Electrochemical coupling reactions of benzyl halides on a powder cathode and cavity cell

Article abstract of DOI:10.1016/j.electacta.2010.09.007

Benzyl halides adsorbed on a cathode made of pure or silver-doped graphite powder were reduced in an undivided cell using an aqueous electrolyte and an inert anode, in the absence or presence of benzaldehyde. The product ratio is influenced by the applied potential, the leaving halide group and presence/absence of silver electrocatalyst. The highest yield of bibenzyl was obtained from the electrolysis of benzyl bromide on silver-doped graphite at a constant potential equal to -1.0 V vs Ag/AgCl (sat. KCl). Benzyl chloride is less prone to dimerization, but undergoes efficient carbonyl addition to benzaldehyde, especially in the presence of silver. The results can be interpreted as a competition of radical and anionic processes.

Full text of DOI:10.1016/j.electacta.2010.09.007

Electrochimica Acta 56 (2010) 575–579
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical coupling reactions of benzyl halides on a powder cathode and
cavity cell
a
a
a,∗
b
Ronny F.M. de Souza , Carlos A. de Souza , Madalena C.C. Areias , Christine Cachet-Vivier ,
b
b
b
a
a
Michel Laurent , Rachid Barhdadi , Eric Léonel , Marcelo Navarro , Lothar Wilhelm Bieber
Departamento de Química Fundamental, CCEN, Universidade Federal de Pernambuco, av. Prof. Luis Freire S/N, 50740-901 Recife, Brazil
a
b
Equipe Electrochimie et Synthèse Organique, Institut de Chimie et des Matériaux Paris-Est UMR CNRS-Université Paris Est-Créteil, 2 rue H. Dunant, 94320 Thiais, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 June 2010
Received in revised form 2 September 2010
Accepted 4 September 2010
Available online 15 September 2010
Benzyl halides adsorbed on a cathode made of pure or silver-doped graphite powder were reduced
in an undivided cell using an aqueous electrolyte and an inert anode, in the absence or presence of
benzaldehyde. The product ratio is influenced by the applied potential, the leaving halide group and
presence/absence of silver electrocatalyst. The highest yield of bibenzyl was obtained from the electrolysis
of benzyl bromide on silver-doped graphite at a constant potential equal to −1.0 V vs Ag/AgCl (sat. KCl).
Benzyl chloride is less prone to dimerization, but undergoes efficient carbonyl addition to benzaldehyde,
especially in the presence of silver. The results can be interpreted as a competition of radical and anionic
processes.
Keywords:
Benzyl halide coupling
Organic electrosynthesis
Graphite powder cathode
Cavity electrode
©
2010 Elsevier Ltd. All rights reserved.
Silver electrocatalysis
1
. Introduction
taining at the same time an electrical current. These conflicting
demands have been reconciled for the first time in the electrochem-
ical, metal-free Reformatsky reaction realized some years ago in our
laboratory [6]. The electrochemical system consisted of a carbon
fiber cathode which adsorbs efficiently the hydrophobic reagents
(bromoester and aldehyde), an aqueous electrolyte in which the
reagents are insoluble and an inert anode.
A main challenge in organic synthesis is the formation of new
C–C bonds in order to construct more complex carbon skeletons in
a defined arrangement. However, in contemporary electrochem-
ical publications, such C–C couplings are still a minor subject
[
1,2]. Due to high reactivity, short lifetime and presence of organic
solvent and supporting electrolyte, the directly electrogenerated
intermediates—carbanions, radicals and carbocations—have a very
low probability to undergo selective bimolecular collisions with the
desired substrate. One strategy to partly overcome these problems
is the use of electrocatalysts, present in the electrolyte (homoge-
neous way) or the electrode material itself (heterogeneous way),
to transform the primary intermediates in less energetic and more
selective species of longer lifetime. Nevertheless, the diffusion con-
trolled transport of neutral and charged reactants to and from the
electrode is still an unsolved limitation in most processes [3–5].
