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The palladium-catalyzed direct arylation of azoles with (hetero)aryl halides is nowadays one of the most versatile and efficient procedures for the selective synthesis of heterobiaryls. Although this procedure is, due to its characteristics, also of great interest in the industrial field, the wide use of a reaction medium such as DMF or DMA, two polar aprotic solvents coded as dangerous according to environmental, health, safety (EHS) parameters, strongly limits its actual use. In contrast, the use of aromatic solvents as the reaction medium for direct arylations, although some of them show good EHS values, is poorly reported, probably due to their low solvent power against reagents and their potential involvement in undesired side reactions. In this paper we report an unprecedented selective C-5 arylation procedure involving anisole as an EHS green reaction solvent. In addition, the beneficial role of benzoic acid as an additive was also highlighted, a role that had never been previously described.

A selective direct arylation of the different Csp2‐H bonds of imidazo[2,1‐b]thiazole with (hetero) aryl halides can be achieved simply by switching from a palladium catalyst system to the use of stoichiometric amounts of copper. The observed selectivity, also rationalized by DFT calculations, can be explained by a change in the mechanistic pathways between electrophilic palladation and base‐promoted C−H metalation. [ABSTRACT FROM AUTHOR]

An effective Pd(II)/Ag(I)‐promoted intramolecular cross‐dehydrogenative coupling (CDC) of substituted 1‐benzylimidazoles under air provides, for the first time, a simple access to several functionalized imidazo[2,1‐a]isoindoles, an interesting class of polycyclic heteroaromatic compounds. The direct involvement of two unactivated carbon–hydrogen bonds, without any directing group, grants an elevated atom economy of the whole process. [ABSTRACT FROM AUTHOR]

An efficient synthesis of symmetrical 1,2-disubstituted 4,5- dialkynylimidazoles by Sonogashira alkynylation of the corresponding 4,5-dibromo derivatives was developed. Moreover, through a careful tuning of the palladium ligand, unsymmetrical 1,2-disusbtituted 4,5-dialkynylimidazoles were also prepared through a regioselective C5 alkynylation of 4,5-dibromoimidazoles, followed by a second alkynylation involving the 4-bromo derivatives so obtained. This interesting class of imidazole-fused enediynes is also able to give thermal Bergman cycloaromatization (BC), as proved by DSC experiments. [ABSTRACT FROM AUTHOR]

Two methods for the one-pot synthesis of 2,5-diarylimidazoles by palladium-catalyzed direct C--H arylation of 1-substituted imidazoles with aryl bromides have been devised. The first method, promoted by copper(I) iodide, is best suited for electron-poor aryl bromides, and also allows the 2,5-diarylation of thiazole and oxazole. The use of xylene instead of DMA is the key for the efficiency of the second method, which gives the best results with electron-rich aryl bromides. [ABSTRACT FROM AUTHOR]

This review with 387 references covers the literature until the end of November 2013 on the synthesis of multiply arylated heteroarenes via highly selective palladium-catalyzed direct C-H arylation reactions of heteroarene derivatives with aryl halides or pseudohalides or aryliodonium salts. Particular attention has been made to describe the synthesis of biologically active compounds and substances that are privileged structural motifs in organic materials. The practicality, versatility, and limitations of the various developed arylation protocols are discussed. 1 Introduction 2 Synthesis of Multiply Arylated Five-Membered Hetero­arenes with One Heteroatom 2.1 Multiply Arylated Furan and Benzo[b]furan Derivatives 2.2 Multiply Arylated Thiophenes and Benzo[b]thiophene Derivatives 2.3 Multiply Arylated Pyrrole and Indole Derivatives 3 Synthesis of Multiply Arylated Five-Membered Hetero­arenes with Two or More Heteroatoms 3.1 Multiply Arylated Pyrazole and 2H-Indazole Derivatives 3.2 Multiply Arylated Isoxazole Derivatives 3.3 Multiply Arylated Oxazole Derivatives 3.4 Multiply Arylated Thiazole and Thiazole N-Oxide Derivatives 3.5 Multiply Arylated Imidazole and Imidazole N-Oxide Derivatives­ 3.6 Multiply Arylated 1,2,3-Triazole and 1,2,3-Triazole N-Oxide Derivatives 3.7 Diarylated 1,2,4-Triazole and 1,3,4-Thiadiazole Derivatives 3.8 Diarylated Sydnone, 2,1,3-Benzothiadiazole, and Tetrazole Derivatives 4 Synthesis of Multiply Arylated Six-Membered Hetero­arenes 4.1 Diarylated Pyridines, Pyridine N-Oxides, and Pyrazine N-Oxides 5 Synthesis of Multiply Arylated Compounds Formed by Five-Membered Heteroarenes Fused to Five-Membered Heteroarenes 5.1 Diarylated Thieno[2,3-b]thiophenes, Imidazo[1,2-b]pyrazoles, and Thiazolo[3,2-b]-1,2,4-triazoles 6 Synthesis of Multiply Arylated Compounds Formed by Six-Membered Heteroarenes Fused to Five-Membered Hetero­arenes 6.1 Diarylated Indolizines, Pyrimido[5,4-b]indolizines, Pyrrolo[2,3-b]pyridines, Pyrazolo[1,5-a]pyrimidines, Imidazo[1,2-a]pyridines, Imidazo[1,5-a]pyridines, Xanthines, Imidazo[1,2-b]tetrazines, Imidazo[1,2-a]pyrazines, and ­Furo[3,2-b]pyridines 7 Conclusions [ABSTRACT FROM AUTHOR]

The palladium/copper cocatalyzed sila-Sonogashira re-action of (hetero)arylethynysilanes with (hetero)aryl bromides in toluene and water at 40 °C under PTC conditions gave the required di(hetero)arylethynes in moderate to high yields. Activated, deactivated and orito-substituted (hetero)aryl bromides are well tolerated. This protocol also allowed the preparation of symmetrical diarylethynes by double arylation of 1,2-bis(trimethylsilyl)ethyne. [ABSTRACT FROM AUTHOR]

Review: < 250 refs. [ABSTRACT FROM AUTHOR]