The construction of aromatic rings by photocatalytic radical-induced cyclization reactions

Abstract

Photocatalytic radical-induced cyclization reactions offer an efficient paradigm for constructing aromatic rings due to their unique environmental friendliness, independence from electronic effects, and high functional group tolerance. However, the high reactivity of intermediates in radical reactions, particularly the challenges of controlling it proceeds in an orderly manner according to the expected pathway and the reaction terminates after the ring is formed, which leads to diminished precision in the construction of aromatic rings. Compared to traditional cyclization methods for synthesizing aromatic rings, photocatalysis enables direct cleavage of high-bond-energy chemical bonds under mild conditions, eliminating the need for high temperature, high pressure, or stoichiometric oxidants. Furthermore, photocatalytic systems can serve as a bridge to connect multiple reaction steps, thereby streamlining the process and enhancing efficiency. Recently, significant progress has been made in applying photocatalytic radical-induced cyclization for the construction of aromatic rings. However, to date, there has been no comprehensive summary of this area reported. In this review, we try to provide a comprehensive perspective on the construction of aromatic rings via photocatalytic radical-induced cyclization. The discussion is organized into five sections based on the type of aromatic ring formed: monoheteroatom-doped aromatic rings, diheteroatom-doped aromatic rings, triheteroatom-doped aromatic rings, all-carbon aromatic rings, and non-classical aromatic rings. This review is particularly focused on elucidating reaction mechanisms, the synergistic effect of catalysts and light sources, and the applications of this strategy in pharmaceutical synthesis and materials science.

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1. Introduction

Aromatic rings, serving as pivotal frameworks in the synthesis of pharmaceutical molecules, natural products, and functional materials, have long been a central focus in synthetic chemistry.¹ Based on their atomic composition, aromatic ring systems can be primarily categorized into three classes:² (1) all-carbon aromatic rings, where all atoms are carbon, forming five- or six-membered rings through sp² hybridization; (2) monoheteroatom-doped aromatic rings (containing N, O, S, etc.), where heteroatom introduction alters electron distribution, conferring specific reactivity and polarity to the molecules; (3) polyheteroatom-doped aromatic rings, containing two or more heteroatoms, whose chemical properties vary depending on the number and type of heteroatoms due to potential synergistic or competitive interactions that markedly influence the electronic characteristics of the aromatic ring. Traditionally, the construction of aromatic rings relies primarily on cyclization reactions, with typical methods including:³ (1) electrophilic cyclizations, such as the Haworth reaction for the construction of polycyclic aromatic compounds. The process is followed by intramolecular attack on a carbocation intermediate by a neighbouring aromatic ring. This is followed by a dehydrogenation reaction that restores aromaticity;⁴ (2) pericyclic reactions, where Diels–Alder [4 + 2] cycloaddition forms six-membered rings, followed by aromatization steps such as dehydrogenation or decarboxylation to generate aromatic rings, two σ-bonds with predictable properties are formed in a single step;⁵ (3) condensation reactions such as the Robinson annulation, which constructs six-membered ketone structures through a Michael addition/aldol condensation sequence⁶ (Fig. 1a). In recent years, the rapid advancement of electrochemical techniques has significantly propelled the development of novel aromatic ring construction strategies, particularly through electrocyclization-mediated synthetic approaches.⁷ Notable progress has been achieved in the electrochemical synthesis of naphthalene derivatives and the generation of benzene rings via cyclohexatriene intermediates, demonstrating the remarkable potential of electrosynthesis in aromatic system fabrication. However, existing methods for constructing aromatic rings face multiple challenges: firstly, the reactions often require harsh conditions such as high temperature, high pressure, or strong acids and bases, which are not only time- and energy-consuming but also prone to triggering side reactions or degrading sensitive functional groups. Secondly, electronic effects limit the expandability of the reaction as electrophilic/nucleophilic substitution places demands on the electronic nature of the aromatic hydrocarbons. Thirdly, these transformations often rely on heavy metal catalytic systems, which are neither efficient nor environmentally friendly. Finally, there is a lack of effective control over regioselectivity and stereoselectivity, leading to the generation of byproducts. In response to the limitations of traditional cyclization reactions for constructing aromatic rings, scientists have turned their attention to radical strategies. Radical reactions, owing to their unique environmental friendliness, independence from electronic effects, and high functional group tolerance, effectively circumvent the aforementioned issues.⁸ As a result, radical strategies have witnessed vigorous development in the construction of aromatic ring frameworks. However, precise control over highly reactive intermediates in radical reactions remains a significant challenge, particularly the effective regulation of stereoselectivity, which has yet to be fundamentally resolved.⁹

Fig. 1 (a) Conventional cyclization reactions for constructing aromatic rings; (b) construction of aromatic rings by photocatalytic radical-induced cyclization.

In recent decades, the advancement of photocatalytic synthesis has provided a novel approach for the construction of aromatic rings. Compared to traditional cyclization methods for synthesizing aromatic rings, this approach combines the advantages of a combination of “molecular scissors” and “intelligent navigation”. Light energy, as a clean driving force, enables the direct cleavage of high-bond-energy chemical bonds under mild conditions, eliminating the need for high temperature, high pressure, or stoichiometric oxidants.¹⁰ Simultaneously, photocatalysis can serve as a bridge to connect multiple reaction steps, not only simplifying complex processes but also overcoming the limitations of substrate pre-functionalization.¹¹ This approach streamlines and enhances the efficiency of cyclization reactions, which traditionally required protecting groups or directing groups, more streamlined and efficient. Like assembling building blocks at the molecular level, light acts as a “molecular adhesive”, splicing and combining the modules to obtain two- or three-dimensional aromatic structures.¹² Through the novel mode of “light driven + radical path”, photocatalytic radical induced cyclization breaks through the limitations of traditional thermochemical and electrochemical systems for the synthesis of aromatic rings, and shows unique value in selective, sustainable and functional molecular synthesis. Its scientific significance lies not only in providing a more sustainable aromatic ring synthesis approach, but also in providing a theoretical and technical foundation for future green chemistry and precision synthesis. This review provides a comprehensive perspective on the construction of aromatic rings via photocatalytic radical-induced cyclization, organized into five sections based on the type of aromatic ring formed: (1) synthetic strategies for monoatomic-substituted heteroaromatic ring systems, such as pyridine, pyrrole, furan, and thiophene rings; (2) synthetic methodologies for dual-heteroatom-substituted heteroaromatic ring systems, including pyrimidine, pyrazole, and triazine rings; (3) synthetic paradigms for triheteroatom-substituted heteroaromatic ring systems, such as furazan, thiadiazole, and triazole rings; (4) synthetic methodologies for all-carbon aromatic ring architectures; and (5) synthetic strategies for non-classical aromatic ring systems, including pyridone, 2-pyrone, and α-pyrone rings (Fig. 1b). This review provides a comprehensive perspective on the construction of aromatic rings via photocatalytic radical-induced cyclization, organized into five sections based on the type of aromatic ring formed: (1) synthetic strategies for monoheteroatom-doped aromatic ring systems, such as pyridine, pyrrole, furan, and thiophene rings; (2) synthetic methodologies for diheteroatom-doped aromatic ring systems, including pyrimidine, pyrazole, and triazine rings; (3) synthetic paradigms for triheteroatom-doped aromatic ring systems, such as furazan, thiadiazole, and triazole rings; (4) synthetic methodologies for all-carbon aromatic ring architectures; and (5) synthetic strategies for non-classical aromatic ring systems, including pyridone, 2-pyrone, and α-pyrone rings (Fig. 1b). This review systematically summarizes recent advances in the construction of aromatic rings via photocatalytic radical-induced cyclization, with a particular focus on elucidating reaction mechanisms, the synergistic design of catalysts and light sources, and the applications of this method in pharmaceutical synthesis and materials science. By analyzing current challenges, such as the control of regioselectivity and the universality of complex substrates, as well as future development directions, this review aims to provide theoretical insights for the development of green and efficient synthetic systems for aromatic rings.

2. Synthetic methodologies for monoheteroatom-doped aromatic ring systems

In the realm of organic chemistry, the synthesis and application of monoheterocyclic compounds, including pyridine, pyrrole, furan, and thiophene, have garnered significant attention as a prominent research focus.¹³ Foundational contributions to heterocycle construction have been made through classical named reactions, such as the Hantzsch pyridine synthesis, the Knorr pyrrole reaction, the Paal–Knorr furan ring closure, and the Gewald thiophene synthesis. These heterocycles adhere to the Hückel aromaticity rule, each containing 6π electrons, yet they exhibit varying degrees of stability. Notably, the nitrogen atom in pyridine exerts a pronounced electron-withdrawing effect, rendering it electron-deficient aromatic, thereby conferring the highest stability and the lowest electrophilic substitution activity among the group. In contrast, pyrrole, furan, and thiophene are electron-rich five-membered heterocycles characterized by heightened reactivity. Thiophene, in particular, demonstrates unique reactivity attributable to the substantial atomic radius and potent polarization capability of sulfur. These compounds find extensive utility in drug molecules, natural products, and functional materials.¹⁴ Nonetheless, their synthesis is fraught with challenges: the incorporation of heteroatoms often exacerbates ring strain, regioselectivity control proves arduous, and certain reactions necessitate stringent conditions (e.g., high-temperature dehydration or strong acid catalysis), which curtail functional group compatibility and impede yield optimization. In the ensuing discussion, we will elucidate the construction of the fifteen most novel classes of monoatomic heteroaromatic ring molecules, examining both intermolecular and intramolecular cyclization strategies.