A completely different strategy consists in the direct elec-
tron transfer to or from the concentrated, solvent free reactants
deposited on the electrode. In principle, this avoids mass transport
problems and side reactions with solvent or electrolyte. However,
there are serious experimental obstacles: how to fix the reagents
on the electrode and avoid their diffusion into the electrolyte main-
A decisive improvement of these general characteristics has
been achieved recently with the construction of a very simple
undivided electrochemical cell in which carbon cathode ends in a
cavity containing compressed graphite powder of high active sur-
face [7]. In this system, the electrochemical Reformatsky reaction
produced preparative yields of ␤-hydroxyesters with several sub-
stituted benzaldehydes. We have also shown that, on silver powder
electrode, the overvoltage of the reduction of halogenated deriva-
tives (trichloromethane) is drastically reduced (about −1.00 V) [8].
In the present work, we describe the application of the same
powder cell to the cathodic reduction of benzyl halides (1) in the
absence or presence of benzaldehyde (2), using pure graphite or
silver-doped graphite as cathodic materials.
2. Experimental
2.1. Reagents
∗ Corresponding author. Tel.: +55 81 2126 7417; fax: +55 81 2126 8442.
E-mail address: madalena@ufpe.br (M.C.C. Areias).
All chemicals were of reagent grade and were used without fur-
ther purification. Distilled water was employed in all experiments.
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.09.007

5
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R.F.M. de Souza et al. / Electrochimica Acta 56 (2010) 575–579
(
A)
(B)
(C)
0
0
-2
b
a
0
-2
a
b
b
a
-
2
4
6
8
-
-
-
-4
-4
-6
-6
-8
-8
Ep(a)=-1.40V
Ep(a)=-1.32V
Ep(a)=-1.62V
-
-
-
10
12
14
-10
-12
-14
Ep(b)=-1.07V
-10
-12
-14
Ep(b)=-0.88V
Ep(b)=-1.32V
-
2,0 -1,8 -1,6 -1,4 -1,2 -1,0 -0,8 -0,6 -0,4
-2,0 -1,8 -1,6 -1,4 -1,2 -1,0 -0,8 -0,6 -0,4
E vs sat. (Ag/AgCl) / V
-2,0 -1,8 -1,6 -1,4 -1,2 -1,0 -0,8 -0,6 -0,4
E vs sat. (Ag/AgCl) / V
E vs sat. (Ag/AgCl) / V
−
1
Fig. 1. Voltammograms recorded with a powder electrode at 0.1 mV s in which the cathode solution was comprised of: (A) 0.20 mmol of benzyl chloride, (B) 0.20 mmol of
benzyl bromide and (C) 0.20 mmol of benzaldehyde. Cathode (a) graphite and (b) silver-doped graphite.
The cathode materials were pure graphite from Aldrich [7782-42-
] used as received or 2% electrochemically silver-doped graphite,
3. Results and discussion
5
prepared using silver nitrate from Aldrich according to the proce-
dure described in Section 2.1.1.
Fig. 1 shows voltammograms recordedfor different reagents (1a,
1b and 2) and for different cathodic materials (graphite or silver-
dopedgraphite). Forminimizingohmicdropandcapacitive current,
very low scan rates were applied when using the powder elec-
trode, thus voltammetries were carried out with the quantitative
electrolysis of the electroactive species. Therefore, experimental
exchanged charge, Qexp, can be deduced from the voltammogram
peak area, A, using the following relation:
2.1.1. Instrumentation, cells and procedures
Voltammetry and controlled-potential electrolyses were car-
ried out using Autolab PGSTAT 30 potentiostat/galvanostat and
a home-made three-electrode undivided cell described in a
previous work [7]. All cathodic materials were pressed under
A
Qexp =
(1)
−
2
1
.90 kg cm in the device cavity. The cell was always filled with
v
−1
a 0.1 mol L KBr solution. The voltammograms were performed at
where v is the scan rate.
The number of exchanged electrons can then be calculated from
the following relation, deduced from Faraday’s law:
−1
0
.1 mV s . Electrolyses were performed at a controlled potential
until transfer of the theoretical charge or until reaching a current
lower than −1.0 mA. All potentials are referred to Ag/AgCl (sat.
KCl).
