Indole, as one of the most important nitrogen-containing heterocyclic compound, is found widely in various natural products and synthetic drugs. 3-Iodoindoles play an important role as intermediate in the selective functionalization of indoles. 3-iodoindoles could be conveniently synthesized by electrochemical annulations of o-alkynylanilines with readily available iodide salt. This sustainable electrolysis strategy provides an efficient access to a series of 3-iodoindoles in good to excellent yields under external oxidant-free, base-free conditions and features broad functional group tolerance.

A series of ester compounds were synthesized by direct oxidative coupling of aldehydes with alcohols. The electrosynthesis reaction can be carried out at room temperature without inert gas protection, external oxidants, N-heterocyclic carbene (NHC) catalyst and alkali (such as DBU (1,5-dizzabicyclo[5.4.0]undecen-5-ene)), etc. The oxidative esterification reaction processes in a simple and easily available undivided electrochemical cell and constant current mode. This protocol features mild reaction conditions, broad substrate scope, cheap and easily available starting materials, and simple operation, providing a new strategy for the green synthesis of ester compounds.

A metal-free visible light-induced decarboxylation of phenylacetic acid derivatives has been successfully developed. Without the additives, direct decarboxylative monodeuteration of acids was successfully achieved using cheap deuterium sources. Besides, in presence of air, similar catalytic systems provided ketone products.

Electrochemical organic synthesis is considered as an environmentally benign method because it employs traceless electrons as redox agents, thereby avoiding the need for chemical oxidants. The electrochemical decarboxylative acylation of isocyanides and relay hydration reaction for the synthesis of α-ketoamides was developed. This reaction can be carried out under mild conditions in the absence of a chemical oxidant and a transition-metal catalyst.

α-Thioetherified carbonyl compounds widely exist in nature and various industries. They are important intermediates for the synthesis of heterocycles. The traditional synthesis methods of carbonyl thioethers usually need the pre- functionalization of substrates to convert them from nucleophiles to electrophiles. The synthesis methods are cumbersome and complex. Therefore, it is very important to explore a new thioetherification reaction of carbonyl compounds. The synthesis of thioether compounds by oxidative dehydrogenation coupling reaction of ketones and thiophenols under visible light was developed. In the photocatalytic system, without any metal, the reaction successfully realizes high atomic economy and wide substrate application range using cheap and easily available air as oxidant, and provides a simpler method for the direct construction of C—S bond.

Trifluoromethylarene compounds are readily available, and the highly selective cleavage of one C(sp³)—F bond in trifluoromethyl group is an important strategy to access pharmaceutical molecules containing the gem-difluoro groups. However, there are still some challenges in this field, such as the difficulty in activating the C(sp³)—F bond and the highly selective cleavage of the single C(sp³)—F bond. In recent years, efficient methods for the construction of gem-difluoro groups have been developed through the transformation of trifluoromethyl group, in which difluoroalkyl radicals or difluoromethyl carbocation intermediates are alway involved. The recent research progress in this field is summarized based on the cleavage strategies of trifluoromethyl group.

Herein, an aerobic oxidation approach for the synthesis of α-keto esters using oxygen as a cheap and pollution-free oxidant has been developed. By using readily available α-diazo esters as starting materials, the transformation was achieved by the inducement of visible light with eosin Y as the light sensitizer. The reactions proceed at room temperature under air atmosphere, tolerating substrates with broad scope, thus providing a green and facile synthetic approach to structurally diverse α-keto esters.

A metal-free visible-light-induced regioselective ortho-C—H phosphonylation of β-naphthols with diarylphosphine oxides has been developed using fluorescein as an organophotoredox catalyst in the presence of K₂S₂O₈ at room temperature under N₂ atmosphere. This feature along with mild reaction conditions, sensitive functional group tolerance, good to excellent yields, high regioselectivity, and scale-up synthesis makes it an ideal and practical alternative modification of β-naphthols. The experimental result suggests that a radical pathway is involved in the reaction.

Nitrones, served as one of the most important 1,3-dipoles in organic synthetic chemistry, can participate in various organic synthetic reactions to synthesize various heterocyclic compounds, such as nucleophilic addition, cycloaddition reaction, rearrangement reaction, C—H bond activation, etc. They are very important organic building blocks for the further construction of complex molecules. In recent years, α,β-unsaturated nitrone has attracted widespread attention from synthetic chemists because of its α,β-unsaturated bond containing rich chemical transformations, and can take part in various new chemical conversions to construct new structural heterocyclic compounds. The new strategies for the preparation of α,β-unsaturated nitrones in the past ten years, and their applications to construct nitrogen heterocyclic compounds, including O-transfer reaction, addition of nucleophiles and radical reagents, and various cycloaddition reactions, are reviewed.

