Regioselective Direct C2 Arylation of Indole, Benzothiophene and Benzofuran: Utilization of Reusable Pd NPs and NHC-Pd@MNPs Catalyst for C–H Activation Reaction

Sustainable protocols for direct C–H bond arylation of (hetero)arenes

“…Very recently, low-cost palladium nanoparticles stabilized by black pepper extract were used by Dateer and co-workers in the direct arylation of indole with aryl iodides, which unfortunately showed low recyclability attributable to severe metal leaching. 55 Supported Pd species have found successful application as recoverable catalysts also in the direct arylation of other bicyclic N-heteroarenes, i.e., imidazopyridines, imidazopyrimidines, adenines and quinolines. In 2013, Lee et al investigated the direct C-H arylation of imidazo[1,2-a]pyridine with aryl bromides of different stereo-electronic properties in the presence of bimetallic Pd-Fe 3 O 4 heterodimer nanocrystals (1 mol%), affording the corresponding arylated products with complete C3 selectivity: in this case no heterogeneity tests were performed, but Pd-Fe 3 O 4 catalyst exhibited high recyclability (10 runs, keeping almost the same product yield).…”

“…Recently, a comparison between N-heterocyclic carbenepalladium magnetic nanoparticles (NHC-Pd@MNPs) and black pepper extract-stabilized palladium nanoparticles in the Scheme 19 C3 regioselective direct C-H bond arylation of quinolines 44 with diaryliodonium salts using palladium nanocatalyst, reported in 2020 Yadav et al 61 Scheme 20 Direct C-H arylation of 2-n-butylfuran 46 with bromoarenes using palladium-polypyrrole nanocomposites Pd@PPy, reported in 2011 by Hierso et al 62 C2 regioselective direct C-H bond arylation of benzofuran 51 with aryl iodides was reported by Dateer et al 55 In both cases, reactions were carried out with 2 mol% of the palladium catalyst, with 1.1 equiv. of sodium acetate as the base, in DMSO at 120 °C for 24 h, affording the corresponding 2-arylbenzofuran 52 in good yields: 66-87% with NHC-Pd@MNPs; 71-90% with black pepper extract-stabilized Pd nanoparticles (Scheme 22).…”

“…C2 arylation of benzo [b]thiophenes with aryl iodides was described by Dateer et al: they compared the performance of black pepper extract-stabilized palladium nanoparticles, showing a modest recyclability due to a severe metal leaching, and N-heterocyclic carbene-Pd magnetic nanoparticles (NHC- 80 Pd@MNPs), which could not be recycled due to their high solubility in the reaction medium. 55 However, the pepper extract-stabilized Pd nanoparticles can be considered more interesting from the point of view of Green Chemistry, since they were based on economically cheap and environmentally friendly materials.…”

Heterogeneous Pd/C‐Catalyzed Ligand‐Free Direct C‐2 Functionalization of Indoles with Aryl Iodides

“…[1] Arylation of indole at the C-2 position offers important synthetic intermediates of different bioactive compounds. [2][3][4][5][6][7] The traditional methods for the C-2 arylation of indole with aryl halide require the prefunctionalization of both coupling partners. [8,9] On the contrary, the development of transition metal-based catalyst systems in recent years leads to the direct arylation of indole through CÀ H bond functionalization.…”

Room Temperature C–H Arylation of Benzofurans by Aryl Iodides

“…Although conventional cross-coupling methods are still widely used, there are a growing number of methods for C–H arylation of benzofurans. In spite of the growing methodology, these methods have limited utility because they require elevated temperatures, preventing use on substrates containing heat-sensitive functional groups. − A small number of mild methods have been reported. − These reports are limited by small substrate scopes, sluggish kinetics under room temperature conditions, and use aryl diazonium salts which have limited commercial availability and can decompose violently …”