Metal-Free Thiocyanation of Imidazoheterocycles through Visible Light Photoredox Catalysis

Oxidative Organic Transformations Photocatalyzed by NDI in Visible Light

In this work, we report the synthesis and photocatalytic properties of N,N-bis(n-hexyl)-2-bromo-6-(n-hexylamino)-1,4,5,8-naphthalenetetracarboxylic diimide photocatalyst, NDI-PC, in visible light. In the presence of air or oxidant, NDI-PC efficiently enables multiple photooxygenations of isoquinolines, thiocyanation of phenylimidazopyridines, functionalization of quinolinones by allowing regioselective installation of an SCN, SeCN, SPh, SePh, Cl, Br, or I group at the C-3 position, and isomerization of alkenes. Mechanistic investigations suggest an oxidative photoredox process for oxygenation and C-H functionalization, while isomerization is believed to proceed through a photosensitization pathway.

Photocascade chemoselective controlling of ambident thio(seleno)cyanates with alkenes via catalyst modulation

Controlling the ambident reactivity of thiocyanates in reaction manifolds has been a long-standing and formidable challenge. We report herein a photoredox strategy for installing thiocyanates and isothiocyanates in a controlled chemoselective fashion by manipulating the ambident-SCN through catalyst modulation. The methodology allows redox-, and pot-economical 'on-demand' direct access to both hydrothiophene and pyrrolidine heterocycles from the same feedstock alkenes and bifunctional thiocyanomalonates in a photocascade sequence. Its excellent chemoselectivity profile was further expanded to access Se- and N-heterocycles by harnessing selenonitriles. Redox capability of the catalysts, which dictates the substrates to participate in a single or cascade catalytic cycle, was proposed as the key to the present chemodivergency of this process. In addition, detailed mechanistic insights are provided by a conjugation of extensive control experiments and dispersion-corrected density functional theory (DFT) calculations.

Electrochemical selective divergent C-H chalcogenocyanation of N-heterocyclic scaffolds

An electrochemical direct selective C-H chalcogenocyanation approach for indolizine derivatives under mild conditions has been described. Cyclic enone-fused, chromone-fused and 2-substituted indolizines possessing EDGs (electron donating groups) and EWGs (electron withdrawing groups) were successfully reacted with NH4SCN and KSeCN under electrochemical conditions to provide a wide array of mono and bis-chalcogenocyanate-indolizines in 75-94% yields. In addition, 1-substituted imidazo[1,5-a]quinolines were also successfully chalcogenocyanated under the optimized reaction conditions providing a platform for the synthesis of pharmaceutically privileged molecules. By switching the reaction conditions, the developed protocol offers selective synthesis of C-3 thiocyanate and 1,3 bis-thiocyanate indolizines in good to excellent yields under catalyst-free conditions. On the basis of control experiments and cyclic voltammetry data, a plausible reaction pathway is also presented.

Recent advances in radical thiocyanation cyclization or spirocyclization reactions

Organic thiocyanates are valuable biological moities and drug-building blocks. They can also transform effectively into thioethers, thiols, alkynyl thioethers, and thiocarbamates in synthetic chemistry. With respect to the merits of thiocyanates, many chemists and our research team have developed diverse strategies to access SCN-revised heterocycles/spirocycles via an effective radical cyclization process. Hence, this review article first describes the importance/application of thiocyanates. Subsequently, it summarizes the reaction conditions, substrate scopes, and plausible mechanism, respectively, of the excellent work stated above.

Visible Light Induced Three-Component 1,2-Dicarbofunctionalization of Alkenes and Alkynes

Harnessing visible-light in organic synthesis is one of the most effective methods that aligns with green and sustainable chemistry principles and hence skyrocketed in the last two decades. Similarly, three-component 1,2-dicarbofunctionalization of alkenes and alkynes has recently been a great choice to construct complex molecular systems in an easy and rapid manner. Therefore, light-induced reactions can be an excellent alternative to carry out 1,2-dicarbofunctionalization reactions, and very recently, organic chemists across the globe have fascinated us with their interesting articles. In this present review, we have summarized the recent advancements in the area of visible light induced three-component 1,2-dicarbofunctionalization of alkenes and alkynes till March 2023. We have categorized the discussion based on the catalysts used to carry out the transformations for better understanding and different important aspects of these transformations have also been covered.

