Advances in photochemical/electrochemical synthesis of heterocyclic compounds from carbon dioxide

Yu-Yang Xie ^a and Ying-Ming Pan *^b
aJiangsu Key Laboratory of New Drug Research and Clinical Pharmacy, School of Pharmacy of Xuzhou Medical University, Xuzhou 221004, People's Republic of China
^bState Key Laboratory for Chemistry and Molecular Engineering of Medicinal Resources, School of Chemistry and Pharmaceutical Sciences of Guangxi Normal University, Guilin 541004, People's Republic of China. E-mail: panym@mailbox.gxnu.edu.cn

First published on 12th August 2024

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Yu-Yang Xie

Yu-Yang Xie is a lectorate at Xuzhou Medical University. He was born in 1991, in Anhui, China. He obtained his MSc degree from Guangxi Normal University and PhD degree in 2020 from Shanghai Jiao Tong University (China). He joined Xuzhou Medical University at the end of 2020. His current research interests focus on new strategies and methods for drug research, lead compound discovery and optimization.

Ying-Ming Pan

Ying-Ming Pan is a professor at Guangxi Normal University. He was born in Jiangxi, China. He received his BSc degree from Gannan Normal University and MSc degree from Guangxi University (China). He obtained his PhD degree from Xianmen University in 2010. He was awarded the Mingde Teacher Award in 2018. His present research interests focus on electrochemical synthesis and heterogeneous catalysis.

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1. Introduction

Since the advent of modern industrialization, the concentration of CO₂ in the atmosphere has continued to rise, causing detrimental impacts on global ecology.¹ Consequently, mitigating the concentration of atmospheric CO₂ has emerged as one of the paramount challenges confronting the world in recent years.² The efficient conversion of CO₂ into valuable chemical compounds presents a unique opportunity for resolving environmental predicaments associated with its presence.³ In the organic synthesis chemistry field, CO₂ serves as an ideal and valuable C1 building block due to its abundance, cost-effectiveness, non-toxicity and renewability.⁴ Currently, researchers are able to convert CO₂ into various compounds,⁵ including alcohols, ketones, carboxylic acids, amides, carbonyl compounds, and heterocyclic compounds among others. Among them, the heterocyclic structure plays an important role in many fields⁶ such as biology, medicine, agriculture and materials science (Fig. 1), as it is a key structure found in many drugs⁷ and organic raw materials.⁸

However, the activation of linear CO₂ presents a significant challenge due to its inherent thermodynamic and kinetic stability.⁹ As a result, conventional CO₂ conversion reactions require the use of highly active reagents or harsh reaction conditions,¹⁰ such as carboxylation with air-sensitive organometallic reagents that have high-energy substrates. Additionally, the incorporation of a metal catalyst¹¹ to activate CO₂ or the starting material can expedite the reaction by reducing the energy barrier associated with the transition state. However, there are still several key issues that need to be addressed in these transformations, such as the utilization of precious metal catalysts and stoichiometric oxidants. Therefore, it is imperative to develop green, efficient, and convenient methods for CO₂ utilization. Furthermore, the significance of environmentally sustainable synthetic chemistry has been increasingly emphasized in recent years. In particular, photochemical/electrochemical organic synthesis has gained significant attention due to its ability to produce highly active intermediates such as free radicals, free radical ions, and electron transfer complexes that are difficult to obtain through conventional methods.¹² Additionally, using electricity instead of various redox reagents in electrochemical synthesis aligns with the current trend and demand for reducing low carbon emissions.¹³ Under photochemical reaction conditions, light irradiation facilitates the conversion of reactants into active free radical intermediates under mild conditions, leading to the formation of final products.¹⁴ Considering these aspects, photoelectrochemical CO₂ fixation appears more appealing and convenient compared to traditional methods.

Although numerous reviews¹⁵ are available in published studies on CO₂ carboxylation reactions,¹⁶ there is a lack of comprehensive summaries regarding the synthesis of heterocyclic compounds¹⁷ using CO₂. This prompts us to further explore the potential of photo/electrochemical synthesis for CO₂ reactions. Here, we will provide a comprehensive overview of the research progress in the synthesis of heterocyclic compounds involving CO₂ participation, with a focus on distinct reaction types within photochemistry and electrochemistry. Additionally, we will elucidate the practicality of these reactions and their resulting products while delving into the underlying reaction mechanisms.

2. The photochemical conversion of CO₂ into heterocycles

In the past decade, the utilization of visible light in chemical reactions has enabled organic chemists to achieve unconventional bond formations and previously elusive chemical processes under mild reaction conditions. The research conducted by MacMillan,¹⁸ Stephenson,¹⁹ Yoon,²⁰ Xiao,²¹ Yu,²² and others in the field of visible light photoredox catalysis has sparked a revolution and paradigm shift within the realm of organic chemistry. The photoredox catalysts consist of transition metal photocatalysts such as polypyridyl complexes of iridium and ruthenium or organo-photocatalysts like rose bengal, eosin Y, rhodamine B, methylene blue, etc. These photocatalysts exhibit excellent reducing/oxidizing properties upon excitation by visible light. The structures and redox potentials of all the photocatalysts mentioned in this review are summarized in Table 1 and Scheme 1, respectively.

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According to the literature, photochemical fixation of CO₂ into heterocycles can be classified into four modes: (i) single electron transfer (SET) processes; (ii) cooperative/dual photocatalysis; (iii) visible light-induced transition metal catalyzed transformations; and (iv) photoexcitation.

2.1 The SET processes

The SET processes usually require the addition of metal-based complexes or organic dyes as photosensitizers, which can then activate substrates into radical species through the SET process.²⁵ A general mechanism of SET processes is shown in Scheme 2. Excited-state photocatalysts are initially produced by irradiation of the photocatalyst, which has an altered electronic distribution caused by the metal-to-ligand charge transfer. Then, this excited state photocatalyst*, which can be considered a more potent oxidant or stronger reductant, reacts with an electron-rich substrate (an electron donor) or electron-deficient substrate (an electron acceptor) by accepting or donating an electron via a SET process. In detail, the oxidized photocatalyst˙⁺ or reduced photocatalyst˙⁻ can then undergo a SET process with either the substrate or other intermediates to regenerate the ground state catalyst (Scheme 2).