An efficient and convenient strategy for the preparation of β-ketosulfone via C—H bond functionalization with sodium sulfite and aryl ethyl ketone/indenone under metal-free condition has been reported. The method has simple operation steps, mild reaction conditions and good functional group compatibility. The reaction enables the direct formation of C(sp³)—H into C—S bonds, thereby avoiding the use of strong oxidants and prefunctionalization of substrates. The β-ketosulfone compounds can be transformed as intermediates in organic synthesis.

A dimerization of N-methylindoles by trimethylsilyl bromide promoted C—C bond cross coupling reactions is described. A series of 2,3'-linked indolylindolines are smoothly installed under mild reaction conditions with up to 95% yields. The gram scale reactions demonstrate the practicality of this protocol. The UV-vis absorption and fluorescence spectra properties of target products are preliminarily studied.

A new photochemical reduction of nitroaromatics mediated by p-toluenethiol and PCy₃ under solvent-free, metal-free, and mild conditions is reported for the first time. This reaction has shown good generalities, and amine derivatives with different substituents and substitution patterns can be obtained in good yields. Therefore, this reduction method represents a potentially valuable new strategy for the efficient synthesis of amine compounds.

A transition-metal-free and chemical oxidant-free method was developed for the uniform synthesis of 4-arylselanylanilines and 4-alkylselanylanilines through electrolytic selenylation of N-unprotected anilines with diorganyl diselenides under mild conditions in an undivided cell. The present approach had good functional group compatibility, which made it to be a green and practical method to synthesize 4-(organylselanyl)anilines.

Carbene is one of the most important synthetic intermediates in organic synthesis. In the past few decades, transition-metal catalyzed carbene transfer reactions have made remarkable development. Recently, visible light-promoted transformation of diazo compounds through the formation of free carbene as key intermediate begun to rise. The reaction only need visible light as the sole energy source which meets the concept of green chemistry. Since the pioneering works developed by the groups of Diaves and Zhou, photo-promoted transformation of diazo compounds has attracted more and more attentions. On the basis of previous work, the latest progress in this field is further improved, which mainly focuses on the recent new transformation reactions of diazo compounds under visible light irradiation and the contributions reported from Chinese research group. The future development direction, as well as challenges in this field is prospected.

As a carboxylic acid, α-keto acid can be used as an acylating reagent by decarboxylation in organic synthesis. However, traditional methods often need transition metal catalysts or high temperatures, which limits the application of α-keto acid to a certain extent. In recent years, organic transformations induced by visible light have attracted extensive attention due to its mild conditions and simple operations. Herein, the recent advances of metal-free photocatalytic application of α-keto acids are summarized, which mainly include acylation reactions and cyclization reactions.

The synthesis of useful 2-acyl benzoxazoles has been realized via the tandem methylene C—H oxygenation and oxazole annulation of N-(o-hydroxyphenyl)phenylacetamides via copper catalysis in one-pot fashion. Besides featuring the incomparable step-economy resulting from the multi-step transformation, the tunable synthesis of diverse 2-methylene functionalized benzoxazoles using identical starting materials as well as the expanded synthesis of corresponding benzothiazoles is also attractive advantage of the work.

Selenium-containing heterocycles are a kind of important organic molecules, which are widely used in medicine, agrochemicals, organic materials and other fields. Therefore, the introduction of selenium atom into organic molecules is of great significance in synthetic chemistry. Visible light and electrochemical technology is one of the green and sustainable synthesis methods, which has opened up broad application prospects in the fields of bifunctionalization and cyclization of unsaturated bonds and direct functionalization of C—H bonds. In recent years, the development of novel selenium cyclization reaction using photoelectric technology to drive unsaturated bonds and direct selenization reaction of C—H bonds has attracted much attention. Therefore, this paper describes the electrochemical-driven and visible-light-induced synthesis reactions of selenium-containing heterocycles, and discusses the applicable scope and mechanism of some reactions.

Quinoxalinone and its derivatives are a class of important nitrogen heterocyclic compounds, which widely exist in natural products, drugs and functional materials. Therefore, the C—H functionalization of quinoxalinone has attracted extensive attention of chemical workers. In recent years, electrochemical synthesis directly uses electron as a “clean reagent” to participate in redox reaction without additional redox reagent, which has the characteristics of mild reaction conditions and good atom economy. This method meets the requirements of green chemistry and sustainable development. With the in-depth study on the mechanism of electrochemical synthesis and the standardization of reaction equipment, this method has become a powerful tool for functionalization of quinoxalineones. The recent advances in the electrochemical C(3)—H functionalization of quinoxaline-2(1H)-one are summarized. The reaction transformation conditions and mechanisms are systematically discussed, and the challenges and future directions of this field are included.

Facile ipso-cyclization and regioselective ortho-capture of N-hydroxylethyl-N-arylpropiolamides are reported for the synthesis of various selenium-containing benzo[b]pyrrolo[2,1-c][1,4]oxazin-3-ones. The reaction works well with high efficiency and broad reaction scope. In the process, it is believed that α-addition of the propiolamide, sequential ipso-cyclization of the aniline, and regioselective ortho-capture with a hydroxyl group are involved.

