Construction of Pyrrolo[1,2-a]indoles via Cobalt(III)-Catalyzed Enaminylation of 1-(Pyrimidin-2-yl)-1H-indoles with Ketenimines and Subsequent Base-Promoted Cyclization

Phototriggered Fluoroalkylation/Cyclization of Unactivated 1-Acryloyl-2-cyanoindoles: Synthesis of RCOCF2-Substituted Pyrrolo[1,2-a]indolediones

A photochemical approach toward RCOCF2-substituted pyrrolo[1,2-a]indolediones was developed by the radical cascade difluoroalkylation/cyclization reaction of unactivated 1-acryloyl-2-cyanoindoles with ethyl iododifluoroacetate or iododifluoramides under visible-light irradiation. This transition-metal- and photosensitizer-free protocol afforded diverse difluoroalkylated pyrrolo[1,2-a]indolediones in moderate to good yields under mild reaction conditions. Most appealingly, the reaction can proceed smoothly under sunlight irradiation, which opens a new avenue toward difluoroalkylated pyrrolo[1,2-a]indolediones.

Visible-Light-Induced Cascade Cyclization of 1-Acryloyl-2-cyanoindole: Access of Difluoroalkylated Pyrrolo[1,2-a]indolediones

An effective method for accessing diverse difluoroalkylated pyrrolo[1,2-a]indolediones via visible-light-induced PhI(OAc)2-promoted cascade difluoroalkylation/cyclization reaction under mild conditions has been established. This method is noteworthy for its use of DMSO-H2O as a green medium at room temperature and avoidance of photocatalysts. The reactions are straightforward to execute and convenient to expand on, provide good to excellent yields, and have good functional group tolerance.

Manganese-catalyzed hydroarylation of multiple bonds

Transition metal-catalyzed C-H activation has become a promising strategy in organic synthesis due to its improved atom-, step- and resource economy. Considering the Earth's abundance, economic benefits, and low toxicity, 3d metal catalysts for C-H activation have received a significant focus. In particular, organometallic manganese-catalyzed C-H activation has proven to be versatile and suitable for a wide range of transformations such as C-H addition to π-components, arylation, alkylation, alkynylation, amination, and many more. Among them, manganese-catalyzed C-H addition to C-C and C-heteroatom multiple bonds exhibited unique and promising reactivity to construct a wide range of complex organic molecules. In this review, we highlight the developments in the field of manganese-catalyzed hydroarylation of multiple bonds via C-H activation with a range of applications until August 2022.

Rhodium(III)-catalyzed regioselective C2-alkenylation of indoles with CF3-imidoyl sulfoxonium ylides to give multi-functionalized enamines using a migratable directing group

A rhodium(III)-catalyzed regioselective C2-alkenylation of indoles for the construction of α-CF₃ substituted enamines has been developed, which utilizes CF₃-imidoyl sulfoxonium ylides (TFISYs) as alkenylating agents for the first time. A wide array of indolyl- and trifluoromethyl-decorated enamine derivatives have been assembled in moderate to good yields.

2-Alkylation of 3-Alkyindoles With Unactivated Alkenes

An acid-catalyzed 2-alkylation of indole molecules is developed. Only catalytic amount of the commercially available, inexpensive and traceless HI is used as the sole reaction promoter. 2,3-Disubstituted indole molecules bearing congested tertiary carbon centers are afforded as the final products in moderate to excellent yields.

Transition Metal-Free, Methoxide-Catalyzed Synthesis of Pyridoindolones

A simple transition metal-free strategy for the synthesis of pyrido[1,2-a]indolone derivatives has been devised through sodium methoxide-catalyzed intramolecular cyclization of 2-alkenylated N-pyrimidyl indoles. The reactions involved a Smiles rearrangement/cyclization cascade, which resulted in a new series of N-fused indoles, potentially applicable skeletons in medicinal chemistry. This reaction presents simple eco-friendly reaction conditions, a high atom- and cost-economy, a short reaction time, and a broad range of substrate scope with high reaction efficiency.

