Palladium-Nanoparticle-Catalysed Ullmann Reactions in Ionic Liquids with Aldehydes as the Reductants: Scope and Mechanism

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Titania Nanorod-Supported Mercaptoundecanoic Acid-Grafted Palladium Nanoparticles as a Highly Reusable Heterogeneous Catalyst for Substrate-Dependent Ullmann Coupling and Debromination of Aryl Bromides

Herein, by implanting palladium nanoparticles (Pd NPs) onto titanium dioxide (TiO₂) nanorods (NRs) through 11-mercaptoundecanoic acid (MUA), we devised a robust heterogeneous catalyst. The formation of Pd-MUA-TiO₂ nanocomposites (NCs) was authenticated using Fourier transform infrared spectroscopy, powder X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray analysis, Brunauer-Emmett-Teller analysis, atomic absorption spectroscopy, and X-ray photoelectron spectroscopy techniques. Pd NPs were synthesized directly onto TiO₂ nanorods without the MUA support for comparative studies. As a means of evaluating the endurance and competency of Pd-MUA-TiO₂ NCs compared to their counterpart (Pd-TiO₂ NCs), both were used as the heterogeneous catalyst for Ullmann coupling of a wide variety of aryl bromides. When Pd-MUA-TiO₂ NCs were used, the reaction produced high yields of homocoupled products (54-88%), whereas the yield was only 76% when Pd-TiO₂ NCs were used. Moreover, Pd-MUA-TiO₂ NCs impressed with their outstanding reusability property, allowing over 14 reaction cycles without losing efficiency. On the flip side, just after seven reaction cycles, the productivity of Pd-TiO₂ NCs dropped around 50%. Presumably, the strong affinity of Pd for the thiol groups of MUA allowed for the substantial control of leaching out of Pd NPs during the reaction. Nonetheless, another crucial feature of the catalyst is that the di-debromination reaction took place with an excellent yield of 68-84% from di-aryl bromides with long alkyl chains instead of macrocyclic or dimerized products. It is worth mentioning that AAS data confirmed that only 0.30 mol % catalyst loading was sufficient to activate a broad substrate scope with large functional group tolerance.

Ligand and Cu free N-arylation of indoles, pyrroles and benzylamines with aryl halides catalyzed by a Pd nanocatalyst

Herein, the N-arylation of aromatic heterocycles like indoles and pyrroles is reported by a Pd nanocatalyst under ligand- and Cu-free conditions.

Reshaping Ullmann Amine Synthesis in Deep Eutectic Solvents: A Mild Approach for Cu-Catalyzed C-N Coupling Reactions With No Additional Ligands

The CuI-catalyzed Ullmann amine cross-coupling between (hetero)aryl halides (Br, I) and aromatic and aliphatic amines has been accomplished in deep eutectic solvents as environmentally benign and recycling reaction media. Under optimized conditions, the reaction proceeds smoothly under mild conditions (60–100°C) in air, in the absence of ligands, with a catalyst (CuI) loading of 10 mol% and K₂CO₃ (aliphatic primary and secondary amines) or t-BuOK (aromatic amines) as the base. A variety of amines have been synthesized in yields up to 98% with a broad substrate scope.

WEPA: a bio-derived medium for added base, π-acid and ligand free Ullmann coupling of aryl halides using Pd(OAc)2

A bio-derived sustainable medium based on water extract of pomegranate ash (WEPA) has, for the first time, been developed for the homocoupling of aryl halides under palladium-assistance. Avoiding the requirement of an external base, ligand and π-acid, the use of the proposed renewable medium offers remarkable attributes like wide substrate scope, good to nearly quantitative yields of biphenyls with exceptional chemoselectivity and scale up viability.

Heck Transformations of Biological Compounds Catalyzed by Phosphine-Free Palladium

The development and optimization of synthetic methods leading to functionalized biologically active compounds is described. Two alternative pathways based on Heck-type reactions, employing iodobenzene or phenylboronic acid, were elaborated for the arylation of eugenol and estragole. Cinnamyl alcohol was efficiently transformed to saturated arylated aldehydes in reaction with iodobenzene using the tandem arylation/isomerization sequential process. The arylation of cinnamyl alcohol with phenylboronic acid mainly gave unsaturated alcohol, while the yield of saturated aldehyde was much lower. Catalytic reactions were carried out using simple, phosphine-free palladium precursors and water as a cosolvent, following green chemistry rules as much as possible.

Palladium-mediated radical homocoupling reactions: a surface catalytic insight

In this work, we report a palladium nanoparticle-promoted reductive homocoupling of haloarenes, exhibiting a broad functional group tolerance. A mechanistic study was carried out, suggesting single-electron transfer processes on the metal surface.

