Ruthenium(II)-Catalyzed Redox-Neutral [3+2] Annulation of Indoles with Internal Alkynes via C–H Bond Activation: Accessing a Pyrroloindolone Scaffold

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.

Iron-Catalyzed Oxidative Rearrangement of Cyclopropanone Hemiaminals: General Access to Pyrroloindolones from Indoles

A concise synthetic approach to medicinally relevant pyrroloindolones and related fused heterocycles is reported via the diastereoselective N-addition of unprotected indoles to readily accessible cyclopropanone equivalents. The resulting stable hemiaminals are shown to smoothly rearrange to pyrroloindolones in mild conditions using Fe(III) catalysis in the presence of inexpensive ammonium persulfate as a stoichiometric oxidant. Experimental evidence points toward the formation of a β-carboxylic radical intermediate prone to cyclization and oxidative rearomatization as the operative mechanistic pathway.

Synthesis of Indole-Fused Oxepines via C-H Activation Initiated Diastereoselective [5 + 2] Annulation of Indoles with 1,6-Enynes

A rhodium-catalyzed diastereoselective formal [5 + 2] annulation of indoles with cyclohexadienone-containing 1,6-enynes has been established via indole 2,3-difunctionalization. The reaction, probably proceeding through tandem indole C2-H alkenylation and intramolecular Friedel-Crafts alkylation relay, provides rapid construction of indole-fused oxepines in good to excellent yields with a broad substrate scope. This method also features concomitant construction of cis-hydrobenzo[b] oxepine scaffolds, a core unit found in numerous natural products of important biological activities.

The C-H functionalization of N-alkoxycarbamoyl indoles by transition metal catalysis

Indole and its congeners are ubiquitous nitrogen-containing organic scaffolds present in a plethora of natural products, marketed drugs, and other organic functional molecules. Recent years have witnessed tremendous advances in the diversification of this motif and its biological applications via transition-metal-catalyzed auxiliary assisted site-selective inert C-H functionalization. In this burgeoning field, N-methoxy/ethoxy/pivaloxy amide functionality has emerged as a most potent auxiliary/DG (directing group) for a wide range of C-C and C-heteroatom bond formations, providing a new advance for forging structurally fabricated polycyclic indole frameworks. This review aims to highlight evolved transformations, like arylation, alkylation, alkenylation, allylation, amidation, difluorovinylation, deuteration, hydroarylation, etc., and the applications of N-alkoxycarbamoyl indole derivatives made within the period of 2014-August 2021. Additionally, explicit mechanistic underpinnings have also been provided in the appropriate places.

Recent advances in the synthesis of pyrrolo[1,2-a]indoles and their derivatives

The pyrrolo[1,2-a]indole unit is a privileged heterocycle found in numerous natural products and has been shown to exhibit diverse pharmacological properties. Thus, recent years have witnessed immense interest from the synthesis community on the synthesis of this scaffold. In light of the ever-increasing demand for pyrrolo[1,2-a]indoles in drug discovery, this review provides an overview of recent synthesis methods for the preparation of pyrrolo[1,2-a]indoles and their derivatives. The mechanistic pathway and stereo-electronic factors affecting the yield and selectivity of the product are briefly explained. Furthermore, we have attempted to demonstrate the utility of the developed methods in the synthesis of bioactive molecules and natural products, wherever offered.

Redox-Neutral Rhodium(III)-Catalyzed Chemospecific and Regiospecific [4+1] Annulation between Indoles and Alkenes for the Synthesis of Functionalized Imidazo[1,5-a]indoles

Exploiting internal alkenes embedded with an oxidizing function/leaving group as a rare and unconventional one-carbon unit, a redox-neutral rhodium(III)-catalyzed chemo- and regiospecific [4+1] annulation between indoles and alkenes for the synthesis of functionalized imidazo[1,5-a]indoles has been achieved. Internal alkenes employed here can fulfill an unusual [4+1] annulation rather than normal [4+2] annulation/C-H alkenylation. This method is characterized by excellent chemo- and regioselectivity, broad substrate scope, good functional group tolerance, good to high yields, and redox-neutral conditions.

