Heterogeneous Palladium Catalyst Research Articles

Heterogenization of Palladium Trimer and Nanoparticles through Polymerization Boosted Catalytic Efficiencies in Recyclable Coupling and Reduction Reactions.

The development of heterogeneous palladium catalysts has shown continuous vitality in the field of catalysis and materials. In this work, we report one concise free radicalpolymerization approach to accomplish the aromatic palladium trimer functionalized polymers PSSy-[Pd3]+(2) and itsderived palladium nanoparticles (3). Full characterizationscould confirmthe successful combination of cationic [Pd3]+or nanoparticles with poly(p-sulfonated styrene) skeleton. Compared to their monomeric tri-palladium precursor (1) and commonPd(dba)2, Pd(PPh3)4, Pd(OAc)2, heterogeneous PSSy-[Pd3]+(2) showsmuch superior catalytic activities (0.15 mol%, TOF = 1333.3h-1) in the SMCC reaction. The identically ligated PdNPs (3)are formed in-suitin the presence of NaBH4andaccomplish quantitative reduction of4-nitrophenol in just 320s(0.50 mol%, TOF = 2250 h-1).Moreover, these heterogeneous catalysts arereused for 5-6times without significant loss of catalytic activity. Their superior catalytic ability isprobably attributed to the synergistic effect of polymer entanglement and the tri-palladium fragment.This work enlightensthat the immobilization of palladiumclustersor nanoparticles by polymerization could offer multiple advantages in stability, efficiencyand recyclabilityfortheir involvedcatalyses and show far-reaching future implications.

Structural Reassignment of Covalent Organic Framework-Supported Palladium Species: Heterogenized Palladacycles as Efficient Catalysts for Sustainable C-H Activation.

Recent decades have witnessed remarkable progress in ligand-promoted C-H activation with palladium catalysts. While a number of transformations have been achieved with a fairly broad substrate scope, the general requirements for high palladium loadings and enormous challenges in catalyst recycling severely limit the practical applications of C-H activation methodologies in organic synthesis. Herein, we incorporate N,C-ligand-chelated palladacycles into rigid, porous, and crystalline covalent organic frameworks for the C-H arylation of indole and pyrrole derivatives. These heterogeneous palladium catalysts exhibit superior stability and recyclability compared to their homogeneous counterparts. We not only produce several highly reactive palladacycles embedded on new framework supports to facilitate C-H activation/C-C bond-forming reactions but also reassign heterogenized palladium species on frameworks containing a benzaldehyde-derived imine moiety as imine-based palladacycles via comprehensive characterization. Our findings provide guidance for the rational design of framework-supported metallacycles in the development of heterogeneous transition-metal catalysis.

Functionalized Phosphorus‐Doped Porous Organic Polymers (FPPOPs)‐Promoted Pd‐Catalyzed Arylation of Hydrosilanes

The development of novel catalyst systems, especially as highly efficient heterogeneous catalysts for Si‐C bond ‐forming arylation reaction of hydrosilanes is highly desirable. Herein, we developed a simple and eco‐friendly methodology for construction of functionalized phosphorus‐doped porous organic polymers (FPPOPs) and its heterogeneous palladium catalyst through anchoring on FPPOPs bearing a phosphine ligand, in which the fabrication of FPPOPs was completed through the vinyl‐substituted triphenylphosphine (ViPPh3) and vinyl‐substituted polyhedral oligomeric silsequioxane (ViPOSS) as well as functional olefins via radical co‐polymerization. All these FPPOPs could act as a phosphine ligand for Pd‐catalyzed arylation of hydrosilanes have been characterized by SEM, FTIR, and X‐RAD. Moreover, the POSS‐based functionalized phosphorus‐doped porous organic polymers (POSS‐PPOPs) proved to be a high efficiency catalyst system for Si‐C bond ‐forming arylation of hydrosilanes with aryl iodides and with good yields as well as excellent chemoselectivity with low amount of catalyst under mild conditions. The mechanistic studies supported the effect of functional group on the catalytic activity of Pd/FPPOPs catalyst, which revealed that the weak coordination as well as non‐covalent interaction between porous organic polymers and substrate played important role in the enhancement of chemoselectivity.

