Visible light organic photoredox catalytic cascade reactions

Visible Light-Induced Radical Cascade Difluoromethylation/Cyclization of Unactivated Alkenes: Access to CF2H-Substituted Polycyclic Imidazoles

An efficient and mild protocol for the visible light-induced radical cascade difluoromethylation/cyclization of imidazoles with unactivated alkenes using easily accessible and bench-stable difluoromethyltriphenylphosphonium bromide as the precursor of the -CF2H group has been developed to afford CF2H-substituted polycyclic imidazoles in moderate to good yields. This strategy, along with the construction of Csp3-CF2H/C-C bonds, is distinguished by mild conditions, no requirement of additives, simple operation, and wide substrate scope. In addition, the mechanistic experiments have indicated that the difluoromethyl radical pathway is essential for the methodology.

Photocatalytic systems: reactions, mechanism, and applications

The photocatalytic field revolves around the utilization of photon energy to initiate various chemical reactions using non-adsorbing substrates, through processes such as single electron transfer, energy transfer, or atom transfer. The efficiency of this field depends on the capacity of a light-absorbing metal complex, organic molecule, or substance (commonly referred to as photocatalysts or PCs) to execute these processes. Photoredox techniques utilize photocatalysts, which possess the essential characteristic of functioning as both an oxidizing and a reducing agent upon activation. In addition, it is commonly observed that photocatalysts exhibit optimal performance when irradiated with low-energy light sources, while still retaining their catalytic activity under ambient temperatures. The implementation of photoredox catalysis has resuscitated an array of synthesis realms, including but not limited to radical chemistry and photochemistry, ultimately affording prospects for the development of the reactions. Also, photoredox catalysis is utilized to resolve numerous challenges encountered in medicinal chemistry, as well as natural product synthesis. Moreover, its applications extend across diverse domains encompassing organic chemistry and catalysis. The significance of photoredox catalysts is rooted in their utilization across various fields, including biomedicine, environmental pollution management, and water purification. Of course, recently, research has evaluated photocatalysts in terms of cost, recyclability, and pollution of some photocatalysts and dyes from an environmental point of view. According to these new studies, there is a need for critical studies and reviews on photocatalysts and photocatalytic processes to provide a solution to reduce these limitations. As a future perspective for research on photocatalysts, it is necessary to put the goals of researchers on studies to overcome the limitations of the application and efficiency of photocatalysts to promote their use on a large scale for the development of industrial activities. Given the significant implications of the subject matter, this review seeks to delve into the fundamental tenets of the photocatalyst domain and its associated practical use cases. This review endeavors to demonstrate the prospective of a powerful tool known as photochemical catalysis and elucidate its underlying tenets. Additionally, another goal of this review is to expound upon the various applications of photocatalysts.

Visible-light-induced photocatalyst-free cascade cyclization of 3-(2-(ethynyl)phenyl)quinazolinones to sulfonated quinolino[2,1-b]quinazolinones

A visible-light-induced K2S2O8-promoted cascade sulfonation/cyclization reaction was established using 3-(2-(ethynyl)phenyl)quinazolinones as efficient substrates under mild conditions. A series of sulfonated quinolino[2,1-b]quinazolinones were successfully synthesized under transition-metal- and photocatalyst-free conditions. Notably, this strategy has the advantages of room temperature and simple operation, easy scale-up, and good functional group tolerance.

Visible-Light-Promoted Radical Cascade Cyclization of 2-Vinyl Benzimidazoles: Access to Benzo[4,5]imidazo[1,2-b]isoquinolin- 11(6H)-ones

A visible-light-enabled photoredox radical cascade cyclization of 2-vinyl benzimidazole derivatives is developed. This chemistry is applicable to a wide range of N-aroyl 2-vinyl benzimidazoles as acceptors, and halo compounds, including alkyl halides, acyl chlorides and sulfonyl chlorides, as radical precursors. The Langlois reagent also serves as an effective partner in this photocatalytic oxidative cascade process. This protocol provides a robust alternative for rendering highly functionalized benzo[4,5]imidazo[1,2-b]isoquinolin-11(6H)-ones.

