Abstract-The electroreduction of aryl halides (bromides or chlorides) allows the coupling reaction with vinylic acetates, in the presence of 2,2-bipyridine and catalytic amounts of cobalt bromide, leading to styrene derivatives in good yields. 

Corinne Gosmini

Jacques Périchon

Paulo Gomes

CiteSeerX

Cobalt-catalyzed electrochemical vinylation of aryl halides using
vinylic acetates
Paulo Gomes, Corinne Gosmini* and Jacques Périchon
Laboratoire d’Electrochimie, Catalyse et Synthèse Organique, UMR 7582, Université Paris 12—C.N.R.S, 2, Rue Henri Dunant,
F-94320 Thiais, France
Received 6 February 2003; revised 11 March 2003; accepted 13 March 2003
Abstract—The electroreduction of aryl halides (bromides or chlorides) allows the coupling reaction with vinylic acetates, in the presence of
2,2-bipyridine and catalytic amounts of cobalt bromide, leading to styrene derivatives in good yields. q 2003 Elsevier Science Ltd. All rights
reserved.
1. Introduction
Styrene derivatives are very useful chemical intermediates.
For preparing both b-substituted or unsubstituted styrene,
the Heck reaction is a powerful method.1 However, the
palladium-catalyzed arylation of enol esters provides
substituted 2-arylaldehydes and ketones as major products.2
These styrene derivatives can also be prepared from
organometallic reagents such as vinylic stannanes3 or
silanes,4 which are synthesized via an intermediate vinyl-
palladium species.5 Direct Pd catalyzed routes have been
described starting from 5-iodopyrimidine,6 however, the
same reaction applied to iodobenzene7 leads to a complex
mixture with less than 20% of styrene. More recently,
various unsymmetrical trans-stilbenes have been syn-
thesized by cross-coupling from reactive phenylvinylacetate
and various substituted iodobenzenes using montmorillonite
supported palladium catalyst in a single step.8 In both cases,
cross-coupling reactions are restricted to aryl iodides
substituted by an electron-donating group and require
palladium complexes catalysis.
In the course of our recent works9 on electrochemical
cobalt-catalyzed reactions, we described the electrochemi-
cal allylation of aryl halides with allylic acetates.10 This
work deals with the cobalt catalyzed coupling of various
aryl bromides or chlorides with vinylic acetates. If the
reaction is carried out in the presence of stoichiometric
amount 2,20-bipyridine, this new efficient carbon–carbon
bond synthesis successfully leads to styrene derivatives.
This new method allows the preparation of a-substituted
styrenes which are hardly accessible by current methods.
This process has also been extended to the coupling of
heteroaromatic halides (X¼Cl or Br) with various vinylic
acetates.
2. Results and discussion
All reactions are conducted in a one-compartment cell
already described,11 using a sacrificial iron anode and a
stainless steel grid as the cathode. The ionic conductivity of
the medium is ensured by addition of NBu4BF4 as
supporting electrolyte. Electrolyses are run at constant
current intensity of 0.2 A (0.01 A/cm2), at room temperature
and under inert atmosphere of argon. During the
electrolysis, the cathodic potential during the electrolysis
remain at ca. 21.6 V/SCE. The consumption of ArX is
monitored by GC analysis. Electrolyses are stopped after
total consumption of ArX. In a typical experiment, 50 mL of
a mixture of solvent (45 mL of acetonitrile and 5 mL of
pyridine) containing 10 mmol of ArX (0.2 M), 25 mmol
of vinyl acetate (0.5 M), 1 or 2 mmol of CoBr2 (0.02 or
0.04 M) depending on the nature of X and 0.5 mmol of
NBu4BF4 (0.01 M) were introduced in the cell. Careful
investigations of the nature of the catalyst provided a clear-
