N-Heterocyclic carbene-palladacyclic complexes: synthesis, characterization and their applications in the C-N coupling and α-arylation of ketones using aryl chlorides

Recent advances in sustainable N-heterocyclic carbene-Pd(II)-pyridine (PEPPSI) catalysts: A review

Various catalysts in homogeneous or heterogeneous catalysis deploy unconventional reaction pathways by lowering the activation energy (AE) barrier, controlling the selectivity, and creating environmental impact, thereby bringing about economic viability. Hence, the study of these methodologies is of immense interest. To develop a new chemistry, there is much scope for the invention of brilliant candidates that could effectively catalyze diverse reaction methodologies. The palladium-catalyzed reactions motivate interesting applications on various organic transformations under mild reaction conditions. Although phosphorous designed ligands or catalysts have been used, despite their expensiveness, sensitivity and other properties, there is the necessity of developing even better cross-coupling ligands or catalysts such as N-heterocyclic carbene (NHC)-based palladium complexes. These palladium-NHCs (Pd-NHC) are novel and universal nucleophilic entities that have come into light as the most successful class of catalysts in organometallic chemistry. In the same class, a specific category of palladium-NHCs such as palladium-pyridine enhanced pre-catalyst preparation by the stabilization initiation (palladium-PEPPSI) complexes, are emerging as versatile alternatives to phosphine containing palladium complexes for various cross-coupling reactions due to their excellent catalytic activity. Further to mention that NHCs are recently extensively used as ancillary ligands in organometallic chemistry, which includes industrial-related catalytic transformations due to strong σ-donors to metal centres. Apart from this, many NHC-metal complexes are the fascinating consideration in material science as probable metallo-pharmaceuticals. The current review offers a brief exploration of palladium-PEPPSI complexes over the past few years. Further, the synthesis of a variety of these types of catalysts, their applications in Suzuki-Miyaura, Buchwald-Hartwig, Sonogashira, Negishi couplings direct C2-arylation, O-C(O) cleavage, α-arylation/alkylation of carbonyl compounds and trans-amidation reactions via cross-coupling methodologies, which are covered. Additionally, reported recent developments on reusable heterogeneous PdPEPPSI complexes and their catalytic applications are being covered. Finally, the chiral Pd complexes and their asymmetric transformations are discussed.

Highly efficient α-arylation of aryl ketones with aryl chlorides by using bulky imidazolylidene-ligated oxazoline palladacycles

α-Aryl derivatives of carbonyl compounds are important building blocks. Herein, we presented an efficient catalytic system for the α-arylation of aryl ketones with inactive aryl chlorides by firstly using N,N'-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr)-ligated chiral oxazoline palladacycles, and tolerated a wide range of substrates at low catalyst loadings, leading to the desired products in good to excellent yields.

A Second-Generation Palladacycle Architecture Bearing a N-Heterocyclic Carbene and Its Catalytic Behavior in Buchwald–Hartwig Amination Catalysis

Palladacyclic architectures have been shown as versatile motifs in cross-coupling reactions. NHC-ligated palladacycles possessing unique electronic and steric properties have helped to stabilize the catalytically active species and provide additional control over reaction selectivity. Here, we report on a synthetic protocol leading to palladacycle complexes using a mild base and an environmentally desirable solvent, with a focus on complexes bearing backbone-substituted N-heterocyclic carbene ligands. The readily accessible complexes exhibit high catalytic activity in the Buchwald–Hartwig amination. This is achieved using low catalyst loading and mild reaction conditions in a green solvent.

Parameterization of Arynophiles: Experimental Investigations towards a Quantitative Understanding of Aryne Trapping Reactions

Arynes are highly reactive intermediates that may be used strategically in synthesis by trapping with arynophilic reagents. However, ‘arynophilicity’ of such reagents is almost completely anecdotal and predicting which ones will be efficient traps is often challenging. Here, we describe a systematic study to parameterize the arynophilicity of a wide range of reagents known to trap arynes. A relative reactivity scale, based on one-pot competition experiments, is presented by using furan as a reference arynophile and 3-chlorobenzyne as a the aryne. More than 15 arynophiles that react in pericyclic reactions, nucleophilic addition, and σ-bond insertion reactions are parameterized with arynophilicity (A) values, and multiple aryne precursors are applicable.

