Multicomponent syntheses of pyrazoles via (3 + 2)-cyclocondensation and (3 + 2)-cycloaddition key steps

Pyrazoles are rarely found in nature but are traditionally used in the agrochemical and pharmaceutical industries, while other areas of use are also actively developing. However, they have also found numerous other applications. The search for new and efficient syntheses of these heterocycles is therefore highly relevant. The modular concept of multicomponent reactions (MCR) has paved a broad alley to heteroaromatics. The advantages over traditional methods are the broader scope and increased efficiency of these reactions. In particular, traditional multistep syntheses of pyrazoles have considerably been extended by MCR. Progress has been made in the cyclocondensation of 1,3-dielectrophiles that are generated in situ. Limitations in the regioselectivity of cyclocondensation with 1,3-dicarbonyls were overcome by the addition-cyclocondensation of α,β-unsaturated ketones. Embedding 1,3-dipolar cycloadditions into a one-pot process has additionally been developed for concise syntheses of pyrazoles. The MCR strategy also allows for concatenating classical condensation-based methodology with modern cross-coupling and radical chemistry, as well as providing versatile synthetic approaches to pyrazoles. This overview summarizes the most important MCR syntheses of pyrazoles based on ring-forming sequences in a flashlight fashion.

Radical N2-Retention Cyclizations of Aryl Diazoniums: Access to 7/8/9-Membered Heterocycles

We report herein a modular approach to synthesizing diverse functionalized 7/8/9-membered poly-N-containing heterocycles via oxidative radical N2-retention cyclizations of allylic aryl diazonium salts using CF3SO2Na as a CF3 radical source. A range of trifluoromethylated benzotriazepines, benzotriazocines, and benzotriazonines were obtained in moderate to good yields. This transition-metal-free protocol demonstrates atom economy, safe conditions, broad functional group tolerance, and availability of readily accessible reagents.

Aryldiazonium-Salt-Triggered Carboxylative Azotization of Pyrroles or Indoles with Polyhalomethanes via Halogen-Atom Transfer (XAT)

A halogen-atom-transfer (XAT)-based method for carbonylazotization of pyrroles or indoles with aryldiazonium salts and polyhalomethanes via dual C(sp2)-H bond functionalization is described. Using aryldiazonium salts realizes carbonylation/azotization of pyrroles or indoles via polyhalomethyl-radical-mediated and electrophilic substitution, thus providing a green, efficient, and step-economy approach for synthesis of multifunctional pyrroles or indoles from the easily available substrates. Notably, this strategy relies on the use of aryldiazonium salts to extend the well-established iodine atom transfer to bromine or chlorine atom transfer.

Multicomponent Reactions between Heteroatom Compounds and Unsaturated Compounds in Radical Reactions

In this mini-review, we present our concepts for designing multicomponent reactions with reference to a series of sequential radical reactions that we have developed. Radical reactions are well suited for the design of multicomponent reactions due to their high functional group tolerance and low solvent sensitivity. We have focused on the photolysis of interelement compounds with a heteroatom-heteroatom single bond, which readily generates heteroatom-centered radicals, and have studied the photoinduced radical addition of interelement compounds to unsaturated compounds. First, the background of multicomponent radical reactions is described, and basic concepts and methodology for the construction of multicomponent reactions are explained. Next, examples of multicomponent reactions involving two interelement compounds and one unsaturated compound are presented, as well as examples of multicomponent reactions involving one interelement compound and two unsaturated compounds. Furthermore, multicomponent reactions involving intramolecular cyclization processes are described.

Evaluation of Anti-Neuroinflammatory Activity of Isatin Derivatives in Activated Microglia

Neuroinflammation plays a crucial role in the progression of Alzheimer's disease and other neurodegenerative disorders. Overactivated microglia cause neurotoxicity and prolong the inflammatory response in many neuropathologies. In this study, we have synthesised a series of isatin derivatives to evaluate their anti-neuroinflammatory potential using lipopolysaccharide activated microglia as a cell model. We explored four different substitutions of the isatin moiety by testing their anti-neuroinflammatory activity on BV2 microglia cells. Based on the low cytotoxicity and the activity in reducing the release of nitric oxide, pro-inflammatory interleukin 6 and tumour necrosis factor α by microglial cells, the N¹-alkylated compound 10 and the chlorinated 20 showed the best results at 25 µM. Taken together, the data suggest that 10 and 20 are promising lead compounds for developing new neuroprotective agents.

Green Synthesis of Aromatic Nitrogen-Containing Heterocycles by Catalytic and Non-Traditional Activation Methods

Recent advances in the environmentally benign synthesis of aromatic N-heterocycles are reviewed, focusing primarily on the application of catalytic methods and non-traditional activation. This account features two main parts: the preparation of single ring N-heterocycles, and their condensed analogs. Both groups include compounds with one, two and more N-atoms. Due to the large number of protocols, this account focuses on providing representative examples to feature the available methods.

