Enolization versus carbonylation at glassy carbon surface through cathodic means

Article abstract of DOI:10.1016/j.elecom.2013.02.016

The cathodic reduction of ω-bromomethylarylketones in aprotic organic solvents (such as propylene carbonate) containing tetraalkylammonium iodides achieved at smooth glassy carbon (GC) permits through a selective one-electron reduction, the scission of th

Full text of DOI:10.1016/j.elecom.2013.02.016

Electrochemistry Communications 31 (2013) 1–4
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Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Enolization versus carbonylation at glassy carbon surface through cathodic means
Jacques Simonet ⁎
Equipe MaCSE, UMR 6226, Université de Rennes, Campus de Beaulieu, 35042 Rennes Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The cathodic reduction of ω-bromomethylarylketones in aprotic organic solvents (such as propylene carbonate)
containing tetraalkylammonium iodides achieved at smooth glassy carbon (GC) permits through a selective
one-electron reduction, the scission of the C\Br bond. Quite unexpectedly, the free radical thus formed is
Received 11 February 2013
Received in revised form 15 February 2013
Accepted 18 February 2013
Available online 24 February 2013
2
prior attached to the carbon surface in its enolic form (GC\O\(Ar)C_CH ). Successive radical addition process-
es may lead to a redox polymer based on the one-electron oxidation of enol ethers. The average superficial enol
concentration at the carbon surface (found to be about 2.5×10− mol cm ) was assessed by the halogen index
iodine or bromine) via halogen addition to the double bonds followed by the micro-coulometric reduction of
the resulting vicinal di-halo compounds.
8
−2
Keywords:
ω-Bromomethylarylketones
Glassy carbon
(
Enol radical grafting
Enolization
© 2013 Elsevier B.V. All rights reserved.
Modified electrodes
1
. Introduction
and innovating processes: until now, most of aromatic π-donors and
π-acceptors (e.g. nitroarenes) were immobilized at electrodes by the
diazonium way.
The cathodic scission of carbon-halogen bonds for organic halides
(
RX and ArX with X=I or Br) continues even nowadays to induce
Here is presently reported a simple and efficient one-electron cathod-
great interest. Many studies were undertaken on the direct and indirect
C\X bond scission processes [1,2]. At carbon electrodes, heterogeneous
reduction processes correspond most generally to a hydrogenolysis via
a two-electron scission.
2
ic cleavage of ArCOCH Br in high dielectric permittivity organic solvents
in the presence of a tetraalkylammonium iodide as supporting salt. A
similar process has been described for the attachment of anthraquinone
2
at carbon via one-electron reduction of 9,10-[AntQ]-CH Br [8]. The key
Conditions for realizing one-electron scissions of C\X bonds and the
concomitant formation of organic free radicals are being intensively
developed. This research is driven by the interest to obtain coupling re-
action and also the attachment of those radicals at solid surfaces. Acti-
vated organic halides, for instance benzyl bromide, were reported to
be reduced at silver in two well-separated one-electron steps [3], and
the immobilization of benzyl radicals at glassy carbon could be
established [4]. Quite similarly, “allylation” and “propargylation” of GC
using radical attachments from corresponding bromides were achieved
step of the process is the in situ conversion of RBr into RI (ERBrbERI
expected to occur at the interface (I adsorbed). Therefore, since the
)
−
global two-electron reduction of the C\I bond occurs according to two
•
neatly separated one-electron steps, one can selectively produce R rad-
icals from organic bromo derivatives.
The great novelty of the present communication is the mode of at-
•
tachment of the radical ArCOCH
2
supposed first to be grafted in its
keto-form. With several substituted ω-bromoacetophenones, it is
shown for the first time that the immobilization occurs through the
enol form (Scheme 1, reactions (1) and (2)) leading to the generation
of GC\O\C_C linkages. The modification, therefore, to a great extent
concerns the layers of aromatic enol ethers reported to be reversibly
oxidized into their radical cations. Attachment of redox polymers
(poly-enols as thick multi-layer deposits) could be established. This
first contribution deals especially to the cathodic immobilization of rad-
[
5]. Doping of carbon electrodes by thin deposits or nano-particles of
0
transition metals (like Pd at the sub-nanomolar concentration) also
permitted one-electron cleavage of 1-haloalkanes [6]. Beyond signifi-
cantly activated organic substrates, free radicals grafting from neutral
organic species appears quite unusual. Alternatively, a general mode
of aryl radical generation has been developed by using organic cationic
substrates such as functionalized aryl diazonium salts widely employed
as convenient and general pathway for solid surface decoration [7].
Still in the field of surface modification, other simple and efficient
grafting ways through arylation of carbons are sought for in many ap-
plications regarding sensing and catalysis. It certainly appeals for new
2 6 5
icals issued from ω-bromoacetophenones (BrCH COC H X with X=4-H
1a, 4-CH 1b, 4-OCH 1c) together with those of bromoacetyl-2-pyrene
3
3
2 and bromoacetyl-2-naphthalene 3.
2. Experimental
Electrolyses were essentially carried out in 0.1 M solutions of
⁎
Tel.: +33 2 23 23 62 92.
E-mail addresses: jacques.simonet@univ-rennes1.fr, jacques.simonet@dbmail.com.
4
Bu NI (TBAI) in propylene carbonate (PC) pure or added of ethylene
1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.elecom.2013.02.016

