Facile synthesis and bonding of 4-ferrocenyl-1,2,4-triazol-5-ylidene complexes

Ferrocene-substituted carbenes have emerged as attractive, redox-active ligands. However, among the compounds studied to date, ferrocenylated 1,2,4-triazol-5-ylidenes, which are closely related to the archetypal imidazol-2-ylidenes, are still unknown. Here, we demonstrate that the triazolium salt [CHN(Me)NCHN(Fc)]I (2; Fc = ferrocenyl), obtained by alkylation of 4-ferrocenyl-4H-1,2,4-triazole (1) with MeI, reacts selectively with metal alkoxide/hydroxide precursors [(cod)Rh(OMe)]2 and [(IPr)Au(OH)] (cod = cycloocta-1,5-diene, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to produce the ferrocene-substituted 1,2,4-triazol-5-ylidene complexes [(cod)RhI{CN(Me)NCHN(Fc)}] and [(IPr)Au{CN(Me)NCHN(Fc)}]I in good yields. The complexes were characterised by NMR and IR spectroscopy, mass spectrometry, cyclic voltammetry, and single-crystal X-ray diffraction analysis. Density function theory (DFT) calculations were used to rationalise the electrochemical behaviour of the carbene complexes and to elucidate the bonding situation in these compounds. An analysis using intrinsic bond orbitals (IBOs) revealed that the 1,2,4-triazol-5-ylidene ligand exerted a strong trans influence and showed a synergistic stabilisation by the negative inductive and positive π-donor effects of the nitrogen atoms adjacent to the carbene carbon atom; these effects were enhanced by conjugation with the CHN bond at the exterior, similar to that in imidazol-2-ylidenes.

Transition Metal Catalyzed Hiyama Cross-Coupling: Recent Methodology Developments and Synthetic Applications

Hiyama cross-coupling is a versatile reaction in synthetic organic chemistry for the construction of carbon-carbon bonds. It involves the coupling of organosilicons with organic halides using transition metal catalysts in good yields and high enantioselectivities. In recent years, hectic progress has been made by researchers toward the synthesis of diversified natural products and pharmaceutical drugs using the Hiyama coupling reaction. This review emphasizes the recent synthetic developments and applications of Hiyama cross-coupling.

Prospects and Applications of Palladium Nanoparticles in the Cross-coupling of (hetero)aryl Halides and Related Analogues

Discovering efficient methods for the formation of carbon-carbon bonds is a central ongoing theme in organic synthesis. Cross-coupling reactions catalysed by metal nanoparticles are attractive alternatives to the traditional use of metal counterparts due to the catalytic tunability, selectivity, recyclability and reusability of the nanoparticles. The ongoing search for sustainable processes demands that reusable and environmentally benign catalysts are used. While the advantages of nanoparticles catalysts over bulk catalysts cannot be overemphasised, the problem of sintering, agglomeration and leaching are drawbacks to their full industrial applications. Hence, efforts are being made towards advancing the efficiency of the catalytic nanoparticle systems over the years. This review presents the progress, the challenges and the prospects of palladium nanoparticle with focus on Heck, Suzuki, Hiyama and Sonogashira cross-coupling reactions involving (hetero) aryl halides and the analogues.

Stabilization of the Pd–NHC framework with 1,2,4-triazol-5-ylidene ligands toward decomposition in alkaline media

New NHC ligands containing a base-ionizable RNH substituent at the C3 atom of the 1,2,4-triazole ring provide superior stability of the Pd–NHC bond against cleavage in strong alkaline media.

Enhanced π-back-donation resulting in the trans labilization of a pyridine ligand in an N-heterocyclic carbene (NHC) PdII precatalyst: a case study

The molecular structure of the benzimidazol-2-ylidene–PdCl2–pyridine-type PEPPSI (pyridine-enhanced precatalyst, preparation, stabilization and initiation) complex {1,3-bis[2-(diisopropylamino)ethyl]benzimidazol-2-ylidene-κC 2}dichlorido(pyridine-κN)palladium(II), [PdCl2(C5H5N)(C23H40N4)], has been characterized by elemental analysis, IR and NMR spectroscopy, and natural bond orbital (NBO) and charge decomposition analysis (CDA). Cambridge Structural Database (CSD) searches were used to understand the structural characteristics of the PEPPSI complexes in comparison with the usual N-heterocyclic carbene (NHC) complexes. The presence of weak C—H...Cl-type hydrogen-bond and π–π stacking interactions between benzene rings were verified using NCI plots and Hirshfeld surface analysis. The preferred method in the CDA of PEPPSI complexes is to separate their geometries into only two fragments, i.e. the bulky NHC ligand and the remaining fragment. In this study, the geometry of the PEPPSI complex is separated into five fragments, namely benzimidazol-2-ylidene (Bimy), two chlorides, pyridine (Py) and the PdII ion. Thus, the individual roles of the Pd atom and the Py ligand in the donation and back-donation mechanisms have been clearly revealed. The NHC ligand in the PEPPSI complex in this study acts as a strong σ-donor with a considerable amount of π-back-donation from Pd to Ccarbene. The electron-poor character of PdII is supported by π-back-donation from the Pd centre and the weakness of the Pd—N(Py) bond. According to CSD searches, Bimy ligands in PEPPSI complexes have a stronger σ-donating ability than imidazol-2-ylidene ligands in PEPPSI complexes.

R. S, Patil SA, Małecki JG, Budagumpi S. Coumarin-substituted 1,2,4-triazole-derived silver(i) and gold(i) complexes: synthesis, characterization and anticancer studies

A series of coumarin-substituted 1,2,4-triazolium salts and their respective silver– and gold– N-heterocyclic carbene complexes have been reported. The complexes displayed promising anticancer activity with GI₅₀ values of up to 0.354 μM and 8.5983 μM against MCF 7 and HT-29 cell lines, respectively.

