Beyond C2 and C3: Transition-Metal-Catalyzed C–H Functionalization of Indole

Red-light mediated formylation of indoles using a helical carbenium ion as a photoredox catalyst

Low-energy photoredox catalysis has gained significant attention in developing organic transformations due to its ability to achieve high penetration depth and minimum health risks. Herein, we disclose a red-light (λ = 640 nm)-mediated C-3 formylation of indoles utilizing a helical carbenium ion as a photocatalyst and 2,2-dimethoxy-N,N-dimethylethanamine as a formylating source. These protocols exhibit a broad substrate scope under mild conditions with efficient scalability for the synthesis of C-3 formylated indoles.

Transient Directing Group-Assisted Palladium-Catalyzed C4-Alkynylation of Indoles

Pd-catalyzed C4-selective alkynylation of indoles was established by employing glycine as a transient directing group. This reaction exhibits high regioselectivity with the tolerance of a wide scope of functional groups to afford diverse alkynylated indoles in moderate to good yields. Moreover, the readily accessible scale-up synthesis and further decorations to achieve multifunctionalized indoles demonstrate the synthetic potential of this protocol.

Synthesis of 2-Sulfonyl Carbazoles via Oxidative C-H Functionalization of Tetrahydrocarbazoles with Sulfonyl Hydrazides

Herein, we report an approach for the synthesis of 2-sulfonyl carbazoles from the oxidative C-H sulfonylation of tetrahydrocarbazoles. The mechanistic study reveals that this special selectivity is realized by the addition of a sulfonyl radical to the 3,4-dihydrocabazole intermediate via dehydrogenative desaturation of tetrahydrocarbazoles. This approach features readily available starting materials, high regioselectivity, broad substrate scope, and attractive synthetic utility.

Cobalt(II)-Catalyzed Selective C2-H Heck Reaction of Native (N-H) Indoles Enabled by Salicylaldehyde Ligand

Direct functionalization of native (N-H) indoles via C-H activation remains a challenge. Herein, we report a salicylaldehyde-promoted cobalt-catalyzed selective C2-H Heck reaction of native (N-H) indoles with both active and unactivated olefins in the presence of free N-H bonds. A series of structurally diverse C2-alkenylated native (N-H) indoles including natural product and drug derivatives were prepared directly and effectively without additional preprotection and deprotection procedures.

Electrochemical Three-Component C-H Functionalization of Indoles with Sodium Bisulfite and Alcohols to Access Indole-Containing Sulfonate Esters

Herein, an efficient electrochemical three-component C-H functionalization of indoles with sodium bisulfite and alcohols is described, providing a sustainable and convenient synthetic route for the construction of structurally valuable indole-containing sulfonate esters in moderate to good yields. This protocol proceeds in an undivided cell without any metal catalysts or oxidants, features a broad substrate scope, and has an excellent functional group tolerance. Preliminary mechanistic studies suggest that a radical-radical pathway may be involved in this three-component reaction system.

Exploring the capabilities of 2-alkynyl aryl/benzyl azides: synthesis approaches for indoles, quinolines, and their derivatives via transition metal catalysis

In recent research, quinoline and indole structures have gained recognition for their significant clinical relevance and effectiveness. These compounds are known for their wide-ranging pharmacological effects, which include anticancer, antibacterial, antifungal, antiviral, and anti-inflammatory properties. Researchers have successfully implemented a variety of innovative synthetic strategies, leading to the creation of numerous compounds that display fascinating biological activities in diverse fields. This has sparked growing interest in developing quinoline and indole-based analogues, given their impressive variety of biological effects. Over the past few years, new, efficient, and more accessible synthetic techniques-such as green chemistry and microwave-assisted synthesis-have been introduced to produce a diverse array of quinoline and indole structures. This development reflects an expanding area of interest in both academic and industrial settings, making it easier to investigate their biological capabilities. In this review, we examine the intriguing transformations of 2-alkynyl aryl and benzyl azide derivatives into indoles and quinolines, emphasizing the role of metal catalysts such as Au, Cu, Rh, Pd, and Ag, from 2011 to 2024. We showcase the variety of substrates involved, highlight notable advancements in this area of research, and address the limitations faced by chemists. Additionally, we offer insights into the mechanisms driving these important reactions, aiming to enhance understanding and inspire future work in this dynamic field.

