English

• Electro-organic synthesis is widely recognized as a powerful and eco-friendly tool in green chemistry given its unique ability to produce radicals and radical ions through a direct single-electron-transfer manner [1-3]. Consequently, tremendous progress has been achieved in electrochemical transformations during the past years [4-12]. However, a lot of redox sensitive molecules cannot be stable at high potential, thus restricting the electrochemical transformation of such molecules. To overcome these limitations, various catalysts and/or mediators have been applied to achieve the production of reactive species at lower redox potential [13-23].

  Direct utilization of naturally abundant chemical feedstocks for synthesizing high-value chemicals has become a long-standing goal in green chemistry. In this regard, α-amino acid and its derivatives have been frequently used by synthetic chemists because of its stable chemical properties [24]. Among these α-amino derivatives, N-arylglycines have been frequently used as the stable α-aminomethyl radical precursor for constructing carbon-carbon bonds though oxidative decarboxylation [25-32].

  Quinoxalin-2(1H)-ones represent a valuable class of N-heterocycles as they are present in many synthetic drugs and biologically active compounds [33]. Therefore, various functionalized quinoxalin-2(1H)-ones have been synthesized through C-H functionalization of quinoxalin-2(1H)-ones during the past years [34-45]. Recent studies have revealed that ring-fused quinoxalinones (the combination of heterocycles and quinoxalin-2(1H)-one moieties) show unique biological activities and physicochemical properties, and thereby have higher applied value [46]. However, the construction of ring-fused quinoxalinones [47,48] from readily available quinoxalin-2(1H)-ones (particularly in a sustainable fashion) have been scarcely exploited. Imidazo[1,5-a]quinoxalin-4-ones represent a significant class of ring-fused quinoxalinones due to their remarkable biological and pharmacological activities [49]. Recently, Yu and Chen's group reported the synthesis of imidazo[1,5-a]quinoxalin-4-ones through visible light-induced oxidative coupling and annulation of quinoxalin-2(1H)‑ones and N-phenylglycines with graphitic carbon nitride [50] and perovskite [51] as the photocatalyst (Scheme 1a). Although both these methods are useful, they are problematic in industrial applications due to the requirement of expensive g-C₃N₄ or toxic leaded photo-catalyst. Therefore, developing more general and more sustainable synthetic methods toward such molecules is highly desirable. As part of our continuous efforts toward green synthesis [52-58], herein we reported an elegant strategy for constructing imidazo[1,5-a]quinoxalin-4-ones through TBAI/H₂O cooperative electrocatalytic decarboxylation coupling-annulation of quinoxalin-2(1H)-ones with N-arylglycines in methanol aqueous solution (Scheme 1b).

  Scheme 1

  

  Scheme 1.  Decarboxylation coupling-annulation of quinoxalin-2(1H) ones with N-arylglycines.

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  The electrochemical coupling-annulation reaction of N-methylquinoxalin-2(1H)-one (1a) and N-phenylglycine (2a) was selected as template reaction for optimizing the reaction conditions (Table 1). After initial optimizations, the optimal results were obtained by conducting this electrolysis with 20 mol% TBAI as the catalyst and methanol aqueous solution (3:1) as the solvent in an undivided cell equipped with a platinum plate anode and a graphite plate cathode under 7 mA constant current, the target product 3aa was obtained in 94% yield (Table 1, entry 1). Changing the Pt(+)/C(-) electrode pair with other electrode pairs led to reduced efficiencies (entries 2-9). Subsequently, lower yields of 3aa was observed when TBAI was replaced by other iodide salts (entries 10-12) or tetrabutylammonium salts (entries 13-16). With anhydrous methanol as the sole solvent, only 34% yield of 3aa was formed (entry 17). Performing the reaction in a solvent mixture of MeOH and H₂O at a 20:1 ratio could gave 3aa in 61% yield (entry 18). Varying the water loading (entries 18-20) suggested that the optimal volume ratio of MeOH/H₂O is 3:1 for the present reaction. Next, a series of aqueous organic solvents were examined and the results revealed methanol aqueous solution was the premium reaction medium (entries 21-25). Control reactions proved that no reaction occurred in the absence of TBAI or constant current (entries 26 and 27). Performing the model reaction in the dark conditions had no effect on the yield of 3aa (entry 28).