Visible light induced regioselective C-3 thiocyanation of imidazoheterocycles through naphthalimide dye based photoredox catalysis

A visible light induced C-3 thiocyanation of imidazo[1,2-a]pyridines by using a naphthalimide based photoredox catalyst has been reported. Tolerance of electron withdrawing and donating groups at different positions of the imidazo[1,2-a]pyridine ring led to a wide substrate accessibility of this method. This methodology is further reproducible with other heterocycles like benzo[d]imidazo[2,1-b]thiazoles, indoles, azaindoles, and anilines.

Straightforward access to α-thiocyanoketones and thiazoles from sulfoxonium ylides

Efficient, versatile, and metal-free strategies for synthesizing α-thiocyanoketones and thiazoles from β-ketosulfoxonium ylides and ammonium thiocyanate have been described. Due to its simplicity, benign reaction conditions, excellent chemoselectivity, and high yield, this method represents a unique approach for divergent synthesis. Finally, the potential value of the developed methods is demonstrated via large-scale reactions and synthesis of Fanetizole, an anti-inflammatory drug.

Blue Light Irradiated Metal-, Oxidant-, and Base-Free Cross-Dehydrogenative Coupling of C(sp2)-H and N-H Bonds: Amination of Naphthoquinones with Amines

Herein, we report a blue-light-driven amination of C(sp²)-H bond of naphthoquinones and quinones with the N-H bond of primary and secondary amines for the synthesis of 2-amino-naphthoquinones and 2-amino-quinones. The coupling of naphthoquinones with a wide array of aliphatic, aromatic, chiral, primary, and secondary amines having electron donating (-CH₃, -OCH₃, -SCH₃), withdrawing (-F, -Cl, -Br, -I), and CO₂H, -OH, -NH₂ groups with acidic protons selectively occurred to afford C-N coupled 2-amino-naphthoquinones in 60-99% yields and hydrogen gas as a byproduct in methanol solvent without using any additional reagents, additives, and oxidant under the blue light irradiation. Mechanistic insight by DFT computation, controlled experiments, kinetic isotopic effect, and substitution effect of the substrates suggest that the reaction proceeds by radical pathway in which naphthoquinone forms a highly oxidizing naphthoquinonyl biradical upon irradiation of blue light (457 nm). Consequently, electron transfer from electron-rich amine to an oxidizing naphthoquinonyl biradical leads to a naphthoquinonyl radical anion and aminyl radical cation, followed by proton transfer and delocalization leading to a carbon-centered naphthoquinonyl radical. The cross-coupling of naphthoquinonyl carbon-centered and aminyl nitrogen radicals forms a C-N bond, with subsequent elimination of hydrogen gas (which was also confirmed by GC-TCD), affording 2-amino-1,4-naphthoquinone under metal-, reagent-, base-, and oxidant-free conditions.

Nonyl Acridine Orange as a Prospective Photocatalyst in Chalcogenylation of Coumarins and Quinolinones

A mild and efficient method for preparation of 3-sulfenyl and 3-selenyl coumarins and quinolinones mediated by artificial light or sunlight is presented. The elaborated protocol highlights the use of nonyl acridine orange as a photocatalyst to generate a sulfenyl radical from thiols that is further trapped by a heterocycle. The utility of the protocol is justified by a diverse scope of thiols, including short cysteine-containing peptides. The same reaction conditions can be applied for preparation of 3-selenyl coumarins and quinolinones. Various protected and unprotected selenocysteine-containing peptides were successfully utilized demonstrating high tolerance for amino acids with sensitive groups (Arg, Lys, Trp, His, and Tyr).