A solvent mediated regioselective C—H iodination of pyrrolo[1,2-a]quinoxaline with N-iodo-succininide (NIS) has been developed. 1-Iodopyrrolo[1,2-a]quinoxalines and 3-iodopyrrolo[1,2-a]quinoxalines could be selectively synthesized by using CHCl₃ and N,N-dimethylformamide (DMF) as solvents, respectively. Furthermore, 1,3-dibromopyrroloquinoxalines were obtained as the main products by the bromination of pyrrolo[1,2-a]quinoxaline with N-bromo-succinimide (NBS). The method features simple and mild reaction conditions, good regioselectivity, broad substrate scope, and gram-scale synthesis. In addition, the further transformation of halogenated pyrrolo[1,2-a]quinoxaline products has also been investigated through palladium and iodine catalyzed C—X (X=C, S) bond formation reactions.

Mechanochemistry has received increasing attention in the past decade. Among which, the mechanochemical C—H bond functionalization has emerged as a hot research topic. In comparison with the traditional solution-based organic reactions, the mechanochemical transformation features solvent-less conditions and short reaction time, constituting a green and efficient synthetic method in the modern chemical society. Notably, given the outstanding features, some long-standing challenges in the previous solution-based reactions could be well-solved by mechanochemical methods. Therefore, a timely comprehensive overview to recognize the present status of the mechanochemical C—H bond functionalization is of great importance to stimulate the development of more efficient synthetic methods. In this review, the state of the art for mechanochemical C—H bond transformation is presented. The limitation in this research field is also analyzed and the outlook of this area is prospected.

The direct oxidative functionalization of the benzylic C—H bonds of alkyl arenes paves a way for the quick synthesis of carbonyl compounds, which is of great significance in organic synthesis. In the past decades, the photooxidative pathways to achieve this reaction have developed rapidly, using relatively green and readily available oxygen as the terminal oxidant under light irradiation. In this paper, a commercially available and inexpensive chemical reagent, tetrabutylammonium tribromide (TBATB), was used to catalyze the photooxidation of the benzylic C—H bonds. Under visible light irradiation and air atmosphere, a catalytic amount of TBATB was utilized to achieve the efficient conversion of alkyl arenes to the corresponding aryl ketones. The reaction is feasible under mild conditions and without any additives, leading to a broad scope of ketone substrates. This method can be an effective supplement to the current benzylic photooxidation reaction systems.

The first UV-light-initiated cyclization of aryl aldehydes or aryl ketones with alkynes has been developed for the preparation of indenones. This reaction could proceed smoothly under UV-light with O₂ as an oxidant and photocatalyst-free condition, which features high efficiency, high atom economy, easily available starting materials, environmental friendliness, and tolerance to broad functional groups.

Aromatic ring- and sulfone-containing compounds are widely used in the fields of synthetic chemistry, medicinal chemistry and material chemistry. Therefore, it is of great significance to develop a simple, green and efficient method to construct aryl and sulfonyl compounds. Arylazo sulfones can be used as arylation or sulfonylation reagents to access aryl and sulfonyl compounds via hemolytic cleavage of C—N and N—S bonds to generate aryl radical and sulfonyl radical. In this review, the recent research progress of arylations and sulfonylations using arylazo sulfones is summarized. The synthetic methods and the related reaction mechanisms are highlighted, and an outlook of this research field is also given.

Diaryliodonium salts have been widely applied in organic transformations. According to the reaction type, the use of diaryliodonium salts in visible light induced arylation reactions is summarized from the following three parts: C—C bond formation, C—S bond formation and C—P bond formation.

A new visible-light-driven photocatalytic Kharasch-type addition of 1,6-enynes was reported. By using the characteristics of the in-situ-generation of trihalomethyl radicals from photocatalytic reduction of trichloromethane bromide and carbon tetrabromide, fac-Ir(ppy)₃-catalyzed Kharasch-type addition of 1,6-enynes of these polyhalogenated hydrocarbon substrates at room temperature was conducted under visible-light irradiation, regioselectively affording 1-indanone derivatives with an exocyclic double bond and a quaternary carbon in moderate to good yields. Based on the experimental results and literature reports, a reasonable reaction mechanism was proposed, which involved the in-situ-generation of trihalomethyl radicals, radical-induced addition-cyclization and radical cross coupling. In addition, this protocol featured wide substrate scope, high functional group compatibility, 100% atomic utilization, and mild conditions as well as simple operation. It provides a green, mild, and efficient synthetic strategy for the construction of 1-indanone skeleton with potential application, which is consistent with the concept of green chemistry.

N-Hydroxyphthalimide (NHP) esters are easily available, stable, inexpensive, and highly photoactive compounds, which are derived from the corresponding carboxylic acids. In recent years, the application of NHP esters in photocatalytic decarboxylative reactions has captured huge attention. Herein, the vigorous development of photocatalytic alkylation reactions using alkyl NHP esters as alkylating reagents is summarized based on the classification of photocatalysts.