Direct electrochemical reduction of organic halide droplets dispersed in water

Abstract

The direct electroreductive homocoupling of benzyl bromide has been efficiently achieved using water as the solvent. This process does not involve any organic co-solvent, transition metal catalyst, oil, surfactants, but only a cheap and non-toxic supporting electrolyte (KCl). Benzyl bromide droplets dispersed in water were reduced at a low current density, in an undivided cell fitted with a sacrificial aluminum anode. Various cathode materials have been tested (Ni, Pt, C, Ag) to favor organic halide reduction versus hydrogen evolution. Moreover, it was shown that the presence of aluminum cations generated by the oxidation of the anode played a crucial role in the efficiency of the electrochemical reduction step. Surprisingly, this property was exalted in acidic solutions (pH = 1). Under such acidic conditions, both bibenzyl proportion and faradaic yields were considerably improved. The electrochemical activation of energetically stronger C–X bonds such as those encountered in benzyl chloride (C^sp3–Cl) or ethyl 4-iodobenzoate (C^sp2–I) could be also achieved in water though resulted in lower faradaic yields. From a mechanistic point of view, both the faradaic yields and the product distribution obtained under various conditions suggested the occurrence of a radical coupling pathway occurring within the organic droplets or at their surface.

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Introduction

Environmental and health concern promoted a search for suitable alternatives to organic solvents and atom-saving reactants in synthesis.¹ In this context, the combination of electrosynthetic methods with eco-friendly solvents is attractive since the use of electrons as reagents does not involve the formation of any side-products observed when using reducing or oxidizing agents. Direct electrochemistry would also benefit from the absence of inorganic or organometallic catalysts involving expensive or toxic metals and ligands. Several approaches combining electrochemistry and non-toxic fluids have been explored for C–C bond formation.^2–20 One interesting alternative to organic solvents is the use of microemulsions. Microemulsions are stable fluids made from water, oil, and surfactants. They are less toxic and less expensive than most organic solvents² and their properties can be tuned for specific applications by adjusting their composition. The group of J. F. Rusling widely explored their use in electrochemical catalysis.^3–5 For example, they notably examined the catalytic reduction of benzyl bromide (bromomethylbenzene) with macrocylic cobalt complexes in a bicontinuous microemulsion made from didodecyldimethylammonium bromide, water, and dodecane (21/39/40 wt%).⁶ It was shown that microemulsions behaved similarly to usual solvent media (DMF or DMF/H₂O) during benzyl bromide reduction. Indeed, a radical or anionic process could be selected depending on the ligand: bibenzyl (1,1′-ethane-1,2-diyldibenzene) was formed through a radical pathway when the reaction was mediated by vitamin B₁₂ whereas toluene was produced via an anionic process in the presence of Co(I)(salen).⁶

More recently, benzyl bromide has been directly activated (i.e., in the absence of a redox mediator) under galvanostatic conditions, at a graphite electrode, in a bicontinuous microemulsion made of cetyl trimethylammonium bromide, n-hexane, n-butanol, and water in a composition of 17.5, 12.5, 35, 35 wt%, respectively.⁷ Under these conditions, bibenzyl was obtained in a 45.6% yield. Interestingly, the authors also attempted heterolytic coupling between benzyl bromide and some activated olefins such as methylmethacrylate, acrylonitrile, cyclohexanone, or cyclopentanone. Nevertheless, the heterocoupling product yields were extremely modest (2–19% range) which was ascribed to the poor selectivity between the reduction of activated olefins and benzyl bromide.

On the other hand, the use of water as solvent in the presence of a surfactant allowed the electrosynthesis of biaryls from aromatic halides in moderate yields.⁸ The reactions were conducted in the presence of nickel-2,2′-bipyridine as the catalyst. A constant current intensity was applied between a nickel foam cathode and a sacrificial nickel rod anode, which may simultaneously compensate the catalyst loss. The same group showed that the nickel mediated electroreductive coupling of aryl halides could be also efficiently achieved in methanol/ethanol mixtures or in pure ethanol.^9,10 Interestingly this evidenced that electroreduction of the nickel(II) catalyst precursor could be performed within the small potential window allowed by alcohols, and also that the intermediate organonickel species were compatible with the proton activity of alcoholic media.

