Cleavage of aryl iodides at carbon cathodes in organic carbonates. Surfaces doped by transition metals at the nanometric scale: Electro or redox catalysis?

Article abstract of DOI:10.1016/j.elecom.2010.06.011

The cathodic reduction of aryl iodides ArIs at carbon electrodes was achieved in cyclic carbonates (e.g., propylene carbonate PC). The novelty of the present method simply lies in the ability of title compounds to create aromatic carbanions that may act as nucleophiles towards carbonates. Consequently, several aromatic esters ArC(O)OR are generated as side products that play the role of redox mediators for indirectly cleaving C-X bond at solid inert electrodes. This procedure surprisingly allows one to obtain, through this self-induced redox catalysis, Ar-Ar dimers sometimes in high yield. Meanwhile, the triggering of such processes could be boosted by doping inert electrodes by electro-deposition of metals such as palladium, nickel, or silver. Extremely thin layers of metals are well shown to be very efficient (even at average thickness < 0.1 nm) to induce catalytic steps presenting quite large potential shifts compared to bare carbon electrodes when employed in inert solvents. The probable concomitance of electrocatalytic and redox catalytic processes is discussed and argued.

Full text of DOI:10.1016/j.elecom.2010.06.011

Electrochemistry Communications 12 (2010) 1262–1265
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Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Cleavage of aryl iodides at carbon cathodes in organic carbonates. Surfaces doped by
transition metals at the nanometric scale: Electro or redox catalysis?
a
b,
Viatcheslav Jouikov , Jacques Simonet ⁎
a
UMR 6510, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France
Laboratoire MaSCE, UMR 6226, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 May 2010
Received in revised form 6 June 2010
Accepted 7 June 2010
Available online 13 June 2010
The cathodic reduction of aryl iodides ArIs at carbon electrodes was achieved in cyclic carbonates (e.g.,
propylene carbonate PC). The novelty of the present method simply lies in the ability of title compounds to
create aromatic carbanions that may act as nucleophiles towards carbonates. Consequently, several aromatic
esters ArC(O)OR are generated as side products that play the role of redox mediators for indirectly cleaving
C–X bond at solid inert electrodes. This procedure surprisingly allows one to obtain, through this self-
induced redox catalysis, Ar–Ar dimers sometimes in high yield. Meanwhile, the triggering of such processes
could be boosted by doping inert electrodes by electro-deposition of metals such as palladium, nickel,
or silver. Extremely thin layers of metals are well shown to be very efficient (even at average
thickness b0.1 nm) to induce catalytic steps presenting quite large potential shifts compared to bare carbon
electrodes when employed in inert solvents. The probable concomitance of electrocatalytic and redox
catalytic processes is discussed and argued.
Keywords:
Propylene carbonate
Redox catalysis
Electrocatalysis
Aryl iodides
Cathodic scission
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
chemical bonds, in particular carbon–iodine bonds and activate
electrochemical reactions in non-classical ways. We report here a
very peculiar process concerning the electro-reduction of ArIs at
carbon electrodes by means of a self-induced redox process via the
The electrochemical cleavage of aryl halides ArX at carbon
cathodes is now a field well documented [1]. The mechanism of the
scission of the carbon–halogen bond with the preliminary formation
of an aryl radical has been discussed [2]. Therefore, in the course of the
electrochemical process, two transients may be generally produced:
the radical Ar and the parent anion Ar . Those intermediates may
lead to specific reactions. Thus, the formation of Ar in the presence of
−
addition of Ar to cyclic carbonates. The process allows the formation
of dimeric diaryls in high yield. The doping of carbon by deposition of
−
metals at the nanometric scale (δb0.1 nm) boosts Ar formation,
•
−
then allowing this method to be applied to a large series of ArIs.
•
nucleophiles may afford SNR1 reactions [3] as well as radical exchange
with the solvent, while the parent anion Ar can act as a nucleophile
−
2. Experimental
or/and a strong base towards the medium. In the meanwhile, ArX
compounds were among the first organic substrates to be reduced
indirectly [4] by means of redox mediators whose anion radical was
considered as a molecular electrode. Some examples of eliminations
of organic poly-halides were reported to lead to formation of self-
produced redox mediators for giving avalanche-type reductions. On
the other hand, the two-electron catalytic cleavage of ArBrs was
achieved in acetonitrile (AN) at smooth silver electrodes [5].
All electrochemical studies were conducted in 0.1 M solutions of
tetrabutylammonium tetrafluoroborate (TBABF ) in propylene car-
4
bonate (PC) or a dimethylformamide (DMF)/ethylene carbonate (EC)
mixture (50/50, v/v). Solutions were kept over activated neutral
alumina although procedures reported therein do not require
especially dry solutions. All potentials refer to aqueous saturated
calomel electrode (SCE) and the electrochemical instrumentation was
previously described [6].
The use of propylene carbonate in organic electrochemistry is far
from being general despite its high dielectric constant. It may lead to
unexpected reactions due to its capability to strongly polarize
Glassy carbon (GC, vitrified at 2500 °C) was obtained from
Carbone Lorraine. Very thin galvanostatic deposits of nickel, gold,
copper, palladium and silver were achieved for doping GC surfaces.
6
Amounts of electricity employed for such deposits were ≤2×10−
−2
C mm . Therefore, the average thickness of layers of deposited
metals was about 0.1 nm. In voltammetry, GC electrodes with surface
areas of 0.8 mm2 were used. Prior to modifying, all electrodes were
⁎
Corresponding author. Tel.: +33 23236292; fax: +33 23236732.
E-mail address: jacques.simonet@univ-rennes1.fr (J. Simonet).
1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.elecom.2010.06.011