Recent advances in transition metal-catalysed cross-coupling of (hetero)aryl halides and analogues under ligand-free conditions

The formation of new bonds is pivotal in organic chemistry and a prerequisite to life because it allows the construction of complex molecules from simple precursors.

One-Pot Tandem Hiyama Alkynylation/Cyclizations by Palladium(II) Acyclic Diaminocarbene (ADC) Complexes Yielding Biologically Relevant Benzofuran Scaffolds

A series of palladium acyclic diaminocarbene (ADC) complexes of the type cis-[(R¹NH)(R²)methylidene]PdCl₂(CNR¹) [R¹ = 2,4,6-(CH₃)₃C₆H₂: R² = NC₅H₁₀ (2); NC₄H₈ (3); NC₄H₈O (4)] were used not only to perform the C_sp² -C_sp Hiyama coupling between aryl iodide and triethoxysilylalkynes but also to subsequently carry out the one-pot tandem Hiyama alkynylation/cyclization reaction between 2-iodophenol and triethoxysilylalkynes, giving a convenient time-efficient access to the biologically relevant benzofuran compounds. The palladium ADC complexes (2-4) were conveniently synthesized by the nucleophilic addition of secondary amines, namely, piperidine, pyrrolidine, and morpholine on the cis-{(2,4,6-(CH₃)₃C₆H₂)NC}₂PdCl₂ in moderate yields (ca. 61-66%).

Sonogashira (Cu and amine free) and Suzuki coupling in air catalyzed via nanoparticles formed in situ from Pd(ii) complexes of chalcogenated Schiff bases of 1-naphthaldehyde and their reduced forms

The activation of the coupling reactions with the Pd(ii)-complexes (0.05–0.01 mol% loading) is significant in 1–2 h under mild conditions.

A General Palladium-Catalyzed Hiyama Cross-Coupling Reaction of Aryl and Heteroaryl Chlorides

A general palladium-catalyzed Hiyama cross-coupling reaction of aryl and heteroaryl chlorides with aryl and heteroaryl trialkoxysilanes by a Pd(OAc)2 /L2 catalytic system is presented. A newly developed water addition protocol can dramatically improve the product yields. The conjugation of the Pd/L2 system and the water addition protocol can efficiently catalyze a broad range of electron-rich, -neutral, -deficient, and sterically hindered aryl chlorides and heteroaryl chlorides with excellent yields within three hours and the catalyst loading can be down to 0.05 mol % Pd for the first time. Hiyama coupling of heteroaryl chlorides with heteroaryl silanes is also reported for the first time. The reaction can be easily scaled up 200 times (100 mmol) without any degasification and purification of reactants; this facilitates the practical application in routine synthesis.

Single-Molecule Investigation of Initiation Dynamics of an Organometallic Catalyst

The action of molecular catalysts comprises multiple microscopic kinetic steps whose nature is of central importance in determining catalyst activity and selectivity. Single-molecule microscopy enables the direct examination of these steps, including elucidation of molecule-to-molecule variability. Such molecular diversity is particularly important for the behavior of molecular catalysts supported at surfaces. We present the first combined investigation of the initiation dynamics of an operational palladium cross-coupling catalyst at the bulk and single-molecule levels, including under turnover conditions. Base-initiated kinetics reveal highly heterogeneous behavior indicative of diverse catalyst population. Unexpectedly, this distribution becomes more heterogeneous at increasing base concentration. We model this behavior with a two-step saturation mechanism and identify specific microscopic steps where chemical variability must exist in order to yield observed behavior. Critically, we reveal how structural diversity at a surface translates into heterogeneity in catalyst behavior, while demonstrating how single-molecule experiments can contribute to understanding of molecular catalysts.

Mono- and dinuclear palladium(ii) complexes containing both N-heterocyclic carbenes and tetrazole ligands as catalysts for Hiyama coupling

Six tetrazole ligand stabilized NHC–Pd complexes have been synthesized and fully characterized. Catalytic performance of the obtained palladium catalysts has been investigated for the Hiyama coupling reaction.

Dendrimer-encapsulated Pd(0) nanoparticles immobilized on nanosilica as a highly active and recyclable catalyst for the copper- and phosphine-free Sonogashira–Hagihara coupling reactions in water

Recyclable dendrimer-encapsulated Pd(0) nanoparticles immobilized on nanosilica in the Sonogashira–Hagihara reaction under copper(i) and phosphine ligand-free conditions in water.

Continuous flow Negishi cross-couplings employing silica-supported Pd-PEPPSI–IPr precatalyst

The synthesis of a triethoxysilyl functionalisedPd-PEPPSI–IPrcomplex preparedviaazide–alkyne cycloaddition is described.

Synthesis, structure and catalysis of a NHC–Pd(ii) complex based on a tetradentate mixed ligand

NHC–Pd(ii) complex 1 has been prepared. The catalytic activity of 1 in C–C coupling reactions was studied.

A palladium salen complex: an efficient catalyst for the Sonogashira reaction at room temperature

A tetradentate Schiff base derived palladium complex works as an efficient catalyst for the room temperature Sonogashira reaction.

Fluoride-free Hiyama coupling by palladium abnormal N-heterocyclic carbene complexes

A series of Pd complexes of the 1,2,3-triazole based abnormal NHC ligands of the type (a-NHC)PdI₂(L) [L = NC₅H₅ and PPh₃] successfully catalyzed the fluoride-free Hiyama coupling in air.