A Simple and Versatile Amide Directing Group for C-H Functionalizations

Palladium Catalyzed C3-(sp2)-H Alkenylation of Pyrroles via a Benzothiazole Directing Group: A Direct Access to Organic Thermally Activated Delayed Fluorescence Materials

A Pd (II)-catalyzed direct C3-(sp2)-H alkenylation of heteroarenes using benzothiazole as a directing group was successfully achieved. A wide range of 2-N-alkylpyrroles undergo an oxidative coupling with a variety of acrylates to furnish highly regio- and chemoselective E-alkenylation products at the C3 position. An important intermediate complex has been isolated and characterized so as to have an insight into the mechanism. This convenient protocol proved to be practical to access thermally activated delayed fluorescence materials (TADF). These molecules proved to be blue-emitting TADF materials (∼ms lifetime). A detailed and systematic investigation has been carried out to study the photophysical properties, and this has been further validated by the time-dependent density functional theory calculations.

Catalyst-controlled directing group translocation in the site selective C-H functionalization of 3-carboxamide indoles and metallocarbenes

Complementary methods toward the selective functionalization of indole and oxindole frameworks employing an alternative strategy in heteroaryl C-H functionalizations are presented herein. This work focuses on a catalyst-controlled, site selective C-H activation/functionalization of 3-acyl indoles, wherein an amide serves as a robust and versatile directing group capable of undergoing concomitant 1,2-acyl translocation/C-H functionalization in the presence of a RhI/AgI co-catalysts to provide the cross-coupled adducts in high yields. In contrast, the use of IrIII/AgI catalysts subverted the 1,2-acyl migration to afford the corresponding C2-functionalized products in good to excellent yields. A notable feature of the catalyst systems was the exceptional level of site selectivity observed in which the corresponding C-H functionalized indoles were obtained exclusively. Mechanistic experiments indicate a concerted 1,2-acyl migration step and indole metallation occurring through an electrophilic aromatic substitution process.

Oxidative Coupling of Primary Benzamide with Alkenes via o-CH Activation Mediated by Cu(II)/Ru(II)

A selective ortho-olefination of primary amides yields valuable motifs for pharmaceuticals. Primary amide-directed ortho-olefination of benzamide using unactivated alkenes was successfully conducted with a ruthenium(II) catalyst. The established protocol demonstrates efficacy across various benzamides, achieving moderate to good yields with high functional group tolerance. Furthermore, the transformation of ortho-olefinated amides into five-membered lactams highlights their synthetic applicability in the pharmaceutical sector.

Rhodium-Catalyzed C-H Arylation of Indoles with Arylsilanes at Room Temperature

The Cp*Rh-catalyzed C-H arylation of indoles with arylsilanes is developed. This C-H activation transformation allows for the Rh-catalyzed indole C2 arylation to overcome the limitations of requiring strong directing group assistance and high-temperature conditions, achieving a room-temperature transformation driven by a weak directing group. Cp*Rh/MeOH catalytic media are considered a key factor enabling this transformation to occur under mild conditions, and experimental studies and theoretical calculations were performed to rationalize the reaction mechanisms and the influence of methanol as a solvent in promoting the reaction.

Direct C-H difluoroallylation of α,β-unsaturated amides and aryl amides by rhodium catalysis

Herein, we report the first example of Rh-catalyzed C(sp2)-H difluoroallylation of α,β-unsaturated amides with 3-bromo-3,3-difluoropropene under mild conditions. The features of this protocol for difluoroallylation are unprecedented vinylic substrates, ethanol as a green solvent, good functional group and air compatibility, and scale-up synthesis with low catalyst loading.

Synthesis of γ-Spirolactams via Rh(III)-Catalyzed C-H Activation/Directing Group Migration/Dearomatization/Spiroannulation of Indoles with 1,3-Enynes

1,3-Enynes are valuable coupling partners in transition-metal-catalyzed C-H functionalizations. Certainly, aliphatic-substituted 1,3-enynes have been thoroughly investigated in C-H functionalizations, whereas aromatic-substituted 1,3-enynes remain underexplored. Herein, we report the realization of this goal, where we achieve an atom-economical protocol for the synthesis of γ-spirolactams via Rh(III)-catalyzed C-H functionalization of N-carbamoylindoles with 1,3-enynes. The reaction proceeds through a unique cascade strategy, such as C-H activation/directing group (DG) migration/indole dearomatization/spiroannulation, to access novel and synthetically challenging spiro[indoline-2,2'-pyrrolidin]-5'-one scaffolds. Moreover, the isolation of intermediate and mechanistic and ESI-HRMS studies further provide valuable insights into the proposed catalytic cycle.

