Reduction of aryl halides at transition metal cathodes. Conditions for aryl-aryl bond formation: The Ullmann's reaction revisited

Article abstract of DOI:10.1016/j.elecom.2010.03.032

The cathodic reduction of some aryl halides ArX (1-naphthyl halides NpI and NpBr, iodo benzene and bromobenzene PhI and PhBr taken as model substrates) was achieved essentially in propylene carbonate (PC) considered for its high dielectric permittivity. Different electrode materials such as copper, silver, palladium, silver palladium alloy and nickel were used. Such conditions permit the activation of the C-X bond by metal (the step featuring similarity with Ullmann's reaction). Electron transfer to organometallic intermediates generated at the metal interface activates the formation of Ar-Ar linkages often in good yields, especially in the case of aryl iodides.

Full text of DOI:10.1016/j.elecom.2010.03.032

Electrochemistry Communications 12 (2010) 781–783
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Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Reduction of aryl halides at transition metal cathodes. Conditions for aryl–aryl
bond formation
The Ullmann's reaction revisited
Viatcheslav Jouikov a, Jacques Simonet b,
⁎
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 9 March 2010
Received in revised form 26 March 2010
Accepted 26 March 2010
Available online 31 March 2010
The cathodic reduction of some aryl halides ArX (1-naphthyl halides NpI and NpBr, iodo benzene and
bromobenzene PhI and PhBr taken as model substrates) was achieved essentially in propylene carbonate
(PC) considered for its high dielectric permittivity. Different electrode materials such as copper, silver,
palladium, silver palladium alloy and nickel were used. Such conditions permit the activation of the C–X
bond by metal (the step featuring similarity with Ullmann's reaction). Electron transfer to organometallic
intermediates generated at the metal interface activates the formation of Ar–Ar linkages often in good yields,
especially in the case of aryl iodides.
Keywords:
Electrochemical Ulmann's reaction
Cathodic reduction
Aryl iodides
© 2010 Elsevier B.V. All rights reserved.
Homo-coupling
1. Introduction
C–X bond scission under electron transfer when using carbon
electrode materials. Studies on dissociative electron transfer were
The easy formation of aryl–aryl linkages is an important goal in
chemistry, particularly in supramolecular chemistry or in the
preparation of redox oligomers. There exist efficient methods for
building aryl–aryl bonds. Thus, metals such as Mg0, Pd0 and Ni0 may
react with C–X bonds of aromatic halides forming, via oxidative
insertion (mainly in the presence of appropriate ligands), organome-
tallic linkages e.g. C–PdII–X known to promote a large palette of C–C
couplings by the reaction with electrophiles like RX or Ar′X [1]. Worth
quoting is Gattermann's method of dediazotation (reduction of
diazonium salts by Cu in acids) which remains a great method for
creating aryl radicals, which was used with success for building Ar–Ar′
linkages or for the functionalization of radicalophilic surfaces [2].
Similarly, ArX (when X=I) can be converted into bi-aryls in the
presence of copper (Ullmann's reaction [3]). The proposed key
intermediate is an organometallic complex that could be written as
ArCuI,X−. The species of this kind were formed [4] by contact of RBr
with highly activated copper (formation of RCuI). These intermediates
are often considered as nucleophilic species for reacting with organic
halides. It is worth underlining that Ullmann's reaction is essentially
performed with activated ArI (Scheme 1, Eq. (2)) at high temperatures.
In general, direct electrochemical reductions do not yield Ar–Ar
linkages [5]. Until now, the data concerning ArXs deal mostly with the
carried out in polar organic solvents [6]. The transient aryl radical Ar•
is very easy to reduce (E0 − of phenyl radical is 0.05 V vs. SCE [7]) so
R/R
the overall process yields the anion Ar−. Recently [8], the voltam-
metric reduction of a large palette of ArBrs was achieved at smooth
silver and the interest of using transition metals as electrode material
was underlined. Large increases in reduction potentials were
expected to be due to the selective affinity of Br− toward silver.
The studies [9,10] using metals as cathode materials (copper, nickel,
silver and all these materials often associated to palladium in form of
alloys) prompted us to re-consider the electrochemical aryl–aryl bond
formation. This approach was based on the oxidative insertion involving
metals now also used as a cathodic material: more precisely, electro-
active species chemically formed according to Eq. (1) (Scheme 1) could
be electrochemically activated for creating more efficient nucleophiles.
