English

• 1. Introduction

  CO₂ radical anion (CO₂^•−) was recently developed as a potent C1 synthon and potent single-electron-reduction (SET) reductant for reductive transformation and radical carboxylation [1–3]. Under electrochemical [4–7] or photochemical [8,9] conditions, CO₂ could be reduced via SET to its radical anion form, which owns reversed polarity compared with CO₂ and therefore undergoes unprecedent radical transformations. Yu [10–21], Jamison [22,23], Qiu [24–26], Zhang [27], Zhou [28], Romo [29], Maiti [30], He et al. [31] have demonstrated the synthetic applications of CO₂ as the CO₂^•− precursor [32,33]. Giving its redox potential (E_red = −2.21 V vs. SCE), direct reduction of CO₂ to CO₂^•− requires deep reduction conditions [34]. To generate CO₂^•− in milder redox condition, formate salts were utilized as the CO₂^•− precursor via a hydrogen-atom-transfer (HAT) process in the presence of HAT catalyst/reagent. Since 2020, Li [35–37], Wickens [38–43], Jui [44,45], Molander [46,47], Glorius [48], Li [49–51], Mita [52,53], Shang [54–56], Zeng [57], Fu [58,59], Zhu [60–65] and Li et al. [66,67] have demonstrated the utilities of formate salts for C-X bond activation and reductive carboxylation reactions. There reactions have been summarized in several decent review articles [1–3,68–70].

  Very recently, reactions involving oxalate salts as the CO₂^•− precursor were developed for organic synthesis. Different with formate as the CO₂^•− precursor that requires existence of a proper HAT catalyst or reagent, the oxidative fragmentation of oxalic dianion could be realized under much milder and cleaner redox conditions or EDA complex formation process with visible-light irradiation. As the oxalate could be synthesized by reduction of CO₂, utilization of oxalate to access value-added molecules is also an attractive strategy for carbon utilization [71]. Oxalic dianion can be very easily oxidized via SET (E_ox = +0.06 V vs. SCE) to release oxalic radical anion (C₂O₄^•−), which subsequently undergoes homolysis of the C-C bond to give CO₂ and CO₂^•− (Scheme 1) [72]. The generated CO₂^•− can work as the C1 source via Giese radical addition to the unsaturated C-C bonds and also can be utilized as a strong reductant to reduce highly-energy-demanded substrates.

  Scheme 1

  

  Scheme 1.  Degradation of oxalic dianion produces two electrons and two CO₂ molecules through CO₂^•− formation to achieve inert C-X bonds activation and reductive carboxylation reactions.

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  Overall, the C₂O₄^•− could consecutively generate two electrons stepwise in one reaction for a SET reduction relay and release CO₂ as the C1 source for carboxylation. Compared with CO₂ or formate salts as the CO₂^•− precursor, C₂O₄^•− showcased distinguished potential to highly enhance the diversity of the reaction patterns and expand the chemical space of the carboxylic acid derivatives.

  Recently, a series of novel radical transformations involving C₂O₄^•− were disclosed. The aim of the current article is to give an overview of the raising examples on applications of C₂O₄^•− in organic synthesis as an electron donor and a C1 source for carboxylation reactions.

  2. Photocatalytic transformations

  2.1 Reduction by CO₂^•− via SET for C-X bonds functionalization

  As early as 1980s, Balzani et al. had already reported that (NH₄)₂C₂O₄ could be utilized as the sacrificial electron donor to reduce Co(Ⅲ) to Co(Ⅱ), releasing H₂ and CO₂ gas as the waste [73]. In 2013, Cho and You et al. develop a coreactant strategy for trifluoromethyl radical generation through SET reduction of trifluoromethyl iodide with tetrabutylammonium oxalate (TBAO) as the electron donor [74]. In 2014, Lodge and Frisbie et al. employed TBAO as a coreactant to interacted with luminescent Ru(bpy)₃(PF₆)₂ complex for the development of the solid-state electro-chemiluminescent (ECL) light-emitting devices [75]. Later on in 2018, when Bard investigated the use of oxalate salts in ECL systems as the coreactant, formation of C₂O₄^•− was observed and the C₂O₄^•− decomposed to produce CO₂^•− species as a very strong reductant [76]. However, the application of oxalic dianion in organic synthesis other than only as the mere sacrificial electron donor was not realized until 2023.

