Electrocatalytic carboxylation of benzyl chlorides at silver cathodes in acetonitrile

Article abstract of DOI:10.1039/b206746c

Silver exhibits powerful electrocatalytic activities towards the reductive carboxylation of benzyl chlorides (RCl): in CO₂-saturated CH₃CN, reduction of RCl occurs at potentials that are about 0.6 V more positive than those of the same process at Hg or carbon electrodes and gives carboxylic acids in good to excellent yields.

Full text of DOI:10.1039/b206746c

Electrocatalytic carboxylation of benzyl chlorides at silver cathodes in
acetonitrile
Abdirisak A. Isse and Armando Gennaro*
Dipartimento di Chimica Fisica, Università di Padova, via Loredan 2, 35131 Padova, Italy.
E-mail: A.Gennaro@chfi.unipd.it; Fax: +39 049 8275135; Tel: +39 049 8275132
Received (in Cambridge, UK) 11th July 2002, Accepted 8th October 2002
First published as an Advance Article on the web 24th October 2002
Silver exhibits powerful electrocatalytic activities towards
the reductive carboxylation of benzyl chlorides (RCl): in
CO2-saturated CH3CN, reduction of RCl occurs at poten-
tials that are about 0.6 V more positive than those of the
same process at Hg or carbon electrodes and gives carboxylic
acids in good to excellent yields.
more positive than that of CO2 and becomes even more positive
when the solution is saturated with CO2. Voltammograms
similar to those shown in Fig. 1 were obtained for all benzyl
chlorides (RCl). The position of the peak, however, depends
strongly on the electrode material. The data obtained at Ag, Hg
and glassy carbon (GC) electrodes are collected in Table 1. The
data obtained on Ag/Pt (not shown in the Table) were the same,
within experimental error, as those reported for Ag. Whereas
the presence of CO2 had no effect on the peak potential (Ep) of
RCl at GC or Hg, significant positive shifts were observed at the
silver electrodes. The last column shows Ep values obtained at
Ag in CO2-saturated CH3CN.
Electron transfer to organic halides, RX, involves rupture of
the carbon–halogen bond.10 The electrode may get involved in
interactions with RX or its reduction intermediates and
products. Because of its inertness to the species involved in the
reaction, GC has been suggested as the best electrode material
for the investigation of the reduction mechanism of organic
halides. We may, therefore, use this electrode as a reference
material to examine the electrocatalytic properties of the
electrodes towards the reduction of RCl. The data reported in
Table 1 show that similar Ep values were obtained at GC and
Hg, suggesting that, for both electrodes, surface processes do
not play an important role in the reaction.11 Instead, the Ep
values obtained at Ag are considerably more positive than those
measured at GC. Such a great difference of Ep between the two
The electrochemical reduction of benzyl chlorides in CO2-
saturated solvents has been extensively investigated as a
synthetic route to carboxylic acids.1 In fact, this method of
synthesis may provide a valuable alternative route to the
manufacture of 2-arylpropanoic acids, which are of great
interest in the pharmaceutical industry.2 A drawback to the use
of benzyl chlorides as starting material is, however, that their
reduction at the most commonly used cathodes occurs at very
negative potentials, where concomitant reduction of CO2 may
take place, resulting in undesired products and a decrease of
current efficiency. This has prompted the search for suitable
catalytic systems capable of mediating the electrocarboxylation
process.
Several nickel and palladium complexes with phosphine
ligands,3 as well as some square-planar cobalt4 and nickel5
complexes, have been used as homogeneous catalysts for the
electrochemical carboxylation of various benzyl chlorides.
Electrogenerated organic radical anions, acting as outer sphere
electron transfer agents, have also been used.5,6 Although good
current efficiencies were obtained with many of these catalytic
systems, only moderate to low turnover numbers were achieved.
A high turnover number has been reported only in one case
where a zeolite-encapsulated cobalt complex was used.4c
A very interesting type of catalysis in electrochemistry is one
in which the electrode, besides being a source or a sink for
electrons, behaves as a catalyst for the redox reaction. Several
electrode materials, e.g., Pt, Ni, Au, stainless steel and graphite,
have been used for the electrocarboxylation of benzyl chlorides.