2
J. Simonet / Electrochemistry Communications 31 (2013) 1–4
TBAI
PC
containing an iodide as a salt), the first reduction step, usually featuring
a two-electron process is clearly split in two steps, I and II (E =−0.97
and −1.19 V, respectively, with 1c) as depicted in Fig. 1, curve A. This
features a process where the first step corresponds to the generation
of free aryl-acetyl radical without its fast ensuing cathodic reduction
to the anion. At more negative potentials, a large reduction step III
Ar-CO-CH -Br
Ar-CO-CH -I
(1)
(2)
2
2
p
-
e/-I
Ar-CO-CH -I
Ar-CO-CH2
2
p
(E =−2.06 V) features the reduction of acetophenone (reactions 1, 2
-
Ar-CO-CH2 + e + SH
Ar-CO-CH2
Ar-CO-CH + S (3)
and 3, Scheme 1). Using other cathodic materials (Pd, Au, and Pt), the
step I is clearly shifted toward less negative potentials (curve B: at
gold). Fig. 2 A depicts results obtained under similar conditions with 2.
Micro-coulometric measurements at the reduction step I were done
3
Ar-C(O )=CH2
(4)
(5)
−
3
−2
(
amounts of charge of about 0.5×10 C mm ) in order to trap free
radicals at the GC surface and to test the nature of the surface modifica-
tion. Quite surprisingly, we do not notice – when running the resulting
electrode in a solution free of organic substrate – a large reduction peak
that would correspond to the immobilized acetophenone. On the con-
trary, a reversible oxidation step at moderate oxidation potentials
appears (Epb+0.5 V) and would suggest that the free radical is
immobilized to a large extent at the GC surface under its enolic form
GC
Ar-C(O )=CH2
Ar-C=CH2
O
Surface
GC
(
Scheme 1, reaction 5). This is confirmed by FTIR spectroscopy (Fig. 3,
n Ar-C(O )=CH2
spectra A, B, and C, relative to bromo-methyl-ketones 1a, 1c, and 2).
As a matter of fact, large νC_O vibrations are not present while νC\O
bands always appear. In order to check the indubitable presence of dou-
ble bonds grafted at the GC surface, we used the classical method of bro-
mine or iodine index simply obtained by the contact of the electrode for
a few tens of seconds with a dilute solution of halogen in cyclohexane
-
n H
Ar
(
6)
C=CH2
O
Ar-C=CH2
n
(
2 2
Br , 3%) or ethanol (I , 5%), followed by the total reduction of the
O
-
e
di-halo-form in the course of the first scan as depicted in Fig. 1 (inset).
The integration of the charge allows estimating the enol superficial con-
centration at the GC surface through the double bond. Thus, coverage
values based on the double bond could be assessed. For example,
using bromine (Fig. 1 C), 1a, 1b and 1c, the coverages of 3.5, 3.4, and
GC
(7)
−
8
−2
1
.8×10
mol cm , respectively, were found. Employing iodine, the
Ar-C--CH2
-1.0
0.0
-1.0
0.0
O
+
E/V
E/V
GC
1
0 µA
A
Scheme 1. Proposal for the electrochemical reactivity of the free radical issued from the
C-Br cleavage of bromoketones.
B
II
I
carbonate (EC) used then in a 50/50 (v:v) mixture since EC is solid at
room temperature. Tetramethylammonium- and tetraethylammonium
iodides could be employed as well. The use of more common solvents
like N,N-dimethylformamide (DMF) or acetonitrile (ACN) did not give
satisfactory results for observing the expected one-electron process. In
principle, experiments described in this work did not need special treat-
ment of electrolytic solutions. Solvents are employed as received.
Potentials are referred to the aqueous Ag/AgCl/KCl(sat). The electro-
chemical instrumentation has been previously reported [6].
III
-1.0
0.0
E/V
2
2
2
5 µA
GC electrodes had an apparent surface area of 0.8 mm and 7 mm
GC). All carbon samples were purchased from Tokai Carbon Co (code:
1
C
(
GC Rod). Prior being modified, all electrodes were carefully polished
with silicon carbide paper: first with P 500 (Struers) and then with
Norton polishing paper (types 02 and 03). Coulometric measurements
and electrolytic deposits were performed using three-electrode cells
separated with a fritted glass.
3
. Results
−
1
3
.1. Cathodic reduction of ω-bromomethylarylketones
Bromomethylacetophenones (phenyl substituted in the 4-position)
Fig. 1. Voltammetry of 1c (concentration: 12.8 mmol L ) in PC+TBAI at solid elec-
trodes. Scan rate: 50 mV s− . Electrode surface area: 0.8 mm . A) Response at a GC elec-
trode. B) Response at a smooth gold electrode. C) Bromination of the electrode modified at
0.9 V by fixed potential electrolysis (total amount of charge: 0.8×10 C). Two scans. In
the inset, principle of the bromination process and its reduction (re-generation of the enol
1
2
−3
−
were tested electrochemically using GC as cathode material. In the con-
ditions given in the experimental (use of a PC-EC mixture as solvent
ether) between −0.1 and −2.0 V.