Room Temperature Ionic Liquids (RTILs) have also been the subject of intense research as alternatives to organic solvents. A wide range of organic syntheses, including catalyzed reactions, have been carried out in these non-flammable ionic media, which are attractive notably for their easy access, thermal stability, low vapor pressure, but also because they allow simple product recovery and recycling.^11–13 A promising approach was developed by R. Barhdadi et al. who showed that direct and nickel-catalyzed electroreductive homocoupling of organic halides, including benzyl bromide, and their coupling with activated olefins could be efficiently conducted by constant current electrolyses in RTILs.¹⁴ However, the use of a small amount of dimethylformamide (DMF; 5–10%) was required to decrease viscosity and increase conductivity, but also to solubilize the nickel complex catalyst. Similar electrochemical reductive dimerizations were also achieved in neat ionic liquids, without any need to add extra solvents.^15,16 Though the reactions were catalyzed by NiCl₂(bipy) or palladium nanoparticles which may be toxic, costly, and, in some cases, sensitive to oxygen or water.

Recently, it has been established that electrochemical dimerization of aromatic bromides is possible in a RTIL without catalysts or extra solvent.¹⁷ This process relies on the use of a silver cathode which acts electrocatalytically through the dramatic reduction overpotential of organic halides due to the silver surface atoms.^18,19,20 Electrolyses were carried out in an undivided electrochemical cell at constant current intensity with a magnesium rod sacrificial anode and a silver cathode. Under these conditions, the electroreduction of benzyl bromide and bromobenzene led to the production of bibenzyl and biphenyl in 61% and 12% yields, respectively.

Herein we wish to show for the first time that the direct electrochemical reduction of carbon-halide bonds is possible in water, when the organic halide is dispersed mechanically as droplets in the aqueous phase, without the use of any organic co-solvent, transition metal catalyst, oil, or surfactant. The process consisted of applying a small current intensity in an undivided cell fitted with a sacrificial aluminum rod anode and a cathode (Ni, Pt, C, or Ag). This allowed the electrochemical activation of benzyl bromide, benzyl chloride, or ethyl 4-iodobenzoate. The role of the nature of both the cathode and the anode as well as the possible mechanism involved in this original process are discussed.

Experimental section

Chemicals

Water was highly purified (resistivity = 18 MΩ cm⁻¹; Mill-Q system; Millipore, Billerica, MA, USA). Potassium chloride, KCl (Acros), was used as the supporting electrolyte at a concentration of 0.1 M. Benzyl bromide (Fluka), benzyl chloride (Aldrich), and ethyl 4-iodobenzoate (Acros) were used as received. Methylmethacrylate (MMA) and styrene were purchased from Acros Chemicals. Prior to use, they were distilled under reduced pressure to remove the stabilizer. If not immediately used, the distilled monomers were stored at −30 °C under argon. Hydrochloric acid (Acros) was used to acidify the reaction mixture. Dichloromethane and dodecane (both from Acros) were used to extract the organic compounds from the water phase and as the internal standard, respectively.

Instrumentation

Electrolyses were performed in an undivided cell equipped with a hollowed cylindrical nickel foam cathode (Goodfellow, Surface of c.a. 6 cm²) surrounding a sacrificial aluminum rod anode (diameter 1 cm). In some experiments, a platinum grid (Goodfellow, 6 cm²), a carbon cloth (Carbon Lorraine, 6 cm²), or a silver grid (Goodfellow, 6 cm²) were used as the cathode. Galvanostatic electrolyses were carried out with a power source (TTi–QL355 Power Supply) by applying a current intensity of 10 mA between the anode and the cathode.