Harnessing Dual Reactivity of N-Chloroamides for Cascade C-H Amidation/Chlorination of Indoles under Cobalt-Catalysis: Overriding Hofmann Rearrangement Pathway Leading to Aminocarbonylation

Herein, we have developed a Cp*Co(III)-catalyzed cascade C-2 amidation/C-3 chlorination of indoles by leveraging the dual functionality of N-chloroamides at ambient temperature. This protocol avoids the aminocarbonylation pathway that may result from the C-H functionalization of isocyanates formed via a potential Hofmann rearrangement of N-chloroamides. In fact, this represents the first example of directed C-H amidation using N-chloroamides as amidating agent. The control experiment indicated that the C-2 C-H amidation occurs prior to C-3 chlorination. Additionally, chloro functionality has been effectively utilized for the construction of C-S and C-N bonds, thereby expanding the molecular diversity of the synthesized compounds.

Rhodium(III)-Catalyzed Regioselective C4 Alkylation of Indoles with Nitroalkenes

The Rh(III)-catalyzed indole C4-H bond addition to nitroalkenes is disclosed under mild and redox-neutral reaction conditions, offering straightforward access to various 4-(2-nitroalkyl)indoles (34 examples) with excellent chemo- and regioselectivity. Furthermore, late-stage diversifications and mechanistic studies were also performed.

Electrochemical Skeletal Indole Editing via Nitrogen Atom Insertion by Sustainable Oxygen Reduction Reaction

Skeletal molecular editing gained considerable recent momentum and emerged as a uniquely powerful tool for late-stage diversifications. Thus far, superstoichiometric amounts of costly hypervalent iodine(III) reagents were largely required for skeletal indole editing. In contrast, we herein show that electricity enables sustainable nitrogen atom insertion reactions to give bio-relevant quinazoline scaffolds without stoichiometric chemical redox-waste product. The transition metal-free electro-editing was enabled by the oxygen reduction reaction (ORR) and proved robust on scale, while tolerating a variety of valuable functional groups.

Stereo- and Regioselective Installation of Vinyl Sulfonyl Fluoride onto Indoles without Transition-Metal Catalyst

Herein, we developed a practical method for synthesizing a class of novel and highly valuable indolyl vinyl sulfonyl fluorides. This protocol has carved out a path for constructing a broad range of vinyl sulfonyl fluorinated indoles with exclusive stereo- and regioselectivity through the Friedel-Crafts/elimination reaction without any transition-metal catalyst. This transformation features mild conditions, high efficiency, excellent selectivity, and rich substrate compatibility, highlighting its significant value in medicinal chemistry and many related disciplines.

Ru(II)-Catalyzed Skeletal Editing of Oxindole with Internal Alkyne To Synthesize C7-Alkylated Indole Derivatives

A Ru(II)-catalyzed skeletal editing of oxindole scaffolds was established to afford C7-alkyl acetate indole derivatives using internal alkyne and alkyl alcohol. The developed method is simple, efficient, and straightforward. The reaction was extended to substrates having wide chemoselective profiles. When unsymmetrical alkynes were used, promising regioselectivity was realized. A preliminary mechanistic study revealed that the reaction pathway proceeded by Ru(II)/Ag(I)-catalyzed amide cleavage and subsequent oxidative annulation.

Borane-catalysed C2-selective indole reductive functionalisation

Indolines are common motifs within pharamceuticals and natural products. Boron catalysis enables the chemoselective allylation of indoles to give allylic indolines in excellent diastereoselectivity. Mechanistic studies revealed in situ formation of the allylic borane, allylation of the imine tautomer of the indole and B-N/B-H transborylation for catalytic turnover.

Computational Design of Ligands for the Ir-Catalyzed C5-Borylation of Indoles through Tuning Dispersion Interactions

The indole moiety is ubiquitous in natural products and pharmaceuticals. C-H borylation of the benzenoid moiety of indoles is a challenging task, especially at the C5 position. We have combined computational and experimental studies to introduce multiple noncovalent interactions, especially dispersion, between the substrate and catalytic ligand to realize C5-borylation of indoles with high reactivity and selectivity. The successful computational predictions of new ligands should be suitable for ligand design in other transition-metal catalyzed reactions.

