Radical annulative carbohalofunctionalization of 1,n-enynes via atom transfer processes constitutes an efficient and practical synthetical strategy for accessing halogenated cyclic structures with complete atom economy through radical cleavage of a pre-existing carbon-halogen σ-bond of an atom transfer reagent and their transposition over the π-bond of enyne motifs. Recently, such reaction has become one of hot topics. A general photocatalytic annulative carbohalogenation of phenol/arylamine-linked 1,6-enynes with perhalogenated methanes such as BrCCl₃ or CBr₄ is disclosed, leading to the atom-economic, efficient synthesis of functionalized (Z)-2,3-dihydrobenzofurans and (Z)-indolines as main stereoisomers with good yields and high stereoselectivity under mild and oxidant-free conditions, in which only (Z)-configuration was observed in most products. This protocol demonstrates wide substrate scope, high functional group tolerance, and high atom economy as well as high stereoselectivity, providing a green, atom-economic and efficient method toward 2,3-dihydrobenzofurans and indolines.

2H-Indazole is a privileged scaffold widely existing in drugs and bioactive molecules. The C—H functionalization of 2H-indazole is the cutting edge in medicinal chemistry and organic chemistry. Compared with traditional synthetic methods, the visible light/electrochemical-promoted synthetic method has mild conditions and is an environmentally friendly synthetic strategy. The C—H functionalization of 2-aryl-2H-indazoles at the C-3 position catalyzed by photochemical/electrochemical strategies in recent years is summarized in terms of catalytic systems, with special emphasis on the corresponding reaction mechanism, which might inspire the further development of new catalytic methods.

Chirality is widely found in many natural products and drug molecules. It is of great significance to obtain optically pure chiral compounds because of the fact that different enantiomers have distinct or even opposite physiological activities. Deracemization is the most direct, efficient and atom-economic approach to obtain a single enantiomer, and the emerging photocatalytic deracemization attracts much attention because of its high efficiency. Herein, the recent advances in photocatalytic deracemization are summarized. The future research direction of this field is also prospected.

In recent years, visible-light photocatalytic asymmetric synthesis has shown considerable potential in the mild and rapid construction of optically active organic molecules with structural diversity. Chiral Lewis acids (CLA), including chiral borane compounds, lanthanum complexes, first-row transition metal complexes, and chiral-at-iridium or rhodium complexes, have been established as one class of the most effective catalysts being capable of controlling the stereochemistry in photo-induced chemical transformations. The recent advances in this emerging field were presented by classifying the reactions into bifunctional CLA photocatalytic reactions and reactions enabled by dual catalysis with a CLA catalyst and an external photosensitizer, expecting that these studies will stimulate progress in organic synthesis, photocatalysis and asymmetric catalysis.

Sulfonium salts are one of the most important class of organosulfur (IV) compounds which have a positive charge on the sulfur center with three C—S bonds. Because of their bench-stable, easy synthesis, broad structural diversity, and rich reactivity, sulfonium salts are playing a significant role in synthetic chemistry. In recent years, visible-light promoted photoredox catalysis is rapidly developing into a powerful tool for organic synthesis. In this paper, the recent advances of different sulfonium salts in the radical type reactions induced by visible light are summarized. The formation reactions of C—C bond and C—X (X=B, N, O, S, Se, Te, F, Cl, I) bonds are introduced, and the applicable scope and mechanism of some reactions are also discussed.

Alkenes and their functionalized derivatives represent a versatile class of building blocks in organic synthesis. The traditional synthetic methods include Wittig olefination, Peterson olefination, Horner-Wadsworth-Emmons reaction and others, and many of these classic reactions have also become textbook knowledge. Most of these methods are based on ionic pathways and some still require pre-functionalization at the sites where C—C double bonds are formed, resulting some limitations on substrate scope or functional group tolerance. Over the past few years, photoredox catalysis has become a powerful platform for new reaction design owing to its green chemistry characteristics and unique activation modes. The recently developed and representative methods for the synthesis of alkenes under photochemical conditions are summarized. Moreover, the prospects of further developments are also discussed.

Visible-light-mediated 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) catalyzed decarboxylic acetalation of quinoxalin-2(1H)-ones with glyoxylic acid acetalde to access 3-acetal quinoxalin-2(1H)-ones has been reported. This reaction provides a green and efficient approach to construct a series of 3-acetal quinoxalin-2(1H)-ones by using of air as the oxidant at room temperature, which has the advantages of clean source of energy, mild conditions, metal-free and green oxidant. Radical trapping experiment and fluorescence quenching experiments showed that visible-light promoted radical reaction process was involved in this reaction system.