Qexp
−
e
=
(2)
nF
2
% Metallic silver-doped graphite was prepared by electrochem-
ical deposition at a constant potential (−0.60 V) from a mixture of
.03 mmol silver nitrate and 0.150 g of pure graphite, previously
where n is the mole number of electrolysed species and F is Faraday
−
1
−
−1
0
constant, equal to 96,485 C mol (e is expressed in F mol ).
mechanically mixed. The resulting silver-doped graphite was col-
lected and dried in an oven (100 C).
The voltammograms of 1a produced a single peak at −1.40 V
◦
−1
with an area corresponding to ∼1 F mol
(Fig. 1A, curve a).
For benzyl halides (benzyl chloride (1a) or benzyl bromide
Reminding the general electrocatalytic effect of silver on alkyl
halides [9], especially the recent studies with benzyl halides in
homogeneous organic phase [10–12], the voltammetric analyses
were repeated using graphite powder containing 2% of silver,
electrolytically deposited in a separate experiment. On this silver-
doped electrode material (Fig. 1A, curve b), the peak potential for
the reduction of 1a is located at −0.88 V, that corresponds to a sig-
nificant shift of 520 mV towards less negative values compared to
the peak obtained on pure graphite. At the same time, the peak
shape becomes sharper, with an increase of the charge consump-
(
1b)) and benzaldehyde (2) reductions, the cathodic compartment
was prepared by compressing 0.150 g of graphite or silver-
doped graphite powder in the cell cavity. The powder was
then impregnated with 0.2 mmol of pure substrate (1a, 1b or
2) or by a mixture of 0.2 mmol of 1a or 1b and 0.2 mmol of
2, without any addition of solvent. The reagent mixtures were
previously prepared and adequate volume was sampled with a
micropipette.
After electrolyses, the electrolyte was removed from the cell.
Reaction products, namely toluene (3) [108-88-3], bibenzyl (4)
−
1
tion up to 1.67 F mol . Thus both effects on potential and charge
are highlighting the electrocatalytic effect of the silver.
[103-29-7], 1,2-diphenylethan-1,2-diol (5) [579-43-1] and 1,2-
diphenylethanol (6) [614-29-9] were extracted from the graphite
powder following the procedure described in a previous work
Voltammograms were also recorded for 1b on a powder elec-
trode using graphite (Fig. 1B, curve a) and silver-doped graphite
(Fig. 1B, curve b) as cathodic materials. The measured potentials
are respectively equal to −1.32 V and −1.07 V and the calculated
[
5
7] and identified by NMR (Unity-Plus-300) and GC–MS (QP
050 Shimadzu) analysis. Product yields were determined by GC
−
1
−1
(
Varian CP-3380 gas chromatograph; Chrompack CP-SPL5CB cap-
charge to 1.77 F mol and ∼2 F mol for each material. In pres-
ence of silver, the less cathodic potential and the charge increase
evidence the electrocatalytic effect of the metal, even though the
effect is less pronounced than for 1a.
◦
illary column 30 m; initial temperature 60 C then increased to
2
◦
◦
−1
20 C at 10 C min ) with 1,3,5-trimethoxybenzene as internal
standard.

R.F.M. de Souza et al. / Electrochimica Acta 56 (2010) 575–579
577
Table 1
Electrochemical dimerization of benzyl chloride (1a) or benzyl bromide (1b).
