Protonation of Aminoindoles

The behaviour of 2- and 3-aminoindoles towards protonation was studied using NMR techniques. The protonation site of 2-aminoindoles 1 depends on the substituent at the adjacent 3-position. Thus, the 2-aminoindoles 1a,b, with a hydrogen or a phenyl in 3-position, were protonated at the 3-position, whereas 1c,d, bearing electron withdrawing groups, were protonated at the exocyclic nitrogen. In contrast, 3-aminoindoles 2 were always protonated at the amino group.