Indoles from simple anilines and alkynes: palladium-catalyzed C-H activation using dioxygen as the oxidant

Recent advances in electrochemically enabled construction of indoles from non-indole-based substrates

Indole motifs are important heterocycles found in natural products, pharmaceuticals, agricultural chemicals, and materials. Although there are well-established classical name reactions for indole synthesis, these transformations often require harsh reaction conditions, have a limited substrate scope, and exhibit poor regioselectivity. As a result, organic synthesis chemists have been exploring efficient and practical methods, leading to numerous strategies for synthesizing a variety of functionalized indoles. In recent years, electrochemistry has emerged as an environmentally friendly and sustainable synthetic tool, with widespread applications in organic synthesis. This technology allows for elegant synthetic routes to be developed for the construction of indoles under external oxidant-free conditions. This feature article specifically focuses on recent advancements in indole synthesis from non-indole-based substrates, as well as the mechanisms underlying these transformations.

Chemo-, regio-, and stereoselective tetrafunctionalization of fluoroalkynes enables divergent synthesis of 5-7-membered azacycles

Alkyne annulation has been widely used in organic synthesis for the construction of azacycles with unique structural and physicochemical properties. However, the analogous transformation of fluoroalkynes remains a challenge and has seen limited progress. Herein we report a 1,2,3,4-tetrafunctionalization of polyfluoroalkynes for the divergent construction of 5-7-membered (E)-1,2-difluorovinyl azacycles. The use of the fluorine atom as a detachable "activator" not only obviates the use of any transition metal catalysts and oxidizing reagents, but also ensures the [3-5 + 2]-annulation and defluorinative functionalization of fluoroalkynes with high chemo-, regio-, and stereoselectivities. This method exhibits a broad substrate scope, good functional group tolerance, and excellent scalability, providing a modular platform for accessing fluorinated skeletons of medicinal and biological interest. The late-stage modification of complex molecules, the multi-component 1,2-diamination of fluoroalkyne, and the synthesis of valuable organofluorides from the obtained products further highlight the real-world utility of this fluoroalkyne annulation technology.

Ru/O2-Catalyzed Oxidative C-H Activation/Alkyne Annulation Using Quinoline-Functionalized NHC as a Directing and Functionalizable Group

The ruthenium/O2-catalyzed oxidative annulation reaction of imidazo[1,5-a]quinolin-2-ium salts with alkynes via N-heterocyclic carbene-directed C-H activation to obtain π-conjugated fused imidazo[1,5-a]quinolin-2-ium derivatives is reported. Molecular oxygen has been explored as an economic and clean oxidant and an alternative to metal oxidants. The current protocol exhibits a wide range of substrate scope including bioactive (±)-α-tocopherol derivatives. Moreover, most of the annulated products show strong fluorescence properties, indicating their potential for making new light-emitting materials.

Palladium Catalysis: Dependence of the Efficiency of C-N Bond Formation on Carboxylate Ligand and Metal Carboxylate or Carboxylic Acid Additive

The Pd-catalyzed inter- and intramolecular reactions of nitrogen compounds are often carried out with palladium carboxylates, sometimes in the presence of carboxylic acids or alkali metal carboxylates. This Mini-Review highlights the dependence of the reaction efficiency on the nature of the ligand and the carboxylate additives. The proposed reaction mechanisms are presented with, as far as possible, personal comments.

Cu-Mediated Tandem 2,3-Disubstituted Indole Synthesis from Simple Anilines and Internal Alkynes via C-H Annulation

A simple, cost-effective, and straightforward method for the synthesis of 2,3-disubstituted indole scaffolds has been developed. The present protocol involves copper-mediated tandem hydroamination followed by C-H annulation of unprotected anilines with a wide range of internal alkynes. In the presence of Cu(OAc)₂·H₂O and trifluoroacetic acid (TFA), the reaction proceeds well to afford a variety of substituted indole derivatives in moderate to good yields. This process was found to be compatible with both primary and secondary anilines coupled with aromatic/aliphatic alkynes. High-purity copper nanoparticles can be recovered after the reaction, revealing the cost-effectiveness and environmentally benign feature of the current protocol.

Copper-Catalyzed Asymmetric Cyanation of Propargylic Radicals via Direct Decarboxylation of Propargylic Carboxylic Acids

Chiral propargylic cyanides are often used as small-molecule feedstocks for the introduction of chiral centers into various valuable products and complex molecules. Here, we have developed a highly atom-economical strategy for the chiral copper complex-catalyzed synthesis of chiral propargylic cyanides. Propargylic radicals can be smoothly obtained by direct decarboxylation of the propargylic carboxylic acids without preactivation. The reactions show excellent selectivity and functional group compatibility. Gram-scale reaction and several conversion reactions from chiral propargylic cyanide have demonstrated the synthetic value of this strategy.