In recent years, chemists have developed various methods for generating different types of radicals through visible-light photoredox catalysis, which offers distinct advantages such as high efficiency,²⁶ mild conditions, and easy operation. In this regard, Yu and his colleagues demonstrated a strategy using allylamines 1, difluoroalkyl reagents 2, and CO₂ via visible-light photoredox catalysis to synthesize functionalized 2-oxazolidinones 3 (Scheme 3).²⁷ The experiment demonstrates that allylamine with aryl substituents is suitable as a viable substrate for the reaction. Furthermore, various difluoroalkyl reagents were investigated and found to be effective in obtaining the desired compound. Additionally, by increasing the amount of substrate to 4 mmol, a product yield of 84% (1.31 g) was successfully achieved. Subsequent functional group transformations were efficiently conducted by hydrolyzing the ester group CF₂COOEt into carboxylic acid 4 while leaving the 2-oxazolidinone skeleton unaffected. Moreover, LiAlH₄ and NaBH₄ served as efficient and selective reducing agents to obtain reducers 5 and 6, respectively. All of these factors indicate great potential for widespread use of this conversion in organic synthesis and medicinal chemistry.

This difluoroalkylation/carboxylative cyclization can use BrCF₂COOEt as the radical precursor (Scheme 4). Mechanistically, DABCO reduction quenches the excited photocatalyst to produce Ru^I, reduces BrCF₂COO to produce the Ru^I catalyst, and reduces BrCF₂COOEt 2 to produce CF₂COOEt radicals B. Carbamate A is formed in situ using 1 and CO₂, followed by the addition of CF₂COOEt radical B to the double bond of carbamate to generate benzylic radical C. The subsequent step involves the oxidation of C by excited *Ru^II species and intramolecular cyclization of D affording product 3.

Carboxylic bifunctionalization of propargyl amines to obtain functionalized vinyloxazolidinones has been well studied.²⁸ The previous methods, although possessing potential advantages, rely on high loadings of metal catalysts, oxidants and/or elevated temperatures and are restricted to propargyl amines. Recently, McGarrigle and co-workers reported a visible light-assisted carboxylative sulfonylation by employing sodium arylsulfinates 8 as radical precursors, which afforded oxazolidinones 9 and vinyloxazinones 10 (Scheme 5).²⁹ α-Ethyl-substituted bulky and aromatic heterocyclic furan-3-ylmethyl and propyl-substituted propargyl amines were also well tolerated in this protocol and yielded the corresponding oxalidones (9b–9d). In contrast, N-phenyl and N-tosyl substituted propargyl amines failed to generate the corresponding sulfonylated oxazolidinones, possibly due to the weak nucleophilicity of the amines. Interestingly, N-substituted homopropargyl amines were also found to be compatible under the reaction conditions. This is the first report on carboxylative functionalization of homopropargyl amines for synthesizing six-membered rings. N-Cyclohexyl homopropargyl amine, halogen (4-F) bearing sulfinates, and simple benzenesulfinate reacted smoothly under this protocol yielding the corresponding compounds 10b–10d.

A plausible mechanism was proposed (Scheme 6). The Ru^II complex is initially excited by the absorption of blue light, which then oxidizes sulfinate 8 to form sulfinyl radical B and Ru^I species. Propargyl amine 7 reacts with CO₂ and a base to form carbamate intermediate A. The addition of carbamate A to sulfinyl radical B produces vinyl radical intermediate C. Intermediate C then undergoes SET with Ru, followed by a cyclization reaction to generate cyclic intermediate D. Intermediate D might undergo SET with Ru^I, followed by the addition of H⁺ to generate a radical intermediate G and regenerate the Ru^II complex. The reaction of intermediate G in the presence of a base and photocatalyst leads to the elimination of H⁺ and affords product 9. Alternatively, in the presence of a base, intermediate D can form anion E/F, which is subsequently protonated with H⁺ to afford the final product 9.

The production of CO₂ as a by-product is commonly observed in nearly all decarboxylation-involving reactions, and attempts to effectively utilize this crucial C1 building block for subsequent conversions have been met with limited success. In 2021, Sun and co-workers reported a new visible light-induced photoredox-catalyzed α-aminomethyl carboxylation of styrenes 11, leading to diverse γ-lactams 13 (Scheme 7).³⁰ The fact that the CO₂ released during the decarboxylation step can be recycled for subsequent carboxylation reactions is noteworthy. The present work extends the reaction substrate to aliphatic carboxylic acids, which is unheard of for transition metal-catalyzed decarboxylative coupling reactions. The mechanism suggested for the formation of γ-lactams is proposed. The decarboxylation reaction, under the influence of photocatalysis, generates aminomethyl radical A, which subsequently undergoes addition to the olefin double bond resulting in the formation of carbon radical B. The photocatalyst reduction and subsequent single-electron transfer result in the generation of carbanion C. Next, sequestration of the released CO₂ with this anion, leading to protonation, forms γ-amino acid D. The presence of a free N–H bond in the amino group facilitates intramolecular lactamidation of intermediate D, resulting in the formation of various γ-lactams 13.

In 2024, the same group reported the synthesis of lactams through visible-light photoredox-catalyzed intermolecular sequential α-aminomethyl/carboxylative dearomatization of indoles 14 with CO₂ and α-aminoalkyl radical precursors (Scheme 8).³¹ The source of CO₂ differed from that in previous reports.

In 2020, Yu and colleagues reported a strategy involving visible-light photoredox-catalyzed successive single electron transfer to enable the dearomative arylcarboxylation of indoles 17 with CO₂ (Scheme 9).³² The present method allows for selective avoidance of ipso-carboxylation of aryl halides and β-hydride elimination, which are the primary by-products associated with conventional aryl halide double bond functionalization processes. This approach offers a comparatively more straightforward means of accessing indoline-3-carboxylic acids. Despite the challenging steric hindrance, the researchers successfully achieved a 50% yield in obtaining biquaternary carbon products 18b at the C2 and C3 positions. Moreover, this strategy extends beyond the formation of five-membered rings, enabling the synthesis of the corresponding six-membered (18c) and seven-membered (18d) products with separation yields of 42% and 14%, respectively, under standard reaction conditions. To further demonstrate the utility of this method, a gram-scale reaction was conducted under standard conditions resulting in an impressive yield of compound 18a at 80%. Additionally, valuable structures can be easily obtained through simple and selective derivatizations such as bromination or reduction of carboxylic acids.

Based on the Stern–Volmer, isotope-labeling, and radical trapping experiments, it has been demonstrated that benzylic radicals and anions can be generated as key intermediates. This provides a pathway for reductive couplings with other electrophiles such as D₂O and aldehyde (Scheme 10). Mechanistically, the photo-activated 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) undergoes reductive quenching by DIPEA leading to the formation of the radical anion 4CzIPN˙⁻ and the radical cation DIPEA˙⁺ in subsequent steps. The reduced 4CzIPN˙⁻ then generates the [ArBr]˙⁻ radical anion A through single-electron reduction while rejuvenating the catalyst 4CzIPN. The generated [ArBr]˙⁻ radical anion subsequently undergoes fragmentation liberating the bromide anion and aryl radical B. Subsequently, aryl radical B readily undergoes intramolecular radical addition to indole's C2–C3 double bond forming benzylic radical C. In the next step, C reacts with 4CzIPN˙⁻ generating carboanion D which further undergoes nucleophilic addition to CO₂, followed by protonation resulting in the dearomatized arylcarboxylation product 18a.

