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Using silver cathodes for organic electrosynthesis and mechanistic studies
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Review Article Using silver cathodes for organic electrosynthesis and mechanistic studies Mohammad S. Mubarak 1 and Dennis G. Peters 2,∗ In the last decade or so, silver electrodes (almost invariably as cathodes) have captured increasing attention for their ability to facilitate (catalyze) the electrochemical reductive cleavage of carbon–halogen bonds in an enormous variety of organic compounds and under remarkably mild conditions. This short review is intended to call attention to and, hopefully, to stimulate further research and applications related to the use of silver electrodes in areas such as electrosynthesis, mechanistic organic electrochemistry, electrocarboxylation reactions (that could utilize excess carbon dioxide in the environment), nanochemistry, surface chemistry, and the remediation of stockpiles of halogenated environmental pollutants such as pesticides, flame retardants, and chlorofluorocarbons. Addresses 1 Department of Chemistry, The University of Jordan, Amman 11942, Jordan 2 Department of Chemistry, Indiana University, Bloomington, IN 47405, USA Corresponding author: Peters, Dennis G. ([email protected])
Current Opinion in Electrochemistry 2017, 2:60–66 This review comes from a themed issue on Organic and Molecular Electrochemistry 2017 Edited by Jean Lessard For a complete overview see the Issue and the Editorial Available online 22th March 2017 http://dx.doi.org/10.1016/j.coelec.2017.03.001 2451-9103/© 2017 Elsevier B.V. All rights reserved.
pounds [2–4] that pertain to (a) electrosynthesis of various organic compounds and (b) reductive dehalogenation (remediation) of some environmental pollutants [5]. Included in our discussion are comparisons between the use of silver cathodes and alternative strategies that involve different electrode materials or transition-metal catalysts electrogenerated in situ. Another topic to be treated is the very rapidly expanding use of silver nanoparticles as a platform for both electrosynthesis and environmental remediation. We hope these examples will stimulate further developments in the arenas of both electrosynthesis and environmental remediation. Cyclic voltammetry can reveal and dramatize the propensity of silver to electrocatalyze the reductive cleavage of carbon–halogen bonds. An excellent example, depicted in Figure 1, provides a comparison of the cyclic voltammetric behavior of 1,1,2-trichloro-1,2,2-trifluoroethane at silver and glassy carbon electrodes in dimethylformamide containing tetramethylammonium tetrafluoroborate (TMABF4 ) as supporting electrolyte. Each cyclic voltammogram exhibits a pair of well defined, irreversible cathodic peaks that correspond, respectively, to (a) twoelectron reduction of the parent compound to afford 1-chloro-1,2,2-trifluoroethene and (b) two-electron reduction of the latter product to yield 1,1,2-trifluoroethene. A comparison of curves A and B shows that the first and second cathodic peaks are shifted toward more positive potentials by 520 and 670 mV, respectively, when silver serves as the cathode material.
Ring-expansion reactions Dowd and Choi [6] reported the first free-radical ringexpansion reaction in 1987, when they treated ethyl 1-(4iodobutyl)-2-oxocyclohexane-1-carboxylate (1) to obtain ethyl cyclodecanone-6-carboxylate (2 ) in 71% yield.
Introduction In the latest edition of the classic work Organic Electrochemistry, Gennaro et al. [1] contributed a chapter that articulates the place of silver as a modern cathode material for the activation and reductive cleavage of carbon– halogen bonds in organic compounds. These authors offer important insights concerning theoretical underpinnings and practical applications of silver for both electrosynthesis and electroanalysis. In the present article, we desire to acquaint readers with a number of recent uses of silver cathodes for the direct electrochemical reduction of halogenated organic comCurrent Opinion in Electrochemistry 2017, 2:60–66
In 2007 our laboratory described the use of electrogenerated nickel(I) salen to promote three- and fourcarbon ring-expansion reactions of several 1-haloalkyl-2oxocycloalkanecarboxylates, but desired products were obtained in no higher than 23% yield [7]. Because of the special ability of silver to facilitate the reduction of carbon–halogen bonds, we decided to explore anew the possibility of electrochemically induced ring-expansion reactions. A recent publication [8] describes the first examples of one-carbon ring-expansion reactions accomplished via the direct reduction of two different 1bromomethyl-2-oxocycloalkane-1-carboxylates (3 and 4 ) www.sciencedirect.com
Using Silver Cathodes for Organic Electrosynthesis and Mechanistic Studies Mubarak and Peters
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Figure 1
Cyclic voltammograms recorded with (A) a glassy carbon or (B) a silver electrode (area of each cathode = 0.071 cm2 ) at 100 mV s−1 for reduction of a 5.0 mM solution of 1,1,2-trichloro-1,2,2-trifluoroethane in oxygen-free DMF containing 0.050 M TMABF4 . Each scan begins and ends at approximately 0 V with respect to a reference electrode consisting of a cadmium-saturated mercury amalgam in contact with DMF saturated with both cadmium chloride and sodium chloride; this electrode has a potential of –0.76 V vs. SCE at 25°C.
OC2H5
O
O
C 4H 8 I OC2H5
Bu3SnH, AIBN O
in C6H6 O
1
at silver cathodes. Unlike the disappointing results of our earlier study, direct reduction of these two starting materials at silver afforded the desired products (5 and 6 ) in yields of 92% and 55%, respectively. Thus, there is good reason to conclude that further studies of ring-expansion reactions at silver cathodes might be worthwhile.
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2
Electrochemical reduction and intramolecular cyclization of bromo propargyloxy esters In earlier work conducted in our laboratory [9], catalytic reduction of ethyl 2-bromo-3-(3 ,4 -dimethoxyphenyl)-3(propargyloxy)propanoate (7 ) by nickel(I) tetramethylcyclam electrogenerated at vitreous carbon cathodes was found to give two products (8 and 9 ).
Current Opinion in Electrochemistry 2017, 2:60–66
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Organic and Molecular Electrochemistry 2017
O Br H5C2O
O
H3CO OCH3 7 O
O
H 5C2O
H5C2O
O
O
H3CO
H3CO OCH3
OCH3
8
9
Under optimal experimental conditions, namely 2 mM 7 and 0.4 mM nickel(II) tetramethylcyclam, the desired compound (8 ), which is a precursor for the synthesis of isogmelinol (10 ), an antitumor agent), was obtained in yields of 80% or better.
