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The aqueous phase was saturated with NaCl and extracted
with Et2O (3 mL × 60 mL). The organic phases were washed
twice with an NaOH solution (pH 8–9) and once with water
(pH 7), dried over magnesium sulfate and concentrated under
vacuum. The pinacol esters were obtained after evaporation.
When necessary, the boronic esters were purified by column
chromatography on silica-gel with pentane–ether mixtures as
the eluent. The purity of the arylboronic pinacol esters was
checked by GC and 1H NMR. The spectroscopic data were
compared, when available, to those of pure compounds.
3 in ratios depending on the amount of electricity supplied.
The results are presented in Table 1.
After 2 F/mol electrolysis, 1a was converted in 55% and
p-fluorophenylboronic ester 2a was formed with 45% yield
based on converted material. Fluorobenzene was formed as
the by-product. If the electrolysis was run up to 4.5 F/mol,
a 92% conversion of 1a could be attained with 35% of 2a
and 15% of 3. The total amount of aryl–boron coupling
reached 50%. The remaining by-products were non-borated
ArH derivatives, benzene and fluorobenzene.
The results indicated that the presence of the p-fluoro
substituent in boronic ester in compound 2a facilitated the
reduction of the aryl–fluorine bond. Thus, after formation of
2a, both the reduction of 1a and 2a occurred almost simul-
taneously. Moreover, the presence of the p-fluoro substituent
in 1a facilitated also the reduction of the Ar–Cl bond. For
comparison, the reaction with chlorobenzene as the substrate
under the same conditions led to a conversion of only 55%
after 6 F/mol, with 83% selectivity of PhBpin [7].
3. Results and discussion
Theaimofthisworkwastoexaminewhereasthereactivity
of some dihalogenated aryl derivatives such as 1 could afford,
using HBpin as the electrophile, the corresponding mono-
halogenated pinacol boronic esters 2 in reasonable yields
and selectivities (Scheme 1). The Bpin functionality being an
electron-withdrawing group, it was expected that the reduc-
tion of the second halide group Y of 2 would be easily reduced
to afford 3 and/or 4. In our electrolyses, we thus carried out
some reactions at low conversion, in order to obtain a higher
selectivity toward compounds 2.
The electroreductions of a series of differently substituted
dihalides 1 were carried out at room temperature under gal-
vanostatic conditions with three equivalents of HBpin per
aryl dihalide, according to Scheme 1.
The different electrolyses were carried out under the same
reaction conditions, in order to get valuable comparisons.
For particular target molecules, the processes can be most
probably further optimised.
The electrolysis of 1-bromo-4-chlorobenzene 1b (Table 1)
led to a mixture of boronic esters 2b, 3 and 4b, together with
minor amounts of benzene and chlorobenzene.
After the passage of 1 F/mol, 1b was converted in 45%
leading selectively to 2b in 75%. The complete conversion
of 1b was reached with 2 F/mol. However, the yield of 2b
decreased in favor of that of 3 and 4b. The yield of 3 increased
up to 70% with further electrolysis with 4 F/mol. Yields up to
14% of p-pinacol diboronic esters 4b were obtained with 84%
of total aryl–boron coupling. When comparing these results
with those of the reaction of bromobenzene with HBpin, a
78% yield of 3 was obtained after 2 F/mol with complete
conversion.
The reactions generally needed 2–4 F/mol for the reduc-
tion of the Ar–X bond and for the complete consumption
of the starting material. The presence of HBpin as an elec-
trophile but also as a reductant did not significantly influence
the electron consumption of the halide for its reductive func-
tionalisation. Mechanistic aspects of this reaction are still
under study.
3.2. Electrolysis of dibromobenzenes with pinacolborane
Disubstituted 1,2-, 1,3- and 1,4-dibromobenzenes 1c–1e
were electrolysed according to Scheme 1. The electroreduc-
tion of 1,2-dibromobenzene, 1c, in the presence of HBpin in
THF afforded, after 1 F/mol a 63% conversion with a mixture
mum yield obtained for 2c reached 26%. The presence of the
second bromine substituent at the ortho-position facilitated
the reduction of 2c to 3 and 4c. The results of these different
electrolyses are presented in Table 2.
3.1. Electrolysis of p-dihalogeno aryl derivatives 1a
(X = Cl, Y = 4-F) and 1b (X = Br, Y = 4-Cl) in the
presence of pinacolborane
The electrolysis of 1-chloro-4-fluorobenzene 1a in THF in
the presence of pinacolborane afforded boronic esters 2a and
When 1,3-dibromobenzene 1d was electrolysed, 3-
bromophenylboronic ester 2d could be selectively obtained
Scheme 1.