Carbon−Carbon Coupling Reactions Catalyzed by Heterogeneous Palladium Catalysts

Pd immobilization biguanidine modified Zr-UiO-66 MOF as a reusable heterogeneous catalyst in Suzuki-Miyaura coupling

In recent days, nanohybrid metal organic frameworks (MOF) have been considered as next generation catalysts due to their unique features like large surface to volume ratio, tailorable geometry, uniform pore sizes and homogeneous distribution of active sites. In this report, we address the biguanidine modified 3D Zr-centred MOF UiO-66-NH2 following a post synthetic modification approach. Utilizing the excellent chelating ability of biguanidine, Pd ions are immobilized over the host matrix MOF. The as-synthesized material was physicochemically characterized using a broad range of analytical techniques like FT-IR, electron microscopy, EDS, elemental mapping, XRD and ICP-OES. Subsequently the material has been catalytically employed in the classical Suzuki-Miyaura coupling towards the synthesis of diverse biphenyl derivatives at sustainable conditions. There are very few reports on the covalently modified MOFs towards the organic coupling reactions. The catalyst has been isolated by centrifugation and recycled in 9 consecutive runs with almost insignificant leaching and minute decrease in reactivity.

Microflowers formed by complexation-driven self-assembly between palladium(ii) and bis-theophyllines: immortal catalyst for C-C cross-coupling reactions

The Pd catalyst for Suzuki–Miyaura or the other C–C coupling reactions is one of the central tools in organic synthesis related to medicine, agricultural chemicals and advanced materials. However, recycling palladium is a bottleneck for developing the extreme potential of Pd in chemistry. Herein, we established a new heterogeneous Pd catalytic system in which the catalyst is a nanopetal-gathered flower-like microsphere self-assembled from PdCl₂ and alkyl-linked bis-theophyllines. The microflowers catalyzed quantitatively the reaction of aryl bromides and phenylboronic acid in aqueous media at room temperature. It was found that the reaction proceeds better in an air atmosphere than in nitrogen gas even though the Pd(ii) species employed was lowered to 0.001 mol% in the substance. Very interestingly, the microflowers could be recycled 20 times without deactivation in the C–C coupling reaction between bromobenzene and phenylboronic acid in the presence of sodium chloride. We found that the sodium chloride added played an important role in maintaining the morphology of microflowers and preventing the formation of metallic Pd particles.

Bis-theophylline-palladium complex exhibit high catalytic activity in the C–C coupling reaction with excellent recyclability in the presence of NaCl.

Application of polyionic magnetic nanoparticles as a catalyst for the synthesis of carbonitriles with both indole and triazole moieties via a cooperative geminal-vinylogous anomeric-based oxidation

Three-component reaction of aldehydes with 3-(1H-indol-3-yl)-3-oxopropanenitrile and 1H-1,2,4-triazol-5-amine under the solvent-free condition at 70 °C was effectively performed in the presence of 2 mg of polyionic magnetic nanoparticles with pyrazine bridge [Fe₃O₄@SiO₂@(CH₂)₃]₂-Pyrazinium-[TCM]₂ as a catalyst for the synthesis of 7-aryl-5-(1H-indol-3-yl)-[1,2,4]triazolo[1,5-a]pyrimidine-6-carbonitriles via a cooperative anomeric-based oxidation. The polyionic magnetic nanoparticles catalyst was simply recovered and reused four successive runs. The morphology and structure of MNPs catalyst were investigated by numerous techniques such as XRD, FT-IR, EDX, WDX, FE-SEM, TEM, TGA, DTA, and VSM. The obtained products are reported for the first time that were identified by various analyses techniques such as melting point, FT-IR, ¹H NMR, ¹³C NMR, and elemental analysis (CHN). A term entitled a cooperative geminal-vinylogous anomeric-based oxidation was introduced for the latter step of the reaction mechanism for the first time. Synthesis of 7-aryl-5-(1H-indol-3-yl)-[1,2,4]triazolo[1,5-a]pyrimidine-6-carbonitriles by using [Fe₃O₄@SiO₂@(CH₂)₃]₂-Pyrazinium-[TCM]₂ MNPs as a catalyst.