Palladium-Catalyzed Tandem Nucleophilic Addition/C-H Functionalization of Anilines and Bromoalkynes for the Synthesis of 2-Phenylindoles

A novel and efficient palladium-catalyzed annulation of anilines with bromoalkynes for the synthesis of 2-phenylindoles has been described. This approach features excellent regio- and stereoselectivities and good functional group tolerance. Preliminary mechanistic studies indicate that anilines undergo anti-nucleophilic addition to bromoalkynes to generate (Z)-N-(2-bromo-1-phenylvinyl) anilines, followed by sequential C-H functionalization to deliver different substituted 2-phenylindoles. This method provides potential applications for the construction of various biologically active compounds.

Bromide-mediated, C2-selective, and oxygenative alkylation of pyridinium salts using alkenes and molecular oxygen

Herein, we report a bromide-mediated, C2-selective, and oxygenative alkylation of pyridinium salts using alkenes and O₂ for the synthesis of important β-2-pyridyl ketones. Notably, a quaternary carbon center was successfully installed at the C2-position of pyridine and the resulting C2-substituents were highly functionalized. The intermediary cycloadduct was isolated and further transformed into the desired product, which indicated that this three-component reaction underwent a reaction cascade including dearomative cycloaddition and rearomative ring-opening oxygenation. Finally, the bromide-mediated mechanism was discussed and active Br(I) species were proposed to be generated in situ and promote the rearomative ring-opening oxygenation by halogen bond-assisted electron transfer.

Research progress on the structure and biological diversities of 2-phenylindole derivatives in recent 20 years

The privileged structure binds to multiple receptors with high affinity, which is helpful to the development of new bioactive compounds. Indole is classified as a privileged structure, which may be one of the most important structural categories in drug discovery. As a special subset of indole compounds, 2-phenylindole seems to be one of most promising forerunners of drug development. In this paper, 106 articles were referenced to review the structural changes, biological activities and structure-activity relationship of compounds in recent 20 years, and classified them according to their pharmacological activities, from several aspects, including anticancer, antibacterial, anti-inflammatory, analgesic, antiviral, anti-parasite, the biological activities target to central nervous system, et al. It also points out the importance of artificial intelligence (AI) technology in discovery of new 2-phenylindole compounds in a broader prospect. This review will provide some ideas for researchers to develop new indole drugs.

Divergent Pd-catalyzed Functionalization of 4-Oxazolin-2-ones and 4-Methylene-2-oxazolidinones and Synthesis of Heterocyclic-Fused Indoles

Palladium-catalyzed functionalization was presently performed on two building blocks: 4-oxazolin-2-ones and 4-methylene-2-oxazolidinones. Direct Heck arylation of 4-oxazolin-2-ones led to a series of 5-aryl-4-oxazolin-2-ones, including analogues with N-chiral auxiliary, in an almost quantitative yield. The Pd(II)-catalyzed homocoupling reaction of 4-oxazolin-2-ones provided novel heterocyclic across-ring dienes. Meanwhile, the intramolecular cross-coupling of N-aryl-4-methylene-2-oxazolidinones furnished a series of oxazolo[3,4-a]indol-3-ones. Further functionalization of 4-methylene-2-oxazolidinones afforded substituted indoles and heterocyclic-fused indoles with aryl, bromo, carbinol, formyl, and vinyl groups. A computational study was carried out to account for the behavior of the formylated derivatives. The currently developed methodology was applied to a new formal total synthesis of ellipticine.

Recent Advances in Transition-Metal-Catalyzed C-H Functionalization of Ferrocene Amides

During the past decades, in synthetic organic chemistry, directing-group-assisted C-H functionalization is found to be a key tool for the expedient and site-selective construction of C-C and hybrid bonds. Among C-H functionalization of ferrocene derivatives, the directed group strategy is undoubtedly the most commonly used method. Compared to the other directing groups, ferrocene amides can be synthesized easily and are now recognized as one of the most efficient devices for the selective functionalization of certain positions because its metal centre permits fine, tuneable and reversible coordination. The family of amide directing groups mainly comprises monodentate and bidentate directing groups, which are categorized on the basis of coordination sites. In this review, various C-H bond functionalization reactions of ferrocene using amide directing groups are broadly discussed.

