TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 4893–4894
Pyridinium bromochromate: a new and efficient reagent for
bromination of hydroxy aromatics
Shivaji B. Patwari, Mohammad A. Baseer, Yashwant B. Vibhute* and Sudhakar R. Bhusare†*
P.G. Department of Chemistry, Yeshwant Mahavidyalaya, Nanded 431 602, India
Received 11 April 2003; accepted 25 April 2003
Abstract—Pyridinium bromochromate (PBC) has been used as an efficient and selective nuclear brominating agent for
bromination of various substituted hydroxy-acetophenones, aldehydes and phenols. © 2003 Published by Elsevier Science Ltd.
Molecular bromine in acetic acid or chloroform and
KBr plus KBrO3 in aqueous solution are good bromi-
nating agents for organic substrates, but molecular
bromine is hazardous and its vapours are irritating.
Moreover, aqueous KBr–KBrO3 cannot be used for
many substrates since they are insoluble in water.
Both brominating agents cause nuclear as well as
side-chain bromination. The present paper reports the
use of pyridinium bromochromate (PBC) in acetic
acid as a selective brominating agent for hydroxy aro-
matic compounds.
Bromination of hydroxyacetophenone by PBC has the
following advantages: the procedure is simple; the iso-
lation of the product is easy; the PBC reagent is easy
to handle, can be weighed and has no hazardous
effect, cf. bromine; only nuclear bromination occurs,
no side-chain bromination being observed for
hydroxy ketones.
Preparation of pyridinium bromochromate: this was
synthesised6 by taking 20 g (0.2 mol) of chromium
trioxide in water (25 ml), cooled to 0°C. To this solu-
tion, 47% HBr (38 ml, 0.21 mol) was added slowly
with constant stirring. The contents were cooled to
0°C and then pyridine (16.3 ml, 0.2 mol) was added
over 15–20 min, to give a brown solid. The reaction
mixture was chilled for 4–5 h. The dark brown crys-
tals were then filtered and dried. The product was
recrystallized from aqueous acetic acid (40:60 v/v)
and its purity was checked by TLC and confirmed by
melting point and elemental analysis (108°C).
The hydroxy aromatics and PBC were dissolved in
the minimum amount of glacial acetic acid and the
reaction mixtures were heated on a water-bath with
constant stirring. Completion of the reaction was
checked by TLC and the colour of the reaction mix-
ture (green) acts as an indicator monitoring the pro-
gress of the reaction. The time required for the
reaction was 20–45 min; the contents were then
poured into water. The brominated products so
obtained were recrystallized from aqueous ethanol.
The structures of the products were confirmed by IR,
1H NMR, halogen analysis and by comparing their
melting points with those of samples prepared by
known literature methods (Table 1).1–4
A typical procedure for bromination: A mixture of 4%-
hydroxyacetophenone (1.36 g, 0.01 mol) and PBC
(2.58 g, 0.01 mol) in glacial acetic acid (8–10 ml) was
heated on a water bath for a few minutes. After the
reaction was completed (TLC and when a green
colour appeared in the reaction mixture) the contents
were poured into ice-cold water. The solid thus
obtained was filtered, washed with water and crys-
tallised from aqueous ethanol.
* Corresponding authors. Fax: 972-2-6757076; e-mail: bhusare71@
yahoo.com
† Present address: Department of Medicinal Chemistry, The Hebrew
University of Jerusalem, Israel.
0040-4039/03/$ - see front matter © 2003 Published by Elsevier Science Ltd.
doi:10.1016/S0040-4039(03)01071-2
Patwari, Shivaji B.