Metal-Free Photoredox Catalyzed Sulfonylation of Phenylhydrazines with Thiols

The sulfonylation method stands out as a simple and efficient approach for synthesizing sulfonamides. Despite the advancements in constructing the sulfonamide framework, the potential use of phenyl hydrazine as an amination source remains unexplored. Herein, we report a metal-free, environment-friendly photoredox-catalyzed sulfonylation of phenylhydrazines using thiols, employing MeCN:H2O as a green solvent and eosin Y as a photoredox catalyst. This strategy exhibits a broad substrate scope and good functional group compatibility, including hetero(aryl) as well as aliphatic phenylhydrazines. Finally, this protocol also demonstrated good application for the synthesis of pharmaceutical analogues.

Multifunctional Metal-Organic Frameworks as Catalysts for Tandem Reactions

Owing to well-defined structure as well as easy synthesis and modification, metal-organic frameworks (MOFs) have emerged as promising catalysts for tandem reactions. In this article, we aim to summarize the development of multifunctional MOFs, including mixed metal MOFs, MOFs that are synergistically catalyzed by metal nodes and organic linkers, MOFs loaded with metal nanoparticles, etc, as heterogenous catalysts for tandem reactions over the past five years. This concept briefly discusses on present challenges, future trends, and prospects of multifunctional MOFs catalysts in tandem reactions.

Multiple-cycle photochemical cascade reactions

Cascade reactions represent an efficient and economical synthetic approach, enabling the rapid synthesis of a wide array of structurally complex organic compounds. These compounds, previously inaccessible, can now be synthesized in a remarkably limited number of steps. Concurrently, the photochemical reactions of organic molecules have gained prominence as a potent strategy for accessing a diverse range of radical species and intermediates. This is achieved in a controlled manner under mild conditions. Owing to the relentless endeavors of chemists, significant strides have been made in the realm of photochemical cascade reactions. These advancements have facilitated the synthesis of novel molecular structures with high complexity, structures that are typically challenging to generate under thermal conditions. In this review, we comprehensively summarize and underscore the recent pivotal advancements in visible-light-induced cascade reactions. Our focus is on the elucidation of multiple photochemical catalytic cycles, emphasizing the catalytic activation modes and the types of reactions involved.

Tunable C-H functionalization and dearomatization enabled by an organic photocatalyst

C-H functionalization and dearomatization constitute fundamental transformations of aromatic compounds, which find wide applications in various research areas. However, achieving both transformations from the same substrates with a single catalyst by operating a distinct mechanism remains challenging. Here, we report a photocatalytic strategy to modulate the reaction pathways that can be directed toward either C-H functionalization or dearomatization under redox-neutral or net-reductive conditions, respectively. Two sets of indoles and indolines bearing tertiary alcohols are divergently furnished with good yields and high selectivity. The key to success is the introduction of isoazatruxene ITN-2 as a novel photocatalyst (PC), which outperforms the commonly used PCs. The ready synthesis and high modulability of isoazatruxene type PCs indicate their great application potential.

Synthesis of Indenones via Persulfate Promoted Radical Alkylation/Cyclization of Biaryl Ynones with 1,4-Dihydropyridines

The oxidative radical cascade cyclization of alkynes has emerged as a versatile strategy for the efficient construction of diverse structural units and complex molecules in organic chemistry. This work reports an alkyl radical initiated 5-exo-trig cyclization of biaryl ynones with 1,4-dihydropyridines to selectively synthesize indenones.

Photoredox-Catalyzed Divergent Radical Cascade Annulations of 1,6-Enynes via Pyridine N-Oxide-Promoted Vinyl Radical Generation

The divergent organophotoredox-catalyzed radical cascade annulation reactions of 1,6-enynes were developed. A series of cyclopropane-fused hetero- and carbo-bicyclic, tricyclic, and spiro-tetracyclic compounds were facilely synthesized from a broad scope of 1,6-enynes and 2,6-lutidine N-oxide under mild and metal-free conditions with blue light-emitting diode light irradiation. The cascade annulation reaction occurs with the intermediacy of a β-oxyvinyl radical, which is produced from photocatalytically generated pyridine N-oxy radical addition to the carbon-carbon triple bond.

