Palladium Species Research Articles

Pd/Cu-Cocatalyzed Asymmetric Cascade Heck/Tsuji-Trost Reaction to Access Non-natural Tryptophans.

A Pd-catalyzed asymmetric Heck cascade reaction involving the intramolecular carbopalladation of unsaturated hydrocarbons, followed by nucleophilic trapping of the resulting palladium species, is a powerful approach for constructing chiral N-heterocycles. However, the use of prochiral nucleophiles in these reactions remains significantly underexplored. Herein, we report a novel Pd/Cu catalytic system for the asymmetric cascade Heck/Tsuji-Trost reaction of allenamides and aldimine esters. This robust method allows for the rapid synthesis of a wide range of enantiopure non-natural α-substituted tryptophans in high yields (up to 99% yield) with excellent enantioselectivities (up to 98% ee). Additionally, the synthetic utility of this protocol is demonstrated through scale-up experiments and diverse valuable transformations.

Regio-, Site- and Stereo-Selective Aziridination of Conjugated Dienes Enabled by Palladium/Copper/Iodide/Oxygen Cooperation.

Vinylaziridines are important building blocks in organic chemistry, especially in the synthesis of nitrogen-containing heterocycles. The direct and efficient transfer of an appropriate nitrogen source to readily accessible conjugated dienes is a notable methodology. The Pd-catalyzed oxidative 1,2-difunctionalization of conjugated dienes through a π-allyl-palladium species should be an ideal method for the selective synthesis of vinylaziridines. However, this method faces the challenge of regioselectivity, often resulting in 1,4-difunctionalization instead. In this study, we developed a Pd-catalyzed aerobic 1,2-difunctionalization of conjugated dienes via a π-allyl-palladium species to achieve regio-, site- and stereo-selective aziridination under the synergistic effects of PdII, CuI, I-, and O2. The π-allyl palladium species formed in the system undergoes an unusual iodination process, leading to the formation of an allyl iodide intermediate. Subsequently, the vinylaziridine is obtained through intramolecular SN2' substitution of the allyl iodide.

Core–Shell–Satellite Au@CeO2–Pd Plasmonical Photocatalysts for Suzuki–Miyaura Coupling Reaction

ABSTRACTIntegration of localized surface plasmon resonance (LSPR) metallic nanocrystals into photocatalysis is an intriguing approach for light‐driven organic transformations. However, uncertain direction of hot carriers is a grand challenge, which fails to take full advantage of the LSPR excitation. In this work, core–shell gold@ceria nanosphere–supported segregated palladium species (Au@CeO2–Pd) have developed to steer the light absorption capability and charge carrier migration. Under simulated solar light irradiation, the core–shell–satellite Au@CeO2–Pd exhibited efficient light harvesting and colossal activity in the Suzuki coupling reaction. The plasmon‐induced hot electrons overpassed the Schottky barrier at the Au@CeO2 interfaces and migrated from Au core to CeO2 shell accordingly. Subsequently, the photogenerated electrons and hot electrons migrated from Au@CeO2 core–shells to Pd, which acted as an electron acceptor. Detailed photocatalytic mechanism studies revealed that both photoexcited electrons and holes were the main active species involved in the photocatalytic Suzuki coupling reaction process. In particular, the diphenyl yield over Au@CeO2–Pd catalyst was ~1.7 times higher than that of core–shell–satellite Au@SiO2–Pd, where a 19‐nm‐thick SiO2 shell was introduced to prevent the migration of hot electrons. This distinctly certified that the hot electrons transfer in the core–shell–satellite Au@CeO2–Pd under illumination played an important role to drive Suzuki reaction. Overall, this design of core–shell–satellite Au@CeO2–Pd plasmonic photocatalyst has the potential in the field of photo‐driven organic transformations.

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.

