Catalytic System Pd Research Articles

Enantioselective 1,2-Carboamination of 1,3-Dienes with N-Hydroxyphthalimide (NHP) Esters Enabled by a Photoinduced Pd Catalysis.

Herein, a photoinduced, Pd-catalyzed direct 1,2-carboamination of conjugated 1,3-dienes has been successfully achieved. Sequential regioselective C-C bond and enantioselective C-N bond formation allows rapid assembly of a wide range of value-added chiral allylic amines from readily available N-hydroxyphthalimide (NHP) esters and 1,3-dienes under mild conditions. This developed protocol further demonstrates the versatility and potency of the photoexcited Pd catalytic system with a bifunctional reagent in the streamlined difunctionalization of C═C bonds.

Mediating trade-off between activity and selectivity in alkynes semi-hydrogenation via a hydrophilic polar layer

As a crucial industrial process for the production of bulk and fine chemicals, semi-hydrogenation of alkynes faces the trade-off between activity and selectivity due to undesirable over-hydrogenation. By breaking the energy linear scaling relationships, we report an efficient additive-free WO3-based single-atom Pd catalytic system with a vertical size effect of hydrogen spillover. Hydrogen spillover induced hydrophilic polar layer (HPL) with limited thickness on WO3-based support exhibits unconventional size effect to Pd site, in which over-hydrogenation is greatly suppressed on Pd1 site due to the polar repulsive interaction between HPL and nonpolar C=C bonds, whereas this is invalid for Pd nanoparticles with higher altitudes. By further enhancing the HPL through Mo doping, activated Pd1/MoWO3 achieves recorded performance of 98.4% selectivity and 10200 h−1 activity for semi-hydrogenation of 2-methyl-3-butyn-2-ol, 26-fold increase in activity of Lindlar catalyst. This observed vertical size effect of hydrogen spillover offers broad potential in catalytic performance regulation.

Unveiling the potential of Pd(II)‐covalent organic polyimide framework in Suzuki coupling reactions: A study on synthesis, characterization, and catalytic efficiency

In this research, a highly effective heterogeneous Pd catalytic system was conceived by immobilizing Pd(II) onto a substrate composed of covalent organic polyimide frameworks (PI‐COF). The synthesized catalyst underwent comprehensive characterization using techniques such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), N2 adsorption analysis (Brunauer–Emmett–Teller [BET]), Fourier transform infrared (FT‐IR), thermogravimetric analysis (TGA), and inductively coupled plasma atomic emission spectroscopy (ICP‐AES). Subsequently, the catalytic activity of this immobilized Pd catalyst was explored in the context of the Suzuki reaction. The reactions at 70°C up to 1 h under a normal atmosphere in water result in biaryls at good to excellent yields (>90%). More importantly, the leaching of Pd(II) during the reaction process could be negligible (about 0.5 ppm), which is crucially important when preparing bioactive species. The loading of Pd(II) ions on the PI‐COF was 0.49 mmol/g. The catalyst exhibited exceptional performance, demonstrating remarkable efficacy in catalyzing a wide range of aryl halides (excluding aryl chlorides) when reacted with phenylboronic acid under environmentally friendly reaction conditions. Notably, the catalyst exhibited robust recyclability, retaining its catalytic activity throughout 5 reaction cycles without any significant decrease.

A Three-Component Arene Difunctionalization: Merger of C(sp3)/(sp2)-H Bond Addition.

A tandem three-component C-H bond addition involving the activation of an inert C(sp3)-H bond is reported. The process enables the direct regioselective synthesis of 1,2-difunctionalized arenes with the formation of C(sp3)- and C(sp2)-C(arene) bonds. 2-Iodobenzoic acid derivatives behave as masked bifunctional reagent (BFR) and react with 2-pyridyl-methyl sulfoximine (MPyS) protected aliphatic acids bearing α,α-disubstituted groups, and alkenes to produce β-aryl-δ-alkenyl amide derivatives in a single operation. The transformation involves Pd(II)/Pd(IV) and Pd(II)/Pd(0) catalytic systems. Detailed mechanistic studies, including density functional theory (DFT) calculations, reveal the formation of large T-shaped palladacycles and the onset of a 1,2-palladium migration via decarboxylation.