When bulk electrolyses of 7 were conducted at a silver gauze cathode, the product distribution consisted of 12 (49%), together with 13 and 14 (10% and 40%, respectively) [11]. This product distribution differs significantly from those achieved when either nickel(I) tetramethylcyclam-catalyzed reduction or when direct reduction at reticulated vitreous carbon is done, once again demonstrating the unique qualities of silver for the reduction of carbon–halogen bonds.
On the other hand, the direct reduction of 7 at carbon cathodes afforded ethyl trans-3-(3 ,4 -dimethoxyphenyl)prop-2-enoate (11 ) as the principal product [10].
OCH3
O
H3CO
H
OH
OCH3
O H3CO 10
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Using Silver Cathodes for Organic Electrosynthesis and Mechanistic Studies Mubarak and Peters
O
63
Cl Cl Cl
OC2H5
Cl
Cl Cl
15
H3CO
to its behavior at silver gauze cathodes [13], and conditions were established for the complete conversion of lindane to benzene.
OCH3 11
Reduction (remediation) of environmental pollutants A major challenge and goal of environmental remediation is dehalogenation of organic pollutants, as the halogen moieties of these compounds are the primary sources of their harm to society. A not-uncommon dilemma pertaining to these pollutants is how to deal with the often distressingly huge stockpiles that have been sequestered around the world. One compound that came to our early attention is lindane (15 ), the use and release of which were cancelled in 2007 by the Environmental Protection Agency of the United States. Although electrochemical reduction of lindane has been the subject of numerous studies [12], our attention turned
Another area of interest has been the electrochemical reduction of flame retardants (decabromodiphenyl ether and 1,2,5,6,9,10-hexabromocyclododecane) at both carbon and silver cathodes [14,15]. Debromination of decabromodiphenyl ether was most successful when the reduction was conducted at a silver gauze cathode [14]. For the reduction of 1,2,5,6,9,10-hexabromocyclododecane (HBCD) at a silver cathode, nearly quantitative debromination of a 2.0 mM solution of the parent flame retardant to a mixture of isomers of 1,5,9-cyclododecatriene was achieved; however, this result was greatly attenuated when the concentration of HBCD was decreased [15]. A few years ago, the direct electrochemical reduction of 1,1,2-trichloro-1,2,2-trifluoroethane at silver cathodes was explored as a way to remediate the large stockpiles of this now-banned chlorofluorocarbon [CFC) [16]. Although the targeted CFC proved relatively easy to destroy electrochemically, some of the resulting products (especially 1chloro-1,2,2-trifluoroethene) still pose a substantial risk to O
OC2H5 H3CO
H3CO 12 O H3CO
H3CO OC2H5
H3CO
O 13
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OC2H5
H3CO 14
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O
O
O
CH2Cl
16
O
O
17 (41% yield)
the ozone layer. Thus, there is plenty of room for more detailed and successful research. In particular, the use of mixtures of an organic solvent with water might be fruitful.
onto silver, followed by dissociative electron transfer that eventually leads to adsorbed phenyl carbanions that react with carbon dioxide. Additional mechanistic details are discussed in the original publication.
Finally, we call attention to studies of the reduction of 4,4 -(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene) (DDT) and 4,4 -(2,2,2-trichloroethane-1,1-diyl)bis(methoxybenzene) [17,18] at silver gauze cathodes. After DDT was banned from use in the period of 2002–2003, the latter compound (known as methoxychlor) was temporarily promoted as a replacement. Undoubtedly, the most compelling result of these two investigations was the observation that DDT can be converted essentially quantitatively to a totally dechlorinated product (1,1 -ethylidenebisbenzene or EBB) when DDT is electrolyzed in solvent–electrolyte containing 1 M D2 O. This last finding definitely merits more scrutiny as a possible route to the remediation of stockpiles of DDT.
Research by Yang et al. [23] demonstrated that silver nanoparticles prepared via direct reduction of silver nitrate with hydrazine hydrate in aqueous solution can be employed as a cathode material for the electrocatalytic carboxylation of 1-phenethyl bromide to afford 2phenylpropanoic acid in 98% yield. This catalyst was employed for the electrocarboxylation of several other substrates to prepare carboxylic acids in high yield.
Synthesis of coumarins Coumarins play an important role in a variety of fields because these compounds exhibit diverse biological activity as well as significant and unique optical behavior. However, electrosynthesis of coumarins has not been extensively explored, and few publications have appeared that deal with this topic [19,20]. A recent paper [21] describes the electrosynthesis of several substituted coumarins in modest yield via direct reduction of phenyl 2-chloroacetates at silver cathodes. A typical reaction is the transformation of 2-formylphenyl 2-chloroacetate (16 ) to 2H-chromen-2-one (17).
Electrocarboxylation Carbon dioxide, the principal greenhouse gas linked to climate change, is readily available and relatively inexpensive. It can be utilized to synthesize carboxylic acids that have both industrial and pharmaceutical applications. Isse et al. [22] used silver for the reduction of carbon– halogen bonds in the presence of carbon dioxide. Employing silver cathodes in carbon dioxide-saturated dimethylformamide, these workers reduced bromobenzene under mild conditions to afford benzoic acid in high yield. To account for this electrocatalytic process, a mechanism was proposed that involves initial adsorption of the substrate Current Opinion in Electrochemistry 2017, 2:60–66
Wang et al. [24] employed cyclic voltammetry and bulk electrolysis to investigate the selective catalytic electrocarboxylation of 2-bromo-, 3-bromo-, and 4-bromostyrene to give the respective substituted benzoic acids in carbon dioxide-saturated dimethylformamide at silver cathodes. Using a silver electrode in an undivided cell, these workers were able to reduce the carbon–bromine bond preferentially over the carbon–carbon double bond of the styrene moiety.
Adsorption of halide ions onto polycrystalline silver cathodes An obvious consequence of the reduction of halogenated compounds at silver cathodes is the release of halide ions (notably I– and Br– ) that are specifically adsorbed, in a potential-dependent way, onto the surface of the electrode, thereby influencing electron-transfer events. Falciola et al. [25] have determined the potential-dependent specific adsorption of Br– and I– onto polycrystalline silver electrodes in acetonitrile, propylene carbonate, and dimethylformamide. It was found that the strength of halide adsorption increases as one goes from Br– to I– and from acetonitrile to propylene carbonate to dimethylformamide.
