Kavala et al.
these problems some environmentally safer procedures
has been envisioned to involve the in situ preparation of
positive bromonium species by oxidation of bromide ion
with suitable oxidants under various homogeneous and
heterogeneous reaction conditions.9
involve organic ammonium bromide and molecular bro-
mine in most cases, thus an indirect use of toxic molec-
ular bromine. Recently, organic ammonium tribromide
has been prepared in an environmentally benign way by
9g
the reaction of V
2
O
5
, aqueous H
2
O
2
, and KBr. However,
9g
this method generates some heavy metal as toxic waste.
Other problems associated with these reagents are the
use of expensive organic ammonium cations and the use
of 1/3 of its total bromine for an aromatic electrophilic
substitution type reaction and 2/3 of its bromine toward
addition to C-C multiple bonds. Some of the organic
ammonium tribromides have phase transfer properties,
hence a substantial amount gets extracted along with the
organic products in an organic solvent during workup,
thereby making the purification tedious and the method
expensive for large-scale reaction. Recovery and recycling
of expensive organic ammonium cations is also poor after
the reaction. Pyridinium tribromide or pyridinium hydro-
bromide perbromide is not so stable compared to other
organic ammonium tribromides and is reported to have
three different bromine compositions with different melt-
ing points.1 To overcome the problems of phase transfer
properties, poor stability, regio- and stereoselectivity,
recovery, and recycling of the spent reagent, we have
synthesized a novel ditribromide reagent. The new
reagent has higher bromine content per molecule, better
bromination efficiency and selectivity and is devoid of
phase transfer property, and the spent reagent can be
recovered and regenerated easily. In this paper we
wished to report the preparation of a new ditribromide
reagent, development of a solvent, metal and ionic-liquid
free bromination protocol, and recovery of the reagent.
The reagent was prepared by refluxing pyridine (2
equiv) with 1,2-dibromoethane (1 equiv). The resultant
Results and Discussion
Bromine free bromination with stable crystalline or-
1
0
ganic ammonium tribromide like tetrabutyl, tetra-
1
1b
9g
9g,12
methyl,
cetyl,
benzyltrimethyl,
pentylpyri-
1
3
14
15
dinium, pyridinium, and DBU has gained consider-
able interest. Tribromides are more suitable than the
liquid bromine because of their crystalline nature, hence
easy for their storage, transport, and maintenance of
desired stoichiometry. Preparations of these reagents
(
5) (a) Sonogashira, K. Comprehensive Organic Synthesis; Trost, B.
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4d
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(
1
,2-dipyridiniumdibromide-ethane (DPDBE) solid was
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(
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k
(
2
4
bromination of aromatic compounds with NH Br. The
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