Rapid Syntheses of Heteroaryl-Substituted Imidazo[1,5-a]indole and Pyrrolo[1,2-c]imidazole via Aerobic C2-H Functionalizations

Photoinduced Regioselective Decarbonylative and Decarboxylative C-O Bond Functionalizations: Approach toward Chemoselective Scissions of Isatoic Anhydride and Unraveling the Enroutes through Control Experiments and DFT Studies

Distinctive, green, innovative, and well-organized photoinduced (metal- or photocatalyst-free) regioselective decarbonylative and decarboxylative C-O bond functionalization protocols to access aryl 2-aminobenzoates and 2-substituted benzoxazinone derivatives in excellent yields have been devised. These are achieved through the chemoselective scission of isatoic anhydride with ketones, diaryliodonium triflate, nitroalkene, phthalazinone, and phenol derivatives, which, in turn, served as the representative "electrophilic and nucleophilic" coupling partners. Control experiments and DFT calculations reveal that electrophilic radical-bearing coupling partners specifically follow the decarbonylation pathway, while nucleophilic radical-bearing conjugates facilitate the decarboxylation process. Thus, the devised methods represent the chemoselective fragmentation of isatoic anhydride, which occurs due to the electronic nature of the coupling partners. Again, the regioselective C-O/O-C bond formation is also a novel outcome of this methodology. We have also devised a green method for synthesizing 2-aminobenzoate-subtituted paracetamol through a decarboxylation route. A fluorescence quenching study indicates that phenyl 2-aminobenzoate specifically detects Fe(II) ions, exhibiting no reactivity toward various other metal ions. Additionally, transition-metal-catalyzed C-H bond functionalization of 2-substituted benzoxazinone with phenyl vinyl sulfone was performed at ease with significant yields, which appreciated the strategy developed by us.

New Chemo-, Regio- and Stereoselective Reactions and Methods in Organic Synthesis

The Special Issue "New Chemo-, Regio- and Stereoselective Reactions and Methods in Organic Synthesis" collects eight articles that have developed advanced approaches to the chemo-, regio- and stereoselective synthesis of novel important compounds, scaffolds, synthons, and practically valuable products [...].

Rhodium-Catalyzed C-H Arylation of Indoles with Arylsilanes at Room Temperature

The Cp*Rh-catalyzed C-H arylation of indoles with arylsilanes is developed. This C-H activation transformation allows for the Rh-catalyzed indole C2 arylation to overcome the limitations of requiring strong directing group assistance and high-temperature conditions, achieving a room-temperature transformation driven by a weak directing group. Cp*Rh/MeOH catalytic media are considered a key factor enabling this transformation to occur under mild conditions, and experimental studies and theoretical calculations were performed to rationalize the reaction mechanisms and the influence of methanol as a solvent in promoting the reaction.

Synthesis of γ-Spirolactams via Rh(III)-Catalyzed C-H Activation/Directing Group Migration/Dearomatization/Spiroannulation of Indoles with 1,3-Enynes

1,3-Enynes are valuable coupling partners in transition-metal-catalyzed C-H functionalizations. Certainly, aliphatic-substituted 1,3-enynes have been thoroughly investigated in C-H functionalizations, whereas aromatic-substituted 1,3-enynes remain underexplored. Herein, we report the realization of this goal, where we achieve an atom-economical protocol for the synthesis of γ-spirolactams via Rh(III)-catalyzed C-H functionalization of N-carbamoylindoles with 1,3-enynes. The reaction proceeds through a unique cascade strategy, such as C-H activation/directing group (DG) migration/indole dearomatization/spiroannulation, to access novel and synthetically challenging spiro[indoline-2,2'-pyrrolidin]-5'-one scaffolds. Moreover, the isolation of intermediate and mechanistic and ESI-HRMS studies further provide valuable insights into the proposed catalytic cycle.

Recent development in the synthesis of imidazo[1,5-a]indole derivatives: an in-depth overview

In the realm of nitrogen-fused heterocycles, imidazo[1,5-a]indole and its derivatives are recognized as privileged structural patterns in various pharmaceutical drugs and biologically active natural products, emphasizing their significance. This review comprehensively explores the synthetic strategies for constructing imidazo[1,5-a]indole scaffolds, with a particular focus on transition metal-catalyzed methodologies. The primary highlighted method is [4 + 1] annulation, along with other notable approaches such as C-H activation/cyclization, enantioselective C-H annulation, intramolecular hydroamination, and double cyclization processes.