  Table 1

  

  Table 1.  Optimization of reaction conditions.^a

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  To evaluate the scope of this reaction, a series of quinoxalin-2(1H)-ones and N-arylglycines were screened under the optimal conditions (Scheme 2). Delightedly, both mono- and disubstituted quinoxalin-2(1H)-ones 1 furnished the annulated products (3aa–3ja) in good to excellent yields, suggesting that the reaction was not sensitive to the electronic effects of the substituents on the phenyl. Quinoxalin-2(1H)-ones 1 possessing various functional groups at N-1 position underwent the electrochemical transformation smoothly and delivered the desired products (3ka–3ta) with 81%-86% yields. Remarkably, a broad range of synthetic important functional groups including alkyl, alkoxy, halides, trifluoromethyl, cyano, alkenyl, alkynyl, ester, benzyloxycarbonyl and p-methoxybenzyl groups are well tolerated. N-Methylbenzo[g]quinoxalin-2(1H)-one was also well applicable to provide the target product (3ua) in good yield. Subsequently, a series of N-arylglycines were investigated. Pleasingly, N-arylglycines with diverse electron-donating or electron-withdrawing substituents on the phenyl ring successfully entered this process furnishing the target products (3ab-3ah) in good yields. It is especially noteworthy that the oxidation-sensitive methylthio group also survived under the electrochemical conditions to deliver to the corresponding product 3ae in 76% yield. Furthermore, methyl groups at each position of the benzene ring of N-phenylglycine 2 were well compatible with the standard conditions, delivering the expected products (3ab, 3ai and 3aj) with good yields.

  Scheme 2

  

  Scheme 2.  Reaction scope. Conditions: Pt (15 mm × 10 mm × 0.1 mm) as the anode, C (15 mm × 10 mm × 2 mm) as the cathode, constant current = 7 mA, 1 (0.2 mmol), 2 (0.5 mmol), TBAI (20 mol%), MeOH (6 mL), H₂O (2 mL), room temperature, undivided cell. Isolated yields.

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  To prove the practicality of this electrocatalytic reaction, a scale‐up reaction (5 mmol) was performed under the standard conditions (Scheme 3). Delightedly, the current reaction gave the desired product 3aa in 73% isolated yield (1.02 g), showing a high potential application for industry scale‐up.

  Scheme 3

  

  Scheme 3.  Gram-scale synthesis of 3aa.

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  To better understand the electrochemical annulation process, a series of mechanistic studies were carried out. Considering that this type of annulation reactions may involve the 3-aminomethyled quinoxalin-2(1H)-one (4aa) intermediate, generated from the coupling of the phenylaminomethyl radical and 1a [50,51], we conducted the reaction of 1a with different amounts of 2a, but no 4aa was observed (Scheme 4a). The LC-MS real-time analysis of the reaction mixture also revealed that no 4aa was observed during the reaction. Both the experimental results highlighted the exclusive chemoselectivities and excellent regioselectivities. The annulation process was fully suppressed in the presence of radical scavenger (TEMPO, BHT and 1,1-diphenylethylene), and the TEMPO-CH₂NHPh adduct (5aa), BHT-CH₂NHPh adduct (5ab) and diphenylethylene-CH₂NHPh adduct (5ac) were detected (Scheme 4b). These results indicated that the phenylaminomethyl radical intermediate was involved in the present reaction. Treatment of 1a and 2a in the presence of molecular iodine (1 equiv.) at room temperature in MeOH aqueous solution could give the desired product 3aa in 45% yield (Scheme 4c).

  Scheme 4

  

  Scheme 4.  (a) LC-MS analysis of the reaction mixture; (b) Radical scavenger experiment; (c) I₂-Promoted annulation reaction.

