T. Raju et al. / Tetrahedron Letters 47 (2006) 4581–4584
4583
consisted of methanol: water (70:30) at a flow rate of
1 ml/min. Samples were analysed using a UV detector
at a wavelength of 254 nm.
can be controlled precisely by simply switching off the
electricity, thereby eliminating the problems of handling,
transporting and storage of liquid bromine. The simple
reaction set-up, cheap and environmentally friendly
reagents, excellent yields of regioselective monobromin-
ated products and simple work-up make this method
valuable from a preparative point of view.
It is believed that electrochemically generated bromine
combines with water, giving one molecule of hypobro-
mous acid and one molecule of HBr. The hypobromous
acid is unstable due to its pronounced ionic nature and
thus in the presence of hydrobromic acid, one molecule
of water is removed from hypobromous acid giving Br+,
which attacks the electron rich aromatic ring, and the
product obtained under these conditions is exclusively
the p-ring-brominated product and no trace of other
regioisomers or dibromo products were detected (Scheme
2). When benzylic positions were present in the starting
materials, no benzylic bromides were detected as prod-
ucts in the crude reaction mixture. Although the pro-
tected aromatic amine and phenol were p-brominated
in excellent yields, aniline and phenol-like aromatic sub-
strates furnished a mixture of o- and p-isomers (o-cresol
gave 30% o- and 30% p-bromo isomers along with 40%
of returned starting material) after passing the theoreti-
cal charge, while highly deactivated compounds, such as
nitrobenzene and chlorobenzene, did not undergo bro-
mination even on prolonged reaction. In the case of bro-
mination of 2-naphthol, the o-brominated material was
the sole product when the –OH group was unprotected.
When protected with a methyl group, the p-brominated
product was formed exclusively.
Acknowledgements
The authors thank Professor A. K. Shukla, Director,
Central Electrochemical Research Institute, Karaikudi,
for Providing NMR facilities.
References and notes
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It is notable that following electrolysis, the brominated
aromatic compound was easily isolated simply by sepa-
ration of the organic layer with the remaining bromide
salt available for further use in anodic bromination by
adding calculated amount of HBr (based on the amount
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Br2 + H2O
HOBr
HOBr + HBr
Br+ + H2O
(1)
(2)
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20. Representative procedure for electrochemical bromina-
tion: A solution of 2-naphthol (10 mmol) in 25 ml of
chloroform was taken in an undivided jacketed cell. To the
above solution, a 50–60% aqueous sodium bromide
solution (50 ml) containing 5 ml of HBr (46% solution,
H+
OMe
OMe
OMe
Br+
+
(3)
-H+
+
Br
H
Br
Scheme 2.