Selective Manipulation of Well-Defined Trinuclear Pd(II)-Complexes

Several strategies are available to design well-defined multimetallic molecular entities bearing functional ligands. Substoichiometric exchange reactions in the coordination sphere of pre-existing multinuclear precursors are relatively underexploited in this context. Palladium(II) acetate is not a mononuclear compound in the solid state but rather exists as a trimer, i. e. [Pd3(OAc)6]. Although this material is ubiquitously used to synthesize mononuclear Pd species, it may principally also lend itself to selective exchange of some of the edge-sharing acetate units in its triangular motif, whilst keeping the overall multinuclear architecture intact. Strikingly, little is known about the controlled manipulation and substoichiometric substitution chemistry of this well-defined conglomerate. We herein conclusively demonstrate that, for the first time, the targeted exchange of two or four acetate units from the Pd3(acetate)6 platform is possible, thereby installing either one or two new tridentate ligands onto this trinuclear architecture. Follow-up exchange and substitution chemistry is available without disrupting the multimetallic nature of the core structure. New complexes 2-7 are all conclusively characterized using multinuclear NMR spectroscopy, UV-vis and IR spectroscopy as well as X-ray diffraction analysis.

Kinetic Aspects of Suzuki Cross-Coupling Using Ligandless Pd Nanoparticles Embedded in Aromatic Polymeric Matrix

During the last decades, palladium nanoparticles (Pd(0) NPs) and Pd(II) compounds were shown to be attractive catalysts for fine organic synthesis. Nanostructured Pd(0) or Pd(II) catalysts have a relatively low environmental impact, but, at the same time, they are indispensable for such processes as Suzuki cross-coupling. This paper describes the preparation of Pd(0) or Pd(II) supported/embedded in hyper-cross-linked polystyrene (HPS) and compares their activity in Suzuki cross-coupling between phenylboronic acid and 4-bromoanisole. Obviously, the palladium charge (Pd(0) ↔ Pd(II)) changes continuously during the reaction catalytic cycle. It would seem that the use of the starting palladium in the form of Pd(0) or Pd(II) should not affect the reaction’s kinetic laws for both catalysts, but their special individuality is manifested between them. Nanoparticulate Pd(0) catalysts are stable during the reaction. In contrast, catalysts based on Pd(II) are extremely active in the initial period of the reaction, but then the “hot form” of the catalyst is rapidly converted into the form of Pd(0), whose activity is identical to that of the preliminarily reduced catalyst. This work discusses the possible nature of this phenomenon. A mathematical model for Suzuki cross-coupling reaction was suggested that was able to adequately describe experimental data. The level of reliability (R2) of the correlation between the experimental and calculated data was R2 = 0.97–0.99.

Aqueous Suzuki couplings mediated by a hydrophobic catalyst

The catalytic activity of [(Ph₂P-o-C₆H₄)₂N]PdCl in aerobic aqueous Suzuki couplings is described. Though hydrophobic, this molecular catalyst is competent in cross-coupling reactions of arylboronic acids with a variety of electronically activated, unactivated, and deactivated aryl iodides, bromides, and chlorides upon heating in aqueous solutions under aerobic conditions to give biphenyl derivatives without the necessity of amphiphiles even in the presence of an excess amount of mercury.

Anchored Palladium Complex-Generated Clusters on Zirconia: Efficiency in Reductive N-Alkylation of Amines with Carbonyl Compounds under Hydrogen Atmosphere

Carbon-nitrogen bond formation is an important method on both laboratory and industrial scales because it realizes the production of valuable pharmaceuticals, agrochemicals, and fine chemicals. Direct reductive N-alkylation of amines with carbonyl compounds via intermediary imine compounds, especially under catalytic hydrogenation conditions, is one of the most convenient, economical, and environmentally friendly methods for this process. Here we report a novel palladium species on zirconia having specific activity towards hydrogenation of imines but other carbonyl groups remaining intact. The present catalytic property offers a practical synthetic method of functionalized secondary amines by reductive N-alkylation under mild conditions with high atom-efficiency. Mechanistic studies revealed that the catalytically active species is the palladium cluster, which is generated in situ from molecular palladium complexes on the support by exposure to atmospheric hydrogen. These fundamental findings are expected to progress in developing novel cluster catalysts for chemical processes directed towards a sustainable society.