Oxindole derivatives are well developed in many fields, and this special heterocyclic scaffold is more and more widely used in natural products and bioactive molecules due to its unique functionality. Therefore, it is very important to explore new synthesis methods for this motif. A visible light-assisted photocatalyst-free protocol for the efficient synthesis of sulfonylated oxindoles under mild condition was reported. The present method starts from simple and readily available N-arylacrylamide and sodium sulfinates, uses cheap K₂S₂O₈ as oxidant, and features moderate to good yields and excellent functional group tolerance.

Because of the redox properties of pyridine salt, it is found that N-amino pyridine salt plays an important role in free radical chemistry. N-Amino pyridinium salts easily undergo single electron reduction and N—N bond fragmentation to deliver N radicals. In this review, the advances of N-amino pyridinium salt serves as a powerful nitrogen radical precursor in visible light induced reactions for C—N bond formation are summarized in the last three years. According to the reaction substrates, the review is divided into three parts: react with arenes, react with olefins, and react with alkanes.

Diazo compounds have rich reactivity and are important synthetic intermediates in organic synthesis. Diazo compounds are commonly used as the precursors of transition metal carbenes, and the relative transition metal catalyzed reactions have achieved many important progresses in the past decades. In sharp contrast, radical reactions with diazo compounds as a key component are still less explored. Owing to the easy generation of diverse radical intermediates under mild conditions, visible-light photoredox catalysis has emerged as a powerful tool for organic synthesis. By the combination of visible light photoredox catalysis and diazo compounds, some novel reactions that different from the classical carbene processes have been developed, which further extend the synthetic applications of these reagents. The progresses of radical reactions of diazo compounds enabled by visible light photoredox catalysis in the past five years are summarized, including: (1) diazo compounds as the nucluophiles, (2) diazo compounds as the equivalents of carbyne precursors, (3) diazo compounds as the radical acceptors, and (4) diazo compounds as the radical precursors. We wish that the detailed discussion on the mechanism of the reactions would help the audiences to understand the features of radical reactions of diazo compounds and the realative visible light chemistry.

As an important synthetic method in organic chemistry research, the difunctionalization of alkenes plays an important role in the field of synthetic chemistry. In recent years, the difunctionalization of alkenes caused by free radicals participating in the migration of intramolecular functional groups has risen rapidly, providing a new idea for the difunctionalization of alkenes. This strategy has the advantages of atom economy and environmental friendliness. As an effective means to generate radical precursors, mild and green photocatalysis has inspired chemistsʼ interest in initiating difunctionalization of alkenes by using photocatalysis to drive group migration. According to the species of near and far aryl, heteroaryl, imine, cyano, acyl, alkynyl, alkenyl and other functional groups involved in photocatalytic radicals, the alkene difunctionalization reactions are classified and summarized, and some reaction mechanisms are discussed.

Isocyanides are a class of valuable C1 building blocks, widely used in organic chemistry, combinatorial chemistry, biomedicine and other fields, and their derivatives are effective building blocks for the synthesis of bioactive molecules and complex natural products. Herein, the recent advances in photocatalytic reactions with isocyanides are summarized according to the types of the reaction mechanism. The photocatalyst, substrate range and reaction mechanism are emphatically discussed. Finally, the challenges and future development trends of this research field are discussed and prospected.

Diazo compounds are important synthons which have attracted condirable attentions by synthetic chemists. They can server as carbene precursors, 1,3-dipoles, C-nucleophiles, terminal N-electrophiles and radical intermediates. In recent years, the photocatalytic generation of radicals from diazo compounds has made remarkable development. In this review, the recent developments in this research field are summarized which mainly focus on the generation of diazomethyl radical, carbon radicals, carbyne radicals and allene radicals from diazo compounds. Moreover, the future development direction, as well as challenges in this field is prospected.

Carbon dioxide (CO₂), as the main components of greenhouse gases, is one of the most abundant carbon sources at present. In organic chemistry, CO₂ is often used as C1 synthon to synthesize carboxylic acids by constructing C—C bond. Transition metal catalysis has developed as a powerful tool for the conversion and utilization of CO₂. In order to achieve the strategic goal of “double carbon”, it is of great significance to develop green and sustainable CO₂ conversion and utilization technology combined with contemporary hot topics in the field of organic chemistry. In recent years, the development of photocatalytic synthesis technology provied a new opportunity for the conversion and utilization of CO₂. The bond breaking of C—H bond, C—O bond, C—N and C—X bond and carboxylation reactions with CO₂ by photocatalysis strategy were systematically described.