Rh-Catalyzed C-H Alkynylation of Indole Derivatives with Silver(I)-Controlled Regiodivergence

We have disclosed silver(I)-induced switching of regioselectivity in rhodium-catalyzed C-H alkynylation of indole derivatives with the help of a pivaloyl directing group by tuning C-H metalation modes. The judicious choice of AgOAc, Ag2O, and Ag2CO3 affords an array of C2-alkynylated indoles, C4-alkynylated indoles, and C2,C4-dialkynylated indoles, respectively. The synthetic utility of the alkyne fragment is demonstrated by derivatization into valuable indole-based compounds.

Designing Cobalt(II) Complex for Chemoselective Synthesis of 2-Aryl-3-Formyl Indoles from Amino Alcohols and Alcohols†

An air-stable, inexpensive, and isolable cobalt(II) complex (C1) of N-((1-methyl-1H-imidazol-2-yl)methyl)-2-(phenylselanyl)ethan amine (L1) was synthesized and characterized. The complex was used to catalyze a one-pot cascade reaction between 2-(2-aminophenyl)ethanols and benzyl alcohol derivatives. Interestingly, 2-aryl-3-formylindole derivatives were formed instead of N-alkylated or C-3 alkylated indoles. A broad substrate scope can be activated using this protocol with only 5.0 mol % catalyst loading to achieve up to 87 % yield of 2-aryl-3-formylindole derivatives. The mechanistic studies suggested that the reaction proceeds through tandem imine formation followed by cyclization.

Ruthenium-catalyzed C-H functionalization of indoles and indolines with 7-azabenzonorbornadienes: access to aminodihydronaphthyl indoles and indolines

Indoles, indolines and hydronaphthylamines are ubiquitous structural motifs in natural products, pharmaceuticals, and biologically active molecules. In this paper, we report the synthesis of aminodihydronaphthyl-substituted indoles and indolines via a Ru-catalyzed carbamoyl-directed C-H functionalization of indoles and indolines with 7-azabenzonorbornadienes. In the presence of Cu(OAc)2 and AgSbF6, [Ru(p-cymene)Cl2]2 catalyzes the reaction of 1-carbamoylindoles with 7-azabenzonorbornadienes to produce 2-(1-amino-1,2-dihydronaphthalen-2-yl)indoles. Under the same conditions, the reaction of 1-carbamoylindolines with 7-azabenzonorbornadienes affords 7-(1-amino-1,2-dihydronaphthalen-2-yl)indolines. In both cases, the reactions yield cis-configured products.

Pd-Catalyzed Direct C7 Trifluoromethylation of Indolines with Umemoto's Reagent

An efficient palladium-catalyzed region-selective C7-trifluoromethylation of indolines using commercially available Umemoto's reagent was reported. The reaction utilizing Umemoto's reagent as CF3 radical precursor, pyrimidine as a removable directing group, Pd(II) as a catalyst, and Cu(II) as an oxidant furnished the required products with excellent regioselectivities and good yields. The present strategy has good region-selectivity, broad substrate scope, and scale-up application. Additionally, the present method was underlined by the direct C-1 trifluoromethylation of carbazoles. Furthermore, C7 trifluoromethylated indole can also be easily obtained via Pd-catalyzed direct C-7 trifluoromethylation/oxidation/deprotection sequential reactions.

HFIP-mediated C-3-alkylation of indoles and synthesis of indolo[2,3-b]quinolines & related natural products

An expeditious metal free C-3 alkylation of indoles and its NIS-mediated deviation to indolo[2,3-b]quinolines is reported. This protocol, executed in aqueous HFIP has broad substrate scope and is well inclined towards the ideas of sustainable chemistry. Applications of these strategies in accessing bioactive natural products like vibrindole, norcryptotakeine, neocryptolepine and indenoindolone scaffolds has also been demonstrated.

Iodine-mediated oxidative triple functionalization of indolines with azoles and diazonium salts

We report an innovative synthetic strategy for the generation of polysubstituted indoles from indolines, aryldiazonium salts, and azoles. The methodology encompasses an electrophilic substitution reaction affording C5-indoline intermediates which undergo an iodine-mediated oxidative transformation coupled with C-H functionalization to yield the indole derivatives.

Rh(III)-Catalyzed C7-Alkylation of Isatogens with Malonic Acid Diazoesters

Rh(III)-catalyzed C7-alkylation of isatogens (indolin-3-one N-oxides) with malonic acid diazoesters has been developed. This strategy utilizes oxygen anion on the N-oxide group of isatogens as a directing group and successfully achieves the synthesis of a series of C7-alkylated isatogens with moderate to good yields (48-86% yields). Moreover, the N-oxides of isatogens can not only serve as the simple directing group for C7-H bond cleavage but also be deoxidized for easy removal.