Synthesis and characterization of bis(amine)palladium(II) carboxylate complexes as precursors of palladium nanoparticles

The synthesis and characterization of the adducts of n-alkyl amine and palladium n-alkyl carboxylate, [Pd(R2NH2)2(R1COO)2] (R1 = 1, 7, and 11; R2 = 8, 12, and 16), as precursors for the synthesis of palladium nanoparticles (PdNPs) was carried out via differential scanning calorimetry, FT-IR, Raman and UV-Vis spectroscopy, NMR spectroscopy (1H, 13C pulsed field gradient spin-echo (PGSE), and 13C CP-MAS), and powder X-ray diffraction. Pd n-alkyl carboxylates were obtained by a ligand exchange reaction from palladium acetate and the appropriate aliphatic carboxylic acid. It is proposed that carboxyl moieties in the presence of amine ligands are bound to palladium ions via monodentate bonding as opposed to bridging bidentate coordination of pure palladium carboxylate which exists in the form of polymer aggregates. All the studied palladium carboxylate/amine complexes form bilayer lamellar structures and exhibit first-order melting transitions. The evidence presented in this study shows that the phase behavior of bivalent metal carboxylates is mainly controlled by the type of coordination of carboxylate head groups. For n-alkyl carboxylates, linear chain type aggregates replace the trimeric units of Pd acetate. In solution, in the presence of amine, palladium salt aggregates disintegrate and the Pd complex is isolated and stabilized by amine molecules. Using bis(amine) palladium carboxylate adducts as precursors, palladium nanoparticles were fabricated. During high temperature thermolysis, the bis(amine) Pd carboxylate complex decomposes to form small sized Pd nanoparticles. Combining NMR techniques with FTIR spectroscopy, it was possible to follow an original stabilization mechanism. PdNPs are stabilized by weakly interacting long chain aliphatic amide and carboxylic acid derived from the palladium precursor.

Microwave-Assisted Regioselective Suzuki Coupling of 2,4-Dichloropyrimidines with Aryl and Heteroaryl Boronic Acids

Suzuki coupling reaction has been often used for the preparation of a diverse set of substituted pyrimidines. In this study, the Suzuki coupling of 2,4-dichloropyrimidines with aryl and heteroaryl boronic acids was investigated. A thorough screening of reaction conditions and the use of microwave irradiation led to a very efficient and straightforward synthetic procedure providing C4-substituted pyrimidines in good to excellent yields. Short reaction time (15 min) and extremely low catalyst loading (0.5 mol%) are the main advantages of our tetrakis(triphenylphosphine)palladium(0) catalyzed microwave-assisted procedure, which could be used for quick and low-cost regioselective preparation of substituted pyrimidine rings.

Study of Deactivation in Suzuki Reaction of Polymer-Stabilized Pd Nanocatalysts

This work is addressed to the phenomenon of catalyst deactivation taking place during the repeated uses in the reaction of Suzuki-Miyaura (S-M) cross-coupling, which is widely applied in industry for C-C bond formation. Ligandless catalysts based on Pd(0) NPs supported on hyper-cross-linked polystyrene (HPS) of two types (non-functionalized and bearing tertiary amino groups) were studied in a model S-M reaction between 4-bromoanisole and phenylboronic acid. Synthesized catalysts were shown to be highly active under mild reaction conditions. HPS allows stabilization of Pd(0) NPs and prevents their agglomeration and detectable Pd leaching. However, the loss of catalytic activity was observed during recycling. The deactivation issue was assigned to the hydrophobic nature of non-functionalized HPS, which allowed a strong adsorption of cross-coupling product during the catalyst separation procedure. A thorough washing of Pd/HPS catalyst by hydrophobic solvent was found to improve to the big extent the observed catalytic activity, while the replacement of non-functionalized HPS by a one containing amino groups increased the catalyst stability at the expense of their activity.

Si-thiol supported atomic-scale palladium as efficient and recyclable catalyst for Suzuki coupling reaction

Atomic-scale catalysts leverage the advantages of both heterogeneous catalysts for their stability and reusability and homogeneous catalysts for their isolated active sites. Here, a palladium catalyst supported by Si-thiol, a commercially available mercaptopropyl-modified and TMS-passivated amorphous silica, was synthesized and characterized by SEM,TEM, aberration-corrected STEM-HAADF, XRD, FT-IR and XPS. Statistical analysis revealed that the catalytic Pd species predominantly consisted of intermediate sized nanoparticles (<2 nm), small amounts of essentially isolated atoms (ca. 0.1 nm), and limited amounts of somewhat larger nanoparticles (<5 nm). The nanoscale atomic clusters dominated the reactivity and served as the key active sites for Suzuki coupling. The outcomes of the reaction were greatly affected by the choice of solvents, and Pd/Si-thiol was demonstrated to be reusable for more than three times without a noticeable loss of catalytic activity. [Formula: see text].

Revisiting the mechanism of the Fujiwara–Moritani reaction

A new perspective of the Fujiwara–Moritani reaction is presented, including its kinetic profile and catalyst deactivation studies.

