Ru(II)-Catalyzed Switchable C-H Alkylation and Spirocyclization of 2-Arylquinoxalines with Maleimides via ortho-C-H Activation

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.

Base-promoted cascade vinylogous Michael/Michael addition of alkylidene succinimides for the construction of penta-substituted cyclopentanes

Herein, an efficient, base catalyzed cascade vinylogous Michael/Michael cycloaddition reaction of α-alkylidene succinimides and oxindole-derived pyrazolones has been successfully developed. A variety of highly functionalized cyclopentanes fused with spirooxindoles were obtained in good yields, with excellent diastereoselectivities and exclusive vinylogous site-selectivities. This strategy represents the first example of α-alkylidene succinimides serving as nucleophilic reagents to trigger a vinylogous cascade reaction.

Ru(II)-Catalyzed C-H Alkylation of N-Benzyltriflamide with Maleimides: Synthesis of o-Succinimide Substituted Benzaldehydes

A ruthenium-catalyzed N-benzyltriflamide assisted C-H alkylation with maleimide followed by hydrolysis of in situ generated imine has been developed for the first time. This synthetic method results in the efficient synthesis of o-succinimide derivatives of benzaldehydes. This reaction involves less expensive and mild reaction conditions and shows excellent site selectivity and good functional group compatibility.

A rhodium-catalyzed cascade C-H activation/annulation strategy for the expeditious assembly of pyrrolidinedione-fused 1,2-benzothiazines

A cascade annulation strategy triggered by rhodium(III)-catalyzed C-H activation has been reported for the expeditious assembly of pyrrolidinedione-fused 1,2-benzothiazines from free NH-sulfoximines with maleimides under mild conditions. Without the need for inert atmosphere protection, a broad range of sulfoximines with maleimides were well tolerated, producing diverse fused-thiazine derivatives in moderate to good yields. Additionally, the late-stage transformation of the target product demonstrated the potential synthetic value of this protocol.

I2-Mediated Site-Selective C-H Functionalization: Access to p-Amino-Substituted Unsymmetrical Benzils and Quinoxalines from Sulfoxonium Ylides

An I2-mediated approach for selective C-H functionalization of unprotected aniline derivatives for synthesizing benzils and quinoxaline derivatives from sulfoxonium ylides has been described. Aniline derivatives and sulfoxonium ylides ornamented with different functional groups showed good compatibility. They afforded the corresponding products with moderate to high yields via a mild and simple procedure. Finally, we validated the practicality of this method by scaling up the reaction and further conversion of the synthesized derivatives into other valuable molecules.

Recent advances in spirocyclization of maleimides via transition-metal catalyzed C-H activation

In recent years, the maleimide scaffold has received a great deal of attention in C-H activation. Several types of products can be constructed using maleimides as a coupling partner. Alkylation, alkenylation, annulation, dehydrogenative annulation and spirocyclization are various reactions shown by maleimides in C-H activation. Thus, the maleimide scaffold has been extensively studied in the last few years in C-H activation owing to its unique reactivity. Among the diverse class of reactions of maleimides, spirocyclization is a less explored reaction. The spirocycles, in particular the spirosuccinimides are interesting candidates in drug discovery and materials chemistry. Therefore the method of spirocyclization of maleimides via C-H activation becomes an important strategy for the synthesis of a diverse array of spirosuccinimides. This review summarizes the reports available in this field from 2015-2023 and also highlights the scopes and prospects of this method.

C-H activation-initiated spiroannulation reactions and their applications in the synthesis of spirocyclic compounds

Spirocyclic skeletons are prevalent in natural products, pharmaceuticals and organic functional materials. Meanwhile, transition-metal-catalyzed C-H activation reactions have demonstrated unparalleled advantages such as high efficiency, excellent atom-economy, good chemoselectivity and regioselectivity for the formation of target organic molecules. In recent years, C-H activation reactions have been creatively utilized in the synthesis of spirocyclic compounds. This review summarizes the most recent progress made in C-H activation-initiated spiroannulation reactions and their applications in the construction of structurally diverse and biologically valuable spirocyclic scaffolds by using alkynes, diazo compounds, maleimides, alkenes, quinones and cyclopropenones as the coupling partners.