Light-Driven Carbon-Carbon Coupling of α-sp3-CH of Aliphatic Alcohols with sp2-CH Bond of 1,4-Naphthoquinones

Here, an α-selective C_sp3-H bond functionalization of primary aliphatic alcohols with 1,4-naphthoquinones yielded C_sp²-C_sp² coupled products driven by blue-LED light under catalyst, metal, base, and reagent-free conditions. In this transformation, cleavage of three C-H bonds (two sp³-C-H, one sp²-C-H, and one O-H) and four new bonds formed, leading to fluorescent 2-acylated-1,4-naphthohydroquinones.

Visible-Light Promoted Regioselective Oxygenation of Quinoxalin-2(1H)-ones Using O2 as an Oxidant

A visible-light-mediated sustainable approach for metal-free oxygenation of quinoxalin-2(1H)-one by employing Mes-Acr-MeClO₄ as a photocatalyst without using any additive or cocatalyst is reported here. O₂ served as the eco-friendly and green oxidant source for this conversion. In addition, the protocol exhibited high regioselectivity and tolerance toward a broad spectrum of functional groups to furnish quinoxaline-2,3-diones in good to excellent yields.

Direct Access to Thiocyano-Thioesters from Cyclic Thioacetals via Photoredox Catalysis: An Introduction of Two Functional Groups in One Pot

The cyanation of organic compounds is an important synthetic transformation and mainly relies on a toxic CN source. Undeniably, thiocyanate salt has emerged as a very mild and environmentally benign CN source, yet its synthetic utility for cyanation is highly limited to very few types of organic compounds. Herein, we report the direct cyanation of cyclic thioacetals for accessing compounds with two different functional groups (thiocyano-thioesters) in one pot using sodium thiocyanate via photoredox catalysis. The protocol has been further extended for the direct cyanation of disulfides and diselenide to access aryl thiocyanates and aryl selenocyanate. A plausible mechanism has been proposed based on a series of control experiments, cyclic voltammetry and Stern-Volmer studies.

Visible-Light-Induced Photocatalytic Trifluoromethylation of Bunte Salts: Easy Access to Trifluoromethylthiolated Synthons

A metal-free visible-light-induced trifluoromethylation of Bunte salts of α-bromoketones/alkyl bromide/benzyl bromides/functionalized allyl (Baylis-Hillman) bromides has been accomplished using Langlois' reagent in the presence of inexpensive eosin Y as a photocatalyst to form the privileged trifluoromethylthiolated synthons. The method is straightforward, operationally simple, and endowed with broad substrate scope and good functional group tolerance.

Iodine-Catalyzed Multicomponent Synthesis of Highly Fluorescent Pyrimidine-Linked Imidazopyridines

Herein, we report a metal-free one-pot three-component reaction of aryl methyl ketones, 2-aminopyridines, and barbituric acids for the synthesis of pyrimidine-linked imidazopyridines using a catalytic amount of molecular iodine in DMSO medium. This process involves a one-pot C-H oxidation, followed by the formation of one C-C and two C-N bonds. A wide variety of aryl methyl ketones and 2-aminopyridines were found to be suitable for this methodology. The UV and fluorescence properties of the synthesized products were studied in water and DMSO media. Most of the synthesized products exhibited very good to excellent fluorescence quantum yield. Among all the products, compounds 4p and 4q showed the maximum fluorescence quantum yield (0.36) in water medium under basic conditions and compound 4c showed the maximum fluorescence quantum yield (0.75) in DMSO medium.

Recent Developments in the Photochemical Synthesis of Functionalized Imidazopyridines

Imidazopyridines constitute one of the most important scaffolds in medicinal chemistry, as their skeleton could be found in a myriad of biologically active molecules. Although numerous strategies were elaborated for imidazopyridine preparation in the 2010s, novel eco-compatible synthetic approaches have emerged, conscious of climate change concerns. In this framework, photochemical methods have been promoted to conceive this heterocyclic motif over the last decade. This review covers the recently published works on synthesizing highly functionalized imidazopyridines by light induction.