Divergent synthesis of pyrrolizine derivatives through C-H bond functionalization of pyrroles

Presented herein is the synthesis of diversely functionalized pyrrolizines from the reaction of N-alkoxycarbamoyl pyrroles with CF3-ynones. The formation of the product is based on a C-H bond activation-initiated cascade process including N-alkoxycarbamoyl group-directed alkenylation of the pyrrole scaffold followed by simultaneous intramolecular nucleophilic addition along with cleavage and transfer of the directing group. By taking advantage of the rich chemistry of the transferred alkoxycarbamoyl moiety, the products could be transformed into a series of structurally and biologically interesting pyrrolizine derivatives. To our knowledge, this is the first example in which the N-alkoxycarbamoyl unit acted as a transferable and transformable directing group for the divergent synthesis of pyrrolizines.

Palladium-Catalyzed Weak-Chelation-Assisted C4-Nitration of Indoles with tert-Butyl Nitrite: Formal Access to Aminated Indoles

Palladium-catalyzed weak-chelation-assisted C4-selective nitration of indoles has been accomplished employing tert-butyl nitrite in the presence of oxone under molecular oxygen at a moderate temperature. Aerobic conditions, C4-selectivity, substrate scope, conversion to valuable aminated indoles, and late-stage natural product modifications are the important practical features.

Pd/Brønsted acid catalysed intramolecular N-allylation of indoles and pyrroles with alkynes for the synthesis of N-fused heterocycles

We, herein, report a Pd(0) and Brønsted acid-catalyzed redox-neutral intramolecular N-allylation of indoles and pyrroles with alkynes for the synthesis of biologically important imidazolidinone-fused N-heterocycles. The allylation is completely atom-economical and is applicable to a wide range of substrates. The methodology eliminates the use of a leaving group or an oxidizing agent, often employed for the allylation of nucleophiles. To the best of our knowledge, N-allylation of indoles and pyrroles with alkynes has not been reported to date.

Contributing to Biochemistry and Optoelectronics: Pyrrolo[1',2':2,3]imidazo[1,5-a]indoles and Cyclohepta[4,5]pyrrolo[1,2-c]pyrrolo[1,2-a]imidazoles via [3+2] Annulation of Acylethynylcycloalka[b]pyrroles with Δ1-Pyrrolines

Available pyrrolylalkynones with tetrahydroindolyl, cycloalkanopyrrolyl, and dihydrobenzo[g]indolyl moieties, acylethynylcycloalka[b]pyrroles, are readily annulated with Δ¹-pyrrolines (MeCN/THF, 70 °C, 8 h) to afford a series of novel pyrrolo[1',2':2,3]imidazo[1,5-a]indoles and cyclohepta[4,5]pyrrolo[1,2-c]pyrrolo[1,2-a]imidazoles functionalized with an acylethenyl group in up to an 81% yield. This original synthetic approach contributes to the arsenal of chemical methods promoting drug discovery. Photophysical studies show that some of the synthesized compounds, e.g., benzo[g]pyrroloimidazoindoles, are prospective candidates for TADF emitters of OLED.

Ruthenium(II)-Catalyzed Highly Chemo- and Regioselective Oxidative C6 Alkenylation of Indole-7-carboxamides

We disclosed the first efficient method for highly chemo- and regioselective C6 alkenylation of indole-7-carboxamides using inexpensive Ru(II) catalyst through chelation assisted C-H bond activation. Electronically diverse indole-7-carboxamides and alkenes react efficiently to produce a wide range of C6 alkenyl indole derivatives. Further the C6 alkenyl indole-7-carboxamides modified to their derivatives through simple chemical transformations. The observed regioselectivity and kinetics has been evidenced by deuterium incorporation and intermolecular competitive studies. In addition, for mechanistic insights, the intermediates were analyzed by HRMS.

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.

