Aromaticity and aminopyrroles: Desmotropy and solution tautomerism of 1 H-pyrrol-3-aminium and 1 H-pyrrol-3(2 H)-iminium cation: A stable σ-complex

Protonation of 3-aminopyrrole at C-2 gave the σ-complex 1H-pyrrol-3(2H)-iminium cation, whereas protonation at the exoamino group gave its 1H-pyrrol-3-aminium tautomer. Both tautomers were isolated as their respective tetrakis(pentafluorophenyl)borate salt, an example of desmotropy. In solution, the NH₃-tautomer was favored in hydrogen-bonding solvents and the CH₂-tautomer in CH₂Cl₂. A combination of effects on the aromaticity of the aminopyrrole ring increased the relative stability of the σ-complexes (conjugate acids) such that they can be readily observed or isolated.