Rh(III)-Catalyzed Chemoselective [4 + 2] Annulations for the Synthesis of [1,3]Oxazinoindolones: A Combined Experimental and Computational Study

The interaction of N-alkoxy-1H-indole-1-carboxamides with transition metals leads to indole-fused heterocyclic scaffolds through directing group leaving/migration, [3 + 2], N-C2 [4 + 1], and [4 + 2] annulations. However, the corresponding O-C2 [4 + 2] annulation reactions have never been reported. Herein, we report the chemoselective annulation of N-alkoxy-1H-indole-1-carboxamides catalyzed by Rh(III), affording [1,3]oxazinoindolones through a hitherto unknown reaction pathway. This unprecedented C2 oxygen cyclization, rather than the known C2 nitrogen cyclization to form oxazinoindolones via five key steps, has been explained using density functional theory.

Repurposing a supramolecular iridium catalyst via secondary Zn⋯O[double bond, length as m-dash]C weak interactions between the ligand and substrate leads to ortho-selective C(sp2)-H borylation of benzamides with unusual kinetics

The iridium-catalyzed C-H borylation of benzamides typically leads to meta and para selectivities using state-of-the-art iridium-based N,N-chelating bipyridine ligands. However, reaching ortho selectivity patterns requires extensive trial-and-error screening via molecular design at the ligand first coordination sphere. Herein, we demonstrate that triazolylpyridines are excellent ligands for the selective iridium-catalyzed ortho C-H borylation of tertiary benzamides and, importantly, we demonstrate the almost negligible effect of the first coordination sphere in the selectivity, which is so far unprecedented in iridium C-H bond borylations. Remarkably, the activity is dramatically enhanced by exploiting a remote Zn⋯O[double bond, length as m-dash]C weak interaction between the substrate and a rationally designed molecular-recognition site in the catalyst. Kinetic studies and DFT calculations indicate that the iridium-catalyzed C-H activation step is not rate-determining, this being unique for remotely controlled C-H functionalizations. Consequently, a previously established supramolecular iridium catalyst designed for meta-borylation of pyridines is now compatible with the ortho-borylation of benzamides, a regioselectivity switch that is counter-intuitive regarding precedents in the literature. In addition, we highlight the role of the cyclohexene additive in avoiding the formation of undesired side-products as well as accelerating the HBpin release event that precedes the catalyst regeneration step, which is highly relevant for the design of powerful and selective iridium borylating catalysts.

Synthesis of 1H-Isoindoles via Ruthenium(II)-Catalyzed Cyclization of Benzimidates with Alkenes

A Ru(II)-catalyzed efficient synthesis of 1H-isoindoles via the cyclization of benzimidates with alkenes has been demonstrated. This methodology exhibits high compatibility with various functionalized activated and unactivated olefins containing different sensitive functional groups. This protocol provides an effective method for synthesizing various 1H-isoindole derivatives in decent to excellent yields. Notably, the ortho-alkenylation of benzimidates with unactivated alkenes was achieved. A potential reaction mechanism has been suggested that involves C-H activation, 1,2 insertion, and β-hydride elimination subsequent to aza-Michael addition.

Assembly of Heterocyclic C-Glycosides by Ru-Catalyzed C-H Activation/Cyclization with Carbonyl Sulfoxonium Ylide Glyco-Reagents

New carbonyl sulfoxonium ylide glyco-reagents have been developed, enabling the synthesis of versatile heteroarene C-glycosides through a Ru-catalyzed C-H activation/annulation strategy. These reactions tolerate various saccharide donors and represent a significant advance in the stereoselective synthesis of heterocyclic C-glycosides. Furthermore, the strategy and methods could be applied to large-scale reactions and late-stage modifications of some structurally complex natural products or drugs.

Accessing 7,8-Dihydroquinoline-2,5-diones via Rh-Catalyzed Olefinic C-H Activation/[4+2] Cyclization

Herein, we report a rhodium-catalyzed C-H activation/[4+2] cyclization reaction between α,β-unsaturated amides and iodonium ylides for the synthesis of novel 7,8-dihydroquinoline-2,5-diones and analogues. This protocol provides a series of pyridones fused with saturated cycles with good functional group compatibility, good water and air tolerance, and good to excellent yields under mild and green reaction conditions. Additionally, scale-up synthesis can be smoothly performed with as low as 0.25 mol % catalyst loading. Recycling experiments and different transformation experiments were also carried out to demonstrate the potential synthetic utility of this protocol.