Correlation between Zeolitic SiO2/Al2O3 Ratio and the Low-Temperature NOx Adsorption Capacity of Pd/SSZ-13 under Realistic Exhaust Conditions

Passive NOx adsorber (PNA) is one of the important means to effectively reduce NOx emission control in the cold start of a diesel engine. A series of Pd/SSZ-13 catalysts with different SiO2/Al2O3 ratios (11, 17, and 25) were prepared using the impregnation method. Furthermore, the effect of the SiO2/Al2O3 ratio on the adsorption performance of Pd/SSZ-13 at low-temperature NOx under realistic exhaust conditions was studied. The results show that the Pd/SSZ-13 catalyst with a low SiO2/Al2O3 ratio after loading Pd has a higher specific surface area and palladium ion content. There is a negative correlation between NOx adsorption performance and the SiO2/Al2O3 ratio. After high-temperature heat treatment, the acid sites closely related to palladium species increase and palladium species will redisperse, producing more palladium ions. The palladium ions coexist in Pd/SSZ-13 in the form of Pd2+ and Pd+, among which Pd2+ is divided into two types: Z−Pd2+Z− and Z−[Pd(II)OH]+. The NOx adsorption performance of the Pd/SSZ-13 catalyst was significantly improved, and the higher the SiO2/Al2O3 ratio, the more obviously the advantage of NOx adsorption performance increased after heat treatment. The NOx adsorption kinetic model of the Pd/SSZ-13 catalyst under realistic exhaust conditions was most suitably described by the pseudo-first-order model.

Silica supported Schiff-based palladium nanocatalyst for n-alkylation at room temperature

This works documents a new silica gel-supported nanocatalyst (Si@NSBPdNPs 3) with low Pd loadings for n-alkylation reactions at room temperature. Post synthesis characterisation using SEM-EDX and ICP techniques provided a quantitative assessment of palladium species. Additionally, TEM analysis unveiled an average palladium nanoparticle size of 5.87 ± 0.2 nm. In-depth X-ray Photoelectron Spectroscopy (XPS) analysis revealed its predominant composition as Pd(0) complexed to a Schiff base ligand on low cost silica matrix. The nanocatalyst exhibited high efficacy in the catalysis of n-alkylation (Michael addition) reactions with various α,β-unsaturated Michael acceptors, yielding the corresponding n-alkyl products at room temperature with exceptional yields. Notably, the catalyst exhibited good stability and could be easily separated from the reaction mixture. Moreover, the catalyst displayed recyclability potential, maintaining its original catalytic efficacy for up to seven cycles without any discernible loss.

A novel “turn-on” NIR fluorescent probe based on benzothianone for the detection of palladium species and its application in cell and zebrafish

The development of an efficient palladium probe holds significant application value, considering the detrimental impact of palladium contaminants on human health. Thus, it is critical to create a sensitive detection method. To this end, a fluorescent probe TM-TPA-Pd based on benzothianone structure was designed, using allyl carbonate as the Pd0 recognition unit. TM-TPA-Pd exhibited high sensitivity (1.4 eq), selectivity, near-infrared (NIR) fluorescence (798 nm), and low detection limit (0.46 μM) for Pd0 with a rapid “turn-on” fluorescence signal (5 min). Furthermore, TM-TPA-Pd has extremely low cytotoxicity and has been successfully applied to detecting cells and zebrafish, which has great potential for palladium detection in biological systems.

Palladium‐Catalyzed Sc(OTf)3/Cs2CO3‐Assisted C−O Cross‐Coupling of Vinylidenecyclopropane‐Diesters with Phenolic Compounds

AbstractWe herein describe a method in the synthesis of aryl ether derivatives from a cross‐coupling reaction of vinylidenecyclopropane‐diesters (VDCP‐diesters) with commercially available phenolic compounds for the formation of C−O bond under palladium catalysis and assisted by Sc(OTf)3/Cs2CO3. This developed transformation proceeds through a nucleophilic addition at the 2‐position of allenic moiety at 50 °C for 4.0 h and is compatible with substituents such as bromine, iodine and borate on the phenolic substrates via a zwitterionic π‐propargyl palladium species, affording the desired products in 29% to 95% isolated yields. A plausible reaction mechanism has been also proposed on the basis of control, deuterium labeling experiments and density functional theory (DFT) calculations.