2.1 Construction of the pyridine ring

Pyridine rings are extensively utilized across diverse industries including pharmaceuticals, agrochemicals, and flavor chemistry, while also serving crucial roles in daily chemical production and organometallic catalysis as versatile ligands.¹⁵ Three classical synthetic methodologies have been particularly impactful:¹⁶ (1) the Hantzsch pyridine synthesis, which employs aldehyde, β-keto ester, and ammonia condensation to assemble the heterocyclic core; (2) the Bohlmann–Rahtz approach, based on the cyclocondensation of ethynyl ketones with cyanoacetate derivatives; (3) the Guareschi–Thorpe reaction that constructs complex pyridine architectures through ketone–cyanoacetate condensation mediated by ammonia. In this section, we critically analyze ten cutting-edge research articles advancing pyridine ring construction methods, highlighting recent innovations in this fundamental area of heterocyclic chemistry.

2.1.1 Intermolecular cyclization to build pyridine rings. Intermolecular cyclization reactions driving multicomponent synergistic cyclization through the photocatalytic generation of radical intermediates are nowadays an important route for the construction of pyridines and their derivatives. In 2020, the research team of Li, Zhao and Luo reported a method for constructing pyridine derivatives. This method involves an intramolecular [5 + 1] cycloaddition reaction between N-vinylaziridine p-toluenesulfonate and difluoroalkyl halides under visible light catalysis (Scheme 1).¹⁷ In contrast to conventional photocatalytic strategies that utilize radical intermediates, this methodology employs BrCF₂CO₂Et as a novel C1 synthon in combination with fac-Ir(ppy)₃ as the photocatalyst and DIPEA as a base, enabling efficient pyridine ring formation under blue light irradiation. This study reports the successful implementation of a [5 + 1] cycloaddition reaction involving vinylaziridine. The developed methodology offers a green and sustainable synthetic strategy for constructing diverse pyridine derivatives under mild reaction conditions. This approach demonstrates remarkable efficiency and practical utility, characterized by a broad substrate scope and excellent functional group tolerance. To further elucidate the proposed mechanism, the authors performed radical trapping experiments on substrates 1 and 2. In the model reaction, the reaction stops completely if 2,2,6,6-tetramethyl-1-piperinedinyloxy (TEMPO) is added, whereas when replacing TEMPO with ethylene-1,1-dibenzylbiphenyl, only a trace amount of product 3 is observed (in less than 5% yield), while the yield of the difluoroalkyl and ethylene-1,1-dibenzylbiphenyl adduct 4 is 28%. Furthermore, classical radical clock experiments showed a yield of 18% for the corresponding product 5 under standard conditions. The results of the control experiments showed the participation of ˙CF₂CO₂Et in the reaction process. The research team proposed a plausible reaction mechanism involving a photoexcited-state Ir^III* catalyst. The photocatalytic cycle initiates with the generation of the ˙CF₂CO₂Et radical through the catalytic action of Ir^III*, which concurrently undergoes oxidation to form an Ir^IV species. Subsequently, the ˙CF₂CO₂Et radical reacts with substrate 1 to produce a primary carbon radical intermediate. This intermediate undergoes ring-opening via C–N bond cleavage, preferentially forming a nitrogen-stabilized secondary carbon radical due to the lower bond dissociation energy (BDE) of C–N bonds (compared to C–C bonds), making this pathway kinetically favored. The resultant nitrogen-stabilized carbon radical then participates in a single-electron transfer process with the Ir^IV species, serving as an electron donor to regenerate the Ir^III catalyst while generating an α,β-unsaturated imine intermediate through concomitant deprotonation. This imine subsequently undergoes a cascade transformation under basic conditions: (1) E2 elimination to form a conjugated diene system, (2) 6π-electrocyclic ring closure to establish the aromatic framework, and (3) defluorinative arylation to furnish the final product 3.

Scheme 1 Intramolecular [5 + 1] cycloaddition between N-vinylaziridine p-toluenesulfonate and difluoroalkyl halides.

In addition to the construction of pyridine rings through intermolecular cyclization of C1 synthons, Huo, Qiu, Bao and their colleagues proposed a synthetic method in 2021 for the preparation of polysubstituted pyrido[3,4-c]quinoline-1,3-diones. This method diverges from traditional pyridine synthesis pathways that rely on imide ion intermediates. Instead, it utilizes single-electron oxidation and employs photocatalysis to achieve an oxidative tandem [4 + 2]-cyclization/aromatization reaction between glycine derivatives and maleimides (Scheme 2).¹⁸ In this process, Ru(bpy)₃Cl₂ serves as the photocatalyst and TBHP as the oxidant. As an atom-economical approach, this reaction synthesizes a variety of pyrrolo[3,4-c]quinoline-1,3-dione compounds under mild conditions with good yields. This work opens new opportunities for the transformation of glycine derivatives with electrophiles and demonstrates the broad application potential of photocatalysis in organic synthesis. Furthermore, it expands the synthetic toolkit for C–C bond formation and heterocyclic compound construction in organic chemistry.

Scheme 2 [4 + 2]-Cyclization/aromatization between glycine derivatives and maleimides.

The intramolecular radical-induced cyclization for the construction of pyridine rings typically involves two key stages: the formation of critical free radical intermediates, followed by cyclization driven aromatization processes, which are typically mediated by transition metal catalyzed reactions. Li and his team in 2023 developed a ruthenium photocatalytic cascade cyclization of benzocyclopentane[b]acridine derivatives via tricyclization of 3-ethynylbiphenyl cyanide with α-bromo-unsaturated carbonyl compounds (Scheme 3).¹⁹ The reaction enables the simultaneous formation of three C(sp³)–C(sp²) bonds and one C(sp²)–N bond through a cascade process involving intramolecular cyclization to construct pyridine rings. Mechanistically, it features the addition of a carbon-centered radical to a C≡C bond followed by three successive radical cyclizations. Notably, this transition metal-catalyzed protocol demonstrates remarkable synthetic advantages, including mild reaction conditions, broad substrate compatibility, and exceptional bond-forming efficiency. The significant decrease in product yield observed upon the addition of TEMPO under optimal reaction conditions indicates its effective inhibition of the cyclization process. Mechanistic analysis suggests that the formation of diene 12 likely originates from intermediate 13 captured by TEMPO, providing critical evidence for a radical-involved reaction pathway. Control experiments employing 2,6-di-tert-butyl-4-methylphenol (BHT) as an alternative radical scavenger further corroborate this radical-mediated mechanism. Notably, the BHT-modified system completely abolished the cyclization process, instead resulting in the exclusive formation of trapped products 14 and 15. Furthermore, light–dark interval experiments revealed that continuous light irradiation is essential for reaction progression, supporting the hypothesis of a non-chain radical mechanism rather than a conventional chain propagation process. Based on controlled experiments, Li proposed a plausible reaction mechanism. The catalyst Ru(II), upon photoexcitation, reacts with unsaturated α-bromo carbonyl compounds to generate carbon-centered radicals. These radicals subsequently undergo addition, cyclization, SET oxidation, and deprotonation processes to yield the final product.

Scheme 3 Tricyclization of 3-ethynylbiphenyl cyanide with α-bromo-unsaturated carbonyl compounds.

Phosphorus-containing pyridine ring analogs can be constructed by conventional methods such as transition metal-catalyzed coupling reactions, phosphorylation reactions, and condensation reactions. In 2022, Wu's team discovered a pathway using cobalt oxime as a catalyst for the synthesis of phosphorylated heteroaromatics (Scheme 4).²⁰ This catalytic system operates under exceptionally mild conditions, exhibits a broad substrate scope, and delivers good to excellent yields. Notably, the reaction eliminates the need for metallic reductants or oxidizing agents, generating only hydrogen or methane as by-products, thereby achieving high atom economy and aligning with the principles of green chemistry. Furthermore, the scalability of the reaction and its compatibility with sunlight demonstrate significant potential for the sustainable synthesis of valuable organophosphorus compounds. Under visible light irradiation, the cobalt oxime complex (Co^III) is excited to its excited state (Co^III*), demonstrating significant oxidative capability to oxidize phosphine oxides, thereby generating phosphorus radicals. Subsequently, the P-radical reacts with isonitrile via a radical addition reaction to form imidazolium intermediates, which are then cyclized intramolecularly to form carbon-centered radical intermediates. Co^II traps these carbon-radical intermediates to form the C–Co^III intermediate, which ultimately generates a phosphorylated heteroaromatic product via β-H depletion and regenerates the Co^III catalyst at the same time. In this strategy, the cobalt-oxime complex not only promotes photo-oxidation but also β-H elimination, thus eliminating the need for additional oxidizing or reducing agents. The by-products are only H₂ or CH₄, providing high atom economy and broad substrate applicability.

Scheme 4 Synthesis of phosphorylated heteroaromatics using cobalt oximide as a catalyst.

Unlike Wu's team,²⁰ Yu, Chen and colleagues focused on the use of 4-CzIPN-^tBu as a photocatalyst. Catalyzed by 4-CzIPN-^tBu with visible light, Yu, Chen, and coworkers generated phosphorus radicals via a visible-light-induced proton-coupled electron transfer (PCET) process to synthesize a variety of phosphorus-containing heterocyclopentenes. This work extends the application of photocatalytic reactions in the construction of phosphorus-containing compounds (Scheme 5).²¹ This approach avoids the use of expensive and toxic transition metal photocatalysts, employing organic dyes as alternatives to make the reaction more environmentally friendly. Using this catalytic system, the authors successfully synthesized a series of phosphorylated nitrogen-containing heteroaromatic compounds, including phenanthridines, quinolines, and benzothiazoles, with yields ranging from moderate to high (45%–85%). Under light irradiation, 4-CzIPN-^tBu becomes excited and undergoes a PCET process with the tautomer of diphenylphosphine oxide, generating a phosphorus radical and a radical anion of the photocatalyst. The resulting 4-CzIPN-^tBu anion then reacts with tert-butyl hydroperoxide (TBHP), producing a hydroxide ion and a tert-butoxy radical while regenerating the photocatalyst. The phosphorus radical undergoes isocyanide addition with the substrate, forming an iminyl intermediate. This intermediate sequentially undergoes: (1) intramolecular cyclization to generate a radical intermediate; (2) electron transfer to the tert-butoxy radical (yielding a tert-butoxide anion and a cationic intermediate); and (3) proton transfer to a base, ultimately forming a stable nitrogen-containing heteroaromatic product. Notably, the cobalt oxime-based photocatalytic system introduced in this study not only advances a radical-driven phosphorylation strategy, enriching the theoretical basis for the combination of photocatalysis and cobalt catalysis, but also provides an efficient and environmentally benign approach for the sustainable synthesis of valuable organophosphorus compounds. This approach holds great promise for industrial-scale applications.