Copper-Promoted Aerobic Oxidative [3+2] Cycloaddition Reactions of N,N-Disubstituted Hydrazines with Alkynoates: Access to Substituted Pyrazoles

A copper-promoted aerobic oxidative [3+2] cycloaddition reaction for the synthesis of various substituted pyrazoles from N,N-disubstituted hydrazines with alkynoates in the presence of bases is developed. This work involves a direct C(sp³)-H functionalization and the formation of new C-C/C-N bonds. In this strategy, inexpensive and easily available Cu₂O serves as the promoter and air acts as the green oxidant. The reaction exhibits the advantages of high atom and step economy, high regioselectivity, and easy operation.

Copper(II)-Catalyzed Direct C-H Trifluoroethylation of Heteroarenes

Trifluoroethyl (CH₂CF₃) is an important functional group in many pharmaceutical and agrochemical compounds. Herein, we report an efficient method for the copper-catalyzed direct trifluoroethylation of heteroarenes. The reaction exhibited good compatibility to various substrates, and the desired products were obtained in good yields. Preliminary mechanistic investigations indicate the trifluoroethyl radical is involved in the catalytic circle. Moreover, the late-stage modification of bioactive molecules further confirmed the practical applications of this method.

DMSO as a C1 Source for [2 + 2 + 1] Pyrazole Ring Construction via Metal-Free Annulation with Enaminones and Hydrazines

A cascade reaction between enaminones, hydrazines, and dimethyl sulfoxide (DMSO) for the synthesis of 1,4-disubstituted pyrazoles catalyzed by molecular iodine in the presence of Selectfluor has been realized. DMSO plays a dual role as the C₁ source and the reaction medium. In addition, the synthesis of 1,3,4-trisubstituted pyrazoles using aldehydes as alternative C₁ building blocks has also been achieved.

Recent Advances in the Green Synthesis of Heterocycles: From Building Blocks to Biologically Active Compounds

Recent advances in the environmentally benign synthesis of common heterocycles are described. This account features three main parts; the preparation of non-aromatic heterocycles, one-ring aromatic heterocycles and their condensed analogs. Due to the great variety of and high interest in these compounds, this work focuses on providing representative examples of the preparation of the target compounds.

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.

Hypervalent iodine(iii)-promoted rapid cascade reaction for the synthesis of unsymmetric azo compounds

A rapid three-component cascade reaction for the synthesis of unsymmetric azo compounds via a radical activation strategy has been reported. Various aryldiazonium salts and unactivated alkenes are well compatible, providing the corresponding products in good to excellent yields. This strategy gives an efficient and practical solution for the synthesis of unsymmetric azo compounds with two C-N bond formation. A free radical pathway mechanism is advised for this transformation.

ZnI2-Catalyzed Aminotrifluoromethylation Cyclization of Alkenes Using PhICF3Cl

We report here an alternatively catalytic aminotrifluoromethylation of alkenes using PhICF₃Cl as a bifunctional reagent along with ZnI₂ as a dual catalyst. A combined catalytic strategy was established for the intramolecular aminotrifluoromethylation of 4-pentenamines. As a result, a set of 2-trifluoroethyl-pyrrolidines was obtained in a high selectivity. Mechanism studies revealed that the reaction included an iodine anion-catalyzed radical chlorotrifluoromethylation of alkenes and a sequential Lewis acid-promoted aminocyclization of the resulting chlorotrifluoromethylated intermediates.

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.

Copper-catalyzed one-pot synthesis of 2-(2,2,2-trifluoroethyl)-substituted benzofused heterocycles

A method for copper-catalyzed synthesis of 2-trifluoroethyl-substituted benzofurans and indoles from the reaction of salicylaldehyde/2-aminobenzaldehyde p-tosylhydrazones with 2-bromo-3,3,3-trifluoropropene has been developed.

Intermolecular radical carboamination of alkenes

Vicinal alkene carboamination is a highly efficient and practical synthetic strategy for the straightforward preparation of diverse and valuable amine derivatives starting from simple compounds. During the last decade that approach has found continuous research interests and various practical methods have been developed using transition-metal catalysis. Driven by the renaissance of synthetic radical chemistry, intermolecular radical alkene carboamination comprising a C-C bond and a C-N bond forming step has been intensively investigated recently culminating in novel strategies and improved protocols which complement existing methodologies. Radical alkene carboamination can be achieved via three different reaction modes. Such cascades can proceed through N-radical addition to an alkene with subsequent C-C bond formation leading to 2,1-carboamination products. Alternatively, the C-C bond can be installed prior to the C-N bond via initial C-radical addition to the alkene with subsequent β-amination resulting in 1,2-carboamination. The third mode comprises initial single electron oxidation of the alkene to the corresponding alkene radical cation that gets trapped by an N-nucleophile and the cascade is terminated by radical C-C bond formation. In this review, the three different conceptual approaches will be discussed and examples from the recent literature will be presented. Further, the reader will get insights into the mechanism of the different transformations.

