Synthesis of Unsymmetrical Biheteroarenes via Dehydrogenative and Decarboxylative Coupling: a Decade Update

Synthesis of Quinoline-Indole Hybrids through Cu(II)-Catalyzed Amination and Annulation between N-Oxides and o-Alkynylanilines

The synthesis of (iso)quinoline-indole hybrids by reacting (iso)quinoline N-oxides with o-alkynylanilines in the presence of a combination of copper(II) catalyst and a bidentate 2,2'-bipyridine ligand is described. The utility of this method was demonstrated through site-selective functionalization of the synthesized products. A plausible reaction pathway for site-selective amination followed by annulative indole formation was elucidated by a series of mechanistic investigations.

C-H Functionalization of Heteroarenes via Electron Donor-Acceptor Complex Photoactivation

C-H Functionalization of heteroarenes stands as a potent instrument in organic synthesis, and with the incorporation of visible light, it emerged as a transformative game-changer. In this domain, electron donor-acceptor (EDA) complex, formed through the pairing of an electron-rich substrate with an electron-accepting molecule, has garnered substantial consideration in recent years due to the related avoidance of the requirement of photocatalyst as well as oxidant. EDA complexes can undergo photoactivation under mild conditions and exhibit high functional group tolerance, making them potentially suitable for the functionalization of biologically relevant heteroarenes. This review article provides an overview of recent advancements in the field of C-H functionalization of heteroarenes via EDA complex photoactivation with literature coverage up to April, 2024.

Visible-light-induced redox-neutral difunctionalization of alkenes and alkynes

The twelve principles of green chemistry illuminate the pathway in the direction of sustainable and eco-friendly synthesis, marking a fundamental shift in synthetic organic chemistry paradigms. In this realm, harnessing the power of visible light for the difunctionalization of various skeletons without employing any external oxidant or reductant, specifically termed as redox-neutral difunctionalization, has attracted tremendous interest from synthetic organic chemists due to its low cost, easy availability and environmentally friendly nature in contrast to traditional metal-catalyzed difunctionalizations. This review presents an overview of recent updates on visible-light-induced redox-neutral difunctionalization reactions with literature coverage up to May 2024.

An Electrochemical Oxo-amination of 2H-Indazoles: Synthesis of Symmetrical and Unsymmetrical Indazolylindazolones

We have established a supporting-electrolyte free electrochemical method for the synthesis of indazolylindazolones through oxygen reduction reaction (eORR) induced 1,3-oxo-amination of 2H-indazoles where 2H-indazole is used as both aminating agent as well as the precursor of indazolone. Moreover, we have merged indazolone and indazole to get unsymmetrical indazolylindazolones through direct electrochemical cross-dehydrogenative coupling (CDC). This exogenous metal-, oxidant- and catalyst-free protocol delivered a number of multi-functionalized products with high tolerance of diverse functional groups.

Aerobic Catalytic Cross-Dehydrogenative Coupling of Furans with Indoles Provides Access to Fluorophores with Large Stokes Shift

Sustainability in chemical processes is a crucial aspect in contemporary chemistry with sustainable catalysis as a vital parameter of the same. There has been a renewed focus on utilizing earth-abundant metal catalysts to expand the repertoire of organic reactions. Furan is a versatile heterocycle of natural origin used for multiple applications. However, it has scarcely been used in cross-dehydrogenative coupling. In this work, we have explored the cross-dehydrogentive coupling of furans with indoles using commonly available, inexpensive FeCl3  ⋅ 6H2 O (<0.25 $/g) as catalyst in the presence of so called 'ultimate oxidant' - oxygen, without the need for any external ligand or additive. The reactions were found to be scalable and to work even under partially aqueous conditions. This makes the reaction highly economical, practical, operationally simple and sustainable. The methodology provides direct access to π-conjugated short oligomers consisting of furan, thiophene and indole. These compounds were found to show interesting fluorescence properties with remarkably large Stokes shift (up to 205 nm). Mechanistic investigations reveal that the reaction proceeds through chemoselective oxidation of indole by the metal catalyst followed by nucleophilic trapping by furan.

Synthesis of fused bis-indazoles/indazoles via a one-pot sequential strategy

We report here an efficient synthesis of fused bis-indazoles/indazoles via a one-pot sequential strategy starting from o-azido aldehydes and amines. This novel method involves the sequential formation of 2H-indazole followed by a Pd-catalyzed intramolecular cross-dehydrogenative coupling reaction. Overall, this one-pot sequential reaction involved the formation of new five bonds, resulting in the formation of three heterocyclic rings.

Transition-metal-catalyzed ortho C-H functionalization of 2-arylquinoxalines

Recently, direct C-H bond activation and functionalization has become a prodigious and hot topic among synthetic organic chemists due to its step-economic nature and substantial synthetic versatility. On the other hand, quinoxaline, a fused bicycle of benzene and pyrazine, has omnipresent applications in medicinal-, industrial- and materials chemistry. The presence of the N-1 atom in 2-arylquinoxaline enables chelation formation with a metal catalyst leading to the formation of ortho-substituted products. In this review, all articles related to the ortho C-H bond functionalization of 2-arylquinoxalines published up to May 2022 are highlighted.