English

• Nitroarenes are significant feedstocks and versatile building blocks in industry and organic synthesis, respectively [1, 2]. However, selective conversion of nitroarenes into a specific type of nitrogen-containing molecules has long been a challenge as the reduction usually provided a wide distribution of different products and intermediates, such as nitroso-, azo, azoxyl, hydrozo-, and amino-derivatives [3-5]. While nitroarenes were widely employed to produce anilines by stoichiometric metals or catalytic hydrogenation [6-9], selective synthesis of arylhydroxylamines from nitroarenes was rarely reported [10]. Conventional strategies require precious metals and reductant (H₂ or NaBH₄) to access the N—O single bond [11-14]. Photochemical reduction of nitroarene to arylhydroxylamine relied on ultraviolet-light irradiation in the presence of toxic methyl hydrazine as the electron donor (Scheme 1A) [15, 16]. In 2022, Werlé group reported an adaptive rodium catalyst for nitroarene reduction, in which the hydrogenation process in the presence of 2 bar hydrogen gas could be controlled by switching the solvent and temperature (Scheme 1B) [17]. However, development of a mild and general protocol for selective reduction of nitroarenes with environmental benign reductant is still highly challenging.

  Scheme 1

  

  Scheme 1.  Reduction of nitro compounds to amines or hydroxylamines.

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  During the past decades, visible-light-induced radical transformation emerged as a powerful tool for redox reaction under mild conditions and a variety type of photocatalysts were developed to achieve different redox potentials [18-25]. In addition, a deeper reduction potential out of the range of conventional photocatalysis could be generated by CO₂ radical anion (CO₂^•−, E_1/2 = −2.21 V vs. SCE) [26-29]. Therefore, the inert energy-demanding substrates could be reduced in the presence of CO₂^•− [30-35]. With CO₂^•− as the reductant, hydrogenation of unsaturated π-bonds, including C=O [36, 37] and C=N [38, 39] double bonds as well as C—C double [40, 41] and triple bonds [42] were realized. However, reduction of N=O double bond, especially for nitroarene reduction that involves multi-electrons transfer process, has not been realized, mainly due to the difficulty to precisely control the reduction potential through the whole reductive transformation. This is also the reason why most of the nitroarene reduction reactions only provided anilines as the final products. Development of a new efficient and tunable catalytic system is required to realize the selective reduction of the nitroarenes.

  Based on our previous studies on formate as the CO₂^•− precursor under various catalytic conditions [43-49], we envisioned that reduction of nitroarenes could be precisely tuned and accurately controlled with the combination of CO₂ radical anion and different photocatalysts (such as metal complexes and dyes) under a dual-catalysis condition. Herein, we disclose a general visible-light-induced approach for selective reduction of nitroarenes to access arylhydroxylamines and anilines, respectively, by simply switching the photocatalyst with formate as the electron donor. Moreover, in the presence of formaldehyde, methylaniline and imidazolidine derivatives could also be realized selectively. Nitroalkanes could produce the corresponding oximes under this reductive reaction system.

  For CO₂ radical anion generation, we used thiol radical to abstract the hydrogen atom from formate salt and started our initial investigation for nitroarene reduction (see Supporting information for detailed screening conditions). As shown in Table 1, when nitroarene 1 was treated with thiol T1 as the HAT (hydrogen atom transfer) catalyst, Rose Bengal sodium ((3-oxo-6-oxido-2, 4, 5, 7-tetraiodo-3H-xanthen-9-yl)−3, 4, 5, 6-tetrachlorobenzoate disodium salt) was proven as the optimal photocatalyst in the presence of 2.5 equiv. of HCO₂Cs in DMSO under 450 nm blue LED irradiation. Arylhydroxylamine 2 was obtained in 64% yield, along with 7% of the aniline byproduct 3 (Table 1, entry 1). The counter cation of the formate, such as Li⁺, Na⁺, and K⁺ were then explored, showing Cs⁺ was the best one (Table 1, entries 2–4). Thiol catalysts substituted with different substituents were also screened. Results showed methyl thiosalicylate (T1) is optimal (Table 1, entries 5–8). Reaction could not occur in the absence of either light, or T1 (Table 1, entries 9 and 10). It is not surprising that the reaction also could not proceed without HCO₂Cs (Table 1, entry 11). Replacement of HCO₂Cs by diisopropylethylamine (DIPEA) as the electron donor failed to convert the nitroarene starting material, indicating the formate is crucial to realize this reductive transformation (Table 1, entry 12). In the absence of photocatalyst, there was 35% of the hydroxylamine 2 was observed, probably due to formation of an EDA (electron-donor-accepter) complex between the nitroarene and the aryl sulfur anion to initiated the electron transfer process.

