Quaternary ammonium polyhalides

Halogenation can be run under milder conditions using more active quaternary ammonium polyhalides such as pyridinium tribromide. The reaction between thiophene and benzyltrimethylammonium tribromide in acetic acid-ZnCl2 provided 2,5-dibromothiophene [6]. Chloro- and iodo-substituted thiophene derivatives may be prepared in the same manner. In comparison, bromination of thiophene employing 2 equivalents of NBS in chloroform gave 2,5-dibromothiophene in 56% yield [7]. 

Free halogens are generally inconvenient to use, owing to their toxic and corrosive nature, but can be replaced by quaternary ammonium polyhalides. Quaternary ammonium tribromides are well established [e.g. 1] as solid, readily handled and relatively non-toxic alternatives for electrophilic bromine. More recently, other quaternary ammonium polyhalides have been produced, which together with the tribromides, have wide application as catalysts or in stoichiometric quantities in electrophilic substitution and addition reactions, oxidations, etc. 

The synthesis of polyhalide salts, R4NX , used in electrophilic substitution reactions, are described in Chapter 2 and H-bonded complexed salts with the free acid, R4NHX2, which are used for example in acid-catalysed cleavage reactions and in electrophilic addition reactions with alkenes, are often produced in situ [33], although the fluorides are obtained by modification of method I.I.I.B. [19, 34], The in situ formation of such salts can inhibit normal nucleophilic reactions [35, 36]. Quaternary ammonium chlorometallates have been synthesized from quaternary ammonium chlorides and transition metal chlorides, such as IrClj and PtCl4, and are highly efficient catalysts for phase-transfer reactions and for metal complex promoted reactions [37]. 

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