Although the role of the cathode was not investigated in depth,
it seems that none of the electrodes so far tested has a particular
electrocatalytic effect towards the reductive carboxylation of
the halides. Very recently, a series of papers dealing with the
reduction of organic halides at silver cathodes has appeared.7
The most important result emerging from such studies is that
silver exhibits extraordinary electrocatalytic activities towards
the reduction process, especially in the case of bromides and
iodides. With the exception of a study on the carboxylation of
some aromatic halides, which gave only poor to moderate
yields,8 the possibility of exploiting the electrocatalytic nature
of silver in electrocarboxylations has not been explored yet.
Here we report the results of an investigation on the
potentialities of silver as an electrocatalyst in the carboxylation
of benzyl chlorides. The process was studied in CH3CN + 0.1 M
Et4NClO4 at Ag, Hg and carbon cathodes at 25 °C.9 Two types
of silver electrode were used: one made from the bulk metal and
the other (denoted Ag/Pt) prepared by electrodeposition of Ag
on Pt. On all three electrodes, a single irreversible peak is
observed in cyclic voltammetry. Fig. 1 shows cyclic voltammo-
grams recorded at Ag for the reduction of PhCH2Cl in the
absence and presence of CO2, as well as for the reduction of
CO2 alone. It is noteworthy that Ep of PhCH2Cl is about 0.5 V
Fig. 1 Cyclic voltammograms recorded at 0.2 V s21 in CH3CN + 0.1 M
Et4NClO4 at a Ag electrode. (a) 3 mM PhCH2Cl, (b) as (a) + 0.28 M CO2,
(c) 4 mM CO2.
Table 1 Voltammetric data for the reduction of benzyl chlorides (3 mM) and
CO2 (4 mM) in CH3CN + 0.1 M Et4NClO4 at v = 0.2 V s21
GC
Hg
Ag
Ep/
Ep/
Ep/
Ep (CO2)a/
Compound
V vs. SCE V vs. SCE V vs. SCE V vs. SCE
CO2
PhCH2Cl
4-CF3C6H4CH2Cl
4-CH3OC6H4CH2Cl
PhCH(CH3)Cl
22.41
22.25
21.99
22.26
22.23
22.46
22.27
21.97
22.31
22.29
22.34
21.79
21.67
21.82
21.81
21.52
21.48
21.52
21.53
a In the presence of 0.28 M CO2.
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CHEM. COMMUN., 2002, 2798–2799
This journal is © The Royal Society of Chemistry 2002

electrodes points to a significant electrocatalytic effect of the
silver electrode towards the reduction of RCl.
carboxylic acids in very high yields without passivation of the
electrode. Silver has shown good catalytic properties in the
electroreduction of a variety of organic halides and it is fairly
likely to be a good cathode material for their electrocarboxyla-
tion. The excellent results reported here for the benzyl chlorides
recommend a broad research in that direction.
This work was supported by the Consiglio Nazionale delle
Ricerche (CNR) and the Ministero dell’Istruzione, dell’Uni-
versità e della Ricerca (MIUR).
The extraordinary electrocatalytic effect of Ag is probably
related to the high affinity of the metal for halide ions, which is
well documented in the literature.12 As reported earlier for
various alkyl halides,7b,d it is very likely that reduction of RCl
at Ag involves interaction of both RCl and its reduction
intermediates and products with the electrode. Such interactions
can affect both the thermodynamics and kinetics of the process.
Although adsorption of the products has a thermodynamic
advantage resulting in a positive shift of the reduction potential,
kinetic effects probably play a major role in the electrocatalytic
process. In fact, interaction of RCl with Ag with the formation
of an activated complex of the form R…Cl…Ag may decrease
significantly the great overpotential associated with the dis-
sociative reduction of the carbon–halogen bond.
The most relevant outcome of the voltammetric investigation
is that, at Ag cathodes, reduction of RCl occurs at potentials
considerably more positive than Ep of CO2. In the least
favourable case, the difference between Ep of CO2 and that
measured for RCl in the presence of CO2 is 0.51 V (Table 1).
