Formation of the 5-Azoniafulvene Ion and its Benzo-annellated Analogue from N-Mannich Bases of Pyrrole and Indole

1-Dialkylaminomethylpyrroles are shown to behave in many respects like aminals. Acylation by an acid chloride, for instance, occurs normally at the amine-type N-atom rather than at the pyrrole ring. Spontaneous cleavage of the resulting quaternary acylammonium salts affords the 5-azoniafulvene ion ( 3 ). This higly reactive iminium ion, and its benzo-annellated analogue ( 4 ) can be trapped by electron rich aromatic compounds such as N-methylpyrrole or N,N-dimethylaniline. More elaborate N-Mannich bases are accessible by addition of indoles to enamines.