cut evidence for the key role of the ligand (2,20-bipyridine).
Indeed, no more than 15% of coupling product was detected
when the reaction was performed with only 0.5 equiv. or
without 2,20-bipyridine. The use of 2,20-bipyridine in
stoichiometric amount (1 equiv. vs ArX) was necessary to
avoid the formation of the reduced product and the yield of
the coupling product increased to 62% in the case of the
reaction with p-bromoanisole. The 2,20-bipyridine is used to
complex cobalt but that 1 equiv. vs ArX is required because
electrogenerated Fe(II) ions undergo competitive complexa-
tion of 2,20-bipyridine. These new results prompted us to
extend the coupling reaction to various aryl bromides using
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)00404-6
Tetrahedron 59 (2003) 2999–3002
* Corresponding author. Tel.: þ33-1-49-78-11-39; fax: þ33-1-49-78-11-
48; e-mail: gosmini@glvt-cnrs.fr
Keywords: electrochemical vinylation; vinylic acetates; aryl halides.
the conditions described in Eq. (1) and results are reported
in Table 1
ð1Þ
It is worth noting that good yields are obtained with
bromobenzene (Table 1, entry 1) and aryl bromides
substituted by an electron-donating group such as OMe,
Me, i-Pr (Table 1, entries 2–6). However, when the
substituent is in ortho position, only traces of the
corresponding methoxystyrene are observed (Table 1,
entry 7). Moreover, the presence of an ortho substituent
strongly affects the coupling reaction with this catalytic
system. Indeed, aryl bromides substituted by an electron-
withdrawing group like ethyl 4-bromobenzoate (Table 1,
entries 8, 9), which are more reactive, lead to a small
amount of coupling product (less than 5%/ArBr), the major
product being the reduced one.
As already mentioned for the electrochemical addition or
vinylation of aryl halides onto olefins9b or with acrylic
esters,12 respectively, the use of a consumable iron anode is
essential for the reaction to remain catalytic in cobalt. With
other metal, e.g. Al, Zn or Mg, only the reduced product
(ArH) was obtained.
In these reactions, electrolyses are run with a cathodic
potential of 21.6 V/SCE which corresponds to the
reduction of bipyridine stabilized ‘ArylCo’ species.
We have thus extended the vinyl acetate coupling process to
aryl chlorides (Eq. (2)). These halides must be activated by
the presence of an electron-withdrawing group on the
aromatic nucleus to react with electrogenerated cobalt
species as already mentioned in our electrochemical
process. Results are reported in Table 2.
In the reaction involving aryl chlorides, 0.2 equiv. of cobalt
catalyst (0.04 M) per ArCl are introduced in the medium to
increase the yield, all other parameters being identical to the
reaction carried out from aryl bromides coupling (Eq. (2))
ð2Þ
Satisfactory yields are obtained with methyl 4-chlorobenzo-
ate (Table 2, entry 10) and p-chloroacetophenone (Table 2,
entry 11). In the case of the reaction with p-chlorobenzo-
nitrile (Table 2, entry 12), a low yield was obtained.
We have also extended the process to other vinylic acetates
with various aromatic halides (bromides and chlorides) as
described in Eq. (3)
ð3Þ
X¼Br: CoBr2: 1 mmol; X¼Cl: CoBr2: 2 mmol
Results are reported in Table 3.
The reaction with commercially available isoprenyl acetate
was first investigated. Good yields are obtained for the
coupling reaction with aryl bromides (Table 3, entries 13,
14). Starting from various aromatic chlorides, coupling
products were obtained in good yields (Table 3, entries
15–21).
We have also investigated the reaction of methyl 4-chloro-
benzoate with other more substituted vinylic acetates.