Scheme 5 Biosynthesis of phosphorus-containing heterocyclopentenes by the PCET process.

In contrast to the catalyst studied by Yu and Chen,²¹ the Zhou and Zhang group focused on the more common organic dye eosin Y. They utilized eosin Y as a photocatalyst to develop a visible-light-induced radical cascade acylation/cyclization/aromatization reaction for constructing acyl oxime esters from N-propargyl aromatic amines and 3-acylquinolines (Scheme 6a).²² In particular, the reaction utilizes an acyl oxime ester as an acyl radical precursor to generate an acyl radical by C–C bond breaking, avoiding the dependence on an acyl chloride or anhydride in conventional acylation reactions. C–C bond breaking of peracyl oximes provides a convenient route to 3-acylated quinolines. Coincidentally, in 2020, Huang and his colleagues discovered another method to synthesize 3-sulfoquinolines through the one-step formation of C–S and C–C bonds, also using eosin Y as a photocatalyst (Scheme 6b).²³ The main feature of this method is the visible-light-promoted radical cascade reaction of N-alkynylaniline with sodium sulfite as the sulfonyl radical precursor under metal-free conditions.

Scheme 6 Eosin Y as a photocatalyst for radical cyclization/aromatization.

Traditional SET processes for constructing pyridine rings in photochemical reactions typically require the assistance of photocatalysts, transition metals, and oxidants.²⁴ Transition metals and organic dyes, often used as photosensitizers, are not only expensive but also environmentally unfriendly. In 2023, Patel's team proposed a visible-light-induced method for synthesizing sulfur-functionalized pyridines from (E)-2-(1,3-diphenylallylidene) malononitrile and thiophenols through the formation of an electron donor–acceptor (EDA) complex (Scheme 7).²⁵ The most remarkable feature of this method is that it does not require photocatalysts, transition metals or oxidizing agents, which provides a new idea for the rapid construction of multi-substituted pyridine backbones with good industrialization potential.

Scheme 7 Synthesis of sulfur-functionalized pyridines from (E)-2-(1,3-diphenylallyl) malononitrile and thiophenol.

Highly valent iodine reagent (ABZ) is often used as an azide radical source in organic reactions due to its property that the I–N₃ bond is susceptible to cleavage under visible light irradiation and in the presence of BIOAc. Waster's group utilized the reactivity characteristics of ABZ to investigate and report a metal-free, visible-light-catalyzed radical nitrogen transposition reaction, which provides a method for the synthesis of quinolines with cyclopropene under visible light irradiation using ABZ as an azide radical source (Scheme 8).²⁶ Notably, this reaction was most effective for 3-trifluoromethylcyclopropenes, yielding the valuable 4-trifluoromethylquinolines.

Scheme 8 Synthesis of cyclopropenylquinolines using ABZ as an azide radical source.

2.1.2 Intramolecular cyclization to form a pyridine ring. In contrast to conventional photocatalysts employing noble metals such as ruthenium (Ru) (as exemplified in prior studies), copper-based catalysts have recently emerged as a promising alternative owing to their distinct advantages in sustainable catalysis. The growing research interest in Cu catalysts stems from their exceptional cost-effectiveness coupled with natural abundance, making them particularly attractive for scalable applications. More significantly, these materials exhibit unique electronic configurations characterized by strong 3d orbital interactions with reactive intermediates, which can fundamentally modify adsorption–desorption dynamics during catalytic processes. Furthermore, copper-based systems demonstrate remarkable photothermal conversion capabilities that enable synergistic utilization of both photonic and thermal energy inputs, potentially overcoming limitations in traditional photocatalytic systems.²⁷ Copper and molecular oxygen (O₂) both play important roles as a cheap, green catalyst and oxidant, respectively, in the production of reactions. In 2021, Xia and colleagues reported an intramolecular oxidative cyclization reaction employing copper catalysis and visible-light induction for the synthesis of 4-carbonylquinoline and indole derivatives (Scheme 9).²⁸ The core of this reaction lies in the synergistic combination of inexpensive copper metal with visible-light photocatalysis, which replaces traditional precious metals (such as Ru). The dual role of O₂ in the reaction is particularly noteworthy: it serves as both a terminal oxidant, regenerating Cu^I to Cu^II to sustain the catalytic cycle, and a participant in the crucial hydrogen atom transfer (HAT) step, generating superoxide radicals to drive the radical cascade. This design ingeniously integrates the photoinduced SET process with the radical cyclization mechanism, effectively constructing heteroaromatic ring frameworks (such as quinoline and indole) through intramolecular cyclization, oxygen capture, and dehydration steps, while also avoiding the use of exogenous oxidants. Subsequently, Xia's team proposed a possible reaction mechanism. Under visible light irradiation, the Cu^II catalyst enters an excited state, initiating a SET with substrate 38′, generating Cu^I and a substrate-derived intermediate. The Cu^I species is reoxidized by molecular oxygen to regenerate Cu^II and produce a superoxide anion radical. Subsequently, the intermediate abstracts a hydrogen atom from the superoxide radical, forming a hydroperoxyl radical (HOO˙) and an alkyl radical. This alkyl radical undergoes three sequential transformations: (1) intramolecular cyclization to construct a cyclic framework; (2) trapping by oxygen to form peroxyl radicals; and (3) HAT to yield a hydroperoxide intermediate. Finally, dehydration of the hydroperoxide generates the target product.

Scheme 9 Copper-catalyzed visible-light-induced synthesis of 4-carbonylquinoline and indole derivatives.

2.2 Construction of the pyrrole ring

Pyrrole rings and their derivatives exhibit significant biological activity, excellent doping conductivity, and remarkable insecticidal, fungicidal, and herbicidal properties, making them widely used in pharmaceuticals, materials, agriculture, natural products, and bioactive molecules.²⁹ This section introduces a novel method for preparing pyrrole rings through a photoinduced radical-initiated process, involving regioselective and chemoselective bond cleavage, structural rearrangement, and functionalization of 2-alkynyl alkylamides.

In 2022, Wang, Mutra and colleagues developed a photocatalyst-free, metal-free, and additive-free strategy for synthesizing chalcone-substituted pyrrolidines and their derivatives through photoinduced amide structural rearrangement and functionalization (Scheme 10).³⁰ The photochemical method proposed in this study addresses critical challenges in ynamide chemistry, eliminating the need for expensive metal/photocatalysts, external oxidants, and long reaction times, while also overcoming issues such as hazardous waste generation and low atom economy in traditional radical systems. Using dichloromethane as the solvent at room temperature under blue light irradiation, the optimized reaction system achieved efficient conversion of 2-alkynyl ynamides with 4-methylbenzenesulfonyl iodide within 2–10 minutes, yielding the target product in 81% yield. Wang and colleagues conducted a series of control experiments to elucidate the reaction mechanism. Radical trapping experiments using TEMPO and ethylene-1,1-diyldibenzene confirmed the involvement of a radical pathway, with a 55% product yield even when BHT was used as the trapping agent. Notably, the ynamide reaction primarily produced α,β-addition products rather than bond-cleavage derivatives, indicating that the 2-alkynyl group plays a crucial role as an effective directing group. ¹³C Labelling experiments revealed two key mechanistic features: (1) the sulfonyl group first undergoes α-carbon addition to the ynamide, followed by intramolecular alkyne migration and (2) sulfonyl derivatives influence the stability and isomerization of the ynamide but do not participate in the migration process. The proposed mechanism involves the generation of a sulfonyl radical from sulfonyl iodide under blue LED irradiation, initiating α-carbon regioselective addition to the ynamide. This triggers a sequential process of cyclization, bond cleavage/migration, and radical addition, ultimately yielding the product through radical recombination with iodine radicals or selenium species.

Scheme 10 Light induced acyl imine structural remodeling cyclization reaction.

2.3 Construction of the furan ring

Furan rings and their derivatives are widely used in pharmaceuticals, materials science, and natural products due to their excellent biological activity, unique electronic structures, optical properties, and widespread occurrence in nature.³¹ This section will introduce two methods for constructing furan rings utilizing visible light and metal synergistic catalysis.

2.3.1 Intermolecular cyclization to form furan rings. Palladium catalysts are typically highly active, enabling many reactions that are challenging under traditional conditions to proceed successfully under relatively mild conditions. For instance, in the Heck reaction, palladium catalysts facilitate the coupling of aryl halides with alkenes at lower temperatures and pressures, yielding arylalkene products of significant synthetic value. In 2023, Wu's team integrated palladium catalysis with photocatalysis to develop a visible-light and palladium synergistic catalytic system, achieving a tandem reaction between alkynylphosphine oxides and bromophenols or bromonaphthols to construct multiaryl furan frameworks (Scheme 11).³² This reaction employs a novel palladium/photoredox dual catalytic system, achieving the challenging cleavage of C(sp³)–P(V) bonds through a radical tandem cyclization. This method exhibits broad substrate compatibility with various aryl substituents (Me, OMe, halogens, heterocycles) except nitro groups, providing an efficient and high-yielding route to access multiaryl furan structures. The synergistic combination of palladium catalysis and photoredox activation expands the toolbox for constructing complex heterocyclic frameworks. Mechanistic studies reveal three critical stages of the catalytic cycle: (1) initial oxidative addition forms an aryl bromide intermediate, which undergoes single-electron transfer with photoexcited palladium(0) to generate a mixed aryl-palladium-radical species; (2) the radical intermediate undergoes sequential cyclization and aryl transfer, regenerating palladium(0) to sustain catalysis; and (3) subsequent photoinduced homolytic cleavage of the C–P bond produces a phosphorus-centered radical, facilitating proton transfer and final arylation.