Visible light-enabled synthesis of phosphorylated indolizine and pyridoindole derivatives via HAT-mediated radical cascade cyclization

A visible light-enabled cascade cyclization strategy is disclosed with concomitant phosphorylation and heterocycle construction. It provides a novel and environmentally benign approach for accessing tetrahydroindolizine-containing phosphonates under metal-free conditions. Mechanistic studies revealed that phosphinoyl radicals were generated from H-phosphonates via a HAT process.

Extent of carbon nitride photocharging controls energetics of hydrogen transfer in photochemical cascade processes

Graphitic carbon nitride is widely studied in organic photoredox catalysis. Reductive quenching of carbon nitride excited state is postulated in many photocatalytic transformations. However, the reactivity of this species in the turn over step is less explored. In this work, we investigate electron and proton transfer from carbon nitride that is photocharged to a various extent, while the negative charge is compensated either by protons or ammonium cations. Strong stabilization of electrons by ammonium cations makes proton-coupled electron transfer uphill, and affords air-stable persistent carbon nitride radicals. In carbon nitrides, which are photocharged to a smaller extent, protons do not stabilize electrons, which results in spontaneous charge transfer to oxidants. Facile proton-coupled electron transfer is a key step in the photocatalytic oxidative-reductive cascade - tetramerization of benzylic amines. The feasibility of proton-coupled electron transfer is modulated by adjusting the extent of carbon nitride photocharging, type of counterion and temperature.

Unified Approach to Diverse Heterocyclic Synthesis: Organo-Photocatalyzed Carboacylation of Alkenes and Alkynes from Feedstock Aldehydes and Alcohols

We report an organo-photocatalyzed carboacylation reaction that offers a springboard to create chemical complexity in a diversity-driven approach. The modular one-pot method uses feedstock aldehydes and alcohols as acyl surrogates and commercially available Eosin Y as the photoredox catalyst, making it simple and affordable to introduce structural diversity. Several biologically relevant skeletons have been easily synthesized under mild conditions in the presence of visible light irradiation by fostering a radical acylation/cyclization cascade. The proposed reaction mechanism was further illuminated by a number of spectroscopic studies. Furthermore, we applied this protocol for the late-stage functionalization of pharmaceuticals and blockbuster drugs.

Organic photoredox catalyzed C(sp3)-H functionalization of saturated aza-heterocycles via a cross-dehydrogenative coupling reaction

Herein we present a novel protocol to access α-functionalized saturated aza-heterocycles, and a variety of nucleophilic groups, such as indole, naphthol, phenol, pyrrole, furyl, nitromethyl, and cyano, could be easily installed into saturated aza-heterocycles. Furthermore, a range of biologically valuable 3,3'-diindolylmethane derivatives could also be readily accessed under mild photocatalytic conditions.

Metal-free polychloromethylation/cyclization of unactivated alkenes towards ring-fused tricyclic indolones and benzoimidazoles

Polychloromethylative cyclization of N-alkenyl indoles was developed under metal-free conditions to afford tricyclic pyridoindolones and pyrroloindolones in moderate to good yields. In the reaction, commercially available CHCl3 and CH2Cl2 were employed as tri- and dichloromethyl radical sources. Moreover, tri- and dichloromethylated polycyclic benzoimidazoles can also be obtained under standard conditions.

Visible Light-Induced Metal-Free Fluoroalkylations

Fluoroalkylation is a crucial synthetic process that enables the modification of molecules with fluoroalkyl groups, which can enhance the properties of compounds and have potential applications in medicine and materials science. The utilization of visible light-induced, metal-free methods is of particular importance as it provides an environmentally friendly alternative to traditional methods and eliminates the potential risks associated with metal-catalyst toxicity. This Account describes our studies on visible light-induced, metal-free fluoroalkylation processes, which include the use of organic photocatalysts or EDA complexes. We have utilized organophotocatalysts such as Nile red, tri(9-anthryl)borane, and an indole-based tetracyclic complex, as well as catalyst-free EDA chemistry through photoactive halogen bond formation or an unconventional transient ternary complex formation with nucleophilic fluoroalkyl source. A variety of π-systems including arenes/heteroarenes, alkenes, and alkynes have been successfully fluoroalkylated under the developed reaction conditions.