This provides a potential window save enough for electro-
carboxylation of RCl to be performed without any interference
from reduction of CO2. Controlled-potential electrolyses were
carried out in CO2-saturated solutions containing ca. 50 mM
RCl (1 mmol) in an undivided cell with an Al sacrificial anode.
Both Ag and Ag/Pt electrodes were used. Also some experi-
ments were performed at Hg and graphite electrodes for
comparison. In each case, the electrolysis was carried out at a
potential just beyond Ep of RCl and was stopped after total
conversion of the halide was achieved. At the end of the
electrolysis, identification and quantification of the products
was done by HPLC. The results are reported in Table 2. The
principal product was always the corresponding carboxylic acid
(RCO2H) although its yield depends, to some extent, on the
experimental conditions. At silver electrodes, whether bulk or
coated on Pt, excellent results both in terms of chemical yields
and current efficiencies were obtained.13 Substituents on the
phenyl ring do not strongly modify the product distribution,
although, as a general trend, the yield of RH increases with the
electron-donating power of the substituent. At graphite and Hg
electrodes (entries 4, 5 and 10), poor current efficiencies
( < 50%) were obtained. Owing to the very negative potentials
required for the reduction of RCl at these electrodes, concomi-
tant reduction of CO2 to give oxalate, CO and carbonate, as
previously reported,14 probably takes place resulting in a high
consumption of charge.
Notes and references
1 G. Silvestri, S. Gambino, G. Filardo and A. Gulotta, Angew. Chem., Int.
Ed. Engl., 1984, 23, 979; O. Sock, M. Troupel and J. Perichon,
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G. Jacob and J. P. Juhasz, J. Appl. Electrochem., 1989, 19, 345; A.
Sasaki, H. O. Kudoh, H. Senboku and M. Tokuda, in, Novel Trends in
Electroorganic Synthesis, ed. S. Torii, Springer, Tokyo, 1998, p. 245.
2 J. D. Genders and D. Pletcher, Chem. Ind., 1996, 682.
3 J. F. Fauvarque, A. Jutand and M. Francois, New J. Chem., 1986, 10,
119; J. F. Fauvarque, A. Jutand and M. Francois, J. Appl. Electrochem.,
1988, 18, 109; J. F. Fauvarque, Y. de Zelicourt, C. Amatore and A.
Jutand, J. Appl. Electrochem., 1990, 20, 338; J. Damodar, S. R. K.
Mohan and S. R. J. Reddy, Electrochem. Commun., 2001, 3, 762.
4 (a) C. Folest, J. M. Duprilot, J. Perichon, Y. Robin and J. Devynck,
Tetrahedron Lett., 1985, 26, 2633; (b) A. A. Isse, A. Gennaro and E.
Vianello, J. Chem. Soc., Dalton Trans., 1996, 1613; (c) A. A. Bessel and
D. R. Rolison, J. Am. Chem. Soc., 1997, 119, 12673; (d) G. Zheng, M.
Stradiotto and L. Li, J. Electroanal. Chem., 1998, 453, 79; (e) W.-H.
Chung, P. Guo, K.-Y. Wong and C.-P. Lau, J. Electroanal. Chem.,
2000, 486, 32.
5 A. Gennaro, A. A. Isse and F. Maran, J. Electroanal Chem., 2001, 507,
124.
6 O. Scialdone, G. Filardo, A. Galia, D. Mantione and G. Silvestri, Acta
Chem. Scand., 1999, 53, 800.
7 (a) S. Rondinini, P. R. Mussini, G. Sello and E. Vismara, J.
Electrochem. Soc., 1998, 145, 1108; (b) S. Rondinini, P. R. Mussini, F.
Crippa and G. Sello, Electrochem. Commun., 2000, 2, 491; (c) S.
Rondinini, P. R. Mussini and M. Specchia, J. Electrochem. Soc., 2001,
148, D102; (d) S. Rondinini, P. R. Mussini, P. Muttini and G. Sello,
Electrochim. Acta, 2001, 46, 3245.
8 C. L. Scortichini and S. J. Babinec, J. Electroanal. Chem., 1994, 379,
111.