Selective α-Methylation of Aryl Ketones Using Quaternary Ammonium Salts as Solid Methylating Agents

We describe the use of phenyl trimethylammonium iodide (PhMe₃NI) as an alternative methylating agent for introducing a CH₃ group in α-position to a carbonyl group. Compared to conventional methylating agents, quaternary ammonium salts have the advantages of being nonvolatile, noncancerogenic, and easy-to-handle solids. This regioselective method is characterized by ease of operational setup, use of anisole as green solvent, and yields up to 85%.

N-Heterocyclic carbene complexes enabling the α-arylation of carbonyl compounds

The considerable importance of α-arylated carbonyl compounds, which are widely used as final products or as key intermediates in the pharmaceutical industry, has prompted numerous research groups to develop efficient synthetic strategies for their preparation in recent decades. In this context, the α-arylation of carbonyl compounds catalyzed by transition-metal complexes have been particularly helpful in constructing this motif. As illustrated in this contribution, tremendous advances have taken place using palladium- and nickel-NHC (NHC = N-heterocyclic carbene) complexes as pre-catalysts for the arylation of a wide range of ketones, aldehydes, esters and amides with electron-rich, electron-neutral, electron-poor, and sterically hindered aryl halides or pseudo-halides. Despite significant progress, especially in asymmetric α-arylations promoted by chiral NHC ligands, there are numerous challenges which have and continue to encourage further studies on this topic. Some of these are presented in this report.

Different effects of metal-NHC bond cleavage on the Pd/NHC and Ni/NHC catalyzed α-arylation of ketones with aryl halides

Fundamental differences in the behavior of Pd/NHC and Ni/NHC catalytic systems in ketones α-arylation were elucidated and exploited.

Design, synthesis and biological activities of pyrrole-3-carboxamide derivatives as EZH2 (enhancer of zeste homologue 2) inhibitors and anticancer agents

A novel series of pyrrole-3-carboxamides targeting EZH2 have been designed and synthesized. The structure–activity relationships were summarized by combining within vitrobiological activity assay and docking results.

The crystal structure of tetrachlorido-bis{1,3-bis(2,6-diisopropylphenyl)-1H-3λ4-imidazol-2-yl}-(μ2-pyrimidine-κ2N:N′)dipalladium(IV) — dichloromethane (1/2), C60H80Cl8N6Pd2

C60H80Cl8N6Pd2, triclinic, P1̄ (no. 2), a = 10.8703(10) Å, b = 17.0284(15) Å, c = 18.9595(17) Å, α = 83.1960(10)°, β = 88.909(2)°, γ = 71.523(2)°, V = 3304.5(5) Å3, Z = 2, Rgt(F) = 0.0488, wRref(F2) = 0.1213, T = 130 K.

The crystal structure of dichlorido(1,3-bis(2,6-diisopropyl-phenyl)-1H-3λ4-imidazol-2-yl)(3-phenyl-pyridine-κN)palladium(IV), C38H45N3Cl2Pd

C38H45N3Cl2Pd, monoclinic, P21/c (no. 14), a = 12.8249(8) Å, b = 16.0552(9) Å, c = 20.5611(10) Å, β = 115.396(3)°, V = 3824.5(4) Å3, Z = 4, R gt(F) = 0.0287, wR ref(F 2) = 0.0879, T = 173(2) K.

Facile synthesis of Pd(ii) and Ni(ii) pincer carbene complexes by the double C–H bond activation of a new hexahydropyrimidine-based bis(phosphine): catalysis of C–N couplings

A new hexahydropyrimidine-based NHC proligand undergoes facile double C–H bond activation to give Pd(ii) and Ni(ii) NHC pincer cationic complexes. The Pd complex catalyzes C–N cross couplings very efficiently.