Later, the same group developed the visible-light photoredox-catalyzed arylcarboxylation of unactivated alkenes 22 with CO₂, leading to a variety of highly valued polycyclic acids 23 in moderate to good yields (Scheme 11).³³ The reaction exhibits high chemo- and regio-selectivities, mild reaction conditions, good compatibility with functional groups, and broad substrate scope, allowing for easy scalability and facile derivatization of products. A wide range of electron-donating groups and electron-withdrawing groups were tolerated at the ortho/meta/para-positions of the arene moiety, providing the desired products in moderate to good yields. Substrates with different substituents on the aliphatic chain and 1,1-disubstituted unactivated alkenes were also suitable for this transformation. Additionally, indolin-3-ylacetic acid derivatives, chroman-4-ylacetic acid derivatives, and thiochromane-4-ylacetic acid derivatives can be prepared using this strategy.

The reaction produced an impressive 84% yield of product 23a, demonstrating its scalability. Furthermore, derivatization of 23a was conducted to showcase its potential synthetic applications. Selective reduction of product 23a using NaBH₄ resulted in a remarkable yield of 92%. The condensation between compound 23a and glycine methyl ester led to the high-yield formation of cycloamide 25. Additionally, the successful decarboxylation of the primary carboxylic acid in compound 23a was enabled by synergistic photoredox and HAT catalysis. Moreover, compound 23a exhibited moderate reactivity in decarboxyl-trifluoromethylation, resulting in the formation of compound 27. It is worth noting that compound 23a readily converted into oxidation-active ester 28, facilitating C–P bond formation through decarboxylphosphorylation as well as C–S bond formation via aryl thioylation (Scheme 12).

A possible mechanism for the overall transformation of unactivated alkenes is proposed (Scheme 13). The irradiated photocatalyst Ir^III generates excited *Ir^III, which can be reductively quenched through a catalytic thiolate to produce Ir^II and a thiyl radical. The Ir^II species may then participate in the reduction of CO₂ through a SET event, resulting in the generation of CO₂˙⁻ and the regeneration of Ir^III, thus completing the photoredox catalytic cycle. The in situ generated CO₂˙⁻ undergoes radical addition to the CC double bond of an unactivated alkene 22 to form an alkyl carbon radical A, which is expected to rapidly undergo cyclization and form the radical intermediate B. Finally, carboxylate B can be obtained through a hydrogen atom transfer (HAT) process involving radical intermediate C and the thiyl radical while simultaneously regenerating the thiol catalyst. The final product 23 is then formed during workup by protonation. Furthermore, it is proposed by the authors that incorporating silane as an additive can effectively facilitate alternative pathways for generating CO₂˙⁻.

In recent years, transition metal-catalyzed functionalized carboxylation of alkynes with CO₂ has been extensively studied by many groups.³⁴ However, the excessive use of metal reagents and the presence of transition metal residues have prevented their wide applications in industry. Moreover, the majority of these reactions proceed through two primary pathways: either the cyclometallation of CO₂ with a low-valent transition-metal catalyst and alkyne or the carboxylation of the in situ generated alkenyl metallic intermediate with CO₂. Both pathways involve two-electron activation processes. Visible-light photochemistry has emerged as a captivating strategy for achieving innovative chemical transformations in an environmentally friendly manner. The hydrocarboxylation of alkynes driven by visible light poses greater challenges compared to that of alkenes due to the difficulty in achieving high levels of chemical, regional, and Z/E selectivity, which may undergo mutual changes under light irradiation. The issue was resolved by Wu and Zhao in 2018 through the utilization of the visible light photooxidation/cobalt dual catalysis technique.³⁵ Subsequently, difunctionalizing carboxylation of alkynes 32 with CO₂ was further extended by Yu's group in 2023 (Scheme 14).³⁶ Various substituted thiophenols 31 and aryl acetylenes 32 could be directly converted into important target thiochromones 33 with high chemoselectivity and good regioselectivity. The involvement of single electron activation of CO₂ in thiocarboxylation has been demonstrated by mechanistic and computational studies, leading to unique β-carboxylation. To demonstrate the synthetic potential of this one-pot scheme, gram-scale reactions and several product derivatization experiments were carried out. The gram-scale reaction succeeded in obtaining the target compound at a yield rate of 75%. Additionally, thiochromones can be derived into CMV protease inhibitors, insecticidal compounds, and antimalarial compounds.

2.2 Cooperative/dual photocatalysis

Cooperative/dual photocatalysis synergistically combines traditional photocatalysis with transition metal catalysis (TM-cat.) (Scheme 15).³⁷ In this case, the sole photo-absorbing species employed as a photocatalyst engages in either a redox- or energy-transfer process with the traditional TM catalyst to facilitate the transformation. Transition metal intermediates trap and stabilize transient radical species generated by photoredox steps, making them available for synthesis.

In 2018, Wu, Zhao, and their colleagues developed a one-pot hydrocarboxylation/isomerization/cyclization protocol that utilizes iridium/cobalt dual catalysis to efficiently produce bicyclic scaffolds from simple alkyne precursors 34 (Scheme 16).³⁵ This approach not only enables facile access to coumarins but also allows for the generation of 2-quinolones and 2-benzoxepinones. A proposed mechanism is outlined in Scheme 16. The visible-light-excited *Ir^III undergoes reductive quenching by ⁱPr₂NEt to form an amine radical cation and a reduced Ir^II species which is then oxidized by the Co^II complex to regenerate Ir^III. The active Co^I species generated in this step reacts with CO₂ and alkyne substrates 34, leading to the formation of five-membered cobaltacycle intermediate A. Protonolysis of the C–Co^III bond by cationic amine radicals results in carboxylatocobalt intermediate C formation. The transient complex C may undergo photoredox-catalyzed reduction facilitated by ZnBr₂ through transmetalation, resulting in carboxylate D formation. Finally, aryl-substituted compound D can undergo reversible isomerization via an Ir-mediated triplet–triplet energy transfer process under visible-light irradiation producing product 37 through acid-mediated intramolecular cyclization. Terminal alkynes will insert another alkyne molecule into five-membered cobaltacycle A forming seven-membered cobaltacycle B, followed by reductive elimination yielding 2-pyrone 36 and regenerating Co^I. In the case of the substrate ortho-ester substituted aryl alkyne, the in situ generated C–Co^III bond undergoes intramolecular addition to the ester substituent, resulting in the formation of carbo-carboxylation intermediate G. Complex G can potentially undergo photoredox-catalyzed reduction and transmetalation with ZnBr₂, leading to the formation of zinc complex H while simultaneously regenerating CoI species. The tautomerization of H yields γ-keto acrylic zinc species I, which further undergoes another tautomerization to achieve the synthesis of γ-hydroxybutenolide 38.