Regioselective C3Alkylation of Indoles for the Synthesis of Bis(indolyl)methanes and 3-Styryl Indoles

Herein, we describe an efficient transition-metal-free regioselective C3alkylation of indoles for the synthesis of bis(indolyl)methanes and 3-styryl indoles. Nitrobenzene is employed as the oxidant to oxidize the alcohols in the presence of a strong base and the reaction avoids the use of transition metals such as Ru and Mn. The protocol provides a favorable route to access biologically active compounds such as arundine, vibrindole A, and turbomycin B.

Palladium-Catalyzed Regioselective C4-H Acyloxylation of Indoles with Carboxylic Acids via a Transient Directing Groups Strategy

The development of an efficient method for the synthesis of C4 oxy-substituted indoles is an appealing yet challenging task. Herein, we report a general palladium-catalyzed TDG approach for the direct C4-H acyloxylation of indoles. The protocol features atom and step economy, excellent regioselectivity, and good tolerance of functional groups. Moreover, the reaction can accommodate a range of carboxylic acids including benzoic acids, phenylacetic acids, and aliphatic acids.

Transition-Metal-Catalyzed Directed C-H Functionalization in/on Water

Directing group assisted C-H bond functionalization using transition-metal-catalysis has emerged as a reliable synthetic tool for the construction of regioselective carbon-carbon/heteroatom bonds. Off late, "in/on water directed transition-metal-catalysis", though still underdeveloped, has appeared as one of the prominent themes in sustainable organic chemistry. This article covers the advancements, mechanistic insights and application of the sustainable directed C-H bond functionalization of (hetero)arenes in/on water in the presence of transition-metal-catalysis.

Site-Selective Synthesis of Antitumor C5-Aminated Indoles via Neighboring Aldehyde Group Assisted Catellani Reaction

A palladium/norbornene (NBE) cooperative catalytic system was developed to access C5-aminated indoles, starting from readily available C4-idonated indoles. Good yields and exclusive site selectivity were achieved for a broad substrate scope, including drug molecule core architectures. Control experiments found that both aldehyde on the C3 position and sulfonyl protecting group on the N1 position were vital for the transformation. Preliminary bioactivity evaluation identified a promising leading compound 3af with potent antitumor proliferative activity against several cancer cells.

Nickel-Catalyzed Tandem Cyclization of 1,6-Diynes with Indolines/Indoles through Dual C-H Bond Activation

A nickel-catalyzed site-selective tandem cyclization of 1,6-diynes with substituted indolines or indoles through consecutive dual C-H bond activation is described. In the reaction, substituted fused indole and carbazole derivatives were observed in good to excellent yields, in which three consecutive C-C bonds formed in one pot. Later, in the presence of DDQ, the aromatization of the indoline derivative was converted to the indole derivative. A possible reaction mechanism involving dual C-H bond activation as a key step was proposed to account for the present reaction.

Acetoxy Group-Directed Regioselective C2 Alkenylation of Indoles via Pd-Ag Bimetallic Catalysis

Regioselective C-H functionalizations of indoles reported to date with directing groups at C3 mainly rely on functional groups that are linked to the indole via C-C bonds. However, groups that are linked to the indole core by C-X linkages are also attractive due to the possibility of further modifications of the C-X bond. Herein, we report a 3-acetoxy directing group for the regioselective C2 alkenylation of indoles via a C-H activation-based, cross-dehydrogenative, oxidative Heck-type reaction. The reaction is catalyzed by Pd(II) and Ag(I) with stoichiometric Cu(II) as the oxidant and provides the 2-alkenylated indoles in yields of 52-84%. The reaction conditions are compatible with several functional groups at different positions as well as different N-protecting groups or free NH groups on the indole core. With respect to the alkene coupling partners, the reactions are successful with acrylates, vinyl sulfates, and phosphates. Specifically designed experiments, as well as density functional theory (DFT) computational studies, reveal that a heterodinuclear [Pd(μ-OAc)3Ag] bimetallic species is the actual catalyst responsible for the C-H alkenylation. A mechanistic path involving this catalytic species was also found to be favorable over other possible pathways for explaining the observed regioselectivity through DFT studies.