Molecular iodine catalyzed C(sp2)-H sulfenylation of biologically active enaminone compounds under mechanochemical conditions and studies on their biocidal activity including molecular docking and DFT

Here we demonstrated a solvent free, mechanochemical I2 catalyzed C(sp2)-H sulfenylation of enaminones under grinding condition. Only catalytic amount of I2 is required on silica surface without any external heating. The reaction time has reduced to a great extent in comparison to their solution based counterpart. The frictional energy created by ball-mill on mesoporous silica materials has attracted much attention towards this mechanochemical approach for molecular heterogeneous catalysis. Their large surface area and well defined porous architecture certainly increase the catalytic ability of iodine in this developed protocol. Anti-microbial activities of our synthesized compounds were investigated against two gram positive (Staphylococcus aureus and Bacillus cereus) and two gram negative (Escherichia coli and Klebsiella pneumonia) bacteria. To understand the potency of these compounds (3a-3m) as antimalarial agents, molecular docking studies were also performed. Density functional theory was also used to investigate the chemical reactivity and kinetic stability of the compound 3a-3m.

Rh(III)-Catalyzed C-H Activation/Annulation for the Construction of Quinolizinones and Indolizines

A catalytic-condition-controlled synthesis strategy was reported to build quinolizinone and indolizine derivatives from the easily available enamide and triazole substrates with high regioselectivity and good functional group tolerance. More especially, this transformation has successfully fulfilled a C-H bond activation of terminal olefin from enamides followed by a [3 + 3] and a [2 + 3] cyclization cascade under different catalytic conditions, respectively, to provide two kinds of potentially biologically active heterocyclic scaffolds with a ring-junction nitrogen atom. Mechanistically, the methoxyamine formyl group serves as either a traceless directing group (DG) or an oxidizing DG via the C-N and C-C cleavage in this protocol.

Cascade [4 + 1] Annulation through Activation of the C(sp2)-H Bond Enabling Benzothiadiazinoisoindolcarboxylate, Benzothiadiazinoisoindole, and Benzothiadiazinoisoindolepyrrolidinedione as Hybrid Spiro-Heterocyclic Frameworks†

Two structurally distinct and biologically privileged succinimide and isoindole heteroarenes bearing benzothiadiazinedioxide motif-centered hybrid conjugates are proficiently achieved through Rh(III)-catalyzed sequential C(sp2)-H bond activation, ortho-alkenylation and finally cascade intramolecular cyclization. The significant feature of this developed protocol is that the resulting diversely decorated heterocycles contain a quaternary carbon center and this has been coursed through atypical [4 + 1] annulation ignoring the prevalent [4 + 2]-cyclization pathway and interestingly the applied coupling partners (e.g., maleimide, maleate, and styrene) to materialize the protocol functioned only as C1 synthon. Furthermore, the selective reduction strategy enables to modify the hybrid conjugate of succinimide and benzothiazine dioxide to benzothiazine dioxide-based spirocyclic isoindolopyrrolidinedione skeleton following preferential reduction of one carbonyl group of imide functionality. Overall this methodology emerges to be easily handled, versatile, time-efficient, and manifests relatively unfamiliar spiro-cyclization and good functional group tolerance so easy to grab a library of the entirely new variant of decorated hybrid spiro-heterocyclic scaffolds.

Non-Directed C-H Arylation of Anisole Derivatives via Pd/S,O-Ligand Catalysis

Non-directed C-H arylation is one of the most efficient methods to synthesize biaryl compounds without the need of the prefuctionalization of starting materials, or the installment and removal of directing groups on the substrate. A direct C-H arylation of simple arenes as limiting reactants remains a challenge. Here we disclose a non-directed C-H arylation of anisole derivatives as limiting reagents with aryl iodides under mild reaction conditions. The arylated products are obtained in synthetically useful yields and the arylation of bioactive molecules is also demonstrated. Key to the success of this methodology is the use of a one-step synthesized S,O-ligand.

Copper-catalysed dehydrogenation or lactonization of C(sp3)-H bonds

Cytochrome P450 enzymes are known to catalyse bimodal oxidation of aliphatic acids via radical intermediates, which partition between pathways of hydroxylation and desaturation1,2. Developing analogous catalytic systems for remote C-H functionalization remains a significant challenge3-5. Here, we report the development of Cu(I)-catalysed bimodal dehydrogenation/lactonization reactions of synthetically common N-methoxyamides through radical abstractions of the γ-aliphatic C-H bonds. The feasibility of switching from dehydrogenation to lactonization is also demonstrated by altering reaction conditions. The use of a readily available amide as both radical precursor and internal oxidant allows for the development of redox-neutral C-H functionalization reactions with methanol as the sole side product. These C-H functionalization reactions using a Cu(I) catalyst with loading as low as 0.5 mol.% is applied to the diversification of a wide range of aliphatic acids including drug molecules and natural products. The exceptional compatibility of this catalytic system with a wide range of oxidatively sensitive functionality demonstrates the unique advantage of using a simple amide substrate as a mild internal oxidant.

Construction of Isoquinolone Scaffolds on DNA via Rhodium(III)-Catalyzed C-H Activation

Isoquinolone is one of the most common heterocyclic core structures in countless natural products and many bioactive compounds. Here, a highly efficient approach to synthesize isoquinolone scaffolds on DNA via rhodium(III)-catalyzed C-H activation has been described. This chemistry transformation is robust and has shown good compatibility with DNA, which is suitable for DNA-encoded library synthesis.