Effect of the length and aromaticity of N3-substituent on adsorption performance of imidazolium-based poly(ionic liquids) towards Pd (II)

Imidazolium-based poly(ionic liquids) (PILs) have been deemed as attractive candidates in the field of precious metal adsorption. However, their further performance optimization is hampered by a lack of an inner understanding of the structure-adsorption performance relationship. In this research, electron and charge distributions of the imidazolium cations are tailored by changing the N3-substitute, and their adsorption performances for PdCl₄^2- were optimized accordingly. Furthermore, the adsorption mechanism is studied by synthesizing corresponding ionic liquid (IL) monomers and their Pd-adducts. Interestingly, longer N3 alkyl chains lead to more hydrogen bonds with PdCl₄^2-, which is beneficial for adsorption. Whereas, it is unfavorable for attracting anions due to a decrease in electrostatic potential (ESP) around cations caused by longer alkyl chains and aromatic substituents at N3 position. It is worth noting that the ESP around the cations plays a more important role in the adsorption process, which determines the adsorption performance of the imidazolium-based PILs. Thus, the performance optimization of imidazolium-based PILs should mainly focus on increasing the ESP of imidazolium cations in the future. This research highlights the potential of the cationic structure-adsorption performance relationship of PILs, which opens a new avenue to develop adsorbents for the metallurgical industry.

Hydrogen Bonding Directed Self-Assembly of a Binuclear Ag(I) Metallacycle into a 1D Supramolecular Polymer

An Ag(I) metallacycle obtained unexpectedly during the preparation of Pd(II) complexes of the bifunctional ligand 5-([2,2′-bipyridin]-5-yl)pyrimidine-2-amine (L) has been characterized using X-ray structure determination as a binuclear, metallacyclic species [Ag₂L₂](SbF₆)₂, where both the bipyridine and pyrimidine-N donors of L are involved in coordination to the metal. The full coordination environment of the Ag(I) defines a case of highly irregular 4-coordination. In the crystal, the Ag-metallacycles assemble into one-dimensional supramolecular metalladynamers linked together by hydrogen-bonding interactions.

Recent Advances of Pd/C-Catalyzed Reactions

The Pd/C-catalyzed reactions, including reduction reactions and cross-coupling reactions, play an irreplaceable role in modern organic synthesis. Compared to the homogeneous palladium catalyst system, the heterogeneous Pd/C catalyst system offers an alternative protocol that has particular advantages and applications. Herein, a review on Pd/C-catalyzed reactions is presented. Both the advances in Pd/C-catalyzed methodologies and the application of Pd/C-catalysis in total synthesis are covered in this review.

Tri-tert-butyl(n-alkyl)phosphonium Ionic Liquids: Structure, Properties and Application as Hybrid Catalyst Nanomaterials

A series of sterically hindered tri-tert-butyl(n-alkyl)phosphonium salts (n-CnH2n+1 with n = 1, 3, 5, 7, 9, 11, 13, 15, 17) was synthesized and systematically studied by 1H, 13C, 31P NMR spectroscopy, ESI-MS, single-crystal X-ray diffraction analysis and melting point measurement. Formation and stabilization palladium nanoparticles (PdNPs) were used to characterize the phosphonium ionic liquid (PIL) nanoscale interaction ability. The colloidal Pd in the PIL systems was described with TEM and DLS analyses and applied in the Suzuki cross-coupling reaction. The PILs were proven to be suitable stabilizers of PdNPs possessing high catalytic activity. The tri-tert-butyl(n-alkyl)phosphonium salts showed a complex nonlinear correlation of the structure–property relationship. The synthesized family of PILs has a broad variety of structural features, including hydrophobic and hydrophilic structures that are entirely expressed in the diversity of their properties

Asymmetric Synthesis of Chiral Bicyclo[2.2.1]hepta-2,5-diene Ligands through Rhodium-Catalyzed Asymmetric Arylative Bis-cyclization of a 1,6-Enyne

A series of novel chiral diene ligands (1R,4S)-L₁, which are based on the bicyclo[2.2.1]heptadiene skeleton and are substituted with methyl and an ester group at the bridgehead carbons, were synthesized through rhodium-catalyzed asymmetric arylative bis-cyclization of 1,6-enyne 1 as a key step. The rhodium catalyst with one of the (1R,4S)-L₁ ligands was used for the asymmetric bis-cyclization of 1 giving bicyclic product (1S,4R)-2 of 99% ee, which is a synthetic precursor of (1S,4R)-L₁ ligands.

