Novel synthesis of arylboronic acids by electroreduction of aromatic halides in the presence of trialkyl borates

Article abstract of DOI:10.1039/b200744b

A novel preparation of aryl and heteroarylboronic acids by an electrochemical coupling reaction is described. It is based on the reductive coupling between aromatic or heteroaromatic halides and a trialkyl borate. The reactions are carried out in DMF or THF with the use of sacrificial aluminium or magnesium anodes in a single-compartment cell. Arylboronic acids are obtained with moderate to good selectivities and isolated yields.

Full text of DOI:10.1039/b200744b

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Novel synthesis of arylboronic acids by electroreduction of aromatic
halides in the presence of trialkyl borates
c
Carine Laza,a Elisabet Dunach,*b Francoise Serein-Spirau,*y Joel J. E. Moreaud
and Luc Vellutinid
a
` ´
Laboratoire Aroˆmes, Syntheses et Interactions, Universite de Nice-Sophia Antipolis,
06108 Nice cedex2, France
Laboratoire de Chimie Bio-Organique (CNRS UMR 6001), Universite de Nice-Sophia
b
c
d
´
Antipolis, 06108 Nice cedex2, France. E-mail: dunach@unice.fr; Fax: þ33 4 92 07 61 51
´
Laboratoire de Chimie Organique et Organometallique (CNRS UMR 5802),
Universite Bordeaux1, 33405 Talence cedex, France
´
´
´
Laboratoire de Chimie Organometallique (CNRS UMR 5076), Ecole Nationale Superieure
de Chimie de Montpellier, 8, Rue de l0Ecole Normale, 34293 Montpellier cedex, France
Received (in Strasbourg, France) 1st December 2001, Accepted 15th January 2002
First published as an Advance Article on the web
A novel preparation of aryl and heteroarylboronic acids by an
electrochemical coupling reaction is described. It is based on
the reductive coupling between aromatic or heteroaromatic
halides and a trialkyl borate. The reactions are carried out in
DMF or THF with the use of sacrificial aluminium or mag-
nesium anodes in a single-compartment cell. Arylboronic acids
are obtained with moderate to good selectivities and isolated
yields.
No example of such an electrochemical coupling reaction
involving trialkyl borates has, to our knowledge, been yet
described in the literature.
The reactions at the electrodes are, at the anode, the oxi-
dation of the Al or Mg rods into the corresponding Al3þ or
Mg2þ ions in solution. At the cathode, the reduction of the aryl
halide should generate ArÀ, which further reacts with the
B(OR)3 electrophile to afford, after hydrolysis, the corres-
ponding arylboronic acid, as shown in Scheme 1.
The electrolyses were carried out either in DMF or in THF,
at room temperature and at constant current intensity. The
results of the electrosynthesis of several aryl and hetero-
arylboronic acids are summarized in Table 1.
The electroreduction of phenyl bromide in the presence of
B(OMe)3 afforded a 45% isolated yield of phenylboronic acid
in 82% conversion after passage of 2 F molÀ1 (entry 1). Similar
results were obtained with o-methoxy or p-methyl substituted
aryl bromide derivatives (entries 2–4), in DMF or THF, using
either B(OMe)3 or B(OiPr)3 as electrophiles. Sterically hin-
dered 2,4,6-trimethylphenyl bromide afforded the corres-
ponding arylboronic acid in 40% isolated yield (entry 5). In all
cases, the main by-product was the Ar–H derivative, which
resulted from the reductive dehalogenation of ArX and which
was formed in yields up to 60%. The other by-products were
the corresponding phenols, ArOH, which were shown to be
formed in the extraction procedure; they could be avoided by a
work-up under inert atmosphere. In reactions run in DMF,
aldehydes, ArCHO, issued from formylation8,10 were also
formed in 5–20% yield.
Aryl boronic acids 1 constitute an important class of com-
pounds, widely used in the last decade as coupling agents in the
Pd-catalyzed Suzuki reaction.1 This reaction constitutes an
efficient and selective method for Ar–Ar0 reductive coupling
between an aryl halide and an arylboronic acid, under mild
conditions.2 Arylboronic acids also participate in various C–C
bond formation reactions,3 including the coupling of vinyl
halides4 and polymerization reactions.5
The access to arylboronic acids is essentially limited to the
reaction of an aryl Grignard6 or an aryl lithium reagent7 with a
trialkyl borate at low temperature. Variable yields of aryl-
boronic acids are obtained. These methods require the pre-
vious preparation of the organometallic species and only
tolerate a restricted number of functional groups, owing to the
basic and nucleophilic reaction conditions. On the other hand,
the preparation of the ArMgX or ArLi reagents with aryl
chloride derivatives is often difficult and the less easily avail-
able bromide derivatives are generally required.
We present here our results on the use of electrochemical
methodology as a novel strategy for the one-step synthesis of
arylboronic acids under mild conditions. The electrochemical
coupling was performed in a single-compartment cell fitted
with a consumable anode of aluminium or magnesium and an
inert cathode of stainless steel or nickel foam.8,9 The coupling
reaction is based on the direct electroreduction of either aryl or
heteroaryl chlorides and bromides in the presence of several
trialkyl borates, such as B(OMe)3 or B(OiPr)3 , according to
eqn. (1).
y Present address: Laboratoire de Chimie Organometallique (CNRS
UMR 5076), Ecole Nationale Superieure de Chimie de Montpellier, 8,
Rue de l’Ecole Normale, 34293 Montpellier cedex, France.
E-mail: fspirau@cit.enscm.fr; Fax: þ33 4 67 14 72 12.
Scheme 1
DOI: 10.1039/b200744b
New J. Chem., 2002, 26, 373–375
373
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ArB(OH)2 % yield
Table 1 Electrosynthesis of arylboronic acids
Entry
1
Ar–X
B(OR)3
Solvent
DMF
Anode=cathode
F molÀ1 of ArX
2.0
% conv. of ArX
82
B(OMe)3
Mg=stainless steel
45a
48a
32a
44a
2
3
4
B(OMe)3
B(OMe)3
B(OiPr)3
DMF
DMF
THF
Mg=stainless steel
Mg=stainless steel
Mg=stainless steel
2.0
2.5
4.0
89
100
100
5
B(OMe)3
DMF
Mg=stainless steel
2.0
78
40a
6
7
B(OiPr)3
B(OMe)3
B(OMe)3
B(OMe)3
B(OMe)3
THF
THF
DMF
DMF
DMF
Mg=stainless steel
Mg=stainless steel
Mg=stainless steel
Mg=nickel foam
Al=nickel foam
4.0
1.6
4.3
5.1
8.0
82
83
46a
40a
36a
36a
41a
8
100
100
63
9
10
11
12
13
14
15
B(OMe)3
B(OMe)3
B(OMe)3
B(OMe)3
B(OMe)3
B(OMe)3
DMF
DMF
DMF
DMF
DMF
DMF
Mg=stainless steel
Al=stainless steel
Mg=stainless steel
Al=stainless steel
Al=stainless steel
Al=stainless steel
3.5
2.2
2.2
2.2
2.2
2.2
100
100
100
80
38a
56b
57b
57b
73b
59b
70
16
100
Isolated yield calculated on converted ArX. Selectivity in ArB(OH)2 evaluated by gas chromatography or 1H NMR. They were isolated as the
a
b
arylboronic esters in 31 to 53% yields, by esterification of the arylboronic acids with 2,20-dimethylpropane-1,3-diol.12
Some aryl chloride derivatives, though more difficult to
reduce, could also be functionalized with B(OMe)3 or B(OiPr)3
in DMF or THF to afford the corresponding arylboronic acids
after acidic hydrolysis, in 36–46% yields (entries 6–11). The
nature of the cathode did not strongly influence the results
(entries 8, 9), and replacement of the Mg by an Al anode led to
a 41% yield of p-tolylboronic acid but with incomplete con-
version (entries 9, 10).
halides using the sacrificial anode methodology.11 However,
chlorosilanes being more electrophilic than trialkyl borates,
the protonation reaction occurs to a lesser extent in electro-
silylation.
A high selectivity was found in the reduction of aromatic
dihalides (entry 15), allowing the access to haloarylboronic
acids, which have proved to be useful monomers in Pd(0)
catalyzed polymerization.5
For thienyl halide derivatives, the aluminium=stainless
steel couple seemed also well suited and led to conversions
ranging from 70 to 100% after passage of 2.2 F molÀ1
(entries 12–16). The selectivities in arylboronic acid varied
from 56 to 73%, the only by-product of the reaction being
the protonation of the starting material. Related observations
were made in the reductive electrosilylation of aromatic
In conclusion, the electrochemical method involving the
reaction of aryl or heteroaryl halides and trialkyl borates
constitutes an alternative for the synthesis of aryl and hetero-
arylboronic acids. The selectivities of arylboronic acids are
in the range of 32–73%. The main by-product, the Ar–H
derivative, arising from competitive reduction, is easy to
eliminate. The reaction is carried out under mild conditions
374
New J. Chem., 2002, 26, 373–375