Typical procedure

In a typical experiment, the organic halide (1 mmol.) was added to the undivided electrochemical cell containing water (25 mL), KCl (0.1 M), and dodecane (300 μL) was used as the internal standard. The electrolysis was conducted under argon by setting a constant current intensity of 10 mA between the cathode (nickel foam, platinum grid, carbon cloth, or silver grid) and a sacrificial aluminium bar anode, under vigorous stirring. The crude electrolytic solution was treated with dichloromethane (10 mL) to extract the organic compounds which were detected and quantified by gas chromatography (Varian, GC3900 model) using internal standards and authentic products.

Results and discussion

Direct electroreductive coupling of benzyl bromide

In a typical experiment, benzyl bromide (6.3 mmol.) was added to an undivided electrochemical cell containing water (25 mL). To ensure proper conductivity, potassium chloride (0.1 M) was also added. The ensuing mixture of PhCH₂Br dispersed droplets in the H₂O electrolyte was then electrolyzed for 24 h at a constant current intensity (10 mA) applied between a nickel foam cathode and a sacrificial aluminum bar anode. Under these conditions, the starting benzyl bromide compound was almost totally consumed, after a charge of 1.4 Faraday mol⁻¹ was passed. The GC analysis (Table 1, Entry 1) showed that bibenzyl (1,1′-ethane-1,2-diyldibenzene) was produced in a 76% yield with respect to the starting benzyl bromide. The rather good faradaic yield (53%) suggested that hydrogen evolution was moderate under these conditions. Surprisingly, only a small amount of toluene (methylbenzene) (2.5%) was recovered at the end of the electrolysis (it should be noted that since argon was bubbled all along the experiment its yield may likely be underestimated due to volatility). Indeed a yield of 17% (since 83% of non-volatile compounds were obtained) in toluene was expected. Interestingly, a small amount of benzyl alcohol (phenylmethanol) was also obtained. Traces of benzaldehyde (phenylmethanal) suggested a possible oxidation of benzyl alcohol at the anode. Finally, traces of benzylchloride (chlorophenylmethane) were obtained which may have arisen from a nucleophilic substitution involving the comparatively large concentration of chloride anions from the supporting electrolyte.

Table 1 Electrolyses of benzylbromide micro-dispersed mechanically in water, under galvanostatic conditions, in undivided or divided cell and as a function of various parameters such as: benzyl halide concentration, the nature of the cathode material, the pH of the solution, and with or without the presence of styrene

Entry	Cathode/anode	Specificity	PhCH2Br (%)^c	PhCH3 (%)^c, ^e	PhCHO (%)^c	PhCH2Cl (%)^c	PhCH2OH (%)^c	(PhCH2)2 (%)^c	(PhCH2)2/PhCH2Brconv. (%)^c,^d	Faradaic Yield (%)^g
a Water (25 mL) + KCl (0.1 mol. L^−1) + PhCH2Br (6.3 mmol.); I = 10 mA; electric charge = 864 C (1.4 F mol^−1 of PhCH2Br). b Water (25 mL) + KCl (0.1 mol. L^−1) + PhCH2Br (1 mmol.); I = 10 mA; electric charge = 153 C (1.6 F mol^−1 of PhCH2Br). c CPG yields determined in the presence of dodecane as internal standard. d Yield of bibenzyl calculated with respect to converted PhCH2Br. e The yield of toluene is very likely underestimated for volatility reasons. f The solutions of both the cathodic and anodic compartments were grouped for the CPG analysis. g Faradaic yield determined for the production of (PhCH2)2.
1^a	Ni/Al		1.5	2.5	< 1	< 1	4.5	75	76	53
2^b	Ni/Al		53	4	2	2	10	18.5	39.5	11.7
3^b	Pt/Al		63	1.5	1	3	7.5	12	32.5	7.6
4^b	C/Al		26	7.4	2	2	8.5	33	44.5	20.8
5^b	Ag/Al		< 1	5	< 1	< 1	3	75	75	47.3
6^b^f	Ni/Al	Divided cell	62	7	2.5	3.5	20	5	13	3.2
7^b	Ni/Al	+ 0.5 mL HCl (pH = 1)	1.5	7.5	1.5	2	7.5	50	51	31.5
8^b	Ni/Al or Ni	+ NiBr2 (1 mmol.)	65.7	8	0.2	1.9	7.5	16.7	48.7	10.5
9^b	Ni/Al	+ 30 mmol. Styrene	96.7	0.75	1.2	0.2	0.4	0.75	22.8	0.5

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The unexpected high yield for benzyl bromide dimerization in such a PhCH₂Br droplets/H₂O mixture led us to further investigate the mechanism that is inevitably different from that usually described in organic solvent (Scheme 1).