Scheme 11 Construction of polyarylfuran frameworks with bromophenols or bromonaphthols by alkynylphosphine oxides.

2.3.2 Intramolecular cyclization to form furan rings. In 2020, Hashim's research group found that the use of dual core gold catalysts and EnT of aryl iodides can achieve alkyne radical carbocyclization/diboronation to construct benzyl diborate salts containing furan rings (Scheme 12).³³ This reaction utilizes visible light as the energy source, avoiding the use of UV light or harsh conditions required in traditional methods, offering milder reaction conditions and providing a catalytic system for the application of gold catalysts in visible-light photocatalysis. Through condition optimization using substrate 51 and B₂Pin₂ as a model, the authors discovered that the target product could be obtained under blue light irradiation in MeCN in the presence of a dinuclear gold catalyst and sodium carbonate. This method exhibits excellent reactivity with aromatic substrates containing various substituents such as acyl, ester, and halogen, and has been proven to efficiently synthesize heterocyclic diborate esters containing benzofuran, indole, benzothiophene, and other heterocyclic compounds. These structures are important intermediates in drug development. To investigate the reaction mechanism, the authors conducted absorption spectroscopy and ³¹P NMR analyses, revealing that [Au₂(μ-dppm)₂](OTf)₂ and aryl iodides exhibited no absorption peaks under blue LED light, and a new gold complex was formed. Through control experiments, a plausible reaction mechanism was proposed, involving steps such as EnT, aryl-iodide bond cleavage, cyclization, and borylation. The photocatalyst [Au₂(μ-dppm)₂](OTf)₂, upon visible light excitation, forms a complex with Na₂CO₃, initiating an EnT process. This process promotes the transition of aryl-iodides to their triplet state, leading to the homolytic cleavage of the C–I bond and generating aryl and iodine radicals. The aryl radical undergoes a 5-exo cyclization to form a vinyl radical intermediate, which then completes its transformation through two pathways: recombination with the iodine radical and interaction with a boronate ester.

Scheme 12 Construction of benzyl diboronic acid salts containing a furan ring by alkynyl radicals.

2.4 Construction of the thiophene ring

Thiophene rings are often prepared using the Hinsberg reaction and Paal–Knorr reaction, which face the problems of insufficient productivity, excessive contamination and harsh conditions of traditional methods. In this section, two advanced and feasible visible light-catalyzed methods for the construction of thiophene rings will be introduced.

Photocatalytic reactions without external photocatalysts have long been a goal in green organic synthesis. In 2023, the Wang and Li team developed a strategy for the light-driven radical cyclization of 2-alkynyl thioanisoles with disulfides to synthesize 3-arylthiobenzothiophenes without the need for external photocatalysts (Scheme 13).³⁴ Mechanistic studies, including radical clock experiments and fluorescence quenching experiments, have confirmed the involvement of sulfur radicals in a cascade process comprising alkyne addition, cyclization, isomerization, and desulfurization. This method demonstrates excellent functional group compatibility and remarkable scalability, providing a practical strategy for the synthesis of 3-arylthio (seleno) benzothiophene/selenophene derivatives without the need for external photocatalysts. This reaction proceeds solely under visible light irradiation. Upon photoexcitation, the substrate R₂SSR₂ generates the excited-state species R₂SSR₂*, which acts as a photosensitizer to transfer energy to the disulfide, forming an excited-state disulfide. The excited-state disulfide subsequently undergoes homolytic cleavage to produce a thiyl radical. This thiyl radical selectively attacks the alkyne moiety of 2-alkynyl thioanisole, forming a highly regioselective vinyl radical intermediate. The intermediate then undergoes an intramolecular cyclization with the XMe group to yield the final product, accompanied by the conversion of the methyl radical to thioanisole as a side reaction. Notably, the generated product can function as a photosensitizer to re-enter the reaction, thereby achieving an efficient catalytic cycle.

Scheme 13 Synthesis of 3-arylthiobenzothiophene from 2-alkynylthioanisole and disulfide.

In contrast to the photocatalyst-free concept proposed by the Wang and Li team,³⁴ the Zhou and Yu team achieved the construction of arylthiothiophenes through a photoinduced copper-catalyzed three-component radical cyclization of thiyl alkynes, cyclobutanone oximes, and boronic acids (Scheme 14).³⁵ The reaction exhibits varying substituent effects across different substrates: (1) for thiyl alkynes, electron-donating groups on the alkyne aryl ring enhance reaction efficiency through favorable electronic and steric effects, while electron-withdrawing groups reduce yields. Notably, alkenyl substituents exhibit significant partial conjugation effects. (2) Cyclobutanone oxime substrates demonstrate broad compatibility with various substitution patterns, where the electronic nature and positional arrangement of substituents critically influence reactivity. (3) In boronic acid substrates, electronic and steric factors on the aryl ring significantly impact reactivity. Specifically, electron-donating groups promote reactivity, while heteroaryl boronic acids reduce activity. This method provides a streamlined approach for constructing highly functionalized aryl-2-thienyl sulfides under mild conditions, with the formation and reductive elimination of the Cu^III intermediate being key steps, highlighting the versatility of copper catalysis in complex transformations. This process offers new insights for constructing complex molecules, particularly with potential applications in photocatalysis. Regarding this reaction, under visible light irradiation, cyclobutanone oxime esters undergo SET with Cu^I* or Cu^I, generating iminyl radicals and Cu^II. The iminyl radicals undergo C–C bond cleavage to form cyanoalkyl radicals, which are subsequently captured by thiyl alkynes, yielding alkyl intermediates. These intermediates cyclize to produce vinyl radicals, facilitating C(sp³)–S bond cleavage, regenerating alkyl radicals, and releasing ethylene. Simultaneously, the thiyl radicals react with the Cu^IIPh complex, forming a high-valent Cu^III intermediate, which ultimately undergoes reductive elimination to afford the target product.

Scheme 14 Construction of arylthioindenes from thioalkynes, cyclobutanone oximes and boronic acids.

3. Synthetic methodologies for diheteroatom-doped aromatic ring systems

Diatomic heteroaromatic compounds and their derivatives play a pivotal role in various technological fields due to their versatile chemical properties.³⁶ In drug development, purine analogs (e.g., 6-mercaptopurine), featuring N,C-heteroaromatic ring systems, exhibit antitumor effects by interfering with nucleic acid synthesis metabolism. Food preservation systems leverage the antibacterial properties of thiazole derivatives, while thiamethoxam, a thiazole-containing neonicotinoid, demonstrates potent insecticidal activity by disrupting nicotinic acetylcholine receptors. Reactive Brilliant Red X-3B dye, with its heteroaromatic structure, forms stable conjugates with textile substrates, serving as a model for chromogenic applications. In energy materials science, heteroaromatic frameworks contribute to the formation of high-potential, cyclically stable anode structures by enhancing π-electron delocalization and redox stability. Traditional methods for synthesizing diatomic heteroaromatic ring molecules typically rely on classical named reactions, such as the Buchwald–Hartwig coupling and Sonogashira coupling. This section will detail eight advanced synthetic strategies for preparing diatomic heteroaromatic ring molecules, focusing on intramolecular and intermolecular cyclization approaches.

3.1 Intermolecular cyclization forms a diatomic heteroaromatic ring

As a metal-free organic dye, eosin Y offers cost-effective and environmentally friendly advantages over metal catalysts, eliminating potential metal ion toxicity and contamination. During the photocatalytic process, its excited state undergoes photoinduced electron transfer: it first self-oxidizes, donating electrons to the substrate, and then accepts electrons from a sacrificial agent, regenerating to its ground state, thereby establishing a sustainable catalytic cycle. Sharada, Bakthadoss, and their colleagues developed a visible-light-mediated, metal-free strategy for synthesizing imidazoles through organic dye-catalyzed [4 + 1] cyclization. In this process, eosin Y triggers a photocatalytic dehydrogenation reaction to produce the key enamine intermediate, followed by the insertion of the nitrogen atom from TMSN₃ to complete the construction of the heterocyclic structure (Scheme 15).³⁷ This metal-free photocatalytic strategy demonstrates notable environmental advantages through its mild reaction conditions, avoidance of stoichiometric oxidants, and exceptional atom economy in constructing CF₃-substituted imidazoles. These structurally unique heterocycles hold significant potential for pharmaceutical development and bioimaging applications due to their distinctive electronic properties and metabolic stability. Under visible-light irradiation, eosin Y undergoes photoexcitation to initiate a SET process, oxidizing THIQ enamine successively to its radical cation and subsequently to an iminium ion. This reactive intermediate undergoes nucleophilic attack by the azide anion (N₃⁻) to generate an α-azido radical intermediate. Subsequent oxidative elimination of nitrogen gas (N₂) from the azido radical produces an imidoyl radical, which ultimately couples to form the final product featuring two new C–N bonds.

Scheme 15 Eosin Y catalyzed [4 + 1] cyclization synthesis of imidazole.