Visible light-induced functionalization of indazole and pyrazole: a recent update

Indazole and pyrazole are renowned as a prodigious class of heterocycles having versatile uses in medicinal as well as industrial chemistry. Considering sustainable approaches, recently, photocatalysis has become an indispensable tool in organic chemistry due to its application for the activation of small molecules and the use of a clean energy source. In this review, we have highlighted the use of metal-based photocatalysts, organic photoredox catalysts, energy transfer photocatalysts and electron-donor-acceptor complexes in the functionalization of indazole and pyrazole. This perspective is arranged based on the types of functionalization reactions on indazole and pyrazole. A detailed discussion regarding the reaction mechanism of each reaction is given to provide a comprehensive guide to the reader. Finally, a summary of existing challenges and the future outlook towards the development of efficient photocatalytic methods for functionalization of these heterocycles is also presented.

Recent advances in photochemical and electrochemically induced thiocyanation: a greener approach for SCN-containing compound formation

Techniques utilizing photo- and electrochemically induced reactions have been developed to accelerate organic processes. These techniques use light or electrical energy (electron transfer) as a direct energy source without using an initiator or reagent. Thiocyanates are found in biologically active and pharmacological compounds and can be converted into various functional groups. It is one of the most prominent organic scaffolds. Significant development in photo- and electro-chemically induced thiocyanation procedures has been made in recent years for the conception of carbon-sulfur bonds and synthesis of pharmaceutically important molecules. This review discusses different photo- and electro-chemically driven thiocyanation C(sp3)-SCN, C(sp2)-SCN, and C(sp)-SCN bond conception processes that may be useful to green organothiocyanate synthesis. We focus on the synthetic and mechanistic characteristics of organic photo- and electrochemically accelerated C-SCN bond formation thiocyanation reactions to highlight major advances in this novel green and sustainable research field.

Photochemical Csp2-H bond thiocyanation and selenocyanation of activated arenes, batch and continuous-flow approaches

Herein, we report an eco-friendly photochemical oxidative C_sp²-H thiocyanation and selenocyanation of activated arenes. The reaction proceeds under Violet LED irradiation in the presence of K₂S₂O₈, which quickly oxidizes KSCN and KSeCN, finally producing arylthio/selenocyanates. Using this benign, atom-economic protocol, the desired chalcogenide products were obtained regioselectively, with isolated yields that range from very good to excellent. Although, mechanistic study indicates that it is difficult to distinguish between a radical to a S_EAr reaction mechanism between the photo-induced formed ^•SCN, for the former, or NCSSCN, for the latter, to the aromatic heterocycles. The inhibition experiment together with the observed reactivity and regioselectivity, would be in agreement with the latter. The synthetic methodology designed could be successfully adapted to continuous-flow systems in a segmented-flow regime, employing the organic phase as the product reservoir. Using this setup, the advantage of the latter can be demonstrated by reducing the reaction time and improving the product yields. Similarly, the scaling up of the reaction to gram scale resulted in favorable outcomes by the flow setup, which installs the photo-flow chemistry as a powerful tool to be included into routine reaction procedures, which have great relevance for the pharmaceutical industry.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class─either sp2 or sp3 C-H functionalization─lends perspective and tactical strategies for use of these methods in synthetic applications.

Oxidative thiocyanation of allylic alcohols: an easy access to allylic thiocyanates with K2S2O8 and NH4SCN

A practical method for the synthesis of allylic thioacyanates from allylic alcohols was disclosed employing K2S2O8 as the oxidant and NH4SCN as the thiocyanate source.

Photochemical regioselective C–H arylation of imidazo[1,2-a]pyridine derivatives using chlorophyll as a biocatalyst and diazonium salts

This communication describes the development of a mild method for the arylation of imidazo[1,2-a]pyridine with diazonium salt derivatives and using chlorophyll as a biocatalyst via visible-light catalysis.