A Three‐Component Arene Difunctionalization: Merger of C(sp3)/(sp2)−H Bond Addition

AbstractA tandem three‐component C−H bond addition involving the activation of an inert C(sp3)−H bond is reported. The process enables the direct regioselective synthesis of 1,2‐difunctionalized arenes with the formation of C(sp3)− and C(sp2)−C(arene) bonds. 2‐Iodobenzoic acid derivatives behave as masked bifunctional reagent (BFR) and react with 2‐pyridyl‐methyl sulfoximine (MPyS) protected aliphatic acids bearing α,α‐disubstituted groups, and alkenes to produce β‐aryl‐δ‐alkenyl amide derivatives in a single operation. The transformation involves Pd(II)/Pd(IV) and Pd(II)/Pd(0) catalytic systems. Detailed mechanistic studies, including density functional theory (DFT) calculations, reveal the formation of large T‐shaped palladacycles and the onset of a 1,2‐palladium migration via decarboxylation.

Palladium-Catalyzed β-C(sp3)-H Arylation of Silyl Prop-1-en-1-ol Ethers with Aryl Halides: Entry to α,β-Unsaturated Ketones.

A palladium(0)-catalyzed β-C(sp3)-H arylation of silyl prop-1-en-1-ol ethers with aryl halides for the synthesis of α,β-unsaturated ketones is presented. In contrast to the reported β-C(sp3)-H arylation of ketones, the chemoselectivity of this current method relies on the Pd(0) catalytic systems and reaction temperatures: While using the Pd(dba)2/DavePhos/KF system at 80 °C resulted in β-C(sp3)-H monoarylation to produce β-monoarylated α,β-unsaturated ketones, harnessing the Pd(OAc)2/t-Bu XPhos/K2HPO4 system at 110 °C induced β-C(sp3)-H diarylation to afford β,β-diarylated α,β-unsaturated ketones. The method provides a versatile route that uses readily available ketone-derivatized α-nonsubstituted silyl prop-1-en-1-ol ethers as the alkene sources and is characterized by a good functional group compatibility, a broad substrate scope, and an excellent selectivity.

Picolinaldehyde-Zinc(II)-Palladium(0) Catalytic System for the Asymmetric α-Allylation of N-unprotected Amino Esters.

Reported in this work is a synergistic ternary achiral picolinaldehyde-Zn(II)-chiral palladium complex system for the highly enantioselective α-allylation of N-unprotected amino esters. By utilizing a variety of allylic carbonates or vinyl benzoxazinanones as substrates, α-allyl α-amino esters were obtained in high yields (up to 96%) with high enantioselectivities (up to 98% ee). Control experiments suggest that the coordination of Zn(II) with the Schiff base intermediate enhances the acidity of the α-C-H bonds of amino esters, thereby favoring α-allylation over intrinsic N-allylation. Furthermore, NMR studies reveal an interaction between the chiral palladium complex and the Zn(II)-Schiff base intermediate, leading to the formation of a picolinaldehyde-Zn(II)-Pd(0) catalytic system.

Pd-catalyzed oxidative amination of 2-alkenylquinazolin-4(3H)-ones: synthesis of methylene and vinyl derivatives of pyrrolo(pyrido)[2,1-b]quinazolinones.

When treated with the catalytic system Pd(OAc)2/PPh3/Cs2CO3/benzoquinone in dioxane or Pd(PPh3)2Cl2/t-BuONa/Cs2CO3/benzoquinone in toluene, 2-butenylquinazolin-4(3H)-ones undergo intramolecular aza-Wacker cyclization to give methylene-substituted pyrrolo(pyrido)[2,1-b]quinazolinones. The latter catalytic system is also efficient in the reaction of pentenyl(hexenyl)quinazolin-4(3H)-ones but, in these cases, the aminopalladation of C-H multiple bonds significantly competed with allylic C(sp3)-H bond activation which leads to hitherto unknown vinyl-substituted pyrrolo(pyrido)[2,1-b]quinazolinones.