Additionally, in order to boost the reactivity of ArX towards this
nucleophile, the use of solvents of high dielectric constants is proposed
for stronger polarization of the C–X bond increasing its electrophilicity.
For this, PC possessing dielectric constant ε=66, – much higher than
that of DMF (ε=37), – was successfully used. This preliminary work is
focused on the capacities of ArXs to form dimers under changing such
experimental conditions as solvents and electrode materials.
2. Experimental
All electrochemical studies were conducted in freshly prepared
⁎
Corresponding author. Tel.: +33 23236292; fax: +33 23236732.
E-mail address: jacques.simonet@univ-rennes1.fr (J. Simonet).
0.1 M solution of tetra n-butylammonium tetrafluoroborate (TBABF4)
1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.elecom.2010.03.032

782
V. Jouikov, J. Simonet / Electrochemistry Communications 12 (2010) 781–783
silver and smooth gold stationary electrodes, half-peak potentials of
−1.24 V, −1.30 V and −1.40 V are respectively observed. Fig. 1
(curves B a, b, c and d) exhibits the reductions obtained at Ag–Pd and
Ag electrodes, and those at glassy carbon surfaces modified by very
thin (δb0.05µm) deposits of nickel and copper. Those catalytic steps
are immediately followed by a reversible step (IV) that was identified
as resulting from the reaction of aryl anion with organic carbonates
(e.g. with PC, it is ring opening with the formation of an aromatic ester
moiety reversibly reducible under these conditions).
PhI behaves quite similarly: Fig. 1 (curves C and D) shows that the
reduction process is more complex than in DMF or AN. At a GC
electrode, a two-electron step I (Ep/2 =−1.93 V) corresponds to the
cleavage of the C–I bond. With Ag–Pd, a catalytic reduction peak (IIIc)
(diffusion controlled) is obtained (Ep/2 =−1.19 V) while deposits of
Cu, Ag, Pt, Pd and Ni onto GC show half-peak potentials equal to
−1.38 V, −1.46 V, −1.46 V, −1.40 V and −1.47 V, therefore within a
rather narrow potential range. Fig. 1, curves D b and c display the
responses of the organic iodide at GC–Ni and GC–Cu surfaces. At more
negative potentials, the reversible step (IV at −2.11 V) specific for the
addition of the aryl anion to PC is observed.
Scheme 1.
in dimethylformamide (DMF), acetonitrile (AN), and PC. All potentials
refer to aqueous saturated calomel electrode (SCE). The electrochem-
ical instrumentation was previously reported [10].
Glassy carbon (GC), nickel, gold, copper and silver disks used as
solid substrates had a surface area of 0.8 mm2. Prior to use, all
electrodes were carefully polished with Norton polishing paper (type
02 and 03). Metals could be also deposited galvanostatically onto GC
from acidic solutions of their salts. By this way, electrodes denoted as
GC-M were obtained. Such depositions were carried out with
electricity amounts of 2×10−3 °C mm−2 at a current density of
500 µA mm−2 (average thickness≪0.1 μm). Before use, the electro-
des were thoroughly rinsed with alcohol and acetone.
ArBrs display similar results at transition metal surfaces. So at an
Ag–Pd electrode, NpBr exhibits the step IIIc at −1.38 V and PhBr
shows this step at −1.40 V. Voltammetry data obtained at solid
surfaces modified by the deposits of Cu, Ag, and Pd are rather close to
those obtained with parent ArIs.
One might expect that the materials clearly leading to potential
shifts in voltammetry, in particular Ag, Ag–Pd, Cu or Pd, preliminarily
react with ArXs (Scheme 1, Eq. (1)). Indeed, the solutions of ArXs in
DMF and PC put in contact with smooth Cu or Ag plates clearly
provoke (as shown in Fig. 2) a form of corrosion of the latter. Copper
surface shows pinholes and copper crystallites, suggesting then a
reaction of the halide with Cu followed by a disproportionation (in
fact a mechanism already evoked for the Ullmann's reaction). At
silver, a similar feature is obtained. The material is hollowed with a lot
of channels of very small size scattered among Ag nano-particles.