  Table 1

  

  Table 1.  Variation of the standard reaction conditions.^a

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  Entry	Variations	Yield (%)b
  		2	3
  1	None	79(64)c	7
  2	HCO2Li	0	0
  3	HCO2Na	13	< 5
  4	HCO2K	66	< 5
  5	T2	37	8
  6	T3	13	< 5
  7	T4	25	< 5
  8	T5	10	5
  9	In dark	0	0
  10	w/o T1	0	0
  11	w/o HCO2Cs	0	0
  12	DIPEA instead of HCO2Cs	0	0
  13	w/o PC	35	trace
  a Reaction conditions: 1a (0.2 mmol), Rose Bengal sodium (3.0 mol%), T1 (10 mol%), HCO2Cs (2.5 equiv.), DMSO (0.1 mol/L), r.t., 24 h under N2 atmosphere. b Yields were determined by 1H NMR analysis with 1, 2-dicloroethane as an internal standard. c Isolated yield.

  With the optimized reaction conditions in hand, the substrate scope for synthesis of arylhydroxylamines was conducted as shown in Scheme 2. The nitroarenes tethering electron withdrawing groups, such as esters, worked smoothly to give the corresponding hydroxylamines 4–6 in good yields. Interestingly, the alkyne moiety in the substrate still remained after the reaction to give a reactive handle for late-stage modification of product 6. The amide functional group were also amendable to give products 7 and 8. Afterward, the electron-donating groups, such as Me and ph, on nitroarenes were tested to give products 9–11 in moderate yields. To further examine the functional group tolerance, the vulnerable Cl, CF₃, and CN groups, which are unstable under reduction conditions, were also investigated under the standard reaction conditions. The desired arylhydroxylamine products 12 and 13 were obtained in 56% and 52% isolated yields, respectively. Even elevated temperature was utilized for synthesis of 12, the dehalogenation was not observed under such strong reductive conditions. The heteroarenes, including thiophene and pyridine, were also tolerated to give product 14 and pyridinyl hydroxylamine 15. The nitroarenes derived from l-menthol, (+)-cedar alcohol, as well as l-(-)-borneol were also converted to the arylhydroxylamine products 16–18 in good yields.

  Scheme 2

  

  Scheme 2.  Synthesis of arylhydroxylamines. Reaction conditions: nitroarenes (0.2 mmol), Rose Bengal sodium (3.0 mol%), T1 (10 mol%), HCO₂Cs (2.5 equiv.), DMSO (0.1 mol/L), r.t., 24 h under N₂ atmosphere. ^a Reaction was conducted at 60 ℃. ^b Reaction was conducted at 50 ℃.

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  To further elaborate the utility of our strategy, selective synthesis of anilines from nitroarenes was the next goal. By simply switching the photocatalyst to fac-Ir(ppy)₃, the nitroarenes could be transformed into anilines in the presence of 5.0 equiv. of HCO₂Cs (see Supporting information for detailed screening conditions). As illustrated in Scheme 3, the para-ester group on the nitroarene was amendable to give aniline 3 as a sole product in 91% yield. The pharmaceutical drugs benzocaine (19) and butamben (20) could be synthesized from the corresponding nitroarenes in over 90% isolated yields. The ester-functionalized nitroarene tethering alkyne moiety was also amendable to give aniline 21 in 75% yield, without touching the C—C triple bond. The amide substituents were then examined to give desired product 22–24, respectively, in over 85% yield. The para-Me, para-isopropyl, ortho-MeO, and multi-substituted nitroarenes were converted into the corresponding anilines in good yields, although elevated temperature was required for certain substrate 25–29. The thiol ether substrate was also tolerated to give product 30 in 68% yield. Substrate with highly conjugated system, such as para-aromatic, was also good candidate to give product 31 in 77% yield at slightly elevated temperature. The steric hindered substrate 2-methyoxyl-6-cyano nitrobenzene gave the desired product 32 in only 23% isolated yield. Afterward, various nitro-group substituted heteroarenes, such as pyridine, quinoline, and thiophene, were examined to give the corresponding amine products 33–36 in moderate to good yields. The amino ester derived substrate was also amendable to give aniline 37 in 46% yield. Substrates derived from natural alcohols, such as L-menthol, (+)-cedar alcohol, and L-(-)-borneol, were smoothly converted to the corresponding aniline products 38–40 in up to 98% yield. The local anesthetic drug procaine (41) was also obtained from the corresponding nitroarenes in 72% yield.