Bimetallic Nanocrystals: Structure, Controllable Synthesis and Applications in Catalysis, Energy and Sensing

In recent years, bimetallic nanocrystals have attracted great interest from many researchers. Bimetallic nanocrystals are expected to exhibit improved physical and chemical properties due to the synergistic effect between the two metals, not just a combination of two monometallic properties. More importantly, the properties of bimetallic nanocrystals are significantly affected by their morphology, structure, and atomic arrangement. Reasonable regulation of these parameters of nanocrystals can effectively control their properties and enhance their practicality in a given application. This review summarizes some recent research progress in the controlled synthesis of shape, composition and structure, as well as some important applications of bimetallic nanocrystals. We first give a brief introduction to the development of bimetals, followed by the architectural diversity of bimetallic nanocrystals. The most commonly used and typical synthesis methods are also summarized, and the possible morphologies under different conditions are also discussed. Finally, we discuss the composition-dependent and shape-dependent properties of bimetals in terms of highlighting applications such as catalysis, energy conversion, gas sensing and bio-detection applications.

Catalytic Liquefaction of Kraft Lignin with Solvothermal Approach

Lignin is a natural biopolymer present in lignocellulosic biomass. During paper pulp production with the Kraft process, it is solubilized and degraded in Kraft lignin and then burned to recover energy. In this paper, the solvolysis of Kraft lignin was studied in water and in water/alcohol mixtures to produce oligomers and monomers of interest, at mild temperatures (200–275 °C) under inert atmosphere. It was found that the presence of alcohol and the type of alcohol (methanol, ethanol, isopropanol) greatly influenced the amount of oligomers and monomers formed from lignin, reaching a maximum of 48 mg·glignin−1 of monomers with isopropanol as a co-solvent. The impact of the addition of various solid catalysts composed of a metal phase (Pd, Pt or Ru) supported on an oxide (Al2O3, TiO2, ZrO2) was investigated. In water, the yield in monomers was enhanced by the presence of a catalyst and particularly by Pd/ZrO2. However, with an alcoholic co-solvent, the catalyst only enhanced the formation of oligomers. Detailed characterizations of the products with FTIR, 31P-NMR, 1H-NMR and HSQC NMR were performed to elucidate the chemical transformations occurring during solvolysis. The nature of the active catalytic specie was also investigated by testing homogeneous palladium catalysts.

Mechanistic insights into carbon-carbon coupling on NiAu and PdAu single-atom alloys

Carbon-carbon coupling is an important step in many catalytic reactions, and performing sp³-sp³ carbon-carbon coupling heterogeneously is particularly challenging. It has been reported that PdAu single-atom alloy (SAA) model catalytic surfaces are able to selectively couple methyl groups, producing ethane from methyl iodide. Herein, we extend this study to NiAu SAAs and find that Ni atoms in Au are active for C-I cleavage and selective sp³-sp³ carbon-carbon coupling to produce ethane. Furthermore, we perform ab initio kinetic Monte Carlo simulations that include the effect of the iodine atom, which was previously considered a bystander species. We find that model NiAu surfaces exhibit a similar chemistry to PdAu, but the reason for the similarity is due to the role the iodine atoms play in terms of blocking the Ni atom active sites. Specifically, on NiAu SAAs, the iodine atoms outcompete the methyl groups for occupancy of the Ni sites leaving the Me groups on Au, while on PdAu SAAs, the binding strengths of methyl groups and iodine atoms at the Pd atom active site are more similar. These simulations shed light on the mechanism of this important sp³-sp³ carbon-carbon coupling chemistry on SAAs. Furthermore, we discuss the effect of the iodine atoms on the reaction energetics and make an analogy between the effect of iodine as an active site blocker on this model heterogeneous catalyst and homogeneous catalysts in which ligands must detach in order for the active site to be accessed by the reactants.