Satisfyingly, the coupling reaction takes place with
excellent yields in the case of cyclopentenyl and 3,4-
dihydronaphthyl acetates (Table 3, entries 22, 23) easily
obtained using known procedures.13 Better yields are
obtained using substituted vinylic acetate considering the
fact that vinyl acetate is poorly soluble in this medium.
The process was finally extended to heteroaromatic
chlorides (Scheme 1).
Good yields were obtained in the reaction of isopropenyl
acetate with 4-chloroquinoleine (65%) and 3-chloropyridine
(73%) as described in Scheme 1.
3. Conclusion
We have achieved a new coupling reaction14 of various
functionalized aromatic and heteroaromatic halides (X¼Br
or Cl) with various vinylic acetates using a cobalt catalyst.
This catalytic process is a convenient route for the
Table 1. Coupling reaction between aryl bromides and vinyl acetate
Entry ArBr Isolated yield (%)/
ArBr
1 C6H5Br 52 [1]
2 p-MeOC6H4Br 62 [2]
3 p-PhOC6H4Br 44 [3]
4 p-MeC6H4Br 53 [4]
5 p-iPrC6H4Br 40 [5]
6 m-MeOC6H4Br 54 [6]
7 o-MeOC6H4Br ,5
8 p-EtOCOC6H4Br ,5
9 p-NCC6H4Br ,5
Table 2. Coupling reaction between aryl chlorides and vinyl acetate
Entry ArCl Isolated yield (%)/ArCl
10 p-MeOCOC6H4Cl 57 [7]
11 p-MeCOC6H4Cl 60 [8]
12 p-NCC6H4Cl 15 [9]
P. Gomes et al. / Tetrahedron 59 (2003) 2999–30023000
preparation of a-substituted styrene derivatives in good
yields. The reaction has been extended to several aryl
bromides and chlorides, compounds which have never been
used before for the direct vinylation using vinylic acetates.
This simple experimental procedure appears to be a mild
and useful method for the synthesis of various vinyl-aryl
compounds and cobalt associated to bipyridine appears
more efficient for this reaction than palladium complexes.
From a mechanistic point of view, Co(II) associated to
bipyridine is reduced either in Co(I) or Co(0).The resulting
low valent cobalt complex most likely reacts with ArX by
oxidative addition to form an ArCo species. This last species
could undergo a substitution reaction with the carbon
attached to the ester group. Studies are now in progress to
try to get insights about the mechanism of this electro-
chemical reaction.
4. Experimental
GC analysis was carried out using a 25-m DB1 capillary
column. Mass spectra were recorded with an ITD spec-
trometer coupled to a gas chromatograph (DB1, 30 m).
Column chromatography was performed on silica gel 60,
70–230 mesh. 1H, 13C and 19F spectra were recorded in
CDCl3 at 200 MHz with TMS as an internal standard.
The electrochemical cell was similar to that described
previously.12 All solvents and reagents were purchased and
used without further purification. Acetonitrile and pyridine
were stored under argon.
4.1. General procedure for the coupling of aryl bromides
with vinylic acetates
In an undivided cell using a consumable iron anode and a
stainless steel grid as the cathode, CoBr2 (0.219 g, 1 mmol),
2,20-bipyridine (1.56 g, 10 mmol), ArBr (10 mmol) and
vinylic acetate (25 mmol) were placed in a mixture solvent
of acetonitrile/pyridine (45 mL/5 mL). The ionic conduc-
tivity of the medium is ensured by addition of NBu4BF4
(0.165 g, 0.5 mmol) as supporting electrolyte. The solution
was electrolyzed under argon at room temperature at
constant current intensity of 0.2 A (0.01 A/cm2) until the
aryl bromide is totally consumed. The solution was
hydrolyzed with HCl (2N) and extracted with diethyl
ether, the organic layer washed with brine, dried and the
solvent evapored under vacuum. Coupling products were
isolated by column chromatography on silica gel with
pentane/ether as eluent.
4.2. General procedure for the coupling of aryl chlorides