Silver nanoparticles Silver nanoparticles have been employed as cathode materials for the electrocatalytic reduction of carbon– halogen bonds. In a relatively early investigation, Isse et al. [26] developed a straightforward potentiostatic pulse procedure for the deposition of silver clusters onto glassy www.sciencedirect.com
Using Silver Cathodes for Organic Electrosynthesis and Mechanistic Studies Mubarak and Peters
C2H 5 COCH2Cl N CH2OCH3 C 2H 5 18
carbon, and these electrodes were used for direct reduction of benzyl chloride at several different potentials in acetonitrile containing tetraethylammonium perchlorate; mixtures of toluene (58–60%) and 3-phenylpropanenitrile (22–28%) were obtained as products. By reduction of an aqueous solution of silver nitrate, Rondinini et al. [27] prepared colloidal silver nanoparticles supported on carbon powder for the conversion of chloroform to methane. In similar fashion, Lugaresi et al. [28] prepared silver nanoparticles (2–15 nm in diameter) supported on carbon as a cathode for the electroreduction of chloroform to methane. In a subsequent publication, Lugaresi et al. [29] described the synthesis of silver nanoparticles of three different sizes and their use as cathodes for the electrocatalytic reduction of trichloromethane in aqueous media. Niu et al. [30] described (a) the assembly of silver nanoparticles onto disposable screen-printed carbon electrodes by means of vacuum sputtering and (b) the use of these devices for the reduction of hydrogen peroxide in neutral aqueous media. It was found that the surface coverage of the assembled silver plays an important role in the electroreduction of hydrogen peroxide. Verlato et al. [31] prepared silver-modified nickel foams and employed these materials as cathodes for the reductive dechlorination of herbicides such as alachlor (18). In addition, silver nanoparticles have been recently employed as catalysts for the electroreduction of carbon dioxide to carbon monoxide. In this context, Back et al. [32] studied the active sites of both silver and gold nanoparticles as catalysts for this important process. These researchers concluded that silver nanoparticles are an efficient and relatively inexpensive alternative to gold as a catalyst for the electroreduction of carbon dioxide. Along with being used as cathode materials for electrochemical reduction, silver nanoparticles have also been incorporated into electrochemical sensors. Because many articles have been published in this field, we thought it would be worthwhile to mention two very recent examples pertaining to this topic. Peng et al. [33] synthesized www.sciencedirect.com
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a metal–organic framework and silver nanoparticle composite as a novel modified electrode for detection of tryptophan at a level as low as 0.14 μM. On the other hand, Fu et al. [34] developed a filtered cathodic vacuum arc technique to prepare a silver film that was loaded onto carbon paper to form an electrode employed as a sensor of tryptophan at a detection limit of 0.04 μM.
New electrode surfaces involving silver cathodes Simonet and coworkers [35–37] have reported exciting new discoveries pertaining to the electrochemical formation and properties of novel surface-modified silver cathodes. When a smooth silver microelectrode (covered with natural graphite particles) in dimethylformamide or acetonitrile containing a tetraalkylammonium salt is polarized cathodically, most of the graphite layer is exfoliated. When employed for the reduction of 1-iodoalkanes, these “silver–graphene” electrodes promote the catalytic, oneelectron cleavage of carbon–iodine bonds, with the possibility that the resulting alkyl radical adds to the adjacent carbon network [35]. Evidence has been provided for the immobilization (grafting) of pyridine moieties onto silver (as well as gold and carbon) surfaces via reduction of 4-(ωhaloalkyl)pyridines in organic solvents such as dimethylformamide. These modified electrodes may open new opportunities for electrochemical studies of proteins and metalloenzymes [36]. Recent research [37] indicates that, when smooth silver electrodes are cathodically polarized in carbon dioxide-saturated dimethyformamide or acetonitrile containing tetraalkylammonium salts, significant insertion of carbon dioxide into the silver surface takes place. This phenomenon manifests itself in the appearance of cathodic peaks at potentials depending on the identity of the cation of the supporting electrolyte. In particular, when tetramethylammonium tetrafluoroborate is the supporting electrolyte, it is proposed that a layer of Ag–CO2 – ,TMA+ forms on the surface of the silver cathode. Furthermore, it is suggested that such a material might serve as a source of carbon dioxide for carboxylation reactions. References 1.
Gennaro A, Isse AA, Mussini PR: Organic Electrochemistry. Edited by Hammerich O, Speiser B. edn 5. London: Taylor & Francis, LLC; 2016:24, 917–940.
2.
Strawsine LM, Sengupta A, Raghavachari K, Peters DG: Direct reduction of alkyl monohalides at silver in dimethylformamide: effects of position and identity of the halogen. Chem Electro Chem 2015, 2:726–736.
3.
Martin ET, Strawsine LM, Mubarak MS, Peters DG: Direct reductions of 1,2- and 1,6-dibromohexane at silver cathodes in dimethylformamide. Electrochim Acta 2015, 186:369–376.
4.
Rose JA, McGuire CM, Hansen AM, Karty JA, Mubarak MS, Peters DG: Direct reduction of 1-bromo-6-chlorohexane and 1-chloro-6-iodohexane at silver cathodes in dimethylformamide. Electrochim Acta 2016, 218:311–317.
5.
Martin ET, McGuire CM, Mubarak MS, Peters DG: Electroreductive remediation of halogeated environmental pollutants. Chem Rev 2016, 116:15198–15234. Current Opinion in Electrochemistry 2017, 2:60–66
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6.
Dowd P, Choi S-C: Free radical ring expansion by three and four carbons. J Am Chem Soc 1987, 109:6548–6549.
7.
Mubarak MS, Barker WF IV, Peters DG: Nickel(I) salen-catalyzed reduction of 1- haloalkyl-2-oxocycloalkanecarboxylates: threeand four-carbon ring expansions. J Electrochem Soc 2007, 154:F205–F210.
8.
Wappes EA, Mubarak MS, Peters DG: Electrochemical reduction of 1-bromomethyl-2- oxocycloalkane-1-carboxylates at silver cathodes in dimethylformamide: one-carbon ring-expansion reactions. J Electrochem Soc 2014, 161:G122–G127.
9.
Esteves AP, Goken DM, Klein LJ, Lemos MA, Medeiros MJ, Peters DG: Electroreductive intramolecular cyclization of a bromo propargyloxy ester catalyzed by nickel(I) tetramethylcyclam electrogenerated at carbon cathodes in dimethylformamide. J Org Chem 2003, 68:1024–1029.