Green Synthesis of Alzheimer's Disease Probes Aftobetin and Analogues Enabled by Flow Technology and Heterogeneous Photocatalysis.

Aftobetin is a non-invasive diagnosis probe of Alzheimer's disease, that can bind with aggregated β-amyloid peptide in eye's lenses, used for early diagnosis of Alzheimer's disease in a rapid and painless mode. The reported synthesis of this probe fell short in the aspects of greenness and economy due to the involvement of toxic Chromium(IV) oxidant, noble palladium catalyst, elevated reaction temperature, the long reaction time as well as the cumbersome workup. Herein, a holistic optimization of the synthetic process was achieved via the employment of flow technology and heterogenous photocatalysis. Firstly, the integration of heterogenous carbon nitrides photocatalysis and circulation flow technology furnished the air oxidation of alcohol and nickel catalyzed C-N coupling at 20-g scale, thus avoiding the use of toxic Chromium and precious palladium species respectively. Flow-intensified esterification between acyl chloride and alcohol, just taking 30 seconds replaced the Steglich esterification of 6 hours, also avoiding the generation of difficult-to-remove dicyclohexylurea. Finally, C-N coupling, esterification and Knoevenagel condensation were telescoped together, thus simplifying the reaction workup. This fully-flow protocol led to the on-demand synthesis of eight probes.

Study of size effect of palladium atoms as single-atom and clusters nanoparticle catalysts for Heck reactions

We reported a series of nitrogen-doped carbon-supported palladium (Pd) species with various sizes for the Heck reaction of iodobenzene and methyl acrylate. By precisely controlling the synthesis procedure, we have successfully synthesized a range of Pd-based catalysts, including single Pd atom, sub-nanometer Pd clusters (∼ 0.4 nm, and ∼1.6 nm), and Pd nanoparticles (∼5.8 nm and 10 nm). Remarkably, these catalysts exhibit a distinct size-dependent catalytic activity in the Heck reaction. In the case of the Heck reaction of 4‑methoxy-iodobenzene and methyl acrylate, the Pd-10 site demonstrated the highest site-specific turnover frequency (TOF), reaching 792.5 h−1. This performance is impressively 5.2 and 15.6 times superior to that of the Pd1 site (152 h-1) and Pd-10 site (50.6 h-1), respectively. Significantly better in the Heck reactions with other aryl halides were achieved with Pd-1.6/N-C (comprising of approximately 1.6 nm-sized Pd clusters), compared to those containing 10 nm-sized nanoparticles (Pd-10/N-C) or Pd1/N-C. These findings suggested that Pd clusters of around 1.6 nm may serve as the potential catalytic sites for the Heck reaction.

Synthesis of 1,2′‐Spirobi[indene]‐1,3‐diones by Pd(II)‐Catalyzed C−H Activation and Alkynes Annulation Reaction

AbstractAn intermolecular annulation reaction of 2‐aryl‐1,3‐indandions with alkynes was reported using Pd(OAc)2 to access spirobi[indene]‐1,3‐diones. Under palladium catalysis, the substrates form a homocoupling dimer intermediate through a catalytic dehydrogenative cross‐coupling process. The palladium(II) species could come from dimer or 2‐aryl‐1,3‐indandion. Notably, this pathway is not typically observed in enol‐directed formal sp3 C−H functionalization/oxidative annulation palladium chemistry. This transformation provides a route to access a class of functionalized spiro carbocyclic indenes.