Free Amine-Directed Redox Neutral Ruthenium(II) Catalysis toward Regioselective Synthesis of Heterobiaryls

A regioselective coupling of ortho-heteroaryl anilines and 7-oxabenzonorbornadienes has been developed by leveraging free amine-directed redox-neutral Ru(II) catalysis. This protocol facilitates formal C-2 arylation of the indole moiety under mild conditions to offer valuable heterobiaryls in high yields. The reaction displays a broad substrate generality and scalability and retains efficacy in the presence of diverse pharmacophore scaffolds. Moreover, products bearing a free amine group were successfully employed in Mg(NTf2)2-catalyzed double Michael addition cascade, which led to the synthesis of intricate indole- and pyrrole-fused azaheterocycles.

Planar-chiral arene ruthenium complexes: synthesis, separation of enantiomers, and application for catalytic C-H activation

Heating tert-butyl-tetraline with [(p-cymene)RuCl2]2 produces the racemic complex [(arene)RuCl2]2, which can be separated into enantiomers by chromatography of its diastereomeric adducts with chiral phosphine ligand. The resolved chiral complex catalyzes C-H activation of N-methoxy-benzamides and their annulation with N-vinyl-pivaloyl amide giving dihydroisoquinolones in 50-80% yields and with 40-80% enantiomeric excess.

Palladium-Catalyzed Directed Carbon-Carbon Bond Activation of Aryl Nitriles for Cyano Transfer

Herein, we report the C-H cyanation of indoles via a palladium-catalyzed directed C-CN activation reaction using aryl nitrile as a cyano source. The employment of the phenoxy-oriented group is the key to the cleavage of the C-CN bond. This protocol features a broad substrate scope, good efficiency, and high regioselectivity. Furthermore, the practical application of this protocol was showcased in the late-stage functionalization and synthesis of indole derivatives, which were derived from drugs and natural products, through the process of cyanation.

Rhodium-Catalyzed Diastereoselective C-H Activation/[4 + 2] Annulation of α,β-Unsaturated Amides with Bicyclic Alkenes

Herein, we report a rare example of rhodium-catalyzed C-H activation/[4 + 2] annulation of alkenyl amides with bicyclic alkenes under mild and green conditions. The reactivity of the rhodium catalyst in this study differed from that observed in cobalt-catalyzed C-H activation/[3 + 2] annulation between vinylic amides and bicyclic alkenes. In addition, the reaction was performed in EtOH at room temperature, which also displayed excellent diastereoselectivity, good functional group tolerance, and air compatibility. A series of novel bridged-ring skeletons were obtained in good to excellent yields. Scale-up experiments were carried out with 1 or 0.75 mol % rhodium catalyst, affording the desired bridged-ring skeleton in excellent yields.

Access to Amides and Lactams via Pyridotriazole as a Transformable Directing Group

Amide and lactam frameworks were synthesized via an efficient two-step strategy. In this protocol, pyridotriazoles were first treated with isocyanates to form the corresponding amides, which were found to be sufficiently reactive to undergo subsequent intramolecular N-H insertion in the absence of any additional reagents or catalysts.

Enaminone-directed ruthenium(II)-catalyzed C-H activation and annulation of arenes with diazonaphthoquinones for polycyclic benzocoumarins

The weakly coordinating enaminone functionality has been leveraged for a C-H bond activation strategy under ruthenium catalysis and employed in the regioselective annulative coupling of arenes with diazonaphthoquinones, offering polycyclic benzocoumarins in very high yields. The enaminone motif plays a dual role and the protocol operates through a Ru(II)/Ru(IV) catalytic pathway which is amenable to the diversification of various pharmacophore-coupled substrates.

Rh(iii)-catalyzed highly site- and regio-selective alkenyl C-H activation/annulation of 4-amino-2-quinolones with alkynes via reversible alkyne insertion

3,4-Fused 2-quinolone frameworks are important structural motifs found in natural products and biologically active compounds. Intermolecular alkenyl C-H activation/annulation of 4-amino-2-quinolone substrates with alkynes is one of the most efficient methods for accessing such structural motifs. However, this is a formidable challenge because 4-amino-2-quinolones have two cleavable C-H bonds: an alkenyl C-H bond at the C3-position and an aromatic C-H bond at the C5-position. Herein, we report the Rh(iii)-catalyzed highly site-selective alkenyl C-H functionalization of 4-amino-2-quinolones to afford 3,4-fused 2-quinolones. This method has a wide substrate scope, including unsymmetrical internal alkynes, with complete regioselectivity. Several control experiments using an isolated key intermediate analog suggested that the annulation reaction proceeds via reversible alkyne insertion involving a binuclear Rh complex although alkyne insertion is generally recognized as an irreversible process due to the high activation barrier of the reverse process.

