The one-electron cleavage of benzylic bromides at palladium and palladized cathodes: Benzyl radicals generation and immobilization onto solid interfaces

Article abstract of DOI:10.1016/j.elecom.2009.12.009

Benzyl bromide 1 and 4-nitrobenzyl bromide 2 are reduced at solid electrodes in propylene carbonate (PC) and dimethylformamide (DMF) containing tetraalkylammonium salts. Palladium electrodes and these covered with layers of Cu-Pd, Ag-Pd, Au-Pd, and Ni-Pd were especially found to favour the one-electron scission of the C-Br bond. Under these conditions, 1 and 2 are reduced in two separated steps assigned to the transient formation of a free radical capable to couple or to add onto the cathodic material. The formation of benzyl radical allows the in situ addition onto unsaturated organic systems (mono- and di-benzylations). The formation of benzylic radical was confirmed by ESR technique (trapping the paramagnetic intermediate with nitrones). Lastly, preliminary experiments were achieved in order to test the grafting feasibility of benzyl radicals onto cathodic interfaces.

Full text of DOI:10.1016/j.elecom.2009.12.009

Electrochemistry Communications 12 (2010) 331–334
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Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
The one-electron cleavage of benzylic bromides at palladium and palladized
cathodes: Benzyl radicals generation and immobilization onto solid interfaces
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:
Benzyl bromide 1 and 4-nitrobenzyl bromide 2 are reduced at solid electrodes in propylene carbonate
Received 27 November 2009
Accepted 8 December 2009
Available online 16 December 2009
(PC) and dimethylformamide (DMF) containing tetraalkylammonium salts. Palladium electrodes and
these covered with layers of Cu–Pd, Ag–Pd, Au–Pd, and Ni–Pd were especially found to favour the one-
electron scission of the C–Br bond. Under these conditions, 1 and 2 are reduced in two separated steps
assigned to the transient formation of a free radical capable to couple or to add onto the cathodic mate-
rial. The formation of benzyl radical allows the in situ addition onto unsaturated organic systems (mono-
and di-benzylations). The formation of benzylic radical was confirmed by ESR technique (trapping the
paramagnetic intermediate with nitrones). Lastly, preliminary experiments were achieved in order to test
the grafting feasibility of benzyl radicals onto cathodic interfaces.
Keywords:
Benzyl radicals
Cleavage of carbon–halogen bond
Organic halides
Cathodic reduction
Surface modification
Ó 2010 Elsevier B.V. All rights reserved.
1
. Introduction
The scission of carbon–heteroatom bonds (C–X) by means of
much less negative potentials to afford selectively free benzyl rad-
icals. Therefore, bibenzyl formation indeed occurs according to a
radical homo-coupling and not via an S
N
2 displacement (Wurtz
metals is a fascinating subject since the reducing metals (transition
metal) can exercise ‘‘oxidative” insertion towards the C–X linkage
reaction) which can be expected in the absence of a proton donor.
Moreover, the interest of producing benzyl radicals directly at the
electrode surface (by metal involvement) prompted us to propose a
new mode for immobilizing such transients at electrode surfaces
(presumably by grafting, the method already developed with dia-
zonium ion reduction [10]).
[
1,2]. Even more fascinating is the electrochemistry that allows
using the metal as cathode material along with the possibility of
modifying its reduction power simply by fixing an appropriate po-
tential corresponding to the cleavage reaction to be achieved. The
present work is focused on the cleavage of carbon–halogen bonds
(
mainly belonging to organic halides) that was abundantly studied
at mercury and glassy carbon cathodes [3]. The recent studies
uncovered the tendency of boosting the ease of the cathodic cleav-
age of these substrates when using transition metals like palla-
dium (alkyl iodides [4]), silver (alkyl and aromatic halides [5,6])
and alloys of palladium (Ag–Pd and Cu–Pd for affording alkyl rad-
icals in reduction of alkyl halides [7,8]). Until now, the use of pal-
ladium, nickel or copper as cathode material for catalytic cleavage
of C–X bonds allowing one-electron scission of benzylic halides has
never been undertaken. If silver was recently used as cathode
material for cleaving benzyl bromide according to one- or two-
electron pathway, depending on the presence of a proton donor
2. Experimental
All electrochemical studies were conducted in a freshly pre-
pared 0.1 M solution of tetra n-butylammonium tetrafluoroborate
(TBABF ), tetra n-butylammonium bromide (TBABr) in dimethyl-
4
formamide (DMF) or propylene carbonate (PC). Benzyl bromides
1 and 2 were obtained from Aldrich with a purity minimum of
95%. Procedures reported therein do not require especially dry
solutions. All potentials refer to aqueous saturated calomel elec-
trode (SCE) and the electrochemical instrumentation was previ-
ously reported [8].
[
9], the present preliminary communication aims to show that pal-
Glassy carbon (GC), nickel, gold, copper and silver used as solid
substrates to be palladized, all had a surface area of 0.8 mm . Prior
to being modified, all electrodes were carefully polished with sili-
con carbide paper (Struers) or with Norton polishing paper (type
2
ladized electrodes may selectively lead, under comparable condi-
tions and within the same potential range, to benzyl radical as
well. The ESR study has shown the macro-reductions achieved at
0
2 and 03). Palladization processes were carried out by galvano-
static deposition of metal from solutions of palladium chloride
*
Corresponding author. Tel.: +33 23236292; fax: +33 23236732.
E-mail address: jacques.simonet@univ-rennes1.fr (J. Simonet).
À1
(10 g L in 0.1 N HNO
3
or HCl aqueous solutions). These deposits
1
388-2481/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.elecom.2009.12.009

