamines to the corresponding nitrones with H₂O₂. Complete conversions of substrates and quantitative yields of products are obtained under environmentally friendly experimental conditions and with the use of simple work‐up procedures. Symmetrically substituted hydroxylamines, and non‐symmetrical 3‐substituted and 2‐substituted hydroxypyrrolidines, precursors of nitrones applied in the synthesis of alkaloids and biologically active congeners, have been considered as substrates. The heterogeneous catalysts are stable under the reaction conditions and can be recovered and recycled for at least five times without any appreciable loss in efficiency.     

Semiconductor‐Gold Nanocomposite Catalysts for the Efficient Three‐Component Coupling of Aldehyde,Amine and Alkyne in Water

An efficient heterogeneous lead sulfide‐gold catalyst has been successfully developed for the synthesis of propargylic amines via a three‐component coupling reaction of aldehyde, amine and alkyne in water. The process is simple and applicable to a diverse range of aromatic and aliphatic aldehydes, amines and alkynes. Furthermore, the catalyst is stable to air and water, and can be easily recovered and reused.     

Highly Efficient and Reusable Polyaniline‐Supported Palladium Catalysts for Open‐Air Oxidative Heck Reactions under Base‐ and Ligand‐Free Conditions

Oxidative Heck reactions of arylboronic acids and alkenes are carried out in the presence of polyaniline‐supported palladium catalysts using air as a co‐oxidant to afford the Heck products with excellent yields and selectivities under base‐ and ligand‐free conditions. The catalyst was recovered by simple filtration and used for several cycles with consistent activity.     

Synthesis of 3‐Iodoquinolines by Copper‐Catalyzed Tandem Annulation from Diaryliodoniums,Nitriles, and 1‐Iodoalkynes

A novel method for the synthesis of 3‐iodoquinolines was developed by copper‐catalyzed tandem annulation from diaryliodoniums, nitriles, and 1‐iodoalkynes. It is a method that is characterized by the most convenient operation and wide molecular diversity.

     

Silica‐Supported Dendrimer‐Palladium Complex‐Catalyzed Selective Hydrogenation of Dienes to Monoolefins

The selective hydrogenation of cyclic and acyclic dienes to monoolefins occurs under very mild conditions, in the presence of silica‐supported PAMAM‐Pd complexes. The activity and selectivity of this reaction is sensitive to the dendrimer structure. These dendritic complexes display excellent recycle properties, retaining activity for up to eight recycles.     

Suzuki Coupling Reactions in Pure Water Catalyzed by Supported Palladium – Relevance of the Surface Polarity of the Support

Heterogeneous (supported) palladium catalysts like palladium on carbon and a variety of metal oxides have been shown to be highly active for Suzuki coupling reactions in neat water under mild reaction conditions (T=65 °C). It has been demonstrated for the first time that hydrophobic effects of the catalyst surface play an important role for the catalyst activity in water. Catalysts possessing hydrophobic surfaces (e.g., palladium on carbon) show higher activity for Suzuki coupling reactions in water than their hydrophilic counterparts (palladium on metal oxides). Tuning of the surface polarity of metal oxide supports (by silylation) results in higher activity under these conditions. Stronger alkaline conditions (three‐fold excess of base) compensate the effect of hydrophobic supports and result in high activity of the catalysts also with hydrophilic supports. The addition of tetrabutylammonium bromide to generate, activate and stabilize the catalytic species (dissolved palladium complexes) is necessary for the conversion of more demanding substrates. The reaction is considered to be homogeneous taking place near the catalyst surface inside a droplet or layer of the reactant.     

Heterogeneous Gold‐Catalyzed Selective Reductive Transformation of Quinolines with Formic Acid

Single phase rutile titania supported gold nanoparticles (Au/TiO₂‐R) are found to be efficient and versatile catalysts for chemo‐ and regioselective transfer hydrogenation of quinoline derivatives to 1,2,3,4‐tetrahydroquinolines (THQs) using formic acid (FA) as a safe and convenient hydrogen source under mild conditions. The activity and chemoselectivity of the Au/TiO₂‐R catalyst towards THQs is excellent, with a substrate to catalyst ratio (S/C) of 1000 being feasible. Furthermore, a straightforward and selective route to N‐formyltetrahydroquinolines (FTHQ) directly from quinoline compounds and FA by one‐pot, gold‐catalyzed reductive N‐formylation protocol is also established.