Palladium-catalysed direct 3- or 4-arylation of 2,5-disubstituted pyrrole derivatives: an economically and environmentally attractive procedure

Palladium-Catalysed Intermolecular Direct C–H Bond Arylation of Heteroarenes with Reagents Alternative to Aryl Halides: Current State of the Art

Abstract:

Abstract: This unprecedented review with 322 references provides a critical up-to-date picture of the Pd-catalysed intermolecular direct C–H bond arylation of heteroarenes with arylating reagents alternative to aryl halides that include aryl sulfonates (aryl triflates, tosylates, mesylates, and imidazole-1-sulfonates), diaryliodonium salts, [(diacetoxy)iodo]arenes, arenediazonium salts, 1-aryltriazenes, arylhydrazines and N’-arylhydrazides, arenesulfonyl chlorides, sodium arenesulfinates, arenesulfinic acids, and arenesulfonohydrazides. Particular attention has been paid to summarise the preparation of the various arylating reagents and to highlight the practicality, versatility, and limitations of the various developed arylation protocols, also comparing their results with those achieved in analogous Pd-catalysed arylation reactions involving the use of aryl halides as electrophiles. Mechanistic proposals have also been briefly summarised and discussed. However, data concerning Pd-catalysed direct C–H bond arylations involving the C–H bonds of aryl substituents of the examined heteroarene derivatives have not been taken into account.

Transition-Metal Catalyst Free Oxidative Radical Arylation of N-Methylpyrrole

This study represents an expansion of the application of catalysis in air through conventional coupling and free radical processes. A reactive free aryl radical intermediate was generated via the oxidation of an activated Ar-NH-NH₂ bond by air as a simple and readily available oxidant. For this purpose, the usability of phenylhydrazine and phenylhydrazine hydrochloride salt reagents for the direct arylation of pyrrole with aryl radicals was investigated. The facile coupling of N-methylpyrrole with aryl radicals was easily applied for the convenient direct synthesis of C-2 arylated pyrroles without a transition-metal catalyst.

Regioselectivity in palladium-catalysed direct arylation of 5-membered ring heteroaromatics

In the past few years, several results using modified catalysts and new reaction conditions have been reported permitting better control of the regioselectivity of Pd-catalysed arylations.

Synthesis of 3-bromosubstituted pyrroles via palladium-catalyzed intermolecular oxidative cyclization of bromoalkynes with N-allylamines

A novel Pd-catalyzed intermolecular oxidative cyclization of bromoalkynes with N-allylamines to construct 3-bromo-pyrrole derivatives has been developed.

Palladium-catalyzed direct ortho-sulfonylation of azobenzenes with arylsulfonyl chlorides via C–H activation

ortho-Sulfonylated azobenzenes were obtained in moderate to high yields by direct ortho-sulfonylation of azobenzenes' C–H bond with arylsulfonyl chlorides.

Tetraaryl-, pentaaryl-, and hexaaryl-1,4-dihydropyrrolo[3,2-b]pyrroles: synthesis and optical properties

Efficient conditions for the synthesis of tetra-, penta-, and hexasubstituted derivatives of 1,4-dihydropyrrolo[3,2-b]pyrrole were developed. The tetraaryl derivatives were obtained in a novel one-pot reaction among aromatic aldehydes, aromatic amines, and butane-2,3-dione. After a thorough examination of various reaction parameters (solvent, acid, temperature) p-toluenesulfonic acid was identified as the crucial catalyst. As a result, 1,4-dihydropyrrolo[3,2-b]pyrroles were obtained in the highest yields reported to date. The scope and limitation studies showed that this new method was particularly efficient for sterically hindered aldehydes (yields 45-49%). Pentaaryl- and hexaaryl-1,4-dihydropyrrolo[3,2-b]pyrroles were prepared from tetraaryl-1,4-dihydropyrrolo[3,2-b]pyrroles via direct arylation by employing both electron-poor and electron-rich aromatic and heteroaromatic haloarenes. Strategic placement of electron-withdrawing substituents at the 2-, 3-, 5-, and 6-positions produced an acceptor-donor-acceptor type fluorophore. The resulting multiply substituted heteropentalenes displayed intriguing optical properties. The relationship between the structure and photophysical properties for all compounds were directly compared and thoroughly elucidated. All synthesized products displayed strong blue fluorescence and exhibited moderate to large Stokes shifts (3000-7300 cm(-1)) as well as high quantum yields of fluorescence up to 88%. Two-photon absorption cross-section values measured in the near-IR region were surprisingly high (hundreds of GM), given the limited conjugation in these propeller-shaped dyes.