Visible Light-Induced Decomposition of Acyl Peroxides Using Isocyanides: Synthesis of Heteroarenes by Radical Cascade Cyclization

Visible light-mediated facile synthesis of heteroarenes, namely, isoquinolines, benzothiazoles, and quinazolines, is demonstrated by employing isocyanides and inexpensive acyl peroxides. It is shown for the first time that singlet-excited isocyanides decompose acyl peroxides into aryl/alkyl radicals. The latter attack isocyanides, yielding imidoyl radicals that subsequently cyclize to afford heteroarene products. The protocol involving radical cascade reactions obviates the requirement of any external photocatalyst, oxidant, additive, and base.

Visible-light-induced cascade reaction: a sustainable approach towards molecular complexity

Photoredox catalysis has demonstrated rapid evolution in the field of synthetic organic chemistry. On the other hand, the splendour of cascade reactions in providing complex molecular architectures renders them a cutting-edge research area. Therefore, the merging of photocatalysis with cascade synthesis brings out a synthetic paradigm with immense potential. The development of photocascade catalysis for a target molecule with a particular molecular skeleton and stereochemical framework presents certain challenges but provides a robust and environmentally benign synthetic alternative. This comprehensive review assembles all the accomplishments and highlights of visible-light-induced cascade reactions with literature coverage up to October 2022.

Organic photoredox catalyzed dealkylation/acylation of tertiary amines to access amides

A mild metal-free C-N bond activation strategy for the direct conversion of inert tertiary amines with acyl chlorides into tertiary amides via organic photoredox catalysis is presented. In this protocol, a novel organic photocatalyst (Cz-NI-Ph) that showed excellent catalytic performance during C-N bond cleavage is developed. Moreover, this reaction features green and mild conditions, broad substrate scope, and readily available raw materials.

TBHP-mediated photochemical coupling/cyclization of N-arylacrylamides with thiols

The effective photoredox-mediated oxidative thiolation and cyclization of N-arylacrylamides with thiols leads to biologically interesting 3-thionated oxindoles through C-S and C-C bond formation. This process represents a straightforward reaction that starts from non-prefunctionalized thiolating reagents. Mechanistic studies demonstrated that the TBHP serves as a key radical initiator with visible-light catalysis.

Radical-Based Route to Functionalized Tetralin: Formal Total Synthesis of (±)-Hamigeran B

A formal synthetic route to hamigeran B, an antiviral marine natural product with a unique tricyclic molecular architecture, has been developed. The key chemical transformations in the present route include a novel zinc(II)porphyrin-catalyzed photoredox radical cascade cyclization to access a functionalized tetralin, a catalyst-free benzylic radical bromination with NBS by visible-light irradiation, and a samarium(II)-induced cyclization of brominated tetralone possibly via an orthoquinodimethane-like intermediate.

Visible-Light-Induced Cascade Cyclization of 3-(2-(Ethynyl)phenyl)quinazolinones to Phosphorylated Quinolino[2,1-b]quinazolinones

3-(2-(Ethynyl)phenyl)quinazolinones were designed and synthesized as a class of novel and efficient skeletons for phosphorylation/cyclization reactions. Under visible light irradiation, a series of phosphorylated quinolino[2,1-b]quinazolinones (35 examples, up to 87% yield) were first synthesized from 3-(2-(ethynyl)phenyl)quinazolinones and diarylphosphine oxides by using 4CzIPN as a photocatalyst under mild conditions. This reaction was also applicable under sunlight irradiation. Moreover, the reaction efficiency could be significantly improved under continuous-flow conditions.

Visible-light-induced alkylation of 2-iminochromene

A metal-free, photochemical protocol for C-4 alkylation of 2-iminochromene has been developed by employing the naturally abundant feedstock carboxylic acid. Selective C-4 alkylation under photoredox conditions to access C-4 alkylated 2-iminochromene in up to 81% yield was achieved. In addition, biologically relevant chromophores can be easily incorporated under this protocol.