One-pot C-C, C-N, and C-S bond construction for synthesis of 3-sulfenylindoles directly from unactivated anilines involving dual palladium catalysis and mechanistic insights from DFT.

An efficient dual Pd-catalytic system was developed for one-pot synthesis of 3-sulfenylindoles via C-C, C-N and C-S bond construction directly from unactivated 2-iodo(NH)anilines under mild reaction conditions. Furthermore, 3-selenyl/halo/carbon-functionalized indoles were synthesized in good yields and a short reaction time. The synthetic utility of 3-sulfenylindole was also demonstrated. The key role of solvent in palladium catalysis was unravelled by DFT.

Orchestrating a β-Hydride Elimination Pathway in Palladium(II)-Catalyzed Arylation/Alkenylation of Cyclopropanols Using Organoboron Reagents.

The scope of chemoselective β-hydride elimination in the context of arylation/alkenylation of homoenolates from cyclopropanol precursors using organoboronic reagents as transmetalation coupling partners was examined. The reaction optimization paradigm revealed a simple ligand-free Pd(II) catalytic system to be most efficient under open air conditions. The preparative scope, which was investigated with 48 examples, supported the applicability of this reaction to a wide range of substrates tolerating a variety of functional groups while delivering β-substituted enone and dienone derivatives in 62-95% yields.

Suzuki–Miyaura arylation of 2,3‐, 2,4‐, 2,5‐, and 3,4‐dibromothiophenes

A convenient and general method for Suzuki–Miyaura double cross‐coupling of various dibromothiophenes was developed. Using a simple and cheap catalytic system Pd(OAc)2/PPh3 in 95% EtOH, various (hetero)arylboronic acids react with dibromothiophenes providing diarylthiophenes, useful in the synthesis of metal–organic frameworks (MOFs), in moderate to excellent yields. The overall efficiency of the catalytic process and slight excess of boronic acids allowed to suppress formation of side products and significantly simplify the purification of products.

Pd‐Catalyzed Cyclization of Alkynyl Norbornene Derivatives for the Synthesis of Benzofused Heteroarenes

AbstractModular approaches, which allow a systematic variation of heteroaromatic cores and substituents, are crucial for the development of heteroaromatic drug candidates and organic functional materials. A new strategy involving the cyclization of heteroarenes tethered with alkynes through a norbornene bridge was developed. The precursors were readily prepared by a three‐component coupling process of heteroaryl halides, norbornadiene, and terminal alkynes. The Pd catalytic system derived from Pd(OAc)2and 2‐(pyrazol‐1‐yl)pyridine transformed a variety of five‐membered heteroarenes to the corresponding benzofused products, including (di)benzothiophene, indazole, carbazole, indole, and benzofuran, with aryl and alkyl substituents at the C4(C7) position. During the cyclization process, the norbornene ring underwent a retro‐Diels‐Alder reaction, serving as an acetylene synthon. This approach was used to synthesize naphthalene derivatives from electron‐rich arenes, demonstrating its versatility in the annulation of (hetero)aromatic rings.magnified image

Pd-catalyzed cross-coupling of arabinogalactan propargyl ethers with 5-bromosalicylic acid