Rh(III)-Catalyzed Divergent Synthesis of Alkynylated Imidazo[1,5-a]indoles and α,α-Difluoromethylene Tetrasubstituted Alkenes

Herein, we report the divergent synthesis of alkynylated imidazo[1,5-a]indoles and α,α-difluoromethylene tetrasubstituted alkenes through Rh(III)-catalyzed [4 + 1] annulation/alkyne moiety migration and C-H alkenylation/DG migration, respectively. This protocol features tunable product selectivity, excellent chemo-, regio-, and stereoselectivity, broad substrate scope, moderate to high yields, good tolerance of functional groups, and mild redox-neutral conditions.

A Water-Soluble Rhenium(I) Catalyst for the Regio- and Stereoselective C(sp2)-H Alkenylation of N-Pyridyl-/N-Pyrimidylindole and the N-H Alkenylation of N-Pyrimidylaniline Derivatives with Ynamides

A water-soluble and low-valent rhenium(I) catalyst for the C2 alkenylation of N-pyridyl/N-pyrimidylindole derivatives with ynamides under mild conditions using water as the solvent has been described. The reaction of N-pyridyl/N-pyrimidyl indole with the ynamide afforded the C2-Z-selective alkenylation derivative as the sole product, and the reactions of N-pyrimidylanilines delivered the corresponding N-alkenylated product rather than the expected C-H alkenylation products in high yields under the same conditions.

Regioselective cascade annulation of indoles with alkynediones for construction of functionalized tetrahydrocarbazoles triggered by Cp*RhIII-catalyzed C–H activation

An efficient regioselective and stereoselective cascade annulation of indoles with alkynediones has been developed for construction of free (NH) tetrahydrocarbazoles with continuous quaternary carbons via Cp*Rh^III-catalyzed indole C2–H activation.

Ruthenium-Catalyzed C–H Activations for the Synthesis of Indole Derivatives

The synthesis of substituted indoles has received great attention in the field of organic synthesis methodology. C–H activation makes it possible to obtain a variety of designed indole derivatives in mild conditions. Ruthenium catalyst, as one of the most significant transition-metal catalysts, has been contributing in the synthesis of indole scaffolds through C–H activation and C–H activation on indoles. Herein, we attempt to present an overview about the construction strategies of indole scaffold and site-specific modifications for indole scaffold via ruthenium-catalyzed C–H activations in recent years.

Sulfoximines-Assisted Rh(III)-Catalyzed C-H Activation and Intramolecular Annulation for the Synthesis of Fused Isochromeno-1,2-Benzothiazines Scaffolds under Room Temperature

A mild and facile Cp*Rh(III)-catalyzed C–H activation and intramolecular cascade annulation protocol has been proposed for the furnishing of highly fused isochromeno-1,2-benzothiazines scaffolds using S-phenylsulfoximides and 4-diazoisochroman-3-imine as substrates under room temperature. This method features diverse substituents and functional groups tolerance and relatively mild reaction conditions with moderate to excellent yields. Additionally, retentive configuration of sulfoximides in the conversion has been verified.

The ruthenium(ii)-catalyzed C–H olefination of indoles with alkynes: the facile construction of tetrasubstituted alkenes under aqueous conditions

An environmentally-friendly and facile protocol for the construction of tetrasubstituted alkenes has been established with Ru(ii)-catalyzed C–H bond functionalizations under mild conditions.

Understanding electronic effects on carboxylate-assisted C-H activation at ruthenium: the importance of kinetic and thermodynamic control

