Mechanism of Direct C–H Arylation of Pyridine via a Transient Activator Strategy: A Combined Computational and Experimental Study

Nucleophilic C4-selective (hetero) arylation of pyridines for facile synthesis of heterobiaryls

The synthesis of heterobiaryl compounds holds significant value in organic chemistry due to their extensive range of applications. Herein, we report a highly efficient strategy for conducting C4-selective (hetero) arylation of pyridines using N-aminopyridinium salts. The reaction proceeds readily at room temperature in the presence of a base, thus eliminating the requirement for catalysts or oxidants. This method allows for the installation of various electron-rich (hetero) aryl groups on pyridines, resulting in the streamlined synthesis of highly valuable C4-(hetero) aryl pyridine derivatives, which are otherwise challenging to acquire via conventional methods. This simple and straightforward method will facilitate access to a range of heterobiaryl compounds thereby promoting their application in various scientific disciplines.

Copper-Catalyzed Oxidative C-H Annulation of Quinolines with Dichloroethane toward Benzoquinoliziniums Using an In Situ Activation Strategy

Described herein is a copper-catalyzed oxidative C-H annulation of quinolines with 1,2-chloroethane (DCE), providing a concise synthetic approach to benzoquinoliziniums. In this protocol, DCE not only serves as a solvent and an in situ activation agent of quinoline C2-H but also works as vinyl equivalents to constitute the six-membered azonia ring. Furthermore, the resultant benzoquinolizinium library exhibits good properties of binding to DNA and low cytotoxicity.

The regioselective annulation of N-methylpyridinium ylides with alkenes enabled by palladium catalysis: access to 3-unsubstituted indolizine derivatives

The first catalytic protocol for the regioselective [3 + 2] annulation of N-methyl pyridinium ylides with alkenes to establish various valuable 3-unsubstituted indolizine derivatives is accomplished via palladium catalysis at the unactivated position.

Strategic Advances in Sequential C-Arylations of Heteroarenes

Sequence-specific C-arylation strategies have important applications in medicinal and material research. These strategies allow C-C bond formations in a regioselective manner to synthesize large molecular libraries for studying structure-activity profiles. The past decade has seen the development of single C-C bond forming reactions using various transition-metal catalysts, cryogenic metalation strategies, and metal-free methods. Sequential arylations of heterocycles allow for the formation of multiaryl derivatives and are a preferred choice over de novo synthetic routes. This perspective sheds light on recent strategic advances to develop various sequential synthetic routes for the multiarylation of heteroarenes. This perspective addresses many challenges in optimizing sequential routes with respect to catalysts, reaction parameters, and various strategies adopted to obtain diversely arylated products.

Regioselective Direct C-H Trifluoromethylation of Pyridine

A highly efficient and regioselective direct C-H trifluoromethylation of pyridine based on an N-methylpyridine quaternary ammonium activation strategy has been developed. A variety of trifluoromethylpyridines can be obtained in good yield and excellent regioselectivity by treating the pyridinium iodide salts with trifluoroacetic acid in the presence of silver carbonate in N,N-dimethylformamide. The protocol features good functional group compatibility, easily available starting materials, and operational simplicity. Controlled experiments showed that the reaction may involve a nucleophilic trifluoromethylation mechanism.

Selective synthesis of pyridyl pyridones and oxydipyridines by transition-metal-free hydroxylation and arylation of 2-fluoropyridine derivatives

An efficient protocol for the construction of various pyridyl pyridone and oxydipyridine derivatives through a hydroxylation and arylation tandem reaction of 2-fluoropyridines is reported. Under simple transition-metal-free conditions, the reaction provided a series of products in good to excellent yields, and their structures were confirmed by crystal diffraction analysis. Furthermore, the controlling effect of 6-position substituents on the highly selective synthesis of pyridone and oxydipyridine was studied.

Palladium-Catalyzed Direct Arylation of Alkylpyridine via Activated N-Methylpyridinium Salts

An efficient Pd-catalyzed arylation of alkylpyridine based on the pyridinium activation strategy has been developed for synthesis of mixed aryl alkylpyridines. It was found that (1) the N-methyl group in the pyridinium salts acted as a transient activator and could be automatically departed after the reaction, (2) CuBr was an indispensable additive for achieving the C₆-selective arylation, (3) the α-branched alkyl chain on the alkylpyridine greatly increased the yield of the product. Deuterium labelling experiment revealed that in the case of the α-branched alkylpyridine, the presence of CuBr completely inhibited the H/D exchange at the benzylic position and thus enabled the selective arylation at the C₆ position. This protocol demonstrates a broad substrate scope, and with respect to both the aryl iodides and the α-branched alkylpyridine, the desired mixed aryl alkylpyridines were obtained in generally good to excellent yields.

The C-H functionalization of organic cations: an interesting and fresh journey

Organic ionic compounds, especially those with organic cations, are commonly applied in ionic liquids (ILs), organocatalysts, (a)NHC ligands, ion recognition, and optoelectronic materials. The direct C-H functionalization of organic cations offers valuable opportunities for the rapid assembly of diverse functionalized cations and for their further exploitation in material science applications. This review summarizes the substantial progress that has been made in the C-H functionalization of organic cations from the 1960s to May 2020, including transition metal-mediated/catalyzed C-H alkylation, arylation, and annulation, and photo-induced C-H functionalization. Substrate scopes, limitations, regio-/chemoselectivity, and reaction mechanisms are discussed. In addition, the applications of some new organic functional materials are briefly exemplified. This review also aims to serve as a reminder that much care should be taken when using organic ionic compounds as solvents, because they can behave as reactants that can break up desired coupling reactions.

Mechanistic insight into the silver-catalyzed cycloaddition synthesis of 1,4-disubstituted-1,2,3-triazoles: the key role of silver

By using DFT calculations, we disclose the reason why silver(i) catalytically promotes the activity and regioselectivity of the cycloaddition of 1,4-disubstituted-1,2,3-triazoles.