Direct comparison of safer or sustainable alternative dipolar aprotic solvents for use in carbon–carbon bond formation

Direct comparison of safer dipolar aprotic solvents for use in carbon–carbon bond formation is of vital importance for industrial applications. Cyrene exhibited high initial reaction rates, yields and solvent recovery in the Baylis–Hillman reaction.

Shining light on the solid–liquid interface: in situ/operando monitoring of surface catalysis

Many industrially important chemical transformations occur at the interface between a solid catalyst and liquid reactants. In situ and operando spectroscopies offer unique insight into the reactivity of such catalytically active solid–liquid interfaces.

Templating metastable Pd2 carboxylate aggregates

Evaluation of the potential for metal-metal (M-M) cooperation to enable catalysis requires access to specific polynuclear aggregates that display appropriate geometry and size. In many cases, exerting synthetic control over the aggregation of simple metal salts is a challenge. For example, Pd(ii) acetate self assembles as a trimer (i.e. Pd3(OAc)6) both in the solid state and in solution and does not feature close Pd-Pd interactions. Related carboxylate-supported Pd2 aggregates (i.e. Pd2(OAc)4), which would feature close Pd-Pd interactions, are thermodynamically metastable in solution phase and thus largely unavailable. Here we demonstrate ion metathesis within pre-formed metal-organic frameworks (MOFs) to prepare metastable Pd2 tetracarboxylates sites. The newly synthesized materials are characterized by elemental analysis, PXRD, SCXRD, EXAFS, XANES, and gas adsorption analysis. In addition, the critical role of network solvation on the kinetics of ion metathesis was revealed by coupled TGA-MS and ICP-MS experiments. The demonstration of templated ion metathesis to generate specific metastable coordination sites that are inaccessible in solution phase chemistry represents a new opportunity to interrogate the chemistry of specific polynuclear metal aggregates.

Ligand-Free Bioinspired Suzuki-Miyaura Coupling Reactions using Aryltrifluoroborates as Effective Partners in Deep Eutectic Solvents

Pd-catalyzed Suzuki-Miyaura cross-coupling between (hetero)aryl halides (Cl, Br, I) and versatile, moisture-stable mono- and bifunctional potassium aryltrifluoroborates proceeded efficiently and chemoselectively in air and under generally mild conditions; a catalyst loading as low as 1 mol % combined with Na₂ CO₃ as a base in choline chloride/glycerol (1:2) deep eutectic solvent (DES) was used as a sustainable and environmentally responsible medium. The catalyst, base, and DES were easily and successfully recycled up to six times with an E-factor as low as 8.74. Valuable biaryls and terphenyl derivatives were furnished in yields of up to 98 %; over 50 reactions were compared and discussed. The methodology was applied for the synthesis of the nonsteroidal anti-inflammatory drugs Felbinac and Diflunisal.

The role of trinuclear species in a palladium acetate/trifluoroacetic acid catalytic system

Reactions catalyzed by palladium(ii) acetate and trifluoroacetic acid (TFA) have a clear preactivation phase. However, the structure of real catalytic species remains unclear. We show that the key species are cyclic trinuclear complexes of composition [Pd₃(OAc)_6-x(OTFA)_x] (x = 1-6) formed by a sequential ligand exchange from [Pd₃(OAc)₆]. Furthermore, we prove that the trinuclear palladium backbone of the precatalyst remains preserved during the first phase of the C-H activation reaction of acetanilides. In other words, the reaction pathway including the trinuclear species should be taken into account in discussion about mechanisms of the reactions catalyzed by palladium acetates.

Computational study on palladium-catalyzed alkenylation of remote δ-C(sp3)–H bonds with alkynes: a new understanding of mechanistic insight and origins of site-selectivity

Palladium-catalyzed alkenylation of δ-C(sp³)–H bonds with alkynes was conducted by density functional theory calculations. The present study shows that the dimeric Pd₂(OAc)₄ mechanism reproduces experimental observations well, including regioselectivity and provides a deep mechanistic insight complementing the monomeric Pd(OAc)₂ mechanism recently reported by Chen's group. In addition, the economical heterodimeric Ni–Pd(OAc)₄ was predicted to be a potential species for such alkenylation of δ-C(sp³)–H bonds.

Palladium-catalyzed alkenylation of δ-C(sp³)–H bonds with alkynes was conducted by DFT calculations, showing that the dimeric Pd₂(OAc)₄ mechanism reproduces experimental observations well.