Iodine(III)-promoted oxidative carbotrifluoromethylation of maleimides with imidazopyridines and Langlois' reagent

A metal-free protocol for oxidative carbotrifluoromethylation of maleimides with imidazopyridines and Langlois' reagent has been developed using (diacetoxyiodo)benzene (PIDA) as an oxidant. This three-component strategy enables one-step construction of 3,4-disubstituted maleimides in good yields with high functional group tolerance. Both experimental and theoretical studies support the proposed radical reaction mechanism.

Synthesis of Spiroisoindolinones via Ru(II)-Catalyzed Spiroannulation of N-Acyl Ketimines with Aryl Isocyanates/Isothiocyanates through Aromatic C-H Bond Activation

Herein, we disclose the first report on Ru(II)-catalyzed amidation/thioamidation of 3-hydroxy-3-arylisoindolinones with isocyanates/isothiocyanates, respectively. The reaction furnishes spiroisoindolinones via sequential C-H functionalization of ortho C-H bond followed by intramolecular cyclization in moderate to high yields (up to 94%). Moreover, the developed strategy is highly atom-economical and site-selective and shows diverse substrate generality. Also, synthesized spiroisoindolinones undergo several chemical transformations.

Spirocyclization and Michael addition of 3-benzylidene succinimides: route to spirocyclopentapyrrolidine-tetraones and benzylidene N-arylpyrrolidine-diones

Reactions of 3-benzylidene succinimides with 2-substituted 2-hydroxy-indane-1,3-diones and unsaturated pyrazolones are carried out under basic conditions to afford spirocyclized derivatives and Michael adducts, respectively, with high regio- and stereo-selectivities. The most notable aspect of the reaction is the ability of highly reactive benzylidene succinimide to act as both an electrophile and a nucleophile causing spirocyclization. The reaction proceeded under mild and metal-free conditions and products were isolated in good to high yields. The current strategy utilizes simple and easily accessible precursors, and provides functionally rich products of medicinal interest with two to four contiguous stereogenic centres and complete regioselectivity with excellent diastereoselectivity.

Palladium-Catalyzed Aerobic Oxidative Spirocyclization of Alkyl Amides with Maleimides via β-C(sp3)-H Activation

An efficient method for the synthesis of bicyclic spirodiamine molecules via β-C(sp3)-H bond activation of aliphatic amides, followed by cyclization with maleimides, has been developed. The reaction proceeds through an amide-directed β-C(sp3)-H bond activation of alkyl amides and subsequent cyclization with maleimides. The methodology is highly compatible with a wide variety of maleimides. Amides derived from biologically active aliphatic and fatty acids were also found to be highly compatible with the protocol. A palladacycle was synthesized and found to be the active intermediate in this reaction. A plausible reaction mechanism was also proposed to account for this spirocyclization.

Redox-Neutral Ruthenium(II)-Catalyzed Enol-Directed Arene C-H Alkylation with Maleimides

An enol-assisted regioselective arene C-H alkylation with maleimides is developed under redox-neutral ruthenium(II) catalysis, offering a wide variety of valuable 3-aryl succinimides including amino acid embedded frameworks in good to excellent yields. The products were also aromatized to produce synthetically useful resorcinol-based biaryls. Mechanistic studies support an organometallic pathway with a reversible C-H metalation step for this reaction.

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.

Transition-metal-catalyzed C-H bond alkylation using olefins: recent advances and mechanistic aspects

Transition metal catalysis has contributed immensely to C-C bond formation reactions over the last few decades, and alkylation is no exception. The superiority of such methodologies over traditional alkylation is evident from minimal reaction steps, shorter reaction times, and atom economy while also allowing control over regio- and stereo-selectivity. In particular, hydrocarbonation of alkenes has grabbed increased attention due its fundamental ability to effectively and selectively synthesise a wide range of industrially and pharmaceutically relevant moieties. This review attempts to provide a scientific viewpoint and a systematic analysis of the recent developments in transition-metal-catalyzed alkylation of various C-H bonds using simple and activated olefins. The key features and mechanistic studies involved in these transformations are described briefly.