Silicate-induced high-temperature-resistant small-crystallite ceria support enhancing palladium-catalyzed low-concentration methane combustion

High-performance Pd/CeO2 catalysts for the purification of low-concentration methane in waste gases by catalytic combustion have long been pursued, given that methane is a potent greenhouse gas. The reduction in ceria oxygen supply and palladium dispersion, caused by ceria crystallite sintering, deactivates the catalyst. Here, we report an enhanced palladium catalyst supported on a silicate-induced small-crystallite ceria (7.2 nm) calcined at 800 °C, compared to the pristine ceria (50.7 nm). The catalyst was coated on a cordierite monolith for 0.1 vol% methane combustion, achieving 90 % methane conversion at 336 °C, which is 84 °C lower than the temperature required for Pd/CeO2 reference. The small-crystallite ceria exhibited a high oxygen supply capacity due to abundant oxygen vacancies, facilitating the conversion of carbon-containing intermediates from methane dissociation on palladium species, thereby enhancing the catalyst’s intrinsic activity. The catalyst, with increased palladium dispersion, demonstrated a methane reaction rate 6.7 times higher than that of the Pd/CeO2 at 280 °C. This study provides new insights into designing highly efficient Pd/CeO2-based catalysts for low-concentration methane combustion.

Palladium‐Catalyzed Selective Syntheses of 2,3‐Allenyl Amines via Double Functionalization Coupling of 2‐Alkynyl‐1,4‐diol Dicarbonates

Comprehensive Summary2,3‐Allenyl amines have shown wide applicability in biomedical and synthetic applications. Due to their enormous potential for applications, researchers have been dedicated to the development of methods for synthesizing 2,3‐allenyl amines. Herein, a palladium‐catalyzed three‐component reaction of 2‐alkynyl‐1,4‐diol dicarbonates, organoboronic acids, and nitrogen nucleophiles forming 2,3‐allenyl amines with excellent regio‐ and chemo‐selectivity has been developed. Substrate compatibility and synthetic applications have been demonstrated. Control experiments supported a mechanism involving 1,2,3‐triene‐Pd species and methylene‐π‐allyl palladium species.

Electronic structure and microenvironment modulation of Pd@UiO-66 enhances direct CO esterification to dimethyl carbonate

While the microenvironment around metal catalytic sites is recognized to be crucial in heterogeneous catalysis, its roles in CO esterification to dimethyl carbonate remain subtle. Herein, Pd nanoparticles are confined into the metal–organic frameworks with TiIV metal substitutions and functional linker groups, namely Pd@Ti-Zr-UiO-66-X (X = H, NH2, NO2). The partial substitution of metal nodes by Ti species can dramatically enhance the activity of CO, and synergistically improves catalytic performance with electron-withdrawing functionalized nitroxide linkers. As a result, Pd@Ti-Zr-UiO-66-NO2 exhibits superior conversion of CO (67.5%), selectivity for DMC (83.5%, based on all products) and WTY for DMC (1725 g·kgcat−1·h−1) much higher than those of Pd@Zr-UiO-66-H with conversion of CO (46.4%), selectivity for DMC (87%) and WTY for DMC (1175 g·kgcat−1·h−1). Based on results of experimental and DFT calculation, the synergistic effect of highly electronegative Ti species and nitro-functional linker groups with electron-withdrawing ability realizes the precise control of the electronic structure and microenvironment of Pd NPs. The number of charges transferred from Pd NPs to Zr-UiO-66 and Ti-Zr-UiO-66-NO2 increased from 1.83 to 2.69 e, respectively. The microenvironment of electron-deficient palladium species and Ti-Zr-UiO-66-NO2 promotes the activation of MN and CO, which exhibits a superior performance in direct CO esterification to dimethyl carbonate. This study not only provides a new approach for rational modulation of the chemical microenvironment of metal centers and optimization of catalytic performance but also offers important insights for the design of heterogeneous catalysts.