Infrared, Electronic Absorption, and Emission Spectra of Tris (4-methyl-1-(1- methyl-1H-benzo [d]imidazol-2-yl)-4H-benzo [d]imidazo[1,5-a]imid- azole-3-yl) methyl: Roles of Molecular Electronic States and Axial Substitution

In order to comparing and explaining the experimental infrared, electronic absorption, and emission spectra of tris (4-methyl-1-(1-methyl-1H-benzo [d] imidazol-2-yl) -4H-benzo [d] imidazo[1,5-a]imid- azole-3-yl) methyl (L6), density functional theory calculations were performed to systematically study the spectral properties of several species with different electronic states (L6⁺ , L6, and HL6⁺ ) as well as the axial substituted derivative with methoxy group (L6-OCH₃ ). The main peaks in the vibrational, electronic absorption, and emission spectra were assigned in detail. Roles of molecular electronic states and axial substitution on the spectral properties are revealed. The calculated spectra are compared with experimental results to distinguish the nature of L6 in crystal and solvent. This work combing experiment and theoretical calculations would be good example for clarifying molecular electronic states and thus promising the practice application of such kind of novel molecules.

Construction of 2-alkynyl aza-spiro[4,5]indole scaffolds via sequential C-H activations for modular click chemistry libraries

Herein, we have developed a strategy of sequential C-H activations of indole to construct novel 2-alkynyl aza-spiro[4,5]indole scaffolds, which incorporated both alkyne and spiro-units into indole. Gram-scale synthesis and a one-pot, three-step synthesis demonstrated the utility of this protocol. Hybrid conjugates with an oseltamivir derivative further offered a powerful tool for the construction of a versatile spiroindole-containing library via click chemistry.

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.

Recent Advances in Functionalization of Pyrroles and their Translational Potential

Among the known aromatic nitrogen heterocycles, pyrrole represents a privileged aromatic heterocycle ranging its occurrence in the key component of "pigments of life" to biologically active natural products to active pharmaceuticals. Pyrrole being an electron-rich heteroaromatic compound, its predominant functionalization is legendary to aromatic electrophilic substitution reactions. Although a few excellent reviews on the functionalization of pyrroles including the reports by Baltazzi in 1963, Casiraghi and Rassu in 1995, and Banwell in 2006 are available, they are fragmentary and over fifteen years old, and do not cover the modern aspects of catalysis. A review covering a comprehensive package of direct functionalization on pyrroles via catalytic and non-catalytic methods including their translational potential is described. Subsequent to statutory yet concise introduction, the classical functionalization on pyrroles using Lewis acids largely following an ionic mechanism is discussed. The subsequent discussion follows the various metal-catalyzed C-H functionalization on pyrroles, which are otherwise difficult to implement by Lewis acids. A major emphasize is given on the radical based pyrrole functionalization under metal-free oxidative conditions, which is otherwise poorly highlighted in the literature. Towards the end, the current development of pyrrole functionalization under photocatalyzed and electrochemical conditions is appended. Only a selected examples of substrates and important mechanisms are discussed for different methods highlighting their scopes and limitations. The aromatic nucleophillic substitution on pyrroles (being an electron-rich heterocycle) happened to be the subject of recent investigations, which has also been covered accentuating their underlying conceptual development. Despite great achievements over the past several years in these areas, many challenges and problems are yet to be solved, which are all discussed in summary and outlook.

Unleash electron transfer in C-H functionalization by mesoporous carbon-supported palladium interstitial catalysts

The functionalization of otherwise unreactive C–H bonds adds a new dimension to synthetic chemistry, yielding useful molecules for a range of applications. Arylation has emerged as an increasingly viable strategy for functionalization of heteroarenes which constitute an important class of structural moieties for organic materials. However, direct bisarylation of heteroarenes to enable aryl-heteroaryl-aryl bond formation remains a formidable challenge, due to the strong coordination between heteroatom of N or S and transitional metals. Here we report Pd interstitial nanocatalysts supported on ordered mesoporous carbon as catalysts for a direct and highly efficient bisarylation method for five-membered heteroarenes that allows for green and mild reaction conditions. Notably, in the absence of any base, ligands and phase transfer agents, high activity (turn-over frequency, TOF, up to 107 h⁻¹) and selectivity (>99%) for the 2,5-bisarylation of five-membered heteroarenes are achieved in water. A combination of characterization reveals that the remarkable catalytic reactivity here is attributable to the parallel adsorption of heteroarene over Pd clusters, which breaks the barrier to electron transfer in traditional homogenous catalysis and creates dual electrophilic sites for aryl radicals and adsorbate at C2 and C5 positions. The d-band filling at Pd sites shows a linear relationship with activation entropy and catalytic activity. The ordered mesopores facilitate the absence of a mass transfer effect. These findings suggest alternative synthesis pathways for the design, synthesis and understanding of a large number of organic chemicals by ordered mesoporous carbon supported palladium catalysts.

