Electrochemical vinylation of aryl and vinyl halides with acrylate esters catalyzed by cobalt bromide

Article abstract of DOI:10.1016/S0040-4039(02)01286-8

A consumable anode process permits the electrochemical Heck reaction between aromatic or vinylic halides and acrylate esters using cobalt bromide as catalyst associated with bipyridine as ligand in a mixture of acetonitrile/triethylamine/pyridine as solvent.

Full text of DOI:10.1016/S0040-4039(02)01286-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 5901–5903
Electrochemical vinylation of aryl and vinyl halides with acrylate
esters catalyzed by cobalt bromide
Paulo Gomes, Corinne Gosmini,* Jean-Yves Ne´de´lec and Jacques Pe´richon
Laboratoire d’Electrochimie, Catalyse et Synthe`se Organique, UMR 7582, Universite´ Paris 12, CNRS, 2, Rue Henri Dunant,
94320 Thiais, France
Received 7 May 2002; accepted 1 July 2002
Abstract—A consumable anode process permits the electrochemical Heck reaction between aromatic or vinylic halides and
acrylate esters using cobalt bromide as catalyst associated with bipyridine as ligand in a mixture of acetonitrile/triethylamine/pyr-
idine as solvent. © 2002 Elsevier Science Ltd. All rights reserved.
Vinylation is a versatile process for the carbon–carbon
bond formation and for the functionalization of a
variety of organic halides. Vinylation of aryl and vinyl
halides, the Heck reaction,1 is generally performed by
the catalytic action of a zero-valent palladium complex
formed in situ by the reduction of a palladium salt.2
Several natural products or biologically active com-
pounds have been synthesized during the last two
decades using this process as a key stage.3 Indeed, this
reaction has the advantage of tolerating various func-
tional groups.
electrochemical vinylation of aryl and vinyl halides with
acrylate esters as substrate in the solvent mixture
acetonitrile/triethylamine/pyridine.
Results and discussion
All reactions were conducted in a one-compartment cell
already described.9 Solvents were used as received, and
the reactions were conducted in a ternary solvent mix-
ture of acetonitrile/triethylamine/pyridine (V/V/V=7/2/
1), under an inert atmosphere of argon at 70°C. The
ionic conductivity was ensured by NBu4BF4 (0.01 M)
added to the reaction mixture. Electrolyses were con-
ducted at constant current intensity of 0.2 A (0.01
A/cm2), with an iron consumable anode, associated to a
stainless steel grid as the cathode. In a typical experi-
ment, 35 ml of acetonitrile, 10 ml of triethylamine, 5 ml
of pyridine containing 7.5 mmol of RX (0.15 M), 0.75
mmol of CoBr2 (0.015 M), 10 mmol of 2,2%-bipyridine
(0.2 M) and 20 mmol of ethyl acrylate (0.4 M) were
introduced into the cell.
Though none of them could rival palladium in synthetic
versatility, a number of other metals can catalyze Heck-
type reactions,4 but in most cases only aryl iodides lead
to good yields. Nickel is a most appealing candidate for
possible replacement of Pd catalysts. Nevertheless, the
arylation of acrylate esters in the presence of nickel
catalysts does not give cinnamates but b-arylpropi-
onates as major products.5 Recently, we have shown
that Co(I) species generated in situ in acetonitrile/pyri-
dine by electroreduction of Co(II) salts react quickly
with aryl halides in an oxidative addition reaction.6
This property associated with the use of the con-
sumable anode process has been applied to the electro-
chemical preparation of arylzinc compounds,7 as well
as the electrochemical addition of aryl halides onto
activated olefins.8
In these reaction conditions, the expected substitution
product is often obtained predominantly, along with
the conjugated addition as in the case of the nickel
catalysis reported by Boldrini.10 In all cases, traces of
dimer (RR) are also observed. The product resulting of
the hydrogenation of the organic halide is the third
main compound. The presence of 2,2%-bipyridine in
excess, i.e. 1.3 equiv. for 1 equiv. of RX, is of a crucial
importance. Indeed, the amount of addition product
increases when this proportion is lowered, and becomes
the major product with only twice the amount of
In this paper, we explore the scope of the use of cobalt
bromide associated to 2,2%-bipyridine to promote the
* Corresponding author. Fax: +33(0)1 49 78 11 48; e-mail:
gosmini@glvt-cnrs.fr
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01286-8