Palladium-catalyzed oxidative C-H activation/annulation of N-alkylanilines with bromoalkynes: access to functionalized 3-bromoindoles

A straightforward approach to the synthesis of 3-bromoindoles via palladium-catalyzed oxidative C-H activation/annulation of N-alkylanilines with bromoalkynes has been described. This protocol features high atom economy, excellent chemo- and regioselectivities, and good functional group tolerance. Moreover, the resultant 3-bromoindoles can be transformed to various functionalized indole derivatives, which demonstrates the practicability of this method in organic synthesis.

Rh-Catalyzed [4 + 2] Annulation with a Removable Monodentate Structure toward Iminopyranes and Pyranones by C-H Annulation

The Rh-catalyzed reactions of N-pyridinyl enaminones with internal alkynes leading to the synthesis of iminopyranes via a key C-H bond activation and subsequent tautomeric O-H bond cleavage are reported. Moreover, the pyridine ring in the amino group acts as an auxiliary monodentate site for this annulation and can be easily removed by a simple hydrolysis to afford pyranones.

Regioselective synthesis of 6-nitroindole derivatives from enaminones and nitroaromatic compounds via transition metal-free C-C and C-N bond formation

Few methods are known for the synthesis of nitroindole derivatives. A simple and practical Cs₂CO₃-promoted method for the synthesis of 6-nitroindole derivatives from enaminones and nitroaromatic compounds has been developed. Two new C-C and C-N bonds were formed in a highly regioselective manner under transition metal-free conditions.

Construction of aziridine, azetidine, indole and quinoline-like heterocycles via Pd-mediated C-H activation/annulation strategies

N-Heterocycles can be found in natural products and drug molecules and are indispensable components in the area of organic synthesis, medicinal chemistry and materials science. The construction of these N-containing heterocycles by traditional methods usually requires the preparation of reactive intermediates. In the past decades, with the rapid growth of transition metal catalysed coupling reactions, syntheses of heterocycles from precursors with inert chemical bonds have become a challenge. More recently, in the field of transition metal associated C-H direct functionalization, efficient methods have been developed for the syntheses of N-heterocyclic compounds such as aziridines, azetidines, indoles and quinolines under the click type of reaction mode. In this review, representative synthetic methodologies developed in the recent 10 years for the preparation of this small class of N-heterocycles via the Pd-catalysed C-H activation and C-N bond formation pathway are discussed. We hope this article will provide new insights from the strategies highlighted into future molecular design, synthesis and applications in medical and materials sciences.

1,2-Difunctionalizations of alkynes entailing concomitant C–C and C–N bond-forming carboamination reactions

Vicinal carboamination of alkynes is a highly reliable and efficient practical strategy for the quick preparation of valuable and diverse amine derivatives starting from simple synthons. The last decade has witnessed numerous practical methods employing transition-metal-based/metal-free carboamination approaches using alkynes for the synthesis of these N-bearing entities. Driven by the renaissance of transition metal catalysis, intermolecular and intramolecular carboamination of alkynes comprising concomitant C–N and C–C bond formation has been studied extensively. In contrast to metal catalysis, though analogous metal-free approaches have been relatively less explored in the literature, they serve as alternatives to these expensive approaches. Despite this significant progress, reviews documenting such examples are sporadic; as a result, most reports of this type remained scattered throughout the literature, thereby hampering further developments in this escalating field. In this review, different conceptual approaches will be discussed and examples from the literature will be presented. Further, the reader will get insight into the mechanisms of different transformations.

The 1,2-difunctionalization of alkynes happening through concomitant C–C and C–N bond formation strategies have provide an unified access to diversely functionalized N-bearing heterocycles.

Cobalt-catalyzed C(sp3)-H bond functionalization to access indole derivatives

Herein, we develop an efficient method of cobalt-catalyzed C(sp3)-H bond functionalization to synthesize indole derivatives. The highlight of this protocol is accomplished by the sequential C-H activation. This “cobalt/ organic...

AlBr3-Promoted stereoselective anti-hydroarylation of the acetylene bond in 3-arylpropynenitriles by electron-rich arenes: synthesis of 3,3-diarylpropenenitriles

Reactions of 3-arylpropynenitriles (ArC≡CCN) with electron-rich arenes (Ar′H, benzene and its polymethylated derivatives) under the action of aluminum bromide (AlBr₃, 6 equiv) at room temperature for 0.5–2 h result in the stereoselective formation of 3,3-diarylpropenenitriles (Ar(Ar′)C=CHCN) in yields of 20–64%, as products of mainly anti-hydroarylation of the acetylene bond. The obtained 3,3-diarylpropenenitriles in triflic acid CF₃SO₃H (TfOH) at room temperature for 1 h are cyclized into 3-arylindenones in yields of 55–70%.

