Electronic Nature of Ketone Directing Group as a Key To Control C-2 vs C-4 Alkenylation of Indoles

Pd-Catalyzed Regioselective Deuteration of Indole's C4-Position with Transient Directing Groups

As a representative scaffold of alkaloids, indoles have been extensively subjected to deuteration, but the regioselective C4 labeling has not been achieved due to its low reactivity. In this work, a Pd-catalyzed deuterium labeling at the indole's C4 position has been developed under the strategy of transient directing, using D2O as a deuterium source. The substituent effect is found to be crucial in facilitating this H/D exchange process, where the reversing C-D bond formation favors an electron-enriched ligation contrary to its C-H halogenation counterpart.

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.

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.

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.

Rhodium(III)-catalyzed regioselective C(sp2)-H activation of indoles at the C4-position with iodonium ylides

Herein, we report a Rh(III)-catalyzed C4-selective activation of indoles by using iodonium ylides as carbene precursors. This protocol proceeded under redox neutral reaction conditions and provided important coupling products with good tolerance of functional groups and high yields. In addition, one-pot synthesis and scale-up and mechanistic studies were also conducted.

Pd(II)-Catalyzed Transient Directing Group-Assisted Regioselective Diverse C4-H Functionalizations of Indoles

The development of a rational strategy for achieving site-selective C4-H halogenation of indoles is an appealing yet challenging task. Herein, we disclose a Pd(II)-catalyzed transient directing group (TDG)-assisted methodology for realizing C4 chlorination/bromination of indoles employing glycine as the TDG and NFSI as a bystanding oxidant. The use of inexpensive and commercially available CuX₂ as the halide source is the key highlight of this protocol. Furthermore, the TDG methodology was also extended to accessing C4 acetoxylated indoles employing acetic acid as the acetate source and 1-fluoro-2,4,6-trimethylpyridinium triflate as the oxidant.

Pd(II)/Lewis acid catalyzed regioselective olefination of indole with dioxygen

Transition metal ion catalyzed indole olefination through C-H activation is a convenient protocol to synthesize versatile bioactive vinylindole compounds; however, in most cases, stoichiometric amounts of oxidants were necessary to accomplish the catalytic cycle. The present study describes a Pd(II)/LA (LA: Lewis acid) catalyzed indole olefination with dioxygen as the sole oxidant. The olefination reaction with electron-rich olefins proceeded smoothly through the pyrrolyl N-carboxamide group directed remote C-H activation at the C3 position of the indole with the Pd(II)/LA catalyst, whereas Pd(II) alone was a very sluggish catalyst under identical conditions. For the electron-deficient olefins, the directing N-carboxamide group was not essential for olefination with this Pd(II)/LA catalyst, demonstrating a different olefination pathway from that of electron-rich olefins. Remarkably, ¹H NMR kinetics disclosed that olefination proceeded much faster with electron-rich olefins than with electron-deficient ones.

Rh(iii)-Catalyzed dienylation and cyclopropylation of indoles at the C4 position with alkylidenecyclopropanes

Herein, we report a Rh(iii)-catalyzed C–H functionalization of indoles at the C4 position with alkylidenecyclopropanes (ACPs).

Site-selective C-H functionalization to access the arene backbone of indoles and quinolines

The site-selective C-H bond functionalization of heteroarenes can eventually provide chemists with great techniques for editing and building complex molecular scaffolds. During the past decade, benzo-fused N-heterocycles such as indoles and quinolines have been among the most widely investigated organic templates. Early developments have led to site-selective C-H bond functionalization on the pyrrole and pyridine cores of indoles and quinolines; however, C-H functionalization on the benzenoid ring has remained a great challenge in catalysis. In this review, we elaborate on recent developments in the highly challenging functionalization of C-H bonds on the less-reactive benzenoid core of indoles and quinolines. These findings are mainly described as selective directing group assisted strategies, remote C-H functionalization techniques and their reaction mechanisms. The underlying principle in each strategy is elucidated, which aims to facilitate the design of a more advanced structure of heterocycles based on bioactive molecules, synthetic drugs, and material aspects. Moreover, the challenges and perspectives for catalytic C-H functionalization to access the arene backbone of indoles and quinolines are also proposed in the conclusion section.

Regioselective C-H Functionalization of the Six-Membered Ring of the 6,5-Fused Heterocyclic Systems: An Overview

The regioselective C-H functionalization of the five-membered ring of the 6,5-fused heterocyclic systems is nowadays well documented due to its high reactivity compared to the six-membered ring. So, developing new procedures of C-H functionalization of the six-membered ring "by thinking out of the box" is extremely challenging, which explains the limited number of reports published to date. This review paper aims to highlight advances achieved in this emerging chemistry research and discusses recently reported methods.