Rh(III)-Catalyzed C-H Activation of Benzamides and Chemodivergent Annulation with Benzoxazinanones: Substrate Controlled Selectivity

Decarboxylative annulation of propargyl carbamates with benzamides has been realized via rhodium-catalyzed C-H bond activation under mild conditions, delivering two distinct classes of heterocycles in high efficiency and selectivity under substrate control. This protocol provides a direct synthetic method for the preparation of functionalized 1,8-naphthyridines and isoindolinones.

The crucial role of silver(I)-salts as additives in C-H activation reactions: overall analysis of their versatility and applicability

Transition-metal catalyzed C-H activation reactions have been proven to be useful methodologies for the assembly of synthetically meaningful molecules. This approach bears intrinsic peculiarities that are important to be studied and comprehended in order to achieve its best performance. One example is the use of additives for the in situ generation of catalytically active species. This strategy varies according to the type of additive and the nature of the pre-catalyst that is being used. Thus, silver(I)-salts have proven to play an important role, due to the resulting high reactivity derived from the pre-catalysts of the main transition metals used so far. While being powerful and versatile, the use of silver-based additives can raise concerns, since superstoichiometric amounts of silver(I)-salts are typically required. Therefore, it is crucial to first understand the role of silver(I) salts as additives, in order to wisely overcome this barrier and shift towards silver-free systems.

Conceptual advances in nucleophilic organophosphine-promoted transformations

Catalysis by trivalent nucleophilic organophosphines has emerged as an essential tool in organic synthesis. Several new organic transformations promoted by phosphines substantiate and complement the existing synthetic chemistry tools. Mere design of the substrate and reagent combinations has introduced new modes of reactivity patterns, which are otherwise difficult to achieve. These design considerations have led to the rapid build-up of complex molecular entities and laid a solid foundation to synthesise bioactive natural products and pharmaceuticals. This article presents an overview of some of the conceptual advances, including our contributions to nucleophilic organophosphine chemistry. The scope, limitations, mechanistic insights, and applications of these metal-free transformations are discussed elaborately.

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.

Rh(III)-Catalyzed C8-Spiroannulation of 1-Aminonaphthalenes with Maleimides

The rhodium(III)-catalyzed C8-spiroannulation of 1-aminonaphthalenes with maleimides is described herein. Initially formed C8-alkenylated 1-aminonaphthalenes can intercept nucleophilic 1-amino groups through the intramolecular aza-Michael reaction, resulting in the formation of spirofused tetracyclic frameworks. This protocol displayed a wide substrate scope and a broad functional group compatibility. The synthetic utility of this process is demonstrated by the gram-scale synthesis, late-stage modification, and synthetic transformations.

Rhodium-catalyzed enantioselective and diastereodivergent access to diaxially chiral heterocycles

N-N axially chiral biaryls represent a rarely explored class of atropisomers. Reported herein is construction of diverse classes of diaxially chiral biaryls containing N-N and C-N/C-C diaxes in distal positions in excellent enantioselectivity and diastereoselectivity. The N-N chiral axis in the products provides a handle toward solvent-driven diastereodivergence, as has been realized in the coupling of a large scope of benzamides and sterically hindered alkynes, affording diaxes in complementary diastereoselectivity. The diastereodivergence has been elucidated by computational studies which revealed that the hexafluoroisopropanol (HFIP) solvent molecule participated in an unusual manner as a solvent as well as a ligand and switched the sequence of two competing elementary steps, resulting in switch of the stereoselectivity of the alkyne insertion and inversion of the configuration of the C-C axis. Further cleavage of the N-directing group in the diaxial chiral products transforms the diastereodivergence to enantiodivergence.

Bronsted Acid-Catalyzed Regioselective Carboxamidation of 2-Indolylmethanols with Isonitriles

A regioselective direct carboxamidation reaction of 2-indolylmethanols with readily available isocyanoesters/isocyanides has been reported in this work. The reaction was catalyzed by Bronsted acid such as p-TsOH to deliver the benzylic regioselective amides in 67-86% yield under mild conditions. The developed methodology provides alternative access to traditional metal-free carboxamidation via C-C and C-O bond formation with high atom economy. Furthermore, the developed approach was diversified to synthesize chiral indole-2-carboxamide derivatives with a moderate enantiomeric excess (61-73% ee) using an (R)-chiral phosphoric acid.