3
32
V. Jouikov, J. Simonet / Electrochemistry Communications 12 (2010) 331–334
ilarly, palladization of Ni and Au was achieved by dipping into a
À1
4 2 4
PdSO -based solution (10 g L in 0.2 N H SO aqueous fresh solu-
tion) during 10 min. Obtaining palladium alloys with Cu and Ag by
means of a redox displacement is expected to occur with gold and
nickel [11].
3
. Results
3.1. Benzyl bromide
Voltammograms of 1 in PC and DMF are strongly dependent on
the cathode material. As exhibited in Fig. 1, the two-electron
reduction step (E
p
= À1.60 V, formation of the benzyl anion) ob-
served at glassy carbon, is strongly shifted towards less negative
potentials and splitted at palladium and palladized surfaces. Thus,
at Cu–Pd (curve B), Ag–Pd (curve C) and palladized palladium
(
curve D), the two-electron transfers are efficiently separated.
These two steps (peak potentials Ep1 = À0.97 V and Ep2 = À1.35 V
in the case of Ag–Pd, curve C) are assigned to the formation of
the free benzyl radical (step I) and the second one (step II) to its
reduction. The sum of currents (i
to the current obtained at GC. The global current is proportional
to 1 concentration. The current inequality (iII < i ) is likely due to
the fast coupling of the benzyl radical. Lastly, the overall process
was checked to be diffusion controlled.
I
+ iII) is more or less equivalent
À1
Fig. 1. Voltammetries of 1 (concentration: 8.6 mmol L ) at several electrodes.
À1
I
Solvent: PC containing 0.1 M TBABr. Scan rate: 50 mV s . (A) Glassy carbon. (B)
Copper–palladium. (C) Silver–palladium. (D) Palladized palladium (average thick-
À2
ness of Pd: 5 Â 10
lm).
Potential of step II fits well with values given previously [12] for
the standard reduction potential of benzyl radical (À1.215 V).
Therefore, reductions achieved at potentials <1 V may easily pro-
À2
À2
needed electricity amounts much smaller than 10 C mm using
a current density of 500
l
A mm2 (average thickness ꢀ 0.2
lm).
Before using, the electrodes were thoroughly rinsed with water,
alcohol and acetone and finally dried in a hot air stream.
Cu–Pd and Ag–Pd electrodes were prepared according to a spe-
cific procedure fully described in previous reports [7,8]. Quite sim-
Fig. 2. ESR spectra of spin-trapping adducts of benzyl radicals during the catalytic
À1
one-electron cleavage of 1 at Ag–Pd (a) and Ag (b) electrodes. The conditions are:
Fig. 3. Voltammetries of 2 at different palladized electrodes. Scan rate: 50 mV s
.
À1
À1
DMF + 0.1 M TBABF
4
, concentration of 1: 10 mmol L . Presence of TBPN is in a
(A) and (B): concentration: 7.4 mmol L . Electrolyte: DMF + 0.1 M TBABF
4
. Elec-
À1
threefold molar excess. Applied potentials: À0.85 V with (a) and À1.2 V with (b).
trode materials: Ni–Pd and Au–Pd respectively. (C): Concentration: 5.3 mmol L
.
Total electricity amounts in both cases: 7 C. Spectrum (c) corresponds to a
PC + 0.1 M TBABr. Palladized glassy carbon electrode (average thickness of palla-
dium layer: 5 nm). Hold at À0.6 V, sonication of the electrode during 2 min, and
thorough rinsing with water and acetone: three first scans exhibited.
simulation on the following bases: g = 2.0066, a
N H
= 14.55 G, a = 2.38 G fitting with
the trapping of a benzyl radical under similar conditions [13].

