Cobalt bromide as catalyst in electrochemical addition of aryl halides onto activated olefins

Article abstract of DOI:10.1016/S0040-4039(00)00420-2

The consumable anode process permits the electrochemical arylation of activated olefins from functionalized aryl halides when cobalt halide is used as catalyst, either associated with bipyridine and pyridine as ligands in DMF as solvent, or with only pyridine in acetonitrile as solvent. (C) 2000 Published by Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00420-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 3385–3388
Cobalt bromide as catalyst in electrochemical addition of aryl
halides onto activated olefins
∗
Paulo Gomes, Corinne Gosmini, Jean-Yves Nédélec and Jacques Périchon
Laboratoire d’Electrochimie, Catalyse et Synthèse Organique, UMR 7582, Université Paris 12-CNRS 2, Rue Henri Dunant,
F-94320 Thiais, France
Received 18 February 2000; accepted 10 March 2000
Abstract
The consumable anode process permits the electrochemical arylation of activated olefins from functionalized
aryl halides when cobalt halide is used as catalyst, either associated with bipyridine and pyridine as ligands in
DMF as solvent, or with only pyridine in acetonitrile as solvent. © 2000 Published by Elsevier Science Ltd. All
rights reserved.
Conjugate additions onto activated olefins are of great utility in organic synthesis for carbon–carbon
1
bond formation. Most applications involve the reaction of either organometallic reagents in the presence
2
of cuprous salt and, in some cases, chlorotrimethylsilane, or cuprate reagents. Thus, these methods re-
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4
quire the preliminary preparation of air and/or moisture-sensitive organometallic magnesium, lithium,
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6
zinc, or manganese species. Moreover, the preparation of these organometallics from organic halides
bearing a functional group like CO, COOR, or CN turns out to be quite difficult, or even impossible.
Consequently, other chemical or electrochemical processes have been conceived to avoid such a difficult
step. These processes involve a transition metal complex as catalyst, and are carried out in mild reaction
conditions. The transition metal can be, for instance, nickel associated with various ligands such as
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cyclam or salen and an excess of zinc as reductant. The cathodic reduction of a catalyst precursor
can be alternatively used, as shown by our previous work on the arylation and alkenylation of activated
olefins based on the combined use of a nickel salt as catalyst and a sacrificial iron anode and in the
10
presence of common ligands like acetonitrile or pyridine. We, and others, have also reported that the
electrochemical conjugate addition of alkyl halides can be realized with cobalt complexes as catalysts.11
But Co(I) species were said to be unreactive toward aromatic halides until we found, quite recently, that
they can be activated by electroreduced Co(II) and we took advantage of this property to devise a new
method of preparation of arylzinc compounds12 in a very simple electrochemical process.
In this paper we explore the scope of the use of cobalt bromide, associated or not to bipyridine, as
catalyst in the electrochemical coupling between aryl halides and activated olefins in solvent mixtures
like DMF/pyridine or acetonitrile/pyridine.
∗
Corresponding author. Fax: 33 1 49 78 11 48; e-mail: gosmini@glvt-cnrs.fr (C. Gosmini)
0
040-4039/00/$ - see front matter © 2000 Published by Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00420-2
tetl 16700