10. Esteves AP, Goken DM, Klein LJ, Medeiros MJ, Peters DG: Direct electrochemical reduction of a bromo propargyloxy ester at vitreous carbon cathodes in dimethylformamide. J Electroanal Chem 2003, 560:161–168. 11. Henderson RJ, Buehler NR, Pasciak EM, Mubarak MS, Peters DG: Electrochemical reduction of a bromo propargyloxy ester at silver cathodes in dimethylformamide. J Electrochem Soc 2014, 161:G128–G132. 12. Merz JP, Gamoke BC, Foley MP, Raghavachari K, Peters DG: Electrochemical reduction of (1R,2r,3S,4R,5r,6S)-hexachlorocyclohexane (lindane) at carbon cathodes in dimethylformamide. J Electroanal Chem 2011, 660:121–126. 13. Peverly AA, Karty JA, Peters DG: Electrochemical reduction of (1R,2r,3S,4R,5r,6S)-hexachlorocyclohexane (lindane) at silver cathodes in organic and aqueous-organic media. J Electroanal Chem 2013, 692:66–71. 14. Peverly AA, Pasciak EM, Strawsine LM, Wagoner ER, Peters DG: Electrochemical reduction of decabromodiphenyl ether at carbon and silver cathodes in dimethylformamide and dimethyl sulfoxide. J Electroanal Chem 2013, 704:227–232. 15. Wagoner ER, Baumberger CP, Peverly AA, Peters DG: Electrochemical reduction of 1,2,5,6,9,10-hexabromocyclododecane at carbon and silver cathodes in dimethylformamide. J Electroanal Chem 2014, 713:136–142. 16. Wagoner ER, Peters DG: Electrocatalytic reduction of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) at silver cathodes in organic and organic-aqueous solvents. J Electrochem Soc 2013, 160:G135–G141. 17. McGuire CM, Peters DG: Electrochemical dechlorination of 4,4’-(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene) (DDT) at silver cathodes. Electrochim Acta 2014, 137:423–430. 18. McGuire CM, Peters DG: Direct electrochemical reduction of 4,4’-(2,2,2-trichloroethane-1,1-diyl)bis(methoxybenzene) (methoxychlor) at carbon and silver cathodes in dimethylformamide. J Electrochem Soc 2016, 163:G44–G49. 19. Dolly, Batanero B, Barba F: Cathodic reduction of hydroxycarbonyl compound trichloroacetyl esters. Tetrahedron 2003, 59:9161–9165. 20. Du P, Mubarak MS, Karty JA, Peters DG: Electrosynthesis of 4-methylcoumarin via cobalt(I)-catalyzed reduction of 2-acetylphenyl 2-chloroacetate or 2-acetyl 2,2-dichloroacetate. J Electrochem Soc 2007, 154:F231–F237.
22. Isse AA, Durante C, Gennaro A: One-pot synthesis of benzoic acid by electrocatalytic reduction of bromobenzene in the presence of CO2 . Electrochem Commun 2011, 13:810–813. 23. Yang H, Wu L, Wang H, Lu J: Cathode made of compacted silver nanoparticles for electrocatalytic carboxylation of 1-phenethyl bromide with CO2 . Chin J Catal 2016, 37:994–998. 24. Wang H-M, Sui G-J, Wu D, Feng Q, Wang H, Lu J-X: Selective electrocarboxylation of bromostyrene at silver cathode in DMF. Tetrahedron 2016, 72:968–972. 25. Falciola L, Mussini PR, Trasatti S, Doubova LM: Specific adsorption of bromide and iodide anions from nonaqueous solutions on controlled-surface polycrystalline silver electrodes. J Electroanal Chem 2006, 593:185–193. 26. Isse AA, Gottardello S, Maccato C, Gennaro A: Silver nanoparticles deposited on glassy carbon. Electrocatalytic activity for reduction of benzyl chloride. Electrochem Commun 2006, 8:1707–1712. 27. Rondinini S, Aricci G, Krpetic´ Ž, Locatelli C, Minguzzi A, Porta F, Vertova A: Electroreductions on silver-based electrocatalysts: the use of Ag nanoparticles for CHCl3 to CH4 conversion. Fuel Cells 2009, 9:253–263. 28. Lugaresi O, Encontre H, Locatelli C, Minguzzi A, Vertova A, Rondinini S, Comninellis C: Gas-phase volatile organic chloride electroreduction: a versatile experimental setup for electrolytic dechlorination and voltammetric analysis. Electrochem Commun 2014, 44:63–65. 29. Lugaresi O, Perales-Rondón JV, Minguzzi A, Solla-Gullón J, Rondinini S, Feliu JM, Sánchez-Sánchez CM: Rapid screening of silver nanoparticles for the catalytic degradation of chlorinated pollutants in water. Appl Catal B 2015, 163:554–563. 30. Niu X, Shi L, Pan J, Qiu F, Yan Y, Zhao H, Lan M: Modulating the assembly of sputtered silver nanoparticles on screen-printed carbon electrodes for hydrogen peroxide electroreduction: effect of the surface coverage. Electrochim Acta 2016, 199:187–193. 31. Verlato E, He W, Amrane A, Barison S, Floner D, Fourcade F, Geneste F, Musiani M, Seraglia R: Preparation of silver-modified nickel foams by galvanic displacement and their use as cathodes for the reductive dechlorination of herbicides. ChemElectroChem 2016, 3:2084–2092. 32. Back S, Yeom MS, Jung Y: Active sites of Au and Ag nanoparticle catalysts for CO2 electroreduction to CO. ACS Catal 2015, 5:5089–5096. 33. Peng Z, Jiang Z, Huang X, Li Y: A novel electrochemical sensor of tryptophan based on silver nanoparticles/metal–organic framework composite modified glassy carbon electrode. RSC Adv 2016, 6:13742–13748. 34. Fu Y, Su W, Wang T, Hu J: Electrochemical sensor of tryptophan based on an Ag/CP electrode prepared by the filtered cathodic vacuum arc technique. J Electrochem Soc 2016, 163:B107–B112. 35. Jouikov V, Stéphant N, Poizot P, Simonet J: The silver–graphene electrode. Building, stability, and catalytic efficiency. Electrochem Commun 2015, 51:125–128. 36. Jouikov V, Simonet J: Facile immobilization of pyridine at gold, silver, and carbon surfaces. Generation of multi-strata versatile electrodes. Electrochem Commun 2015, 56:20–23. 37. Simonet J: Electrochemical insertion of CO2 into silver in a large extent. Electrochem Commun 2015, 58:11–14.