Additionally, the formation of an organometallic transient (Eq. (1)) is
corroborated by reduction of organic halides in the presence of fine
metal powders when taking a bare carbon electrode as a cathode and
polarizing it at the potential at which ArX is not cleaved in the absence
of catalyst. Stirring the solution allows the formation of a 3D electrode
with metal particles playing the role of vector in this process. Thus, by
a preceding reaction of ArX at their surfaces followed by a discharge of
modified grains at the cathode, metal nano-particles of copper or
silver could be deposited at the GC surface.
Ag–Pd alloy electrodes were prepared according to a specific
procedure fully described in a previous report [10].
3. Results
The voltammetric reduction of aryl iodides in PC is summarized by
the data shown in Fig. 1. At a glassy carbon electrode (curve A), two
main reduction steps (I and II) are obtained with NpI, corresponding
to the two-electron cleavage of the C–I bond (step I) and the
reversible reduction of the produced naphthalene (step II); these
steps have Ep/2 =−1.78 V and −2.48 V, respectively. With palladium,
silver, copper, nickel and gold electrodes, a catalytic step (IIIc) is then
observed and all these materials display very large positive potential
shifts relative to the potentials at carbon. Thus, at Ag–Pd, smooth
Lastly, potentiostatic electrolyses with different cathode materials
were performed in a solvent like PC (or other organic carbonates) but
also in DMF and AN in order to check the solvent effect. Most striking
results obtained in PC are gathered in Table 1. Especially, reductions of
NpI in PC lead to 1,1′-binaphthyl in quite high yields (runs 2 and 3).
Electrolyses with different concentrations and in the presence of
several tetraalkyl ammonium salts confirm these results. It was also
found that fairly dry PC (kept under activated alumina) allowed higher
yields of Np–Np without formation of naphthalene. Small amounts of a
1-naphthoate ester correspond to the addition of the anion Ar− to PC.
This side reaction occurs whatever the experimental conditions. PhI
similarly affords Ph–Ph (entry 1) as major product and certainly in a
good yield. Still with NpI, the use of smooth Cu and Ag also led to the
dimer but unexpectedly, quite large amounts of NpH were concom-
itantly obtained (entries 4 and 5). Entry 6 confirmed that the use of less
dry PC (not maintained under activated neutral alumina) favors the
two-electron scission at Ag with an increase in the relative amount of
NpH. Additionally, reductive cross-couplings of mixtures of [NpI+PhI]
were successfully achieved (entry 7). Phenyl acetylene considered as a
radical trap (entry 8) could not confirm free aryl radicals as transients
(no addition across the triple bond was observed). All experiments
performed with ArBrs in PC yielded dimers in trace amounts only
Fig. 1. Voltammetry of ArIs at solid electrodes (0.8 mm2) in PC+TBABF4. Scan rate
50 mV s−1. (A) — reduction of NpI (5.5 mmol L−1) at GC and (B) — at Ag–Pd (a), smooth
silver (b), and at modified carbon surfaces: GC–Ni (c) and GC–Cu (d). (C) — reduction of
PhI (6.3 mmol L−1) at GC and (D) — at Ag–Pd electrode (a), GC–Cu (b), GC–Ni (c), and
GC–Pd (d) electrodes.

V. Jouikov, J. Simonet / Electrochemistry Communications 12 (2010) 781–783
783
Table 1
Potentiostatic electrolyses of ArX in PC and in mixtures of diethylcarbonate (DEC) with
DMF in a divided cell. Electrolyte: TBABF4. Amounts of ArX: about 0.2 mmol. Applied
potential: −1.6 V vs. SCE. Average amount of electricity for a total conversion of ArI:
1.5 F mol−1. (a) — the relative proportion of reduction products was obtained by ether
extractions followed by GC/MS analyses. (b)
— the reaction with solvent (here
essentially cyclic carbonate, PC) corresponds to the nucleophilic attack of the
a
electrogenerated Nu (Ar−) on the C O of the carbonate group concomitantly with the
ring opening. Here, the predominant isomer is ArC(O)OCH2CH(CH3)OH (c) — PC not
kept over activated alumina. (d) — equimolar mixture of two ArXs. (e) — PAC:
phenylacetylene in a 1:3 molar ratio to ArX. (f) — the reaction was not totally
completed and a large amount of the starting compound left. (g) — formation of
ethylbenzoate.