Metal-free C-H Borylation and Hydroboration of Indoles

The C-H borylation and hydroboration reactions have emerged as promising synthetic tools to construct organoboron compounds. Organoboron compounds of N-heterocycles, particularly indole derivatives, have found widespread application in a variety of fields. As a result, considerable advancement in the area of C-H borylation and hydroboration reactions of indoles was observed in the last few decades. Among the various synthetic methods applied, the metal-free approach has received special attention. This mini-review discusses the recent progress in the area of C-H borylation and hydroboration reactions of indoles under metal-free conditions, their scope, and brief mechanistic studies.

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.

A redox-neutral weak carbonyl chelation assisted C4-H allylation of indoles with vinylcyclopropanes

A weak acyl chelation-assisted distal C4-H allylation of indoles has been accomplished using vinylcyclopropanes as an allylating agent under redox-neutral ruthenium(II) catalysis. The regioselectivity, removable directing group, substrate scope and diastereoselectivity are the important practical features.

Weakly coordinating tert-amide assisted Rh(III)-catalyzed C4-cyanation of indoles: application in photophysical studies

A rhodium(III)-catalyzed indole C4-selective cyanation is described using the bench-stable, user-friendly electrophilic cyanation agent N-cyano-N-phenyl-p-toluenesulfonamide (NCTS) as a coupling partner. A suitably positioned weakly coordinating tert-amide group was utilized for this site selectivity. The developed protocol proceeded with a broad scope. [Cp*Rh(MeCN)3][SbF6]2 was found to be an effective Rh(III) catalyst for this transformation. An initial study was carried out to know the photophysical properties of the C4-cyanated indole frameworks.

Aryldiazonium Salt-Triggered [2 + 2 + 1] Heteroannulation of Indoles by an Arylhydrazone Radical-Relayed 1,5-Hydrogen Atom Transfer

An unprecedented three-component [2 + 2 + 1] annulation cascade of indoles with aryldiazonium salts and polyhalomethanes or acetone is presented by dual hydrogen atom transfer (HAT) and C-H functionalization. By employing readily accessible aryldiazonium salts as the radical initiators and electrophiles and polyhalomethanes and acetone as the C1 units, this method unprecedentedly constructs a pyrazole ring on an indole ring skeleton through the formation of two C-N bonds and a C-C bond in a single reaction.

B(C6F5)3-mediated direct intramolecular C7-alkenylation of N-propargylindoles

B(C6F5)3-mediated direct C7-alkenylation of N-propargylindoles without directing groups was developed. This reaction proceeds via the π-activation of the alkynyl group with B(C6F5)3/Friedel-Crafts alkenylation/proton transfer reaction sequence. Interestingly, C7-alkenylation products could further convert into the fused indoles by deprotonation and finally polyaromatic N-heterocycles by the hydride abstraction.

Direct Electrophilic Attack of Compound I on the Indole Ring in the Peroxygenase Mechanism of Dehaloperoxidase DHP B in Degrading Haloindole: Electron Transfer Promotes the Reaction

The H2O2-dependent degradation of haloindole catalyzed by the dehaloperoxidase (DHP) from Amphitrite ornate has been reported to employ the peroxygenase mechanism, and the two oxidized products 5-halo-2-oxindole and 5-halo-3-oxindole have a similar amount. According to a previous experimental study, compound I (Cpd I) was suggested to be responsible for triggering the reaction, and the reaction may undergo three possible intermediates; however, the reaction details are still unclear. To clarify the reaction mechanism of DHP, the computational model was constructed on the basis of the high-resolution crystal structure, and a series of the quantum mechanical/molecular mechanical calculations were performed. Based on our calculation results, it is confirmed that the reaction starts from the direct electrophilic attack of Cpd I on the indole ring of the substrate, and the resulted intermediate contains both a carbocation and an oxygen anion, whereas the common hydrogen abstraction by Cpd I was calculated to correspond to a relatively higher barrier. In addition, a net electron transfer from the substrate to the iron center is observed during the attack of Cpd I on the indole ring; therefore, the carbocation/oxygen anion intermediate can easily undergo an intramolecular hydride transfer to form the product 5-halo-2-oxindole or isomerize to the epoxide intermediate which finally generates another product 5-halo-3-oxindole. It is the zwitterionic characteristic of the intermediate that makes the intermolecular hydride transfer quite easy, and it is the high electron affinity of the iron center that promotes the single-electron oxidation of the reaction intermediate. Our calculations well explain the formation of two oxidized products 5-halo-2-oxindole and 5-halo-3-oxindole.