with vinylic acetates
The procedure used for aryl chlorides is the same that aryl
bromides excepted for amount of CoBr2 (0.438 g, 2 mmol).
4.3. Registry numbers (provided by the authors)
Styrene (1), 100-42-5; 4-methoxy-styrene (2), 637-69-4;
4-phenoxy-styrene (3), 4973-29-9; 4-methyl-styrene (4),
622-97-9; 4-isopropyl-styrene (5), 2055-40-5; 3-methoxy-
styrene (6), 626-20-0; 4-ethenyl-benzoic acid methyl ester
(7), 1076-96-6; 1-(4-ethenyl-phenyl)ethanone (8), 10537-
63-0; 4-ethenyl-benzonitrile (9), 3435-51-6; 4-isopropenyl-
1-methoxy-benzene (10), 1712-69-2; 4-isopropenyl-1-
methyl-benzene (11), 1195-32-0; 4-isopropenyl-benzoic
Table 3. Coupling reaction between aromatic halides and vinylic acetates
Entry ArX Isolated yield (%)/ArX
13 p-MeOC6H4Br 61 [10]
14 p-MeC6H4Br 47 [11]
15 p-MeOCOC6H4Cl 92 [12]
16 p-MeCOC6H4Cl 76 [13]
17 p-F3CC6H4Cl 81 [14]
18 p-NCC6H4Cl 20 (56 CC) [15]
19 p-FC6H4Cl 67 [16]
20 m-F3CC6H4Cl 74 [17]
21 70 [18]
22 p-MeOCOC6H4Cl 70 [19]
23 p-MeOCOC6H4Cl 57 [20]
Scheme 1.
P. Gomes et al. / Tetrahedron 59 (2003) 2999–3002 3001
acid methyl ester (12), 26581-23-7; 1-(4-isopropenyl-
phenyl)ethanone (13), 5359-04-6; 1-isopropenyl-4-
trifluoromethyl-benzene (14), 55186-75-9; 4-isopropenyl-
benzonitrile (15), 19956-03-7; 4-fluoro-1-isopropenyl-
benzene (16), 350-40-3; 1-isopropenyl-3-trifluoromethyl-
benzene (17), 368-79-6; 1-isopropenyl-naphthalene (18),
1855-47-6; 3-isopropenyl-pyridine (22), 15825-89-5.
4.4. Product analysis
4.4.1. 4-(Cyclopent-1-enyl)-benzoic acid methyl ester
(19). Yield: 70%; 1H NMR (200 MHz) d (ppm): 8.12 (d,
2H, J¼8.4 Hz), 7.60 (d, 2H, J¼8.4 Hz), 6.45 (t, 1H,
J¼2.0 Hz), 4.05 (s, 3H), 2.76 (m, 4H), 2.19 (m, 2H); 13C
NMR (50 MHz) d (ppm): 166.7, 141.5, 140.9, 129.4, 129.0,
128.0, 125.1, 51.7, 33.3, 32.8, 23.0; EI/MS, m/z (%): 202
(71) [M], 171 (20), 170 (17), 169 (11), 144 (11), 143 (100),
142 (17), 141 (18), 128 (43), 115 (23). Anal. calcd for
C13H14O2: C, 77.20; H, 6.98; O, 15.82. Found: C, 77.08; H,
6.91; O, 15.43.
4.4.2. 4-(3,4-Dihydro-1-naphtyl)-benzoic acid methyl
ester (20). Yield: 57%; 1H NMR (200 MHz) d (ppm):
8.24 (d, 2H, J¼8.3 Hz), 7.59 (d, 2H, J¼8.3 Hz), 7.25 (m,
4H), 6.30 (t, 1H, J¼4.7 Hz), 4.09 (s, 3H), 3.01 (t, 2H,
J¼7.9 Hz), 2.58 (td, 2H, J¼7.9, 4.7 Hz); 13C NMR
(50 MHz) d (ppm): 166.9, 145.7, 139.4, 136.8, 134.5,
129.7, 129.0, 128.9, 128.8, 127.8, 127.4, 126.4, 125.4, 52.0,
28.3, 23.6; EI/MS, m/z (%): 265 (22) [Mþ1], 264 (100) [M],
249 (26), 233 (14), 231 (10), 206 (14), 205 (80), 204 (22),
203 (38), 202 (36), 190 (16), 189 (14), 149 (10). Anal. calcd
for C18H16O2: C, 81.79; H, 6.10; O, 12.11. Found: C, 82.19;
H, 6.02; O, 11.84.
4.4.3. 4-Isopropenyl-2-methyl-quinoleine (21). Yield:
65%; 1H NMR (200 MHz) d (ppm): 7.75 (d, 1H,
J¼8.4 Hz), 7.62 (dd, 1H, J¼8.4, 0.9 Hz), 7.27 (m, 1H),
7.05 (m, 1H), 6.71 (s, 1H), 5.05 (t, 1H, J¼1.6 Hz), 4.72 (s,
1H), 2.36 (s, 3H), 1.79 (s, 3H); 13C NMR (50 MHz) d
(ppm): 158.2, 150.2, 148.1, 141.9, 128.9, 125.3, 125.0,
124.1, 119.7, 117.0, 25.0, 24.2; EI/MS, m/z (%): 184 (12)
[Mþ1], 183 (89) [M], 182 (33), 181 (19), 169 (14), 168
(100), 167 (31), 166 (11). Anal. calcd for C13H13N: C,
85.21; H, 7.15; N, 7.64. Found: C, 85.23; H, 7.35; N, 7.42.
Acknowledgements
We gratefully acknowledge the financial support provided
by Rhodia industry.
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P. Gomes et al. / Tetrahedron 59 (2003) 2999–30023002


Cobalt-catalyzed electrochemical vinylation of aryl halides using vinylic acetates

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Cobalt-catalyzed electrochemical vinylation of aryl halides using vinylic acetates

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