Regioselective oxidative Pd-catalysed coupling of alkylboronic acids with pyridin-2-yl-substituted heterocycles

Arenes and Heteroarenes C-H Functionalization Under Enabling Conditions: Electrochemistry, Photoelectrochemistry & Flow Technology

C-H bond functionalization generates molecular complexity in single-step transformation. However, the activation of C-H bonds requires expensive metals or stoichiometric amounts of oxidizing/reducing species. In many cases, they often require pre-functionalization of starting molecules. Such pre-activating measures cause waste generation and their separation from the final product is also troublesome. In such a scenario, reactions activating elements generating from renewable energy resources such as electricity and light would be more efficient, green, and cost-effective. Further, incorporation of growing flow technology in chemical transformation processes will accelerate the safer accesses of valuable products. Arenes & heteroarenes are ubiquitous in pharmaceuticals, natural products, medicinal compounds, and other biologically important molecules. Herein, we discussed enabling tools and technologies used for the recent C-H bonds functionalization of arenes and heteroarenes.

Peroxide-mediated site-specific C–H methylation of imidazo[1,2-a]pyridines and quinoxalin-2(1H)-ones under metal-free conditions

An effective approach to realize the direct methylation of imidazo[1,2-a]pyridines and quinoxalin-2(1H)-ones with peroxides under metal-free conditions is described.

Ir(iii)-catalyzed ortho C-H alkylations of (hetero)aromatic aldehydes using alkyl boron reagents

Transition-metal-catalyzed C-H alkylation reactions directed by aldehydes or ketones have been largely restricted to electronically activated alkenes. Herein, we report a general protocol for the Ir(iii)-catalyzed ortho C-H alkylations of (hetero)aromatic aldehydes using alkyl boron reagents as the coupling partner. Featuring aniline as an inexpensive catalytic ligand, the method was compatible with a wide variety of benzaldehydes, heterocyclic aldehydes, potassium alkyltrifluoroborates as well as a few α,β-unsaturated aldehydes. An X-ray crystal structure of a benzaldehyde ortho C-H iridation intermediate was also successfully obtained.

Palladium-Catalyzed Decarboxylative ortho-Acylation of Anilines with Carbamate as a Removable Directing Group

An efficient palladium-catalyzed decarboxylative ortho-acylation of anilines with α-oxocarboxylic acids has been realized by using carbamate as a directing group (DG). The reaction proceeds smoothly with high regioselectivity to afford diverse acylation products of aniline derivatives in moderate to good yields under mild conditions. This transformation exhibits broad substrate scope and highly functional group tolerance. In addition, the employed DG can be easily removed to give the corresponding 2-amino aromatic ketones. Importantly, several transformations of the synthesized ortho-acylated anilines into several synthetically valuable products have been demonstrated for their utilities.

Palladium-catalyzed C-H activation/C-C cross-coupling reactions via electrochemistry

Palladium-catalyzed C-H activation/C-C cross-coupling reactions typically require stoichiometric chemical oxidants and exogenous ligands. However, there are significant disadvantages associated with the use of traditional stoichiometric oxidants. To overcome these issues, we have developed an electrochemical strategy to achieve methylation and acylation.

Palladium-catalyzed C-H alkylation of 2-phenylpyridines with alkyl iodides

Palladium-catalyzed C-H alkylation reaction of 2-phenylpyridines with alkyl iodides has been successfully developed. The palladacycles obtained from 2-phenylpyridines should act as the key intermediates in the alkylation reaction.

Regioselective synthesis of C3 alkylated and arylated benzothiophenes

Benzothiophenes are heterocyclic constituents of important molecules relevant to society, including those with the potential to meet modern medical challenges. The construction of molecules would be vastly more efficient if carbon-hydrogen bonds, found in all organic molecules, can be directly converted into carbon-carbon bonds. In the case of elaborating benzothiophenes, functionalization of carbon-hydrogen bonds at carbon-number 3 (C3) is markedly more demanding than at C2 due to issues of regioselectivity (C3 versus C2), and the requirement of high temperatures, precious metals and the installation of superfluous directing groups. Herein, we demonstrate that synthetically unexplored but readily accessible benzothiophene S-oxides serve as novel precursors for C3-functionalized benzothiophenes. Employing an interrupted Pummerer reaction to capture and then deliver phenol and silane coupling partners, we have discovered a directing group-free method that delivers C3-arylated and -alkylated benzothiophenes with complete regioselectivity, under metal-free and mild conditions.

DDQ-promoted direct C5-alkylation of oxazoles with alkylboronic acids via palladium-catalysed C–H bond activation

The first protocol for the direct C5-alkylation of oxazoles through transition-metal-catalysed C(5)–H bond activation.

Pd/Ni catalyzed selective N–H/C–H methylation of amides by using peroxides as the methylating reagents via a radical process

A palladium/nickel catalyzed selective N–H/C–H methylation of amides by using peroxides is described, providing N-methylation products and C-methylation products in moderate to good yields.

Rhodium(III)-Catalyzed Site-Selective C-H Alkylation and Arylation of Pyridones Using Organoboron Reagents

In this study we developed a method for the pyridine-directed, rhodium-catalyzed, site-selective C-H alkylation and arylation of pyridones using commercially available trifluoroborate reagents. This simple and versatile transformation proceeded smoothly under relatively mild conditions with perfect site selectivity. The coupling groups in the boron reagents can be extended to primary alkyl, benzyl, and cycloalkyl. Moreover, direct C-H arylation products could also be obtained under similar conditions.

Palladium(II)-Catalyzed ortho-Arylation of Aromatic Alcohols with a Readily Attachable and Cleavable Molecular Scaffold

A palladium(II)-catalyzed C-H arylation process of alcohols has been developed. The strategy utilizes a novel quinoline-based hemiacetal scaffold that can direct the selective C-H bond functionalization. This reaction provides a useful method to construct biaryl compounds of benzyl alcohols in good to excellent yields. The new molecular scaffold can be readily attached, removed, and recovered.

Amino acid-promoted C–H alkylation with alkylboronic acids using a removable directing group

Palladium-catalyzed C–H alkylation reaction with alkylboronic acids has been developed using a removable pyridyldiisopropylsilyl directing group.

Direct access to 2-(hetero)arylated pyridines from 6-substituted 2-bromopyridines via phosphine-free palladium-catalyzed C–H bond arylations: the importance of the C6 substituent

A Pd(OAc)₂ catalyst promotes the arylation of heteroarenes with 6-substituted 2-bromopyridines, via C–H bond activation, providing straightforward access to 2-(hetero)arylpyridines.

Palladium-catalysed direct C-2 methylation of indoles

A direct C-2 methylation reaction of indoles bearing an N-2-pyrimidyl directing group has been developed with a palladium catalyst.

Metal-free methylation of a pyridine N-oxide C–H bond by using peroxides

Metal-free methylation of a pyridine N-oxide C–H bond was developed using peroxide as a methyl reagent under neat conditions. Pyridine N-oxide derivatives with various groups (e.g., Cl, NO₂, and OCH₃) were all suitable substrates.