Scheme 1 Reduction of benzyl bromide in organic solvents.

For convenience reasons, a reference experiment of the electrolysis of only 1 mmol. of benzyl bromide was performed under the same conditions as those described for the first experiment, and was stopped after a charge of 1.6 Faraday mol⁻¹ was passed (Entry 2). Under these conditions, a lower faradaic yield was obtained when compared to the first experiment, since only half of the starting compound was converted, most likely due to a more efficient contribution of proton reduction under diluted conditions. As a consequence, the production of bibenzyl (with respect to the converted benzyl halide) was about half that obtained under more concentrated conditions (Compare Entries 1 and 2). This is in agreement with the expected influence of the concentration of the reactants on the rate of bi-molecular reactions leading to the dimer.²¹

The effect of the cathode material was first evaluated by comparing the outcome of three otherwise identical electrolyses performed in the presence of either a nickel foam, a platinum grid, or a carbon cloth (Table 1, Entries 2–4). As expected, the balance between proton and benzyl bromide reduction depended on the cathode material. The highest faradaic yields for both the consumption of benzyl bromide and the production of bibenzyl were thus obtained in the presence of a carbon cloth. Carbon is expected to display the highest overpotential towards proton reduction, compared to nickel and platinum (C > Ni > Pt). On the other hand, taking into account the special catalytic properties of silver towards the reduction of organic halides in non-aqueous solvents^18–20 an additional electrolysis was performed in the presence of a silver grid as the cathode (Table 1, Entry 5). That electrolysis gave the highest selectivity and bibenzyl was obtained with the highest faradaic yield (compare Entry 5 with Entries 2–4) demonstrating that the remarkable electrocatalytic properties of silver²⁰ apply under our conditions.

The role played by the aluminum salts produced all along the electrolyses at the sacrificial anode was then investigated (Table 1, Entry 2). Nickel was chosen as the cathode material even if the best results, in terms of bi-benzyl production, were obtained in the presence of silver or carbon cathode materials. When the electrolysis of benzyl bromide was performed in a divided electrochemical cell, the benzyl bromide conversion was slightly lower than that obtained when the anodic and the cathodic compartment were not separated (Compare Entries 2 and 6). Benzyl alcohol was the main product and the yield of bibenzyl produced with respect to the consumed benzyl bromide was about a third of that obtained for an electrolysis performed in an undivided cell. These results suggested that benzyl alcohol is formed through nucleophilic substitution involving the hydroxyl anions formed by the reduction of water. This reaction would be indeed disfavored in the presence of aluminum salts since the hydroxyl anions would be scavenged by Al³⁺ to produce Al(OH)₃.

Unexpectedly, the electrolysis of benzylbromide under acidic conditions (pH = 1) in an undivided cell and a nickel cathode, improved both the reduction and the homocoupling processes (Table 1, Entry 7). Accordingly, the reduction of benzyl bromide was almost complete after 1.6 Faraday mol⁻¹ and bibenzyl was produced in a 51% yield with respect to the converted organic halide. This suggests a possible activation of the C–Br bond by Al³⁺, which would act as a Lewis acid. However, other hypotheses may also be invoked. For instance, the acidity could prevent a possible passivation of the electrode.