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P. Gomes et al. / Tetrahedron Letters 43 (2002) 5901–5903
2,2%-bipyridine. Triethylamine and pyridine as bases are
not strictly necessary, but enables a higher substitution
versus addition product ratio.
As already mentioned for the electrochemical addition
of aryl halides onto olefins,8 the use of a consumable
iron anode is necessary for the reaction to be catalytic
Table 1. Coupling of aryl and vinyl halides with acrylate esters catalyzed by CoBr2
CH3CN/NEt3/pyridine (7/2/1)
e, CoBr2: 0.75 mmol
COOR'
COOR'
+
RX
+
R
RH
(1)
+
Bpy: 10 mmols
Iron anode
R
B
COOR'
A
70°C, I= 0.2 A
7.5 mmols
20 mmols

P. Gomes et al. / Tetrahedron Letters 43 (2002) 5901–5903
5903
We have extended the reaction to a vinyl chloride
(entry 17) and found that in the conditions described
before, the Heck product is the major one.
in cobalt. Similar results were obtained with a con-
sumable cobalt anode. With other metals, e.g. Al, Zn or
Mg, only the reduction product (RH) was obtained.
The results reported here illustrate that cobalt bromide
is an efficient catalyst for the electrochemical vinylation
of aromatic bromides (especially substituted by an elec-
tron-donating group) and vinyl chlorides with acrylate
esters. This reaction must now be extended to other
types of olefins (acrylonitrile, methylvinylketone, sty-
rene…), and the role of iron as anode which has
already been noticed previously will have to be studied
more deeply.
Results are reported in Table 1 according to Eq. (1).
The reaction is stereoselective thus giving only E-
olefins in all the studied cases. Bromobenzene (entry 1)
and aryl bromides substituted by an electron-donating
group in para position (entries 2–6) give the best
results. With these reagents, the amount of addition
product is about half that of the substitution product.
This selectivity is reversed with aryl bromides substi-
tuted by electron-withdrawing groups: the addition
product is generally obtained predominantly (entries
8–9), except with ethyl bromobenzoate (entry 7). In all
cases, if the electrolysis is continued after total con-
sumption of the organic halide, the substitution
product is reduced into the corresponding b-arylpropi-
onate ester.
References
1. Heck, R. F.; Nolley, J. P., Jr. J. Org. Chem. 1972, 37,
2320–2322.
2. Ozawa, F.; Kubo, A.; Hayashi, T. Chem. Lett. 1992, 11,
2177–2180.
3. (a) Hong, C. Y.; Overman, L. E. Tetrahedron Lett. 1994,
35, 3453–3456; (b) Ashimori, A.; Matsuura, T.; Overman,
L. E.; Poon, D. J. J. Org. Chem. 1993, 58, 6949–6951.
4. Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev. 2000,
100, 3009–3066.
It also comes out that the presence of an ortho sub-
stituent makes the coupling reaction very difficult
(entry 10) with this catalytic system.
5. Lebedev, S. A.; Lopatina, V. S.; Petrov, E. S.; Beletskaya,
I. P. J. Organomet. Chem. 1988, 344, 253–259.
6. Buriez, O.; Ne´de´lec, J. Y.; Pe´richon, J. J. Electroanal.
Chem. 2001, 506, 162–169.
7. (a) Gosmini, C.; Rollin, Y.; Pe´richon, J. Patent applica-
tion in France, no. 99/08480, July 1st, 1999; (b) Gosmini,
C.; Rollin, Y.; Ne´de´lec, J. Y.; Pe´richon, J. J. Org. Chem.
2000, 65, 6024–6026.
Aryl iodides which are very reactive towards electro-
generated Co(I) gave mainly the reduction product,
along with the addition product (entries 11–12). With
aryl chlorides (entries 13–14), which are less reactive
than corresponding bromides, the chloride is not con-
sumed and only traces of the Heck and the conjugated
addition products are formed.
8. Gomes, P.; Gosmini, C.; Ne´de´lec, J. Y.; Pe´richon, J.
We have also studied the nature of the substituent on
the ester group in the acrylate. With a butyl group
(entry 15), yield was similar to that obtained with an
ethyl group. On the other hand, with the methyl (entry
16), the Heck reaction product was formed in a minor
way. No explanation was found until now.
Tetrahedron Lett. 2000, 41, 3385–3388.
9. Chaussard, J.; Folest, J. C.; Ne´de´lec, J. Y.; Pe´richon, J.;
Sibille, S.; Troupel, M. Synthesis 1990, 5, 369–381.
10. Boldrini, G. P.; Savoia, D.; Tagliavini, E.; Tombrini, C.;
Umani Ronchi, A. J. Organomet. Chem. 1986, 301, C62–
C64.