21. Pasciak EM, Peters DG: Reduction of substituted phenyl 2-chloroacetates at silver cathodes: electrosynthesis of coumarins. J Electrochem Soc 2014, 161:G98–G102.
Current Opinion in Electrochemistry 2017, 2:60–66
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Review Article Using silver cathodes for organic electrosynthesis and mechanistic studies Mohammad S. Mubarak 1 and Dennis G. Peters 2,∗ In the last decade or so, silver electrodes (almost invariably as cathodes) have captured increasing attention for their ability to facilitate (catalyze) the electrochemical reductive cleavage of carbon–halogen bonds in an enormous variety of organic compounds and under remarkably mild conditions. This short review is intended to call attention to and, hopefully, to stimulate further research and applications related to the use of silver electrodes in areas such as electrosynthesis, mechanistic organic electrochemistry, electrocarboxylation reactions (that could utilize excess carbon dioxide in the environment), nanochemistry, surface chemistry, and the remediation of stockpiles of halogenated environmental pollutants such as pesticides, flame retardants, and chlorofluorocarbons. Addresses 1 Department of Chemistry, The University of Jordan, Amman 11942, Jordan 2 Department of Chemistry, Indiana University, Bloomington, IN 47405, USA Corresponding author: Peters, Dennis G. ([email protected])
Current Opinion in Electrochemistry 2017, 2:60–66 This review comes from a themed issue on Organic and Molecular Electrochemistry 2017 Edited by Jean Lessard For a complete overview see the Issue and the Editorial Available online 22th March 2017 http://dx.doi.org/10.1016/j.coelec.2017.03.001 2451-9103/© 2017 Elsevier B.V. All rights reserved.
pounds [2–4] that pertain to (a) electrosynthesis of various organic compounds and (b) reductive dehalogenation (remediation) of some environmental pollutants [5]. Included in our discussion are comparisons between the use of silver cathodes and alternative strategies that involve different electrode materials or transition-metal catalysts electrogenerated in situ. Another topic to be treated is the very rapidly expanding use of silver nanoparticles as a platform for both electrosynthesis and environmental remediation. We hope these examples will stimulate further developments in the arenas of both electrosynthesis and environmental remediation. Cyclic voltammetry can reveal and dramatize the propensity of silver to electrocatalyze the reductive cleavage of carbon–halogen bonds. An excellent example, depicted in Figure 1, provides a comparison of the cyclic voltammetric behavior of 1,1,2-trichloro-1,2,2-trifluoroethane at silver and glassy carbon electrodes in dimethylformamide containing tetramethylammonium tetrafluoroborate (TMABF4 ) as supporting electrolyte. Each cyclic voltammogram exhibits a pair of well defined, irreversible cathodic peaks that correspond, respectively, to (a) twoelectron reduction of the parent compound to afford 1-chloro-1,2,2-trifluoroethene and (b) two-electron reduction of the latter product to yield 1,1,2-trifluoroethene. A comparison of curves A and B shows that the first and second cathodic peaks are shifted toward more positive potentials by 520 and 670 mV, respectively, when silver serves as the cathode material.
Ring-expansion reactions Dowd and Choi [6] reported the first free-radical ringexpansion reaction in 1987, when they treated ethyl 1-(4iodobutyl)-2-oxocyclohexane-1-carboxylate (1) to obtain ethyl cyclodecanone-6-carboxylate (2 ) in 71% yield.
Introduction In the latest edition of the classic work Organic Electrochemistry, Gennaro et al. [1] contributed a chapter that articulates the place of silver as a modern cathode material for the activation and reductive cleavage of carbon– halogen bonds in organic compounds. These authors offer important insights concerning theoretical underpinnings and practical applications of silver for both electrosynthesis and electroanalysis. In the present article, we desire to acquaint readers with a number of recent uses of silver cathodes for the direct electrochemical reduction of halogenated organic comCurrent Opinion in Electrochemistry 2017, 2:60–66
In 2007 our laboratory described the use of electrogenerated nickel(I) salen to promote three- and fourcarbon ring-expansion reactions of several 1-haloalkyl-2oxocycloalkanecarboxylates, but desired products were obtained in no higher than 23% yield [7]. Because of the special ability of silver to facilitate the reduction of carbon–halogen bonds, we decided to explore anew the possibility of electrochemically induced ring-expansion reactions. A recent publication [8] describes the first examples of one-carbon ring-expansion reactions accomplished via the direct reduction of two different 1bromomethyl-2-oxocycloalkane-1-carboxylates (3 and 4 ) www.sciencedirect.com
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Figure 1
Cyclic voltammograms recorded with (A) a glassy carbon or (B) a silver electrode (area of each cathode = 0.071 cm2 ) at 100 mV s−1 for reduction of a 5.0 mM solution of 1,1,2-trichloro-1,2,2-trifluoroethane in oxygen-free DMF containing 0.050 M TMABF4 . Each scan begins and ends at approximately 0 V with respect to a reference electrode consisting of a cadmium-saturated mercury amalgam in contact with DMF saturated with both cadmium chloride and sodium chloride; this electrode has a potential of –0.76 V vs. SCE at 25°C.
OC2H5
O
O
C 4H 8 I OC2H5
Bu3SnH, AIBN O
in C6H6 O
1
at silver cathodes. Unlike the disappointing results of our earlier study, direct reduction of these two starting materials at silver afforded the desired products (5 and 6 ) in yields of 92% and 55%, respectively. Thus, there is good reason to conclude that further studies of ring-expansion reactions at silver cathodes might be worthwhile.
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2
Electrochemical reduction and intramolecular cyclization of bromo propargyloxy esters In earlier work conducted in our laboratory [9], catalytic reduction of ethyl 2-bromo-3-(3 ,4 -dimethoxyphenyl)-3(propargyloxy)propanoate (7 ) by nickel(I) tetramethylcyclam electrogenerated at vitreous carbon cathodes was found to give two products (8 and 9 ).
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O Br H5C2O
O
H3CO OCH3 7 O
O
H 5C2O
H5C2O
O
O
H3CO
H3CO OCH3
OCH3
8
9
Under optimal experimental conditions, namely 2 mM 7 and 0.4 mM nickel(II) tetramethylcyclam, the desired compound (8 ), which is a precursor for the synthesis of isogmelinol (10 ), an antitumor agent), was obtained in yields of 80% or better.
