A Versatile Palladium/Triphosphane System for Direct Arylation of Heteroarenes with Chloroarenes at Low Catalyst Loading

Abstract

Palladium-catalyzed cross-coupling reactions between organometallic nucleophilic reagents and electrophilic organic halides or pseudohalides emerged as powerful synthetic tools for the construction of C À C bonds.[1-2] Such catalytic coupling processes are applied to a wide array of fields, which range from biological sciences to materials chemistry. [3][4] Their applications to heteroaromatic substrates set the stage for convergent synthetic routes to valuable substituted heterocyclic structures.[5] Because of the necessity of limiting costly and contaminant metallic reagents, the research focus has shifted to the direct arylation of heteroaromatic substrates by the combined C À H/C À X activation (X = halide or pseudohalide).[6] This type of methodology presents the advantage of circumventing the preparation of organometallic nucleophilic reagents. It also avoids stoichiometric formation of metallic side products, from which undesired contamination could be appalling for pharmaceutical, agrochemical, and related biological applications.Reports of palladium-catalyzed direct arylations of heteroaromatics have described the use of organic bromides, [7] iodides, [8] triflates, [9] mesylates and tosylates, [10] sulfamates and phosphates, [11] iodonium salts, [12] and potassium trifluoroborates [13] as useful reagents. Organic chlorides remained noticeably uncommon partners, [14] despite the fact that among halides and pseudohalides, chlorides are arguably the most useful single class of substrates because of their straightforward access, their lower cost, and the wider diversity of available compounds. However, chloroarenes are most often unreactive under the conditions employed to couple other more reactive starting materials. Few monodentate electron-rich catalysts, [15] and a catalytic system based on a chelating diphosphane [16] have achieved a limited number of intermolecular couplings [17] between mostly unsubstituted or electron-deficient aryl chlorides and heteroaromatic compounds. Despite this remarkable progress, more sustainable catalytic systems, employing significantly less palladium/ligand catalyst, for the coupling of a wide array of diversely substituted aryl chlorides to heteroaromatic compounds have not yet been reported. Herein, we disclose a new air-stable, moisture and temperature tolerant palladium/ triphosphane system that is highly efficient for the direct arylation of substituted furan, pyrrole, thiophene, and thiazole substrates. Notably, these findings represent an economically attractive direct arylation of hetero-and diheteroaromatic substrates with chloroarenes by using less than 1 mol % of the palladium/ligand catalyst. This versatile system highlights also the potential of tridentate ferrocenyl polyphosphane ligands as air-stable, easy to handle auxiliaries in demanding intermolecular C À H/C À Cl activations.As a part of our program directed towards the development of robust polydentate auxiliaries for various crosscoupling reactions, [18] we probed various ferrocenyl polyphosphane ...

N-Heterocyclic carbene–palladium catalysts for the direct arylation of pyrrole derivatives with aryl chlorides

“…We had previously observed that with this pyrrole derivative a high yield of 89% could be obtained in the presence of only 0.5 mol % of a triphosphine associated to Pd(OAc) 2 as the catalyst [27]. With complexes 2 , 3 , 8 and 9 , a high conversion of 4-chlorobenzonitrile ( 11 ) and good yields of the coupling product 16 were obtained (Table 1, entries 1–9).…”

“…However, there are still limitations for these reactions in terms of aryl halide or heteroaromatic tolerance. Up to now, very few examples of palladium-catalyzed direct arylations of pyrroles by using aryl chlorides have been reported, [26–27]. Daugulis and co-workers recently described the arylation of pyrrole derivatives with a variety of aryl chlorides using 5 mol % of Pd(OAc) 2 associated to 10 mol % of Cy 2 P- o -biphenyl as the catalyst [26].…”

Recent Developments in Pd-Catalyzed Direct Arylations of Heteroarenes with Aryl Halides

“…The use of a particular ferrocenyl triphosphine in the direct arylation of heteroarenes with aryl chlorides that allowed low palladium loadings was described by Doucet and co-workers [21]. The couplings were carried out in DMA at 150 C for 16 h, employing 2.0 equiv.…”

O desafio da ativação das ligações C-H em síntese orgânica

“…Estes haletos orgânicos são os menos reativos de sua série, mas são também os mais baratos, justificando assim o esforço de levar a metodologia até sua aplicação final. Roy e colaboradores, 15 por exemplo, realizaram a arilação de diversos anéis heteroaromáticos na presença de 0,5 mol% de Pd(OAc) 2 e de um ligante do tipo fosfina. As reações foram feitas em N,N-dimetilacetamida (DMAc), com aquecimento a 150 °C por 16 h. Os rendimentos variaram entre 15 e 93%, para os diversos casos (Esquema 3).…”