In 2023, Guo, Zheng et al. developed a photocatalytic method for synthesizing imidazo[2,1-a]isoquinolines through [3 + 2] cyclization-aromatization of amidines with isoquinolinium N-ylides (Scheme 16).³⁸ Under optimized conditions, the reaction proceeds via direct C(sp²)–H activation, enabling sequential C–C and C–N bond formation in a one-pot process. This mechanistically distinct approach achieves the construction of the imidazo[2,1-a]isoquinoline framework through three key steps: photocatalytic generation of amidinyl radicals, radical addition/cyclization with N-ylides, and oxidative aromatization. The protocol demonstrates broad substrate compatibility across diverse substituted amidines and isoquinolinium salts. These studies establish different synthetic paradigms for heterocyclic ring formation through radical-mediated cascade reactions.

Scheme 16 Synthesis of imidazo[2,1-a]isoquinoline from amidine and isoquinolinium N-ylide.

2022, Zhu, Chen's group discovered a method for the synthesis of pyrazine rings by photoredox/copper co-catalyzed reaction of oxime esters with trimethylcyanosilane domino cyclization (Scheme 17).³⁹ The reaction utilizes a photoactivated Ir^III catalyst to reduce oxime ester substrates, generating nitrogen-centered radicals to initiate the process. These nitrogen-centered radicals undergo a series of steps involving catalytic cycles of copper and iridium complexes, radical coupling, reductive elimination, and 6-exo-dig cyclization with Cu^I. Subsequent proton transfer and isomerization ultimately yield tetrasubstituted pyrazine products. This method represents the successful synthesis of structurally novel tetrasubstituted pyrazines from oxime esters, demonstrating exceptional advantages in bond-forming efficiency (forming up to four bonds per catalytic cycle), step economy, and broad substrate compatibility. The success of gram-scale synthesis underscores the scalability of this approach. The research team led by Zhu and Chen conducted mycelial growth inhibition assays to evaluate the antifungal activity of pyrazine-derived products against six plant pathogenic fungi (Rhizoctonia solani, Fusarium graminearum, Alternaria solani, Botrytis cinerea, Colletotrichum orbiculare, and Alternaria alternata), with boscalid as the positive control. The half-maximal effective concentration (EC₅₀) values were systematically determined. Notably, at a concentration of 50 μg mg⁻¹, the tested products exhibited significantly superior inhibitory activity against Colletotrichum orbiculare and Botrytis cinerea compared to the positive control. These findings highlight the enhanced antifungal efficacy of the synthesized pyrazine compounds and suggest their potential applicability in the development of novel antifungal agents.

Scheme 17 Domino synthesis of the pyrazine ring from oxime ester and trimethylcyanosilane.

For two consecutive years, Ma and Zhao published two articles on the construction of heteroatomic aromatic rings over Ir-catalysts. One article introduced a novel visible-light-driven [3 + 2 + 1] cyclization strategy, converting diazo carbonyl compounds into various nitrogen-containing heterocycles such as fluorinated pyrimidines (Scheme 18a).⁴⁰ The other article presented a method for synthesizing substituted pyrazolo[1,5-a]pyridine derivatives using ethyl diazoacetate, pentafluoroethyl iodide, and 1-aminopyridinium ylide as model substrates (Scheme 18b).⁴¹ The first article utilizes α-diazoketones as denitrogenation agents, reacting with fluoroalkyl radicals under visible light irradiation to construct fluorine-containing azidoheterocycles, offering a novel pathway for the synthesis of fluorinated heterocycles. The second article employs diazo compounds as atypical radical acceptors, developing a photo-driven radical-polar crossover cyclization strategy for synthesizing N-heterocycles bearing perfluoroalkyl substituents. These methods exhibit excellent functional group tolerance and broad substrate scope under mild reaction conditions, expanding the application of diazo compounds in photochemical synthesis.

Scheme 18 Construction of pyridine rings in Ir-catalyzed systems.

In 2024, Sureshkumar's team developed a metal-free photocatalytic strategy for constructing tetrasubstituted pyrazoles through visible-light-driven [4 + 1] cyclization (Scheme 19).⁴² This protocol enables efficient synthesis under mild conditions (visible light irradiation, ambient temperature) using readily available starting materials, without the need for transition metals or external oxidants. By leveraging a controlled radical-polar crossover process, this method establishes an environmentally benign route for constructing structurally diverse pyrazole derivatives. The key mechanistic steps include: photo-excitation of a 9,10-dimethylacridine catalyst to generate a strong oxidant, which mediates the one-electron oxidation of the in situ-formed ketoimine, releasing carbon dioxide and acyl radicals; addition of the acyl species to the 1,2-diaza-1,3-diene radical to form a nitrogen radical intermediate; one-electron reduction to regenerate the photocatalyst, which also generates a nitrogen anionic intermediate; and condensation via an intramolecular cyclization to produce the pyrazole product.

Scheme 19 [4 + 1] cyclization reaction of tetrasubstituted pyrazoles.

Photocatalysis without metal involvement has been the goal of green organocatalysis. Moorthy's group proposed a metal-free catalytic method to generate phenanthridines and their derivatives from the reaction of isonitrile and acyl peroxides in acetonitrile solvent using irradiation of blue LEDs (Scheme 20).⁴³ In this study, the authors proposed a green synthesis method for heterocyclic compounds (phenanthridines and their derivatives) without metals, photocatalysts, or additives, utilizing visible-light-induced decomposition of isonitrile-initiated acyl peroxides. The authors elucidated the mechanism of radical-mediated photochemistry by UV-visible/fluorescence spectroscopy and photoswitching experiments, demonstrating that a photodrive is necessary for the reaction to proceed and that the reaction involves a SET process. Combined with control experiments, a possible reaction mechanism was obtained: visible light excitation drives isonitrile into a singlet excited state, followed by electron transfer with the acyl peroxide. Then, alkyl radicals attack isocyanates to form imidazole radicals, which undergo intramolecular cyclization to produce various heterocyclic frameworks. The reaction mechanism involves the dual role of the isonitrile as a photosensitizer and a substrate. Specifically, the generated imino radicals undergo regioselective aromatic ring attack followed by cyclization reactions at different positions to form their respective heterocycles. Although the triplet state is involved to a lesser extent, its effect is still negligible. This tandem radical generation/cyclization strategy provides an efficient and environmentally benign pathway for the construction of various heterocyclic frameworks, demonstrating the unique ability of isonitriles to mediate photochemical initiation in a single synthetic operation.

Scheme 20 The reaction between isonitrile and acyl peroxide to generate phenanthrene derivatives.

3.2 Intramolecular cyclization to form a diatomic heteroaromatic ring

In 2020, Brachet and his coworkers reported an approach for the synthesis of pyridazine derivatives using an intramolecular cyclization reaction in a photocatalytic system with Ru(bpy)₃(PF₆)₂, sodium tert-butanol, methanol, and a photocatalytic system, using phosphonylhydrazones as raw materials (Scheme 21).⁴⁴ For the first time, it has been proposed that phosphoramidates can serve as effective precursors for nitrogen-centered radicals under visible light irradiation, with the precursors undergoing in situ cleavage post-reaction, eliminating the need for additional deprotection steps. This method enables the synthesis of a diverse array of pyridazines, significantly enriching the synthetic approaches to pyridazine frameworks.

Scheme 21 Synthesis of pyridazine derivatives by intramolecular cyclization.

4. Synthetic methodologies for triheteroatom-doped aromatic ring systems

Classic reactions for constructing triatomic heteroaromatic rings include the Corey–Chaykovsky reaction, Darzens condensation, and Feist–Benary reaction.⁴⁵ In this section, we will focus on the construction of oxadiazole, thiadiazole, and triazole rings, thereby introducing six advanced synthetic approaches for triatomic heteroaromatic rings. These modern methods overcome the limitations of classical approaches by enhancing regioselectivity, atom economy, and functional group tolerance, reflecting current trends in heterocycle synthesis.

4.1 Intermolecular cyclization forms a triatomic heteroaromatic ring

High-valent iodine(III) reagents enable the efficient synthesis of 2,5-disubstituted-1,3,4-oxadiazoles through a photoinduced oxidative cyclization mechanism with aldehydes under catalyst-free conditions. These reagents simultaneously act as both the oxidant and electrophilic coupling partner, facilitating the formation of the heteroaromatic ring system through radical-mediated C–O and C–N bond formation processes in Li's photocatalytic protocol (Scheme 22).⁴⁶ This catalyst-free approach enables the direct conversion of commercially available aldehydes to oxadiazole derivatives under mild conditions, thus establishing an efficient visible-light-driven strategy for the construction of unique heterocyclic scaffolds. Under visible light irradiation, homolytic cleavage of a high-valent iodine reagent generates iodine-activated radicals, which initiate the HAT process with aldehydes such as benzaldehyde. The generated carbonyl radical then undergoes a radical–radical coupling reaction with a diazo derivative to form a key bicyclic intermediate. This intermediate undergoes oxidative cyclization to form 2,5-disubstituted-1,3,4-oxadiazole derivatives via consecutive N–N bond cleavage and heterocycle formation.

Scheme 22 Synthesis of oxadiazole from a high valent iodine(III) reagent and aldehyde.

In the same year, a research group led by Han and Chen proposed another photocatalytic method to generate diazotrifluoroethyl radicals using α-keto acids and high-valent iodine(III) reagents, followed by 2,5-disubstituted-1,3,4-oxadiazole derivatives (Scheme 23).⁴⁷ The reaction utilizes diazotrifluoroethyl radicals as CF₃-containing building blocks to achieve the synthesis of 2,5-disubstituted-1,3,4-oxadiazole derivatives under mild conditions without the need for photocatalysts, metals, or additives, under green reaction conditions, with a wide range of substrates and the possibility of gram-scale synthesis and later derivatization. Under blue LED irradiation, the C–I bond of the high-valent iodine reagent undergoes homolytic cleavage, generating diazotrifluoroethyl radicals and iodoacyl radicals. The acyl radical undergoes a HAT process with an α-keto acid (e.g., 2-oxo-2-phenylacetic acid) to produce a carboxylic acid radical and O-iodobenzoic acid. The carboxylic acid radical undergoes intramolecular cyclization following a coupling reaction with the resonance structure of the diazotrifluoroethyl radical, culminating in the formation of 2,5-disubstituted-1,3,4-oxadiazole derivatives.