a
Entry
PhCH2–X
Cathodea
Eb/V
Product compositionc
(mmol)
1
3 (mmol)
4 (mmol)
Ratio 4:3
1
2
3
1a
1a
1a
Graphite
Graphite
Graphite
−(.00
−(.(0
−(.(0
0.054
0.062
0.002
0.092
0.118
0.182
0.027
0.010
0.008
1:3.41
1:11.8
1:22.8
4
5
6
7
1b
1b
1b
1b
Graphite
Graphite
Graphite
Graphite
−0.80
−(.00
−(.(0
−(.(0
0.036
0.006
0.008
0.032
0.002
0.048
0.096
0.092
0.081
0.073
0.048
0.038
1:0.02
1:0.66
1:2.00
1:2.42
8
9
0
1
2
1a
1a
1a
1a
1a
Graphite/silver
Graphite/silver
Graphite/silver
Graphite/silver
Graphite/silver
−(.60
−(.80
−(.00
−(.(0
−(.(0
0.072
0.054
0.016
0.024
0.020
0.026
0.026
0.056
0.128
0.110
0.051
0.060
0.064
0.024
0.035
1:0.51
1:0.43
1:0.88
1:5.33
1:3.14
1
1
1
1
1
1
1
1
3
4
5
6
7
1b
1b
1b
1b
1b
Graphite/silver
Graphite/silver
Graphite/silver
Graphite/silver
Graphite/silver
−(.60
−(.80
−(.00
−(.(0
−(.(0
0.094
0.022
0.012
0.010
0.020
0.008
0.022
0.028
0.050
0.060
0.049
0.078
0.080
0.070
0.060
1:0.16
1:0.28
1:0.35
1:0.71
1:1.00
a
−2
General reaction conditions: cathode compressed at 1.90 kg cm ; graphite powder cathode, 0.20 mmol 1; each reaction was continued until transfer of around the
theoretical charge or until the current reach 1.0 mA. 3, toluene; 4, bibenzyl.
b
vs sat. (Ag/AgCl).
Determined by GC.
c
After this analytical exploration, 1a and 1b were submitted
to electrolyses at constant potentials in the same cell used for
voltammetry. The electrolyses were performed on a graphite cath-
ode applying potentials between −0.80 and −1.40 V and then on
silver-doped graphite between −0.60 and −1.40 V in 0.20 V inter-
vals (Table 1). As expected from the voltammetric behaviour, for the
same electrode material and applied potential, 1b was consumed
more completely and faster than 1a. In all cases, only two reaction
products were observed: toluene (3) and bibenzyl (4) (Scheme 1).
On pure graphite, for both halides, the yield of 4 increased con-
tinuously from more negative to less negative potentials, reaching
because of its good reactivity in related aqueous Barbier-type reac-
tions [15]. First, the reduction potential of 2 in the cavity cell was
determined by voltammetry (Fig. 1C). On pure graphite a sharp peak
was observed at −1.32 V (Fig. 1C, curve a). The one-electron trans-
fer to 2 results in the quantitative formation of a mixture of two
diastereoisomers of 1,2-diphenylethan-1,2-diol (d, l and meso) (5)
as described in a previous work [7]. Surprisingly, this reduction
peak shifted to a more negative value, −1.62 V, on silver-doped
graphite powder (Fig. 1C, curve b), exactly the opposite effect to the
one found for 1a and 1b. This effect should facilitate, in principle, a
selective electron transfer to the halide and prevent hydrodimer-
ization, normally expected in the cathodic reduction of 2 [16]. In
the following cross-coupling experiments between 1a or 1b and
2, three new products were expected in addition to 3 and 4: 1,2-
diphenylethanol (6) and two diastereomers of 5 (Scheme 2). Indeed,
depending on the reaction conditions, all five compounds were
observed simultaneously. The main observation arising from the
analysis of Table 2 is the preferential carbonyl addition to form
product 6 in almost all experiments with 1a. On pure graphite
the highest yield of 6, 0.080 mmol (40%), was obtained at −1.40 V
(Table 2, entry 3). 1,2-Diphenylethan-1,2-diol, 5, as a 1:1 mixture
of diastereomers, was observed as byproduct from 1a at −1.40 V
(Table 2, entry 3). In all experiments with 1b (Table 2, entries 4–6),
4 was the main product, but 6 and 5 were also important. 3 was a
minor product in all experiments on pure graphite.
0.081 mmol (81%) in the case of 1b (Table 1, entry 4). Consequently,
3
was favored by more cathodic potentials and was the main prod-
uct in all experiments with 1a (Table 1, entries 1–3).