New salicylate derivatives of arabinogalactan have been synthesized in up to 90% yield by cross-coupling of propargyl ethers of arabinogalactan with 5-bromosalicylic acid salts. The reaction proceeds in the presence of the catalytic system Pd(PPh3)4/PPh3 with CuBr (CuI), and piperidine or 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) as a base in DMSO at 70–95 °С. The propargyl groups of the starting arabinogalactan ethers are converted into propinylsalicylate ones with 35 and 50% conversion depending on the nature of the base used. Degree of arabinogalactan substitution with propynyl salicylate fragments reaches 1.0 and 0.7 in the reactions with piperidine and DBU, respectively. In the case of piperidine, along with the main process, intensive unprecedented (for Sonogashira reaction) hydroamination of propargyl groups is also observed, followed by hydrolysis of unstable enamines. The products have been characterized by IR, 1Н and 13С NMR, UV spectroscopy as well as elemental analysis and gel permeation chromatography. Salicylate derivatives of arabinogalactan show high activity in prolonging the activated partial thromboplastin time (APTT), whereas neither the arabinogalactan nor propargyl ethers of arabinogalactan nor salicylic acid salts have obvious effects.

Addition copolymerization of norbornene lactone catalyzed by Pd complexes

AbstractThe addition copolymerization of norbornene (NB) with functionalized monomers can lead to the modification of physical properties of poly(NB). Herein, the synthesis of new copolymer of NB with exo‐norbornene lactone (exo‐NBL) is reported. The copolymerization proceeded by four Pd catalytic systems, and of these, Pd(allyl)IDippCl/AgSbF6 (IDipp = 1,3‐bis[2,6‐diisopropylphenyl]imidazolin‐2‐ylidene)) was the most effective for the incorporation of exo‐NBL. Specifically, the copolymerization with exo‐NBL/NB feed ratio of 50/50 at r.t. by 0.1 mol% of the Pd catalyst produced poly(NB‐co‐exo‐NBL) with Mn of 87,000, and Mw/Mn of 1.2 in 40% yield, incorporating exo‐NBL of 18 mol%. The time–conversion plots and 1H diffusion ordered spectroscopy (DOSY) NMR analysis of the copolymer suggest that it has a random sequence. In contrast, no copolymer was formed from endo‐NBL. This is because of steric hindrance of the endo‐lactone moiety by considering the (co)polymerization of endo‐5‐norbornene‐2‐carboxylic acid methyl ester (endo‐NBCO2Me). The incorporation of exo‐NBL improves the solubility of poly(NB‐co‐exo‐NBL) in several chlorinated solvents and gives high thermal stability with 10% weight loss at a temperature of more than 400°C. Two amorphous halos corresponding to intra‐ and interchain distances were observed in the WAXD patterns, allowing to calculate d‐spacing values, which are higher than that of poly(NB).

Activation of Aryl Chlorides in the Suzuki-Miyaura Reaction by “Ligand-Free” Pd Species through a Homogeneous Catalytic Mechanism: Distinguishing between Homogeneous and Heterogeneous Catalytic Mechanisms

A distinction between the homogeneous and heterogeneous catalytic mechanisms of a myriad of Pd catalytic systems for the Suzuki-Miyaura reaction has been made based on soluble metal complexes or he...

Formation and stabilization of nanosized Pd particles in catalytic systems: Ionic nitrogen compounds as catalytic promoters and stabilizers of nanoparticles

• Pd catalytic systems usually contain nanoparticles regardless of the type of pre-catalyst and require special stabilizers. • Ionic nitrogen compounds stabilize Pd nanoparticles and open up new fields of application in organic synthesis. • The structure of the ionic compound affects the stabilization mechanism and the efficiency of the catalysts. • The catalytic action of the ionic nitrogen compound can outweigh the action of the ligands. • The overall effect of stabilization can be highly dependent on the nature of catalysis and reaction conditions. Actual palladium catalysts in synthetic transformations in reaction mixtures are usually represented by dynamic catalytic systems that contain various interconvertible forms of metal particles, including molecular complexes, metal clusters, and nanoparticles. The low thermodynamic stability of Pd nanoparticles can lead to their aggregation and, as a consequence, to the deactivation of the catalytic systems. Therefore, stabilization of nanosized Pd particles is of key importance to ensure efficient catalysis. This review discusses the main pathways for the formation of Pd nanoparticles and clusters from various precatalysts in catalytic systems, as well as current views on the mechanisms of stabilization of these nanosized Pd particles using various types of ionic nitrogen compounds, such as ammonium, amidinium, azolium, and pyridinium salts. The use of ionic nitrogen compounds as specially added or in situ formed stabilizers, ligands, catalytic promoters, heterogenized catalysts (supported ionic liquid phase, SILP) and reaction media (ionic liquids) is exemplified by several important catalytic reactions. The main effects of ionic nitrogen compounds on catalytic processes are also discussed, including possible involvement in catalytic cycles and unwanted side reactions.