Transition-Metal-Catalyzed C-H Bond Activation for the Formation of C-C Bonds in Complex Molecules

Site-predictable and chemoselective C-H bond functionalization reactions offer synthetically powerful strategies for the step-economic diversification of both feedstock and fine chemicals. Many transition-metal-catalyzed methods have emerged for the selective activation and functionalization of C-H bonds. However, challenges of regio- and chemoselectivity have emerged with application to highly complex molecules bearing significant functional group density and diversity. As molecular complexity increases within molecular structures the risks of catalyst intolerance and limited applicability grow with the number of functional groups and potentially Lewis basic heteroatoms. Given the abundance of C-H bonds within highly complex and already diversified molecules such as pharmaceuticals, natural products, and materials, design and selection of reaction conditions and tolerant catalysts has proved critical for successful direct functionalization. As such, innovations within transition-metal-catalyzed C-H bond functionalization for the direct formation of carbon-carbon bonds have been discovered and developed to overcome these challenges and limitations. This review highlights progress made for the direct metal-catalyzed C-C bond forming reactions including alkylation, methylation, arylation, and olefination of C-H bonds within complex targets.

Redox-neutral C-H annulation strategies for the synthesis of heterocycles via high-valent Cp*Co(III) catalysis

A variety of biologically active molecules, pharmaceuticals, and natural products consist of a nitrogen-containing heterocyclic backbone. The majority of them are isoquinolones, indoles, isoquinolines, etc.; thereby the synthesis and derivatization of such heterocycles are synthetically very relevant. Also, certain naphthol derivatives have high synthetic utility as agrochemicals and in dye industries. Previous approaches have utilized ruthenium, rhodium, or iridium which may not be desirable due to the high toxicity, low abundance, and high cost of such 4d and 5d metals. Moreover, the need for an external oxidant during the reaction also adds by-products to the system. A high-valent cobalt-catalyzed redox-neutral C-H functionalization strategy has emerged to be a far better alternative in this regard. The use of the non-noble metal cobalt allows for selectivity and specificity in product formation. Also, the redox-neutral concept avoids the use of an external oxidant either due to the presence of a metal in a non-variable oxidation state throughout the catalytic cycle or due to the presence of an oxidizing directing group or an oxidizing coupling partner. Such an oxidizing directing group not only directs the catalyst to a specific reaction site by chelation but also regenerates the catalyst at the end of the cycle. Certain bonds such as N-O, N-N, N-Cl, N-S, and C-S are the main game-players behind the oxidizing property of such directing groups. In the other case, the directing group only chelates the catalyst to a reaction center, whereas the oxidation is carried out by the upcoming group/coupling partner. Overall, merging the redox-neutral concept with the high-valent cobalt catalysis is paving the way forward toward a sustainable and environmentally friendly approach. This review critically describes the mechanistic understanding, scope, limitations, and synthesis of various biologically relevant heterocycles via the redox-neutral concept in the high-valent Cp*Co(III)-catalyzed C-H functionalization chemistry domain.

Condition-Controlled O-Acylation and N-O Bond Reduction of Hydroximic Acids with Thioacetic Acid

Condition-dependent transformations between hydroximic acids and thioacetic acid were achieved. Using NH₄HCO₃ in the ethanol solvent, efficient N-O bond cleavage of hydroxamic acids occurred to afford primary amides with high functional group compatibility. The reaction was switched to O-acylation when NEt₃ and H₂O were used as the base and solvent, respectively. These facile transformations could be scaled up to the gram level smoothly. Preliminary mechanistic studies suggested that the N-O bond cleavage involves a cascade process of acylation/reduction.

2-Butyne Biscarbonate as a "Bridge" in Rhodium(III)-Catalyzed [4 + 2] Cyclization and Diels-Alder Reaction

Described herein is the development of an unprecedented approach to construct multiple heterocycles with high selectivity through Rh(III)-catalyzed two- or three-component cyclization reaction from simple and readily available starting materials: N-methoxybenzamides, 2-butyne biscarbonate, and maleimides. This methodology provides an efficient strategy for the synthesis of diverse and complicated heterocycles in a one-pot manner and displays excellent features of extremely mild reaction conditions, easy operation, excellent regioselectivity, and good functional group compatibility.

A Cheap and Efficient Oxidant (n-Bu)4NNO3-Enabled C(sp2)- and C(sp3)-H Olefination at Room Temperature

To further promote the widely practical application of C-H activation, developing green and mild reaction conditions has invariably been the objective of researchers, especially when it comes to remote C-H activation reactions. Herein, we report a new cheap and powerful (n-Bu)₄NNO₃ oxidant. This oxidant is efficient and universal for Pd(II)-catalyzed sp² and sp³ C-H olefination and allows the reaction to be carried out at room temperature. Because of this, we attempted to make C-H functionalization more economical and environmentally benign.

Iridium catalysed C2 site-selective methylation of indoles using a pivaloyl directing group through weak chelation-assistance

Here we present an iridium catalysed C2-selective methylation of indoles using methyltrifluoroborate as a source of methyl group. The iridium catalyst selectively discriminates the indole C2 and C4 C-H bonds by coordination with a pivaloyl directing group.

