Reaction of 3-Substituted Imidazopyridines with Br+ and the Alleged 5-Bromo-Substituted Product

Article abstract of DOI:10.1021/jo01307a003

The reaction of 3-methylimidazo<1,2-a>pyridine with NBS was reinvestigated and is shown to give products formed by apparent nucleophilic substitution at the 2-position.NBS in CHCl3 gave compounds 4 and 6, while NBS in CCl4 or Br2 in CHCl3 gave exclusively 4.Mechanisms and differences in product formation are discussed; evidence that the previously reported NBS product was in fact 3-bromo-5-methylimidazo<1,2-a>pyridine, rather than the alleged 5-bromo-3-methyl derivative 3, is presented.Compound 3 was prepared by diazotization of 5-amino-3-methylimidazo<1,2-a>pyridine in the presence of HBr and by condensation of 2-bromopropanal (or its acetal) with 2-amino-6-bromopyridine (12).This latter reaction of the weakly basic aminopyridine 12 is shown to follow the normal pattern in which the amino nitrogen condenses with the carbonyl carbon.Structures are established by infrared, mass, and 1H-NMR spectral analyses, mechanistic considerations, and diagnostic reactions.Experimental and computer-generated 1H-NMR spectra of compounds 3 and 13 are reproduced.