On silver-doped graphite, more elevated potentials (−0.60 V and
−
0.80 V for 1a, Table 1, entries 8 and 9, and −0.60 V for 1b, entry 13)
lead to higher amounts of unreacted reagent. For a potential equal
to −1.0 V and for lower values, the halide is converted almost quan-
titatively. The highest dimer yield was obtained at −1.0 V (Table 1,
entries 10 and 15), giving 0.064 mmol (64%) and (0.080 mmol) 80%
for 1a and 1b, respectively. Nevertheless, more negative potentials
brought no further increase of conversion and a significant decrease
in the dimer/toluene ratio. Finally, the comparison of results for
graphite and silver-doped graphite shows that the reduction of 1a
to dimer 4 is significantly improved in presence of silver putting in
evidence the electrocatalytic effect of silver. However, this effect is
not observed on the yields for 1b, since high amounts of dimer are
already found on graphite (entry 4).
The use of silver-doped cathode material increased the reaction
rate and conversion of reagents in all cases. The shift of the halides
1a and 1b peak potentials to less cathodic values made possible effi-
cient electrolyses even at −0.80 V. The best yield of 6, 0.132 mmol
(66%), was obtained at −1.0 V (Table 2, entry 8). No important effect
of silver on the formation of 4 was observed for 1a, but toluene for-
mation was favored by more negative potentials (Table 2, entries
After the success of homocoupling of benzyl halides on the pow-
der cathode, the reductive addition of these halides to the carbonyl
group was investigated. Benzaldehyde, 2, was chosen as substrate
9
and 10), as already observed in Table 1. In the case of 1b, silver
electrocatalysis brought no improvement of the coupling reaction
to 6, but favored dimerization of 4 at all potentials. As anticipated
in the discussion of the voltammetry of 2, no pinacol, 5, formation
was observed in the silver-electrocatalyzed experiments except at
−
1.40 V (Table 2, entry 15).
From the synthetic point of view, the results described above
Scheme 1. Dimerization of benzyl halides.
demonstrate that electrochemical reduction of 1b on a graphite

5
78
R.F.M. de Souza et al. / Electrochimica Acta 56 (2010) 575–579
Scheme 2. Benzyl halides and benzaldehyde coupling.
powder cathode, doped or not by silver, can be used for the effi-
cient dimerization to 4, the parent compound of an important
class of intermediates in the synthesis of biologically active com-
pounds for agriculture and medicine [17–19]. In comparison with
other electrochemical procedures in homogeneous organic phase
1
4
-
X- e-
e-
e-
-
2
-
[
10–14,20], our method is of similar efficiency, but more attractive
Ph
CH2
Ph
CH
Ph CH
O
by its experimental simplicity and the low environmental impact,
as a consequence of the elimination of organic solvents and sup-
porting electrolyte. Regarding these latter characteristics, it can be
compared to the zinc-promoted chemical dimerization in aqueous
medium reported previously [21].
A
B
C
A + A
A + C
3
B +
1
4
-
-
X
+
+
On the other hand, the electrochemical heterocoupling with 2 is
best performed with 1a on silver-doped graphite and is described
here for the first time. Since the pioneer work of Baizer on the
electrochemical carbonyl addition [22], little progress has been
reported in this field [23,24]. Only very recently a related carboxy-
lation of 1a [25,26] and an allylation of 2 on a zinc cathode [27] have
been described. It is worthwhile to mention that even the classical
Grignard reaction gives higher amounts of bibenzyl as byproduct
when compared to the electrochemical process described here [28].
The results presented in this work allow also important con-
clusions about the mechanisms involved in the reactions. It is well
established by previous studies in homogeneous phase that ben-
zyl halides can undergo one and two-electron reductions giving
rise to benzyl radical A and carbanion B [29–31]; one-electron
transfer to 2 will produce anion-radical C [16] (Scheme 3). These
three electrogenerated reactive intermediates can interact with
each other or with the starting materials 1a, 1b and 2. In the exper-
iments on pure graphite, the large difference between 1a and 1b in
the yield of dimer 4 excludes an exclusive common intermediate
such as A in the rate determining step. The best results obtained
H
H
5
2
6
B +
B + H+
C + C
+
2
H
Scheme 3. Benzyl halides and benzaldehyde coupling mechanism.
with 1b suggest a decisive influence of the leaving halogen, prob-
ably in a S 2 type substitution of carbanion B on 1a and 1b. The
important quantities of 3 have to be explained also by an anionic
N
precursor.