Mechanistic Insights Into the Rhodium-Catalyzed C-H Alkenylation/Directing Group Migration and [3+2] Annulation: A DFT Study

The mechanism of the rhodium-catalyzed C-H alkenylation/directing group migration and [3+2] annulation of N-aminocarbonylindoles with 1,3-diynes has been investigated with DFT calculations. On the basis of mechanistic studies, we mainly focus on the regioselectivity of 1,3-diyne inserting into the Rh-C bond and the N-aminocarbonyl directing group migration involved in the reactions. Our theoretical study uncovers that the directing group migration undergoes a stepwise β-N elimination and isocyanate reinsertion process. As studied in this work, this finding is also applicable to other relevant reactions. Additionally, the role of Na⁺ versus Cs⁺ involved in the [3+2] cyclization reaction is also probed.

Synthesis of Selenoflavones via Ruthenium-Catalyzed Selenylation of Unsaturated Acids

An efficient method for pharmaceutically useful selenoflavones via a ruthenium-catalyzed selenylation reaction is demonstrated. The ruthenium-catalyzed selenylation was applied to synthesize diverse alkenyl selenides from simple unsaturated acids/amides and diaryl diselenides. A wide range of differently substituted diaryl diselenides can be applied in this protocol with a good functional group with excellent stereo- and regioselectivity.

Convergent synthesis of triarylamines via Ni-catalyzed dual C(sp2)-H amination from benzamides with benzohydroxamic acids

An unprecedented method of nickel-catalyzed dual C(sp²)-H amination of N-quinolylbenzamides with benzohydroxamic acids is developed to access triarylamines in one pot. For the first time, broad-spectrum hydroxylamine is employed as an amino source for C-H amination, featuring good chemo-selectivity and functional group tolerance. Furthermore, the catalytic system could be further extended to N-(pivaloyloxy)benzamide, dioxazolone, isocyanate and aniline for C-H amination.

Palladium-Catalyzed C-H Allylation/Annulation Reaction of Amides and Allylic Alcohols: Regioselective Construction of Vinyl-Substituted 3,4-Dihydroisoquinolones

A palladium-catalyzed highly regioselective C-H allylation/annulation reaction of N-sulfonyl amides with secondary or tertiary allylic alcohols has been developed to construct 3,4-dihydroisoquinolones bearing a synthetically valuable vinyl substituent. This cascade cyclization approach of allylic alcohols involving C-H allylation has not been reported previously. The commercially available allylic alcohol substrates, the only by-product of water, and the used terminal oxidant of O₂ provide environmentally benign advantages.

Transition metal-catalyzed C-H/C-C activation and coupling with 1,3-diyne

This review provides a broad overview of the recent developments in the field of transition metal-catalyzed C-H/C-C bond activation and coupling with 1,3-diyne for assembling alkynylated heterocycles, bis-heterocycles, and 1,3-enynes. Transition metal-catalyzed inert bond (C-H/C-C) activation has been the focus of attention among synthetic chemists in recent times. Enormous developments have taken place in C-H/C-C bond activation chemistry in the last two decades. In recent years the use of 2π-unsaturated units as coupling partners for the synthesis of heterocycles through C-H/C-C bond activation and annulation sequence has received immense attention. Among the unsaturated units employed for assembling heterocycles, the use of 1,3-diynes has garnered significant attention due to its ability to render bis-heterocycles in a straightforward manner. The C-H bond activation and coupling with 1,3-diyne has been very much explored in recent years. However, the development of strategies for the use of 1,3-diynes in the analogous C-C bond activation chemistry is less explored. Earlier methods employed to assemble bis-heterocycle used heterocycles that were preformed and pre-functionalized via transition metal-catalyzed coupling reactions. The expensive pre-functionalized halo-heterocycles and sensitive and expensive heterocyclic metal reagents limit its broad application. However, the transition metal-catalyzed C-H activation obviates the need for expensive heterocyclic metal reagents and pre-functionalized halo-heterocycles. The C-H bond activation strategy makes use of C-H bonds as functional groups for effecting the transformation. This renders the overall synthetic sequence both step and cost economic. Hence, this strategy of C-H activation and subsequent reaction with 1,3-diyne could be used for the larger-scale synthesis of chemicals in the pharmaceutical industry. Despite these advances, there is still the possibility of exploration of earth-abundant and cost-effective first-row transition metals (Ni, Cu, Mn. Fe, etc.) for the synthesis of bis-heterocycles. Moreover, the Cp*-ligand-free, simple metal-salt-mediated synthesis of bis-heterocycles is also less explored. Thus, more exploration of reaction conditions for the Cp*-free synthesis of bis-heterocycles is called for. We hope this review will inspire scientists to investigate these unexplored domains.