2.3 Visible light-induced transition metal-catalyzed transformations

In this case, “traditional” transition-metal complexes were used as photocatalysts. For visible light-induced transition metal-catalyzed reactions, the catalysts are typically transition metal complexes such as palladium, iron, gold, cobalt, platinum, and copper with ligands like phosphine-, amine-, oxime-, or acetylides. This reaction does not require an exogenous photosensitizer to promote the photocatalytic transformations. The bond-forming/breaking process of visible light-induced transition-metal-catalyzed transformations occurs with direct involvement of the metal center (i.e., substrate–transition-metal interaction), which is a feature not commonly found in conventional visible light photocatalysis. Additionally, these transformations take place through a single catalytic cycle in which the transition metal complexes serve a dual role by absorbing visible light and enabling catalysis (Scheme 17).

In 2018, Yu's group developed oxy-alkylation of allylamines 39 with CO₂ and unactivated alkyl bromides via visible light-driven palladium catalysis (Scheme 18a).³⁸ Exclusively utilizing commercially available Pd(PPh₃)₄ as the photocatalyst, blue LED light is employed for irradiation purposes. A variety of primary, secondary and tertiary alkyl bromides are applied in this reaction to give 2-oxazolidinones 41 under 1 atm of CO₂. In 2019, Cheng and co-workers adapted a similar catalytic system to the radical oxy-alkylation of 2-(1-arylvinyl)anilines 42 with unactivated alkyl bromides and CO₂, which further expanded the substrate to 1,4-dihydro-2H-3,1-benzoxazin-2-ones 43 (Scheme 18b).³⁹

The visible light-induced transition metal-catalyzed reactions, in which the catalytic cycle is initiated by photoexcitation of the Pd⁰ catalyst, involve several steps (Scheme 19). First, the excited Pd⁰L* undergoes oxidation by alkyl bromide 40, resulting in the formation of active Pd^I species C and an alkyl radical. Then, substrate 39a rapidly reacts with CO₂ to form carbamate A. The introduction of the radical into the alkene within carbamate A leads to the formation of a more stabilized benzylic radical B. Subsequently, a SET occurs between intermediate B and Pd^I species, regenerating the Pd⁰ catalyst and forming the stabilized cation D. This cation is then quickly trapped intramolecularly by a nucleophilic carbamate, resulting in the desired product 41a.

2.4 Photoexcitation

Unlike photoredox and energy transfer, no additional photosensitizer (transition metal complexes or organic dyes) is required for photoexcitation. Some substrates, such as ketones, carboxylic acids, halides, etc., can absorb light and transform themselves into reactive intermediates to initiate the photochemical reaction (Scheme 20).⁴⁰

In 1972, Giezendanner and his colleagues initially demonstrated the pioneering application of photochemistry in the fixation of CO₂ to heterocycles. They successfully achieved the catalyst-free synthesis of substituted 2H-oxazol-5-ones 45 through the reaction between various arylazirines 44 and CO₂ under mercury lamp irradiation (Scheme 21a).⁴¹ The tunable structures of arylazirines 46 can lead to an enhanced isolated yield of 2H-oxazol-5-ones 47 in the catalyst-free system, thereby improving overall efficiency (Scheme 21b).⁴² The reaction of 2,3-diphenyl-2H-arizine 44a resulted in the formation of an equimolar mixture comprising two inseparable isomers.

The Minakata group has reported the conversion of CO₂ into 2-oxazolidinone for the first time.⁴³ Subsequent advancements in this reaction model primarily focused on expanding active iodinating reagents or starting materials. Expanding the application of photochemical synthesis to CO₂ fixation represents a highly promising field. In 2017, He et al. demonstrated the utilization of perfluoroalkyl iodides as radical and fluorine sources in a photoinduced radical-initiated protocol for the carboxylative cyclization of allyl amines 48 with CO₂ (Scheme 22).⁴⁴ The perfluoroalkylated cyclic carbonate 51 is successfully obtained via this protocol from allyl alcohol and CO₂, which is particularly noteworthy.

A plausible mechanism for the metal-free and visible light-driven carboxylative cyclization of allyl amines 48 with CO₂ is illustrated in Scheme 22 through a set of control experiments. The carbamate A is readily formed from 48 and CO₂, facilitated by TBD in the initial stage. Under the initiation of visible light, the carbon–iodine bond in nC₄F₉I undergoes homolytic cleavage, resulting in the generation of ˙nC₄F₉ and I˙ radical species. The addition of the ˙nC₄F₉ radical to the π-bond of carbamate A yields a perfluoroalkylated secondary carbon radical B, which is subsequently captured by the I˙ radical to form the iodine perfluoroalkylated carbamate intermediate C (Path I). Alternatively, the intermediate C could potentially be generated through a radical propagation of B with nC₄F₉I (Path II). Finally, the perfluoroalkylated 2-oxazolidinone 49 is obtained through intramolecular nucleophilic cyclization of C, accompanied by the release of [TBDH]⁺I⁻.

Shortly thereafter, the same group demonstrated a unique strategy for synthesizing exo-iodomethylene 2-oxazolidinones using propargylic amines 52 as substrates and CO₂ as a C1 synthon, and initiating the carboxylative cyclization of propargylic amines with CO₂ through molecular iodine (Scheme 23).⁴⁵ The carbamate A was formed through TMG-mediated coupling of amine 52 and CO₂ in the initial stage, according to the proposed mechanism illustrated in Scheme 23. Concurrently, under visible light irradiation, the generation of iodine radical I˙ from I₂ was achieved. Subsequently, insertion of I˙ species into the triple bond of intermediate A occurred to form radical intermediate B. The interaction between B and another radical I˙ ultimately resulted in liberation of [TMGH]⁺I⁻, along with carboxylative annulation, thereby yielding the desired target product 53.

To our satisfaction, the desired product 53a was conveniently obtained with a separation yield of 88% through an amplification reaction using 4 mmol of the starting material (Scheme 24).

Furthermore, the researchers explored various transformations of iodinated 2-oxazolidinones in order to demonstrate the versatility of this visible light-driven metal-free scheme in the carboxylative cyclization synthesis of propylamine with CO₂. Fortunately, a successful Buchwald–Hartwig reaction, Suzuki reaction, and Sonogashira reaction could be carried out using 53a as the substrate, resulting in good yields of products 54, 55, and 56, respectively. In addition, 53a is capable of photoreducing and photocatalyzing the “ene Reaction”, resulting in the formation of compounds 57 and 58, respectively. The extensive array of transformations observed in this study serves to illustrate the versatility of exo-iodomethyl 2-oxazolidinone as a chemical substrate. Consequently, it can be regarded as a foundational compound that readily lends itself to the synthesis of numerous valuable derivatives.

