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.

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.

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.

Synthesis of Indoles via Intermolecular and Intramolecular Cyclization by Using Palladium-Based Catalysts

As part of natural products or biologically active compounds, the synthesis of nitrogen-containing heterocycles is becoming incredibly valuable. Palladium is a transition metal that is widely utilized as a catalyst to facilitate carbon-carbon and carbon-heteroatom coupling; it is used in the synthesis of various heterocycles. This review includes the twelve years of successful indole synthesis using various palladium catalysts to establish carbon-carbon or carbon-nitrogen coupling, as well as the conditions that have been optimized.

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.

Indoles from Alkynes and Aryl Azides: Scope and Theoretical Assessment of Ruthenium Porphyrin-Catalyzed Reactions

A symbiotic experimental/computational study analyzed the Ru(TPP)(NAr)₂ -catalyzed one-pot formation of indoles from alkynes and aryl azides. Thirty different C₃ -substituted indoles were synthesized and the best performance, in term of yields and regioselectivities, was observed when reacting ArC≡CH alkynes with 3,5-(EWG)₂ C₆ H₃ N₃ azides, whereas the reaction was less efficient when using electron-rich aryl azides. A DFT analysis describes the reaction mechanism in terms of the energy costs and orbital/electronic evolutions; the limited reactivity of electron-rich azides was also justified. In summary, PhC≡CH alkyne interacts with one NAr imido ligand of Ru(TPP)(NAr)₂ to give a residually dangling C(Ph) group, which, by coupling with a C(H) unit of the N-aryl substituent, forms a 5+6 bicyclic molecule. In the process, two subsequent spin changes allow inverting the conformation of the sp² C(Ph) atom and its consequent electrophilic-like attack to the aromatic ring. The bicycle isomerizes to indole via a two-step outer sphere H-migration. Eventually, a 'Ru(TPP)(NAr)' mono-imido active catalyst is reformed after each azide/alkyne reaction.

Synthesis of Indoles through Domino Reactions of 2-Fluorotoluenes and Nitriles

Indoles are essential heterocycles in medicinal chemistry, and therefore, novel and efficient approaches to their synthesis are in high demand. Among indoles, 2-aryl indoles have been described as privileged scaffolds. Advanced herein is a straightforward, practical, and transition-metal-free assembly of 2-aryl indoles. Simply combining readily available 2-fluorotoluenes, nitriles, LiN(SiMe₃ )₂ , and CsF enables the generation of a diverse array of indoles (38 examples, 48-92 % yield). A range of substituents can be introduced into each position of the indole backbone (C4 to C7, and aryl groups at C2), providing handles for further elaboration.

Carboxylic Acid-Promoted Single-Step Indole Construction from Simple Anilines and Ketones via Aerobic Cross-Dehydrogenative Coupling

The cross-dehydrogenative coupling (CDC) reaction is an efficient strategy for indole synthesis. However, most CDC methods require special substrates, and the presence of inherent groups limits the versatility for further transformation. A carboxylic acid-promoted aerobic catalytic system is developed herein for a single-step synthesis of indoles from simple anilines and ketones. This versatile system is featured by the broad substrate scope and the use of ambient oxygen as an oxidant and is convenient and economical for both laboratory and industry applications. The existence of the labile hydrogen at C-3 and the highly transformable carbonyl at C-2 makes the indoles versatile building blocks for organic synthesis in different contexts. Computational studies based on the density functional theory (DFT) suggest that the rate-determining step is carboxylic acid-assisted condensation of the substrates, rather than the functionalization of aryl C-H. Accordingly, a pathway via imine intermediates is deemed to be the preferred mechanism. In contrast to the general deduction, the in situ formed imine, instead of its enamine isomer, is believed to be involved in the first ligand exchange and later carbopalladation of the α-Me, which shed new light on this indolization mechanism.

Computational Characterization of the Mechanism for the Oxidative Coupling of Benzoic Acid and Alkynes by Rhodium/Copper and Rhodium/Silver Systems

DFT calculations were applied to study the oxidative coupling between benzoic acid and 1-phenyl-1-propyne catalyzed by [CpRhCl₂ ]₂ (Cp=cyclopentadienyl) by using either Cu(OAc)₂ (H₂ O) or Ag(OAc) as the terminal oxidant, a process that has been experimentally shown to have subtleties related to regioselectivity (placement of the phenyl substituent of the alkyne in the isocoumarin product) and chemoselectivity (isocoumarin or naphthalene derivatives). Calculations reproduced the experimental results and showed the involvement of the oxidant throughout the catalytic cycle. The regioselectivity was found to be decided in the alkyne insertion step, in particular by the relative arrangement of the two phenyl groups. The high chemoselectivity towards isocoumarin associated to Cu(OAc)₂ (H₂ O) could be explained by the fact that the copper moiety blocks the CO₂ extrusion pathway, which would lead to naphthalene derivatives, something that does not happen if Ag(OAc) is used.

Chloroacetate Promotes and Participates in the Oxidative Annulation of Pyridines/Isoquinoline by Using Oxygen as the Oxidant

The aerobic oxidative annulation of chalcones, pyridines/isoquinoline and ethyl chloroacetate to indolizines was achieved by cascade reaction. Various functional groups on chalcones were tolerated. And different pyridines derivatives could also be suitable substrates. Ethyl chloroacetate is an essential component in participating of the oxidative annulation process. Overall, this protocol is very practical and efficient by using molecular oxygen as oxidant with high selectivity for the annulation product.

Electrochemical C-H/N-H Functionalization for the Synthesis of Highly Functionalized (Aza)indoles

Indoles and azaindoles are among the most important heterocycles because of their prevalence in nature and their broad utility in pharmaceutical industry. Reported herein is an unprecedented noble-metal- and oxidant-free electrochemical method for the coupling of (hetero)arylamines with tethered alkynes to synthesize highly functionalized indoles, as well as the more challenging azaindoles.

Cationic Cobalt(III) Catalyzed Indole Synthesis: The Regioselective Intermolecular Cyclization of N-Nitrosoanilines and Alkynes

The unique regioselectivity and reactivity of cobalt(III) in the direct cyclization of N-nitrosoanilines with alkynes for the expedient synthesis of N-substituted indoles is demonstrated. In the presence of a cobalt(III) catalyst, high regioselectivity was observed when using unsymmetrical meta-substituted N-nitrosoanilines. Moreover, internal alkynes bearing electron-deficient groups, which are almost unreactive in the [Cp*Rh(III)]-catalyzed system, display good reactivity in this transformation.

Synthesis of indoles and tryptophan derivatives via photoinduced nitrene C–H insertion

Functionalized indoles and tryptophans can be obtained from stannylated alkenes and o-iodoanilines via Stille coupling.