The electrocatalytic effect of silver shifts the reduction poten-
tials of 1a and 1b to less negative values. Although the voltammetric
analysis did not show in any case separated peaks (Fig. 1A and
B), it can be hypothesized that a single electron transfer, produc-
ing radical A, will be more probable at less cathodic potentials,
exactly where the best ratios 4:3 are observed (Table 1, entries
8–10, 14–17). This indicates that the coupling of two radicals A
Table 2
a
Electrochemical coupling of benzyl chloride (1a) or benzyl bromide (1b), with benzaldehyde (2).
Entry
PhCH2–X
Cathodea
Eb/V
Product compositionc
1
(mmol)
3 (mmol)
4 (mmol)
2 (mmol)
5 (mmol)
6 (mmol)
Ratio 6:4:3
1
2
3
1a
1a
1a
Graphite
Graphite
Graphite
−1.00
−1.20
−1.40
0.051
0.068
0.033
0.021
0.044
0.054
0.029
0.022
0.029
0.109
0.096
0.044
0.000
0.003
0.025
0.080
0.072
0.080
1:0.36:0.26
1:0.30:0.61
1:0.36:0.68
4
5
6
1b
1b
1b
Graphite
Graphite
Graphite
−1.00
−1.20
−1.40
0.002
0.000
0.012
0.012
0.020
0.024
0.095
0.086
0.078
0.160
0.160
0.120
0.000
0.004
0.028
0.018
0.020
0.016
1:5.28:0.67
1:4.30:1.00
1:4.88:1.50
7
8
9
1a
1a
1a
1a
Graphite/silver
Graphite/silver
Graphite/silver
Graphite/silver
−0.80
−1.00
−1.20
−1.40
0.018
0.012
0.068
0.033
0.014
0.008
0.040
0.054
0.024
0.025
0.022
0.029
0.088
0.065
0.096
0.044
0.000
0.000
0.003
0.025
0.098
0.132
0.073
0.080
1:0.24:0.14
1:0.19:0.06
1:0.30:0.55
1:0.36:0.68
1
0
1
1
1
1
1
1
2
3
4
5
1b
1b
1b
1b
1b
Graphite/silver
Graphite/silver
Graphite/silver
Graphite/silver
Graphite/silver
−0.60
−0.80
−1.00
−1.20
−1.40
0.032
0.000
0.000
0.000
0.000
0.008
0.012
0.012
0.020
0.016
0.088
0.088
0.092
0.086
0.090
0.176
0.164
0.172
0.172
0.144
0.000
0.000
0.000
0.000
0.008
0.004
0.024
0.016
0.018
0.022
1:22.0:2.00
1:3.67:0.50
1:5.75:0.75
1:4.78:1.11
1:4.09:0.72
a
−2
General reaction conditions: cathode compressed at 1.90 kg cm ; graphite powder cathode, 0.20 mmol 1 and 2; each reaction was continued until transfer of the
theoretical charge or until the current reach 1.0 mA. 3, toluene; 4, bibenzyl; 5, 1,2-diphenylethan-1,2-diol; 6, 1,2-diphenylethanol.
b
vs sat. (Ag/AgCl).
Determined by GC.
c

R.F.M. de Souza et al. / Electrochimica Acta 56 (2010) 575–579
579
competes successfully with the SN2 process reducing, but not abol-
Acknowledgements
ishing, the difference in yields from the two halides. Accordingly,
the formation of toluene is also inhibited, especially at less negative
potentials.
Similar trends can be detected and rationalized in the addition
to 2. In the absence of silver, B is the most probable interme-
diate and the two electrophiles 1 and 2 will compete in the
following step. In the case of 1a this competition is largely
favorable to the formation of the coupling product 6, but the
better leaving group of 1b allows the preferential formation of
The authors wish to thank granting authorities CNPq and
CAPES/COFECUB (Proc.532/06) for fellowship to Ronny F. M. Souza
and financial support; and Rogério T. Ribeiro for fruitful discussions.
References
[
[
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1
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[
[
[
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