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386
1. Results and discussion
1.1. Electroreductive coupling between aryl halides and methyl vinyl ketone
0
In DMF/pyridine with CoBr in the presence of 2,2 -bipyridine (Bpy): All reactions are conducted
2
under an inert atmosphere of argon at 70°C in commercial solvents without further purification. The
ionic conductivity is ensured by NBu BF (0.01 M) added to the reaction mixture.
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4
2
Electrolyses are conducted at constant current intensity of 0.2 A (0.01 A/cm ) in an undivided
1
3
cell already described, with an iron consumable anode associated to a nickel foam as the cathode.
Electrolyses are stopped after consumption of 2.2 F per mol of ArX. The cathodic potential during
the electrolysis is constant between −1.2 V and −1.4 V versus SCE, i.e. close to the potential of the
14
formation of the Co(I) complex from CoBr Bpy . In a typical experiment, 45 ml of DMF, 5 ml of
2
2
0
pyridine containing 7.5 mmols ArX (0.15 M), 1 mmol CoBr , 2 mmols 2,2 -bipyridine and 10 mmols
2
methyl vinyl ketone (MVK) are introduced into the cell. When half of ArX is consumed, 5 mmols MVK
are added to the solution. This overall excess of MVK is necessary to avoid the formation of biaryl.
MVK is, however, preferably added in two portions to avoid the formation of a double adduct. Also,
0
2
,2 -bipyridine as ligand of cobalt is of crucial importance since no addition product is observed when it
is omitted. This contrasts with the formation of arylzinc species via cobalt catalysis where the presence
0
12
of 2,2 -bipyridine is not necessary. Finally, we have noticed that the presence of pyridine as co-solvent
allows a slightly higher yield to be obtained. The results are reported in Table 1 according to Eq. (1).
(1)
Aryl bromides substituted by an electron-withdrawing group (entries 1–5 and 8) give better results
than bromobenzene (entry 6). The adduct is also formed from p-bromobenzaldehyde, although in
moderate yield. Aryl chlorides did not react at all. With an electron-donating group on the ring, only the
iodo compounds are, but slightly, reactive (entry 7). With o-bromoacetophenone (entry 8), a Robinson
annelation reaction leads to the cyclic product (H).
In CH CN/pyridine with CoBr : We also carried out the reaction in acetonitrile/pyridine. In this solvent
3
2
0
mixture, 2,2 -bipyridine is not necessary. The experimental conditions are similar to those used above
except that all MVK (i.e. 2 equiv. versus ArX) is now present at the beginning of the electrolysis, and the
reactions are conducted at 60°C instead of 70°C in DMF. The results are reported in Table 2 according
to Eq. (2).
(2)
The results are better than in DMF/pyridine (entries 9 and11). No Ar–Ar is formed, ArH being the
only by-product. The yields can even be improved if ArX is added with a syringe pump. Thus, with
p-bromobenzoate we obtain 78% instead of 70%. With p-chloroacetophenone the addition product is
formed contrary to DMF/pyridine, but in low yield.

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387
Table 1
Cobalt-catalyzed electrochemical addition of substituted aryl halides on MVK in DMF/pyridine
Table 2
3
Cobalt-catalyzed electrochemical addition of substituted aryl halides on MVK in CH CN/pyridine
So, the solvent CH CN/pyridine offers advantages over DMF/pyridine of affording slightly better
3
0
results and avoiding the use of 2,2 -bipyridine as ligand.
1.2. Electroreductive coupling between para-bromobenzoate and other activated olefins in
CH CN/pyridine
3
The reaction conditions optimized with MVK are then applied to the arylation of other activated olefins
according to Eq. (3).

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388
(3)
In the case of ethyl acrylate (Z_COOEt), the addition product is obtained in 35% yield along with the
reduction product ArH and the dimer Ar–Ar. With acrylonitrile (Z_CN) the major product is ArH; the
electrolysis also leads to the polymerization of acrylonitrile, and only traces of the addition product are
found (<10%).
The results reported here illustrate that cobalt bromide is an efficient catalyst for the electrochemical
addition of aryl bromides, substituted by electron-withdrawing groups, on activated olefins in a mixture
of either DMF or acetonitrile and pyridine. It is worth mentioning that the use of iron as anode is also
necessary. Indeed, with other metals, e.g. Al, Zn or Mg, no addition product is obtained. The possible role
of the iron ions, already noticed in other reactions,1 remains to be elucidated. Work is also in progress
to generalize and optimize these reactions which use a very simple electrochemical process, commercial
solvents and reagents in very mild conditions.
1b
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