Transition metal-catalyzed synthesis of N, O−Heterocycles via C–H functionalization

Direct Access to Benzofurans and Indoles from Ethylene with 2-Methylphenols/Anilines through Iridium-Catalyzed Dehydrogenative Annulation

The direct conversion of o-cresol and 2-methylphenols to 2-methylbenzofurans was achieved by an Ir/DTBE-DPPE-catalyzed reaction under an atmosphere of ethylene (1 atm). The reaction involves the initial selective formation of a C-C bond at the benzylic C(sp3)-H bond of 2-methylphenols with ethylene without the accommodation of the C(sp2)-H bonds at the 6-positions, which is followed by subsequent C-O bond-forming annulation along with associated dehydrogenation and double bond migration steps. The reaction conditions also allowed for the direct conversion of o-toluidine and 2-methylanilines into 2-methylindoles.

C(sp3)-H Bond Functionalization of 8-Methylquinolines

Quinolines have emerged as essential components in various medicinal agents, playing a key role in treating various ailments. Numerous drugs with a quinoline core have been recognized for their antimalarial, antibacterial, and anticancer activities and have been successfully commercialized, including chloroquine, ciprofloxacin, topotecan, etc. Over the past two decades, a tremendous expansion in the C-H bond functionalization of quinoline scaffolds to widen this chemical space for drug discovery have been witnessed. This review article summarizes the efforts toward C(sp3)-H functionalization of 8-methylquinolines for C(sp3)-C/X bond formation under metal and metal-free strategies. Each section briefly overviews the C(sp3)-H functionalization of 8-methylquinoline, highlighting the metal and metal-free approaches.

Iodonium ylides: an emerging and alternative carbene precursor for C-H functionalizations

The metal-catalyzed successive activation and functionalization of arene/heteroarene is one of the most fundamental transformations in organic synthesis and leads to privileged scaffolds in natural products, pharmaceuticals, agrochemicals, and fine chemicals. Particularly, transition-metal-catalyzed C-H functionalization of arenes with carbene precursors via metal carbene migratory insertion has been well studied. As a result, diverse carbene precursors have been evaluated, such as diazo compounds, sulfoxonium ylides, triazoles, etc. In addition, there have been significant developments with the use of iodonium ylides as carbene precursors in recent years, and these reactions proceed with high efficiencies and selectivities. This review provides a comprehensive overview of iodonium ylides in C-H functionalizations, including the scope, limitations, and their potential synthetic applications.

Recent Strategies in Nickel-Catalyzed C–H Bond Functionalization for Nitrogen-Containing Heterocycles

N-heterocycles are ubiquitous in natural products, pharmaceuticals, organic materials, and numerous functional molecules. Among the current synthetic approaches, transition metal-catalyzed C–H functionalization has gained considerable attention in recent years due to its advantages of simplicity, high atomic economy, and the ready availability of starting materials. In the field of N-heterocycle synthesis via C–H functionalization, nickel has been recognized as one of the most important catalysts. In this review, we will introduce nickel-catalyzed intramolecular and intermolecular pathways for N-heterocycle synthesis from 2008 to 2021.

C-H bond functionalization by high-valent cobalt catalysis: current progress, challenges and future perspectives

Over the last decade, high-valent cobalt catalysis has earned a place in the spotlight as a valuable tool for C-H activation and functionalization. Since the discovery of its unique reactivity, more and more attention has been directed towards the utilization of cobalt as an alternative to noble metal catalysts. In particular, Cp*Co(III) complexes, as well as simple Co(II) and Co(III) salts in combination with bidentate chelation assistance, have been extensively used for the development of novel transformations. In this review, we have demonstrated the existing trends in the C-H functionalization methodology using high-valent cobalt catalysis and highlighted the main challenges to overcome, as well as perspective directions, which need to be further developed in the future.

Recent advances in directed sp2 C-H functionalization towards the synthesis of N-heterocycles and O-heterocycles

Heterocyclic compounds are widely present in the core structures of several natural products, pharmaceuticals and agrochemicals, and thus great efforts have been devoted to their synthesis in a mild and simpler way. In the past decade, remarkable progress has been made in the field of heterocycle synthesis by employing C-H functionalization as an emerging synthetic strategy. As a complement to previous protocols, transition metal catalyzed C-H functionalization of arenes using various directing groups has recently emerged as a powerful tool to create different classes of heterocycles. This review is mainly focussed on the recent key progress made in the field of the synthesis of N,O-heterocycles from olefins and allenes by using nitrogen based and oxidizing directing groups.

Three-Component Couplings among Heteroarenes, Difluorocyclopropenes, and Water via C-H Activation

Three-component couplings have been realized for efficiently constructing various nitrogen-containing skeletons via C-H activation, where difluorocyclopropenes have been first identified as coupling partners. Many substrates including sp² and sp³ C-H substrates were well tolerated, furnishing the corresponding products in good yields. Furthermore, a catalyst-dependent reaction was also developed, enabling divergent construction of two different frameworks. The application value of these reactions was demonstrated in gram-scale experiments with as little as 1 mol % catalyst.