Thios/selenium sulfonates are key structural skeletons in natural products, synthetic bioactive molecules and marketed therapeutic drugs, as well as important building material in organic synthesis, and are widely used in drug molecules and materials science. With the rapid development of transition metal-catalyzed, photocatalytic and electrocatalytic radical reactions, breakthroughs in radical chemistry have been achieved in the field of synthesis. Therefore, the applications of thios/selenium sulfonates as radical reagent have attracted extensive attention. The applications of thios/selenium sulfonates as radical reagent in metal catalysis, visible light catalysis, metal-light synergistic catalysis and other types of catalysis, which have been developed rapidly in recent years, are reviewed, and the mechanisms of some reactions are discussed in detail.

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.

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.

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.

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.

Bifunctionalization of alkenes is a hot topic in organic synthesis, which can introduce different functional groups into the carbon-carbon double bond (C=C) of olefins quickly and economically, thereby increasing the complexity and application value of molecules. The trifluoromethyl groups can influence lipophilicity, permeability, and metabolic stability of organic compounds. Meanwhile, C—N bond widely exists in various organic molecules. The construction of C—CF₃/C—N bond simultaneously through bifunctionalization of alkenes is of great significant in the field of organic synthesis and medicinal chemistry. In this review, we focus on the progress in C—CF₃/C—N bond formation reactions of alkenes involving free radical intermediates based on different “N” sources. The possible radical reaction mechanism and their possible applications are also highlighted.

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.

Organosilanes are versatile intermediates and products in material chemistry, pharmaceutical chemistry and synthetic chemistry because of their unique physical, chemical and physiological properties. In the past century, the synthesis of organosilanes containing C—Si bonds has gradually become one of the research hotspots. Due to the high tolerance of functional groups, good atomic economy and chemoselectivity of radical chemistry, the construction of C—Si bond via free radical silylation reaction has achieved great progress in recent years. The research progress of the radical silylation reactions exploring silane as the precursor of silicon radical in the last decade is summarized, and the related reaction mechanism is discussed.

Unactivated cycloalkane is one of the most extensive natural resources, and it is of great significance to develop mild and efficient strategies to achieve C(sp³)—H of unactivated cycloalkanes. At the same time, the preparation of ring compounds is one of the hotspots in organic synthesis and medicinal chemistry. Because of the advantages of mild conditions and high reactivity, radical cascade cyclization involving C(sp³)—H functionalization of unactivated cycloalkanes can simultaneously meet the above requirements and synthesize cyclic molecules with potential applications in high atomic- and step- economy manner. In this review, from the point of view of iron catalysis, copper catalysis, and metal-catalyst-free systems, the advances in radical cascade cyclization involving C(sp³)—H functionalization of unactivated cycloalkanes in recent ten years are summarized.

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.

The difunctionalization of unsaturated hydrocarbons (alkenes or alkynes) is an important strategy to introduce functional groups, and is also an important method to synthesize polysubstituted hydrocarbons, which is widely used in the fields of total synthesis of natural products and drug synthesis. The latest research progress on the electrochemically enabled difunctionalization of alkenes or alkynes in recent five years is summarized. According to the classification of difunctionalization of alkenes and cyclization of unsaturated hydrocarbon, the application and the related reaction mechanism are introduced. It is hoped that this review can be referred to the future application in organic synthesis of electrochemic- mediated difunctionalization of alkenes/alkynes.

As one of the central functional groups in organic chemistry, carbonyl skeleton can be achieved by many ways, but the intermolecular radical reaction using acyl phosphonate as acyl donor is rarely reported. The acylation reaction between acyl phosphonate as a free radical acceptor and three free radical donors, alkyl aldehyde, Hans ester and alkane, was studied under thermochemical and photochemical conditions, respectively. When alkyl aldehydes and alkanes were used as radical precursors, the intermolecular acylation reaction could proceed smoothly under the conditions of microwave heating and the presence of oxidants. When Hantzsch esters were used as the radical precursors, the intermolecular acylation reaction could take place smoothly under UV irradiation at room temperature.

Ethers, aldehydes, and amides are widely found in natural products and pharmaceutical molecules, and employed as inexpensive materials in synthetic chemistry. Radical-mediated heteroatomic α-C—H bond functionalization provides an efficient strategy for structural modification of ethers, aldehydes, and amides. New approaches of C—H alkynylation and allylation through phenyliodine bis(trifluoroacetate) (PIFA) induced hydrogen atom transfer (HAT) under visible-light irradiation are reported. The method is operationally simple, leading to the corresponding alkynylated and allylated products in moderate to good yields. The protocol features mild and transition-metal free conditions, broad functional group compatibility, and is adapted to a diversity of ethers, aldehydes, and amides as well as various alkynylating and allylating reagents.

