Synthesis ofN-Containing Heterocyclic Compounds Using Visible-light Photoredox Catalysis

Palladium catalyzed ortho-C(sp2)-H activation/cyclization of aryl amines assisted by imine and vinylacetic acid

Palladium-catalyzed directed C - H functionalization/cyclization is an effective approach for synthesizing nitrogen heterocycles. Imine, known for its ease of installation/removal, has been extensively used in the C-H activation of aldehydes, ketones, and alkylamines. Nevertheless, it has been rarely explored in the C(sp2)-H activation of aryl amines because of the generation of a strained four-membered palladacycle. Herein, an imine directed palladium catalyzed C(sp2)-H functionalization of aryl amines assisted by vinylacetic acid is established, providing access to a variety of γ-lactone fused tetrahydroquinolines under mild reaction conditions. The methodology demonstrates broad substrate scope and good functional group tolerance, representing notable advancement in organic synthesis. Mechanistic experiments are performed to clarify how the C(sp2)-H activation occurs, indicating the crucial role of vinylacetic acid. DFT calculations supports the observations, elucidating the strained four-membered ring C-H activation barrier is overcome via coordination and hydrogen bond interaction of vinylacetic acid.

Photocatalytic radical cyclization of N-(o-cyanobiaryl)acrylamides with oxime esters

A visible-light-mediated radical cascade cyclization of N-(o-cyanobiaryl)acrylamides with oxime esters for the assembly of acyl-containing pyrido[4,3,2-gh]phenanthridines has been developed. The present protocol tolerates a wide range of oxime esters through a single reaction via fragmentation, radical addition, nitrile insertion, and cyclization under mild conditions.

Visible light-mediated difluoromethylation/cyclization in batch and flow: scalable synthesis of CHF2-containing benzimidazo- and indolo[2,1-a]isoquinolin-6(5H)-ones

We report here a practical and cost-effective method for the synthesis of CHF2-containing benzimidazo- and indolo[2,1,a]-isoquinolin-6(5H)-ones through a visible light-mediated difluoromethylation/cyclization cascade. The method, which affords functionalized multifused N-heterocyclic scaffolds in moderate to high yields under mild reaction conditions, is also easily scalable using low-cost 3D printed photoflow reactors.

A Recent Update on the Visible Light-promoted Organic Transformations - A Mini-review

Visible light-induced reactions are a rapidly developing and powerful technique to promote organic transformations. They provide green and sustainable chemistry and have recently received increasing attention from chemists due to their wide application in organic synthesis. Light energy is eco-friendly, cheap, green, and inexhaustible with potential industrial and pharmaceutical applications. In this review, the most recent advances in visible light-induced reactions (2021-till date) have been highlighted.

Photoredox catalysis enabling decarboxylative radical cyclization of γ,γ-dimethylallyltryptophan (DMAT) derivatives: formal synthesis of 6,7-secoagroclavine

An unusual photoredox-catalyzed radical decarboxylative cyclization cascade reaction of γ,γ-dimethylallyltryptophan (DMAT) derivatives containing unactivated alkene moieties has been developed, providing green and efficient access to various six-, seven-, and eight-membered ring 3,4-fused tricyclic indoles. This type of cyclization, which was hitherto very difficult to comprehend in ergot biosynthesis and to accomplish by more conventional procedures, enables the synthesis of ergot alkaloid precursors. In addition, this work describes a mild, environmentally friendly method to activate, reductively and oxidatively, natural carboxylic acids for decarboxylative C-C bond formation by exploiting the same photocatalyst.

Visible-Light-Promoted Cyanoalkylation/Cyclization Cascade Reaction to Assemble Polyheterocycles in Continuous Flow

This study describes a visible-light-induced cascade reaction for preparing cyanoalkyl-containing polyheterocycles initiated by the photoinduced radical cascade addition of N-arylacrylamide derivatives using cyclic oxime esters as radical sources followed by cyanoalkyl-mediated cyclization. This protocol features outstanding functional group compatibility, providing a variety of desired phenanthridine derivatives in moderate to good yields. Moreover, the application of a microflow technique enhanced these reactions compared with the equivalent batch reaction, significantly reducing reaction times to 10 min.

The Renaissance of Organo Nitriles in Organic Synthesis

In the arena of functional group-oriented organic synthesis, the nitrile or cyano functionality is of immense importance. The presence of nucleophilic N -atom, π-coordinating ability of the triple bond, and electrophilic C-center imparts unique and interesting reactivity. Owing to the ability of the nitrile to transform into various other functional groups or intermediates, the chemistry is very rich and diverse. In particular, the involvement of nitrile in numerous organic reactions such as inter- or intramolecular alkyne insertion, [2 + 2 + 2] cycloaddition with alkynes, [3 + 2] cycloaddition with azides, [4 + 2] cycloaddition with diene allow the synthesis of many important carbocycles and heterocycles. Furthermore, the nitrile serves as a directing group in many C-H bond functionalization reactions to introduce diverse functionality and participate as a radical acceptor in radical cascade strategies to obtain a large variety of functional molecules. This review mainly focuses on the reactivity and diverse synthetic application of the nitrile including C-H bond functionalization, alkyne insertion, cycloaddition, and thermal or photochemical cascade strategy. The objective of the current review aims at bringing out the striking collection of various nitrile-triggered organic transformations.