Electrochemical reactor dictates site selectivity in N-heteroarene carboxylations

Pyridines and related N-heteroarenes are commonly found in pharmaceuticals, agrochemicals and other biologically active compounds1,2. Site-selective C-H functionalization would provide a direct way of making these medicinally active products3-5. For example, nicotinic acid derivatives could be made by C-H carboxylation, but this remains an elusive transformation6-8. Here we describe the development of an electrochemical strategy for the direct carboxylation of pyridines using CO2. The choice of the electrolysis setup gives rise to divergent site selectivity: a divided electrochemical cell leads to C5 carboxylation, whereas an undivided cell promotes C4 carboxylation. The undivided-cell reaction is proposed to operate through a paired-electrolysis mechanism9,10, in which both cathodic and anodic events play critical roles in altering the site selectivity. Specifically, anodically generated iodine preferentially reacts with a key radical anion intermediate in the C4-carboxylation pathway through hydrogen-atom transfer, thus diverting the reaction selectivity by means of the Curtin-Hammett principle11. The scope of the transformation was expanded to a wide range of N-heteroarenes, including bipyridines and terpyridines, pyrimidines, pyrazines and quinolines.

1,3-Diynes: A Versatile Precursor in Transition-Metal Catalyzed (Mediated) C-H Functionalizations

Transition metal-catalyzed C-H functionalization of diverse arenes with alkyne units has attracted enormous attention for decades since they provide straightforward access to various functionalization/annulations, which are commonly present in bioactive compounds and natural products. Recently, conjugated alkynes (1,3-diynes) have been utilized as key coupling partner in many C-H activation reactions due to their versatile characteristic properties. The presence of two C≡C bonds in conjugated 1,3-diyne brings the new diversity in synthetic transformations, such as chemo-, regioselective pathways, mono-bis functionalizations, cascade annulations, etc. Herein, we summarized the latest developments in the realm of transition-metal-catalyzed C-H functionalizations of diverse arenes with 1,3-diynes. Moreover, we highlighted the diverse transformations, conditions, mechanisms and applications of the corresponding reaction in detail.

Transition Metal-Catalyzed C-H Activation/Annulation Approaches to Isoindolo[2,1-b]isoquinolin-5(7H)-ones

The isoindolo[2,1-b]isoquinolin-5(7H)-one scaffold is widely present in lots of bioactive natural products. Diverse types of strategies have been developed to construct this scaffold. Recently, transition metal-catalyzed C-H activation/annulation is emerging as a powerful and straightforward method to construct diverse polyheterocycles with high atom- and step-economy. It also has been employed for the synthesis of the isoindolo[2,1-b]isoquinolin-5(7H)-one scaffold. This review provides an introduction to recent advances for the preparation of isoindolo[2,1-b]isoquinolin-5(7H)-ones by using transition metal-catalyzed C-H activation/annulation. It will help researchers to find hidden opportunities and accelerate the discovery of novel transformations based on C-H activation/annulation.

Base-catalyzed Effective C2-Amidation of Azolium Salts Using Isocyanates under Mild Conditions

An unprecedented base-catalyzed hydroarylation of isocyanates with azolium salts was developed, which follows a simple reaction pathway and provided facile access to diverse C2-amidated azolium salts under mild conditions. Importantly, this methodology can also be applied for the successive C2-amidation of a bisimidazolium salt with two different isocyanates to provide the corresponding unsymmetrically substituted bisamide derivatives. Notably, the obtained amidated salts can also serve as a prominent carbene surrogate for the synthesis of metal-NHC complexes.

Palladium-Catalyzed C(sp2)-H Arylation of Peptides Directed by Aspartic Acid

Herein, we report a palladium-catalyzed C(sp²)-H di- or monoarylation of short peptides containing N-terminal benzamide groups using aspartic acid (Asp) as an endogenous directing group. This strategy has the following merits: a broad substrate scope, selective diarylation of peptides, and gram-scale synthesis. Furthermore, this strategy can be successfully utilized to synthesize peptide-peptide conjugates.

Microwave assisted regioselective halogenation of benzo[b][1,4]oxazin-2-ones via sp2 C-H functionalization

A microwave assisted, palladium-catalyzed regioselective halogenation of 3-phenyl-2H-benzo[b][1,4]oxazin-2-ones has been demonstrated using inexpensive and readily available N-halosuccinimide. The reaction utilizes the nitrogen atom present in the heterocyclic ring as the directing group to afford regioselective halogenated products in good to moderate yields. The established protocol provides wide substrate scope, high functional group tolerance, and high atom and step economy. The reaction proved to be cost-effective and time-saving as it required only a few minutes for completion and is amenable to gram scale. The halogen atoms present in synthesized products provide further scope for post-functionalization. Several post-functionalized products have also been synthesised to demonstrate the high utility of the reaction in the field of drug discovery and late-stage functionalization.

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.