At this stage, it is important to mention that the catalytic activation of benzyl bromide by nickel salts coming from a possible acidic attack of the nickel foam cathode can be disregarded. Indeed, an electrolysis performed in the presence of a nickel salt (NiBr₂) (Table 1, Entry 8), did not exalt the electrochemical process, even in the presence of a nickel sacrificial anode generating nickel salts all along the experiment (Compare Entries 2 and 8). Instead, the benzyl bromide conversion remained low, the majority of electricity passed was engaged in nickel salt reduction, which took place at a less negative potential value than that of the halogenated derivative.

Extension to the electrochemical activation of C–Cl and C–I bonds

The very good yields obtained for the electrochemical activation of benzyl bromide in water prompted us to perform electrolyses of benzyl chloride (chlorophenylmethane) and ethyl 4-iodobenzoate which possess energetically stronger carbon–halide bonds.

The corresponding results are summarized in Table 2. The conversion of benzyl chloride was lower than that of benzyl bromide even under concentrated conditions (Compare Table 2, Entry 1 and Table 1, Entry 1). Nevertheless, 8.5% of bibenzyl was formed with respect to the converted benzyl chloride. Interestingly, similar results were obtained under diluted condition but at low pH value suggesting that acidic conditions counterbalanced the dilution effect (Table 2, Entry 2).

Table 2 Electrolyses of benzyl chloride and ethyl 4-iodobenzoate in H₂O + KCl (0.1 M) performed at a constant current of 10 mA in an undivided cell

Entry	RCl (R = PhCH2) or R′I (R′ = EtCO2Ph)	Cathode/Anode	Charge	R′I (%)^a	RH or R′H (%)^a	ROH (%)^a	RCl (%)^a	RR or R′R′ (%)^a	RR/RClconv. or R′H/R′Iconv. (%).	Faradaic Yield (%)
a CPG yields determined in the presence of dodecane as internal standard. b The yield of toluene is very likely underestimated because of its volatility. c In the presence of HCl (0.5 mL—pH = 1). d Faradaic yield determined for the production of RR. e Faradaic yield determined for the production of R′H.
1	PhCH2Cl (6.3 mmol.)	Ni/Al	864 C (1.4 F mol^−1 of RCl)		2^b	2	41.5	5	8.5	3.5^d
2^c	PhCH2Cl (1 mmol.)	Ni/Al	153 C (1.6 F mol^−1 of RCl)		6^b	2.5	40	4	6.7	2.5^d
3	EtCO2PhI (1 mmol.)	C/Al	864 C (9 F mol^−1 of ArI)	89	10			traces	91	1.1^e
4	EtCO2PhI (1 mmol.)	Ag/Al	153 C (1.6 F mol^−1 of ArI)	90	10				100	6.3^e
5^c	EtCO2PhI (1 mmol.)	C/Al	864 C (9 F mol^−1 of ArI)	99	traces

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The activation of ethyl 4-iodobenzoate could be also observed in water but led to very poor faradaic yields. The reduction of this aromatic halide was carried out in the presence of either a carbon cloth or a silver grid as the cathode. As already observed with benzyl bromide, the use of a silver cathode increased the proportion of the aryl halide reduction with respect to proton reduction (compare Entries 3 and 4 in Table 2). In both cases, 10% of the starting iodide compound was converted into the corresponding 2e⁻ reduction product, but the presence of the silver cathode provided higher faradaic yields. Unlike PhCH₂Br, the activation of ethyl 4-iodobenzoate was not affected by the acidity of the medium (Entry 5) since the conversion of the iodide remained minor even after 24 h, proton reduction being the main cathodic reaction in this case.

Mechanistic considerations

In organic solvents such as dimethylformamide (DMF) or acetonitrile, direct electrolyses of benzyl bromide yielded toluene as the main product,^22,23via the protonation of the corresponding benzyl anion.^23–25 Under these conditions, the reduction of benzyl bromide occurs via a concerted electron transfer–bond cleavage mechanism, as depicted in Scheme 1. Accordingly, the first electron transfer is followed by the rapid reduction of the electrogenerated benzyl radical. Indeed, the standard potential of the ArCH₂^•/ArCH₂⁻ couple is more positive than that of the starting halide compound. This prevents a radical–radical coupling leading to bibenzyl.²¹ Toluene is then produced via the immediate protonation of the basic benzyl carbanion from either residual water or the solvent itself (AH in Scheme 1) and no S_N2 reaction between the carbanion and the parent substrate is observed. This is not surprising since the pK_a of toluene is more than 10 orders of magnitude greater than that of water.