Efficient and environmentally sustainable domino protocol for the synthesis of diversified spiroheterocycles with privileged heterocyclic substructures using bio-organic catalyst in aqueous medium

An efficient and environmentally sustainable synthetic protocol has been presented to synthesize structurally diverse spiroxindoles spiroannulated with indenopyrroloimidazoles, pyranopyrroloimidazoles, chromenopyrroloimidazoles, and imidazopyrrolopyrimidines involving three-component reaction of isatins, hydantoin, and β-diketones in the presence of green and sustainable bio-organic catalyst, β-amino acid, 2-aminoethanesulfonic acid (taurine), in aqueous media. The synthetic efficiency, operational simplicity, and reusability of catalyst make the present synthetic protocol cost effective, time efficient, and eco-friendly to synthesize molecules with structural diversity and molecular complexity and expected to contribute significantly not only to drug discovery research but also to pharmaceutical and medicinal chemistry.

One-pot synthesis of substituted pyrrole-imidazole derivatives with anticancer activity

A facile and efficient method to synthesize pyrrole-imidazole was developed via a post-Ugi cascade reaction followed by one purification procedure. Synthesized pyrrole-imidazole was collected by performing a mild reaction and a simple procedure, which could be applicable to a broad scope of functionalized anilines. The screening results demonstrated that compound 7e exhibited a high potency of anticancer activity in human pancreatic cancer cell lines PANC and ASPC-1. Our work shed light on the potential of post-Ugi cascade reaction in combinatorial and medicinal chemistry.

A route to access imidazol[1,5-a]indole-1,3-diones and pyrrolo[1,2-c]imidazole-1,3-diones

A novel and practical route to synthesize imidazol[1,5-a]indoles and pyrrolo[1,2-c]imidazoles via N-H functionalization has been developed. Indole-2-carboxylic acid or pyrrole-2-carboxylic acid with diverse aniline groups and carbonyldiimidazole (CDI), in the presence of a base under microwave conditions, resulted in imidazol[1,5-a]indoles and pyrrolo[1,2-c]imidazoles, respectively. The present method is free of work-up and no need for column chromatography. Both title scaffolds can serve as useful heterocyclic scaffolds in medicinal chemistry as such, or they can be used as building blocks to construct different classes of useful compounds.

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.

Palladium-Catalyzed Multicomponent Reaction of Alkynes, Carboxylic Acids, and Isocyanides: A Direct Approach to Captodative Olefins

A palladium-catalyzed multicomponent reaction of alkynes, carboxylic acids, and isocyanides has been developed with the assistance of silver salt under mild conditions. Highly functionalized captodative olefins are synthesized efficiently by this method, which can find many applications as versatile synthons in organic synthesis.

Silver-promoted regioselective [4+2] annulation reaction of indoles with alkenes to construct dihydropyrimidoindolone scaffolds

An Ag^I-promoted regioselective [4+2] annulation reaction of indoles with alkenes has been established.

Deep eutectic solvent promoted synthesis of structurally diverse hybrid molecules with privileged heterocyclic substructures

Structurally diverse hybrid molecules; indenopyrroloimidazoles, imidazoindoles, chromenopyrroloimidazoles and imidazopyrrlopyrimidines, have been synthesized using DES as a sustainable solvent and promoter.

Copper mediated C-H amination with oximes: en route to primary anilines

Here we report an efficient Cu(i)-mediated C-H amination reaction with oximes as amino donors to introduce NH2 groups directly. Various strongly coordinating heterocycles including quinoline, pyrimidine, pyrazine, pyrazole and triazole were tolerated well. The potential utility was further demonstrated in a late-stage modification of telmisartan (an antagonist for the angiotensin II receptor).