Recent Advances in Palladium-Catalyzed Cross-Coupling Reactions at ppm to ppb Molar Catalyst Loadings

Palladium Comprising Dicationic Bipyridinium Supported Periodic Mesoporous Organosilica (PMO): Pd@Bipy–PMO as an Efficient Hybrid Catalyst for Suzuki–Miyaura Cross-Coupling Reaction in Water

In this study, we developed a novel catalysts consisting of periodic mesoporous organosilica functionalized with bipyridinium ionic liquid supported palladium. The physiochemical properties of the hybrid catalyst were investigated using Fourier transform infrared spectroscopy, small angle X–ray powder diffraction, field emission scanning electron microscope, transmission electron microscope, nitrogen adsorption–desorption analyses, and atomic absorption spectroscopy. The stabilized Pd species inside the mesochannels provided good catalytic efficiency for the Suzuki–Miyaura coupling reactions in water. The activity of the designed catalysts retained for several consecutive recycle runs. The stability, recoverability, and reusability of the designed heterogeneous catalyst were also studied under various reaction conditions.

Synthesis, structural properties and reactivity of ruthenocene-based pincer Pd(ii) tetrahydroborate

Non-covalent interactions determine the structure, crystal packing and reactivity of isolated ruthenocene-based pincer Pd(ii) complexes. Bifurcate dihydrogen-bonded complexes are active intermediates of tetrahydroborate alcoholysis.

Simple fabrication of a carbaldehyde based fluorescent “turn-on” probe for the selective and sole detection of Pd2+: application as test strips

A simple fluorescent “turn-on” probe (DHMC) has been designed for selective and sole detection of Pd²⁺.

Enantioselective conjugate addition of an α,α-dithioacetonitrile with nitroalkenes using chiral bis(imidazoline)–Pd complexes

The first highly enantioselective reaction of α,α-dithioacetonitriles with nitroolefins as electron deficient olefins using chiral Phebim–Pd catalysts was developed.

Decarboxylative Suzuki–Miyaura coupling of (hetero)aromatic carboxylic acids using iodine as the terminal oxidant

A novel methodology for the decarboxylative Suzuki–Miyaura-type coupling of carboxylic acids with arylboronic acids has been developed.

Recent advances in the application of nano-catalysts for Hiyama cross-coupling reactions

This mini-review highlights the recent developments in the field of metal nanoparticle (NP) catalyzed Hiyama cross-coupling reactions.

Dimethylformamide-stabilised palladium nanoclusters catalysed coupling reactions of aryl halides with hydrosilanes/disilanes

N,N-Dimethylformamide-stabilised Pd nanocluster (NC) catalysed cross-coupling reactions of hydrosilane/disilane have been investigated. In this reaction, the coupling reaction proceeds without ligands with low catalyst loading. N,N-Dimethylacetamide is a crucial solvent in these reactions. The solvent effect was considered by various techniques, such as transmission electron microscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The Pd NCs can be recycled five times under both hydrosilane and disilane reaction conditions.

A highly efficient and reusable Pd NCs catalyst system for silylation of aryl halides was developed.

Controlling the selectivity and efficiency of the hydrogen borrowing reaction by switching between rhodium and iridium catalysts

Fundamental insights into rhodium and iridium catalysed hydrogen borrowing reactions have enabled strategies for controlling product selectivity.

Recyclable and Reusable Heteroleptic Nickel Catalyst Immobilized on Metal-Organic Framework for Suzuki-Miyaura Coupling

Metal-organic frameworks (MOFs) provide highly versatile platforms to stabilize molecular catalysts that are not readily accessible under homogeneous conditions, thus enabling access to a new set of catalytic materials. Herein, we describe a recyclable and highly active nickel catalyst immobilized on MOF for Suzuki-Miyaura coupling reaction, which operates under mild conditions. This mixed ligand catalyst forms from the combination of 1 equiv of MOF-immobilized ligand, 1 equiv of nickel source, and 1 equiv of PPh₃. The nature of the catalyst was verified through a series of analytical tests and catalysis experiments. The immobilized catalyst was reusable for at least up to 7 cycles without decrease in the yield of the coupled product. We also verified that this reaction does not work under homogeneous conditions and that the reaction is truly heterogeneous through "hot filtration" experiments. We identified that the reaction is first order in arylborane concentration and negative order in arylbromide concentration through the effect of substrate concentrations on the initial rate. This informed us to conduct the catalysis under slow addition of the arylbromide and reduce the catalyst loading to 1% from 3%, without detriment to the yield or rate of the reaction. The catalyst gave good to excellent isolated yields with a range of functionalities, including heterocycles on aryl bromide with widely varying electronic properties.