When bulk electrolyses of 7 were conducted at a silver gauze cathode, the product distribution consisted of 12 (49%), together with 13 and 14 (10% and 40%, respectively) [11]. This product distribution differs significantly from those achieved when either nickel(I) tetramethylcyclam-catalyzed reduction or when direct reduction at reticulated vitreous carbon is done, once again demonstrating the unique qualities of silver for the reduction of carbon–halogen bonds.
On the other hand, the direct reduction of 7 at carbon cathodes afforded ethyl trans-3-(3 ,4 -dimethoxyphenyl)prop-2-enoate (11 ) as the principal product [10].
OCH3
O
H3CO
H
OH
OCH3
O H3CO 10
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Using Silver Cathodes for Organic Electrosynthesis and Mechanistic Studies Mubarak and Peters
O
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Cl Cl Cl
OC2H5
Cl
Cl Cl
15
H3CO
to its behavior at silver gauze cathodes [13], and conditions were established for the complete conversion of lindane to benzene.
OCH3 11
Reduction (remediation) of environmental pollutants A major challenge and goal of environmental remediation is dehalogenation of organic pollutants, as the halogen moieties of these compounds are the primary sources of their harm to society. A not-uncommon dilemma pertaining to these pollutants is how to deal with the often distressingly huge stockpiles that have been sequestered around the world. One compound that came to our early attention is lindane (15 ), the use and release of which were cancelled in 2007 by the Environmental Protection Agency of the United States. Although electrochemical reduction of lindane has been the subject of numerous studies [12], our attention turned
Another area of interest has been the electrochemical reduction of flame retardants (decabromodiphenyl ether and 1,2,5,6,9,10-hexabromocyclododecane) at both carbon and silver cathodes [14,15]. Debromination of decabromodiphenyl ether was most successful when the reduction was conducted at a silver gauze cathode [14]. For the reduction of 1,2,5,6,9,10-hexabromocyclododecane (HBCD) at a silver cathode, nearly quantitative debromination of a 2.0 mM solution of the parent flame retardant to a mixture of isomers of 1,5,9-cyclododecatriene was achieved; however, this result was greatly attenuated when the concentration of HBCD was decreased [15]. A few years ago, the direct electrochemical reduction of 1,1,2-trichloro-1,2,2-trifluoroethane at silver cathodes was explored as a way to remediate the large stockpiles of this now-banned chlorofluorocarbon [CFC) [16]. Although the targeted CFC proved relatively easy to destroy electrochemically, some of the resulting products (especially 1chloro-1,2,2-trifluoroethene) still pose a substantial risk to O
OC2H5 H3CO
H3CO 12 O H3CO
H3CO OC2H5
H3CO
O 13
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H3CO 14
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O
O
O
CH2Cl
16
O
O
17 (41% yield)
the ozone layer. Thus, there is plenty of room for more detailed and successful research. In particular, the use of mixtures of an organic solvent with water might be fruitful.
onto silver, followed by dissociative electron transfer that eventually leads to adsorbed phenyl carbanions that react with carbon dioxide. Additional mechanistic details are discussed in the original publication.
Finally, we call attention to studies of the reduction of 4,4 -(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene) (DDT) and 4,4 -(2,2,2-trichloroethane-1,1-diyl)bis(methoxybenzene) [17,18] at silver gauze cathodes. After DDT was banned from use in the period of 2002–2003, the latter compound (known as methoxychlor) was temporarily promoted as a replacement. Undoubtedly, the most compelling result of these two investigations was the observation that DDT can be converted essentially quantitatively to a totally dechlorinated product (1,1 -ethylidenebisbenzene or EBB) when DDT is electrolyzed in solvent–electrolyte containing 1 M D2 O. This last finding definitely merits more scrutiny as a possible route to the remediation of stockpiles of DDT.
Research by Yang et al. [23] demonstrated that silver nanoparticles prepared via direct reduction of silver nitrate with hydrazine hydrate in aqueous solution can be employed as a cathode material for the electrocatalytic carboxylation of 1-phenethyl bromide to afford 2phenylpropanoic acid in 98% yield. This catalyst was employed for the electrocarboxylation of several other substrates to prepare carboxylic acids in high yield.
Synthesis of coumarins Coumarins play an important role in a variety of fields because these compounds exhibit diverse biological activity as well as significant and unique optical behavior. However, electrosynthesis of coumarins has not been extensively explored, and few publications have appeared that deal with this topic [19,20]. A recent paper [21] describes the electrosynthesis of several substituted coumarins in modest yield via direct reduction of phenyl 2-chloroacetates at silver cathodes. A typical reaction is the transformation of 2-formylphenyl 2-chloroacetate (16 ) to 2H-chromen-2-one (17).
Electrocarboxylation Carbon dioxide, the principal greenhouse gas linked to climate change, is readily available and relatively inexpensive. It can be utilized to synthesize carboxylic acids that have both industrial and pharmaceutical applications. Isse et al. [22] used silver for the reduction of carbon– halogen bonds in the presence of carbon dioxide. Employing silver cathodes in carbon dioxide-saturated dimethylformamide, these workers reduced bromobenzene under mild conditions to afford benzoic acid in high yield. To account for this electrocatalytic process, a mechanism was proposed that involves initial adsorption of the substrate Current Opinion in Electrochemistry 2017, 2:60–66
Wang et al. [24] employed cyclic voltammetry and bulk electrolysis to investigate the selective catalytic electrocarboxylation of 2-bromo-, 3-bromo-, and 4-bromostyrene to give the respective substituted benzoic acids in carbon dioxide-saturated dimethylformamide at silver cathodes. Using a silver electrode in an undivided cell, these workers were able to reduce the carbon–bromine bond preferentially over the carbon–carbon double bond of the styrene moiety.
Adsorption of halide ions onto polycrystalline silver cathodes An obvious consequence of the reduction of halogenated compounds at silver cathodes is the release of halide ions (notably I– and Br– ) that are specifically adsorbed, in a potential-dependent way, onto the surface of the electrode, thereby influencing electron-transfer events. Falciola et al. [25] have determined the potential-dependent specific adsorption of Br– and I– onto polycrystalline silver electrodes in acetonitrile, propylene carbonate, and dimethylformamide. It was found that the strength of halide adsorption increases as one goes from Br– to I– and from acetonitrile to propylene carbonate to dimethylformamide.