  Scheme 3

  

  Scheme 3.  Synthesis of arylamines. Reaction conditions: Nitroarenes (0.2 mmol), fac-Ir(ppy)₃ (1.0 mol%), T1 (10 mol%), HCO₂Cs (5.0 equiv.), DMSO (0.1 mol/L), r.t., 24 h under N₂ atmosphere. ^a Reaction was conducted at 50 ℃.

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  Moreover, the nitroalkanes [50-52] were also examined with the developed protocol with 4CzIPN (2, 4, 5, 6-tetra-9H-carbazol-9-yl-1, 3-benzenedicarbonitrile) as the optimal photocatalyst (see Supporting information for details on screening of the reaction conditions). Various nitroalkanes were converted to the corresponding oximes 42–48 selectively in good to excellent yields (Scheme 4). The electron density on the arene did not affect the nitro group reduction. The secondary nitro groups could also be reduced to give the corresponding oximes 49 and 50, respectively, in good yields. These results further illustrated the synthetic application of the reduction system created by the collaborative usage of CO₂ radical anion and the dual-catalysis process.

  Scheme 4

  

  Scheme 4.  Synthesis of oximes from alkyl nitro compounds. Reaction conditions: Nitroalkanes (0.2 mmol), 4CzIPN (2.0 mol%), T1 (10 mol%), HCO₂Cs (2.5 equiv.), DMSO (0.1 mol/L), r.t., 6–18 h under N₂ atmosphere.

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  To further elaborate the synthetic application of our methods, several transformations were realized as shown in Scheme 5A. Inspired by Zhang's work [53, 54], we surmised that the hydroxylamine intermediate during the nitroarene reduction process could be trapped by formaldehyde via rapid condensation to give nitrones, which should be able to access value-added aniline derivatives in one-pot manner. To our delight, the nitroarenes tethering electron-withdrawing substituents could be converted to the corresponding N-methyl anilines 51–56 in good yields with 3DPAFIPN (2, 4, 6-tris(diphenylamino)−5-fluoroisophth alonitrile) as the photocatalyst in the presence of formaldehyde (18 wt% in H₂O).

  Scheme 5

  

  Scheme 5.  Synthetic applications.

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  These results were in stark contrast with Zhang's work, in which the imidazolidines were obtained as the final products. Interestingly, when the reaction temperature was elevated to 50 ℃ in the presence of excess amounts of formaldehyde (18 wt% in H₂O), the imidazolidine 57 could be synthesized in 58% isolated yield in one-pot manner (Scheme 5B, middle). By treatment of 57 with NBS (N-bromosuccinimide), the oxidized imidazolinium salt 58 was formed in 45% yield, which demonstrates the potentials of our protocol to be further applied in the field of organo- and metal-complex catalysis (Scheme 5B, top). In addition, cyclic thiourea 59 was synthesized from imidazolidine 57 when it was treated with sublimed sulfur (S₈) under solvent-free conditions at 150 ℃ (Scheme 5B, bottom).

  When 1, 4-diketone was added into the reaction under condition B, the heterocycle 60 could be constructed in 71% yield in one step. After following treatment as shown in the reference [55], the biological anti-mycobacterial molecule 61 could be obtained, indicating the application potential of our protocol for biological molecule synthesis in a more step-economic manner (Scheme 5C). In addition, arylhydroxylamine is the key backbone structure of many pharmaceutical or agricultural drugs and a versatile synthon for synthesis of nitroxide or multi-functionalized aromatic molecules. As illustrated in Scheme 5D (top), product 2 could be acylated easily to give 62, which could be further transformed into tri-substituted aromatics 63 and 64 that easy to be further modified to access value-added complex molecules. In addition, the product 48 was scaled up to 5 mmol and it could be converted to the corresponding nitrile 65 and amine 64, respectively, following the reported procedures (Scheme 5D, bottom). In fact, all the conditions disclosed in this work could be scaled up to 2–5 mmol without decrease of the isolated yields (Scheme 5E). All the above transformations indicated the application potentials of our strategy for highly selective nitroarenes reduction to access various nitrogen-containing molecules.