Profiling the oxidative activation of DMSO-F6 by pulse radiolysis and translational potential for radical C-H trifluoromethylation

The oxidative activation of the perfluorinated analogue of dimethyl sulfoxide, DMSO-F₆, by hydroxyl radicals efficiently produces trifluoromethyl radicals based on pulse radiolysis, laboratory scale experiments, and comparison of rates of reaction for analogous radical systems. In comparison to commercially available precursors, DMSO-F₆ proved to be more stable, easier to handle and overall more convenient than leading F₃C-reagents and may therefore be an ideal surrogate to study F₃C radicals for time-resolved kinetics studies. In addition, we present an improved protocol for the preparation of this largely unexplored reagent.

Transition-Metal- and Light-Free Directed Amination of Remote Unactivated C(sp3)-H Bonds of Alcohols

Due to the great value of amino alcohols, new methods for their synthesis are in high demand. Abundant aliphatic alcohols represent the ideal feedstock for the method development toward this important motif. To date, transition-metal-catalyzed approaches for the directed remote amination of alcohols have been well established. Yet, they have certain disadvantages such as the use of expensive catalysts and limited scope. Very recently, transition-metal-free visible-light-induced radical approaches have emerged as new powerful tools for directed remote amination of alcohols. Relying on 1,5-HAT reactivity, these methods are limited to β - or δ-amination only. Herein, we report a novel transition-metal- and visible-light-free room-temperature radical approach for remote β -, γ-, and δ-C(sp3)-N bond formation in aliphatic alcohols using mild basic conditions and readily available diazonium salt reagents.

Copper-catalyzed tandem annulation/enol nucleophilic addition to access multisubstituted indoles

A method to access various multisubstituted indoles from propargylic alcohols and readily available enol nucleophiles by copper-catalyzed tandem annulation/enol nucleophilic addition has been developed. Compared to the expensive metal catalysts such as platinum, gold, silver, and palladium used previously, the most economical copper(i) catalyst could achieve this reaction efficiently. The fused heterocyclic compounds, pyrrolo[1,2-a] indoles, could be afforded by further transformation of the products. The allyl cation intermediate may be involved in the mechanism.

Trifluoromethanesulfonic Anhydride as a Low-Cost and Versatile Trifluoromethylation Reagent

A large number of reagents have been developed for the synthesis of trifluoromethylated compounds. However, an ongoing challenge in trifluoromethylation reaction is the use of less expensive and practical trifluoromethyl sources. We report herein the unprecedented direct trifluoromethylation of (hetero)arenes using trifluoromethanesulfonic anhydride as a radical trifluoromethylation reagent by merging photoredox catalysis and pyridine activation. Furthermore, introduction of both the CF₃ and OTf groups of the trifluoromethanesulfonic anhydride into internal alkynes to access tetrasubstituted trifluoromethylated alkenes was achieved. Since trifluoromethanesulfonic anhydride is a low-cost and abundant chemical, this method provides a cost-efficient and practical route to trifluoromethylated compounds.

Nickel-catalyzed direct C-H trifluoroethylation of heteroarenes with trifluoroethyl iodide

A highly selective nickel-catalyzed C-H trifluoroethylation of heteroarenes was developed with the assistance of a monodentate directing group. This protocol provides efficient access to various trifluoroethyl-substituted heteroarenes, including indoles, pyrroles, furans, and thiophenes, with commercially available CF₃CH₂I as an alkylation reagent. This robust catalytic procedure is scalable and tolerates a broad range of functional groups. Moreover, multifluoroalkylation of indoles is also viable.

Aromatic substitutions of arenediazonium salts via metal catalysis, single electron transfer, and weak base mediation

Radical aromatic substitutions of arenediazonium salts can be initiated by various methods. The recent developments of weak base-mediated protocols provide great advantages over conventional metal-mediated or photoredox reactions by their operational simplicity, price, hazard potential and scalability.

CF3SO2X (X = Na, Cl) as reagents for trifluoromethylation, trifluoromethylsulfenyl-, -sulfinyl- and -sulfonylation. Part 1: Use of CF3SO2Na

Sodium trifluoromethanesulfinate, CF3SO2Na, and trifluoromethanesulfonyl chloride, CF3SO2Cl, are two popular reagents that are widely used for the direct trifluoromethylation of a large range of substrates. Further, these two reagents are employed for the direct trifluoromethylsulfenylation and trifluoromethylsulfinylation, the introduction of the SCF3 and the S(O)CF3 group, respectively. In addition to the aforementioned reactions, the versatility of these two reagents is presented in other reactions such as sulfonylation and chlorination. This first part is dedicated to sodium trifluoromethanesulfinate.

Palladium-catalyzed regioselective C–H fluoroalkylation of indoles at the C4-position

An exclusive catalytic C4-selective fluoroalkylation of indoles with highly active (1H, 1H-perfluoroalkyl)mesityliodonium triflate has been described.