Meta- and para-substituted 1-phenylpyrazoles (R-phpyz-H) react with [RuCl2(p-cymene)]2 in the presence of NaOAc to form cyclometallated complexes [M(R-phpyz)Cl(p-cymene)] (where R = NMe2, OMe, Me, H, F, CF3 and NO2). Experimental and DFT studies indicate that product formation can be reversible or irreversible depending on the substituents and the reaction conditions. Competition experiments show that the kinetic selectivity favours electron-donating substituents and correlate well with the Hammett parameter, giving a negative slope (ρ = -2.4) that is consistent with a cationic transition state. However, surprisingly, the thermodynamic selectivity is completely opposite, with substrates featuring electron-withdrawing groups being favoured. These trends are reproduced with DFT calculations that locate a rate-limiting transition state dominated by Ru-O bond dissociation and minimal C-H bond elongation. Detailed computational analysis of these transition states shows that C-H activation proceeds by an AMLA/CMD mechanism through a synergic combination of a C-H→Ru agostic interaction and C-HO H-bonding. NBO calculations also highlight a syndetic bonding term, and the relative weights of these three components vary in a complementary fashion depending on the nature of the substituent. With meta-substituted ligands H/D exchange experiments signal kinetically accessible ortho-C-H activation when R = NMe2, OMe and Me. This is also modelled computationally and the calculations highlight the kinetic relevance of the HOAc/Cl exchange that occurs post C-H bond cleavage, in particular with the bulkier NMe2 and Me substituents. Our study highlights that the experimental substituent effects are dependent on the reaction conditions and so using such studies to assign the mechanism of C-H activation in either stoichiometric or catalytic reactions may be misleading.

Cp*Rh(III)-Catalyzed C-H Bond Difluorovinylation of Indoles with α,α-Difluorovinyl Tosylate

Unprecedented Rh(III)-catalyzed C-H bond difluorovinylation of indoles has been successfully developed, and this method provided an example of direct difluorovinylation reaction through C-H bond activation which was rarely reported. In this context, N-ethoxycarbamoyl served as the directing group and 2,2-difluorovinyl tosylates were used for the construction of difluorovinyl-substituted indoles. This method provided a practical strategy for difluorovinylation of indoles with moderate to good yields and is characterized by the broad synthetic utility, mild conditions, and high efficiency.

Ruthenium(ii)-catalyzed selective C–H bond activation of imidamides and coupling with sulfoxonium ylides: an efficient approach for the synthesis of highly functional 3-ketoindoles

Ruthenium-catalyzed selective C–H bond activation of imidamides and annulation of sulfoxonium ylides were achieved, which afforded a series of 3-ketoindole derivatives in good yields, with good functional group compatibility.

Highly selective C–H bond activation of N-arylbenzimidamide and divergent couplings with diazophosphonate compounds: a catalyst-controlled selective synthetic strategy for 3-phosphorylindoles and 4-phosphorylisoquinolines

A highly efficient synthetic strategy to construct 3-phosphorylindole and 4-phosphorylisoquinoline was achieved.

Rhodium(III)-Catalyzed C-H Activation of α-Iminonitriles or α-Imino Esters and Cyclization with Acrylates to 2 H-Isoindoles

Rh(III) catalysts have played increasingly important roles in the activation of C-H bonds to build heterocyclic scaffolds. However, there are few reports on the more challenging synthesis of pharmaceutically important 2 H-isoindoles and fused 2 H-isoindoles. The process reported herein is an effective strategy to produce 2 H-isoindole or fused 2 H-isoindole derivatives via a Rh(III)-catalyzed transformation of α-iminonitriles or α-imino esters with acrylates.

Rhodium-Catalyzed [4 + 1] Cyclization via C-H Activation for the Synthesis of Divergent Heterocycles Bearing a Quaternary Carbon

The development of an efficient approach to construct fused polycyclic systems bearing a quaternary carbon center represents a great challenge to synthetic chemistry. Herein, we report a Rh(III)-catalyzed [4 + 1] annulation of propargyl alcohols with various heterocyclic scaffolds under an air atmosphere. Diverse fused heterocycles containing a quaternary carbon center were obtained in moderate to good yields. Additionally, this method features a high atom-economy, metal oxidant free, simple operation, and compatibility with various functional groups.

Palladium(II)-Catalyzed Mono- and Bis-alkenylation of N-Acetyl-2-aminobiaryls through Regioselective C-H Bond Activation

We developed palladium-catalyzed oxidative coupling of olefins with N-acyl 2-aminobiaryls through a sequence of ortho C-H bond activation/alkene insertion/reductive elimination. Furthermore, we controlled the selectivity of mono- and bis-alkenylation products with the solvent effect. The developed protocol was promising for a broad substrate scope ranging from activated olefins with a wide variety of functional groups to unactivated olefins.

Synthesis of indoles from aroyloxycarbamates with alkynes via decarboxylation/cyclization

Palladium-catalyzed decarboxylation/cyclization of aroyloxycarbamates with alkynes to construct C–N bonds.