Electrosynthesis of functionalized organodizinc compounds from aromatic dihalides via a cobalt catalysis in acetonitrile/pyridine as solvent

Article abstract of DOI:10.1016/S0040-4039(01)00597-4

Electroreduction of aryl-dichlorides or -dibromides in an electrochemical cell fitted with a sacrificial zinc anode and in the presence of cobalt halides as catalyst in acetonitrile/pyridine as solvent affords the corresponding organodizinc species in moderate to excellent yields with high selectivity.

Full text of DOI:10.1016/S0040-4039(01)00597-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 3843–3846
Electrosynthesis of functionalized organodizinc compounds from
aromatic dihalides via a cobalt catalysis in acetonitrile/pyridine
as solvent
Hyacinthe Fillon, 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,
F-94320 Thiais, France
Received 9 March 2001; accepted 8 April 2001
Abstract—Electroreduction of aryl-dichlorides or -dibromides in an electrochemical cell fitted with a sacrificial zinc anode and in
the presence of cobalt halides as catalyst in acetonitrile/pyridine as solvent affords the corresponding organodizinc species in
moderate to excellent yields with high selectivity. © 2001 Elsevier Science Ltd. All rights reserved.
Recent work in this laboratory has shown that CoI
formed by electroreduction of CoX2 (X=Br or Cl) in
dimethylformamide or acetonitrile as solvent containing
pyridine as ligand for cobalt reacts rapidly with aryl
bromides.1 This explains the success in the electrosyn-
thesis of arylzinc halides catalyzed by CoX2 if the
electroreduction is conducted in the presence of ZnBr2
in a one-compartment cell fitted with a zinc sacrificial
anode2 (Scheme 1).
difficulties in obtaining the aromatic diiodo com-
pounds. To our knowledge, no method of synthesis of
organodizinc compounds from aromatic dibromides or
dichlorides has been reported. Therefore, we tried to
extend our electrochemical process to the preparation
of organodizinc compounds from aryl dibromides or
dichlorides bearing functional groups. The combination
of consumable zinc anode and nickel–bipyridine cataly-
sis that we have already reported6 was found unsuitable
for the synthesis of the compounds it was aimed at. In
all cases monozinc derivatives were the main products.
Thus, for example, from 2,5-dibromothiophene 5-
bromo-2-thienylzinc bromide was the only product.7
Aryldizinc compounds XZn-Ar-ZnX are also interest-
ing synthetic intermediates. For example dibromoben-
zene appears to be an ideal starting material for the
preparation of symmetrical and/or unsymmetrical dis-
ubstituted benzene derivatives by cross coupling of its
dizinc derivative with aromatic halides in the presence
of Pd(0).3 If Rieke magnesium in excess (4 equiv. of
Mg*) is sufficiently reactive to form the di-Grignard
reagent by reaction with dibromobenzene at room tem-
perature,4 the corresponding dizinc reagent can only be
prepared from diiodobenzene,5 using the same proce-
dure. This seriously limits the process owing to the
Results and discussion
Diorganozinc compounds were synthesized by the same
method as described before for monoaryl halides using
a cobalt catalysis. All reactions were conducted in a
one-compartment cell already described,8 using a con-
sumable zinc anode and a nickel foam cathode, under
an inert atmosphere of argon, at room temperature in
e, CoCl2 0.1 eq
ZnBr2 0.3 eq
Br
BrZn
FG
FG
Zn anode
CH3CN / Pyridine (v:v=9/1)
RT
Scheme 1.
* Corresponding author. Fax: 33(0)1 49 78 11 48; e-mail: gosmini@glvt-cnrs.fr
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00597-4