Applications of SBA-15 supported Pd metal catalysts as nanoreactors in C-C coupling reactions

Nanoreactors are material structures with engineered internal cavities which create exclusive confined nanoscale surroundings for chemical reactions. The cavities of mesoporous silica SBA-15 can be used as nanoreactors for incorporating catalytic species such as metal nanoparticles, complexes etc. Since SBA-15 silica has a neutral framework, organic functional groups and heteroatoms have been embedded by direct or post-synthesis approaches in order to modify their functionality. Palladium is the most used transition metal for C-C bond formations. Because of the great importance of C-C coupling reactions, this review article aims at providing a deep insight into the state of art in the field of the synthesis and the application of mesoporous SBA-15 silica-supported Pd catalysts in C-C coupling transformations. In most cases, synthesis and modification of the catalyst, time and yield of reactions, recyclability and leaching of the Pd species from the SBA-15 support are discussed to reveal the role of SBA-15 in C-C coupling reactions.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Pseudo-tetrahedral Rhodium and Iridium Complexes: Catalytic Synthesis of E-Enynes

The reactions of the rhodium(I) and iridium(I) complexes [M(PhBP₃ )(C₂ H₄ )(NCMe)] (PhBP₃ =PhB(CH₂ PPh₂ )₃ ^- ) with alkynes have resulted in the synthesis of a new family of pseudo-tetrahedral complexes, [M(PhBP₃ )(RC≡CR')] (M=Rh, Ir), which contain the alkyne as a four-electron donor. The reactions of these unusual compounds with two-electron donors (L=PMe₃ , CNtBu) produced a change in the "donicity" of the alkyne from a 4e^- to a 2e^- donor to give five-coordinate complexes. These were the final products with the iridium complexes, whereas further reactions took place with the rhodium complexes. In particular, C(sp)-H bond activation of the alkyne occurred leading to hydrido alkynyl complexes. This process is essential for the further reactivity of the alkynes, and if the alkyne itself was used as reagent, E-enyne complexes were obtained. As a consequence of this chemistry, we show that the complex [Rh(PhBP₃ )(C₂ H₄ )(NCMe)] is a very efficient pre-catalyst for the regioselective di- and trimerization of terminal alkynes to E-enynes and benzene derivatives, respectively. Interestingly, acetonitrile significantly enhanced the catalytic activity by facilitating the C(sp)-H bond activation step. A hydrometalation mechanism to account for these experimental observations is proposed.

Synthesis of Pyridazine Derivatives by Suzuki-Miyaura Cross-Coupling Reaction and Evaluation of Their Optical and Electronic Properties through Experimental and Theoretical Studies

A series of π-conjugated molecules, based on pyridazine and thiophene heterocycles 3a–e, were synthesized using commercially, or readily available, coupling components, through a palladium catalyzed Suzuki-Miyaura cross-coupling reaction. The electron-deficient pyridazine heterocycle was functionalized by a thiophene electron-rich heterocycle at position six, and different (hetero)aromatic moieties (phenyl, thienyl, furanyl) were functionalized with electron acceptor groups at position three. Density Functional Theory (DFT) calculations were carried out to obtain information on the conformation, electronic structure, electron distribution, dipolar moment, and molecular nonlinear response of the synthesized push-pull pyridazine derivatives. Hyper-Rayleigh scattering in 1,4-dioxane solutions, using a fundamental wavelength of 1064 nm, was used to evaluate their second-order nonlinear optical properties. The thienylpyridazine functionalized with the cyano-phenyl moiety exhibited the largest first hyperpolarizability (β = 175 × 10⁻³⁰ esu, using the T convention) indicating its potential as a second harmonic generation (SHG) chromophore.