Ruthenium(ii)-catalyzed regioselective direct C4- and C5-diamidation of indoles and mechanistic studies

A ruthenium(ii)-catalyzed regioselective direct diamidation of 3-carbonylindoles at the C4- and C5-positions using various dioxazolones is described. This novel protocol allows for the effective installation of two amide groups on the benzene ring in indole. A remarkably broad substrate scope, excellent functional group tolerance, and mild reaction conditions are notable features of this protocol. Further explorations reveal that benzo[b]thiophene-3-carboxaldehyde is a viable substrate and affords its corresponding diamidation products. The diamido indoles are further converted into various functionalized products and used as sensors for metal ion detection. Density functional theory studies are also conducted to propose a reaction mechanism and provide a detailed understanding of the regioselectivity observed in the reaction.

Ruthenium(ii)-catalyzed regioselective C4-/C5-diamidation of 3-carbonylindoles is described and a DFT study is conducted to understand the observed regioselectivity and the mechanism.

Palladium-Catalyzed Direct C2-Biarylation of Indoles

Biaryl and indole units are important structural motifs in several bioactive molecules and functional materials. We have accomplished straightforward access to C2-biarylated indole derivatives through palladium-catalyzed C-H activation strategy with a broad range of substrate scope in yields of 24 to 92%. Besides, the UV/visible absorption and fluorescence properties of the ensuing products were explored. The calculated higher dihedral angle and rotational barrier values for the selected C2-biarylated indoles show that these compounds may display atropisomerism at room temperature.

Palladium-Catalyzed Remote C-H Phosphonylation of Indoles at the C4 and C6 Positions by a Radical Approach

Palladium-catalyzed direct C-H activation of indole benzenoid moiety has been achieved in the past decade. However, palladium-catalyzed remote C-H activation of indoles is rare. Herein, we report a challenging palladium-catalyzed remote C4-H phosphonylation of indoles by a radical approach. The method provides access to a series of C4-phosphonylated indoles, including tryptophan and tryptophan-containing dipeptides, which are typically inaccessible by direct C4-H activation due to its heavy reliance on C3 directing groups. Notably, unexpected C6-phosphonylated indoles were obtained through blocking of the C4 position. The preliminary mechanistic studies indicated that the reactions may proceed via a C7-palladacycle/remote-activation process. Based on the strategy, examples of remote C4-H difluoromethylation with BrCF₂ COOEt are also presented, suggesting that the strategy may offer a general blueprint for other cross-couplings.

Tandem Iridium-Catalyzed Decarbonylative C-H Activation of Indole: Sacrificial Electron-Rich Ketone-Assisted Bis-arylsulfenylation

Described herein is a decarbonylative tandem C–H bis-arylsulfenylation of indole at the C2 and C4 C–H bonds through the use of pentamethylcyclopentadienyl iridium dichloride dimer ([Cp*IrCl₂]₂) catalyst and disulfides. A new sacrificial electron-rich adamantoyl-directing group facilitates indole C–H bis-functionalization with a traceless in situ removal. Various differently substituted disulfides can be easily accommodated in this reaction by a coordination to Ir(III) through the formation of six- and five-membered iridacycles at the C2 and C4 positions, respectively. Mechanistic studies show that a C–H activation-induced C–C activation is involved in the catalytic cycle.

Theoretical studies on the mechanism of Pd2+-catalyzed regioselective C-H alkylation of indole with MesICH2CF3OTf

The reaction mechanism of Pd²⁺-catalyzed regioselective C-H alkylation of indole with MesICH₂CF₃OTf has been investigated by the density functional theory calculations. The reaction mechanism mainly contains four steps: C-H activation, oxidative addition, reductive elimination, and ligands substitution. From our calculations, we find that the C-H activation step was realized by the acetate anion (^-OAc) assisted concerted metalation deprotonation (CMD) process and the transition state of C-H activation process is a square planar configuration. Moreover, the calculation results suggest that the regioselectivity of C-H bond alkylation of indole with MesICH₂CF₃OTf can be ascribed to the different stability of the CMD transition states in C-H activation step and the relative stabilities of deprotonated intermediates.

Synthesis of 4-Alkylindoles from 2-Alkynylanilines via Dearomatization- and Aromatization-Triggered Alkyl Migration

A simple method for rapid synthesis of 4-alkylindoles from 2-alkynylanilines was reported. The protocol involves an oxidative dearomatization and an aromatization triggered regioselective alkyl migration. A range of alkyl groups including linear, branched, or cycloalkyl groups can be introduced into the C4 position of indole.

Rhodium(III)-Catalyzed Aerobic Oxidative C-H Olefination of Unsaturated Acrylamides with Unactivated Olefins

A rhodium(III)-catalyzed aerobic oxidative cross-coupling of acrylamides with unactivated alkenes via vinylic C-H activation has been developed. The present cross-coupling reaction was examined with a variety of differently functionalized acrylamides and unactivated olefins. In these reactions, highly valuable amide-functionalized butadienes were prepared in good to excellent yields. This protocol was also compatible with Weinreb amides. A possible reaction mechanism involving the chelation-assisted vinylic C-H activation via a carboxylate-assisted deprotonation pathway is proposed.

