Room-temperature ionic liquids as new solvents for organic electrosynthesis. The first examples of direct or nickel-catalysed electroreductive coupling involving organic halides

Article abstract of DOI:10.1039/b302944a

Direct or Ni-catalysed electroreductive homocouplings of organic halides and couplings of organic halides with activated olefins are efficiently conducted by constant current electrolyses in an undivided cell in room-temperature ionic liquids as the solvent-electrolyte media.

Full text of DOI:10.1039/b302944a

Room-temperature ionic liquids as new solvents for organic
electrosynthesis. The first examples of direct or nickel-catalysed
electroreductive coupling involving organic halides
Rachid Barhdadi,* Coralie Courtinard, Jean Yves Nédélec and Michel Troupel
Laboratoire d’Electrochimie Catalyse et Synthèse Organique (UMR 7582), CNRS-Université Paris 12 Val
de Marne, 2 rue Henry Dunant, 94320 Thiais, France. E-mail: barhdadi@glvt-cnrs.fr; Fax: (+33)149 78 11
48
Received (in Cambridge, UK) 14th March 2003, Accepted 9th April 2003
First published as an Advance Article on the web 16th May 2003
Direct or Ni-catalysed electroreductive homocouplings of
organic halides and couplings of organic halides with
activated olefins are efficiently conducted by constant
current electrolyses in an undivided cell in room-tem-
perature ionic liquids as the solvent–electrolyte media.
carbons and EtO2. These features allow easy product separation
and solvent recycling. Thus, following electrolysis, the organic
products are extracted by treating the electrolytic solution with
pentane or EtO2, and then the solvent is washed with an aqueous
solution of NaBF4† in order to eliminate metallic salts arising
from the anodic reaction. After drying in a vacuum oven the
RTIL is used again in further experiments.
Environmental considerations now constrain chemists to in-
tegrate the control of risks (and costs) into their procedures and
the recent literature reflects various approaches aiming at eco-
friendly processes.
In a typical experiment, 20 mmol of RX were added to 40 mL
of [octyl-mim][BF4]. The electrolysis was conducted by setting
a constant current intensity between the nickel grid cathode and
a magnesium or aluminium bar anode until GC analysis of
samples indicated the consumption of RX. We noticed that, in
spite of its ionic structure, the RTIL was not efficient enough as
an electric conductor, very likely on account of its high
viscosity, which would limit the ion mobility. This results in an
important ohmic drop, and, therefore a high cell voltage when
the current intensity is increased. This drawback can be easily
overcome either by heating the solution to 40–50 °C or by
adding a small amount of an organic co-solvent (e.g. DMF,
5–10% v/v) or even better, when necessary, by combining these
two ways. In these reaction conditions, the electrolyses were
conducted under a cathodic current density up to 5 mA cm22
with a moderate cell voltage (1.5–10 V), which are usual values
for electrosynthetic processes in organic media. Results are
given in Table 1.
Bibenzyl was obtained in good yields from benzyl bromide or
benzyl chloride after a reaction time involving an electric
charge of 1.1 Faraday per mol of organic halide, which means
a current efficiency close to 100%. The yield is also good in the
case of dichlorodiphenylmethane, which leads to an olefinic
coupling product after a four-electron reduction (2 F per mole of
starting halide). For an alkyl bromide, R–R is formed with a
moderate yield, the alkane RH and the alkene R(–H) were
detected as by-products.
In this context, electrosynthesis remains an attractive method
since the use of electrons as a reagent does not involve the
formation of any side-products. Another approach aspiring to
“green chemistry” is the use of eco-friendly solvents in organic
syntheses. This explains the rapidly growing interest in the
properties of room-temperature ionic liquids (RTILs) in recent
years. Indeed, it has already been demonstrated that a wide
range of organic syntheses, including catalysed reactions, can
be carried out in these alternative non-flammable solvents,
which are attractive for their easy access, thermal stability, low
vapour pressure but also because they allow simple product
recovery and solvent recycling.1–3
Regarding electrochemical processes, RTILs are also promis-
ing solvents owing to their inherent conductivity which avoids
the addition of a supporting electrolyte. Moreover, provided that
both cation and anion are chosen appropriately, a non-protic
medium with a large electrochemical window can be obtained.