  To gain further insighs of the reaction mechanism, a series control experiments were conducted as shown in Scheme 6. First, when the reaction for N-methylaniline synthesis was stopped after just 2 h in the presence of excess amounts of paraformaldehyde, a mixture of N-methyl aniline 51 and the unexpected aninoalcohol product 67 was isolated in 17% and 18% yields, respectively. We proposed that formation of aminoalcohol 67 probably through generation of the intermediate 66, which could undergo nucleophilic attack to formaldehyde. Under the standard conditions in Scheme 5A, where only 3 equiv. of formaldehyde was used, the intermediate 68 was subsequently reduced to give N-methylanilines as the sole products. Therefore, when hydroxylamine 2 was treated with standard condition D, the desired product 51 was formed in 41% yield, indicating intermediates 68 and 69 were formed during the trasformation. When the syntesis of imidazolidine 57 was interupted by quenching the reaction after 2 h, only trace amounts of 57 was observed. Interestingly, the HRMS analysis showed detection of intermediates 70 and 71. Prolonging the reaction time to 16 h produced diamine 72 in 37% yield, alongwith desired product 57 in 34% yield. These results matched with our proposed reaction pathways.

  Scheme 6

  

  Scheme 6.  Control experiments for mechanistic studies.

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  Afterward, the Sterm-Volmer quenching experiments with different photocatalysts were conducted as shown in Fig. 1. The thiol catalyst effectively quenched the excited state of photocatalyst Rose Bengal sodium. However, when HCO₂Cs was added, the quenching did not happen, indicating the direct interaction of the photcatalyst with the ArSH not the ArS^- (the anion form) was the initiation step. Therefore, the HAT process, other than the photoredox process, should be plausible. When fac-Ir(ppy)₃ was tested, the excited state of the Ir^Ⅲ catalyst could be quenched directly by the substrate of nitrobenzene, indicating the Ir^Ⅲ/Ir^Ⅳ cycle is might be possible in this reaction. Interestingly, the sulfur anion (ArS^-) was also able to quench the photocatalyst fac-Ir(ppy)₃. These results showed evidences that multi-initiation pathways were possibly involved in this condition, more details on Stern-Volmer quenching experiments see Supporting information.

  Figure 1

  

  Figure 1.  The Sterm-Volmer quenching experiments.

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  Based on these results and the reported literatures [7, 53, 54, 56-58], we proposed that different mechanisms for nitroarene reduction were involved in the presence of various photocatalysts. When Rose Bengal sodium was employed as the photocatalyst for synthesis of aryl hydrox-ylamines, a HAT catalytic cycles was proposed as illustrated in Scheme 7A because the excited state of Rose Bengal sodium could be quenched by the ArSH. We proposed that the photocatalyst was first excited by blue light irradiation to form the diradical A. Afterward, subsequent HAT process occurred from the thiol to diradical A to give radical B, which underwent a second HAT process with various radical intermediates generated in the reaction system to its original state. Once the sulfur radical ArS^• was generated, subsequent HAT process between it and the formate could generate CO₂ radical anion and regenerate the ArSH catalyst. The CO₂ radical anion is a strong reductant (E_1/2 = −2.21 V vs. SCE) that could easily release one more electron and became CO₂ gas (escaped from the reaction system as the traceless waste). Successive SET-protonation-dehydration reduced nitroarenes to nitrosoarenes and then hydroxylamines. At this moment, the redox process could not be exclusively ruled out. Under such conditions, the reduction potential is not strong enough (Rose Bengal sodium: E_ox = −0.96 V vs. SCE) to further reduce the aryl hydroxylamines (E_red = −1.29 V vs. SCE), therefore the aniline product was not observed.

  Scheme 7

  

  Scheme 7.  Proposed reaction mechanism.