Ortho C-H Functionalization of 2-Arylimidazo[1,2-a]pyridines

C-H activation and functionalization is quite promising in recent days as the strategy offers a go-to general method for different bond formations and hence grants synthetic versatility. At the same time, imidazopyridine, a fused bicycle of imidazole moiety with pyridine ring, has a profound impact due to its ubiquitous and prodigious application in medicinal as well as material chemistry. The presence of N-1 atom in 2-arylImidazo[1,2-a]pyridine facilitates the coordination with metal catalysts leading to the formation of ortho-substituted products. This review summarizes all the articles on ortho C-H functionalization of 2-arylImidazo[1,2-a]pyridines published till August 2021.

Recent progress in the radical α-C(sp3)-H functionalization of ketones

The direct use structurally simple ketones as α-ketone radical sources for α-C(sp³)-H functionalization is a sustainable and powerful approach for constructing complex and multifunctional chemical scaffolds with diverse applications. The reactions of α-ketone radicals with alkenes, alkynes, enynes, imides, and imidazo[1,2-a]pyridines have broadened the structural diversity and complexity of ketones. Through chosen illustrative examples, we outline the recent progress in the development of methods that enable the radical α-C(sp³)-H functionalization of ketones, with an emphasis on radical initiation systems and possible mechanisms of the transformations. The application of these strategies is illustrated by the synthesis of several biologically active molecules and drug molecules. Further subdivision is based on substrate type and reaction type.

Progress and recent trends in the direct selenocyanation of (hetero)aromatic C-H bonds

This review covers recent advances in the direct selenocyanation of (hetero)aromatic C-H bonds with an emphasis on the reaction mechanisms. This novel approach is an effective means of preparing a variety of aromatic and heteroaromatic selenocyanates, which are extremely versatile synthetic precursors of selenium-containing compounds, such as selenols, seleninic acids, selenides, diselenides, and trifluoromethyl selenides.

Visible-Light-Induced Photocatalyst-Free Aerobic Hydroxyazidations of Indoles: A Highly Regioselective and Stereoselective Synthesis of trans-2-Azidoindolin-3-ols

A visible-light-promoted aerobic hydroxyazidation of indole derivatives with TMSN₃ is described. The reaction proceeded under photocatalyst-free conditions to furnish trans-2-azidoindolin-3-ols with high regioselectivity and stereoselectivity. The protocol is operationally simple, and the starting materials (e.g., 1-(pyrimidin-2-yl) indoles, azidotrimethylsilane, and air) are readily available. The proposed mechanism in which substrates act as photocatalysts upon excitation using blue light-emitting diodes (LEDs) was supported by experimental studies.

Design and Use of Electrophilic Thiocyanating and Selenocyanating Reagents: An Interesting Trend for the Construction of SCN- and SeCN-Containing Compounds

Organothiocyanate and organoselenocyanate compounds are of paramount importance in organic chemistry as they are key intermediates to access sulfur- and selenium-containing compounds. Therefore, among the different synthetic pathways to get SCN- and SeCN-containing molecules, original methodologies using electrophilic reagents have recently been explored. This Minireview will showcase the recent advances that have been made. In particular, the design of several electrophilic sources and their applications for the thiocyanation and the selenocyanation of various classes of compounds will be highlighted and discussed.

K2S2O8-mediated regio- and stereo-selective thiocyanation of enamides with NH4SCN

A direct and straightforward thiocyanation of enamides with NH₄SCN under metal-free conditions has been accomplished. A variety of (E)-β-thiocyanoenamides are readily produced in a regio- and stereo-selective manner. The protocol features mild reaction conditions, good functional group tolerance and operational simplicity. The potential utility of this strategy was further demonstrated by transformation of thiocyanate into thiotetrazole-containing compounds and a Pd-catalyzed cross-coupling reaction to afford six- or seven-membered sulfur-containing heterocycles. Mechanistic insights into the reaction indicate that the reaction may proceed via a radical mechanism.