Many investigations have already been devoted to identifying
RTILs as suitable media notably in batteries, fuel cells,
photovoltaic cells or electroplating processes. However, to our
knowledge, only one organic reaction combining RTILs and
electrochemistry has been reported. Moreover, this synthesis is
a non-faradaïc reaction in which a small amount of electricity
engaged in the electroreduction of CO2 generates catalytic
species responsible for the addition of CO2 to epoxides to yield
cyclic carbonates.4
We have also developed in the past many C–C bond forming
reactions involving electrochemistry and the use of transition
metal complexes as homogeneous catalysts.7 Thus we started to
investigated electroreductive nickel-catalysed couplings in a
RTIL with the homocoupling of an aryl halide into a biaryl (eqn.
(2)).
We want to exemplify in this paper that organic electro-
syntheses can be efficiently achieved in RTILs and we report
herein our preliminary results. As a first example we performed
the electroreductive homocoupling of some alkyl- or benzyl-
halides RX (eqn. (1)).
(2)
2RX + 2e2 ? R–R + 2X2
(1)
The electrochemical cell is identical to the one previously
used with common organic solvents:5 electroreduction occurs at
a metallic grid cathode while the anode is a sacrificial metallic
bar (e.g. Mg, Al, Fe) which is electrooxidized, without any
diffusion limit, into the corresponding cation (Mg2+, Al3+,
Fe2+). In addition, the use of sacrificial anodes makes
electroreductive synthesis a cheap process which can be easily
conducted in undivided cells. We selected as the solvent–
electrolyte media, salts of the type [R-mim][BF4], where R is a
linear alkyl chain and, in particular, we used octyl-methylimida-
zolinium tetrafluoroborate [octyl-mim][BF4]. These salts are
easily prepared by conventional methods6 and they dissolve
organic halides while being immiscible water, saturated hydro-
Table 1 Direct electroreductive coupling of organic halides in [octyl-
mim][BF4]a
R–X
Isolated R–R
Yield (%)
C8H17Brb
Ph–CH2Br
Ph–CH2Cl
Ph–CCl2–Ph
C16H34
48
75
78
68
Ph–CH2–CH2–Ph
Ph–CH2–CH2–Ph
Ph2CNCPh2
a General conditions: [octyl-mim][BF4] (40 mL) + DMF (4 mL), RX (20
mmol), nickel grid cathode (20 cm2), Mg or Al bar anode, RT, I = 0.1 A
unless otherwise stated. b I = 0.05 A.
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Table 3 Ni-catalysed electroreductive arylation of activated olefins in
This reaction has been previously carried out in aprotic as
well as in protic solvents.8 The catalytic species is generated by
the electroreduction of the divalent Ni(bpy)2+ complex into the
zerovalent Ni0(bpy) complex which reacts by oxidative addition
to the aromatic halide. More details on the catalytic cycle have
been described elsewhere.9
[octyl-mim][BF4]a
ArBr
Activated olefin
Product and isolated yield (%)
Ph–Br
Ph–Br
CH2NCH–CO–Me Ph–CH2–CH2–CO–Me, 58
CH2NCH–CO2–Bu Ph–CH2–CH2–CO2–Bu, 61
For these experiments, the solvent–electrolyte medium
[octyl-mim][BF4]–DMF (90 : 10 v/v) was added to ArX and
NiBr2bpy (5% vs. ArX) as the catalyst precursor. The
electrosyntheses conducted in the undivided cell under a
constant current (5 mA cm22) were efficient in these conditions.
Fig. 1 shows that ArX (e.g. PhBr) was almost fully consumed
within the theoretical time (one mol of electron engaged per mol
of PhBr), corresponding to a faradaïc yield close to 100%. As
expected, biphenyl is the major product formed in 80% isolated
yield.
The results were very similar with either a nickel, a stainless
steel or an iron bar as the sacrificial anode.
Such excellent current efficiencies and good chemical yields
were observed with various other aryl halides as indicated in
Table 2. These results are as good as those previously obtained
in classical organic solvents.8a
Ph–Br
, 41
3-MeO–C6H4Br CH2NCH–CO–Me 3-MeO–C6H4–(CH2)2–CO–Me, 42
a General conditions: [octyl-mim][BF4] (40 mL) + DMF (4 mL), ArX (20
mmol), activated olefin (30 mmol), NiBr2 (1 mmol), nickel grid cathode (20
cm2), stainless steel bar anode, T ≈ 40 °C, I = 0.1 A, electric charge =
2.8–3 F per mol of ArBr.
Using the experimental device and electrolysis conditions
identical to those described above, the conjugate additions were
carried out in solutions containing a mixture of an aromatic
bromide, an activated olefin in slight excess and a catalytic
amount of NiBr2·3H2O. The results presented in Table 3 show
that the reaction is also possible in [octyl-mim][BF4] since the
arylated olefins are obtained in moderate yields, ArH being the
major side-product. Yields and current efficiencies (60–70%)
are however slightly lower than those obtained at 70–100 °C in
DMF–pyridine mixtures10 but experimental conditions in
RTILs have not been optimized.