Structure-guided Mutagenesis Reveals the Catalytic Residue that Controls the Regiospecificity of C6-Indole Prenyltransferases

Indole is a significant structural moiety and functionalization of the C-H bond in indole-containing molecules expands their chemical space, and modifies their properties and/or activities. Indole prenyltransferases (IPTs) catalyze the direct regiospecific installation of prenyl, C5 carbon units, on indole-derived compounds. IPTs have shown relaxed substrate flexibility enabling them to be used as tools for indole functionalization. However, the mechanism by which certain IPTs target a specific carbon position is not fully understood. Herein, we use structure-guided site-directed mutagenesis, in vitro enzymatic reactions, kinetics and structural-elucidation of analogs to verify the key catalytic residues that control the regiospecificity of all characterized regiospecific C6 IPTs. Our results also demonstrate that substitution of PriB_His312 to Tyr leads to the synthesis of analogs prenylated at different positions than C6. This work contributes to understanding of how certain IPTs can access a challenging position in indole-derived compounds.

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.

Green One-Pot Syntheses of 2-Sulfoximidoyl-3,6-dibromo Indoles Using N-Br Sulfoximines as Both Brominating and Sulfoximinating Reagents

A green one-pot 2,3,6-trifunctionalization of N-alkyl/aryl indoles was achieved by adding three equivalents of N-Br sulfoximine to the indole solution. A variety of 2-sulfoximidoyl-3,6-dibromo indoles were prepared with 38-94% yields using N-Br sulfoximines as both brominating and sulfoximinating reagents. Based on the results of controlled experiments, we propose that a radical substitution involving 3,6-dibromination and 2-sulfoximination occurs in the reaction process. This is first time that 2,3,6-trifunctionalization of indole in one pot has been achieved.

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.

Palladium-Catalyzed Chemoselective Oxygenation of C(sp2)-H and C(sp3)-H Bonds in Isatins

The palladium-catalyzed chemoselective C(sp²)-H and C(sp³)-H bond oxygenation of substituted isatin derivatives is reported. This mild protocol exhibits the C5 C(sp²)-H oxygenation of isatins through electrophilic intermolecular C-H palladation in concentrated solutions using PhI(OAc)₂ or Selectfluor as an oxidant, whereas it exhibits-N-CH₃ C(sp³)-H oxygenation in dilute solutions via carbonyl-assisted intramolecular palladation in the presence of K₂S₂O₈. This oxygenation reaction provides a direct and unified approach for synthesizing diverse oxygenated isatins with sensitive functionalities, including biorelevant compounds.

Recent Advances on Direct Functionalization of Indoles in Aqueous Media

Indoles and their derivatives have dominated a significant proportion of nitrogen-containing heterocyclic compounds and play an essential role in synthetic and medicinal chemistry, pesticides, and advanced materials. Compared with conventional synthetic strategies, direct functionalization of indoles provides straightforward access to construct diverse indole scaffolds. As we enter an era emphasizing green and sustainable chemistry, utilizing environment-friendly solvents represented by water demonstrates great potential in synthesizing valuable indole derivatives. This review aims to depict the critical aspects of aqueous-mediated indoles functionalization over the past decade and discusses the future challenges and prospects in this fast-growing field. For the convenience of readers, this review is classified into three parts according to the bonding modes (C-C, C-N, and C-S bonds), which focus on the diversity of indole derivatives, the prominent role of water in the chemical process, and the types of catalyst systems and mechanisms. We hope this review can promote the sustainable development of the direct functionalization of indoles and their derivatives and the discovery of novel and practical organic methods in aqueous phase.