Ortho C-H Functionalizations of 2-Aryl-2H-Indazoles

C-H Functionalization is ubiquitously considered as a powerful, efficient and handy tool for installing various functional groups in complex organic heterocycles in an easier and step-economic way. Similarly, indazole is endowed as a potent heterocycle and is eminent for its profound impact in biological, medicinal and industrial chemistry. In this scenario, C-H functionalization at the selective ortho position of 2-arylindazole in assistance of a metal catalyst is also becoming an appealing approach in synthetic organic chemistry. This review addressed the recent findings and developments on ortho C-H functionalization of 2-aryl-2H-indazazoles with literature coverage extending from 2018 to May 2022.

Pd(II)-Catalyzed Synthesis of Furo[2,3-b]pyridines from β-Ketodinitriles and Alkynes via Cyclization and N-H/C Annulation

A Pd(II)-catalyzed synthesis of furopyridines has been developed from β-ketodinitriles and alkynes via an unusual N-H/C annulation. The participation of both the nitrile groups and the concurrent construction of furan and pyridine rings through the formation of C-C, C═C, C-O, C-N, and C═N bonds are the important features. The synthetic applicability is further demonstrated through a series of postsynthetic alterations.

Copper(I)-Catalyzed Direct Oxidative Annulation of 1,3-Dicarbonyl Compounds with Maleimides: Access to Polysubstituted Dihydrofuran Derivatives

An efficient annulation method for the synthesis of polysubstituted dihydrofurans from 1,3-dicarbonyl compounds and maleimides is described. The reactions can afford furo[2,3-c]pyrrole derivatives with satisfactory yields. The developed strategy realizes the direct oxidative double C(sp³)-H functionalization in the presence of copper(I) salts and 2-(tert-butylperoxy)-2-methylpropane. Meanwhile, this protocol features a mild reaction condition and simple catalytic system. A reaction mechanism involving a single electron oxidation is also proposed.

Rh(III)-Catalyzed Cascade Annulation to Produce N-acetyl Chain of Spiropyrroloisoquinoline Derivatives

A new rhodium(III)-catalyzed three-component multistep cascade spirocyclization approach was developed to synthesize nolvel N-acetyl chain of spiropyrroloisoquinoline derivatives using oxadiazoles as the directing group. This one-pot reaction also isolates aryloxadiazole...

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

The design and development of robust and efficient methods for installing one heterocycle with another is endowed as a ubiquitous and powerful synthetic strategy to access complex organic biheterocycles in recent days due to their pervasive applications in medicinal as well as material chemistry. This perspective presents an overview on the recent findings and developments for the synthesis of unsymmetrical biheteroarenes via dehydrogenative and decarboxylative couplings with literature coverage mainly extending from 2011 to 2021. For simplification of the readers, the article has been subcategorized based on the catalysts used in the reactions.

Synthesis of spirosuccinimides via annulative cyclization between N-aryl indazolols and maleimides under rhodium(III) catalysis

The rhodium(III)-catalyzed spiroannulation reaction between N-aryl indazol-3-ols and maleimides is described herein. The developed method is showcased by the construction of spirosuccinimides using bioactive molecule-linked and chemical probe-linked maleimides. Combined mechanistic investigations including the determination of an isolable rhodacycle complex aided the elucidation of a plausible reaction mechanism.

Switchable regioselective hydroalkylation of 2-arylindoles with maleimides

A condition-based switchable regioselective hydroalkylation of 2-arylindoles with maleimides has been developed. The reaction in the presence of a Ru(ii)-catalyst resulted in hydroalkylation at the ortho-position of the C2-aryl ring via C-H activation whereas the reaction in the absence of the catalyst in TFE resulted in C3-hydroalkylation. Various functional groups both on the indole ring and on the 2-phenyl ring were tolerated and a wide range of hydroalkylated products were obtained in moderate to high (37-88%) yields.

Ruthenium-Catalyzed Oxidative Cross-Coupling Reaction of Activated Olefins with Vinyl Boronates for the Synthesis of (E,E)-1,3-Dienes

An oxidative cross-coupling reaction between activated olefins and vinyl boronate derivatives has been developed for the highly stereoselective construction of synthetically useful (E,E)-1,3-dienes. The highlight of this reaction is that exclusive stereoselectivity (only E,E-isomer) was achieved from a base-free, ligand-free, and mild catalytic condition with a less expensive [RuCl₂(p-cymene)]₂ catalyst.