Isoindigo has played an important role in organic optoelectronic materials as an excellent acceptor building block due to its planarity and electron-withdrawing capability. The modification, of isoindigo has also become the focus of academia. Recently, great achievements have been made in the synthesis of nitrogen-containing isoindigo derivatives and their applications as optoelectronic materials. The incorporation of nitrogen atom can further adjust the electron push/pull ability and the planarity of isoindigo derivatives, thereby tune their optoelectronic properties. The development of nitrogen-containing isoindigo derivatives, as well as their applications in optoelectronic materials, which we hope would provide useful information for the design of conjugated optoelectronic materials, is summarized.

Near-ultraviolet and visible light-induced organic synthesis without photosensitizer has attracted much attention for the organic chemists. Since no photosensitizer is needed in the reaction, it makes the reaction system more simple and feasible, and indicates the future development direction for visible-light-induced organic transformations. Visible-light-promoted organic synthesis without photosensitizer is reviewed in this paper. It covers organic photosyntheses in which the substrate/product as a light-absorbing substance, the electron donor acceptor (EDA) complex as a light-absorbing substance, and the homolysis of weak covalent chemical bonds under the visible-light irradiation conditions. Moreover, other organic photosynthetic reactions without external photocatalyst are also discussed.

A relay Brønsted acid/visible-light photoredox catalysis strategy for the synthesis of quinazolin(thi)ones is developed. In this procedure, 2-aminoacetophenones and iso(thio)cyanates undergo Brønsted acid catalyzed [4+2] annulation, followed by photocatalyzed C—C bond cleavage reaction. A wide range of diversely substituted quinazoline-2,4-diones and quinazoline-2-thione-4-ones were synthesized under mild conditions. The protocol features eco-benign energy source and solvent, metal-free catalysts, simple starting materials and air as green oxidant.

Visible light is an ideal “tool” for the green chemical synthesis. Visible light-induced reactions show such advantages as mild reaction conditions, simple equipments and less by-products. Therefore, it has gradually become an important method in organic synthesis. Herein an blue light induced LiBr-promoted carbene insertion reaction of diazoesters into C(CO)—C bonds of 1,3-diones for the synthesis of a wide range of all-carbon quaternary center-containing 1,4-dione derivatives is reported. The method features good substrate universality and functional group compatibility, and therefore is an ideal and efficient strategy for the synthesis of 1,4-diones.

A photosensitizer-free visible-light-promoted isocyanide insertion protocol for the synthesis of selenylated spiro[indole-3,3'-quinoline] derivatives has been developed, which has the advantages of functional group tolerance, broad substrate scope, simple operation, and good yield. 2-Phenyl-2'-(phenylselanyl)-6'-(trifluoromethyl)-4'H-spiro[indole-3,3'-quinoline] (3g), 5-bromo-2'-((4-fluorophenyl)selanyl)-2-phenyl-4'H-spiro[indole-3,3'-quinoline] (4d) and 2-phenyl-2'-((4-(trifluoromethyl)phenyl)selanyl)-4'H-spiro[indole-3,3'-quinoline] (4g) showed potent cancer-cell-growth inhibitory activities by methyl thiazolyl tetrazolium (MTT) assay.

Photocatalysis has become the forefront of international chemical research due to its green and environmentally friendly characteristics. A new protocol to construct S—S(O)₂ bonds using selectfluor as oxidant and eosin Y as photocatalyst under blue light irradiation was reported. This method has advantages of mild conditions, good compatibility of functional groups, and easily preparation of various symmetric or asymmetric thiosulfonates various from the corresponding mercaptans with good conversion rates. Thus, this protocol provides a very valuable reference for developing a catalytic system with mild conditions, high atomic economy and strong universality to achieve efficient and controllable conversion of sulfur-containing organic compounds.

The photo-transition metal synergistically catalyzed coupling reaction provides a new research strategy for the development of metal-catalyzed coupling reactions. In this catalytic system, the excited state photosensitizer controls the valence state of the transition metal intermediate through a single electron transfer process, there by regulating the coupling reaction process, especially for the process that is difficult to occur in traditional transition metal catalysis. In addition, the excited photosensitizers could also promote the coupling reaction through the energy transfer process. Simultaneously, the light-promoted transition metal-catalyzed C—X bond coupling reactions without external photosensitizers have also been rapidly developed. Light and transition metals synergistically catalyzed coupling reactions provide an important tool for the construction of C-heteroatom bonds, showcasing broad application prospects in synthetic chemistry.