V. Jouikov, J. Simonet / Electrochemistry Communications 12 (2010) 1262–1265
1263
carefully polished with silicon carbide paper (Struers) or with Norton
polishing paper (types 02 and 03). Before use, the electrodes were
thoroughly rinsed with water, alcohol and acetone and finally dried in
a hot air stream.
Macro-electrolyses performed at a GC electrode in PC at the
threshold of step I (i.e., −1.6 V) result in total disappearance of ArI
−1
(after passing ≈1.5 F mol ). The analysis (GC–MS) of the electrol-
ysis solution shows the addition of the aryl anion onto the carbonate
(for PC, two isomers are obtained according to Eq. 2). Additionally, it
has been checked that parent methyl and ethyl esters of 1-naphthyl
carboxylic acid Ar–C(O)OR give a reversible system at a potential very
close to steps III and IV (Fig. 2). As shown in Fig. 2 B, a pair of
voltammetric steps is obtained after macro-reduction. In the course of
the first sweep, only steps III and IV (assigned with INp to the
Electrolyses were performed in small two-compartment cells on
2
large sheets of GC (3 to 4 cm ). Solutions were extracted with diethyl
ether and analyzed by GC/MS. Yields correspond to percentages of
soluble extracts and do not take into account possible polymer
deposition, which was found to be very small.
reversible reduction of the two isomers of 2, E
p
=−1.80 V and
3
. Results
−
1.90 V) are seen, while an additional scan affords a new minor peak
V (E
p
=−1.38 V) fully reversible if there is no ArI left in solution. This
3
.1. Behavior of ArIs at carbon electrodes
strongly supports that this new reduction step corresponds to the
formation of an unsaturated compound by action of the electro-
generated bases possibly produced in the course of adduct(s) 2
reduction (Eq. (3) Scheme 1). Adding a strong base (e.g., tetrame-
thylammonium hydroxide) to the electrolysis solution also leads to
step V. After many experiments of this kind achieved with ArIs in PC
and EC, it appears that several side products (3 but also 2 in a minor
way) may complicate the overall process by acting as redox medi-
ators. This is strongly reinforced by the fact that standard reduction
potentials of ArIs are expected to be located at about −1.0 V [7], case
of 4-iodobiphenyl, therefore much less negative than data relative to
some aryl bromides found around −2 V [8]. In such a case, the driving
force of the redox catalysis is due to standard potential differences.
In order to check the occurrence of redox processes provoked by the
presence of hydroxyethyl benzoate 2 and vinyl benzoate 3, ArI was
added to the electrolysis solution (Fig. 2 C). The first scan strongly
resembles the scan in Fig. 1 A, while the second scan reveals a large
peak at the potential of V (Ep5 =−1.46 V) together with steps III and
It is very unexpected that many aryl iodides such as 1-
iodonaphthalene (INp), 9-iodophenanthrene (IPA), 2-iodofluorene
IFl), 4-iodobiphenyl (IBp) and others, usually reduced in two two-
electron steps in solvents like acetonitrile and DMF, present a very
different cathodic process when reductions are achieved in cyclic
carbonates like PC and ethylene carbonate (EC). As exhibited in Fig. 1,
the feature (two-electron reduction of the carbon–iodine bond,
(
reaction 1 in Scheme 1, for INp, E =−1.90 V vs. SCE) dramatically
p
changed in the course of repetitive scans. Thus, when the scan starts
from a potential less negative than −1.0 V vs. SCE, the main peak I
may present a very small pre-peak II that may almost totally vanish
during the second scan (Fig. 1 A). Yet, after a rather large number of
scans (B), the pre-peak II (Ep2 =−1.59 V) increases regularly while
step I decays, progressively disappears, and is replaced by a reversible
step. A limit is usually reached after 10scans.
IV then restored. This supports the formation of 3 at the end of the
−
first scan and the triggering of a redox catalysis process by 3•
.
Independently, the response of INp at a smooth silver electrode gives,
under identical conditions, step VI at −1.41 V. It is then very
intriguing why two modes of catalysis, in principle so different, may
lead to such similar potential shifts (ΔE≈+0.4 V at bare carbon).
3.2. Macro-electrolyses at GC
The crucial question is whether step I (absence of redox catalysis)
and step II (strong catalytic effect with a shift of about +0.5 V) as
exhibited in Fig. 1 both yield ArH compounds in high yield, as found at
GC in DMF or acetonitrile. Surprisingly, the answer is no. As a matter
of fact, the reduction of INp at GC in PC at −1.7 V (after a hold at −2 V
to trigger the redox reaction) yields, beside NpH (8%) and Np–PC–OH
(
2 isomers of hydroxyethyl benzoate, 3%), the coupling product Np–
Np (77%). All the INp was reduced. Additionally, the electrode is
covered by a red-brownish deposit, insoluble in acetone, probably
arising from the polymerization of vinyl benzoate. Its structure is
assigned to a redox polymer (polyaryl ester according to Eq. 7) and
this is supported by the existence of a slightly reversible reduction
step (E
p
=−1.89 V). On the contrary, the electrolysis accomplished at
−
2.1 V affords NpH (52%), Np–Np (43%) and Np–Pc–OH (4%). This
behavior is rather general for other aryl iodides but the progressive
deposition of polymer may slow down the reduction process, and a
certain amount of the starting substrate may be recovered.
3.3. Boosting the redox catalytic process
There are at least two additional ways to set up a fast redox
catalytic process as presented above. First, one can add to the solution
an “external” redox mediator the E° of which should be less negative
−
(
i.e., ≈0.4 V) than step I; then, Ar derived from acenaphthylene
Fig. 1. Voltammetry of INp (8.2 mmol L−1) in PC. GC electrode (0.8 mm ); v=50 mV s−1.
2
(E°=−1.60 V) is a very efficient catalyst towards INp. Second, one
can dope the carbon electrode with very small amounts of transition
(A) Two first scans. (B) Repetitive scans.