3. The electrochemical conversion of CO₂ into heterocycles

Under electrochemical conditions, the synthesis of heterocyclic compounds utilizing CO₂ as a C1 source can be classified into two distinct processes: direct electrolysis⁴⁶ and indirect electrolysis⁴⁷ (Scheme 25). Direct electrolysis involves the direct oxidation or reduction of the substrate on the electrode, followed by subsequent conversion of the obtained active intermediate. Indirect electrolysis entails oxidizing or reducing the medium on the electrode to generate an active substance that mediates the oxidation or reduction of the substrate. Indirect electrolysis can be further categorized into four types based on different media, including transition metal, halogen, organic molecule and EGB-mediated CO₂ electrochemical cyclization reaction.

3.1 Direct electrolysis

In 2008, Jiang and co-workers reported the electrochemical dicarboxylation/cyclization of arylacetylene with CO₂ (Scheme 26).⁴⁸ The reaction was carried out in undivided cells with a Ni cathode and an Al sacrificial anode containing n-Bu₄NBr-DMF as the supporting electrolyte, under 3 MPa CO₂ pressure at constant current and room temperature. This resulted in the formation of unsaturated arylmaleic anhydride with excellent yield. When the electrolysis was performed under anhydrous conditions, phenylacetylene or CO₂ on the surface of a nickel cathode first got electrons to form radical anions almost at the same time. Then, the attack of CO₂˙⁻ on phenylacetylene 34 or the reaction of phenylacetylene radical anions A with CO₂ gave an intermediate B (carboxylate radical anion). Further reaction of B with CO₂˙⁻, or reaction of B with CO₂ after getting one electron, afforded a dicarboxylate anion 59, followed by capturing of the anion by Al₃⁺ ions generated at the anodic Al metal to form stable aluminum carboxylates C. Finally, a phenylmaleic anhydride 60 was formed via the dehydration of phenylmaleic acids D during the acidification of the metal carboxylates with 2 mol L⁻¹ aqueous HCl.

However, when catalytic amounts of CuI were introduced into the system (with 4 MPa CO₂), tricarboxylation product 61 was furnished instead.⁴⁹ It is speculated that the copper salt can coordinate to the double bond of the dicarboxylate intermediate 59, facilitating the attack of CO₂ radical anions on the double bond, followed by the attack of the CO₂ radical anions to produce F. Furthermore, F is reduced to G. After being acidified and dehydrated, the product 61 is obtained.

Buckley, Wijayantha, and co-workers reported the electrosynthesis of cyclic carbonates from epoxides and atmospheric CO₂ in the absence of expensive catalysts (Scheme 27).⁵⁰ The reaction was carried out in an undivided cell equipped with acetonitrile as the solvent, n-Bu₄NBr as the electrolyte, and a 60 mA current. Excellent conversion into cyclic carbonate was achieved using a copper cathode/magnesium anode combination. In addition, the use of tetrabutylammonium bromide is essential for the reaction; only 17% conversion to the cyclic carbonate was observed when the bromide counterion was replaced by tetrafluoroborate. Without passing a current, the reaction did not occur. A wide range of substrates are suitable for the approach; excellent yields of cyclic carbonate 63 were produced, including electron-rich and electron-poor aromatic and aliphatic epoxides 62. The approach is easy to set up, is reliable, requires no expensive catalyst, and runs under atmospheric CO₂ pressure and at ambient temperature.

The electrochemical production of enantiomerically pure ring carbonates was achieved by employing chiral epoxides as starting materials in a similar fashion. In this reaction, stainless steel served as the cathode and magnesium as the anode.⁵¹ Scheme 28 provides a description of potential mechanisms involved. Sacrificial anodization of magnesium rods resulted in the formation of Mg²⁺. Simultaneously, CO₂ underwent reduction to form anion radicals which were stabilized by Mg²⁺ ions, leading to the formation of complex A. A subsequent reaction with epoxides involved a nucleophilic attack by bromide anions on the β-C atom, resulting in the formation of compound C. Following an intramolecular nucleophilic attack by oxygen atoms on the epoxide and electron transfer with another molecule of CO₂, compound E was formed and the stable complex A was regenerated. Carboxyl oxide anions acted as nucleophilic substitutes for bromine ions within the molecule, ultimately yielding ring carbonate 63. This proposed mechanism elucidates the crucial roles played by Mg²⁺ and bromine ions.

In 2013, Jiang, Yuan and co-workers reported the electrochemical synthesis of alkylidene lactones 65 from CO₂ and 1,4-diarylbutyl-1,3-diacetylene 64 using copper as a catalyst (Scheme 29).⁵² The reaction proceeded smoothly in an undivided electrochemical cell equipped with a nickel cathode and an aluminum anode, employing an n-Bu₄NBr-DMF electrolyte under constant current conditions at a CO₂ pressure of 4 MPa. In the presence of a CuI catalyst, the desired alkylidene lactones were obtained in satisfactory yields.

The electrochemical reactions can occur not only through direct cathode reduction but also via anodic oxidation. In 2023, Liang, Pan and co-workers developed a sophisticated electrochemically mediated three-component reaction involving proparamide 66, CO₂, and diselenides for the synthesis of selenium-containing oxazolidin-2,4-dione 67 (Scheme 30).⁵³ The oxazolidinone derivatives were obtained through electrolysis using a platinum plate as the cathode material, tetrabutylammonium hexafluorophosphate as the electrolyte, 20 mol% Cu(OAc)₂ as the catalyst, K₂CO₃ as the alkaline additive, and DMSO as the reaction solvent under constant current conditions of 20 mA for 5 hours. The utilization of the precious metal palladium is circumvented by this method, in contrast to the conventional synthesis approach. In the investigation of substrate universality, the authors not only explored the impact of electrical and spatial factors on the yield by incorporating various substituents but also extended the applicability of the substrate by introducing a range of natural products or drug molecular groups (67a–67f). Among them, mosapride, dopamine, tryptamine, and abietic acid derivatives were obtained with yields ranging from 50% to 63%. Aminoester derivatives were obtained with a yield of 75%. Methetine derivatives were obtained in 36% yield.

The diphenyldiene can undergo anodization to generate Se ions in this reaction (Scheme 31). Alternatively, the combination of selenides and [Cu] catalysts forms the copper–selenium complex, which enhances the electrophilicity of selenium atoms. Subsequently, the Se species undergoes an electrophilic addition reaction with the carbamate anion intermediate A through path A/B to obtain the intermediate B. Finally, an intramolecular nucleophilic attack takes place to yield the corresponding product 67.

