Pd(OAc)2-catalyzed C–H activation of indoles: a facile synthesis of 3-cyanoindoles

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.

Recent advances in theoretical studies on transition-metal-catalyzed regioselective C-H functionalization of indoles

Indole compounds are widely found in natural products and drug candidates. The transition-metal-catalyzed regioselective C-H bond functionalization of indoles as the most efficient method for the synthesis of various functionalized indoles has been extensively studied in the past two decades due to its advantages of step economy and atom economy. In general, the catalysts included the transition-metals (Pd, Rh, Ru, Cu, Co, Fe, Zn, and Ga) and these reactions were accomplished with a remarkably wide range of coupling reagents for construction of various C-C and C-X (X = N, O, S) bonds. However, the general and important rules of the regioselectivity are not clear to date. Therefore, a comprehensive analysis through previous reported theoretical studies on transition-metal-catalyzed regioselective C-H bond functionalization of indoles was crucial and significant. In this review, we found that when the C-H bond activation process was the rate-determining step, the regioselectivity ordinarily occurred at the C7 or C4 positions (on benzene ring), and otherwise, the regioselectivity often occurred at C2 position (on pyrrole ring). For indoline substrates, the C-H bond functionalization occurred at the benzene ring. General rules of the regioselectivities for transition-metal-catalyzed C-H bond functionalization of indoles. This review collects major advances in the transition-metal-catalyzed C-H bond functionalization of indoles and indolines.

Organocatalytic dimensions to the C–H functionalization of the carbocyclic core in indoles: a review update

A review highlighting important research findings in remote C–H activation processes using effectual organocatalytic perspectives. The challenging indole carbocyclic ring positions were successfully accessed with proper regio- and stereocontrols.

Transition metal-catalyzed C–H functionalizations of indoles

This review summarises a wide range of transformations on the indole skeleton, including arylation, alkenylation, alkynylation, acylation, nitration, borylation, and amidation, using transition-metal catalyzed C–H functionalization as the key step.

Simple, novel, and efficient synthesis of ethyl 2-(2-(1H-indol-3-yl)benzo[d]thiazol-3(2H)-yl)-2-cyanoacetate or acetate via a three-component reaction

Ethyl 2-(2-(1 H-indol-3-yl)benzo[ d]thiazol-3(2 H)-yl)-2-cyanoacetate or acetate 6a–f as a series of novel compounds were synthesized from the reactions of benzothiazole, 2-methylbenzothiazole, or 2,5-dimethylbenzothiazole and ethyl bromocyanoacetate with indole derivatives. The reactions proceeded in acetone as a solvent under reflux for 5 h in good yields. The structures of new compounds were confirmed by 1H nuclear magnetic resonance, 13C nuclear magnetic resonance, infrared, elemental analyses, and mass spectroscopy.

Efficient construction of diverse 3-cyanoindoles under novel tandem catalysis

A novel and rapid construction of 3-cyanoindoles by palladium-catalyzed tandem reactions has been developed. "N-H" free unprotected, N-alkyl and N-aryl 3-cyanoindoles are obtained with good to excellent yields. The usefulness of this synthetic approach is further demonstrated by the successful synthesis of practical compounds such as the therapeutic estrogen receptor ligand A precursor. Mechanism study shows that the tandem catalysis exploits a Suzuki cross-coupling with subsequent base-induced isoxazole fragmentation, followed by the aldimine condensation.

The palladium-catalyzed direct C3-cyanation of indoles using acetonitrile as the cyanide source

The palladium-catalyzed C3-cyanation of indoles via direct C–H functionalization was achieved utilizing CH₃CN as the cyanide source through transition-metal-catalyzed C–CN bond cleavage.

Theoretical studies on Rh(iii)-catalyzed regioselective C-H bond cyanation of indole and indoline

Density functional theory calculations were carried out to study the reaction mechanism of the Rh(iii)-catalyzed regioselective C-H cyanation of indole and indoline with N-cyano-N-phenyl-para-methylbenzenesulfonamide (NCTS). This mechanism involves four major steps: C-H activation, cyano group insertion, β-N elimination, and regeneration of active species. How different indole and indoline substrates affect the regioselectivity of C-H bond cyanation has been examined and analyzed in detail. Our calculation results indicate that the regioselectivity of C-H bond cyanation of indole depends on the nucleophilicity of carbon atoms in C-Rh(iii) bonds to the cyano group. For indoline, it can be attributed to the different hybridization platforms of the C-H bond activation.

A Pd(ii)/Mg–La mixed oxide catalyst for cyanation of aryl C–H bonds and tandem Suzuki–cyanation reactions

We report direct cyanation of aromatic C–H bonds and tandem Suzuki–cyanation reactions catalyzed by a heterogeneous palladium impregnated magnesium–lanthanum mixed oxide catalyst.

Copper-mediated cyanation of indoles and electron-rich arenes using DMF as a single surrogate

The copper-mediated cyanation of indoles with DMF as a single surrogate of “CN” has been realized.

Microwave-assisted synthesis of 3-nitroindoles from N-aryl enamines via intramolecular arene-alkene coupling

A variety of N-aryl β-nitroenamines were effectively transformed into 3-nitroindoles in good yields and with complete regioselectivity via a rapid microwave (μW) assisted intramolecular arene-alkene coupling reaction. This report further demonstrates the versatility of this method by constructing 3-carboalkoxy- and 3-cyanoindoles. Optimization data, substrate scope, and applications are discussed.

Copper-mediated selective cyanation of indoles and 2-phenylpyridines with ammonium iodide and DMF

Copper-mediated regioselective cyanation of indoles and 2-phenylpyridines was developed by using ammonium iodide and DMF as the combined source of a cyano unit under "Pd-free" conditions. Mechanistic studies indicate that the reaction of indoles proceeds through a two-step sequence: electrophilic initial iodination and then cyanation. The cyanation has a broad substrate scope, high functional group tolerance, and excellent regioselectivity.