Visible-Light-Mediated Additive-Free Decarboxylative Ketonization Reaction of Acrylic Acids: An Access to α-Thiocyanate Ketones

Visible-light-mediated additive-free decarboxylative functionalization of acrylic acids has been developed. The reaction uses inexpensive organic dye 9,10-dicyanoanthracene as a photocatalyst and uses the ubiquitous dioxygen as both an oxygen source and an oxidant. Through this mild and environmentally friendly method, a series of important α-thiocyanate ketones can be generated from easily available acrylic acids and ammonium thiocyanate. In addition, the facile transformation of product α-thiocyanate ketones makes this method have great potential for application in organic and pharmaceutical chemistry.

Photocatalytic C-H Thiocyanation of Corroles: Development of Near-Infrared (NIR)-Emissive Dyes

A new method of activating corrole macrocycles via an in situ generated SCN radical has been developed at very mild conditions at room temperature. This photoredox reaction resulted in the generation of tetrathiocyanatocorroles in good yields. The synthesis of tetrathiocyanatocorroles was never reported earlier. Single-crystal XRD analysis reveals that the insertion of four thiocyanate moieties at the four β-pyrrolic positions has imparted significant distortion to the corrole macrocycle. The generated tetrathiocyanatocorroles are different from the parent corroles in many ways. The photophysical properties of the newly synthesized tetrathiocyanatocorroles are dramatically altered from the parent corroles. The absorption feature of these modified corrole derivatives (both position and intensity) bears a nice similarity with the chlorophyll-a macrocycle. Thus, these newly synthesized molecules can be considered as spectroscopic model systems for chlorophyll-a pigments. The observed absorption and emission spectra of these tetrathiocyanatocorroles certainly point out that these newly developed ligand scaffolds and their various metal complexes will have immense potential as pigments in solar cells and also as NIR-emissive dyes. The observed C-H···Au weak interactions in a representative Au(III)-corrole complex point out that these complexes are capable of activating the unfunctionalized C-H groups and thus will have potential implications in C-H activation reactions.

Functionalization of imidazo[1,2-a]pyridines via radical reactions

The recent advances in radical reactions for the direct functionalization of imidazo[1,2-a]pyridines are reviewed.

Light-induced borylation: developments and mechanistic insights

Organoboron compounds are very important derivatives because of their profound impacts on medicinal, biological as well as industrial applications. The development of several novel borylation methodologies has achieved momentous interest among synthetic chemists. In this scenario, eco-friendly light-induced borylation is progressively becoming one of the best synthetic tools in recent days to prepare organoboronic ester and acid derivatives based on green chemistry rules. In this article, we have discussed all the UV- and visible-light-induced borylation strategies developed in the last decade. Furthermore, special attention is given to the mechanisms of these borylation methodologies for better understanding of reaction insights.

One-pot photocatalytic transformation of indolines into 3-thiocyanate indoles with new Ir(iii) photosensitizers bearing β-carbolines

Herein, we harness the combination of two photocatalytic reactions, promoted by new Ir(iii) photosensitizers, for the direct access to 3-thiocyanato indoles from indolines in a one-pot process.

Hydrogen Peroxide-Mediated Rapid Room Temperature Metal-Free C(sp2)-H Thiocyanation of Amino Pyrazoles, Amino Uracils, and Enamines

A rapid metal- and additive-free room temperature method for C(sp²)-H thiocyanation of aminopyrazoles, aminoisoxazole, aminoisothiazole, amino uracils, and aliphatic enamines has been developed in an aqueous medium using hydrogen peroxide as a benign oxidant and ammonium thiocyanate as a thiocyanating agent. On the other hand, the reaction of hydrogen peroxide and ammonium thiocyanate followed by one-pot addition of NaOH provides the corresponding disulfides in the case of amino azoles, and pyrimidine-fused 2-amino thiazoles were observed in the case of aminouracils. The salient features of this method are the use of an eco-friendly oxidant, reaction tunability to access different products, wide substrate scope, and good to very good yields.