C(sp2)‒Halide activation for efficient Ullmann-type C‒O coupling in water by in situ reduction of supramolecular Cu(II) species stabilized on a modified GO nanosheets support

In the present research, the new supramolecular Cu(II) species immobilized on a magnetic benzotriazole-grafted GO nanosheets (CuO/mGO-TA-Me-BTA), with appropriate geometry of ligands for stronger chealation, was fabricated by using the Click Chemistry concept as a new magnetic nanocomposite. The obtained nanomaterial was characterized by using different spectroscopic, microscopic or analytic techniques and methods such as FT-IR, EDX, XRD, FESEM, elemental mapping, TGA-DTG, AAS and VSM. Furthermore, an attempt has been made for efficient synthesis of diverse derivatives of diaryl ethers in the presence of in situ produced heterogeneous Cu(I) species supported on the modified GO nano-structured catalyst by using 10 mol% loading of sodium ascorbate (NaAsc) in water. In situ reduction of heterogeneous Cu(II) species was found to exhibit exceptional catalytic activity for the activation of C(sp2)‒I and C(sp2)‒Br and even less reactive C(sp2)‒Cl bonds in the Ullman-type C‒O coupling of various substituted phenols and iodo/bromo/chloro arenes in the presence of cesium carbonate and water. Moderate to excellent yields (78 % up to 93 %) of the corresponding diaryl ethers were formed under mild conditions for chloro‑ and bromo/iodo- arenes, respectively. High performance as well as easy recoverability and reusability for at least five times make the reduced CuO/mGO-TA-Me-BTA a robust catalytic system in comparison with other conventional heterogeneous and homogeneous catalysts containing Cu or palladium species. To the best of our knowledge, this is the first application of in situ reduction of heterogeneous Cu(II) species in the C‒X activation for the Ullman-type C‒O coupling reaction. It can address the problem of residual transition metals remaining in the synthesis of diaryl ethers, especially the biologically active compounds.

Characterization of the H2/NOx reaction process over the La0.9Ce0.1Co0.9Pd0.1O3-BaO/Al2O3 catalyst

An excellent textual properties and performance La0.9Ce0.1Co0.9Pd0.1O3-BaO/Al2O3 catalyst was synthesized. The reaction mechanism of H2/NOx over the La0.9Ce0.1Co0.9Pd0.1O3-BaO/Al2O3 catalyst was investigated by temperature programmed reduction/ desorption/ surface reaction (TPR/D/SR) technologies and in-situ diffuse reflectance Fourier transform (DRIFT) technology. The results show that cerium or palladium species are inserted into the cells of LaCoO3, as well as they synergetic promote the redox properties of the La0.9Ce0.1Co0.9Pd0.1O3-BaO/Al2O3 catalyst. Surface activated nitrates exist over the La0.9Ce0.1Co0.9Pd0.1O3-BaO/Al2O3 catalyst, with thermal stable increasing in the order: absorbed N2O4 < monodentate nitrates < chelating bidentate nitrates < nitrates unidentate < free ionic nitrates < bulk free ionic nitrates. H2 preferentially reacted with absorbed N2O4 and monodentate nitrates at low temperatures, due to their high activity. The concentration of generated NH3 from the redox reaction of H2/NOx achieves the maximum value between 350 and 450 °C over the La0.9Ce0.1Co0.9Pd0.1O3-BaO/Al2O3 catalyst. Compared with the NOx adsorption process at 50 °C, monodentate nitrates and absorbed N2O4 disappeared due to their low thermal stability, chelating bidentate nitrates become stronger, as well as free ionic nitrates converted to bulk free ionic nitrates with higher thermal stability at 350 °C. When H2 is exposed to NOx species adsorbed on La0.9Ce0.1Co0.9Pd0.1O3-BaO/Al2O3, chelating bidentate nitrates and bulk free ionic nitrates are consumed gradually, indicating that although the bulk free ionic nitrates own high stability, it also could be consumed by involving in the H2/NOx reaction at 350 °C. The quantitative H2/NO reaction experiments confirmed the results of H2-TPSR and NSR. It is beneficial to the formation of NH3 when the H2/NO ratio is more than 2.5. Comparing traditional Pt-BaO/Al2O3 catalyst, the La0.9Ce0.1Co0.9Pd0.1O3-BaO/Al2O3 catalyst exhibit an excellent performance, including considerable NH3 production property, lower N2O selectivity, and the precious metal saving.

