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

Abstract 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.