Palladium supported on alumina as a highly active heterogeneous catalyst for direct arylation of 6-substituted imidazo[2,1-b]thiadiazole via C[sbnd]H bond activation reaction

Fast, atom-economical, phosphine-free, and eco-friendly microwave-irradiated reactions permitting the “green synthesis” of 5,6-disubstituted imidazo[2,1-b]thiadiazole derivatives using heterogeneous palladium catalyst in triethylammonium 2-(1,1-dioxido-3-oxobenzo[d]isothiazol-2(3H)-yl)acetate ([Et3NH][Sac-CH3CO2]) ionic liquid via CH bond activation reaction of 6-substituted imidazo[2,1-b]thiadiazole derivatives, are reported. This method provides a straightforward route to a wide variety of substituted imidazo[2,1-b]thiadiazoles and has the advantages of reusability of the heterogeneous catalyst, simple methodology, high yields, and easy work-up.

Recyclable Palladium-Catalyzed Carbonylative Coupling of Aryl Halides and Organoaluminum Compounds with tert-Butyl Iso­cyanide as CO Equivalent Leading to 1,2-Diketones

AbstractAn efficient heterogeneous palladium-catalyzed carbonylative coupling of aryl halides and organoaluminum compounds has been developed using tert-butyl isocyanide as CO equivalent. The carbonylation reaction proceeds smoothly in toluene with KOtBu as a base at 100 °C by using 10 mol% of an SBA-15-anchored bidentate phosphine palladium(0) complex [2P-SBA-15-Pd(0)] as the catalyst and provides a general and practical approach for the assembly of 1,2-diketones in good to excellent yields. This heterogenized palladium catalyst can be readily separated and recovered via a simple centrifugation process and reused for more than seven cycles with almost consistent catalytic efficiency.

Dimensional Reduction of Metal-Organic Frameworks for Photocatalytic Synthesis of Fused Tetracyclic Heterocycles.

Heterogeneous palladium catalysts with high efficiency, high Pd atom utilization, simplified separation, and recycle have attracted considerable attention in the field of synthetic chemistry. Herein, we reported a zirconium-based two-dimensional metal-organic framework (2D-MOF)-based Pd(II) photocatalyst (Zr-Ir-Pd) by merging the Ir photosensitizers and Pd(II) species into the skeletons of the 2D-MOF for the Pd(II)-catalyzed oxidation reaction. Morphological and structural characterization identified that Zr-Ir-Pd with a specific nanoflower-like structure consists of ultrathin 2D-MOF nanosheets (3.85 nm). Due to its excellent visible-light response and absorption capability, faster transfer and separation of photogenerated carriers, more accessible Pd active sites, and low mass transfer resistance, Zr-Ir-Pd exhibited boosted photocatalytic activity in catalyzing sterically hindered isocyanide insertion of diarylalkynes for the construction of fused tetracyclic heterocycles, with up to 12 times the Pd catalyst turnover number than the existing catalytic systems. In addition, Zr-Ir-Pd inhibited the competitive agglomeration of Pd(0) species and could be reused at least five times, owing to the stabilization of 2D-MOF on the single-site Pd and Ir sites. Finally, a possible mechanism of the photocatalytic synthesis of fused tetracyclic heterocycles catalyzed by Zr-Ir-Pd was proposed.

Reductive N-Alkylation of Amines with Ketones Using Heterogeneous Polysilane-Palladium Catalysts under Continuous-Flow Conditions.

This work reports a continuous-flow reductive N-alkylation of amines with ketones using molecular hydrogen. The reaction, performed with highly active polysilane-modified heterogeneous palladium catalysts, enables the efficient synthesis of diversely substituted amines under mild flow conditions. The developed catalyst exhibits sustained activity for 5 days (turnover number of >2400). Moreover, the utility of the method is demonstrated by the synthesis of a key intermediate of the active pharmaceutical ingredient teneligliptin.