J. Simonet / Electrochemistry Communications 31 (2013) 1–4
3
-0.5
+0.5
-1.0
0.0
E/V
I
GC
5
A
4
µA
II
2
B
I
Pt
1
II
III
1
Pd
I
2
II
4
µA
3
-
0.2V
+1.3V
2
µA
Fig. 2. Voltammetries of bromomethyl-ketopyrene 2 (concentration: 3.6 mmol L−1) in
−
1
PC-EC (50:50 v/v) containing TBAI. San rate: 50 mV s . Surface area of used solid elec-
2
trodes: 0.8 mm . A) Voltammetry at GC, Pt, and Pd electrodes. B) Initial reduction
(
E
R
4
=−0.6 V) of the deposit after bromination (scan 1) in DMF/TBABF . The regeneration
of enol ether moieties (double reversible step) is followed by scans 2 to 5. B) For compar-
ison, in the inset: oxidation (first three scans) of the layer obtained from 1b.
coverage values are significantly smaller (about 10 times) presumably
owing to a slower diffusion of the halogen inside the grafted layers.
3
.2. Anodic oxidation of immobilized aryl enol ethers
Fig. 3. FTIR spectra of different layers obtained from bromo-ketoarenes at smooth GC. A, B
and C: 3, 2, and 1a, respectively.
Enol ethers are known to be oxidized in two steps [9] in polar or-
ganic solvents at the potentials lower than +1 V. First one-electron
exchange is electrochemically reversible (formation of the radical cat-
ion) leading to a slow radical coupling. On the contrary, the produced
di-cation was reported to react rather fast with nucleophiles (mois-
ture) present in the liquid electrolyte. In the case of the attached to
the carbon surface enol, one may also expect a reversible anodic re-
sponse. This way, first oxidation step was used for estimating the cov-
erage ratio of enolic forms into the attached layer.
(
scan 1) into the enol-ether (scans 2–5). One checks that the enol
ether is reversibly oxidized in two steps (E =+0.04 V and +0.
0 V). The self-inhibition of oxidation is possibly related to the thick-
p
2
ness of the layer, so as the currents of the both steps decrease upon
recurrent scans because of the radical cation coupling as well as of
the reaction of the di-cation with moisture. This experiment permits
to check that the deposit (in the course of its building) is not oxidized.
The integration of the first scan between −0.2 and +0.4 V permits to
Two precise examples are focused on:
−
8
−2
a) The one-electron reduction of bromo-methylpyrene 2 (as pictured in
Fig. 2) permitted to obtain a deposit after an electrolysis at −0.6 V
obtain a superficial concentration Γenol =1.35×10
mol cm . A
fixed potential electrolysis of the α-dibromoenol ether at −0.6 V
−
3
−2
with an amount of electricity of 0.35×10 C mm . After rinsing
with acetone, the layer is immediately allowed to react with bromine.
As exhibited in Fig. 2C, the bromine index estimation is complicated
by the electrochemical conversion of the α-dibromoenol ether
(no oxidation of the enol form) provides a very similar value
−
8
−2
(1.6×10
mol cm ). Such experiments suggest the formation of
a thick layer having the structure of a redox polymer (based on the
reversible oxidation of {\O(Pyrene)C_C} moiety). The motion of

4
J. Simonet / Electrochemistry Communications 31 (2013) 1–4
electrons inside the layer is slow, presumably being limited by the
The high reactivity of alkenoxy radicals is certainly based on polymeri-
zation of the original deposit via successive radical additions (Scheme 1,
reactions (6)). Thus, obtained redox polymers (as poly-enol ethers) cor-
respond roughly to one hundred layer forms. Specifically, the reactivity
of enol forms makes possible chemical modifications making this at-
tached redox polymer highly versatile.
−
concomitant diffusion of BF
4
ions.
b) Another example is also displayed in Fig. 2 (inset) by direct reduc-
−
3
−2
tion of 1b at −0.8 V (amount of electricity: 3×10 C cm ).
After sonication, a large reversible signal is revealed (apparently rel-
ative to one-electron exchange) with a progressive decay of the sig-
nal upon recurrent scans. The reduction step (first scan) is sharp and
symmetrical. On the basis of one-electron transitions, it allows to es-
Acknowledgments
−
8
−2
timate the coverage as Γenol=1.4×10
mol cm
.
The author is extremely grateful to Pr V. Jouikov (Université de
Rennes) for helpful comments about the manuscript.
4
. Conclusion
The electrochemical reduction of ω-bromomethylketoarenes in or-
References
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especially towards GC known to contain large amounts of polyarenes.
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[
(