1264
V. Jouikov, J. Simonet / Electrochemistry Communications 12 (2010) 1262–1265
Scheme 1. Reduction of an aryl halide reduced at a solid electrode in the hypothetical case where the produced aryl anion reacts with the cyclic carbonate (here ethylene carbonate).
This process stresses the case of an aryl ester generation easily electroactive (X=I) and therefore capable (if E°
indirect reduction of the starting ArX.
1 3
NE° ) to trigger, at the solid interface, a redox catalysis permitting the
metals such as Pd, Ni, Ag and Cu already reported to give elec-
trocatalytic activation processes with aromatic halides. Thus, as
displayed in Fig. 3, the use of Ag° and Pd° with IDP (A) and of Ni°
and Pd° with IPA (B) shows, after at least two scans, the voltammetric
responses very close to those already obtained with self-induced
redox processes (for that, B3 or B4 can be compared with Fig. 1 B).
This latter way appears to work nicely with some substituted phenyl
iodides.
Fig. 3. Voltammetry of ArIs in DMF–EC mixtures at GC electrodes doped with metals.
2
−1
−1
Area of stationary electrodes: 0.8 mm ; v=50 mV s . A: IBP (6.4 mmol L ). (1) GC
electrode (two sensitivities in current). (2) Same electrode after galvanostatic deposit
Fig. 2. INp (initial concentration 12 mmol L− ) in PC. Voltammetry before and after
1
of Ag° corresponding to an electricity amount of 2×10 C, two first scans. (3) Same
−6
2
−1
constant potential electrolysis (E
(
r
=−1.6 V at a GC electrode (3 cm )); v=50 mV s
.
experiment with a deposit of Pd° using the same amount of electricity. B: IPA
(6.2 mmol L− ). (1) GC electrode (two first scans). (2) Repetitive scans at a smooth
gold electrode. (3) and (4) Voltammetries at a GC electrode doped with Ni° and Pd°
1
A) Solution at the start of electrolysis at GC electrode. (B) Response at electrolysis
completion. First and second scans. (C) Response of the solution after adding INp. (D)
Independently, response of INp alone at a smooth silver electrode.
6
respectively (In both cases, electricity amounts used: 2×10− C).

V. Jouikov, J. Simonet / Electrochemistry Communications 12 (2010) 1262–1265
1265
4
. Conclusion
References
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[
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1