Chitosan-functionalized poly(3-hydroxybutyrate) bead as a novel biosupport for palladium in the Suzuki cross-coupling reaction

The utilization of environmentally friendly biopolymer poly(3-hydroxybutyrate) (PHB) as a support for metal catalysts is an unexplored area. Herein, exploring a stable and recyclable heterogeneous palladium (Pd) catalyst based on PHB for the Suzuki cross-coupling reaction is an attractive topic. In this study, a novel hydrophobic and hydrophilic core-shell bead supported Pd catalyst was developed by using PHB particle as a hydrophobic rigid framework and hydrophilic chitosan (CS) as the surface of the supporting matrix to fix Pd nanoparticles (NPs). The results of structural characterization showed that the Pd NPs with a small average size of 2.3±0.82 nm were uniformly distributed on the surface of the PHB-CS bead with a Pd content of 0.12 wt%. This demonstrated that the PHB-CS bead created a stable coordinated environment for Pd fixation, preventing Pd leaching. Furthermore, the Pd@PHB-CS catalyst exhibited high catalytic efficiency with 99% conversion in the Suzuki reaction under mild conditions, using a solvent mixture of EtOH and H2O (1:1) for 0.5 h. It is worth noting that the catalyst exhibits excellent stability, and reusability. It can be reused five times without a significant reduction in activity while maintaining its structural integrity. The successful construction of Pd@PHB-CS will further expand the application of PHB in the field of metal catalysts and may help establish a green and environmentally friendly multifunctional platform for the development of highly recyclable heterogeneous catalysts containing metal nanoparticles.

Asymmetric transfer hydrogenation of 2,3-diphenylpropenoic acids over heterogeneous palladium catalysts modified by cinchona alkaloids

Asymmetric catalytic methods are among the convenient syntheses procedures of optically pure organic compounds, many of which are valuable pharmaceutical intermediates. Applying heterogeneous catalysts may contribute to their economic and environmentally benign synthesis. Among the easiest and often used methods of introducing chiral centres in molecules are the reduction of prochiral unsaturated compounds. Although, several heterogeneous catalytic systems are known in which in situ chirally-modified metals are used, such catalysts have never been applied in asymmetric transfer hydrogenations. Here we report the enantioselective transfer hydrogenations of (E)-2,3-diphenylpropenoic acids over supported palladium catalysts modified by dihydrocinchonidine carried out in ordinary laboratory glassware using easily available, cheap hydrogen donor, i.e. potassium formate. High conversions and good enantioselectivities were obtained in transfer hydrogenations of various acids substituted on phenyl rings. Results obtained with various acid derivatives and with few cinchona alkaloid modifiers indicated that the transfer hydrogenations occur through a similar surface intermediate as the enantioselective hydrogenations with molecular hydrogen. A deuterium tracer study showed low deuterium incorporation in the product possibly due to dissociation of formate and exchange with the surface mobile hydrogens, however, these processes are strongly influenced by the donor cation. The asymmetric transfer hydrogenation may be carried out at over 2 g scale, thus may be used as an advantageous laboratory synthetic procedure without applying special precautions characteristic of gaseous reactants.

A novel magnetic nanoparticle as an efficient and recyclable heterogeneous catalyst for the Suzuki cross-coupling reaction

A novel magnetically recyclable heterogeneous palladium catalyst Fe3O4@FSM@Pd was constructed and characterized, which is highly efficient and reusable for the Suzuki–Miyaura cross-coupling reaction.

Carbonylative synthesis of 2-(trifluoromethyl)quinazolin-4(3H)-ones from trifluoroacetimidoyl chlorides and amines based on an activated carbon fiber supported palladium catalyst

Activated carbon fibers, known for their unique physical and chemical properties, are increasingly utilized in diverse industries, including textiles and fabrics, and can serve as ideal supports of heterogeneous palladium catalysts.

Highly reactive and reusable heterogeneous activated carbons-based palladium catalysts for Suzuki−Miyaura reaction

Suzuki−Miyaura reaction of aryl halides with phenylboronic acid using a heterogeneous palladium catalyst based on activated carbons (AC) was systematically investigated in this work. Two different reaction modes (batch procedure and continuous-flow procedure) were used to study the variations of reaction processing. The heterogeneous catalysts presented excellent reactivity and recyclability for iodobenzene and bromobenzene substrates in batch mode, which can be attributed to stabilization of Pd nanoparticles by the thiol and amino groups on the AC supports. However, significant dehalogenation in the reaction mixture and Pd leaching from the heterogeneous catalysts were observed in continuous-flow mode. This unique phenomenon in continuous-flow mode resulted in a dramatic decline in reaction selectivity and durability of heterogeneous catalysts comparing with that of batch mode. In addition, the heterogeneous Pd catalysts with thiol- and amino-modified AC supports exhibited different reactivity and durability in batch and continuous-flow mode owing to the difference of interaction between Pd species and AC supports.