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  The cyclic voltammograms (CV) of related compounds were next investigated. N-Phenylglycine 2a presented a higher oxidative potential (E_onset = 0.82 V, E_p/2 = 1.20 V, Fig. 1, curve a) with anhydrous methanol as the solvent, whereas the lowered potential of 2a (E_onset = 0.42 V, E_p/2 = 0.91 V, Fig. 1, curve b) was observed after the addition of water. These results are in accordance with the yield of 3aa in anhydrous methanol and the mixed solvent of MeOH/H₂O (Table 1, entry 1 vs. 17). We believe that the reduction of oxidative potential was due to the formation of hydrogen bond association between glycine and water. TBAI displayed two obvious oxidation peaks at 0.61 V and 1.80 V (Fig. 2, curve a), which can be assigned to the oxidation of iodide ion into triiodide ion (Iˉ to I₃ˉ) and the oxidation of triiodide ion into molecular iodine (I₃ˉ to I₂) [59]. An un-conspicuous oxidative peak of 1a was appeared at 1.66 V, which was much higher than that of 2a and TBAI, indicating that the oxidation of 2a and TBAI might occur preferentially.

  Figure 1

  

  Figure 1.  Cyclic voltammograms with Pt (25 mm × 10 mm × 0.1 mm) glassy carbon as the working electrode, C (25 mm × 10 mm × 2 mm) as the counter electrode, Ag/AgCl (KCl) as the reference electrode in 0.1 mol/L LiClO₄ with different solvents, scan rate 50 mV/s: (a) 5 mmol/L of 2a in MeOH, (b) 5 mmol/L of 2a in MeOH/H₂O (3:1).

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  Figure 2

  

  Figure 2.  Cyclic voltammograms with Pt (25 mm × 10 mm × 0.1 mm) as the working electrode, C (25 mm × 10 mm × 2 mm) as the counter electrode, and Ag/AgCl (KCl) as the reference electrode in 0.1 mol/L LiClO₄ with MeOH/H₂O (3:1) at 50 mV/s: (a) 5 mmol/L of TBAI, (b) 5 mmol/L of 1a and (c) 5 mmol/L of 2a.

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  Based on the above mechanistic investigations and previous related reports [50,51], a possible mechanism for the cooperative electrocatalytic decarboxylation coupling-annulation reaction was proposed, as shown in Scheme 5. Firstly, an energetically favorable six-membered ring intermediate A was produced due to the formation of intermolecular hydrogen bond between of N-phenylglycine 2a and H₂O, by which the covalent O-H of 2a was effectively activated. The cathodic reduction of intermediate A then gave the N-phenylglycine anion B and H₂. Meanwhile, the anodic-oxidation of an iodide ion generated the molecular iodine, which reacted with intermediated B to form a hypoiodite intermediated C, followed by the homolytic cleavage to produce the oxygen-centred radical D accompanied with regeneration of iodide ion. The aminomethyl radical E, generated from the decarboxylation of radical D, regioselectively attacked the C3 position of 1a to deliver a nitrogen-centred radical F, followed by coupling with another molecular radical E to yield the intermediate G. Eventually, the target product 3aa was formed through the intra-molecular nucleophilic substituent. Considering the moderate oxidative potential of N-phenylglycine, we can not rule out the possibility that the direct anodic oxidation of intermediate B to generate the carboxyl radical D as a minor pathway. A trace amount of iminium species H (generated through the oxidation of radical E) was detected by GC-MS. Because no compound 4aa was detected during the model reaction process, the formation of intermediate G through the addition of 4aa to intermediate H can be ruled out.

  Scheme 5

  

  Scheme 5.  Plausible reaction mechanism.

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  To summarize, we have developed a TBAI/H₂O cooperative electrocatalytic decarboxylation coupling-annulation of quinoxalin-2(1H) ones with N-arylglycines. A broad range of tetrahydroimidazo[1,5-a]quinoxalin-4(5H)-ones (30 examples, 67%-88%) were obtained in good to excellent yields with exclusive chemoselectivities and excellent regioselectivities. The reaction proceeds under chemical oxidant-, additive-, exogenous electrolyte-free and mild conditions with high functional-group tolerance, as demonstrated by the acid-, base- and oxidant-sensitive groups can be well tolerated. Mechanistic studies revealed that the generated H-bond between N-arylglycine and water served as a key factor for yielding α-aminomethyl radical at lower oxidative potential. Both the water (co-solvent and co-catalyst) and TBAI (catalyst and electrolyte) played dual functions in the electrolysis system.

  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.

  Acknowledgment

  We are grateful for financial support from the University of South China.