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H. Fillon et al. / Tetrahedron Letters 42 (2001) 3843–3846
commercial solvents used without purification. The
presence of a supporting electrolyte is not necessary, the
ionic conductivity of the medium being ensured by zinc
bromide added in the medium as source of zinc(II)
species.
the formation of a mixture of the mono (Br-ArZnBr)
and dizinc (BrZn-Ar-ZnBr) compounds.
At 4 F/mol, all the starting compound ArX2 is con-
sumed and the mono zinc compound formed at 2
F/mol is transformed into the diorgano zinc species,
BrZnArZnBr, which is the only organometallic present
in the medium.
Electrolyses were conducted at a constant current inten-
sity of 0.2 A (0.01 A/cm2), and stopped after consump-
tion of 4 F per mole of ArX2. The cathodic potential
during the electrolysis was between −1.1 V and −1.3
V/SCE, which is the potential of the formation of a
Co(I) complex from CoX2–pyridine.
In all cases, good yields are obtained except in the case
of ortho-dibromobenzene (entry 3). In this case, at 2
F/mol, only 16% of mono and dizinc compound are
formed, at 4 F/mol, only the monozinc species formed
is further transformed in dizinc compound. The reac-
tion proceeds efficiently with aryl dibromides substi-
tuted by methyl, fluorine, or methoxy group (entries 4,
5, 6). An important point is the absence of biaryl
compounds. The only by-product of the electrolysis (4
F/mol) is the reduced product ArH2, likely formed by
decomposition of the arylcobalt intermediate.1
In a typical experiment, 50 ml of mixture of solvent (45
ml acetonitrile and 5 ml of pyridine) containing 10
mmols of ArX2 (0.2 M), 2 mmols of CoCl2 (0.04 M)
and 2 mmols (0.04 M) of ZnBr2 (formed by electrore-
duction of 1,2-dibromoethane in the presence of a zinc
anode) were introduced into the cell.
The obtained organozinc species were reacted with
iodine, for GC measurement of the resulting aryl diio-
dide, and using an internal standard (alcane). In some
case, the aromatic organozinc species were coupled with
phenyl iodide using PdCl2(PPh3)2 as catalyst. The
resulting products were isolated.
We have also studied the formation of dizinc com-
pounds from dichlorobenzenes and bromochloroben-
zene. Results are reported in Table 2
In all cases, the monozinc compound is the major
product at 2 F/mol. At more than 2 F/mol, the reaction
does not proceed further, even when all the starting
reagent is not consumed (entry 7). However, all the
starting reagent is consumed at 2 F/mol when p-
dichlorobenzene is activated with an electron-withdraw-
ing group (entry 8). On the other hand, with
p-bromochlorobenzene, only the monozinc compound
In Table 1, we have reported the results obtained with
various aryl dibromides.
In all cases, only the diorganozinc compound is
obtained at 4 F/mol. Analysis of the solution after
consumption of 2 F/mol of ArX2, showed in all cases,
Table 1. Formation of dizinc species from dibromobenzene

H. Fillon et al. / Tetrahedron Letters 42 (2001) 3843–3846
3845
Table 2. Formation of dizinc species from dichlorobenzene and bromochlorobenzene
Table 3. Formation of dizinc species from 2,5-dibromo or 2,5-dichlorothiophene
reaction is the reduction of Zn2+ formed by anode
oxidation. This is accompanied by disappearance of the
CoI by disproportionation which occurs here more
rapidly than the oxidative addition into an ArꢀCl
bond.1
BrZnPhCl shows up at 2 F/mol and a part of this is
transformed in dizinc species at 4 F/mol (entry 9).
Contrary to aryl dibromides, the monozinc intermedi-
ate is always major from dichloro compounds.
We have also tried to form organozinc compounds
from dibromo- or dichlorothiophene but yields are not
satisfactory, as shown in Table 3.
Conclusion
The electrosynthesis of diarylzinc species can be
achieved using dibromobenzenes or activated
dichlorobenzenes with moderate to excellent yields. In
the case of o-dibromobenzene, the yield is poor. The
mono and dizinc compounds are obtained simulta-
neously for these compounds at 2 F/mol except for
bromochlorobenzene where only the monozinc species
is detected. At 4 F/mol, only the dizinc species is
2,5-Dibromothiophene and the 2,5-dichlorothiophene
behave basically like the corresponding benzene
reagents. At 2 F/mol, 2,5-dibromothiophene (entry 10)
gives a mixture of mono and dizinc compounds, and at
4 F/mol only the dizinc compound is obtained. All the
starting compound disappears at 2 F/mol. Reduction
products are probably formed, thiophene and/or poly-
thiophene. We have already reported the formation of
thienyl zinc species from 3-substituted-2,5-dihalothio-
phene catalyzed by NiBr2Bpy, where only the monozinc
compound was obtained.7 With a cobalt catalysis, we
obtained a mixture of the mono and the dizinc com-
pounds which give only the dizinc compound with an
extra charge. On the contrary, dichlorothiophene (entry
11) gives mostly the mono zinc species at 2 F/mol
(mono/di=10) and all the starting reagent is not con-
sumed. With an extra charge, there is no evolution, as
with dichlorobenzene.
obtained
from
dibromobenzenes.
Contrary,
dichlorobenzene reagents give a mixture of mono and
dizinc compounds. This behavior is not surprising con-
sidering the most important reactivity of CꢀBr versus
CꢀCl bond. This is the reason why non-activated
dichloro derivatives are not consumed and the mono-
zinc species is the major organometallic product at the
end of the reaction.
References
We can suppose that in the case of dichloro reagents
(benzene and thiophene), the major electrochemical
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