Construction of a MOF-Supported Palladium Catalyst via Metal Metathesis.

A new MOF-supported heterogeneous palladium catalyst Pd/NBB-1 has been synthesized successfully through the effective metal metathesis between Pd(CF3 COO)2 and NBB-1. NBB-1 is a two-dimensional zinc metal-organic framework constructed from 2-aminoterephthalate (NH2 -H2 BDC) and 2,2'-bipyridine-5-carboxylate (HBPC) by solvothermal method. The replacement efficiency of Pd(II) to Zn(II) is up to 72% after only 24 hours, which is beneficial to the catalytic application. Pd/NBB-1 with a low loading of 2 mol% works efficiently in the 1,4-addition reaction of arylboronic acids with α,β-unsaturated ketones in air, and its catalytic activity keeps unchanged after 3 reaction cycles. This work provides a new strategy to effectively prepare supported noble metal/MOF catalysts, which would further increase the practical applications of metal-organic frameworks.

Arylation of indole at C2 catalyzed by palygorskite grafted covalent organic frameworks supported palladium catalyst

A heterogeneous palladium catalyst, palygorskite grafted covalent organic frameworks supported palladium (Pd-Pal/COF), was successfully prepared by a simple method. A series of C2 aromatic indole derivatives were synthesized under mild reaction conditions. Pd-Pal/COF catalyst can be reused at least 5 times with inappreciable metal leaching. Undoubtedly, facile synthesis, mild reaction conditions, a wide scope of reactants, and recyclability make the catalyst systems potentially useful in organic synthesis. For the synthesized heterogeneous catalysts, palygorskite as a matrix, the unique structure and good dispersion of palygorskite result in the excellent catalytic activity and good cycle stability of the Pd-Pal/COF catalyst, comparing with the traditional clay catalyst. The result further expands the application scope of clay in the catalytic field.

A Novel Palladium-Based Heterogeneous Catalyst for Tandem Annulation: A Strategy for Direct Synthesis of Acridones

AbstractIn order to develop an efficient, rapid, and modular cascade strategy for the direct synthesis of acridones, palladium supported on sulfated alumina and microwave activation are employed. Multifunctional heterogeneous palladium catalysts were prepared in order to carry out the sequential annulation via a Buchwald–Hartwig amination followed by an intramolecular annulation in a one-pot process. This new protocol represents the first report on a catalytic tandem synthesis of acridone derivatives from commercially available starting materials, under ligand-free conditions. The scope of the present methodology is extended to the generation of a library of functionalized acridones, showing high functional group compatibility, in moderate to excellent yields. The applicability of this novel transformation was demonstrated by the concise total synthesis of the natural product arborinine.

Tailor-Made POLITAG-Pd0 Catalyst for the Low-Loading Mizoroki–Heck Reaction in γ-Valerolactone as a Safe Reaction Medium

In this contribution, we have reported our study on the fine-tuned design and synthesis of new POLITAGs-Pd0 (POLymeric Ionic TAG) catalytic systems with the intention of defining highly efficient heterogeneous palladium catalysts stabilized by ionic tags able to operate at a very low loading while being fully recoverable/reusable. By varying both the support and the ratio between the metal and immobilized ionic tag, we have accessed five different heterogeneous catalytic systems that have been fully characterized (EA, XPS, TEM, and HR-TEM) to investigate their morphological differences. The catalytic efficiency of the newly prepared POLITAGs-Pd0 materials was then tested and compared in the representative Mizoroki–Heck cross-coupling reaction, a universally investigated and widely useful process. A close correlation between the catalyst support and catalytic performances has been highlighted. The best system POLITAG-Pd0-HM features a high TOF value (26,786 h–1), affording the final products in high isolated yields by using as little as only 0.0007 mol % of Pd. Moreover, this high catalytic activity coupled with the use of γ-valerolactone (GVL) as a green reaction medium has led to the definition of a very efficient waste-minimized protocol. The newly designed catalyst has been utilized in the representative synthesis of Amiloxate, a widely used sunscreen agent, that could be prepared with negligible metal contamination and very low E-factor.