Revealing the Structure Evolution of Heterogeneous Pd Catalyst in Suzuki Reaction via the Identical Location Transmission Electron Microscopy

The mechanism of palladium nanoparticles (Pd NPs)-catalyzed cross-coupling reactions has been the subject of intense debate since the recognition of catalytic active sites involving a wide array of dynamic changed Pd species. Here, through the combination of the hot filtration experiment together with the recently developed identical location transmission electron microscopy (IL-TEM) method, the delicate structure evolution of highly dispersed Pd NPs supported on oxygen-functionalized carbon nanotubes (Pd/oCNTs) as well as the kinetics properties of derived dissolved species in liquid phase were systemically investigated in the Suzuki-Miyaura reaction. The result indicates that the leached Pd components caused by the strong adsorption of reactants might have a significant contribution to the coupling products, and the degree for different substrates follows the order of iodobenzene > phenylboronic acid > bromobenzene. Meanwhile, the typical three sequential behaviors of supported Pd NPs, including dissolution, deposition, and growth, along with the increase of the conversion throughout the reaction were spatiotemporally observed by tracking the evolution of individually identifiable NPs. The performed work not only provides direct evidence for the interaction between Pd NPs surface with reactants on atomic scale but also gives a valuable reference for fundamentally understanding the mechanism of the heterogeneous Pd-catalyzed Suzuki coupling process as well as rational design of next-generation catalysts with high efficiency and reusability for synthetic applications.

A robust polyfunctional Pd(II)-based magnetic amphiphilic nanocatalyst for the Suzuki-Miyaura coupling reaction

Herein, a robust Pd(II)-based polyfunctional magnetic amphiphilic artificial metalloenzyme was prepared by anchoring a Pd(2,2'-dipyridylamine)Cl2 bearing hydrophilic monomethyl ether poly(ethylene glycol) (mPEG) chains on the surface of amino-functionalized silica-coated magnetic nanoparticles. The 2,2'-dipyridylamine (dpa) has shown excellent complexation properties for Pd(II) and it could be easily anchored onto functionalized magnetic support by the bridging nitrogen atom. Moreover, the bridging nitrogen atom at the proximity of Pd(II) catalytic center could play an important role in dynamic suppramolecular interactions with substrates. The leaching, air and moisture resistant [Pd(dpa)Cl2] complex endow the dynamic and robust structure to the designed artificial enzyme. Moreover, the water dispersibility of designed artificial metalloenzyme raised from mPEG chains and the magnetic nanoparticles core which could function as protein mimics endow it other necessary characters of artificial enzymes. The prepared artificial metalloenzyme displayed remarkable activity in Suzuki-Miyaura cross-coupling reaction employing low-palladium loading under mild conditions, with the exceptionally high turnover frequency, clean reaction profile, easy work-up procedure, good to excellent products yields and short reaction times. The designed air- and moisture-stable artificial metalloenzyme could recycle more than fifteen times with easy separation procedure in aqueous solution under aerobic conditions without any noticeable loss in activity.

Efficient Heterogeneous Palladium Catalysts in Oxidative Cascade Reactions

Palladium-catalyzed oxidations involving cascade processes provide a versatile platform for streamlined conversion of simple feedstocks into functional molecules with high atom and step economy. However, the achievement of high palladium efficiency and selectivity in Pd-catalyzed oxidative cascade reactions is still challenging in many cases, as a result of the aggregation of active palladium species to Pd black and the possible side reactions during each bond-forming step. The two current solutions for addressing these issues are either to utilize oxidant-stable ligands or to use electron transfer mediators (ETMs). The former solution, which includes the use of amines, pyridines, sulfoxides, and carbene derivatives, inhibits aggregation of Pd⁰ during the catalytic cycle, while the latter solution facilitates reoxidation of Pd⁰ to Pd^II to improve the activity and selectivity. Following our long-standing interest in Pd-catalyzed oxidations, very recently we developed heterogeneous catalysts to resolve the issues mentioned above in oxidative cascade reactions. The heterogeneous palladium catalysts (Pd-AmP-MCF or Pd-AmP-CNC) comprise palladium nanoclusters (1–2 nm) immobilized on amino-functionalized siliceous mesocellular foam (MCF) or on crystalline nanocellulose (CNC), exhibiting high activity, selectivity as well as excellent recycling ability.