Copper-Catalyzed (4+1) Cascade Annulation of Terminal Alkynes with 2-(Tosylmethyl)anilines: Synthesis of 2,3-Disubstituted Indoles

A novel strategy based on Cu-catalyzed (4+1) cascade annulation of terminal alkynes as one-carbon synthons with 2-(tosylmethyl)anilines has been developed for the expeditious synthesis of 2,3-disubstituted indoles, in which in situ generations of aza-o-quinone methides and alkynyl-copper(I) species are involved. This annulation provides an effective method for the assembly of synthetically and structurally interesting 2,3-disubstituted indoles.

Organocatalytic Direct Asymmetric Indolization from Anilines by Enantioselective [3 + 2] Annulation

We report the efficient syntheses of chiral tetrahydroindole pyrazolinones by the asymmetric [3 + 2] cascade cyclizations (indolizations) of simple aniline derivatives with pyrazolinone ketimines as 2C synthons. The chiral phosphoric-acid-catalyzed system uses a concerted π-π interaction/dual H-bond control strategy to catalytically direct the asymmetric aniline, which undergoes a highly chemo-, regio-, and enantioselective [3 + 2] cascade annulation, furnishing a series of optically active tetra-hydroindole pyrazolinones with two contiguous chiral aza-quaternary carbon centers in excellent yields with excellent enantioselectivities. This method features a relatively broad substrate scope for amines and 2-naphthylamines and highlights the emerging value of direct chiral indolizations from simple amine sources in organic synthesis.

Rhodium-Catalyzed Annulation of N-Acetoxyacetanilide with Substituted Alkynes: Conversion of Nitroarenes to Substituted Indoles

A general and efficient rhodium-catalyzed redox-neutral annulation of N-acetoxyacetanilides, readily accessible from nitroarenes, with alkynes has been accomplished for the synthesis of substituted indole derivatives. A wide range of substituted 2,3-diarylindoles were achieved from various substituted N-acetoxyacetanilides and symmetrical/unsymmetrical alkynes in good to excellent yields. The developed method was successfully integrated with the synthesis of N-acetoxyacetanilides for the efficient one-pot synthesis of indoles from nitroarenes. The important features are the introduction of N-acetoxyacetamide as a new directing group, redox-neutral annulation, an additive-free approach, wide functional group tolerance, an intramolecular version, and a one-pot reaction of nitroarenes. The method was further extended to the synthesis of potent higher analogues of indole, viz., pyrrolo[3,2-f]indoles and dibenzo[a,c]carbazoles. In addition, a plausible mechanism was proposed based on the isolation and stoichiometric study of a potential aryl-Rh intermediate.

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.

Free Amine, Hydroxyl and Sulfhydryl Directed C-H Functionalization and Annulation: Application to Heterocycle Synthesis

Transition metal-catalyzed direct C-H bond functionalization is recognized as an efficient strategy to assemble heterocyclic frameworks. For this purpose, directing groups (DGs) installation on an organic molecule has remained a widely exploited strategy for the years. The installation of directing groups, especially for the amine, alcohol and thiol containing reactants and their removal after the reaction need additional steps. In this regard, the use of free amine, hydroxyl and sulfhydryl as directing groups in native form is advantageous and in recent times, these transformations have stirred undisputable advancements for applications to heterocycle synthesis. In this review, the aromatic sp² -C attached free amine, hydroxyl and sulfhydryl as native functionalities are shown to be useful for the construction of five to seven-membered N-, O- and S-heterocycles.

Regioselective Mercury(I)/Palladium(II)-Catalyzed Single-Step Approach for the Synthesis of Imines and 2-Substituted Indoles

An efficient synthesis of ketimines was achieved through a regioselective Hg(I)-catalyzed hydroamination of terminal acetylenes in the presence of anilines. The Pd(II)-catalyzed cyclization of these imines into the 2-substituted indoles was satisfactorily carried out by a C-H activation. In a single-step approach, a variety of 2-substituted indoles were also generated via a Hg(I)/Pd(II)-catalyzed, one-pot, two-step process, starting from anilines and terminal acetylenes. The arylacetylenes proved to be more effective than the alkyl derivatives.

Electrooxidative Rhodium-Catalyzed [5+2] Annulations via C-H/O-H Activations

Electrooxidative annulations involving mild transition metal-catalyzed C-H activation have emerged as a transformative strategy for the rapid construction of five- and six-membered heterocycles. In contrast, we herein describe the first electrochemical metal-catalyzed [5+2] cycloadditions to assemble valuable seven-membered benzoxepine skeletons by C-H/O-H activation. The efficient alkyne annulation featured ample substrate scope, using electricity as the only oxidant. Mechanistic studies provided strong support for a rhodium(III/I) regime, involving a benzoxepine-coordinated rhodium(I) sandwich complex as the catalyst resting state, which was re-oxidized to rhodium(III) by anodic oxidation.

