Rhodium(III)-Catalyzed Redox-Neutral CH Annulation of Arylnitrones and Alkynes for the Synthesis of Indole Derivatives

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.

Rh(III)-Catalyzed Spiroannulation Reaction of N-Aryl Nitrones with 2-Diazo-1,3-indandiones: Synthesis of Spirocyclic Indole-N-oxides and Their 1,3-Dipolar Cycloaddition with Maleimides

An efficient strategy for the preparation of spirocyclic indole-N-oxide compounds through a Rh(III)-catalyzed [4 + 1] spiroannulation reaction of N-aryl nitrones with 2-diazo-1,3-indandiones as C1 synthons under extremely mild conditions is presented. From this reaction, 40 spirocyclic indole-N-oxides were easily obtained in up to 98% yield. In addition, the title compounds could be successfully used for the construction of structurally intriguing maleimide-containing fused polycyclic scaffolds via a diastereoselective 1,3-dipolar cycloaddition reaction with maleimides.

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.

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.

Iridium-Catalyzed Redox-Neutral C2 and C3 Dual C-H Functionalization of Indoles with Nitrones toward 7H-Indolo[2,3-c]quinolines

An iridium-catalyzed redox-neutral C-2 and C-3 dual C-H functionalization of indoles with nitrones has been developed, furnishing a range of 7H-indolo[2,3-c]quinolines with high efficiency and regioselectivity under mild reaction conditions. Notably, sequential multiple C-H bond cleavage and C-C bond formation constitute the key events of this process, in which nitrone serves as a building block and an oxidant. Distinct from the previous methods toward 7H-indolo[2,3-c]quinolines, this newly developed reaction features readily available substrates, operational simplicity, broad scope, good to high efficiency, and excellent regioselectivity.

Gold(i)-catalyzed redox transformation of o-nitroalkynes with indoles for the synthesis of 2,3′-biindole derivatives

A gold(i)-catalyzed cascade reaction of o-nitroalkynes with indoles has been reported for the rapid assembly of 2-indolyl indolone N-oxides, which exhibit high anticancer potency against SCLC cells.

Copper(ii)-catalyzed synthesis of multisubstituted indoles through sequential Chan–Lam and cross-dehydrogenative coupling reactions

Starting from arylboronic acids and ester (Z)-3-aminoacrylates, one-pot syntheses of diverse indole-3-carboxylic esters have been described through copper(ii)-catalyzed sequential Chan–Lam N-arylation and cross-dehydrogenative coupling (CDC) reactions. The initial Chan–Lam arylation can proceed in DMF at 100 °C for 24 h to give ester (Z)-3-(arylamino)acrylate intermediates in the presence of Cu(OAc)₂/tri-tert-butylphosphine tetrafluoroborate, a catalytic amount of myristic acid as the additive, KMnO₄ and KHCO₃. Sequentially, these in situ arylated intermediates can undergo an intramolecular oxidative cross-dehydrogenative coupling process in mixed solvents (DMF/DMSO = 2 : 1) at 130 °C to give C3-functionalized multi-substituted indole derivatives.

One-pot syntheses of diverse indole-3-carboxylic esters have been described through copper(ii)-catalyzed sequential oxidative Chan–Lam N-arylation and cross-dehydrogenative coupling (CDC) reaction.

Recent advances in the synthesis of indoles from alkynes and nitrogen sources

This review highlights ten years of success in the synthesis of indoles using alkynes and nitrogen sources as substrates.

Step and redox efficient nitroarene to indole synthesis

A step and redox efficient nitroarene to indole synthesis was herein developed, in sharp contrast to the rich literature on the construction of indoles. Elemental Zinc was found to be best terminal reductant.

Rh(iii)-catalyzed spiroannulation of 3-arylquinoxalin-2(1H)-ones with alkynes: practical access to spiroquinoxalinones

The use of imines as a H acceptor for Rh(iii)-catalyzed spirocyclization of 3-arylquinoxalinones and alkynes via a C–H functionalization/[3 + 2] annulation sequence has been achieved.