This reaction can also be conducted in RTILs other than
[octyl-mim][BF4], e.g. Ni-catalysed electrocoupling between
bromobenzene and butyl acrylate in octylpyridinium tetra-
fluoroborate gave butyl 3-phenylpropionate in 52% yield.
In conclusion, we have demonstrated with these preliminary
experiments that direct or Ni-mediated electrosyntheses by
reductive coupling can successfully achieve C–C bond forma-
tion in simple and mild conditions by using RTILs as solvent–
electrolyte media. This shows that three approaches, electro-
chemistry, homogeneous catalysis and eco-friendly solvents,
aimed at “green chemistry” can be advantageously combined.
A second example of a Ni-catalysed electroreductive cou-
pling in a RTIL is the arylation of an activated olefin (eqn.
(3)).
(3)
This reaction has been previously reported, using NiBr2 as
catalyst precursor in mixed solvents DMF–pyridine (9 : 1) or
DMF–MeCN (1 : 1).10 Although the mechanism of the reaction
remains to be clarified, it should be pointed out that pyridine or
acetonitrile probably interact with the electrogenerated low
valent Ni-species, thus preventing precipitation of nickel metal.
The addition of a nitrogeneous co-solvent is no longer required
since the RTIL [octyl-mim][BF4] can act similarly owing to the
presence of one nitrogen atom bearing a free electron-pair.
Notes and references
† The use of NaBF4 (0.1 mol L21) in the aqueous washing solutions allows
good separation of [octyl-mim][BF4] which is immiscible in water while
[octyl-mim]Br is partially miscible.
1 Ionic Liquids Industrial Application of Green Chemistry, Ed. R. D.
Rogers and K. R. Seddon, A. C. S. Symposium Series, no. 818), 2002,
and references cited therein.
2 Ionic Liquids in Synthesis, Ed. P. Wasserscheid and T Welton,
Chichester, Wiley Europe, 2003, and references cited therein.
3 H. O. Bourbigou and L. Magna, J. Mol. Catal., 2002, 182–183,
419–437and references cited therein.
4 H. Yang, Y. Gu, Y. Deng and F. Shi, Chem. Commun, 2002,
274–275.
5 (a) J. Chaussard, J. C. Folest, J. Y. Nédélec, J. Périchon, S. Sibille and
M. Troupel, Synthesis, 1990, 369–381; (b) J. Y. Nédélec, J. C. Folest
and J. Périchon, J. Chem. Res. (S), 1989, 394.
Fig. 1 Progress vs. time of the Ni(bpy) catalysed conversion of PhBr into
Ph–Ph in [C8H17-mim][BF4]. (Experimental conditions are as indicated in
Table 1.) —:—:— residual PhBr, —0—0— yield of Ph–Ph, - - - - -
theoretical curves.
Table 2 Ni(bpy) catalysed electrosynthesis of biaryls from aryl halides in
[octyl-mim][BF4]a
6 J. D. Holbreyand and K. R. Seddon, J. Chem. Soc., Dalton Trans., 1999,
2133–2139.
7 J. Y. Nédélec, J. Périchon and M. Troupel, Top. Curr. Chem., 1997, 185,
141–173 and references cited therein.
ArX
Ar–Ar and isolated yield (%)
8 (a) Y. Rollin, M. Troupel, D. G. Tuck and J. Périchon, J. Organomet.
Chem., 1986, 303, 131–137; (b) V. Courtois, R. Barhdadi and M.
Troupel, Tetrahedron, 1997, 53, 11569–11576.
9 (a) M. Durandetti, M. Devaud and J. Perichon, New J. Chem., 1996, 20,
659–661; (b) T. Yamamoto, S. Wakabayashi and K. Osakada, J.
Organomet. Chem, 1992, 428, 223–228.
Ph–Br
4-CF3–C6H4Cl
3-CF3–C6H4Br
Ph–Ph 80
4-CF3–C6H4–C6H4–4A-CF3 86
3-CF3–C6H4–C6H4–3A-CF3 72
a General conditions: [octyl-mim][BF4] (40 mL) + DMF (2 mL), ArX (20
mmol), NiBr2(bpy) (1 mmol), nickel grid cathode (20 cm2), iron bar anode,
T ≈ 40 °C, I = 0.1 A, electric charge = 1.1 F mol21
10 S. Condon, D. Dupré, G. Falgayrac and J. Y. Nédélec, Eur. J. Org.
Chem., 2002, 105–111 and references cited therein.
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