Electrophile-Promoted Nucleophilic Cyclization of 2-Alkynylindoles to Give 4-Substituted Oxazinoindolones

A method for the synthesis of 4-organoselanyl oxazinoindolone derivatives by the cascade cyclization of N-(alkoxycarbonyl)-2-alkynylindoles using iron(III) chloride and diorganyl diselenides as promoters was developed. This protocol was applied to a series of N-(alkoxycarbonyl)-2-alkynylindoles containing different substituents. The reaction conditions also tolerated a variety of diorganyl diselenides having both electron donating and electron withdrawing groups. However, the reaction did not work for diorganyl disulfides and ditellurides. The reaction mechanism seems to proceed via an ionic pathway and the cooperative action between iron(III) chloride and diorganyl diselenides is crucial for successful cyclization. We also found that using the same starting materials, by simply changing the electrophilic source to iodine, led to the formation of 4-iodo-oxazinoindolones. The high reactivity of Csp² -Se and Csp² -I bonds were tested under cross-coupling conditions leading to the preparation of a new class of functionalized indole derivatives. In addition, the absorption, emission and electrochemical properties of 4-organoselanyl oxazinoindolones showed an important relationship with the substituents of the aromatic rings. The advantages of the methodology include the use of electrophilic to promote the cyclization reaction and functionalization of the indole ring, and the electronic properties presented by the prepared compounds can be exploited as probes, analyte detectors and optical materials.

Rhodium(III)-catalyzed regioselective C2-alkenylation of indoles with CF3-imidoyl sulfoxonium ylides to give multi-functionalized enamines using a migratable directing group

A rhodium(III)-catalyzed regioselective C2-alkenylation of indoles for the construction of α-CF₃ substituted enamines has been developed, which utilizes CF₃-imidoyl sulfoxonium ylides (TFISYs) as alkenylating agents for the first time. A wide array of indolyl- and trifluoromethyl-decorated enamine derivatives have been assembled in moderate to good yields.

Increased catalytic activity through ZnMo7O24/g-C3N4 heterostructured assemblies for greener indole condensation reaction at room temperature

As an economical conjugated polymer, graphitic carbon nitride (g-C3N4) has recently attracted much attention due to its exciting chemical and thermal stability and easy availability. Herein, we constructed a metal-coordinated graphitic carbon nitride (M-g-C3N4) catalyst through simple impregnation and calcination methods and used it as a new heterogeneous catalyst for the efficient synthesis of bis (indolyl) methanes and trisindolines under mild conditions. This reaction is performed efficiently in water as an environmentally friendly solvent at ambient conditions. The ZnMo7O24/g-C3N4 nanocomposite was synthesized by a simple method by immobilizing Mo7O24(NH4)6·4H2O and ZnCl2 on the surface of g-C3N4 under hydrothermal conditions. It was characterized by FT-IR, EDS, and electronic scanning microscopy (SEM). The metal doping of Mo and Zn on the surface of graphitic carbon nitride leads to the formation of a green catalyst that gives good to excellent yields of products in short reaction times with an easy working procedure. In addition, the ZnMo7O24/g-C3N4 catalyst could be reused at least five runs without apparent loss of efficiency.

Mechanistic Insights into Cobalt-Catalyzed Regioselective C4-Alkenylation of 3-Acetylindole: A Detailed Theoretical Study

A detailed mechanistic study of Co(III)-catalyzed C4-alkenylation of 3-acetylindole (1a) was done based on calculations at density functional theory (DFT) and correlated wave function levels. The whole catalytic cycle consists of four steps: C-H activation, olefin insertion, β-hydride elimination, and regeneration of the catalyst. The theoretical results support olefin insertion as the rate-determining step leading to the experimentally observed regioselectivity of the C4 site over the C2 site. By the analysis of three-dimensional (3D) geometries and the NCl plot, the preference for the C4 site over the C2 site could be attributed to the weaker repulsive interaction between the indole moiety and olefin in the transition states of the olefin insertion step for the former. The reliability of the theoretical mechanistic results is further confirmed through the DFT calculation of other related indole derivatives and olefin substrates.

Selective Iron Catalyzed Synthesis of N-Alkylated Indolines and Indoles

Whereas iron catalysts usually promote catalyzed C3-alkylation of indole derivatives via a borrowing-hydrogen methodology using alcohols as the electrophilic partners, this contribution shows how to switch the selectivity towards N-alkylation. Thus, starting from indoline derivatives, N-alkylation was efficiently performed using a tricarbonyl(cyclopentadienone) iron complex as the catalyst in trifluoroethanol in the presence of alcohols leading to the corresponding N-alkylated indoline derivatives in 31-99 % yields (28 examples). The one-pot, two-step strategy for the selective N-alkylation of indolines is completed by an oxidation to give the corresponding N-alkylated indoles in 31-90 % yields (15 examples). This unprecedented oxidation methodology involves an iron salt catalyst associated with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) and a stoichiometric amount of t-BuOOH at room temperature.