Reaching Green: Heterocycle Synthesis by Transition Metal-Catalyzed C-H Functionalization in Sustainable Medium

Catalytic C-H functionalization has emerged as an efficient alternative to traditional coupling reactions. However, some of these reactions depend on environmentally harmful solvents, weakening the overall green nature of these methods. As organic processes consume large amount of solvents, the use of less harmful solvents enhance the sustainability of these reactions. Herein, we present an overview of transition metal-catalyzed C-H functionalization reactions for the synthesis of heterocycles in sustainable solvents based on CHEM21 solvent selection guide.

Cascade intramolecular imidoylation and C–H activation/annulation of benzimidoyl chlorides with alkynes: one-pot synthesis of 7H-dibenzo[de,h]quinoline analogues

Described herein is a cascade Lewis acid-promoted intramolecular Friedel–Crafts-type imidoylation and Rh(iii)-catalyzed C–H activation/annulation of benzimidoyl chlorides and alkynes, providing a divergent synthetic shortcut to 7H-dibenzo[de,h]quinoline analogues.

Recent Progress in the Transition Metal Catalyzed Synthesis of Indoles

Indole is the most frequently found heterocyclic core structures in pharmaceuticals, natural products, agrochemicals, dyes and fragrances. For about 150 years, chemists were absorbed in finding new and easier synthetic strategies to build this nucleus. Many books and reviews have been written, but the number of new syntheses that appear in the literature, make necessary continuous updates. This reviews aims to give a comprehensive overview on indole synthesis catalyzed by transition metals appeared in the literature in the years 2016 and 2017.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

RhIII-Catalyzed site-selective amidation with nitrone as a traceless directing group: an approach to functionalized arylaldehydes

A unique Rh^III-catalyzed C–H amidation of nitrones with 1,4,2-dioxazol-5-ones has been developed, providing a new approach to functionalized arylaldehydes.

Rhodium-catalyzed redox-neutral coupling of phenidones with alkynes

A switchable synthesis ofN-substituted indole derivatives from phenidonesviarhodium-catalyzed redox-neutral C–H activation has been achieved.

Silver-catalyzed cyclization of nitrones with 2-azetine: a radical approach to 2,3-disubstituted quinolines

A silver-catalyzed intermolecular cyclization of nitrones with 2-azetine has been developed for the synthesis of 2,3-disubstituted quinolines under mild conditions.

Recent advances using [Cp*Co(CO)I2] catalysts as a powerful tool for C–H functionalisation

This perspective highlights recent applications of Cp*Co^III catalysts in C–H functionalisation protocols, exemplifying both terminal couplings and heterocycle formation.

Three-step synthesis of 2,5,7-trisubstituted indoles from N-acetyl-2,4,6-trichloroaniline using Pd-catalyzed site-selective cross-coupling

A facile three-step synthesis of 2,5,7-trisubstituted indoles was realized by using Cy-DHTP as an effective ligand for site-selective cross couplings.

Silver-mediated oxidative annulation of N-arylthio succinimides with alkynes: direct access to benzo[b]thiophenes

A convenient synthetic route to 2,3-diarylbenzo[b]thiophene derivatives via Ag-catalyzed intermolecular oxidative cyclization between bench-stable N-arylthio succinimides and unactivated internal alkynes is demonstrated herein.

Cu(i) mediated Kinugasa reactions of α,β-unsaturated nitrones: a facile, diastereoselective route to 3-(hydroxy/bromo)methyl-1-aryl-4-(-styryl)azetidin-2-ones

A facile and highly diastereoselective synthesis ofcis-3-(hydroxyl/bromo)methyl-1-aryl-4-(-styryl)azetidin-2-onesviacopper(i) mediated Kinugasa reactions of previously unexplored functionalized α,β-unsaturated nitrones.

Intramolecular addition of diarylmethanols to imines promoted by KOt-Bu/DMF: a new synthetic approach to indole derivatives

KOt-Bu/DMF promoted intramolecular addition of diarylmethanols to imines was developed.

Rh(iii)-catalyzed chemoselective C–H functionalizations of tertiary aniline N-oxides with alkynes

Two novel Rh(iii)-catalyzed chemoselective functionalizations of tertiary aniline N-oxides with alkynes, including annulation via the sequential C(sp²)–H and C(sp³)–N activation and an O-atom transfer (OAT) process, have been described.