N-Heterocyclic Carbene Modified Palladium Catalysts for the Direct Synthesis of Hydrogen Peroxide.

Heterogeneous palladium catalysts modified by N-heterocyclic carbenes (NHCs) are shown to be highly effective toward the direct synthesis of hydrogen peroxide (H2O2), in the absence of the promoters which are typically required to enhance both activity and selectivity. Catalytic evaluation in a batch regime demonstrated that through careful selection of the N-substituent of the NHC it is possible to greatly enhance catalytic performance when compared to the unmodified analogue and reach concentrations of H2O2 rivaling that obtained by state-of-the-art catalysts. The enhanced performance of the modified catalyst, which is retained upon reuse, is attributed to the ability of the NHC to electronically modify Pd speciation.

Influence of Silica-Supported Alkylaluminum on Heterogeneous Zwitterionic Anilinonaphthoquinone Nickel and Palladium-Catalyzed Ethylene Polymerization and Copolymerization with Polar Monomers

The development of the heterogeneous late transition metal-catalyzed olefin (co)polymerization is a challenge in current research. Here, we report the preparation of heterogeneous anilinonaphthoquinone nickel and palladium catalysts by the binding of SiO2-supported alkylaluminums to a quinone oxygen of the ligand (Mt-AlR3/SiO2). The heterogeneous nickel catalyst exhibited a very high activity (up to 10.4 × 106 g mol–1 h–1) for ethylene polymerizations to produce semicrystalline high-molecular-weight (Mn up to 54.3 × 104 g mol–1) polyethylenes with a low degree of branching (<11.1/1000 C), high Tm values (121–131 °C), and well-regulated particle morphologies. The polymerization behavior depended on the electron density of the metal center modulated by the Lewis acidity of the remotely bound alkylaluminum as can be analyzed and verified through model optimizations and density functional theory (DFT) calculations. Most importantly, these nickel catalysts were able to produce semicrystalline ethylene/5-hexene-1-yl copolymers with a high activity and high molecular weight. The heterogeneous palladium catalyst can promote the copolymerization of ethylene with commercial methyl acrylate or polar norbornene derivatives with a moderate activity, affording a polar functionalized polyethylene and a cyclic olefin copolymer.

Palladium-Catalyzed trans-Hydroalkoxylation: Counterintuitive Use of an Aryl Iodide Additive to Promote C-H Bond Formation.

We report an enantioselective palladium-catalyzed trans-hydroalkoxylation of propargylic amines with a trifluoroacetaldehyde-derived tether to build chiral oxazolidines. Diastereoselective hydrogenation using a heterogeneous palladium catalyst then gave access to protected benzylic amino alcohols in 45–87% yields and 84–94% ee values. Hydroalkoxylation of the alkynes required a catalytic amount of aryl iodide, highlighting the counterintuitive key role played by a putative Pd(II)/ArI oxidative addition complex to promote oxypalladation/protodemetalation.

Correlation of adsorbed and embedded palladium species in chitosan composite nanofibers with their catalytic activities for Suzuki reactions

Heterogeneous palladium catalysts play an important role in the synthetic organic chemistry. Palladium species have been adsorbed on the surface of solid matrices or embedded inside solid matrices to prepare recyclable heterogeneous catalysts. Due to the different surrounding environment, these two type palladium species had different electronic properties and catalytic performances. In this study, we fabricated three series of chitosan composite nanofibers with adsorbed and embedded palladium species. Their fiber morphologies and chemical structures were characterized by SEM and FT-IR, respectively. Positron annihilation lifetime spectra (PALS) showed that the intensities (I3) of o-Ps annihilations were all linearly decreased with the increase of palladium contents in the chitosan composite nanofibers. Moreover, compared with the palladium adsorbed chitosan composite nanofibers, the palladium embedded chitosan composite nanofibers had stronger o-Ps annihilation ability, which could be ascribed to the stronger electron donating property of embedded palladium species and higher crosslinking of chitosan molecules by embedded palladium species. At last, the I3 values were successfully correlated with the rate constants of chitosan supported palladium composite nanofibers catalyzed Suzuki reactions of iodobenzene with phenylboric acid. Therefore, we have developed a facile method to distinguish the adsorbed and embedded palladium species by PALS, which can be further related with their catalytic performance.