Recent advances in 3-aminoindazoles as versatile synthons for the synthesis of nitrogen heterocycles

Nitrogen-based heterocycles are an important class of structural scaffolds distributed in biologically active natural products, medicinal chemistry, and agrochemicals. Hence, there is increasing interest in the development of novel synthetic strategies for the construction of these privileged structural motifs. Recently, 3-aminoindazoles have emerged as versatile synthons participating in a variety of condensation annulation, denitrogenative transannulation and rearrangement ring expansion reactions, which provide efficient synthetic routes for the formation of nitrogen heterocycles. This review systematically highlights for the first time the most recent advances in 3-aminoindazoles to provide a deep understanding of using 3-aminoindazoles as versatile synthons in organic transformations for synthetic and medicinal chemists.

Stereoselective intermolecular radical cascade reactions of tryptophans or ɤ-alkenyl-α-amino acids with acrylamides via photoredox catalysis

The radical cascade reaction is considered as one of the most powerful methods to build molecular complexity. However, highly stereoselective intermolecular radical cascade reactions that can produce complex cyclic compounds bearing multiple stereocenters via visible-light-induced photocatalysis have been challenging yet desirable. Herein we report a facile and efficient synthesis of multi-substituted trans-fused hexahydrocarbazoles via a stereoselective intermolecular radical cascade reaction of readily available tryptophans and acrylamides enabled by visible-light-induced photoredox catalysis. The trans-fused hexahydrocarbazoles with up to five stereocenters including two quaternary ones can be accessed in up to 82% yield, >20/1 diastereoselectivity, and 96% ee. Interestingly, the tetrahydrocarbazoles are favorably formed when the reaction is performed under air. Moreover, by simply switching the starting material from tryptophans to ɤ-alkenyl substituted α-amino acids, this protocol can be further applied to the stereoselective syntheses of 1,3,5-trisubstituted cyclohexanes which are otherwise challenging to access. Preliminary mechanistic studies suggest that the reaction goes through radical addition cascade and radical-polar crossover processes.

Photocatalytic radical cascade reactions enable the facile construction of diverse cyclic compounds, though they rely on templated precursors. In this paper, the authors report on stereoselective intermolecular radical cascade reaction between tryptophan or ɤ-alkenyl substituted amino acids and acrylamides to synthesise multi-substituted trans-fused hexahydrocarbazoles or 1,3,5-trisubstituted cyclohexanes.

Photoinduced Construction of a Benzothienopyridine-S,S-dioxide Framework Enabled by Polychloropyridyl Multifunctional Motifs

Reported herein is a visible-light-induced, catalyst-free intramolecular cyclization of 4-phenylsulfonyl-2,3,5,6-tetrachloropyridine, leading to rapid assembly of a series of unprecedented benzo[4,5]thieno[3,2-c]pyridine 5,5-dioxide scaffolds under mild conditions. The rational introduction of a perchloropyridin-4-yl module significantly facilitated this photoinduced process and offers a versatile platform for broad structural variation. Mechanistic studies revealed that a newly identified charge-transfer complex with carbonate is crucial to this photoinduced process.

Recent Advances in C-F Bond Cleavage Enabled by Visible Light Photoredox Catalysis

The creation of new bonds via C-F bond cleavage of readily available per- or oligofluorinated compounds has received growing interest. Using such a strategy, a myriad of valuable partially fluorinated products can be prepared, which otherwise are difficult to make by the conventional C-F bond formation methods. Visible light photoredox catalysis has been proven as an important and powerful tool for defluorinative reactions due to its mild, easy to handle, and environmentally benign characteristics. Compared to the classical C-F activation that proceeds via two-electron processes, radicals are the key intermediates using visible light photoredox catalysis, providing new modes for the cleavage of C-F bonds. In this review, a summary of the visible light-promoted C-F bond cleavage since 2018 was presented. The contents were classified by the fluorosubstrates, including polyfluorinated arenes, gem-difluoroalkenes, trifluoromethyl arenes, and trifluoromethyl alkenes. An emphasis is placed on the discussion of the mechanisms and limitations of these reactions. Finally, my personal perspective on the future development of this rapidly emerging field was provided.