Scheme 23 Synthesis of 2,5-disubstituted-1,3,4-oxadiazole derivatives from α-ketoacids and high-valent iodine(III).

Similarly utilizing hypervalent iodine reagents, Liu, Li proposed a novel method in 2020 for synthesizing 2,5-disubstituted-1,3,4-oxadiazole derivatives through photooxidative decarboxylative cyclization between commercially available α-oxocarboxylic acids and hypervalent iodine(III) reagents (Scheme 24).⁴⁸ Not only does it avoid the limitations of transition metal-mediated or strong oxidation in conventional methods, but it also involves the coupling of two different radicals as well as the formation of C–N and C–O bonds by photocatalysis to achieve the reaction under mild conditions. In order to understand the reaction mechanism, the authors also carried out controlled experiments such as free radical capture experiments, free radical bells, etc. which resulted in no oxadiazole product, proving that the reaction is a free radical reaction.

Scheme 24 Synthesis of oxadiazole derivatives by cyclization of α-oxocarboxylic acid with high valent iodine(III).

In the same year, Wang, Li, and their colleagues prepared a fully conjugated donor–acceptor COF (Py-BSZ-COF) containing a benzothiadiazole unit for photocatalytic oxidative amine coupling and thioamide cyclization to produce 1,2,4-thiadiazoles (Scheme 25a).⁴⁹ A COF has high stability and strong charge separation ability, and the use of its generated superoxide radicals to realize photocatalytic oxidative amine coupling and thioamide cyclization reactions has the advantages of a wide substrate range, high yield and good recyclability. In 2023, Namrata Rastogi's team realized an organic photoredox-mediated formal [3 + 2] cycloaddition reaction of 2H-azepanes with aryl diazonium tetrafluoroborates to synthesize 1,3,5-trisubstituted-1,2,4-triazoles (Scheme 25b).⁵⁰ In this reaction, aryl diazonium salts act as radical acceptors under visible-light redox conditions, replacing the usual aryl radical precursors, thus enriching the synthesis of 1,2,4-triazoles.

Scheme 25 Synthesis of 1,2,4-thiadiazole and 1,2,4-triazole.

4.2 Intramolecular cyclization to form a triatomic heteroaromatic ring

1,3,4-Oxadiazoles are important structural units in pharmaceutical molecules and natural products, and their stable heterocyclic structure makes them highly valuable for applications. Currently widely used synthetic approaches make the synthesis of 1,3,4-oxadiazoles much more difficult due to the need for high temperatures, toxic gas systems (e.g., phosgene), and other harsh reaction conditions. Niu's team developed an oxidant-free photoredox-catalyzed cascade cyclization strategy for constructing 1,3,4-oxadiazoles, addressing inherent limitations in conventional synthetic methods through sequential C–O/N–N bond formation mediated by radical-polar crossover processes (Scheme 26).⁵¹ It is worth mentioning that Niu and his colleagues proposed a possible reaction mechanism in which the nitrogen-centered radical undergoes a resonance decarbonylation reaction with the carbon-centered radical, triggering an intramolecular radical cyclization to produce a reaction intermediate. This intermediate coordinates with Co^II and undergoes a SET to form a Co^I-stabilized radical cation. Subsequent deprotonation and tautomerization reactions yield the 1,3,4-oxadiazole product. This redox-neutral strategy is an environmentally benign synthesis that does not require external oxidizing agents, while the only byproduct produced is hydrogen gas.

Scheme 26 Construction of 1,3,4-oxadiazole by photocatalytic oxidation–reduction.

5. Synthetic methodologies for all-carbon aromatic ring systems

As one of the most common aromatic rings, carbocyclic rings play an important role in materials science, medicinal chemistry, organic synthesis, etc.⁵² The proposal of the Diels–Alder reaction in 1928 set off a wave of carbocyclic research and application, and a series of carbocyclic synthesis methods were developed and popularized in the following decades, such as the Robinson annulation reaction, Bergman cyclization reaction, Nazarov cyclization reaction and Claisen rearrangement. Nowadays, the rise of photocatalytic technology has brought new opportunities for free radical cyclization reactions, which can trigger unique reaction pathways under relatively mild conditions to achieve previously inaccessible carbon ring construction, and scientists have made great efforts in this regard.

5.1 Intermolecular cyclization forms a carbon ring

In 2021, Miao, Wang, and co-workers developed a novel photoinduced [3 + 3] intermolecular cyclization strategy enabling the efficient synthesis of 3-hydroxyphenanthro[9,10-c]furan-1(3H)-ones through radical-mediated coupling of α-keto acids with alkyne (Scheme 27).⁵³ The authors investigated the optical absorption spectra of benzoylformic acid (99′), diphenylacetylene (100′), and their mixtures in 1,1,2-trichloroethane solution. Experimental results revealed that the visible light-induced excited state of 99′ plays a crucial role in driving the reaction. The reaction demonstrated broad substrate adaptability, successfully accommodating various α-keto acids bearing both electron-donating and electron-withdrawing groups, along with diverse symmetric and asymmetric alkynes. Under photoexcitation, the carboxyl radical produced by homolytic cleavage of the O–H bond of α-keto acids is added to the C≡C bond of alkynes, and then the product is generated by the acid-promoted protonation and dehydration reactions, as well as by the cation-driven nucleophilic addition and deprotonation processes.

Scheme 27 Synthesis of 3-hydroxyphenanthro[9,10-c]furan-1 (3H)-one from α-ketoacids and alkynes.

In traditional intramolecular Friedel–Crafts alkylation reactions, substrates containing alkyl and aromatic moieties undergo cyclization catalyzed by Lewis acids to form fused cyclic structures. Building on this mechanism, the Chuah team recently developed an innovative [1 + 5] intermolecular cyclization strategy utilizing visible light and iron mediation (Scheme 28).⁵⁴ This method enables the efficient synthesis of 1-bromonaphthalenes by controlling the reaction between allylbenzene and polyhalomethanes. The reaction utilizes inexpensive commercially available allylbenzene and polyhalomethanes as feedstocks for the facile synthesis of polysubstituted 1-bromonaphthalenes and chlorinated naphthalenes under visible light irradiation and iron mediation. Under visible light, CBr₄ reacts with metallic iron to form the complex [BrFe^IICBr₃], which facilitates the Halasch addition reaction of CBr₄ with allylbenzene (similar to the conventional intramolecular Friedel–Crafts alkylation) and the dehydrogenation of the target products to be generated with the help of CBr₄ catalyzed by Fe^III.

Scheme 28 Synthesis of 1-halogenated naphthalene from allyl benzene and polyhalogenated methane.

The following year, Zhou and his research group utilized vinyl diazo compounds as radical acceptors to synthesize 4-fluoroacridines through a visible-light-promoted tandem radical cyclization reaction (Scheme 29a).⁵⁵ Concurrently, Liu's team achieved the synthesis of benzo[j]phenanthridin-6(5H)-one derivatives via a domino radical relay process using 1,7-enynes and aryl diazonium salts under visible-light-driven photocatalytic conditions, employing a P/N heteroleptic Cu^I photosensitizer (Scheme 29b).⁵⁶ Among Liu's experiments, the aryl radical species played the dual role of radical initiator–terminator, which efficiently realized the assembly of the target compounds, and perfectly solved the limitations of the traditional synthesis methods, such as the cumbersome steps, the narrow range of substrates, etc.

Scheme 29 The synthesis of benzo[j]phenanthridin-6 (5H)-one and 4-fluoroacridine.

In 2021, the research group of Zeng and Ke reported a visible-light-driven photocatalytic cyclization reaction, utilizing an Ir^III catalyst to achieve the cyclization-aromatization of ortho-alkynyl aryl vinylsilanes with aryl sulfonyl azides, efficiently constructing naphtho-fused benzosilole compounds (Scheme 30).⁵⁷ In the study, the authors indirectly validated the generation of sulfonyl radicals from acyl azides via unstable aryl sulfonyl diimide intermediates through solvent screening experiments. The complete inhibition of the reaction and detection of sulfonyl esters upon addition of the radical scavenger TEMPO confirmed the involvement of sulfonyl radicals in the reaction. This work not only represents the realization of a vinyl-silane radical Smiles rearrangement, providing a novel method for constructing complex silicon-containing heterocycles, but also introduces a rapid approach for assembling polycyclic fused silicon heterocycles with broad functional group tolerance. In the reaction, aryl sulfonyl azides generate sulfonyl radicals via unstable aryl sulfonyl diimide intermediates, which then undergo C=C addition with vinylsilane substrates to form α-silyl carbon radicals 115 and 115′. These α-silyl carbon radicals participate in radical cyclization with C≡C bonds, forming vinyl radicals 116 and 116′. The vinyl radicals undergo intramolecular cyclization on the benzene ring, followed by Smiles rearrangement, generating β-silyl radicals through spirocyclic intermediates and subsequent desulfonylation. The β-silyl radicals then undergo cascade radical cyclization to form intermediate 117, which, through SET processes and aromatization, yields the final aromatized product 114-1.

Scheme 30 Construction of naphthalene fused phenylsilanol compounds using acetylarylvinylsilane and arylsulfonyl azide.