From a mechanistic point of view, bibenzyl formation from the electrochemical reduction of benzyl bromide or benzyl chloride droplets dispersed in water may proceed either through a radical–radical coupling of the electrogenerated benzyl radicals, or via an S_N2 reaction of the benzyl anion on the starting halogenated molecule. Long-lived benzyl anions cannot be envisioned in water (even more in acidic solutions) but may be formed at the periphery of the hydrophobic benzyl halide droplets whenever the benzyl halide is reduced without leaving the droplet.^26,27 On the other hand, the hydrophobic benzylic radicals should be more stabilized in the organic droplet than in the aqueous electrolyte. If so, direct reduction of benzyl halide droplets at the cathode surface may generate high local concentrations of reduced intermediates (radicals and/or anions) within the droplets near their surface,²⁷ thus enhancing the kinetics of 2nd order processes (among which feature S_N2 or dimerization) since it is not clear if bibenzyl results from a radical or a nucleophilic pathway. In this context, it is worth mentioning that a cyclic voltammetry study of benzyl bromide microdispersions in water at a glassy carbon electrode, performed under conditions identical to those used for preparative electrolyses, established that no adsorption of the organic halide droplets occurred on the electrode surface. Hence, the reaction does not proceed within droplets permanently adsorbed as considered in previous works^26,27 but most likely through a series of rapid droplet/electrode contacts as observed for multi-charged redox dendrimers.

In order to get further insight in the dimerization mechanism, an electrolysis was first performed in the presence of benzyl bromide and methylmethacrylate (MMA). If radical intermediates were to be involved, one would expect the activated olefin to trap the radicals and consequently decrease the yield in bibenzyl. For that purpose, the additive was used in large excess (30 molar equivalents with respect to benzyl bromide). Nevertheless, MMA was partially soluble in water and was reduced at a more positive potential value than benzyl bromide (data not shown).²⁸ This led us to replace MMA by styrene (Table 1, Entry 9). Under these conditions (large excess of styrene) a small amount of the starting halide compound could be activated. The low R–R/RX_conv ratio observed suggests that a part of the electrogenerated benzyl radicals reacted with styrene. Therefore, at these low concentration levels no coupling product could be detected.

Conclusion

This work established that the electrochemical reduction of organic halides such as benzyl bromide, benzyl chloride, and ethyl 4-iodobenzoate mechanically micro-dispersed in water is possible using very simple experimental conditions (no co-solvent, no surfactant nor any solubilizing agent) by applying a small constant current between a cathode and a sacrificial aluminum anode. Compared to other approaches developed in aqueous media, this process featured the cheapest, most environmentally-friendly and most straightforward conditions used in the field of the electrochemical activation of organic halides.

As expected, the nature of the cathode material was of high importance to minimize hydrogen evolution and increase faradaic yields. Besides, it was demonstrated that aluminum salts, which were electrogenerated by the oxidation of the sacrificial anode, played a crucial role in the efficiency of the electrochemical reduction step. Interestingly, this property was exalted in acidic media. From the mechanistic point of view, the results obtained in the presence of styrene afforded evidences for a radical pathway for the dimerization process.

Acknowledgements

This work was supported in part by the Centre National de la Recherche Scientifique (UMR CNRS-ENS-UPMC 8640) and the Ministère de la Recherche (Ecole Normale Supérieure). The French Ministry of Research (MESR) is also thanked for supporting the PhD grant of ED.