  In 2023, Connell et al. developed a catalytic photoredox protocol for dehalogenation of aryl halides with CO₂^•− released from oxalic dianion via oxidative degradation [77]. Reactions were conducted with [Ir] photocatalyst and different electron donors in acetonitrile as the solvent under blue light irradiation (457 nm) for 8 h. As shown in Scheme 2 for the selected examples, when triethylamine (TEA) was used as the electron donor, the conversions of the corresponding bromoarenes for dehalogenation dropped significantly as the energy demand increased. For the most challenging substrate 1d (4-bromoanisole, E_p = −2.72 V vs. SCE), TEA was completely ineffective for the dehalogenation while the oxalate salt TBAO could give 86% conversion, indicating TBAO as a powerful electron donor could dramatically extend the reduction potential of the photocatalytic system via generation of CO₂^•−, which is a potent reductant (E_1/2 = −2.21 V vs. SCE).

  Scheme 2

  

  Scheme 2.  TBAO for C-Br bond activation.

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  Later on, Matsumoto and Maruoka et al. disclosed a bidirectional elongation for synthesis of 1,4-dicarbonyls using ambiphilic radical linchpin via sequential photocatalysis (Scheme 3) [78]. The intermediate 5, prepared from Wittig reagent 3 and electron-deficient alkene 4 in the presence of 4CzIPN as the photocatalyst, was treated with oxalic acid and thiolarene S1 in DMSO/H₂O as solvent under blue light irradiation. In this reaction, the CO₂^•− was utilized as the reductant for C-P bond activation, followed by alkylation with alkenes. The proposed reaction mechanism was illustrated in Scheme 3. Once the intermediate 5 was obtained, it was subjected directly to the next step without any further purification. In the presence of oxalic acid, the phosphonium oxalate salt Ⅰ was in-situ generated. Under blue light irradiation, the photocatalyst in the excited state (4CzIPN*) was subsequently quenched by salt Ⅰ and became its reductive state 4CzIPN^•−. In the meantime, the oxalate decomposed to release CO₂ and CO₂^•−. The later one lost one electron to the phosphonium cation Ⅱ and became CO₂, triggering the C-P bond mesolytic cleavage to give carbon-centered radical intermediate Ⅲ. Afterward, radical addition of intermediate Ⅲ to the alkene 6 occurred to generate intermediate Ⅳ, which abstracted H atom from S1 to give the final product. The HAT catalyst S1 could be regenerated via SET reduction by 4CzIPN^•− and protonation process under the acidic reaction conditions.

  Scheme 3

  

  Scheme 3.  Oxalate salt for C-P bond activation and alkylation.

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  With CO₂^•− as the strong reductant, Zhu et al. then investigated the reductive deoxygenation-carboxylation cascade with various acylated alcohols (Scheme 4) [79]. The reactions were conducted with TBAO as the CO₂^•− donor in the presence of TMG (tetramethylguanidine) as the base and 4CzIPN as the photocatalyst in DMF as the solvent. As illustrated in Scheme 4, the acylated alcohol 8 was first activated by the tetrabutylammonium cation (TBA⁺) to facilitate the single electron transfer by CO₂^•− in-situ generated from C₂O₄²⁻ and the 4CzIPN*. When the radical anion intermediate Ⅱ was formed, the β-scission of intermediate Ⅱ to give the radical intermediate Ⅲ. This intermediate was reduced for the second time to yield the carbon anion intermediate Ⅳ, which trapped the CO₂ in the reaction and produce the final product 9. The deuteration experiment with D₂O indicated formation of the key intermediate Ⅲ and the radical-radical coupling mechanism should be excluded.

  Scheme 4

  

  Scheme 4.  TBAO for C-O bond activation and carboxylation.

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  During such transformation, the counter cation (TBA⁺) was proven to be non-innocent. It could be the carbonyl group activator to promote the single electron transfer from CO₂^•− to the substrate and also stabilize the radical anion intermediate Ⅱ to facilitate the C-O bond scission to achieve the final product. It is worthy to note, different with the previous carboxylation procedures, this reaction was conducted under N₂ atmosphere without any “sacrificial” electron donors. The CO₂ released from C₂O₄²⁻ worked as the C1 source, meaning that the carboxylation reaction efficiency was dramatically improved compared with those reaction with CO₂ gas (usually CO₂ balloon) as the C1 source. Various value-added carboxylic acid derivatives such as Ibuprofen, Naproxen, and Flurbiprofen, even the steric hindered all quaternary carboxylic acid could be synthesized in high efficiency. This strategy demonstrated the outstanding advantages of TBAO as both the SET reductant and the C1 source, providing an efficient way for synthesis of diverse carboxylic acid derivatives.