C-H bond functionalization by high-valent cobalt catalysis: current progress, challenges and future perspectives

Over the last decade, high-valent cobalt catalysis has earned a place in the spotlight as a valuable tool for C-H activation and functionalization. Since the discovery of its unique reactivity, more and more attention has been directed towards the utilization of cobalt as an alternative to noble metal catalysts. In particular, Cp*Co(III) complexes, as well as simple Co(II) and Co(III) salts in combination with bidentate chelation assistance, have been extensively used for the development of novel transformations. In this review, we have demonstrated the existing trends in the C-H functionalization methodology using high-valent cobalt catalysis and highlighted the main challenges to overcome, as well as perspective directions, which need to be further developed in the future.

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.

Rhodium(III)-Catalyzed C-H Bond Functionalization of 2-Pyridones with Alkynes: Switchable Alkenylation, Alkenylation/Directing Group Migration and Rollover Annulation

Cp*Rh(III)-catalyzed chelation-assisted direct C-H bond functionalization of 1-(2-pyridyl)-2-pyridones with internal alkynes that can be controlled to give three different products in good yields has been realized. Depending on the reaction conditions, solvents and additives, the reaction pathway can be switched between alkenylation, alkenylation/directing group migration and rollover annulation. These reaction manifolds allow divergent access to a variety of valuable C6-alkenylated 1-(2-pyridyl)-2-pyridones, (Z)-6-(1,2-diaryl-2-(pyridin-2-yl)vinyl)pyridin-2(1H)-ones and 10H-pyrido[1,2-a][1,8]naphthyridin-10-ones from the same starting materials. These protocols exhibit excellent regio- and stereoselectivity, broad substrate scope, and good tolerance of functional groups. A combination of experimental and computational approaches have been employed to uncover the key mechanistic features of these reactions.

Directing group migration strategy in transition-metal-catalysed direct C-H functionalization

Very recently, directing group (DG) migration has emerged as a practical strategy for transition-metal-catalysed direct C-H activation, resulting in a highly atom-economical process and enabling the reusage of DG. Therefore, great progress has been made in developing multitasking DGs. In this tutorial review, we present the rapid advances of this novel strategy by analyzing and comparing the different types of migratable DGs (including N-O, N-C, N-N or O-C bond cleavage to trigger DG migration). The related mechanisms, as well as synthetic applications, are also mentioned.

Manganese- and rhenium-catalyzed C–H enaminylation: expedient access to novel indole–purine hybrids with anti-tumor bioactivities

The C–H enaminylation of novel 6-(1H-indol-1-yl)-purines with ketenimines was accomplished by means of aqueous manganese and rhenium catalysis, which sets the stage for the facile synthesis of indole–purine hybrids with anti-tumor bioactivities.

Recent advances in cobalt-catalysed C–H functionalizations

Ready availability, low cost and low toxicity of cobalt salts have redirected the attention of researchers away from noble metals, such as Pd, Rh, and Ir, towards Co in the field of C–H functionalisation.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Hydroarylations by cobalt-catalyzed C-H activation

As an earth-abundant first-row transition metal, cobalt catalysts offer a broad range of economical methods for organic transformations via C–H activation. One of the transformations is the addition of C–H to C–X multiple bonds to afford alkylation, alkenation, amidation, and cyclization products using low- or high-valent cobalt catalysts. This hydroarylation is an efficient approach to build new C–C bonds in a 100% atom-economical manner. In this review, the recent developments of Co-catalyzed hydroarylation reactions and their mechanistic studies are summarized.

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.