References

1. L. Á. Cienfuegos, R. Robles, D. Miguel, J. Justicia and J. M. Cuerva, ChemSusChem, 2011, 4, 1035.
2. S. Friberg, Adv. Colloid Interface Sci., 1990, 32, 167.
3. J. F. Rusling, in: Modern Aspects of Electrochemistry, ed. B. E. Conway and J. O'M. Bockris, No. 26, Plenum, New York, 1994, pp. 49–104.
4. J. F. Rusling, in: Electroanalytical Chemistry, ed. A. J. Bard, Vol. 18, Marcel Dekker, New York, 1994, 1–88.
5. J. F. Rusling and D.-L. Zhou, J. Electroanal. Chem., 1997, 439, 89.
6. D. L. Zhou, H. Carrero and J. F. Rusling, Langmuir, 1996, 12, 3067.
7. R. Sripriya, M. Chandrasekaran and M. Noel, J. Appl. Electrochem., 2008, 38, 597.
8. F. Raynal, R. Barhdadi, J. Périchon, A. Savall and M. Troupel, Adv. Synth. Catal., 2002, 344, 45.
9. V. Courtois, R. Barhdadi, M. Troupel and J. Périchon, Tetrahedron, 1997, 53, 11569.
10. V. Courtois, R. Barhdadi, S. Condon and M. Troupel, Tetrahedron Lett., 1999, 40, 5993.
11. Ionic Liquids Industrial Application of Green Chemistry, ed. R. D. Rogers and K. R. Seddon, A.C.S. Symposium Series, No 818, 2002, and references cited therein.
12. Ionic Liquids in Synthesis, ed. P. Wasserscheid and T. Welton, Chichester, Wiley Europe, 2003 and references cited therein.
13. H. O. Bourbigou and L. Magna, J. Mol. Catal. A: Chem., 2002, 182, 419 and references cited therein.
14. R. Barhdadi, C. Courtinard, J.-Y. Nédélec and M. Troupel, Chem. Commun., 2003, 1434.
15. M. Mellah, S. Gmouh, M. Vaultier and V. Jouikov, Electrochem. Commun., 2003, 5, 591.
16. L. D. Pachon, C. J. Elsevier and G. Rothenberg, Adv. Synth. Catal., 2006, 348, 1705.
17. D. Niu, A. Zhang, T. Xue, J. Zhang, S. Zhao and J. Lu, Electrochem. Commun., 2008, 10, 1498.
18. S. B. Rondinini, P. R. Mussini, P. Muttini and G. Sello, Electrochim. Acta, 2001, 46, 3245.
19. C. Bellomunno, D. Bonanomi, L. Falciola, M. Longhi, P. R. Mussini, L. M. Doubova and G. Di Silvestro, Electrochim. Acta, 2005, 50, 2331.
20. Y. F. Huang, D. Y. Wu, A. Wang, B. Ren, S. Rondinini, Z. Q. Tian and C. Amatore, J. Am. Chem. Soc., 2010, 132, 17199.
21. C. Amatore and J.-M. Savéant, J. Electroanal. Chem., 1981, 125, 1.
22. M. D. Hawley, Acyclic Aliphatic Halides. In Encyclopedia of Electrochemistry of the Elements, Organic Section; ed. A. J. Bard and H. Lund; Marcel Dekker: New York, 1980, 14, pp 83–103.
23. O. R. Brown, H. R. Thirsk and B. Thornton, Electrochim. Acta, 1971, 16, 495.
24. M. M. Baizer and J. L. Chruma, J. Org. Chem., 1972, 37, 1951.
25. C. P. Andrieux, A. L. Gorande and J.-M. Savéant, J. Am. Chem. Soc., 1992, 114, 6892.
26. C. E. Banks, T. J. Davies, R. G. Evans, G. Hignett, A. J. Wain, N. S. Lawrence, J. D. Wadhawan, F. Marken and R. G. Compton, Phys. Chem. Chem. Phys., 2003, 5, 4053.
27. C. Amatore, A. Oleinick and I. Svir, J. Electroanal. Chem., 2005, 575, 103.
28. V. Bonometti, E. Labbé, O. Buriez, P. Mussini and C. Amatore, J. Electroanal. Chem., 2009, 633, 99.

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