Modular Counter-Fischer-Indole Synthesis through Radical-Enolate Coupling

A single-electron transfer mediated modular indole formation reaction from a 2-iodoaniline derivative and a ketone has been developed. This transition-metal-free reaction shows a broad substrate scope and unconventional regioselectivity trends. Moreover, important functional groups for further transformation are tolerated under the reaction conditions. Density functional theory studies reveal that the reaction proceeds by metal coordination, which converts a disfavored 5-endo-trig cyclization to an accessible 7-endo-trig process.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

A direct synthesis method towards spirocyclic indazole derivatives via Rh(iii)-catalyzed C–H activation and spiroannulation

A rhodium(iii)-catalyzed [4 + 1] spiroannulation of N-aryl phthalazine-diones (pyridazine-diones) with diazo compounds to construct spirocyclic indazole derivatives with diverse structures is described.

One-Pot Tandem ortho-Naphthoquinone-Catalyzed Aerobic Nitrosation of N-Alkylanilines and Rh(III)-Catalyzed C-H Functionalization Sequence to Indole and Aniline Derivatives

The nitroso group served as a traceless directing group for the C-H functionalization of N-alkylanilines, ultimately removed after functioning either as an internal oxidant or under subsequent reducing conditions. The unique ability of o-NQ catalysts to aerobically oxidize the N-alkylanilines without using solvents and stoichiometric amounts of oxidants has rendered the new opportunity to develop the telescoped catalyst systems without a need for directly handling the hazardous N-nitroso compounds.

Molecular engineering enabling reversible transformation between helical and planar conformations by cyclization of alkynes

Molecular engineering enabling reversible transformation between helical and planar conformations is described herein. Starting from easily available 2-(pyridin-2-yl)anilines and alkynes, a one-pot strategy is set up for the synthesis of aza[4]helicenes via two successive rhodium-catalyzed C–H activation/cyclizations. Helical pyrrolophenanthridiziniums can be transformed into planar conformations through the cleavage of acidic pyrrole N–H, leading to turn-off fluorescence. NMR spectra, single crystal X-ray diffraction and DFT calculations demonstrate that the formation of an intramolecular C–H⋯N hydrogen bond is beneficial to stabilize the pyrrole nitrogen anion of the planar molecule and provide increased planarity. The reversible conformation transformations can be finely adjusted by the electron-donating and -withdrawing groups on the π⁺-fused pyrrole skeleton in the physiological pH range, thus affording an opportunity for pH-controlled intracellular selective fluorescence imaging. Pyrrolophenanthridiziniums show turn-on fluorescence in lysosomes owing to the acidic environment of lysosomes and turn-off fluorescence out of lysosomes, indicating the occurrence of the deprotonation reaction outside lysosomes and the corresponding transformation from helical to planar conformations.

One-pot synthesis of aza[4]helicenes is accomplished through two successive C–H activation/cyclizations, which exhibit on/off fluorescence switching through reversible transformation between helical and planar conformations.

Synthesis of 1H-Indole-2,3-dicarboxylates via Rhodium-Catalyzed C-H Annulation of Arylhydrazines with Maleates

This work describes a one-step synthesis of 1H-indole-2,3-dicarboxylates through C-H activation. Rhodium-catalyzed tandem C-H activation and annulation of 2-acetyl-1-phenylhydrazines with maleates proceeded smoothly in the presence of additive NaOAc and oxidant Ag₂CO₃ and produced the corresponding indole derivatives, 1H-indole-2,3-dicarboxylates, in satisfactory to good yields. A variety of useful functional groups were tolerated on the benzene ring including halogen atoms (F, Cl, Br, and I) and methoxycarbonyl groups.

Catalyst and solvent switched divergent C-H functionalization: oxidative annulation of N-aryl substituted quinazolin-4-amine with alkynes

The development of site-selective C-H functionalizations/annulations is one of the most challenging practices in synthetic organic chemistry particularly for substrates bearing several similarly reactive C-H bonds. Herein, we describe catalyst and solvent controlled ortho/peri site-selective oxidative annulation of C-H bonds of N-aryl substituted quinazolin-4-amines with internal alkynes. The ortho C-H selective annulation was observed using Pd-catalyst in DMF to give indole-quinazoline derivatives, while, Ru-catalyst in PEG-400 favoured the peri C-H bond annulation exclusively to furnish pyrido-quinazoline derivatives.