Redox activated amines in the organophotoinduced alkylation of coumarins

The coumarin core represents the quintessential scaffold of many natural products. While C-3 alkylation is easily achievable, effective greener strategies for C-4 alkylation have been less forthcoming. Herein, we report a metal-free photoinduced deaminative strategy for C-4 alkylation of coumarins using redox activated secondary and benzylic amine derived Katritzky pyridinium salts.

Photomediated Tandem Sulfonyl Addition-Chemoselective N-Cyclization of o-Alkenyl Aryl Ureas: Direct Assembly of Functionalized Dihydroquinazolinones

Photoredox-mediated tandem addition-chemoselective cyclization of o-alkenyl aryl ureas is reported for the synthesis of sulfonyl and activated alkyl-decorated dihydroquinazolinones. By a careful choice of o-alkenyl aryl urea starting materials, we achieved chemoselective N-cyclization in the presence of more reactive amidic oxygen. We have demonstrated the scope of the methodology with a variety of sulfonyl chlorides and activated alkyl halides. Finally, large-scale synthesis of sulfonyl-substituted dihydroquinazolinone showcases the synthetic utility of the methodology.

Stereoselective Cyclization Cascade of Dihydroquinoxalinones by Visible-Light Photocatalysis: Access to the Polycyclic Quinoxalin-2(1H)-ones

An intriguing stereoselective visible-light photocatalysis of dihydroquinoxalinone derivatives has been realized via cyclization with or without the solvolysis cascade. The reactions provided the polycyclic ring structures with efficient formation of multiple bonds and with high stereoselectivity. X-ray crystallography unequivocally determined the structures of five polycyclic products.

Organic photoredox catalytic amino-heteroarylation of unactivated olefins to access distal amino ketones

Here we describe a metal-free amino-heteroarylation of unactivated olefins via organic photoredox catalysis, providing a concise and efficient approach for the rapid synthesis of various δ (β, ε)-amino ketones under mild conditions. This protocol demonstrates that the new photocatalyst Cz-NI developed by our group has an excellent photoredox catalytic performance. Finally, a series of mechanistic experiments and DFT calculations indicate that this transformation undergoes a photoredox catalytic sequential radical addition/functional group migration process.

Photoredox catalyzed cascade CF3 addition/chemodivergent annulations of ortho-alkenyl aryl ureas

Herein, we report a substrate-controlled cascade cyclization of o-alkenyl aryl ureas, an ambident nucleophile for constructing functionalized heterocycles such as 2-amino-1,3-benzoxazines and dihydroquinazolinones in a chemodivergent fashion using photoredox catalysis under mild conditions. The versatility of the method has been successfully demonstrated by applying this strategy to a wide range of substrates and for the synthesis of functionalized etifoxine drug derivatives.

Visible Light-Induced Aerobic Oxidative Dehydrogenation of C-N/C-O to C=N/C=O Bonds Using Metal-Free Photocatalysts: Recent Developments

Performing synthetic transformation using visible light as energy source, in the presence of a photocatalyst as a promoter, is currently of high interest, and oxidation reactions carried out under these conditions using oxygen as the final oxidant are particularly convenient from an environmental point of view. This review summarizes the recent developments achieved in the oxidative dehydrogenation of C-N and C-O bonds, leading to C=N and C=O bonds, respectively, using air or pure oxygen as oxidant and metal-free homogeneous or recyclable heterogeneous photocatalysts under visible light irradiation.

Organic photoredox catalytic α-C(sp3)-H phosphorylation of saturated aza-heterocycles

A metal-free C(sp³)-H phosphorylation of saturated aza-heterocycles via the merger of organic photoredox and Brønsted acid catalyses was established under mild conditions. This protocol provided straightforward and economic access to a variety of valuable α-phosphoryl cyclic amines by using commercially available diarylphosphine oxide reagents. In addition, the D-A fluorescent molecule DCQ was used for the first time as a photocatalyst and exhibited an excellent photoredox catalytic efficiency in this transformation. A series of mechanistic experiments and DFT calculations demonstrated that this transformation underwent a sequential visible light photoredox catalytic oxidation/nucleophilic addition process.