Visible-light-induced denitrification oxygenation reaction of α-diazoesters with N-hydroxyimides to access α-oxyimido esters has been developed. This reaction provides an efficient approach to synthesize a series of N-hydroxyimides at room temperature in 1,4-dioxane under irradiation of 3W blue LED lamps without any photocatalyst. Interestingly, when tetrahydrofuran (THF) was used as the solvent, structurally diverse α-oxyimido esters could be rapidly and efficiently obtained through a three-component coupling reaction of α-diazoesters, THF, and N-hydroxyimides. The reaction could conduct on a gram scale without a significant loss of reaction efficiency, indicating that this method could serve as a practical protocol to synthesize α-oxyimido esters.

An efficient and sustainable visible light-promoted cascade radical cyclization of α-oxocarboxylic acids with o-(allyloxy)arylaldehydes towards the construction of 1,4-diketone containing chromanone derivatives has been developed. The reaction occurred under mild reactions, e.g. room temperature, blue LED irradiation, photoredox catalyst free, and provided the final heterocycles in moderate to good yields.

A transition-metal-free β-selective hydrocarboxylation of alkenes under mild conditions has been developed using a formate salt as the reductant, carbonyl source. A broad range of alkenes were compatible with this method and can be efficiently converted into value-added carboxylic acid products. This method also features low catalyst loading, good functional group tolerance, and easy scalability.

The importance of azaarene derivatives in many arenas, especially pharmaceutical and material industry, has attracted increasing attention of chemists to develop highly efficient synthetic methods. To date, functionalization of commercially available azaarenes and their simple derivatives as feedstocks by exploiting the electronic properties of azaarenes to trigger transformations has been appreciated as a powerful tool. Among them, due to the advantages of mild reaction conditions, good functional group tolerance and diverse reaction types, the methods via visible light-driven photocatalysis have been extensively explored. Notably, a number of asymmetric manifolds towards the synthesis of enantioenriched imine-containing azaarene derivatives have been established with satisfactory results. The advances of this important area are summarized and discussed according to the relative positions of the formed stereocenters to azaarenes.

A photoredox-copper dual-catalyzed cross dehydrogenative coupling reaction of glycosides with benzylic C—H substrates has been developed. The reaction proceeds smoothly under mild reaction conditions and features the using of readily accessible starting materials, which allows the highly site-selective synthesis of diverse glycosides O-alkylation products in 27%~72% yields, providing a new synthetic tool for the site-selective modification of glycosides.

Benzylated quinones widely exist in natural products and drug molecules. Thus their convenient synthesis with high efficiency brings great significance. However, hard conditions are still essential among the present reports. Herein, the development of visible light-induced metal-free benzylation of quinones via cross dehydrogenation coupling reaction with methylene blue as photocatalyst, tert-butyl hydroperoxide as oxidant, and toluene derivatives as benzylation reagent is reported. The reaction exhibits many advantages, including mild conditions, a broad scope with good functional group tolerance, low cost, and avoidance of metal remaining in products. This method may bring novel inspiration and approach for synthesis of bioactive quinones.

The biological importance of quinoxalin-2(1H)-ones has inspired increasing pursuit for the development of efficient and expedient synthetic methods to access these entities. Among them, C3—H functionalization of quinoxalin-2(1H)- ones has been a sought-after strategy in recent years, especially using photocatalysis platform to trigger radical intermediates. Besides the direct addition of photocatalysis-yielding radicals to 3-position of quinoxalin-2(1H)-ones, a handful of three- component cascade reactions have been described, wherein olefins were used to preferentially accept radicals to generate new radical intermediates to experience the second addition process. Given the robust ability of cascade strategy to enrich the varieties of quinoxalin-2(1H)-ones, the exploration of new protocols remains a highly attractive task. Herein, we report photoredox catalytic reactions of quinoxalin-2(1H)-ones with 2-bromo-1-arylethan-1-ones as the partners, leading to a series of valuable 1,4-diketone-functionalized quinoxalin-2(1H)-one derivatives with satisfactory results. This is an unprecedented cascade reaction strategy, involving radical addition to quinoxalin-2(1H)-ones and the resultant intermediates to accept a second addition of radicals generated from bromide substrates or to experience S_N2 reaction with bromides.

An efficient visible light-induced direct cross-dehydrogenative coupling of heteroarenes with aliphatic alcohols in aqueous solution at ambient temperature was developed. The reaction is initiated by persulfate salts and does not require the use of stoichiometric acid for activation of the heterocycle. α-Hydroxyalkyl radicals transiently generated from alcohols via hydrogen atom transfer (HAT) undergo Minisci-type reactions with a range of heteroarenes. This protocol was highlighted by photocatalyst-free, green solvent, mild conditions, readily available starting materials, and wide functional group tolerance.