  
  
• 1. [1]

     N. Chen, H.C. Xu, Chem. Rec. 21 (2021) 2306-2319. doi: 10.1002/tcr.202100048

  2. [2]

     N. Chen, H.C. Xu, Green Synth. Catal. 2 (2021) 165-178. doi: 10.1016/j.gresc.2021.03.002

  3. [3]

     Z. Yang, Y. Yu, L. Lai, et al., Green Synth. Catal. 2 (2021) 19-26. doi: 10.1016/j.gresc.2021.01.009

  4. [4]

     D. Chen, X. Nie, Q. Feng, et al., Angew. Chem. Int. Ed. 60 (2021) 27271-27276. doi: 10.1002/anie.202112118

  5. [5]

     H. Shen, D. Cheng, Y. Li, et al., Green Synth. Catal. 1 (2020) 175-179. doi: 10.1016/j.gresc.2020.10.002

  6. [6]

     J. Jiang, Z. Wang, W.M. He, Chin. Chem. Lett. 32 (2021) 1591-1592. doi: 10.1016/j.cclet.2021.02.067

  7. [7]

     H. Wu, X. Yu, Z. Cao, Chin. J. Org. Chem. 41 (2021) 4712-4717. doi: 10.6023/cjoc202111010

  8. [8]

     S. Guo, L. Liu, K. Hu, et al., Chin. Chem. Lett. 32 (2021) 1033-1036. doi: 10.1016/j.cclet.2020.09.041

  9. [9]

     S. Cheng, C. Ou, H. Lin, et al., Chin. J. Org. Chem. 41 (2021) 4718-4724. doi: 10.6023/cjoc202110019

  10. [10]

      Z.L. Wu, J.Y. Chen, X.Z. Tian, et al., Chin. Chem. Lett. 33 (2022) 1501-1504. doi: 10.1016/j.cclet.2021.08.071

  11. [11]

      Z. Shi, Y. Li, N. Li, et al., Angew. Chem. Int. Ed. 61 (2022) e202206058.

  12. [12]

      J.Y. Chen, H.X. Li, S.Y. Mu, et al., Org. Biomol. Chem. 20 (2022) 8501-8505. doi: 10.1039/d2ob01612e

  13. [13]

      K. Liu, C. Song, A. Lei, Org. Biomol. Chem. 16 (2018) 2375-2387. doi: 10.1039/c8ob00063h

  14. [14]

      H.T. Tang, J.S. Jia, Y.M. Pan, Org. Biomol. Chem. 18 (2020) 5315-5333. doi: 10.1039/d0ob01008a

  15. [15]

      M. Saito, Y. Kawamata, M. Meanwell, et al., J. Am. Chem. Soc. 143 (2021) 7859-7867. doi: 10.1021/jacs.1c03780

  16. [16]

      Z.H. Wang, P.S. Gao, X. Wang, et al., J. Am. Chem. Soc. 143 (2021) 15599-15605. doi: 10.1021/jacs.1c08671

  17. [17]

      Z. Tan, X. He, K. Xu, C. Zeng, ChemSusChem 15 (2022) e202102360.

  18. [18]

      X. Meng, H. Xu, R. Liu, Y. Zheng, S. Huang, Green Chem. 24 (2022) 4754-4760. doi: 10.1039/d2gc01129h

  19. [19]

      T. Feng, S. Wang, Y. Liu, S. Liu, Y. Qiu, Angew. Chem. Int. Ed. 61 (2022) e202115178. doi: 10.1002/anie.202115178

  20. [20]

      M. Chen, Z.J. Wu, J. Song, H.C. Xu, Angew. Chem. Int. Ed. 61 (2022) e202115954.

  21. [21]

      P.S. Gao, X.J. Weng, Z.H. Wang, et al., Angew. Chem. Int. Ed. 59 (2020) 15254-15259. doi: 10.1002/anie.202005099

  22. [22]

      W. Li, Y. Tang, W. Ouyang, et al., Chin. J. Org. Chem. 41 (2021) 4766-4772. doi: 10.6023/cjoc202109044

  23. [23]

      H. Xu, X. Meng, Y. Zheng, J. Luo, S. Huang, Chin. J. Org. Chem. 41 (2021) 4696-4703. doi: 10.6023/cjoc202112016

  24. [24]

      M. Nourisefat, F. Panahi, A. Khalafi-Nezhad, Mol. Divers. 23 (2019) 317-331. doi: 10.1007/s11030-018-9861-0

  25. [25]

