Abstract
The complexation of KBr3 with acyclic poly (ethylene glycol)-400 was carried out in which the chain of polyether suitably wrapped around the cation in a host-guest manner. The resulting complex was found to be an efficient brominating agent for the selective regioselective monobromination of various aromatic compounds in excellent yields under mild reaction conditions. In another protocol, bromination was carried out by using the PEG-embedded KBr3 as a catalyst in the presence of hydrogen peroxide. The presence of hydrogen peroxide enhanced the reaction rates and provided selective bromination within very short reaction times.
Highlights
After the synthesis of crown ethers and the discovery of their complexing properties toward alkali metal cations in 1967, the hostguest chemistry has developed rapidly and being used worldwide in various fields such as supra-molecular chemistry, biomimetic chemistry and materials science [1,2]
Le et al [17] reported the regioselective monobromination of activated aromatics using 1-butyl-3-methylimidazolium tribromide ([Bmim]Br3) under solvent free condition
In continuation to our previous work on polyethylene glycols [19,20,21,22,23], we report a simple, economically affordable and efficient methodology for the selective mono-bromination of aromatic compounds under solvent-less conditions (Scheme 1)
Summary
After the synthesis of crown ethers and the discovery of their complexing properties toward alkali metal cations in 1967, the hostguest chemistry has developed rapidly and being used worldwide in various fields such as supra-molecular chemistry, biomimetic chemistry and materials science [1,2]. A variety of improved protocols by using expensive transition-metal based catalysts [10], alkali metal halides associated with NaIO4 [11] or the combination of aqueous TBHP or H2O2 together with a hydrohalic acid [12] have been reported for this transformation These methods have certain limitations like use of expensive heavy transition metals, toxic/ volatile chlorinated organic solvents and formation of polysubstituted and other side products. To encourage the development of sustainable synthetic methodologies, the use of eco-friendly reagents has become a subject of immense interest in present day chemistry [13,14] In this regard, some ionic liquid tribromides (IL-Br3) have been reported for the bromination of aromatic substrates and synthesis of bromoesters from aromatic aldehydes [15]. In continuation to our previous work on polyethylene glycols [19,20,21,22,23], we report a simple, economically affordable and efficient methodology for the selective mono-bromination of aromatic compounds under solvent-less conditions (Scheme 1)
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