5.2 Intramolecular cyclization to form a carbon ring

In 2022, Niu and co-workers achieved a significant breakthrough in aromatic compound synthesis through the development of a tandem photocyclization/dehydroaromatization strategy (Scheme 31a).⁵⁸ This innovative approach employed a dual photoredox/cobalt oxime catalytic system that synergistically combined energy and electron transfer mechanisms. The methodology enabled the efficient construction of 10-phenanthrenol derivatives from O-aryl-substituted 1,3-dicarbonyl compounds under visible light irradiation. Mechanistic investigations revealed that the fac-Ir(ppy)₃ photocatalyst served dual catalytic functions: facilitating EnT-mediated photocyclization while simultaneously participating in photoredox-catalyzed dehydroaromatization through cobalt oxime cooperation. Wang's research group subsequently reported an advanced visible-light-driven protocol in 2023 for synthesizing functionalized 9-fluorenols and naphthylcyclopropanecarboxaldehydes (Scheme 31b).⁵⁹ Their intramolecular regionally dispersive tandem radical reaction features a unique cyclization/expansion-cyclocondensation sequence starting from simple conjugated alkenyne precursors. The methodology demonstrates remarkable control over reaction pathways through Lewis acid modulation – BF₃·OEt₂ was found to direct radical cyclization trajectories via carbonyl group activation through ligand coordination. This strategic intervention ensures exceptional chemoselectivity while maintaining complete atom economy (100%), representing a notable advancement in sustainable radical chemistry.

Scheme 31 Synthesis of phenylphenol derivatives and naphthalene cyclopropyl formaldehyde derivatives.

6. Synthetic methodologies for non-classical aromatic ring systems

In addition to the monatomic heteroaromatic rings, polyatomic heteroaromatic rings, and carbocycles described above, there are also some uncommon aromatic rings that can be constructed by photocatalytic reactions.⁶⁰ In this section, we introduce in detail the construction methods of three types of aromatic rings, namely, pyridinone, 2-pyrone, and α-pyrone, with the combination of three recent articles.

The XAT strategy using α-aminoalkyl radicals allows the generation of aryl radicals at room temperature, which can be used in intramolecular cyclization reactions to generate biologically relevant alkaloids. Based on this strategy, Baidya's team proposed a method to realize the construction of 2-pyrone derivatives from simple halogenated benzamides under visible light irradiation and in the presence of organic photocatalysts 4-CzIPN and ⁿBu₃N (Scheme 32).⁶¹ To elucidate the reaction mechanism and validate the role of α-aminoalkyl radicals in XAT processes, the authors conducted systematic investigations including radical trapping experiments, Stern–Volmer quenching studies (using 4-CzIPN and ⁿBu₃N as quenchers), and light-switching experiments. Experimental evidence confirms the radical-involved annulation pathway and demonstrates the facile generation of XAT-essential α-aminoalkyl radicals from 4-CzIPN. Building on controlled experiments, a plausible mechanism was proposed: photoexcited 4-CzIPN undergoes SET with ⁿBu₃N to produce key α-aminoalkyl radicals. These radicals initiate XAT with iodobenzamide, forming intermediate 122′ which subsequently undergoes secondary XAT to yield the final phenanthridone product. Notably, this Baidya-developed XAT strategy enables the efficient synthesis of bioactive 2-pyrone alkaloids under mild conditions, offering an economical and sustainable approach with precise reaction control.

Scheme 32 Construction of 2-pyrrolidones using simple halogenated benzamides.

In 2021, Ye, Sun, Qian and coworkers proposed a photoredox-catalyzed N-radical cyclization cascade reaction through addition of N-centered radicals to alkynes (C≡C) (Scheme 33).⁶² This methodology facilitates the efficient, controlled synthesis of functionalized benzoxazinones and 2-hydroxy-3-indolones from readily available O-ethynylacetamides under mild conditions. Notably, the reaction represents the successful implementation of a 5-endo-dig N-radical cyclization cascade featuring unprecedented triple bond cleavage.

Scheme 33 Synthesis of functionalized benzoxazinone and 2-hydroxy-3-indolone from O-ethynyl acetamide.

1,n-Diynes serve as versatile synthons in photocatalytic Kharasch-type cyclization cascades for constructing cyclic compounds via transition-metal catalysis or radical initiation. In 2021, Jiang, Hao, Zhang et al. developed a novel photocatalytic strategy using heteroatom-tethered 1,7-diynes and cost-effective CBrCl₃ to synthesize diverse tricyclic 2-pyrones (Scheme 34).⁶³ The mechanism involves: visible-light excitation of a ground-state Ir^III complex to its photoactive *Ir^III species, which reduces CBrCl₃ via SET to generate a carbon radical; regioselective radical addition to the terminal alkyne of 1,7-diyne, forming intermediate 131; 6-exo-dig cyclization creating an alkenyl radical that undergoes bromo-radical cross-coupling; base-mediated 1,5-SN₂ substitution with hydroxide from water to yield intermediate 132; and sequential elimination, 6π-electrocyclization, and dehydration to afford the final product. This cascade achieves complete C–X bond cleavage and dual annulation using water as an oxygen source, demonstrating excellent functional group tolerance, structural convergence, and sustainability.

Scheme 34 Synthesis of tricyclic 2-pyrrolidone from 1,7-diyne and CBrCl₃ connected by heteroatoms.

7. Conclusions

Photocatalytic radical-induced cyclization has opened a new pathway for the efficient and green synthesis of aromatic rings. By precisely controlling the generation and transformation of radical intermediates, researchers have successfully constructed diverse structural systems, including mono-, di-, and tri-heteroatom-substituted heteroaromatic rings, carbocycles, and non-classical aromatic rings. As outlined in this review, photocatalytic systems enable the activation of high-energy bonds under mild conditions, circumventing the reliance on high temperature, high pressure, or stoichiometric oxidants required by traditional methods, while demonstrating excellent functional group tolerance and regioselectivity. Furthermore, the synergistic design of photocatalysis with transition metal catalysis or PCET has further expanded the reaction types and application scenarios. This review systematically summarizes recent advancements in constructing aromatic ring systems through photocatalytic radical-induced cyclization reactions, with comprehensive discussions focusing on three key aspects: (1) in-depth mechanistic investigations, (2) synergistic design strategies for catalysts and light sources, and (3) evaluation of application potential for target products. Based on the atomic composition characteristics of aromatic systems, the article innovatively categorizes the construction approaches into five structural types: monoheteroatom-doped aromatic rings, diheteroatom-doped aromatic rings, triheteroatom-doped aromatic rings, all-carbon aromatic rings and non-classical aromatic rings. Through systematic analysis of cutting-edge research achievements in each category, the review highlights groundbreaking methodological breakthroughs in photocatalytic aromatic ring construction, while proposing crucial development directions and technical challenges for future exploration in this field.

Despite significant achievements in the construction of aromatic rings via photocatalytic radical-induced cyclization strategies, current research still faces numerous challenges: (1) stereoselectivity control: the high reactivity of radicals makes stereochemical control difficult, necessitating future efforts to integrate chiral catalysts or dynamic kinetic resolution strategies for precise stereocontrol; (2) universality for complex substrates: existing methods exhibit limited applicability to multi-substituted or sterically hindered substrates, requiring the development of more universal catalytic systems. Triplet–triplet energy transfer (TTET) and redox-tunable photocatalysts may activate recalcitrant substrates, while modular ligand design in dual catalytic systems may address regioselectivity in polyheterocycles; (3) mechanistic elucidation of photocatalysis: the detailed mechanisms of certain reactions, such as excited-state energy transfer pathways, remain unclear and require deeper understanding through in situ spectroscopy and theoretical calculations; (4) scalability challenges: optimizing light penetration efficiency, catalyst recovery, and continuous-flow reaction technologies are critical for industrial-scale applications. Through the above summary, we can gain insight into the current status and future prospects of photocatalytic radical-induced cyclization strategies in the construction of aromatic rings. With interdisciplinary collaboration and technological innovation, we believe that photocatalytic radical-induced cyclization strategies hold the potential to become a cornerstone of green synthetic chemistry, providing robust support for sustainable chemical manufacturing and innovative molecular design.

Data availability

No new data are presented. References are made to the original work reviewed and summarised in this manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the National Key Research and Development Program (2023YFC3304203), the Yangtze River Delta Science and Technology Innovation Community Joint Research Project (2023CSJGG1800), the Key Research and Development Program of Zhejiang Province (2025C01200(SD2), 2024C03266 and 2024C03101), the Research Project of Education Department of Hunan Province (21A0605), the Science and Technology Innovation Program of Hunan Province (No. 2022RC1119), and the National 111 Project of China (D 16013) for financial support.