V. Jouikov, J. Simonet / Electrochemistry Communications 12 (2010) 331–334
333
duce this radical. In order to support the formation of benzyl rad-
ical as intermediate, the ESR technique employing N-tert-butyl-
a-
phenylnitrone (TBPN) as trap in excess, allows checking the forma-
tion of benzyl radical as transient (Fig. 2). The six-ray ESR response
obtained (spectra (a) and (b) obtained at Ag–Pd and Ag surfaces,
respectively) matches with a nitroxide already quoted in literature
implying benzyl radical [12]. Additionally, in situ electrogenerated
benzyl radicals can be allowed to add to unsaturated organic rad-
icalophilic systems. In concrete terms, generation of benzyl radical
at Ag–Pd (fixed potential: À0.8 V) in the presence of 2,5-dimethyl-
hexa-2,4-diene yields exclusively dibenzylhexamethyldecadiene
(
28%) together with the expected bibenzyl of which the formation
9
À1 À1
from the radical was reported [12] to be fast (10 M
tion to acenaphthylene provides 10% of mono-benzylation and
% of di-benzylation. Lastly, phenyl acetylene affords monoben-
zyl-styrene (3%) and dibenzylstyrene (7%).
s ). Addi-
7
3.2. 4-Nitrobenzyl bromide
The second species involved in this preliminary study may pro-
Scheme 1.
duce an electrophoric intermediate (nitrobenzyl radical) whose
fate can be followed by electrochemistry. Voltammetry in DMF at
several palladized electrodes (Pd and palladized GC, Ag–Pd, Cu–
Pd, Au–Pd, and Ni–Pd) exhibits (Fig. 3, curves A and B) two steps
I and II that are assigned to the formation of the 4-nitrobenzyl rad-
ical and its reduction. Similarly to the data given above with 1, re-
sults obtained in ESR and radical additions to some unsaturated
systems do confirm such assumption. While peak I potentials
slightly vary with the cathode material (maximum with Ag–Pd:
E
p
= À0.56 V), potentials of peak II are almost the same (E
p
=
À0.83 ± 0.02 V) suggesting that this value might be considered
close to that of the standard reduction potential of the radical.
The potential shift induced by surface catalysis relative to glassy
carbon electrode is much less than in case of 1. This is quite cer-
tainly due to the polarization induced by the nitro group. If now
PC is used as solvent (high dielectric constant), peak separation
could be observed even with Pt and GC. This finding is of great
importance because it renders possible the formation of substi-
tuted benzyl radicals at a wide range of cathode materials, and
therefore it allows considering their addition onto carbons or their
immobilization (or stabilization) at metal surfaces. A first example
is given in Fig. 3, Curve 3 where the benzyl radical appears to be
reversibly grafted at a GC–Pd surface. Finally, the SEM images of
carbon as well as of several metallic surfaces modified by the pres-
ent method (Fig. 4) clearly show that great changes (islands of dif-
ferent conductivity with GC and light veil on Pd) are obtained.
4
. Conclusion
The use of palladium and palladized surfaces may catalyze the
one-electron scission of benzyl bromides to a large extend. The cat-
alytic process is expected to be due to the insertion of Pd into the
C–Br bond (Scheme 1). Potentials’ shifts were found to be huge (at
least with benzyl bromide) and even larger than the ones recently
reported with pure silver [9]. The transient formation of free radi-
cals was shown by ESR (spin-trapping technique) and by addition
across double and triple bonds. The use of PC as solvent permits
boosting the cathode effect and makes possible a general method
for benzyl radical generation allowing grafting onto a large palette
of conductors.
Fig. 4. Surface modification after generating 4-nitrobenzyl radical at (a) glassy
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À2
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C. Both samples were carefully sonicated and rinsed with water and then acetone
before SEM examination.
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