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(one-step, room temperature) with a simple electrochemical
set-up. Although the reaction is still limited in terms of func-
tional group compatibility, work is in progress to widen its
scope, to investigate the reaction mechanism in order to gain a
better insight into this novel electrochemical reaction and to
better control its yield and selectivity.
References
1
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Experimental
The general electrochemical procedure is the following: in a
single-compartment cell fitted with an Al or Mg consumable
anode, the substrates ArX (1 mmol) and B(OR)3 (3 mmol) are
added to a DMF or THF solution (20 mL) containing either
KBr or n-Bu4NBr as supporting electrolyte in DMF (2 Â 10À2 M)
or (CF3SO2)2NLi in THF (7 Â 10À2 M) at room temperature.
The electrolysis was carried out at constant current density
(i ¼ 0.06 to 0.2 A, j ¼ 0.3 to 1.0 A dmÀ2); the charge involved
during the electrochemical process was evaluated by the time
of the electrolysis. After electrolysis, the solvent and B(OR)3 in
excess could be evaporated under vacuum. The medium was
slowly hydrolyzed at 0 ꢀC with an HCl (0.1 M) or H2SO4
(10%) solution. After extraction with Et2O (3 Â 20 mL if the
solvent was evaporated or 3 Â 60 mL otherwise), the organic
phase was dried over sodium or magnesium sulfate and con-
centrated under vacuum. The purity of the arylboronic acids
was evaluated by 1H NMR spectra. Thienylboronic acids were
recrystallized in CH2Cl2 .
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Acknowledgements
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`
We thank Rhodia Organique Fine for financial support of this
work. F. Serein-Spirau would like to thank the CNRS for a
position ‘‘en delegation’’.
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