Cleavage via Selective Catalytic Oxidation of Lignin or Lignin Model Compounds into Functional Chemicals

Lignin, a complex aromatic polymer with different types of methoxylated phenylpropanoid connections, enables the sustainable supply of value-added chemicals and biofuels through its use as a feedstock. Despite the development of numerous methodologies that upgrade lignin to high-value chemicals such as drugs and organic synthesis intermediates, the variety of valuable products obtained from lignin is still very limited, mainly delivering hydrocarbons and oxygenates. Using selective oxidation and activation cleavage of lignin, we can obtain value-added aromatics, including phenols, aldehydes, ketones, and carboxylic acid. However, biorefineries will demand a broad spectrum of fine chemicals in the future, not just simple chemicals like aldehydes and ketones containing simple C = O groups. In particular, most n-containing aromatics, which have found important applications in materials science, agro-chemistry, and medicinal chemistry, such as amide, aniline, and nitrogen heterocyclic compounds, are obtained through n-containing reagents mediating the oxidation cleavage in lignin. This tutorial review provides updates on recent advances in different classes of chemicals from the catalytic oxidation system in lignin depolymerization, which also introduces those functionalized products through a conventional synthesis method. A comparison with traditional synthetic strategies reveals the feasibility of the lignin model and real lignin utilization. Promising applications of functionalized compounds in synthetic transformation, drugs, dyes, and textiles are also discussed.

Recent developments and perspectives in the copper-catalyzed multicomponent synthesis of heterocycles

Heterocyclic compounds have become an inevitable part of organic chemistry due to their ubiquitous presence in bioactive compounds. Copper-catalyzed multicomponent synthesis of heterocycles has developed as the most convenient and facile synthetic route towards complex heterocyclic motifs. In this review, we discuss the advancements in the field of copper-catalyzed multicomponent reactions for the preparation of heterocycles since 2018.

Heterocycles are abundant in several pharmaceutical and naturally occurring compounds. Copper-catalyzed multicomponent reactions are a convenient method for easy access to heterocycles. In this review, we focus on the advancement in this field for the past two years.

Rhodium(III)-Catalyzed Redox-Neutral [3+3] Annulation of N-nitrosoanilines with Cyclopropenones: A Traceless Approach to Quinolin-4(1H)-One Scaffolds

A traceless approach to quinolin-4(1H)-one scaffolds through Rh(III)-catalyzed redox-neutral [3+3] cyclization of N-nitrosoanilines with cyclopropenones has been achieved. This protocol features short reaction time and atom-economical combination without extra additives, which can be further applied in the construction of privileged heterocyclic compounds in pharmaceutical chemistry.

Synthesis of 3-sulfonylquinolines by visible-light promoted metal-free cascade cycloaddition involving N-propargylanilines and sodium sulfinates

A visible-light promoted radical cascade reaction of N-propargylanilines with sodium sulfinates as sulfonyl radical precursors was developed under metal-free conditions.

N-Arylation of (hetero)arylamines using aryl sulfamates and carbamates via C–O bond activation enabled by a reusable and durable nickel(0) catalyst

An effective and general aryl amination protocol has been developed using a magnetically recoverable Ni(0) based nanocatalyst.

Dehydrogenation and α-functionalization of secondary amines by visible-light-mediated catalysis

A visible-light-mediated process for dehydrogenation of amines has been described. The given protocol showed a broad substrate scope, mild reaction conditions and excellent results without the requirement of tedious purification.

Discovery of γ-lactam derivatives containing 1,3-benzodioxole unit as potential anti-phytopathogenic fungus agents

A series of γ-lactam analogs containing 1,3-benzodioxole moiety were designed, and these derivatives were synthesized from the lead compound of lactam via a structural diversity-oriented synthesis, their structures were confirmed by ¹HNMR,¹³CNMR, ESI-MS spectrum. Their antifungal activities were evaluated against four serious and typically crop-threatening agricultural fungi, including Rhizoctonia solani, Alternaria tenuis Nees, Gloeosporium theae-sinensis, and Fusarium graminearum. Some of these derivatives exhibited activity against Alternaria tenuis Nees higher than that of commercial fungicides carbendazim, such as compounds 7a, 7b, and 7i, compared with the blank control, some of these derivatives showed good antifungal activities against Gloeosporium theae-sinensis and Fusarium graminearum. The systematic study provides evidences for further structural modification and application of lactam analogues as antifungal agents for agriculture.