      H. Yang, G. Wei, Z. Jiang, ACS Catal. 9 (2019) 9599-9605. doi: 10.1021/acscatal.9b03567

  26. [26]

      W. Hu, Q. Zhan, H. Zhou, S. Cao, Z. Jiang, Chem. Sci. 12 (2021) 6543-6550. doi: 10.1039/d1sc01470f

  27. [27]

      Y. Song, H. Zhang, J. Guo, et al., Eur. J. Org. Chem. 2021 (2021) 5914-5921. doi: 10.1002/ejoc.202101242

  28. [28]

      H. Xin, Z.H. Yuan, M. Yang, et al., Green Chem. 23 (2021) 9549-9553. doi: 10.1039/d1gc03230e

  29. [29]

      X. Huang, Y.Q. Hu, C. Zhou, Y. Zheng, X. Zhang, Green Chem. 24 (2022) 5764-5769. doi: 10.1039/d2gc01410f

  30. [30]

      W. Zeng, W. Li, H. Chen, L. Zhou, Org. Lett. 24 (2022) 3265-3269. doi: 10.1021/acs.orglett.2c01117

  31. [31]

      X. Chai, X. Hu, X. Zhao, et al., Angew. Chem. Int. Ed. 61 (2022) e202115110.

  32. [32]

      S. Li, X. Li, T. Wang, et al., Adv. Synth. Catal. 364 (2022) 2346-2351. doi: 10.1002/adsc.202200339

  33. [33]

      M. Montana, V. Montero, O. Khoumeri, P. Vanelle, Molecules 25 (2020) 2784. doi: 10.3390/molecules25122784

  34. [34]

      J.W. Yuan, J.L. Zhu, H.L. Zhu, et al., Org. Chem. Front. 7 (2020) 273-285 doi: 10.1039/c9qo01322a

  35. [35]

      K. Niu, L. Ding, P. Zhou, et al., Green Chem. 23 (2021) 3246-3249. doi: 10.1039/d1gc00861g

  36. [36]

      D. Li, X. Wang, S. Li, et al., Chin. J. Org. Chem. 41 (2021) 4610-4622. doi: 10.6023/cjoc202107042

  37. [37]

      M. Cao, Y.L. Fang, Y.C. Wang, et al., ACS Comb. Sci. 22 (2020) 268-273. doi: 10.1021/acscombsci.0c00012

  38. [38]

      L.Y. Xie, Y.S. Liu, H.R. Ding, et al., Chin. J. Catal. 41 (2020) 1168-1173. doi: 10.1016/S1872-2067(19)63526-6

  39. [39]

      J. Xu, L. Huang, L. He, et al., Green Chem. 23 (2021) 6632-6638. doi: 10.1039/d1gc01899j

  40. [40]

      N. Meng, Y. Lv, Q. Liu, et al., Chin. Chem. Lett. 32 (2021) 258-262. doi: 10.1016/j.cclet.2020.11.034

  41. [41]

      L. Yao, D. Zhu, L. Wang, et al., Chin. Chem. Lett. 32 (2021) 4033-4037. doi: 10.1016/j.cclet.2021.06.005

  42. [42]

      Z. Wang, Q. Liu, R. Liu, et al., Chin. Chem. Lett. 33 (2022) 1479-1482. doi: 10.1016/j.cclet.2021.08.036

  43. [43]

      L. Ding, K. Niu, Y. Liu, Q. Wang, Chin. Chem. Lett. 33 (2022) 4057-4060. doi: 10.1016/j.cclet.2021.12.053

  44. [44]

      K. Sun, F. Xiao, B. Yu, W.M. He, Chin. J. Catal. 42 (2021) 1921-1943. doi: 10.1016/S1872-2067(21)63850-0

  45. [45]

      Q.Q. Han, D.M. Chen, Z.L. Wang, et al., Chin. Chem. Lett. 32 (2021) 2559-2561. doi: 10.1016/j.cclet.2021.02.018

  46. [46]

      X. Jiang, K. Wu, R. Bai, P. Zhang, Y. Zhang, Eur. J. Med. Chem. 229 (2022) 114085. doi: 10.1016/j.ejmech.2021.114085

  47. [47]