  Alkyne functionalization could provide diverse chemical bonds modification patterns for synthesis of various complex molecules. Wu and Lu et al. developed a reductive condition to reduce the aryl alkynes 10 to the corresponding alkyne radical anion species, which could be further converted to a serious of structurally diverse products [80]. Interestingly, reactions proceeded in different route depending on the electron-withdrawing groups tethering on the alkyne substrates. Mechanistically, the alkyne 10 could be first reduced by CO₂^•−, which was in-situ generated from TMG and oxalic acid in the presence of 4DPAIPN as the photocatalyst, and protonated by oxalic acid to give the vinyl radical Ⅰ. When the sulfonyl group was tethered on the alkyne substrate, the reaction proceeded through route a as shown in Scheme 5. In the presence of 1,1-diaryl alkene 11, the vinyl radical I added to the alkene to generate radical Ⅱ, which underwent 6-endo-trig radical cyclization to produce the cyclized radical intermediate Ⅲ. Subsequent radical elimination (-PhSO₂^•) and re-aromatization by TMG resulted the final product 12. On the other hand, when the sulfonyl group on alkynes 10 was replaced by esters, the corresponding vinyl radical Ⅰ added to alkenes 13 to give the new carbon-centered radical intermediate Ⅴ, which was further reduced by the 4DPAIPN^•− via the second SET to give ester 14 as the final products. These transformations could produce various naphthyl compounds 12 and aryl propionic acid esters 14 with good functional groups tolerance in good yields. The authors also investigated the reactivities of terminal alkynes, which will be discussed in the next section.

  Scheme 5

  

  Scheme 5.  Oxalate salt for alkynes reductive functionalization.

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  2.2 Giese radical addition of CO₂^•− to alkenes or alkynes

  The reactivity of CO₂^•− as the strong reductant could realize the above-mentioned C-X bonds activation and alkynes reductive transformations. On the other hand, as a carbonyl radical, CO₂^•− can undergo Giese radical addition to the unsaturated carbon-carbon bonds and install the carboxyl groups. In 2023, Wu and Lu et al. reported an elegant and pioneering work, where the C₂O₄²⁻ could be used as a traceless linchpin for cross coupling of different alkenes [81]. As illustrated in Scheme 6, the key step was the formation of the carboxylic acid intermediate 16, which was originally isolated and used as the substrate to screen the reaction conditions for the second step. As shown in Scheme 6, the photocatalyst 4DPAIPN was first excited by blue light irradiation and then oxidize the C₂O₄²⁻ to release CO₂ and CO₂^•−, which underwent Giese radical addition to alkene 15 and generated the carbon-centered radical intermediate Ⅰ. the radical Ⅰ was further reduced by 4DPAIPN^•− to give the carboxylic acid 16. After this transformation was accomplished, the second photocatalyst Ir(dFCF₃ppy)₂(dtbbpy)PF₆ was subjected to the reaction mixture that was then heated to 100 ℃ to oxidize the intermediate 16 and trigger the decarboxylation to give the new carbon-centered radical Ⅱ. In the presence of electron deficient alkenes 17, radical addition to the C-C double bond resulted in radical intermediate Ⅲ, which was further reduced by the photocatalyst in its reductive state to give the anion species. Under the acidic conditions, the anion intermediate was protonated immediately to give the alkenes cross coupling product 18. Overall, C₂O₄²⁻ was employed as a traceless linchpin and two molecules of CO₂ were released. However, this reaction showed the great reactivity of oxalate salt as the C1 source for carboxylation reactions.

  Scheme 6

  

  Scheme 6.  Oxalate salt as linchpin for alkenes cross coupling.