C-H functionalization by high-valent Cp*Co(iii) catalysis

Significant progress has been accomplished in directed C-H functionalization through the use of earth-abundant and inexpensive first-row transition metals. Among these base metals, Co is especially attractive in view of its versatile applications in C-H functionalization, in both low- and high-valent states. In this vein, catalytic Co(iii) species can be generated from the dissociation of a Cp*Co(iii) catalyst or through the oxidation of a low-valent cobalt catalyst in the presence of an oxidant. In this feature article, we will discuss the breakthroughs in Cp*Co(iii)-promoted C-H functionalization. In this field, C(sp²)-H functionalization has been extensively studied and developed. In contrast, few C(sp³)-H functionalization reactions have been reported.

Elemental sulfur accelerated the reactivity of the 3-position of indole for the construction of chromeno[2,3-b]indoles

Elemental sulfur not only acted as an additional oxidant but also facilitated the reactivity of the 3-position of indole to construct chromeno[2,3-b]indoles.

Cobalt-catalyzed C–H activation: recent progress in heterocyclic chemistry

Cobalt-catalyzed C–H activation has gone through some major advancements in the past couple of decades.

Cobalt(iii)-catalyzed 1,4-addition of C–H bonds of oximes to maleimides

An oxime directed cobalt-catalyzed sp² C–H bond addition reaction was developed with a broad substrate scope and no external additives were needed.

α-Oxo-Ketenimines from Isocyanides and α-Haloketones: Synthesis and Divergent Reactivity

The palladium-catalyzed reaction of α-haloketones with isocyanides afforded α-oxo-ketenimines through β-hydride elimination of the β-oxo-imidoyl palladium intermediates. Reaction of these relatively stable α-oxo-ketenimines with nucleophiles such as hydrazines, hydrazoic acid, amines, and Grignard reagent afforded pyrazoles, tetrazole, β-keto amidines, and enaminone, respectively, with high chemoselectivity. Whereas amines attack exclusively on the ketenimine functions, the formal [3+2] cycloaddition between N-monosubstituted hydrazines and α-oxo-ketenimines was initiated by nucleophilic addition to the carbonyl group.

Manganese(I)-Catalyzed Regio- and Stereoselective 1,2-Diheteroarylation of Allenes: Combination of C-H Activation and Smiles Rearrangement

Heteroarenes are important structural motif in functional molecules. A Mn^I -catalyzed 1,2-diheteroarylation of allenes via a C-H activation/Smiles rearrangement cascade is presented. The reaction occurred under additive-free or even solvent-free conditions, which allowed the creation of two C-C and one C-N bonds in a single operation. A series of structurally diverse bicyclic or tricyclic compounds bearing an exocyclic double bond were constructed in good to excellent efficiency. The decarboxylative ring-opening of the products led to the facile synthesis of vicinal biheteroaryls. Synthetic applications were demonstrated and preliminary mechanistic studies were conducted.

Direct Functionalization of C-H Bonds by Iron, Nickel, and Cobalt Catalysis

Non-precious-metal-catalyzed reactions are of increasing importance in chemistry due to the outstanding ecological and economic properties of these metals. In the subfield of metal-catalyzed direct C-H functionalization reactions, recent years have shown an increasing number of publications dedicated to this topic. Nickel, cobalt, and last but not least iron, have started to enter a field which was long dominated by precious metals such as palladium, rhodium, ruthenium, and iridium. The present review article summarizes the development of iron-, nickel-, and cobalt-catalyzed C-H functionalization reactions until the end of 2016, and discusses the scope and limitations of these transformations.

Rhodium-catalyzed C2 and C4 C–H activation/annulation of 3-(1H-indol-3-yl)-3-oxopropanenitriles with internal alkynes: a facile access to substituted and fused carbazoles

Rhodium-catalyzed C2 and C4 C–H activation/annulation of 3-(1H-indol-3-yl)-3-oxopropanenitriles with internal alkynes has been developed. Substituted and fused carbazole derivatives were obtained through formal rhodium-catalyzed (4+2) or tandem (4+2) and (5+2) cycloaddition reactions under mild reaction conditions in good yields.

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.