Run Electrode/ Substrate Products, %a
solvent
ArX
ArH
Ar–Ar
Ar–solv– Other products, %
OHb
1
2
Ag–Pd/PC PhI
–
87
72
13
22
–
–
Ag–Pd/
NpI
6
PCc
3
4
5
6
7
8
Ag–Pd/PC NpI
–
78
21
43
18
20
26
14
13
Di- and tetrahydro
forms of Np–Np b2
–
Smooth
Cu/PCc
Smooth
Ag/PC
NpI
NpI
NpI
53
42
68
–
–
Smooth
Ag/PCc
Ag–Pd/PC PhI+
NpId
NpH 24 (Ph) 4
(Ph) 22 Cross-coupling
(Np) 27 (Np) 11 Np–Ph 12
52
Ag–Pd/PC NpI+
24
20
(1-naphthyl)-phenyl-
acetylene 3
PACe
Dihydro forms b1
–
9
10
Ag–Pd/PC NpBrf
68
–
0.1
–
32
Ag–Pd/
PhBr
100g
DEC-DMF
(90:10)
and Cu. Best yields are obtained with Ag–Pd presumably because of a
synergy of the concomitant action of two metals. If the use of such
metallic surfaces is necessary, employing organic solvents of high
dielectric permittivity such as PC appears essential. It has been shown
that the coupling process does not involve aryl radicals as inter-
mediates. It is expected that the key reaction is the one given by Eq.
(4) although the intrinsic mode of reactivity remains unclear. The
activation of the organometallic intermediate by electron transfer
seems strongly increasing its nucleophilicity (decreasing ΔG≠) to
allow its much faster reactivity on the starting halide. Thus,
electrochemical activation has been already used for accelerating
the reaction of Pd–aryl complexes [11]. Along with this activation, the
ArX electrophilicity must be concomitantly boosted and this is
apparently achieved by higher dissociative ability of PC. For the
moment, this kind of “electrochemical” Ullmann's reaction remains
valid for aryl iodides only.
Fig. 2. MEB images showing the reactivity of ArXs with metals. (A) — smooth copper in
the contact with NpI during 4 h. (B) — smooth commercial silver after the contact with
4-iodobiphenyl (5×10−3 mol L−1) in PC during 6 h in the dark. (C) — smooth silver in
contact with pure NpBr in the dark during 6 h. After reaction, all samples were
consecutively rinsed with ethanol and acetone.
References
[1] E. Negishi, General Discussions and Organomatallics of Main Group Metals in
Organic Synthesis, Organometallics in Organic Synthesis, Vol. 1, John Wiley and
Sons, N.Y, 1980.
[2] J. Pinson, F. Podvorica, Chem. Soc. Rev. 34 (2005) 429.
[3] M. Sainsbury, Tetrahedron 36 (1980) 3327.
[4] R.D. Rieke, D.E. Stack, B.T. Dawson, T.-C. Wu, J. Org. Chem. 58 (1993) 2483.
[5] D.G. Peters, in: H. Lund, O. Hammerich (Eds.), Organic Electrochemistry, Fourth
Edition, Marcel Dekker, N.Y, 2001, p. 343.
[6] J.M. Saveant, in: T.T. Tidwell (Ed.), Advances in Physical Organic Chemistry,
Academic Press, N.Y, 2000, p. 117.
(NpBr, entry 9). The use of ethylene carbonate EC (with 20% of PC
added to maintain the solution liquid) only allowed 2% of Np–Np. PhBr
did not lead to any traces of biphenyl (entry 10). Lastly, it has been
checked that DMF and AN under the conditions of Table 1 were unable
to afford any coupling products with the studied ArIs, leading instead
to the quasi-selective formation of ArHs.
[7] C.P. Andrieux, J. Pinson, J. Amer. Chem. Soc. 125 (2003) 14801.
[8] A. Isse, P.R. Mussini, A. Gennaro, J. Phys. Chem. C 113 (2009) 14983.
[9] J. Simonet, P. Poizot, L. Laffont, Platinum Metals Rev. 52 (2008) 84.
[10] P. Poizot, L. Laffont-Dantras, J. Simonet, J. Electroanal. Chem. 624 (2008) 52.
[11] C. Amatore, E. Carré, A. Jutand, H. Tanaka, D. Qinghua, S. Torii, Chem. Eur. J. 2
(1996) 957.
4. Conclusion
This preliminary report emphasizes that the electro-coupling of
ArX can be achieved by using metallic electrodes such as Ag–Pd, Ag, Ni