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  The same research group then investigated the alkyne hydrocarboxylation reaction with oxalate salt as both the reductant and C1 source [80]. As illustrated in Scheme 7, the CO₂^•− was first produced in the presence of 4DPAIPN, oxalic acid, and TMG under blue light irradiation. The intermediate Ⅰ was then protonated to give vinyl radical Ⅱ, which was reduced via the second SET reduction and followed by protonation to give intermediate Ⅲ. At this moment, the first catalytic cycle was finished. Once more CO₂^•− was formed in the reaction, the Giese radical addition of CO₂^•− to alkene intermediate Ⅲ occurred to produce radical anion intermediate Ⅳ. The third SET reduction process converted the intermediate Ⅳ to the dianion V, which were protonated to yield products 20. The functional groups including Cl, OCF₃, CN, even heteroaryl groups could be well tolerated during such strong reductive conditions, indicating good chemo selectivity of this reaction. Interestingly, by controlling the amounts of oxalic acid, the symmetric aryl dialkyne 19e could be selectively converted to acid 20e with one of the alkynyl group remained.

  Scheme 7

  

  Scheme 7.  Oxalate salt for alkynes reductive hydrocarboxylation.

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  In 2024, Zhu et al. employed TBAO as reductant and also the C1 source for the global deuterocarboxylation of terminal and internal alkynes [82]. In this case, various fully deuterated propionic acids were synthesized with D₂O as the deuteration reagent under blue light irradiation. Mechanistically, the authors proposed radical addition (not SET reduction) of CO₂^•− to the alkyne substrate. As shown in Scheme 8, under such basic condition, the H-D exchange of the terminal alkyne and D₂O occurred first to give the deuterated alkyne as the initial reactant. Oxidation of TBAO with the photocatalyst generated CO₂^•−, which added to the alkyne substrate to give intermediate Ⅰ. The vinyl radical Ⅰ was then reduced by 4DPAIPN to give vinyl anion intermediate Ⅱ. Subsequent deuteration happened to form vinyl acid Ⅲ. At this moment the second catalytic cycle started to reduce the intermediates. After two reduction-deuteration cascades, the multi-deuterated final product was realized through formation of the dianion intermediate Ⅴ. The reaction could tolerate the CN group that is normally vulnerable under strong reductive conditions (22b). The indole ring on the alkyne substrate was also remained after the transformation (22d). Not only terminal alkenes, various terminal alkynes were also investigated and the corresponding triple deuterated carboxylic acids were obtained in moderate to good yields (22e-22h).

  Scheme 8

  

  Scheme 8.  TBAO for deutero-carboxylation of alkynes.

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  3. EDA complex-enabled reductive carboxylation

  Due to the low oxidative potential of the oxalic dianion (E_ox = +0.06 V vs. SCE), SET from the dianion to electron deficient substrate could proceed in the absence of any photocatalyst or additives. In 2024, Zhu and co-workers demonstrated that electron transfer could happen between TBAO and electron-deficient N-benzoyl-imines via formation of the EDA complexes [83]. As shown in Scheme 9, when the imine 23 was treated with TBAO in the presence of guanidine as the base (t-Bu-TMG) in DMF as the solvent, the amino ester 24 could be obtained after 12 h blue light irradiation. This is the first example to show TBAO as the electron donor to form the EDA complexes with electron acceptor and trigger the electron transfer to release CO₂^•− as a strong reductant. This SET reduction relay process is significantly different with traditional photoredox reactions. In this reaction, the EDA complexes Ⅰ were first excited by visible-light irradiation and trigger the intramolecular electron transfer between the donor and the acceptor to give radical anion intermediate Ⅱ. Afterward, the carbon radical intermediate Ⅱ could form the well stabilized adduct Ⅲ with CO₂ released from C₂O₄^•−. Afterward, the CO₂^•− reduced the benzyl radical intermediate Ⅲ to form the carbon anion intermediate Ⅳ, which captured the CO₂ molecule and installed the carboxyl group. However, the transformations proceeded in relatively lower efficiency, giving only moderate yields of the desired products 24 (pathway a). The authors then explored the effect of photocatalyst. When 3DPAFIPN was subjected to the reactions, the reaction efficiency was significantly improved to give the corresponding amino ester products in up to 99% yields in shorter reaction time. Although novel, the reaction substrates were limited to diaryl ketones derived imines, which means the highly conjugated systems were required to form the EDA complexes.

  Scheme 9

  

  Scheme 9.  TBAO for reductive carboxylation of imines.