The synergistic effect of rigid and flexible substituents on insertion polymerization with α-diimine nickel and palladium catalysts

The introduction of flexible cycloalkyl groups greatly enhanced the chain growth in the Ni(ii) catalytic system and facilitated the insertion of polar monomers in the Pd(ii) catalytic system.

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.

Vicinal hydroxyl group-inspired selective oxidation of glycerol to glyceric acid on hydroxyapatite supported Pd catalyst

Selective oxidation of glycerol provides a feasible route towards the sustainable synthesis of high value-added chemicals. Herein, the hydroxyapatite (HAP) supported palladium (Pd) species were fabricated by impregnation and subsequent calcination. The as-obtained heterogeneous Pd catalyst afforded not only excellent selectivity to glyceric acid (GLA) up to 90% with 59% conversion of glycerol but also good recyclability by using molecular oxygen as an oxidant under mild conditions. The characterization of catalysts indicated that both the surface basicity and Pd sites on the catalyst played a crucial role in promoting glycerol oxidation. Notably, it demonstrated that the presence of the vicinal hydroxyl group of glycerol molecule can assist the oxidation reaction via forming a coordination between the vicinal hydroxyl group and Ca2+ sites on HAP-derived catalysts. In this catalytic process, the secondary hydroxyl of glycerol kept untouched and the primary hydroxyl of glycerol was converted into carboxyl group, while the Pd species acted as active centers for cooperatively promoting the subsequent oxidation to generate GLA. Additionally, this catalytic system can be extended widely for the oxidative conversion of other vicinal diols into the corresponding α-hydroxycarboxylic acids selectively. Isotope labeling experiment using H218O confirmed that H2O not only acted as solvent but also was involved in the catalytic cycles. On the basis of the results, a possible reaction mechanism has been proposed. The HAP-supported Pd catalytic system has been shown to serve as an effective approach for the upgrading of bio-derived vicinal diols to high value-added chemicals.

Extension of an Encapsulating Macrobicyclic Ligand Using the Palladium-Catalyzed Suzuki–Miyaura Reaction of a Diiodoclathrochelate Iron(II) Tris-Glyoximate with Reactive Halogen Atoms in Its Apical Substituents

The ability to extend the apical substituents at a macrobicyclic tris-glyoximate framework using a palladium-catalyzed Suzuki–Miyaura C–C-cross-coupling was tested. The reaction of an iron(II) diiodoclathrochelate with phenylboronic acid was successfully performed in toluene using the catalytic system Pd(CH3COO)2/SPhos and CsF as inorganic base. The reflux at 110°C gave a macrobicyclic iron(II) complex with two apical biphenyl substituents, characterized using the spectral methods and by the single crystal X-ray diffraction. Asymmetric unit of its crystal consists of two symmetrically independent clathrochelate molecules. Their FeN6-coordination polyhedra with heights h of 2.37 A, Fe–N distances of 1.903(3)–1.916(2) A, and the bite (chelate) angles α of 78.4°–78.6° possess a distorted trigonal prismatic–trigonal-antiprismatic geometry with the distortion angles φ of 18.5° and 18.0°, respectively; the encapsulated iron(II) ion is situated almost in their centers. The crystal packing is mainly governed by the intermolecular hydrophobic H···H, H···C, and C···C interactions and by the H···O, and H···N hydrogen bonds as well.