Preparation of catalytic materials using ionic liquids as the media and functional components

Ionic liquids (ILs) have attracted much attention due to their unique properties and wide application potential in a variety of fields. The unusual properties of ILs provide numerous opportunities to design and prepare arious advanced materials, including highly efficient catalysts. In recent years, synthesis of different kinds of catalytic materials and their applications in chemical reactions have been studied extensively and have become a very interesting area. Herein, we present a review on the synthesis of catalytic materials using ILs as the media and/or functional components; the important and widely investigated topics are discussed, including mainly metal nanocatalysts/IL, functional IL/support, metals or metal oxides/IL/support, polymeric ILs (PILs) catalysts, and the performances of catalytic systems are highlighted. An outlook for this interesting area is also given at the end of the article.

Palladium-catalyzed cross-coupling of styrenes with aryl methyl ketones in ionic liquids: direct access to cyclopropanes

The combined use of Pd(OAc)2 , Cu(OAc)2 , and dioxygen in molten tetrabutylammonium acetate (TBAA) promotes an unusual cyclopropanation reaction between aryl methyl ketones and styrenes. The process is a dehydrogenative cyclizing coupling that involves a twofold CH activation at the α-position of the ketone. The substrate scope highlights the flexibility of the catalyst; a reaction mechanism is also proposed.

Fast track to nanomaterials: microwave assisted synthesis in ionic liquid media

Herein we present a general approach to metal and metal oxide nanoparticles using simple metal salts as starting materials.

Nanometallic chemistry: deciphering nanoparticle catalysis from the perspective of organometallic chemistry and homogeneous catalysis

Nanoparticle (NP) catalysis is traditionally viewed as a sub-section of heterogeneous catalysis. However, certain properties of NP catalysts, especially NPs dispersed in solvents, indicate that there could be benefits from viewing them from the perspective of homogeneous catalysis. By applying the fundamental approaches and concepts routinely used in homogeneous catalysis to NP catalysts it should be possible to rationally design new nanocatalysts with superior properties to those currently in use.

Gold nanoparticles generated by thermolysis of "all-in-one" gold(I) carboxylate complexes

Consecutive synthesis methodologies for the preparation of the gold(I) carboxylates [(Ph(3)P)AuO(2)CCH(2)(OCH(2)CH(2))(n)OCH(3)] (n = 0-6) (6a-g) are reported, whereby selective mono-alkylation of diols HO(CH(2)CH(2)O)(n)H (n = 0-6), Williamson ether synthesis and metal carboxylate (Ag, Au) formation are the key steps. Single crystal X-ray diffraction studies of 6a (n = 0) and 6b (n = 1) were carried out showing that the P-Au-O unit is essentially linear. These compounds were applied in the formation of gold nanoparticles (NP) by a thermally induced decomposition process and hence the addition of any further stabilizing and reducing reagents, respectively, is not required. The ethylene glycol functionalities, providing multiple donating capabilities, are able to stabilise the encapsulated gold colloids. The dependency of concentration, generation time and ethylene glycol chain lengths on the NP size and size distribution is discussed. Characterisation of the gold colloids was performed by TEM, UV/Vis spectroscopy and electron diffraction studies revealing that Au NP are formed with a size of 3.3 (±0.6) to 6.5 (±0.9) nm in p-xylene with a sharp size distribution. Additionally, a decomposition mechanism determined by TG-MS coupling experiments of the gold(i) precursors is reported showing that 1(st) decarboxylation occurs followed by the cleavage of the Au-PPh(3) bond and finally release of ethylene glycol fragments to give Au-NP and the appropriate organics.

Palladium/zirconium oxide nanocomposite as a highly recyclable catalyst for C-C coupling reactions in water

Palladium nanoparticles have been electrochemically supported on zirconium oxide nanostructured powders and all the nanomaterials have been characterized by several analytical techniques. The Pd/ZrO(2) nanocatalyst is demonstrated to be a very efficient catalyst in Heck, Ullmann, and Suzuki reactions of aryl halides in water. The catalyst efficiency is attributed to the stabilization of Pd nanophases provided by tetra(alkyl)- ammonium hydroxide, which behaves both as base and PTC (phase transfer catalyst) agent.

Carbon-carbon cross coupling reactions in ionic liquids catalysed by palladium metal nanoparticles

A brief summary of selected pioneering and mechanistic contributions in the field of carbon-carbon cross-coupling reactions with palladium nanoparticles (Pd-NPs) in ionic liquids (ILs) is presented. Five exemplary model systems using the Pd-NPs/ILs approach are presented: Heck, Suzuki, Stille, Sonogashira and Ullmann reactions which all have in common the use of ionic liquids as reaction media and the use of palladium nanoparticles as reservoir for the catalytically active palladium species.