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  By far, the utilization of oxalate still provided at least one molecule of CO₂ as the waste. To overcome such challenge and make the reaction more atom economic, very recently, Zhu and co-workers disclosed a dicarboxylation reaction of various alkenes, dienes, and trienes with TBAO in DMF under blue light irradiation [84]. No photocatalyst or any other additives was required. The two electrons and the two carboxyl groups of C₂O₄²⁻ were full transferred to the substrates to give this reaction in very high efficiency and 100% atomic economy. As shown in Scheme 10, the 1,1-diaryl alkenes 25 or dienes 26 could form the EDA complexes with TBAO. Once the complex was formed, the C-C bond of C₂O₄²⁻ could not freely rotate anymore and the electron transfer to the electron deficient alkenes from the dianion was facilitated. After shading blue light to the reaction, the alkene radical anion intermediates Ⅰ and C₂O₄^•− were formed. Within the aggregate, decomposition of C₂O₄^•− occurred to give CO₂ and CO₂^•−, which underwent radical-radical coupling with intermediate Ⅰ to give the anion intermediate Ⅱ. In the presence of CO₂, the anion Ⅱ was carboxylated immediately to give the final diacid products 27 after acidic workup. The deuteration experiment only produced 27′, which means the benzyl anion intermediate Ⅱ was formed during the transformation. The radical-radical coupling step was fast and happened on the terminal carbon, which is also the reason why the terminal position was not deuterated in the presence of additional D₂O. As illustrated in the selected examples, various multi-substituted terminal and internal alkenes could be dicarboxylated and substrate including dienes, acrylates, and trienes could also be converted to the corresponding diacids.

  Scheme 10

  

  Scheme 10.  TBAO for dicarboxylation of alkenes and dienes.

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  4. Miscellaneous reactions involving CO₂^•− as reductant

  As an important component for many industrial chemicals, p-phenylenediamine (PPD) was often obtained by the catalytic hydrogenation of 4-nitroaniline, which suffered from many limitations during the past decades. In 2012, Wu's research group employed nanocrystalline PbBi₂Nb₂O₉ as the photocatalyst to selectively reduce 4-nitroaniline to p-phenylenediamine using oxalate under N₂ atmosphere [85]. Based on the electron spin resonance (ESR) analysis results, it was proven that a large amount of CO₂^•− intermediates existed during the reaction process. It was also demonstrated that the C₂O₄²⁻ was a precursor to CO₂^•−. As shown in Scheme 11, when nanocrystalline PbBi₂Nb₂O₉ was irradiated under visible light (420 nm), electron-hole pairs could be easily generated. Afterward, the photoinduced electrons transfer to the surface of the photocatalyst happened and participated in the redox reaction. Photoinduced holes could react with C₂O₄^2- to produce CO₂^•−. Both of CO₂^•− and electrons were capable of reducing 4-nitroaniline in aqueous solution to access p-phenylenediamine as the multi-electron reduction product.

  Scheme 11

  

  Scheme 11.  Oxalate salt for reduction of 4-nitroaniline to access p-phenylenediamine.

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  Highly chlorinated compounds (HCCs), such as dichlorodiphenyltrichloroethane (DDT), are highly oxidized pollutants seriously threatening ecological environment security and human health. Recently, various techniques have been developed for organic pollutants removal, among which persulfate (PS) was utilized as an emerging strong oxidant with easy migration in subsurface environment. Multiple approaches can be employed for PS activation, where heating the activated PS has received extensive attentions owing to its easy operation and cost-effectiveness as a clean method, thus making heat/PS-based in situ chemical oxidation (ISCO) technologies more appropriate for contaminated sites remediation. However, corresponding degradation performance of HCCs by heat/PS system is extremely limited. In 2022, Zhang and co-workers employed oxalic acid to reductively degrade DDT successfully, therefore achieving the heat/PS-based chemical oxidation for soil and groundwater remediation [86]. As shown in Scheme 12, heat/PS based system could generate hydroxyl radical that abstracted hydrogen atom from oxalic acid to give CO₂^•−, initiating the reductive dichlorination process to convert DDT to DDD or DDE. It is worthy to note, the oxygen in the system would quench the CO₂^•−, hence this reaction exhibited higher DDT degradation performance in an anaerobic environment. The following oxidative mineralization process could finally provide CO₂, H₂O, and Cl⁻ as the final degradation products.

  Scheme 12

  

  Scheme 12.  Oxalate acid for degradation of DDT.