Reactivity of bromofluorenes in palladium-catalysed direct arylation of heteroaromatics

The palladium-catalysed direct arylation using 2-bromofluorene or 2,7-dibromofluorene and heteroaromatics as the coupling partners proceeded using only 0.1–0.5 mol% catalyst.

Solvent-free palladium-catalyzed direct arylation of heteroaromatics with aryl bromides

Solvent is one of the major sources of waste in the course of catalyzed direct arylations. Some palladium-catalyzed direct arylations of heteroaromatics can be advantageously performed without any solvent. In the presence of palladium catalysts (1 mol %) and potassium acetate as the base, the direct 5-arylation of some thiazoles, thiophenes, furans, or pyrroles with aryl bromides as coupling partners proceeds highly regioselectively and in moderate to high yields. However, the use of these solvent-free conditions is limited to electron-deficient aryl bromides.

Palladium-catalysed direct arylation of a tris-cyclometallated Ir(III) complex bearing 2,2'-thienylpyridine ligands: a powerful tool for the tuning of luminescence properties

Palladium-catalysed direct 5-arylation of metallated thiophenes of fac-Ir(N^C(3')-thpy)(3) with aryl bromides via C-H bond functionalisation allows the synthesis of a variety of new Ir complexes in only one step (thpyH = 2,2'-thienylpyridine). The method offers simple modification of the nature of the ligand and hence of the photophysical properties of such complexes.

Palladium-catalyzed direct arylation of thiophenes bearing SO2R substituents

The palladium-catalyzed direct arylation of SO(2)R-substituted thiophene derivatives was found to proceed regioselectively at C5 and in high yields using a variety of aryl bromides and as low as 0.5-0.1 mol % of phosphine-free Pd(OAc)(2) as the catalyst. For these reactions, sulfonyls, sulfonamides, or even a sulfonic ester as the thiophene substituents were successfully employed.

Cyclopentyl methyl ether: an alternative solvent for palladium-catalyzed direct arylation of heteroaromatics

Some ethers, such as cyclopentyl methyl ether and di-n-butyl ether, which can be considered as "greener" solvents than N,N-dimethylacetamide (DMAc) or DMF, can be advantageously employed for the palladium-catalyzed direct arylation of heteroaromatics. In the presence of such ethers and only 0.5-1 mol % of palladium catalysts at 125-150 °C, the direct 5-arylation of thiazoles, thiophenes, or furans by using aryl bromides as coupling partners proceeds in moderate to high yields.

Palladium catalyzed direct 3-arylation of benzofurans using low catalyst loadings

The palladium-catalyzed direct 3-arylation of benzofurans provides a cost-effective and environmentally attractive route for the preparation of 3-arylbenzofuran derivatives. The reactions were carried out using a wide variety of electronically and sterically diverse aryl or heteroaryl bromides with low catalyst loadings. In the presence of only 0.1-0.5 mol % catalyst, products in moderate to good yields were obtained. The aryl bromide reactants were able to tolerate a wide range of functionalities, such as acetyl, propionyl, formyl, ester, nitrile, trifluoromethyl, or fluoro groups. Higher yields were obtained using electron-deficient aryl bromides as reactants compared to using electron-rich aryl bromides. Functionalized benzofuran derivatives, bearing formyl or hydroxymethyl on C2, were also successfully employed.