Ä
Carine Laza, Elisabet Dunach
COMMUNICATIONS
arylboronic ester 3entry 9). It is noteworthy that a good ature, no catalyst, no previous preparation of organo-
yieldwas also obtainedwith 2,4,6-trimethylphenyl metallic species. Work is in progress to investigate the
bromide 3entry 10, yield of 81% based on halide reaction mechanism in order to get a better insight into
conversion), indicating that the reaction was compatible this novel electrochemical reaction.
with sterically hindered aryl derivatives.
With either ortho- or para-methoxy-substitutedaryl
bromide derivatives 3entries 11 and 12) yields of boronic
esters of 68 and79% 3basedon halide conversion) were
obtained, respectively.
Experimental Section
General Electrochemical Procedure
The electrolysis of p-trimethylsiloxyphenyl bromide
afforded the corresponding arylboronic pinacol ester in
65% yieldwith a neutral work-up. The corresponding p-
hydroxyphenylboronic pinacol ester 3p-HOC6H4Bpin)
could be obtained with the same yield under an acidic
work-up 3entry 13).
In the case of p-trifluoromethylphenyl bromide 3entry
14), the p-trifluoromethylphenylboronic ester was ob-
tainedin 58% yield3calculatedon convertedhalide)
after 1 F/mol at a partial conversion. In the presence of
the CF3 groups, the electroreduction of the benzylic C-F
bonds occurred as a competitive reaction. When the
electrolysis was continuedup to 2 F/mol, the yieldof p-
trifluoromethylphenylboronic ester per convertedhal-
ide was lowered to 48% and p-pinBCH2C6H4Bpin was
formedas a by-product.
The reduction of other difunctional aromatic halides
was also examined3entries 15 ± 17) andallowedthe
access to haloarylboronic acids in 45 ± 75% yields at
partial conversions. Thus, p-fluorophenyl-, p-chloro-
phenyl- and p-bromophenylboronic esters were ob-
tained in 45, 75 and 60% yields per converted halide,
respectively, in entries 15 ± 17, after 1 ± 2 F/mol con-
sumption. The further electroreduction of these deriv-
atives afforded mixtures of phenylboronic pinacol ester
and p-phenylene-bis-boronic pinacol diester.
The yieldof arylboronic pinacol esters obtainedby
electrosynthesis with several aryl bromides are slightly
better than those reportedby a chemical Pd-catalysed
procedure.[11,12]
Under inert atmosphere in a single-compartment cell fitted
with a consumable Mg anode and a nickel foam cathode, the
ArX substrate 31 mmol) and HBpin 33 mmol) were added to a
distilled THF solution 320 mL) containing 3CF3SO2)2NLi 37 Â
10À2 M). The electrolysis was carriedout at room temperature
andat constant current ednsity 3i 0.03 A, j 0.15 A Â
dmÀ2), the charge involvedduring the electrochemical process
being calculatedby the time of the electrolysis. After
electrolysis, the solvent in excess was evaporatedunedr
vacuum. The medium was slowly hydrolysed at 08C with a
10% H2SO4 solution until pH 5. The aqueous phase was
saturatedwith NaCl andextractedwith Et 2O 33 Â 60 mL). The
organic phases were washedtwice with an NaOH solution
3pH 8 ± 9) andonce with water 3pH 7), driedover magnesium
sulphate andconcentratedunder vacuum. In most cases pure
pinacol esters were obtainedafter evaporation of the Ar-H by-
products. When necessary, 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 checkedby 1H NMR andthe proudcts were
comparedto authentic samples.
Acknowledgements
We thank Rhodia Organique, Lyon, France for financial
support of this work.
References and Notes
Concerning the mechanism of this aryl-boron cou-
pling reaction, a first aryl halide reduction is proposed to
proceedat the cathode, parallel to the oxidation of the
magnesium rodinto magnesium ions occurring at the
anode. An ate-type complex between a formal ArMgX
andpinacolborane is likely to be formedas reaction
intermediate, before the final hydrolysis to the arylbor-
onic pinacol ester.
In conclusion, the electrochemical methodinvolving
the direct reductive coupling of aryl halides and
pinacolborane constitutes a novel alternative for the
synthesis of arylboronic pinacol esters. The reaction can
be appliedto non-activatedaryl chlorides as well as to
hindered aryl halides. With aryl bromides, the yields of
arylboronic esters are in the range of 58 ± 89% basedon
converted halides. The reaction is carried out under mild
conditions, with a very simple electrochemical set-up
andpresents several advantages: one step, room temper-
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