In this Account, we will discuss the synthesis and characterizations of the heterogeneous palladium catalysts, as well as their catalytic behaviors, and the mechanisms involved in their reactions. An important aspect of these catalysts in oxidation reactions is the generation of active Pd(II) species within the heterogeneous phase. Typical oxidative cascade reactions of our recent research on this topic include oxidative carbocyclization-carbonylation, oxidative carbocyclization-borylation, oxidative alkynylation-cyclization, oxidative carbonylation-cyclization, and oxidative carbocyclization-alkynylation. These reactions provide access to important compounds attractive in medicinal chemistry and functional materials, such as γ-lactone/γ-lactam-based poly rings, cyclobutenols, highly substituted furans, and oxaboroles. During these processes, the heterogeneous catalysts exhibited much higher turnover numbers (TONs) than their homogeneous counterparts (e.g., Pd(OAc)₂) as well as unique selectivity that cannot be achieved by homogeneous palladium catalysts. The origin of the high efficiency and unique selectivity of the heterogeneous catalysts was also investigated. Asymmetric syntheses for the construction of optically pure compounds were realized based on the excellent selectivity in these heterogeneous processes. Kinetic studies revealed that the rate and yield of the reactions were essentially maintained during recycling, which demonstrates that Pd-AmP-MCF and Pd-AmP-CNC are robust and highly active in these oxidative cascade reactions. In addition, inductively coupled plasma optical emisson spectroscopy (ICP-OES) analysis and hot filtration test suggest that these processes most likely proceed via a heterogeneous pathway.

Recent progress in our group has shown that the activity of Pd-AmP-MCF and Pd-AmP-CNC could be improved even further by the addition of Ag⁺ to generate cationic Pd(II). Furthermore, intriguing solvent effects were observed in a Pd-AmP-MCF-catalyzed oxidative cascade process, and solvent-controlled chemoselective transformations were developed based on this property of the catalyst. The heterogeneous strategy of this Account provides solutions to palladium deactivation and selectivity issues in Pd(II)-catalyzed oxidative cascade reactions and enables efficient catalyst recycling, which will open up new opportunities in oxidative cascade reactions.

Controlled synthesis of conjugated polymers in dendritic mesoporous silica nanoparticles

A controlled polymerization strategy is developed by confining the step-growth polycondensation to take place exclusively in the nanochannels of dendritic mesoporous silica nanoparticles. A variety of conjugated polymers with rich structural patterns were obtained in high yields. The molecular weights were precisely controlled with narrow molecular weight distributions. The obtained conjugated polymers were freely processed in solution and casted in film, showing bright fluorescence emission. All the features of this controlled polymerization method endow the conjugated polymers great potential for future applications.

Nickel-Fe3O4 Magnetic Nanoparticles Supported on Multiwalled Carbon Nanotubes: Effective Catalyst in Suzuki Cross Coupling Reactions

Nickel-Fe3O4 nanoparticles supported on multi-walled carbon nanotubes (Ni-Fe3O4/MWCNTs) were synthesized by mechanical grinding of a sample of nickel salt, Fe3O4 and MWCNTs using a ball-mill mixer. The preparation method allows for bulk production of Ni-Fe3O4 nanoparticles at room temperature without the necessity of any solvent or chemical reagent. The nanoparticles prepared by this method exhibit small particles size of 5–8 nm with uniform dispersion of nickel nanoparticles on the surface of multi-walled carbon nanotubes. The Ni-Fe3O4/MWCNTs demonstrated remarkable catalytic activity for Suzuki cross coupling reactions of functionalized aryl halides and phenylboronic acids with excellent turnover number and turnover frequency (e.g., 76,000 h−1) using Monowave 50 conventional heating reactor at 120 °C within a very short reaction time of 15 min. The catalyst is air-stable and exhibits easy removal from the reaction mixture due to its magnetic properties, recyclability with no loss of activity, and significantly better performance than the other well-known commercial nickel catalyst. The Ni-Fe3O4/MWCNTs nanoparticles were fully characterized by a variety of spectroscopic techniques including X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). Since nickel offers similar properties to other more expensive transition metals including the most widely used palladium counterpart in cross coupling catalysis, this work demonstrates a promising lower-cost, air-moisture stable and efficient alternative catalyst based on nickel nanoparticles for cross coupling reactions.

A regioselective C7 bromination and C7 palladium-catalyzed Suzuki-Miyaura cross-coupling arylation of 4-substituted NH-free indazoles

A direct and efficient regioselective C7-bromination of 4-substituted 1H-indazole has been achieved. Subsequently, a successful palladium-mediated Suzuki–Miyaura reaction of C7-bromo-4-substituted-1H-indazoles with boronic acids has been performed under optimized reaction conditions. A series of new C7 arylated 4-substituted 1H-indazoles was obtained in moderate to good yields.

A regioselective C7-bromination of 4-substituted 1H-indazoles followed by a palladium-catalyzed Suzuki–Miyaura reaction with boronic acids is described.