Solvent-dependent nuclearity, geometry and catalytic activity of [(SPhos)Pd(Ph)Cl]2

The nuclearity and structures of the palladium complex [(SPhos)Pd(Ph)Cl]₂ in the solid and solution states are revisited using a combination of Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, NMR spectroscopy, mass spectrometry, DFT calculations and trapping experiments. The complex was tested for its catalytic activity in the coupling reaction between chlorobenzene and n-hexylamine, where different deactivation behaviours were observed in toluene, 1,4-dioxane and DMF.

Role of Lewis acid additives in a palladium catalyzed directed C-H functionalization reaction of benzohydroxamic acid to isoxazolone

Metallic salts as well as protic additives are widely employed in transition metal catalyzed C-H bond functionalization reactions to improve the efficiency of catalytic protocols. In one such example, ZnCl₂ and pivalic acid are used as additives in a palladium catalyzed synthesis of isoxazolone from a readily available benzohydroxamic acid under one pot conditions. In this article, we present some important mechanistic insights into the role of ZnCl₂ and pivalic acid, gained by using density functional theory (M06) computations. Two interesting modes of action of ZnCl₂ are identified in various catalytic steps involved in the formation of isoxazolone. The conventional Lewis acid coordination wherein zinc chloride (ZnCl₂·(DMA)) binds to the carbonyl group is found to be more favored in the C-H activation step. However, the participation of a hetero-bimetallic Pd-Zn species is preferred in reductive elimination leading to C_aryl-N bond formation. Pivalic acid helps in relay proton transfer in C-H bond activation through a cyclometallation deprotonation (CMD) process. The explicit inclusion of ZnCl₂ and solvent N,N-dimethyl acetamide (DMA) stabilizes the transition state and also helps reduce the activation barrier for the C-H bond activation step. The electronic communication between the two metal species is playing a crucial role in stabilizing the C_aryl-N bond formation transition state through a Pd-Zn hetero-bimetallic interaction.

Aqueous Suzuki Coupling Reactions of Basic Nitrogen-Containing Substrates in the Absence of Added Base and Ligand: Observation of High Yields under Acidic Conditions

A series of aqueous heterogeneous Suzuki coupling reactions of substrates containing basic nitrogen centers with phenylboronic acid in the absence of added base and ligand is presented. High yields of products were obtained by employing aryl bromides containing aliphatic 1°, 2°, and 3° amine substituents, and good to high yields were obtained by employing a variety of substituted bromopyridines. In the former series, the pH of the aqueous phase changed from basic to acidic during the course of the reaction, while in the latter series the aqueous phase was on the acidic side of the pH scale throughout the entire course of reaction. A mechanistic interpretation for these observations, which generally preserves the oxo palladium catalytic cycle widely accepted in the literature, is presented.

Catalytic C-H bond functionalisation chemistry: the case for quasi-heterogeneous catalysis

This feature article examines the potential of heterogeneous Pd species to mediate catalytic C-H bond functionalisation processes employing suitable substrates (e.g. aromatic/heteroaromatic compounds). A focus is placed on the reactivity of supported and non-supported Pd nanoparticle (PdNPs) catalysts, in addition to the re-appropriation of well-established heterogeneous Pd catalysts such as Pd/C. Where possible, reasonable comparisons are made between PdNPs and traditional 'homogeneous' Pd precatalyst sources (which form PdNPs). The involvement of higher order Pd species in traditional cross-coupling processes, such as Mizoroki-Heck, Sonogashira and Suzuki-Miyaura reactions, allows the exemplification of potential future topics for study in the area of catalytic C-H bond functionalisation processes.

Vapour-induced solid-state C–H bond activation for the clean synthesis of an organopalladium biothiol sensor

Room-temperature accelerated aging in the solid state has been applied for quantitative azobenzene C–H bond activation by Pd(OAc)₂. Water-soluble dicyclopalladated methyl orange is a selective chromogenic biothiol sensor at physiologically-relevant micromolar concentrations in aqueous media.

A new mechanism for allylic alcohol isomerization involving ruthenium nanoparticles as a ‘true catalyst’ generated through the self-assembly of supramolecular triruthenium clusters

A new five step mechanism has been established for allylic alcohol isomerization involving ruthenium nanoparticles as the ‘true catalyst’.

Recent Advances in the C-H-Functionalization of the Distal Positions in Pyridines and Quinolines

This review summarizes recent developments in the C-H-functionalization of the distal positions of pyridines, quinolines and related azaheterocycles. While the functionalization of the C2 position has been known for a long time and is facilitated by the proximity to N1, regioselective reactions in the distal positions are more difficult to achieve and have only emerged in the last decade. Recent advances in the transition metal-catalyzed distal C-H-functionalization of these synthetically-important azaheterocycles are discussed in detail, with the focus on the scope, site-selectivity and mechanistic aspects of the reactions.