Regioselective symmetrical bromination of protected 2,2′-biimidazole

Treatment of the benzyl and (trimethylsilylethoxymethyl) SEM protected 2,2′-biimidazoles, 2a and 2b, with 2 equivalents of N-bromosuccinimide (NBS) allows obtaining the 5,5′-dibromo and 4,4′-dibromo substituted biimidazoles, 3a and 5b respectively. The use of 4 equivalents of NBS, followed by treatment of the corresponding tetrabromoderivatives 4a and 5b with butyl lithium (BuLi), yields the 4,4′-dibromoderivatives 5a (G=Bn) and 5b (G=SEM).