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.

Access to the C2 C–H olefination, alkylation and deuteration of indoles by rhodium(iii) catalysis: an opportunity for diverse syntheses

A concise approach for a diversity-oriented synthesis via regioselective C2 C–H olefination, alkylation, and deuteration of indoles by Rh(iii) catalysis is described.

Transient directing group controlled regiodivergent C(sp3)–H and C(sp2)–H polyfluoroalkoxylation of aromatic aldehydes

A novel method for achieving regiodivergent C(sp3)–H and C(sp2)–H polyfluoroalkoxylation in the o-methyl benzaldehyde framework by altering the transient directing group is disclosed.

Regioselective C5-H direct iodination of indoles

An efficient regioselective C5-H direct iodination of indoles is reported herein for the first time, which offers a general and practical access to the difficult C5 functionalization of indoles.

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.

Biomimetic Total Syntheses of Ergot Alkaloids via Decarboxylative Giese Coupling

Biomimetic total syntheses of Festuclavine and Pyroclavine were achieved by a sequential radical coupling. The key steps include intramolecular decarboxylative Giese reaction to form the central C ring and 4-nitrobenzenesulfonyl (Ns)-directed indole C4-H olefination to introduce the indole C4 component. In addition, D-ring formation was completed by decarboxylative alkenylation and intramolecular S_N2 reaction.

Copper-catalyzed arylation of polycyclic aromatic hydrocarbons by the P[double bond, length as m-dash]O group

The first example of a directed and regioselective arylation of polycyclic aromatic hydrocarbons (PAHs) by using a P[double bond, length as m-dash]O directing group is reported herein. The protocol uses a cheap copper catalyst, and results in a breakthrough meta-selective C-H functionalization of arylphosphine oxide compounds. Substrates with potential fluorescence properties, for example, pyrene and fluoranthene, were successfully arylated under the system, thus achieving an efficient modification of fluorescent molecules containing the P[double bond, length as m-dash]O functional group.

Rhodium(III)-catalyzed C4-amidation of indole-oximes with dioxazolones via C-H activation

A novel method for the Rh(iii)-catalyzed oxime-directed C-H amidation of indoles with dioxazolones has been developed. This strategy provides an exclusive site selectivity and the directing group can be easily removed. This transformation features a wide substrate scope, good functional group tolerance and excellent yields, and may serve as a significant tool to construct structurally diverse indole derivatives for the screening of potential pharmaceuticals in the future.

Catalyst and solvent switched divergent C-H functionalization: oxidative annulation of N-aryl substituted quinazolin-4-amine with alkynes

The development of site-selective C-H functionalizations/annulations is one of the most challenging practices in synthetic organic chemistry particularly for substrates bearing several similarly reactive C-H bonds. Herein, we describe catalyst and solvent controlled ortho/peri site-selective oxidative annulation of C-H bonds of N-aryl substituted quinazolin-4-amines with internal alkynes. The ortho C-H selective annulation was observed using Pd-catalyst in DMF to give indole-quinazoline derivatives, while, Ru-catalyst in PEG-400 favoured the peri C-H bond annulation exclusively to furnish pyrido-quinazoline derivatives.

Weak Coordinating Carbonyl-Directed Rhodium(III)-Catalyzed C-H Activation at the C4-Position of Indole with Allyl Alcohols

A weakly coordinating carbonyl-directed coupling of allyl alcohols at the C-4 position of indole derivatives under the C-H activation conditions catalyzed by Rh(III) is reported. This results in alkylation at the C-4 position of indole derivatives exclusively. The obtained product forms a tricyclic derivative under aldol reaction conditions, which can be a potential precursor for synthesizing a few alkaloid molecules such as ergot, hapalindole alkaloids, and related heterocyclic compounds.

Iodine-Catalyzed Aerobic Diazenylation-Amination of Indole Derivatives

A mild strategy for consecutive diazenylation and amination of indole moieties has been demonstrated. The functionalization occurs at C3 and C2 carbon atoms, respectively, at the indole scaffold in the presence of catalytic iodine and air at 40 °C in the 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) solvent. It is noteworthy that the aromatic amines are generated in situ by the reaction of aryl hydrazine with iodine. In general, bright red products are obtained in moderate to good yield. Control reactions are conducted to establish the reaction mechanism.

Rh(iii)-Catalyzed tandem indole C4-arylamination/annulation with anthranils: access to indoloquinolines and their application in photophysical studies

An efficient Rh(iii)-catalyzed straightforward strategy was developed for the site-selective tandem C4 arylamination/annulation of indole derivatives with anthranil to provide indoloquinoline moieties.

Rh(iii)-Catalyzed regioselective C4 alkylation of indoles with allylic alcohols: direct access to β-indolyl ketones

A Rh(iii)-catalyzed direct C4 alkylation of indoles with allylic alcohols to access β-indolyl ketones was developed.