Analytical and ecotoxicological studies on degradation of fluoxetine and fluvoxamine by potassium ferrate

A large amount of pharmaceuticals are flushed to environment via sewage system. The compounds are persistent in environment and are very difficult to remove in drinking water treatment processes. Degradation of fluoxetine (FLU) and fluvoxamine (FLX) by ferrate(VI) were investigated. For the 10 mg/L of FLU and FLX, 35% and 50% of the compounds were degraded in the presence of 50 mg/L FeO₄^2- within 10 minutes, respectively. After 10 minutes of the reaction, degradation of FLU and FLX is affected by formation of by-products which were likely more reactive with ferrate and competed in the reaction with FeO₄^2-. In the case of FLU, the identified degradation by-products were hydrofluoxetine, N-methyl-3-phenyl-2-propen-1-amine, 4-(trifluoromethyl)phenol and 1-{[(1R,S)-1-Phenyl-2-propen-1-yl]oxy}-4-(trifluoromethyl)benzene. In the case of FLX, the degradation by-products were fluvoxamine acid and 5-methoxy-1-[4-(trifluoromethyl)phenyl]pent-2-en-1-imine. The results of the ecotoxicological study based on protozoa Spirostomum ambiguum have shown that 50 mg/L FeO₄^2- reduced toxicity of 10 mg/L of FLU and FLX by around 50%. However, in the case of FLX, the results of the ecotoxicological study suggested formation of slightly more toxic compound(s) than FLX during reaction with FeO₄^2-. Application of ferrate(VI) is a viable option for drinking water treatment process; however, caution is needed due to formation of by-products with unknown human health risk.

Recent Advances on Metal-Free, Visible-Light- Induced Catalysis for Assembling Nitrogen- and Oxygen-Based Heterocyclic Scaffolds

Heterocycles are important class of structures, which occupy a major space in the domain of natural and bioactive compounds. For this reason, development of new synthetic strategies for their controllable synthesis became of special interests. The development of novel photoredox systems with wide-range application in organic synthesis is particularly interesting. Organic dyes have been widely applied as photoredox catalysts in organic synthesis. Their low costs compared to the typical photocatalysts based on transition metals make them an excellent alternative. This review describes proceedings since 2015 in the area of application of metal-free, visible-light-mediated catalysis for assembling various heterocyclic scaffolds containing five- and six-membered rings bearing nitrogen and oxygen heteroatoms.

Visible-light-induced radical trifluoromethylthiolation of N-(o-cyanobiaryl)acrylamides

An efficient and general visible-light-mediated trifluoromethylthiolation of N-(o-cyanobiaryl)acrylamides has been successfully accomplished using N-trifluoromethylthiosaccharin as an effective source of SCF3 radicals. The reaction was proposed to proceed via a domino radical trifluoromethylthiolation/cyano insertion/cyclization to afford the corresponding SCF3-containing ring-fused phenanthridine derivatives in moderate to good yields.

Oxidative Deprotection of p-Methoxybenzyl Ethers via Metal-Free Photoredox Catalysis

An efficient and greener deprotection method for p-methoxybenzyl (PMB) ethers using a metal-free visible light photoredox catalyst and air and ammonium persulfate as the terminal oxidants is presented. Various functional groups and protecting groups were tolerated in the developed method to achieve good to excellent yields in short reaction times. Significantly, the developed method was compatible with PMB ethers derived from primary, secondary, and tertiary alcohols and a gram-scale reaction. Mechanistic studies support a proposed reaction mechanism that involves single electron oxidation of the PMB ether.

Intramolecular Reductive Cyclization of o-Nitroarenes via Biradical Recombination

A visible-light-induced/thiourea-mediated intramolecular cyclization of o-nitroarenes under mild conditions is realized for the first time, which provides an efficient and environmentally friendly way to access pharmaceutical relevant quinazolinone derivatives. The reaction can be easily extended to gram level by using a continuous-flow setup with high efficiency. Mechanistic investigation including control experiments, transient fluorescence, UV-vis spectra, and DFT calculations suggests that the formation of active biradical intermediates via intramolecular single electron transfer (SET) is key stage in the catalytic cycle.

Synthesis of (E)-2-amino-N'-benzylidenehexahydroquinoline-3-carbohydrazide

A one-pot, multi-component protocol for the synthesis of a new class of functionalized quinoline carbohydrazide derivatives via reaction of various anilines, dimedone, aromatic aldehydes, and cyanoacetohydrazide is described. The reactions are completed in the presence of catalytic amount of piperidine, respectively, in melt conditions and then in ethanol/water (1:1) as green solvent at 80 °C. Mild conditions, green medium, short reaction times, simple workup and purification process with no chromatographic technique, and good yields are the main advantages of this method.

Organo-Photoredox Catalyzed Oxidative Dehydrogenation of N-Heterocycles

We report here for the first time the catalytic oxidative dehydrogenation of N-heterocycles by a visible-light organo-photoredox catalyst with low catalyst loading (0.1-1 mol %). The reaction proceeds efficiently under base- and additive-free conditions with ambient air at room temperature. The utility of this benign approach is demonstrated by the synthesis of various pharmaceutically relevant N-heteroarenes such as quinoline, quinoxaline, quinazoline, acridine, and indole.