However, the C–Te bond in tellurium is prone to cleavage due to its low bond energy. Moreover, the strong coordination ability of tellurium atoms often hinders transition metal-based catalysts during coupling reactions. These factors have resulted in synthetic methods for preparing organic tellurium molecules lagging far behind those used for sulfur-containing compounds. Therefore, Pan et al. developed a straightforward and efficient method for synthesizing tellurium-containing 2-oxazolidinones 69 from CO₂, ditelluride, and propylpargyne 68 (Scheme 32).⁵⁴ The reaction exhibits a high degree of substrate universality, enabling the synthesis of target products with moderate to excellent yields across diverse functional groups, natural product scaffolds, and drug-like molecular structures.

The bioactivity of these tellurium-containing oxazolidinones was investigated by evaluating the antitumor activity of all products in vitro using the MTT assay. The experimental results demonstrated that the oxazolidinones containing tellurium exhibited superior inhibitory activity against tumor cells compared to 5-fluorouracil (5-FU), which served as a positive control (Table 2). Among them, compound 69f displayed potent cytotoxicity against T-24, MDA-MB-231, MGC-803, and MIA PaCa-2 cells with IC₅₀ values of 0.4 ± 0.7, 0.6 ± 0.9, 0.9 ± 1.1, and 0.5 ± 2.0 μM, respectively. These findings present a novel approach for the synthesis of organotellurium compounds and offer new insights into exploring the biological activity of sulfur-containing organic compounds.

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3.2 Transition metal-mediated electrochemical cyclization with CO₂

In 1999, Duñach and co-workers reported the electrochemical reduction of 2-haloaryl ethers containing allyl and propargyl groups under CO₂ to synthesize benzofuranoacetic acid derivatives (Scheme 33).⁵⁵ Such an intramolecular cyclization–carboxylation reaction occurs in unicellular cells and is catalyzed by [Ni(cyclam)Br₂]. The reaction begins with the electrochemical reduction of Ni(cyclam)²⁺ to form Ni^I species, which (coordinated to CO₂) preferentially undergoes oxidative addition to the C(aryl)–halogen bond of 72a. After the oxidative addition, Ni^III intermediate A is proposed to be formed, which induces aryl radicals, leading to a rapid intramolecular cyclization of the double bond in the side chain to form B. Further one-electron reduction of Ni^III intermediate species B should result in the formation of alkyl Ni^II species C, which is capable of absorbing CO₂ to form a Ni^II carboxylate. The entire reaction is a two-electron process. In the course of a single-compartment battery using a magnesium anode, the magnesium ions produced during the anodic oxidation process are able to form magnesium carboxylate D, which releases nickel for further recycling. Magnesium carboxylate D is stable under the reaction conditions; it accumulates during electrolysis and is simply hydrolyzed at the end of the reaction. In a competitive reaction, intermediate A can also be directly reduced and carboxylated by intermediate E to produce the corresponding benzoic acid derivative.

The subsequent investigation involved the examination of diverse metallic nickel complexes and various reactants, including epoxides, resulting in the production of cyclic carbonates and other heterocyclic compounds.⁵⁶ In addition, the use of metal nanoparticle electrodes as cathodes avoids any additional catalysts for the electrosynthesis of ring carbonates from CO₂ and epoxides.⁵⁷

3.3 Halogen-mediated electrochemical cyclization with CO₂

In 2013, Jiang, Yuan and co-workers reported an efficient electrochemical pathway for the direct conversion of CO₂ with olefins to cyclic carbonates using NH₄I as the carrier electrolyte and graphite as the inert anode (Scheme 34).⁵⁸ Under the synergistic effect of I₂ and NH₃ generated by in situ electrochemistry, CO₂ with various olefins 75 can be smoothly converted into cyclic carbonates 63 with high yields under mild conditions. The generation of I₂ occurs in situ at the anode when NH₄I is employed as the supporting electrolyte. Subsequently, I₂ reacts with olefins and water to form a crucial intermediate iodohydrin B. Simultaneously, the cathodically produced NH₃ acts as a deprotonation reagent. Assisted by NH₃, the intermediate product B can undergo further reaction with CO₂, followed by intramolecular cyclization to yield the desired cyclocarbonate product 63. It is noteworthy that during electrolysis, both I⁻ and NH₄⁺ ions are capable of cycling, thereby ensuring that only electrons in addition to the substrate (CO₂, olefin, and H₂O) are consumed in the formation of cyclic carbonates.

In 2016, Yuan et al. successfully synthesized 1,3,4-oxadiazol-2(3H)-1 derivatives 78 in a one-pot reaction using NaI as the medium (Scheme 35).⁵⁹ This was achieved through a three-component coupling reaction of CO₂ with arylhydrazine and paraformaldehyde. Electrolysis was conducted at room temperature in a non-splitting battery setup with a nickel foil cathode and graphite rod anode. By avoiding the use of toxic phosgene or CO commonly employed in traditional methods, this process offers an environmentally efficient route for obtaining 1,3,4-oxadiazol-2(3H)-1 derivatives. The proposed reaction mechanism involves the initial formation of an intermediate product A by reacting phenylhydrazine 76 with paraformaldehyde 77, followed by anodic oxidation of I⁻ ions to form I₂ which then reacts to produce the key intermediate N-(alpha-iodoethylene)-N′-phenylhydrazine B. CH₃OH is electroreduced at the cathode to generate CH₃O⁻ base species while N-(alpha-iodoethylene)-N′-phenylhydrazine B is converted into its corresponding anion form C through electrogenic base CH₃O⁻ and additional t-BuOK base treatment. Finally, an intermolecular nucleophilic attack occurs between N-(alpha-iodoethylene)-N′-phenylhydrazine B and CO₂ resulting in the formation of product 78 while eliminating I⁻ ions from the system.

In 2021, an efficient method for the electrocatalytic synthesis of new carbamate compounds based on CO₂, N-(2-vinylphenyl)-sulfonamide 79 and amines has been developed by Liang, Tang and co-workers (Scheme 36).⁶⁰ The reaction conditions were a current density of 25 mA cm⁻², a temperature of −10 °C, platinum electrodes as the cathode and anode, NH₄I as the electrolyte and CH₃CN as the solvent. Researchers tested multiple amines coupled with N-(2-vinylphenyl) toluenesulfonamide and CO₂. Various amines, such as pyrrolidine, dialkylamine, primary amine, etc., can produce the desired product with satisfactory yield (80a–80d). It is worth noting that some amino esters (80i, 80j) that can participate in the reaction are also used. The cyclization of N-(2-vinylphenyl) sulfonamide derivatives containing additional sulfonyl groups can obtain high yields (80e). However, the presence of electron-withdrawing groups in substrates with chlorine, bromine, and cyanide substituents on the benzene ring leads to reduced yields (80f–80h). Additionally, the synthesized compounds were subjected to pharmacological investigation. The antitumor activity of these compounds was assessed using the MTT assay. Interestingly, carbamate derivatives exhibited relatively superior antitumor activity compared to both the raw materials and products obtained from the reaction between raw materials and amines. These findings suggest that this novel carbamate demonstrates enhanced potential for anti-tumor effects through CO₂ fixation with amines and N-alkenylsulfonamide as substrates.