9 For voltammetric investigations a silver disk of ~ 0.5 mm diameter, a Pt
sphere coated with silver, a glassy carbon disk of ~ 3 mm diameter and
a Hg sphere were used. Before use, the Ag and GC disks were polished
with a 0.25 mm diamond paste and ultrasonically rinsed in CH3CN and
ethanol, respectively. For macro-scale electrolyses a Ag foil (12 cm2), a
Hg pool (8 cm2), a compact graphite rod (6 cm2) and a Ag-coated Pt
plate (2.2 cm2) were used; the thickness of the Ag film on the latter was
~ 30 mm. The preparation of the Hg microelecrode and the Ag-coated Pt
electrodes was based on electrodeposition of Ag on Pt from aqueous
KAg(CN)2 solutions, as previously described.9a (a) A. A. Isse, A.
Gennaro and E. Vianello, J. Electroanal. Chem., 1998, 444, 241.
10 See, for example, J.-M. Savéant, in, Advances in Electron Transfer
Chemistry, ed. P. S. Mariano, JAI press, Greenwich, 1994, vol. 4, p. 53
and the references cited therein.
In conclusion, we have shown that electrocarboxylation of
benzyl chlorides can be successfully achieved without resorting
to the use of homogeneous catalysts. The process at Ag cathodes
occurs at potentials similar to, or even more positive than, those
of the most efficient catalysts so far reported3–5 and gives
Table 2 Electrocarboxylation of RCl in CH3CN + 0.1 M Et4NClO4
11 The reduction of many organic halides, especially bromides and iodides,
at Hg involves organomercury compounds arising from adsorption of
the intermediate radicals on the electrode, see: D. G. Peters, in, Organic
Electrochemistry, eds. H. Lund and M. M. Baizer, Dekker, New York,
1991, p. 361.
12 See, for example, G. Valette, A. Hamelin and R. Parsons, Z. Phys.
Chem., 1978, 113, 71; M. J. Weaver, J. T. Hupp, F. Barz, J. G. Gordon
and M. R. Philpott, J. Electroanal. Chem., 1984, 160, 321; M. L. Foresti,
M. Innocenti, F. Forni and R. Guidelli, Langmuir, 1998, 14, 7008; J. E.
Pemberton and A. Shen, Phys. Chem. Chem. Phys., 1999, 1, 5671.
13 An exception is when the experiment was conducted in a divided cell
(entry 2). In that case, only a moderate yield of phenylacetic acid was
obtained together with PhCH2CO2CH2Ph arising from esterification of
the carboxylate with PhCH2Cl. Owing to the stabilisation of the
carboxylate ions by the Al3+ cations arising from the dissolving anode,
the undesirable esterification reaction is avoided when an undivided cell
is used.
Product yieldsc (%)
nb
RH
6
RCO2H
a
Entry RCl
Cathode Eapp
1
2d
3
4
5
6
7
8
9
PhCH2Cl
PhCH2Cl
PhCH2Cl
PhCH2Cl
PhCH2Cl
4-CF3C6H4CH2Cl
4-CH3OC6H4CH2Cl Ag
PhCH(CH3)Cl
PhCH(CH3)Cl
PhCH(CH3)Cl
Ag
Ag
Ag/Pt
C
Hg
Ag
21.65 2.11
94
53
90
81
90
95
79
80
81
82
21.65 2.00 18
21.65 2.01
5
22.30 4.49 16
22.24 3.70
21.55 2.32
4
2
21.70 2.26 16
21.68 2.01 15
Ag
Ag/Pt
C
21.68 2.00
6
10
22.29 5.59 14
a Applied potential (V vs. SCE). b e2 per molecule of RCl. c Yield = 100 3
(moles of product/moles of RCl). d A two-compartment cell was used; 10%
benzyl phenylacetate, which accounts for 20% of the starting RCl, was also
obtained.
14 S. Ikeda, T. Takagi and K. Ito, Bull Chem. Soc. Jpn., 1987, 60, 2517; A.
Gennaro, A. A. Isse, M.-G. Severin, E. Vianello, I. Bhugun and J.-M.
Savéant, J. Chem. Soc., Faraday Trans., 1996, 92, 3963.
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