Silver nanoparticles Silver nanoparticles have been employed as cathode materials for the electrocatalytic reduction of carbon– halogen bonds. In a relatively early investigation, Isse et al. [26] developed a straightforward potentiostatic pulse procedure for the deposition of silver clusters onto glassy www.sciencedirect.com
Using Silver Cathodes for Organic Electrosynthesis and Mechanistic Studies Mubarak and Peters
C2H 5 COCH2Cl N CH2OCH3 C 2H 5 18
carbon, and these electrodes were used for direct reduction of benzyl chloride at several different potentials in acetonitrile containing tetraethylammonium perchlorate; mixtures of toluene (58–60%) and 3-phenylpropanenitrile (22–28%) were obtained as products. By reduction of an aqueous solution of silver nitrate, Rondinini et al. [27] prepared colloidal silver nanoparticles supported on carbon powder for the conversion of chloroform to methane. In similar fashion, Lugaresi et al. [28] prepared silver nanoparticles (2–15 nm in diameter) supported on carbon as a cathode for the electroreduction of chloroform to methane. In a subsequent publication, Lugaresi et al. [29] described the synthesis of silver nanoparticles of three different sizes and their use as cathodes for the electrocatalytic reduction of trichloromethane in aqueous media. Niu et al. [30] described (a) the assembly of silver nanoparticles onto disposable screen-printed carbon electrodes by means of vacuum sputtering and (b) the use of these devices for the reduction of hydrogen peroxide in neutral aqueous media. It was found that the surface coverage of the assembled silver plays an important role in the electroreduction of hydrogen peroxide. Verlato et al. [31] prepared silver-modified nickel foams and employed these materials as cathodes for the reductive dechlorination of herbicides such as alachlor (18). In addition, silver nanoparticles have been recently employed as catalysts for the electroreduction of carbon dioxide to carbon monoxide. In this context, Back et al. [32] studied the active sites of both silver and gold nanoparticles as catalysts for this important process. These researchers concluded that silver nanoparticles are an efficient and relatively inexpensive alternative to gold as a catalyst for the electroreduction of carbon dioxide. Along with being used as cathode materials for electrochemical reduction, silver nanoparticles have also been incorporated into electrochemical sensors. Because many articles have been published in this field, we thought it would be worthwhile to mention two very recent examples pertaining to this topic. Peng et al. [33] synthesized www.sciencedirect.com
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a metal–organic framework and silver nanoparticle composite as a novel modified electrode for detection of tryptophan at a level as low as 0.14 μM. On the other hand, Fu et al. [34] developed a filtered cathodic vacuum arc technique to prepare a silver film that was loaded onto carbon paper to form an electrode employed as a sensor of tryptophan at a detection limit of 0.04 μM.
New electrode surfaces involving silver cathodes Simonet and coworkers [35–37] have reported exciting new discoveries pertaining to the electrochemical formation and properties of novel surface-modified silver cathodes. When a smooth silver microelectrode (covered with natural graphite particles) in dimethylformamide or acetonitrile containing a tetraalkylammonium salt is polarized cathodically, most of the graphite layer is exfoliated. When employed for the reduction of 1-iodoalkanes, these “silver–graphene” electrodes promote the catalytic, oneelectron cleavage of carbon–iodine bonds, with the possibility that the resulting alkyl radical adds to the adjacent carbon network [35]. Evidence has been provided for the immobilization (grafting) of pyridine moieties onto silver (as well as gold and carbon) surfaces via reduction of 4-(ωhaloalkyl)pyridines in organic solvents such as dimethylformamide. These modified electrodes may open new opportunities for electrochemical studies of proteins and metalloenzymes [36]. Recent research [37] indicates that, when smooth silver electrodes are cathodically polarized in carbon dioxide-saturated dimethyformamide or acetonitrile containing tetraalkylammonium salts, significant insertion of carbon dioxide into the silver surface takes place. This phenomenon manifests itself in the appearance of cathodic peaks at potentials depending on the identity of the cation of the supporting electrolyte. In particular, when tetramethylammonium tetrafluoroborate is the supporting electrolyte, it is proposed that a layer of Ag–CO2 – ,TMA+ forms on the surface of the silver cathode. Furthermore, it is suggested that such a material might serve as a source of carbon dioxide for carboxylation reactions. References 1.
Gennaro A, Isse AA, Mussini PR: Organic Electrochemistry. Edited by Hammerich O, Speiser B. edn 5. London: Taylor & Francis, LLC; 2016:24, 917–940.
2.
Strawsine LM, Sengupta A, Raghavachari K, Peters DG: Direct reduction of alkyl monohalides at silver in dimethylformamide: effects of position and identity of the halogen. Chem Electro Chem 2015, 2:726–736.
3.
Martin ET, Strawsine LM, Mubarak MS, Peters DG: Direct reductions of 1,2- and 1,6-dibromohexane at silver cathodes in dimethylformamide. Electrochim Acta 2015, 186:369–376.
4.
Rose JA, McGuire CM, Hansen AM, Karty JA, Mubarak MS, Peters DG: Direct reduction of 1-bromo-6-chlorohexane and 1-chloro-6-iodohexane at silver cathodes in dimethylformamide. Electrochim Acta 2016, 218:311–317.
5.
Martin ET, McGuire CM, Mubarak MS, Peters DG: Electroreductive remediation of halogeated environmental pollutants. Chem Rev 2016, 116:15198–15234. Current Opinion in Electrochemistry 2017, 2:60–66
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Dowd P, Choi S-C: Free radical ring expansion by three and four carbons. J Am Chem Soc 1987, 109:6548–6549.
7.
Mubarak MS, Barker WF IV, Peters DG: Nickel(I) salen-catalyzed reduction of 1- haloalkyl-2-oxocycloalkanecarboxylates: threeand four-carbon ring expansions. J Electrochem Soc 2007, 154:F205–F210.
8.
Wappes EA, Mubarak MS, Peters DG: Electrochemical reduction of 1-bromomethyl-2- oxocycloalkane-1-carboxylates at silver cathodes in dimethylformamide: one-carbon ring-expansion reactions. J Electrochem Soc 2014, 161:G122–G127.
9.
Esteves AP, Goken DM, Klein LJ, Lemos MA, Medeiros MJ, Peters DG: Electroreductive intramolecular cyclization of a bromo propargyloxy ester catalyzed by nickel(I) tetramethylcyclam electrogenerated at carbon cathodes in dimethylformamide. J Org Chem 2003, 68:1024–1029.