Under the condition of constant current electrolysis, NH₄I serves as both an electrolyte and a catalyst (Scheme 37). Initially, iodide is oxidized to form iodine molecules. With the assistance of iodine, substrate 79 undergoes cyclization to generate intermediate A. Subsequently, intramolecular cyclization of intermediate A leads to the formation of crucial intermediate B.

The reaction between CO₂ and amines yields carbamate anion C. Intermediate B reacts with carbamate anion C to yield the final product 80. This reaction process employs paired electrolysis without requiring additional oxidants or alkalis, making it environmentally friendly and pollution-free.

In recent years, the prevailing synthetic approach for 2-oxazolidinone has involved the reaction of β-amino alcohols with various carbonylation reagents. However, these methods require the use of phosgene or CO, and their application is limited due to the toxicity associated with these substances. Pan et al. introduced a method for synthesizing pharmacologically active 2-oxazolidinone 83 by reacting CO₂ with allylamine 81 (Scheme 38).⁶¹ This methodology effectively avoids the need for toxic reagents while preserving the unsaturated double bond in the resulting 2-oxazolidinone. Consequently, the resulting product exhibits enhanced malleability and facilitates subsequent structural modifications. Furthermore, this methodology demonstrates its practicality as it allows for the synthesis of six-membered heterocyclic rings (1,3-oxazinone-2-ones) and can accommodate low concentrations of CO₂ in the reaction. Notably, moderate yields were achieved when utilizing three natural products or drug molecules (83e–83g): dehydroabietylamine derivative, amino ester derivative, and dopamine derivative.

The metal-free electrochemical coupling of aromatic hydrocarbons 84 with NH₃, CO₂ or CS₂ to form 2-oxazolidinones or 2-thiazolidinones 86 in the condensation sequence was reported by Vos and co-workers (Scheme 39).⁶² The reaction proceeds through N–H azopyridine, followed by ring enlargement using CO₂ or CS₂. This two-step reaction is catalyzed by a simple iodide catalyst that exhibits excellent regional selectivity compared to olefins, resulting in a total yield of up to 91%. The sustainable metal-free approach demonstrates exceptional atomic efficiency, facilitating the direct synthesis of these unprotected heterocycles from simple aromatic alkenes using renewable energy sources and readily available abundant materials.

3.4 Organic molecule-mediated electrochemical cyclization with CO₂

N-Heterocyclic carbons (NHCs) are formed by direct reduction of imidazole cations from single-electron cathodes and are well-known organic catalysts.⁶³ These electrically generated NHCs have found extensive applications as organic catalysts in the Henry reactions, benzobenzoic acid condensation, β-lactam synthesis, and the anodic oxidation of aldehydes to esters and thioesters.⁶⁴ NHCs can also be used to fix CO₂ onto organic compounds.

In 2002, Deng and coworkers reported the synthesis of cyclic carbonates 63 from CO₂ and epoxides 62 in pure ionic liquids without the need for additional supporting electrolytes and catalysts (Scheme 40a).⁶⁵ The experiments were conducted under mild conditions, employing copper as the working electrode and Al or Mg rods as the anode, in an undivided battery setup. The CO₂ cycloaddition properties of various epoxides 62 such as propylene oxide, epichlorohydrin, and styrene epoxide were examined. When [BMIm][BF₄] was used as the reaction medium with propylene oxide as the substrate, a yield of 92% was achieved. By utilizing inert electrodes in a split cell, Lu et al. successfully achieved the electrochemical reduction of [BMIm][BF₄] to produce NHC, while also accomplishing the conversion of CO₂ and diol 87 into ring carbonates 88 (Scheme 40b).⁶⁶

In 2011, Senboku and co-workers developed benzoate-mediated electrochemical reactions for the reductive radical cyclization of aryl halides with olefins followed by tandem carboxylation (Scheme 41).⁶⁷ Cyclic voltammetry studies showed that 4-tert-butyl benzoate has a reversible peak relative to Ag/Ag+ at −2.9 V, which is higher than that of the aryl bromo substrate (3.2 V), thus supporting the benzoate-mediated substrate reduction pathway. The probable reaction pathway is that 4-tert-butyl benzoate undergoes a one-electron reduction to the radical anion, reducing aryl bromide 72a to the aryl radical A. Upon addition of the intramolecular radical to the pendant olefinic portion of the substrate, the resulting carbon-centered radical B is reduced to carbon C, which is then captured by CO₂ to complete the reaction. On the other hand, dissolution of an Mg anode as the magnesium ion proceeds results in the prevention of any species from oxidizing at the anode.

Later, Senboku et al. reported constant-current electrolysis of 2-(2-propyl oxygen) bromobenzene 89 in DMF using an undivided cell equipped with a Pt cathode and a Mg anode in the presence of CO₂ and the electron transfer medium methyl 4-tert-butylbenzoate (Scheme 42).⁶⁸ This leads to aryl cyclization with a carbon–carbon triple bond and then immobilizes two molecules of CO₂ to obtain 2,2-ring fused succinic acid derivatives 90, in moderate to good yields. Through aryl cyclization, the framework of dihydrobenzofuran, indoline, dihydrobenzothiophene, indenane and tetrahydropyrane was successfully constructed, and succinic acid was successively produced by a unique series carboxylation reaction.

They also reported that vinyl radicals can also be produced by electrochemical reduction in the presence of CO₂ (Scheme 43).⁶⁹ Continuous radical cyclization and carboxylation of vinyl radicals were carried out successfully, and cyclized unsaturated carboxylic acid 92 was obtained in medium and high yield. Summarizing the data from the authors’ study, we find that methyl 4-tert-butylbenzoate is not a necessary electron transfer mediator in this study.

3.5 EGB-mediated electrochemical cyclization with CO₂

Cyanoethyl anion (⁻CH₂CN), as a classically electrogenerated base (EGB), has been widely used in many alkali-induced reactions. To be specific, cathodic reduction of acetonitrile solutions containing tetraalkylammonium salts leads to the formation of cyanoethyl anions (Scheme 44).⁷⁰ The tetraalkylammonium salt is reduced at the cathode, resulting in the generation of a carbon ion that subsequently abstracts a proton from the solvent acetonitrile to yield ⁻CH₂CN. Due to its pronounced alkalinity, cyanoethyl anion can deprotonate weakly acidic substrates, facilitating subsequent carboxylation–cyclization reactions with CO₂ and leading to the production of corresponding cyclization products. Importantly, this process allows for regeneration of acetonitrile as a solvent molecule without any by-product formation.