Structural, Luminescent and Thermal Properties of Heteronuclear PdII⁻LnIII⁻PdII Complexes of Hexadentate N₂O₄ Schiff Base Ligand

New Pd^II–Ln^III–Pd^II complexes of hexadentate N₂O₄ Schiff base ligand (H₄L: N,N′-bis(2,3-dihydroxybenzylidene)-1,3-diamino-2,2-dimethylpropane) with Eu (1), Tb (2), Er (3) and Yb (4) ([Pd₂Eu(H₂L)₂NO₃](NO₃)₂∙2H₂O∙2CH₃OH 1, [Pd₂Ln(H₂L)₂H₂O](NO₃)₃∙3H₂O, where Ln = Tb 2, Er 3, [Pd₂Yb(H₂L)₂H₂O](NO₃)₃∙5.5H₂O 4) were synthesized and characterized structurally and physicochemically by thermogravimetry (TG), differential thermogravimetry (DTG), differential scanning calorimetry (DSC) and luminescence measurements. The compounds 1–4 are built of cationic heterometallic Pd^II–Ln^III–Pd^II trinuclear units. The palladium(II) centers adopt a planar square geometry occupying the smaller N₂O₂ cavity of the Schiff base ligand. The lanthanide(III) is surrounded by two Schiff base ligands (eight oxygen atoms) and its coordination sphere is supplemented by a chelating bidentate nitrate ion in 1 or by a water molecule in 2–4. The complexes have a bent conformation along the Pd^II–Ln^III–Pd^II line with valence angles in the ranges of 162–171°. The decomposition process of the complexes results in mixtures of: PdO, Pd and respective lanthanide oxides Eu₂O₃, Tb₂O₃, Tb₄O₇, Er₂O₃, Yb₂O₃. The luminescent measurements show low efficiency intramolecular energy transfer only in the complex of terbium(III) (2).

The Palladium-Catalyzed Ullmann Cross-Coupling Reaction: A Modern Variant on a Time-Honored Process

Cross-coupling reactions, especially those that are catalyzed by palladium, have revolutionized the way in which carbon-carbon bonds can be formed. The most commonly deployed variants of such processes are the Suzuki-Miyaura, Mizoroki-Heck, Stille, and Negishi cross-coupling reactions, and these normally involve the linking of an organohalide or pseudohalide (such as a triflate or nonaflate) with an organo-metallic or -metalloid such as an organo-boron, -magnesium, -tin, or -zinc species. Since the latter type of coupling partner is often prepared from the corresponding halide, methods that allow for the direct cross-coupling of two distinct halogen-containing compounds would provide valuable and more atom-economical capacities for the formation of carbon-carbon bonds. While the venerable Ullmann reaction can in principle achieve this, it has a number of drawbacks, the most significant of which is that homocoupling of the reaction partners is a competitive, if not the dominant, process. Furthermore, such reactions normally occur only under forcing conditions (viz., often at temperatures in excess of 250 °C). As such, the Ullmann reaction has seen only limited application in this regard, especially as a mid- to late-stage feature of complex natural product synthesis. This Account details the development of the palladium-catalyzed Ullmann cross-coupling reaction as a useful method for the assembly of a range of heterocyclic systems relevant to medicinal and/or natural products chemistry. These couplings normally proceed under relatively mild conditions (<100 °C) over short periods of time and, usually, to the exclusion of (unwanted) homocoupling events. The keys to success are the appropriate choice of coupling partners, the form of the copper metal employed, and the choice of reaction solvent. At the present time, the cross-coupling partners capable of engaging in the title reaction are confined to halogenated and otherwise electron-deficient arenes and, as complementary reactants, α- or β-halogenated, α,β-unsaturated aldehydes, ketones, esters, lactones, lactams, and cycloimides. Nitro-substituted (and halogenated) arenes, in particular, serve as effective participants in these reactions, and the products of their coupling with the above-mentioned carbonyl-containing systems can be manipulated in a number of different ways. Depending on the positional relationship between the nitro and carbonyl groups in the cross-coupling product, the reduction of the former group, which can be achieved under a range of different conditions, provides, through intramolecular nucleophilic addition reactions, including Schiff base condensations, access to a diverse range of heterocyclic systems. These include indoles, quinolines, quinolones, isoquinolines, carbazoles, and carbolines. Tandem variants of such cyclization processes, in which Raney cobalt is used as a catalyst for the chemoselective reduction (by dihydrogen) of nitro and nitrile groups (but not olefins), allow for the assembly of a range of structurally challenging natural products, including marinoquinoline A, (±)-1-acetylaspidoalbidine, and (±)-gilbertine.