Metal-organic framework/polyurea composite microreactors prepared in oil-in-oil emulsions

A new technique was developed to create composite microcapsules consisting of metal–organic framework (MOF) and polyurea. This method employs oil-in-oil (O/O) emulsion as a template for forming these microcapsules. It involves emulsifying a polar solvent containing MOF reagents within a nonpolar phase containing a suitable surfactant and an isocyanate monomer. Subsequently, the addition of the amine monomer initiates two simultaneous processes: the formation of MOF crystals in the core of polar droplets and the interfacial polymerization to create polyurea shell. Optimization of the method involved adjusting various parameters affecting the composite microcapsules' structure and morphology. These parameters include the surfactant’s type and concentration, the emulsion phases’ type and concentration, and quantity of the MOF component.This method was applied to create palladium-based MOF/polyurea microreactors by encapsulating sodium tetrachloropalladate and then reducing the palladium salt with hydrogen to generate reactive palladium species. The produced catalysts were utilized in the selective hydrogenation of terminal alkynes. It was observed that the selectivity of the catalyst is influenced by the quantity of encapsulated MOF, the amount of encapsulated palladium, and the solvent type used for the reaction.

Highly Regio- and Diastereoselective Synthesis of 6,7-Dihydro-4H-furo[3,4-c]pyran Derivatives through Pd-Catalyzed Formal (3 + 3) Allylic Cycloaddition of 2-Butene-1,4-diols with 2-(1-Alkynyl)-2-alken-1-ones.

A highly efficient synthesis of 7-vinyl-6,7-dihydro-4H-furo[3,4-c]pyran derivatives from 2-butene-1,4-diols and 2-(1-alkynyl)-2-alken-1-ones has been achieved with high regio- and diastereoselectivity (dr > 20:1) by Pd-catalyzed tandem heterocyclization/cross-coupling. The π-allyl palladium species Int II generated from 2-butene-1,4-diol by direct cleavage of the C-OH bond is the key to the success in this formal (3 + 3) cycloaddition reaction.

The study of strong metal-support interaction enhanced PdZn alloy nanocatalysts for methanol steam reforming

Methanol steam reforming faces a significant challenge due to CO formation, which can lead to poisoning of fuel cell electrodes. This study introduces a series of Pd/ZnO catalysts prepared by ethylene glycol reduction, exhibiting remarkable selectivity in methanol steam reforming (MSR). The majority of palladium species exist in the form of PdZn alloy, attributed to the dual reduction process. This process, along with the generation of zinc vacancies and oxygen vacancies, enhances the interaction between the metal-carrier, promoting the formation of PdZn alloy, significantly improving CO2 selectivity and catalytic activity. Even at a high temperature of 400 °C, the active phase remains stable. After 5 hours of MSR, the 3% Pd/ZnO-300 H2 catalyst achieves a hydrogen production rate of 1628.0 mmol·gcat−1·h−1, with methanol conversion stabilized at 94%, CO2 selectivity reaching 97.7%, and CO content as low as 0.5%. These results outperform recent studies on hydrogen production. Furthermore, DFT calculations elucidate the complete reaction pathway (111) on PdZn. This study provides initial insights into the influence of metal-carrier interaction on the formation of PdZn alloy, suggesting a new direction for palladium-based catalysts in methanol steam reforming.

Synthesis of 1,1'-Bicarbazoles by Sequential Iron(III)- and Palladium(II)-Catalyzed Oxidative Coupling Reactions.

The iron(III)-catalyzed oxidative coupling of diarylamines to 2,2'-bis(arylamino)-1,1'-biaryls and subsequent twofold palladium(II)-catalyzed oxidative cyclization provide a convergent synthetic route to 1,1'-bicarbazoles. Screening a range of different palladium(II) salts led to palladium(II) acetate, pivalate, and hexafluoroacetylacetonate as the most efficient catalysts. Remarkably, the twofold palladium(II)-catalyzed oxidative cyclization can also be performed under argon. The mechanism for the oxidative cyclization under an inert gas presumably involves regeneration of the catalytically active palladium(II) species by oxidative addition of pivalic acid.