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  As shown in Scheme 7B, when fac-Ir(ppy)₃ was utilized as the photocatalyst, two initiation pathways were possible involved based on the Sterm-Volmer quehching experiments as shown in Fig. 1. First, the photocatalyst was excited by the blue light irradiation. Afterward, direct oxidative quenching of the Ir^Ⅲ* by the nitroarene substrate occurred to generate Ir^Ⅳ species and the radical anion form of the nitroarene. The sulfur anion (ArS^-) generated under basic conditions would be oxidized by Ir^Ⅳ to form the sulfur radical (ArS^•), which abstracted the hydrogen from formate and formed the CO₂ radical anion species. This is the major pathway for such transformation. In the meantime, the reductive quenching of the excited photocatalyst Ir^Ⅲ* by ArS^- could not be rule out (Fig. 1). Under such conditions, an electron pool could be generated continually with this dual-catalytic system (Ir^Ⅱ: E_ox = −1.88 V vs. SCE) and fulfill the requirement of nitroarene reduction that needed multi-electrons transfer process to access the anilines as the final products. In the presence of formaldehyde, value-added nitrogen-containing molecules including N-methylanilines and imidazolidine derivatives could also be obtained.

  In summary, a visible-light-induced dual catalysis strategy with thiol and different photocatalyst was established for highly selective reduction of nitroarenes. The detailed mechanisms for each transformation were also deeply investigated and fully discussed. This protocol could also be applied for reduction of nitroalkanes to exclusively produce the corresponding oximes. Various useful synthetic building blocks and biological active molecules were also constructed with this strategy. The CO₂ gas and water were the only traceless wastes during the transformation, which made this versatile protocol efficient, clean, and sustainable.

  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

  Pei Xu: Methodology, Conceptualization. Tian-Zi Hao: Investigation, Data curation. Zhi-Tao Liu: Methodology, Investigation, Formal analysis. Yi-Qin Liu: Methodology, Formal analysis. Hui-Xian Jiang: Methodology, Data curation. Dong Guo: Writing – review & editing, Writing – original draft, Supervision. Xu Zhu: Writing – review & editing, Writing – original draft, Supervision, Project administration, 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.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.110899.

  
  
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  Scheme 1  Reduction of nitro compounds to amines or hydroxylamines.

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  Scheme 2  Synthesis of arylhydroxylamines. Reaction conditions: nitroarenes (0.2 mmol), Rose Bengal sodium (3.0 mol%), T1 (10 mol%), HCO₂Cs (2.5 equiv.), DMSO (0.1 mol/L), r.t., 24 h under N₂ atmosphere. ^a Reaction was conducted at 60 ℃. ^b Reaction was conducted at 50 ℃.

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  Scheme 3  Synthesis of arylamines. Reaction conditions: Nitroarenes (0.2 mmol), fac-Ir(ppy)₃ (1.0 mol%), T1 (10 mol%), HCO₂Cs (5.0 equiv.), DMSO (0.1 mol/L), r.t., 24 h under N₂ atmosphere. ^a Reaction was conducted at 50 ℃.

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  Scheme 4  Synthesis of oximes from alkyl nitro compounds. Reaction conditions: Nitroalkanes (0.2 mmol), 4CzIPN (2.0 mol%), T1 (10 mol%), HCO₂Cs (2.5 equiv.), DMSO (0.1 mol/L), r.t., 6–18 h under N₂ atmosphere.

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  Scheme 5  Synthetic applications.

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  Scheme 6  Control experiments for mechanistic studies.

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  Figure 1  The Sterm-Volmer quenching experiments.

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  Scheme 7  Proposed reaction mechanism.

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  Table 1.  Variation of the standard reaction conditions.^a

  Entry	Variations	Yield (%)b
  		2	3
  1	None	79(64)c	7
  2	HCO2Li	0	0
  3	HCO2Na	13	< 5
  4	HCO2K	66	< 5
  5	T2	37	8
  6	T3	13	< 5
  7	T4	25	< 5
  8	T5	10	5
  9	In dark	0	0
  10	w/o T1	0	0
  11	w/o HCO2Cs	0	0
  12	DIPEA instead of HCO2Cs	0	0
  13	w/o PC	35	trace
  a Reaction conditions: 1a (0.2 mmol), Rose Bengal sodium (3.0 mol%), T1 (10 mol%), HCO2Cs (2.5 equiv.), DMSO (0.1 mol/L), r.t., 24 h under N2 atmosphere. b Yields were determined by 1H NMR analysis with 1, 2-dicloroethane as an internal standard. c Isolated yield.

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