10. Esteves AP, Goken DM, Klein LJ, Medeiros MJ, Peters DG: Direct electrochemical reduction of a bromo propargyloxy ester at vitreous carbon cathodes in dimethylformamide. J Electroanal Chem 2003, 560:161–168. 11. Henderson RJ, Buehler NR, Pasciak EM, Mubarak MS, Peters DG: Electrochemical reduction of a bromo propargyloxy ester at silver cathodes in dimethylformamide. J Electrochem Soc 2014, 161:G128–G132. 12. Merz JP, Gamoke BC, Foley MP, Raghavachari K, Peters DG: Electrochemical reduction of (1R,2r,3S,4R,5r,6S)-hexachlorocyclohexane (lindane) at carbon cathodes in dimethylformamide. J Electroanal Chem 2011, 660:121–126. 13. Peverly AA, Karty JA, Peters DG: Electrochemical reduction of (1R,2r,3S,4R,5r,6S)-hexachlorocyclohexane (lindane) at silver cathodes in organic and aqueous-organic media. J Electroanal Chem 2013, 692:66–71. 14. Peverly AA, Pasciak EM, Strawsine LM, Wagoner ER, Peters DG: Electrochemical reduction of decabromodiphenyl ether at carbon and silver cathodes in dimethylformamide and dimethyl sulfoxide. J Electroanal Chem 2013, 704:227–232. 15. Wagoner ER, Baumberger CP, Peverly AA, Peters DG: Electrochemical reduction of 1,2,5,6,9,10-hexabromocyclododecane at carbon and silver cathodes in dimethylformamide. J Electroanal Chem 2014, 713:136–142. 16. Wagoner ER, Peters DG: Electrocatalytic reduction of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) at silver cathodes in organic and organic-aqueous solvents. J Electrochem Soc 2013, 160:G135–G141. 17. McGuire CM, Peters DG: Electrochemical dechlorination of 4,4’-(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene) (DDT) at silver cathodes. Electrochim Acta 2014, 137:423–430. 18. McGuire CM, Peters DG: Direct electrochemical reduction of 4,4’-(2,2,2-trichloroethane-1,1-diyl)bis(methoxybenzene) (methoxychlor) at carbon and silver cathodes in dimethylformamide. J Electrochem Soc 2016, 163:G44–G49. 19. Dolly, Batanero B, Barba F: Cathodic reduction of hydroxycarbonyl compound trichloroacetyl esters. Tetrahedron 2003, 59:9161–9165. 20. Du P, Mubarak MS, Karty JA, Peters DG: Electrosynthesis of 4-methylcoumarin via cobalt(I)-catalyzed reduction of 2-acetylphenyl 2-chloroacetate or 2-acetyl 2,2-dichloroacetate. J Electrochem Soc 2007, 154:F231–F237.
22. Isse AA, Durante C, Gennaro A: One-pot synthesis of benzoic acid by electrocatalytic reduction of bromobenzene in the presence of CO2 . Electrochem Commun 2011, 13:810–813. 23. Yang H, Wu L, Wang H, Lu J: Cathode made of compacted silver nanoparticles for electrocatalytic carboxylation of 1-phenethyl bromide with CO2 . Chin J Catal 2016, 37:994–998. 24. Wang H-M, Sui G-J, Wu D, Feng Q, Wang H, Lu J-X: Selective electrocarboxylation of bromostyrene at silver cathode in DMF. Tetrahedron 2016, 72:968–972. 25. Falciola L, Mussini PR, Trasatti S, Doubova LM: Specific adsorption of bromide and iodide anions from nonaqueous solutions on controlled-surface polycrystalline silver electrodes. J Electroanal Chem 2006, 593:185–193. 26. Isse AA, Gottardello S, Maccato C, Gennaro A: Silver nanoparticles deposited on glassy carbon. Electrocatalytic activity for reduction of benzyl chloride. Electrochem Commun 2006, 8:1707–1712. 27. Rondinini S, Aricci G, Krpetic´ Ž, Locatelli C, Minguzzi A, Porta F, Vertova A: Electroreductions on silver-based electrocatalysts: the use of Ag nanoparticles for CHCl3 to CH4 conversion. Fuel Cells 2009, 9:253–263. 28. Lugaresi O, Encontre H, Locatelli C, Minguzzi A, Vertova A, Rondinini S, Comninellis C: Gas-phase volatile organic chloride electroreduction: a versatile experimental setup for electrolytic dechlorination and voltammetric analysis. Electrochem Commun 2014, 44:63–65. 29. Lugaresi O, Perales-Rondón JV, Minguzzi A, Solla-Gullón J, Rondinini S, Feliu JM, Sánchez-Sánchez CM: Rapid screening of silver nanoparticles for the catalytic degradation of chlorinated pollutants in water. Appl Catal B 2015, 163:554–563. 30. Niu X, Shi L, Pan J, Qiu F, Yan Y, Zhao H, Lan M: Modulating the assembly of sputtered silver nanoparticles on screen-printed carbon electrodes for hydrogen peroxide electroreduction: effect of the surface coverage. Electrochim Acta 2016, 199:187–193. 31. Verlato E, He W, Amrane A, Barison S, Floner D, Fourcade F, Geneste F, Musiani M, Seraglia R: Preparation of silver-modified nickel foams by galvanic displacement and their use as cathodes for the reductive dechlorination of herbicides. ChemElectroChem 2016, 3:2084–2092. 32. Back S, Yeom MS, Jung Y: Active sites of Au and Ag nanoparticle catalysts for CO2 electroreduction to CO. ACS Catal 2015, 5:5089–5096. 33. Peng Z, Jiang Z, Huang X, Li Y: A novel electrochemical sensor of tryptophan based on silver nanoparticles/metal–organic framework composite modified glassy carbon electrode. RSC Adv 2016, 6:13742–13748. 34. Fu Y, Su W, Wang T, Hu J: Electrochemical sensor of tryptophan based on an Ag/CP electrode prepared by the filtered cathodic vacuum arc technique. J Electrochem Soc 2016, 163:B107–B112. 35. Jouikov V, Stéphant N, Poizot P, Simonet J: The silver–graphene electrode. Building, stability, and catalytic efficiency. Electrochem Commun 2015, 51:125–128. 36. Jouikov V, Simonet J: Facile immobilization of pyridine at gold, silver, and carbon surfaces. Generation of multi-strata versatile electrodes. Electrochem Commun 2015, 56:20–23. 37. Simonet J: Electrochemical insertion of CO2 into silver in a large extent. Electrochem Commun 2015, 58:11–14.
21. Pasciak EM, Peters DG: Reduction of substituted phenyl 2-chloroacetates at silver cathodes: electrosynthesis of coumarins. J Electrochem Soc 2014, 161:G98–G102.
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