      Q. Yang, Y. Zhang, Q. Sun, et al., Adv. Synth. Catal. 360 (2018) 4509-4514. doi: 10.1002/adsc.201801076

  48. [48]

      J. Wen, W. Zhao, X. Gao, et al., J. Org. Chem. 87 (2022) 4415-4423. doi: 10.1021/acs.joc.2c00135

  49. [49]

      A. Talukdar, D. Ganguly, S. Roy, N. Das, D. Sarkar, J. Med. Chem. 64 (2021) 8010-8041. doi: 10.1021/acs.jmedchem.1c00300

  50. [50]

      Y.F. Si, X.L. Chen, X.Y. Fu, et al., ACS Sustain. Chem. Eng. 8 (2020) 10740-10746.

  51. [51]

      Y.F. Si, K. Sun, X.L. Chen, et al., Org. Lett. 22 (2020) 6960-6965. doi: 10.1021/acs.orglett.0c02518

  52. [52]

      J. Jiang, F. Xiao, W.M. He, L. Wang, Chin. Chem. Lett. 32 (2021) 1637-1644. doi: 10.1016/j.cclet.2021.02.057

  53. [53]

      J.Y. Chen, H.Y. Wu, Q.W. Gui, et al., Chin. J. Catal. 42 (2021) 1445-1450. doi: 10.1016/S1872-2067(20)63750-0

  54. [54]

      J.Y. Chen, C.T. Zhong, Q.W. Gui, et al., Chin. Chem. Lett. 32 (2021) 475-479. doi: 10.1016/j.cclet.2020.09.034

  55. [55]

      Y. Wu, J.Y. Chen, H.R. Liao, et al., Green Synth. Catal. 2 (2021) 233-236. doi: 10.1016/j.gresc.2021.03.006

  56. [56]

      Y. Wu, J.Y. Chen, J. Ning, et al., Green Chem. 23 (2021) 3950-3954. doi: 10.1039/d1gc00562f

  57. [57]

      W.B. He, S.J. Zhao, J.Y. Chen, et al., Chin. Chem. Lett. 34 (2023) 107640. doi: 10.1016/j.cclet.2022.06.063

  58. [58]

      H. Li, P. Chen, Z. Wu, et al., Chin. J. Org. Chem. 42 (2022) 3398-3404. doi: 10.6023/cjoc202207009

  59. [59]

      V.A. Macagno, M.C. Giordano, A.J. Arvía, Electrochim. Acta 14 (1969) 335-357. doi: 10.1016/0013-4686(69)85005-X

• 

  Scheme 1  Decarboxylation coupling-annulation of quinoxalin-2(1H) ones with N-arylglycines.

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  Scheme 2  Reaction scope. Conditions: Pt (15 mm × 10 mm × 0.1 mm) as the anode, C (15 mm × 10 mm × 2 mm) as the cathode, constant current = 7 mA, 1 (0.2 mmol), 2 (0.5 mmol), TBAI (20 mol%), MeOH (6 mL), H₂O (2 mL), room temperature, undivided cell. Isolated yields.

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  Scheme 3  Gram-scale synthesis of 3aa.

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  Scheme 4  (a) LC-MS analysis of the reaction mixture; (b) Radical scavenger experiment; (c) I₂-Promoted annulation reaction.

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  Figure 1  Cyclic voltammograms with Pt (25 mm × 10 mm × 0.1 mm) glassy carbon as the working electrode, C (25 mm × 10 mm × 2 mm) as the counter electrode, Ag/AgCl (KCl) as the reference electrode in 0.1 mol/L LiClO₄ with different solvents, scan rate 50 mV/s: (a) 5 mmol/L of 2a in MeOH, (b) 5 mmol/L of 2a in MeOH/H₂O (3:1).

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  Figure 2  Cyclic voltammograms with Pt (25 mm × 10 mm × 0.1 mm) as the working electrode, C (25 mm × 10 mm × 2 mm) as the counter electrode, and Ag/AgCl (KCl) as the reference electrode in 0.1 mol/L LiClO₄ with MeOH/H₂O (3:1) at 50 mV/s: (a) 5 mmol/L of TBAI, (b) 5 mmol/L of 1a and (c) 5 mmol/L of 2a.

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  Scheme 5  Plausible reaction mechanism.

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  Table 1.  Optimization of reaction conditions.^a

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•