References

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25. H. N. Dhara, A. Rakshit, D. Barik, K. Ghosh and B. K. Patel, Visible-Llight Driven Electron-Donor-Acceptor (EDA) Complex-Initiated Cynthesis of Thio-Functionalized Pyridines, Chem. Commun., 2023, 59, 7990–7993.
26. V. Smyrnov, B. Muriel and J. Waser, Synthesis of Quinolines via the Metal-Free Visible-Light-Mediated Radical Azidation of Cyclopropenes, Org. Lett., 2021, 23, 5435–5439.
27. G. Quach, H. Iranmanesh, E. T. Luis, J. B. Harper, J. E. Beves and E. G. Moore, Mechanistic and Kinetic Insights into Intermolecular [2 + 2] Photocycloadditions, ACS Catal., 2024, 14, 8758–8766.
28. Q. Huang, M. Zhao, Y. Yang, Y. N. Niu and X. F. Xia, Visible-Light-Induced and Copper-Catalyzed Oxidative Cyclization of Substituted O-Aminophenylacetylene for the Synthesis of Quinoline and Indole Derivatives, Org. Chem. Front., 2021, 8, 5988–5993.
29. For selected papers: (a) X. Duan, J. Wang, H. Li, F. Du, R. Chen, W. Lian and M. Shi, Tandem Site-Selective Bromination and Highly Regioselective Heck Reaction of N-allyl Enaminones: Chemodivergent Synthesis of Polysubstituted Pyrroles and Pyridines, Org. Chem. Front., 2024, 11, 5532–5537; (b) F. Yu, Y. Sun, H. Yang, Z. Chai, X. Lu and D. Liu, Progress in Construction of 2H-Pyrrol-2-Ones Skeleton, Chin. J. Org. Chem., 2023, 43, 57–73.
30. M. R. Mutra and J. J. Wang, Photoinduced Ynamide Structural Reshuffling and Functionalization, Nat. Commun., 2022, 13, 2345–2359.
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32. Y. Li, S. Y. Zhang, X. L. Yan, J. Zhu, K. Luo and L. Wu, Visible-Light-Induced Palladium-Catalyzed Construction of Polyarylfuran Skeletons via Cascade Aryl Radical Cyclization and C(sp³)−P(V) Bond Cleavage, Org. Lett., 2023, 25, 4720–4724.
33. L. Zhang, X. Si, F. Rominger and A. S. K. Hashmi, Visible-Light-Induced Radical Carbo-Cyclization/gem-Diborylation through Triplet Energy Transfer between a Gold Catalyst and Aryl Iodides, J. Am. Chem. Soc., 2020, 142, 10485–10493.
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41. P. Zhao, L. Wang, X. Guo, J. Chen, Y. Liu, L. Wang and Y. Ma, Visible Light-Driven alpha-Diazoketones as Denitrogenated Synthons: Synthesis of Fluorinated N-Heterocycles via Multicomponent Cyclization Reactions, Org. Lett., 2023, 25, 3314–3318.
42. K. Pal, V. Srinivasu, S. Biswas and D. Sureshkumar, Visible Light-Induced Organo-Photocatalyzed Route to Synthesize Substituted Pyrazoles, Org. Biomol. Chem., 2024, 22, 2384–2388.
43. M. De-Abreu, M. Selkti, P. Belmont and E. Brachet, Phosphoramidates as Transient Precursors of Nitrogen–Centered Radical Under Visible–Light Irradiation: Application to the Synthesis of Phthalazine Derivatives, Adv. Synth. Catal., 2020, 362, 2216–2222.
44. N. Yadav, S. R. Bhatta and J. N. Moorthy, Visible Light-Induced Decomposition of Acyl Peroxides Using Isocyanides: Synthesis of Heteroarenes by Radical Cascade Cyclization, J. Org. Chem., 2023, 88, 5431–5439.
45. M. Huang, G. Wang, H. Li, Z. Zou, X. Jia, G. Karotsis, Y. Pan, W. Zhang, J. Ma and Y. Wang, EDA Complex-Mediated [3 + 2] Cyclization for the Synthesis of CF₃-Oxadiazoles, Green Chem., 2025, 27, 413–419.
46. J. Li, J. X. Wen, X. C. Lu, G. Q. Hou, X. Gao, Y. Li and L. Liu, Catalyst-Free Visible-Light-Promoted Cyclization of Aldehydes: Access to 2,5-Disubstituted 1,3,4-Oxadiazole Derivatives, ACS Omega, 2021, 6, 26699–26706.
47. W. W. Zhao, Y. C. Shao, A. N. Wang, J. L. Huang, C. Y. He, B. D. Cui, N. W. Wan, Y. Z. Chen and W. Y. Han, Diazotrifluoroethyl Radical: A CF₃-Containing Building Block in [3 + 2] Cycloaddition, Org. Lett., 2021, 23, 9256–9261.
48. J. Li, X. C. Lu, Y. Xu, J. X. Wen, G. Q. Hou and L. Liu, Photoredox Catalysis Enables Decarboxylative Cyclization with Hypervalent Iodine(III) Reagents: Access to 2,5-Disubstituted 1,3,4-Oxadiazoles, Org. Lett., 2020, 22, 9621–9626.
49. S. Li, L. Li, Y. Li, L. Dai, C. Liu, Y. Liu, J. Li, J. Lv, P. Li and B. Wang, Fully Conjugated Donor-Acceptor Covalent Organic Frameworks for Photocatalytic Oxidative Amine Coupling and Thioamide Cyclization, ACS Catal., 2020, 10, 8717–8726.
50. P. Mishra, P. Kumar, O. S. Srivastava and N. Rastogi, Organophotoredox-Mediated Formal [3 + 2]-Cycloaddition of 2H-Azirines with Aryldiazonium Salts: Direct Access to Trisubstituted 1,2,4-Triazoles, Chem. – Asian J., 2023, 18, e202300007.
51. J. L. Li, H. Y. Li, S. S. Zhang, S. Shen, X. L. Yang and X. Niu, Photoredox/Cobalt-Catalyzed Cascade Oxidative Synthesis of 2,5-Disubstituted 1,3,4-Oxadiazoles under Oxidant-Free Conditions, J. Org. Chem., 2023, 88, 14874–14886.
52. For selected papers: (a) Y. B. Chen, L. G. Liu, C. M. Chen, Y. X. Liu, B. Zhou, X. Lu, Z. Xu and L. W. Ye, Construction of Axially Chiral Arylpyrroles via Atroposelective Diyne Cyclization, Angew. Chem., Int. Ed., 2023, 62, e202303670; (b) Q. Q. Kang, Z. Y. Wang, S. J. Hu, C. M. Luo, X. E. Cai, Y. B. Sun, T. Li and W. T. Wei, Copper-Catalyzed Switchable Cyclization of Alkyne-Tethered α-Bromocarbonyls: Selective Access to Quinolin-2-ones and Quinoline-2,4-diones, Org. Chem. Front., 2022, 9, 6617–6623.
53. B. Zhao, Z. Zhang, Y. Ge, P. Li, T. Miao and L. Wang, Photochemical Synthesis of 3-Hydroxyphenanthro [9,10-c] Furan-1(3H)-Ones from α-Keto Acids and Alkynes, Org. Chem. Front., 2021, 8, 975–982.
54. I. I. Roslan, H. Zhang, K. H. Ng, S. Jaenicke and G. K. A. Chuah, Visible Light and Iron–Mediated Carbocationic Route to Polysubstituted 1−Halonaphthalenes by Benzannulation using Allylbenzenes and Polyhalomethanes, Adv. Synth. Catal., 2020, 363, 1007–1013.
55. W. Li and L. Zhou, Vinyldiazo Compounds as 3-Carbon Radical Acceptors: Synthesis of 4-Fluoroacridines via Visible-Light-Promoted Cascade Radical Cyclization, Org. Lett., 2021, 23, 4279–4283.
56. S. Xiang, Q. Ni, Q. Liu, S. Zhou, H. Wang, Y. Zhou and Y. Liu, Approach to Access Benzo[j]Phenanthridinones from 1,7-Enynes and Aryldiazonium Salts via a Domino Radical Relay Process Enabled by a P/N-Heteroleptic Cu(I)-Photosensitizer, J. Org. Chem., 2023, 88, 13248–13261.
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58. J. L. Li, X. L. Yang, S. Shen and X. Niu, Synthesis of 10-Phenanthrenols via Photosensitized Triplet Energy Transfer, Photoinduced Electron Transfer, and Cobalt Catalysis, J. Org. Chem., 2022, 87, 16458–16472.
59. B. S. Gore, L. W. Pan, J. H. Lin, Y. C. Luo and J. J. Wang, Visible Light-Driven Highly Atom-Economical Divergent Synthesis of Substituted Fluorenols and Cyclopropylcarbaldehydes, Green Chem., 2024, 26, 513–519.
60. For selected papers: (a) Z. Liu, Z. Chen, H. Tong, M. Ji and W. Chu, A β-Ketoenamine-Linked Covalent Organic Framework as a Heterogeneous Photocatalyst for the Synthesis of 2-Arylbenzothiazoles by Cyclization Reaction, Green Chem., 2023, 25, 5195–5205; (b) J. Zhang, P. Wang, Y. Li and J. Wu, Asymmetric Sulfonylation with Sulfur Dioxide Surrogates: A New Access to Enantiomerically Enriched Sulfones, Chem. Commun., 2023, 59, 3821–3826; (c) P. Zhou, H. Z. Wu, Q. Li, W. J. Hao, M. S. Tu and B. Jiang, Cu(II)-Catalyzed Annulative Sulfonylimination of alpha-Carbonyl-Gamma-Alkynyl Sulfoxonium Ylides through Sulfoxonium Ylide Elimination for Access to (Z,)-1H-Isochromenes, J. Org. Chem., 2025, 90, 1663–1670.
61. K. Mondal, S. Mallik, S. Sardana and M. Baidya, A Visible-Light-Induced alpha-Aminoalkyl-Radical-Mediated Halogen-Atom Transfer Process: Modular Synthesis of Phenanthridinone Alkaloids, Org. Lett., 2023, 25, 1689–1694.
62. T. D. Tan, T. Y. Zhai, B. Y. Liu, L. Li, P. C. Qian, Q. Sun, J. M. Zhou and L. W. Ye, Controllable Synthesis of Benzoxazinones and 2-Hydroxy-3-Indolinones by Visible-light-promoted 5-endo-dig N-Radical Cyclization Cascade, Cell Rep. Phys. Sci., 2021, 2, 100577–100283.
63. L. Wang, T. Xu, Q. Rao, T. S. Zhang, W. J. Hao, S. J. Tu and B. Jiang, Photocatalytic Biheterocyclization of 1,7-Diynes for Accessing Skeletally Diverse Tricyclic 2-Pyranones, Org. Lett., 2021, 23, 7845–7850.

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