Inesi et al. reported a method for the electrochemical synthesis of chiral oxazolidin-2-ketones 94 from chiral amino alcohols 93 and CO₂ (Scheme 45).⁷¹ Oxazolidin-2-ketones were obtained with good yield under mild conditions, without the use of toxic, polluting or harmful chemicals and without the addition of any base or precursor base.

The Inesi team also reported the efficient electrochemical synthesis of 5-methylene-1,3-oxazolidin-2-one 95 (Scheme 46).⁷² The reaction uses ethylamine 52 and CO₂ as raw materials by direct electrolysis of MeCN and Et₄NPF₆ solutions containing ethylamine, followed by CO₂ bubbling and heating. The yield ranges from medium to excellent, the conditions are mild, and the use of toxic and harmful chemicals and catalysts is avoided.

In 2010, Yuan's group reported a convenient and effective electrochemical method for the cyclization reaction of CO₂ and terminal propargyl alcohol 96 at room temperature (Scheme 47).⁷³ In an integrated electrolytic cell containing n-Bu₄NBr-MeCN electrolyte, propargyl alcohol can be successfully electrosynthesized with CO₂ at a constant current under a CO₂ pressure of 3 MPa, and α-alkylene cyclic carbonates 97 can be obtained with good to excellent separation yield. The experimental results show that under the synergistic action of acetonitrile and n-Bu₄NBr, the Cu⁺ ions and strong bases generated by electrolysis can effectively catalyze or promote the coupling reaction, and then the cyclization reaction occurs. In the electrolysis process, the tetrabutylammonium ion R₄N⁺ on the surface of the nickel cathode initially acquires electrons to form tributylamine R₃N and an alkyl radical. Subsequently, the alkyl radical further gains electrons to generate the alkyl anion R⁻. Notably, the alkyl anion R⁻ exhibits sufficient basicity to deprotonate propargyl alcohol's hydroxyl group, resulting in the formation of intermediate A. This intermediate A then undergoes a reaction with CO₂ to yield carbamate anion intermediate B. Simultaneously, continuous oxidation of a sacrificial copper anode leads to Cu^I formation which can activate intramolecular ring closure reaction on acetylene leading to production of compound D, subsequently releasing the Cu^I catalyst and forming the corresponding ring carbonate 97.

4. Conclusions and outlook

The application of electrochemical and photochemical reactions is undeniably captivating for the following reasons: (1) high compatibility with a variety of functional groups; (2) light energy and electric energy as the driving forces for the reaction, without relying on stoichiometric oxidants or reductants, facilitate efficient and highly selective redox reactions under relatively mild conditions; and (3) decrease in environmental pollution, saving of energy and realization of sustainable chemistry. The carbonylation of CO₂ has been explored through photochemical and electrochemical methods, providing direct access to bioactive heterocycles and pharmaceuticals. This study mainly summarizes the cyclization of alkenes, alkynes, aromatics, alcohols, epoxides, and other compounds with CO₂ as the green C1 source under photocatalytic or electric conditions.

Although significant advancements have been achieved in utilizing CO₂ as a C1 source in photochemistry/electrochemistry, there still exist certain limitations. (1) Currently, photocatalysts primarily consist of precious metal complexes such as Ir and Ru, along with organic dyes. The catalytic system operates by absorbing visible light to induce electron transition from the ground state to the excited state, followed by a catalytic cycle through SET with the substrate. This intramolecular charge transfer induced by visible light necessitates a large π delocalization structure or conjugation in metal–ligand complexes to create a band gap for absorption in the low energy range of visible light. Consequently, complex molecular structures are required to achieve visible light-excited electron transition, inevitably increasing the cost of photocatalysts. In view of this, many chemists have endeavored to utilize Earth's abundant first-row transition metals⁷⁴ as cost-effective alternatives to rare elements like Ru and Ir that are scarcely found in the Earth's crust. Alternatively, chemists have explored intermolecular charge transfer for photoredox catalysis and developed non-metallic anion complex photocatalytic systems⁷⁵ capable of achieving decarboxylation coupling reactions under mild conditions; these systems also hold promise for CO₂ synthesis and conversion. (2) The main catalysts employed in photoredox catalysis include Ru, Ir, and other metals or small molecule photocatalysts. However, their homogeneity renders them non-reusable and economically impractical. Consequently, researchers have been exploring more cost-effective and reusable alternatives to these photocatalysts, aiming to develop protocols that can be readily implemented in the industrial sector. In this context, scientists have proposed various nanomaterials as sustainable and economical substitutes. For instance, [Ru(bpy)₃]²⁺ can be incorporated into metal–organic frameworks (MOFs) to facilitate the separation and reusability of precious metals. Additionally, in electrochemical reactions, scientists fabricate metal catalysts into nanoelectrodes which exhibit high catalytic efficiency while maintaining reusability without any loss of activity. Furthermore, by employing ionic liquids as reaction media for CO₂ electrocatalytic cycloaddition reactions, smooth operation can be achieved without the need for additional supporting electrolytes or catalysts. (3) In the field of electrocatalytic reactions, the development of novel electrocatalysts to enhance chemical conversion efficiency and accuracy remains a prominent research topic in organic chemistry. Due to the fact that most electrochemical transformations solely occur at either the anode or cathode, the complementary half-reaction at the other electrode is often disregarded or wasted within an electrochemical system. For instance, during an electrochemical reduction reaction, a sacrificial anode is typically employed. Consequently, exploring paired electrolytic reactions utilizing both cathodes and anodes has emerged as another focal point in electrochemical synthesis. (4) From the perspective of green chemistry, asymmetric photoelectrochemical synthesis offers a novel approach for the construction of chiral compounds. Despite the development of numerous sophisticated transformations in asymmetric photoelectrochemical synthesis by chemists, there remains a dearth of reactions utilizing small molecules like CO₂ for performing high-value asymmetric photoelectrochemical transformations. The exploration and advancement in this field pose significant and formidable challenges. (5) The photocatalytic and electrocatalytic pathways provide a cost-effective means of converting CO₂ into valuable organic compounds. However, practical applications necessitate the development of an efficient and durable reusable catalyst capable of withstanding impurities in CO₂ feed during industrial processes, while maintaining high activity and selectivity. Therefore, scientists continue to strive towards designing efficient pathways for photocatalytic and electrocatalytic cycloaddition reactions suitable for industrial applications.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the National Natural Science Foundation of China (No. 22161008) for financial support. This work was also supported by the Jiangsu Province Shuangchuang PhD Program (Yu-Yang Xie, JSSCBS20211266) and the start-up funding provided by Xuzhou Medical University.

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