Heterogeneous Catalytic Composites from Palladium Nanoparticles in Montmorillonite Intercalated with Poly (Vinyl Pyrrolidone) Chains

In this study, poly (vinyl pyrrolidone) (PVP) chains intercalated montmorillonite (MMT) matrices has been demonstrated as an excellent scaffolding material for the immobilization of palladium (Pd) nanoparticles to prepare efficient heterogeneous catalysts for Heck reactions. Multiple layers (up to four) of PVP chains can intercalate the interlayer space of the MMT, resulting in an increase therein from 1.25 to 3.22 nm. MMT/PVP with PVP loading (20%) was selected as the platform for the immobilization of Pd. The in-situ reduction of the chelated Pd²⁺ into Pd⁰ in the interlayer space of MMT/PVP composite could be easily achieved. For the prepared Pd@MMT/PVP catalytic composite, a unique maze-like microstructure of Pd nanoparticles tightly encaged by PVP chains and by lamellae of layered silica has been detected by high resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD). Furthermore, the microstructure is well elucidated in molecular level by positron annihilation lifetime analysis of the Pd@MMT/PVP catalytic composite. The prepared Pd@MMT/PVP catalysts were highly active for the Heck coupling reactions between aromatic halides and alkenes, and could be recycled 9 times without significant decreases in coupling yields. The excellent comprehensive catalytic performances of the Pd@MMT/PVP catalytic composites are mainly attributed to their unique maze-like microstructure.

Palladium immobilized on in situ cross-linked chitosan superfine fibers for catalytic application in an aqueous medium

Chitosan composite superfine fibers with a diameter of 321 ± 99 nm were prepared by electrospinning with PEO as the co-spinning polymer and itaconic acid as the in situ cross-linking agent.

Liquid-phase catalysis by single-size palladium nanoclusters supported on strontium titanate: size-specific catalysts for Suzuki–Miyaura coupling

Size-specific catalysis by single-size palladium nanoclusters.

Synthesis of a palladium acetylide-based tubular microporous polymer monolith via a self-template approach: a potential precursor of supported palladium nanoparticles for heterogeneous catalysis

A monolithic, palladium acetylide-based conjugated microporous polymer, Pd-CMP, was synthesized from a palladium dichloride and a trialkyne. The polymerization proceeded in two different ways, the dehydrohalogenation reaction between the alkyne and the palladium halide and the homocoupling reaction of the alkyne. Pd-CMP had a rigid hollow tubular structure. The in situ formed crystalline triethylammonium chloride (TEACl) rod played a critical role in the formation of the tubular morphology as a template. Through the attachment of the polymer particles to the surface of the rod and their reactions with soluble alkynes, a core–shell structure with a TEACl core and a polymer shell formed. The TEACl core was removed by washing with methanol to yield a hollow polymer tube. Pd-CMP showed a hierarchical pore structure and reversible compressibility. Supported Pd nanoparticles were prepared by one-step thermolysis of Pd-CMP as a heterogeneous catalyst. The average diameters of NPs in the products thermolyzed at 300 (Pd-CMP₃₀₀) and 500 °C (Pd-CMP₅₀₀) were 2.6 and 4.1 nm, respectively. Pd-CMP₃₀₀ was used in the heterogeneous catalysis of the 4-nitrophenol reduction reaction and Suzuki–Miyaura coupling between iodobenzene and phenylboronic acid. The reaction yields were higher than 95%. The catalyst could be used for a flow reaction and easily recycled without significant activity loss.

A monolithic palladium acetylide-based tubular microporous polymer was synthesized as a promising precursor of a palladium heterogeneous catalyst.