Organic reactions regioselective bromination with bromine

Recently, several selective bromination reagents for reactive aromatic amines have been developed, for example, 2,4,4,6-tetrabromocyclohexa-2,5-dienone (35), iV-bromosuccinimide-dimethylformamide (36), and hexabro-mocyclopentadiene (37). Although molecular bromine is too reactive to perform selective bromination (mono- versus polybromination), the combined used of bromine and zeolites X and Y has been reported to be applicable to the selective bromination of halobenzenes and alkylbenzenes (38). This zeolite method, however, was not successful in the selective bromination of highly active aromatic compounds. Bromine preadsorbed on zeolite 5A (Ca type) was found to monobrominate aniline in carbon tetrachloride with excellent regioselectivity (91-93% para selectivity) in the presence of organic base, pyridine or 2,6-lutidine (Table XII) (39). The preadsorption of bromine on zeolite 5A is necessary for selective bromination, because the inverse procedure of adding bromine to aniline that had been adsorbed on zeolite beforehand caused a nonselective reaction. 

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