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  Very recently, Okumura and Uozumi et al. utilized oxalate salts as the electron donor for multielectron reduction of esters [87]. As shown in Scheme 13, when aryl esters 32 were treated with the photocatalyst N-BAP in the presence of ammonium oxalate in the mixed solvent of MeCN and water under blue light irradiation, the ester substrates were reduced to the corresponding alcohols 33 at room temperature for 12 h. In this transformation, utilization of the N-BAP as the photocatalyst was crucial to realize the reduction of the ester groups. Firstly, the N-BAP was excited by light and form the N-BAP*, which was quenched by the esters (not oxalate salt) to give the corresponding radical anion intermediates Ⅰ. It is worthy to note that, activating of the carbonyl group of the ester substrate by the counter cations in the reaction was essential for the first SET step. Afterward, the oxidized N-BAP could oxidize the C₂O₄²⁻ to afford CO₂ and CO₂^•−. The authors proposed that the acetal radical intermediate Ⅰ could be further reduced by the photocatalyst from the second catalytic cycle to generate the anion intermediate Ⅱ. In the meantime, direct reduction of intermediate Ⅰ to intermediate Ⅱ by the CO₂^•− released from C₂O₄^•− could not be excluded. Intermediate Ⅱ prefer to lose the H₂O molecule to give aldehyde Ⅲ, which could be reduced by the third and fourth SET reduction to afford the final alcohol product 33. Various aryl and heteroaryl esters could be reduced, affording structurally diverse primary alcohols tethering functional groups such as lactone, nitrile, phosphonate, and fluoride.

  Scheme 13

  

  Scheme 13.  Oxalate salt for reduction of esters.

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  5. Conclusion and outlook

  In summary, the oxalate salts could release two electrons stepwise and the CO₂^•− generated from oxalates not only served as the C1 source but also acts as the single-electron reductant, achieving significant development in the fields of organic synthesis and the degradation of harmful substances. However, current research efforts are still facing significant challenging: (1) Asymmetric transformations for chiral compounds synthesis involving oxalates as the CO₂^•− donor have not yet achieved; (2) Most of the substrates for the related transformations are activated reactants, and the substrate expansion for non-activated substrates, such as un-activated alkenes or alkynes, is still rare; (3) Reactions combining oxalates with transition metals could generate CO₂^•− via ligand-to-metal charge-transfer, however the application of such strategy for organic synthesis has not been reported; (4) The potential for enzymatic catalysis for biochemical reactions with t water soluble oxalate salts remains to be explored; (5) The application of oxalates in the reductive degradation of polymers and harmful substances has of great application potentials; (6) Designing of efficient catalysts or photosensitizers for broad unsaturated substrates is also attractive. It is believed that in the near future, oxalate salts will see more great developments in the fields of organic synthetic chemistry, biochemistry, and materials science.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Hui-Xian Jiang: Writing – original draft, Investigation. Zhi-Tao Liu: Writing – original draft, Investigation. Pei Xu: Writing – review & editing, Writing – original draft, Investigation, Conceptualization. Xu Zhu: Writing – review & editing, Supervision, Project administration, Investigation, Funding acquisition, Conceptualization.

  Acknowledgments

  This work is financially supported by the Jiangsu Province Shuangchuang Ph.D. award (No. JSSCBS20211267, Pei Xu), and the Natural Science Research Project of Jiangsu Universities (No. 23KJB150037, Pei Xu). This work is also sponsored by the Jiangsu Specially-Appointed Professor Program (Xu Zhu) and the Start-up Funding provided by Xuzhou Medical University. The Public Experimental Research Center of Xuzhou Medical University is also acknowledged.

  
  
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• 

  Scheme 1  Degradation of oxalic dianion produces two electrons and two CO₂ molecules through CO₂^•− formation to achieve inert C-X bonds activation and reductive carboxylation reactions.

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  Scheme 2  TBAO for C-Br bond activation.

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  Scheme 3  Oxalate salt for C-P bond activation and alkylation.

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  Scheme 4  TBAO for C-O bond activation and carboxylation.

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  Scheme 5  Oxalate salt for alkynes reductive functionalization.

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  Scheme 6  Oxalate salt as linchpin for alkenes cross coupling.

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  Scheme 7  Oxalate salt for alkynes reductive hydrocarboxylation.

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  Scheme 8  TBAO for deutero-carboxylation of alkynes.

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  Scheme 9  TBAO for reductive carboxylation of imines.

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  Scheme 10  TBAO for dicarboxylation of alkenes and dienes.

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  Scheme 11  Oxalate salt for reduction of 4-nitroaniline to access p-phenylenediamine.

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  Scheme 12  Oxalate acid for degradation of DDT.

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  Scheme 13  Oxalate salt for reduction of esters.

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