Pyrazoles are the most important five-membered N-heterocycles as they are present in many natural products and synthetic drugs. Among various pyrazole derivatives, 4-bromopyrazoles have attracted great attention because they not only display significant biological activities and physicochemical properties but also serve as valuable synthons in organic chemistry. In this manuscript, an efficient and eco-friendly method for the synthesis of various 4-bromopyrazoles through three- component reaction of hydrazine, acetylacetone and 2-bromomalonate was reported and the proposal reaction mechanism was revealed. According to the mechanism, hydrazine reacts with acetylacetone to afford pyrazole firstly, which then converts to 4-bromopyrazole through bromination. This method could also be well applicable for the bromination of other heterocyclic compounds. Given the high oxidation state of diethyl 2-bromomalonate, the peroxidation can be avoided through the electrolysis process, thus the excess amount of brominating agent was not be required.

A mild approach to prepare functionalized azetidines by intermolecular aza Paternò-Büchi reaction using suitable auxochrome (pyrazole) to promote 3-pyrazolyl quinoxalin-2(1H)-ones into its excited state has been developed. Overall, pyrazolyl plays a key role in reducing the excited state energy of the triplet state of quinoxalinones, changing the redox potential and subsequent transformation as a good leaving group. The cross-dehydrogenative coupling (CDC)-amination, aza Paternò-Büchi reaction, and elimination reaction were also investigated.

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.

Benzazepines are a kind of important seven-membered nitrogen-containing heterocyclic compounds, which are extensively exisited in natural products and pharmaceuticals, and have versatile biological activity. Therefore, great attention has been focused on the green and efficient synthesis of the benzazepines. In recent years, radical cascade cyclization reactions have gradually been developed as a powerful tool for the construction of benzazepines owing to their unique advantages. In this minireview, according to the different driving force of reaction, the research progress on the synthesis of benzazepines by radical cascade cyclization reactions in the past five years is systematically summarized in terms of photochemical reactions, electrochemical reactions and thermochemical reactions.

Owing to the special chemical structure of the seven-membered heterocyclic skeleton and the unique chemical properties of the heteroatoms contained, it is widely used in natural products and drug molecules. However, the development of synthetic methodology for these structures is challenging due to the unique thermodynamic and kinetic characteristics of nitrogen/oxygen-containing seven-membered heterocyclic frameworks, as well as their own unique cross-ring forces. Therefore, it is of great significance to develop simple and efficient methods for the construction of seven-membered heterocyclic compounds. Compared with traditional synthetic methods, radical reactions can avoid the limitations of poor atom economy and harsh reaction conditions. In this review, the recent synthetic strategies for the construction of seven-membered nitrogen/oxygen-containing heterocyclic compounds using radical tandem cyclization reactions are summarized.

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.

The 5-endo-trig radical closure has emerged as an ideal method for the fast aseembly of five-membered rings. It is believed to be kinetically disfavored due to the stereoelectronic disadvantages, so it is recognized as a disfavored process according to the Baldwin-Beckwith rules. Despite the challenges, continuous efforts have been devoted to develop effective strategies that enable 5-endo-trig radical cyclization. The recent advances on 5-endo-trig closure of 4-pentenyl radicals are summarized and the common strategies for accelerating the cyclization are analyzed, which will be valuable for the design and development of novel radical 5-endo-trig processes.

Decarboxylation of fluoroalkyl carboxylic acid derivatives is an effective way to obtain fluoroalkyl radicals, which subsequently participates in the tandem and addition reactions of olefins, isonitriles and aromatic (hetero) rings, providing an important method to obtain divegent fluorine-containing molecules. A variety of fluoroalkyl cyclic compounds such as phenanthridines, dihydroflavonoid, quinolinones and indolones could be constructed using the tandem reaction initiated by fluoroalkyl radicals generated from fluoroalkyl carboxylic acids. The research progress of radical reactions induced by decarboxylation of fluoroalkyl carboxylic acids in recent years is summarized, which includes reaction design, reaction mechanism and outlook.

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.

Thiocyano and sulfonyl groups as important functional groups are widely existing in natural products, agrochemicals and medicines. They are also versatile synthetic intermediates that could be converted into various functionalities. There are limited reports on the synthesis of compounds that contain these two functional groups. A simple three-component thiocyanatosulfonylation of aryl acetylenes with sodium sulfinates and NH₄SCN through electrochemical oxidation to construct thiocyanated vinylsulfones has been established. The reaction employs easily accessible starting materials and features the characterization of external oxidant-free, mild reaction conditions, and high stereoselectivity. Moreover, this method enables two C—S bonds to be simultaneously formed in a one-step reaction.

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.

Aryldiazonium salts are widely used in organic synthesis due to their high reactivity and readily availability. As a common arylation reagent, aryl diazonium salts have been widely used in material chemistry, dye chemistry and medicinal chemistry by retaining and leaving N₂ skeleton. In recent years, with the rapid development of radical chemistry, the cyclization reaction involving aryl radicals, which produced by aryldiazonium salts, has also been developed rapidly, and this strategy also stimulates the enthusiasm of organic chemists to construct various ring compounds. In this review, the addition of unsaturated carbon-carbon bonds and carbon-nitrogen bonds to construct various cyclic compounds from aryl radicals is summarized and classified according to the size of the cyclic compounds. Finally, the current research progress is summarized.