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

Protonation of 3-aminopyrrole at C-2 gave the sigma-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 sigma-complexes (conjugate adds) such that they can be readily observed or isolated.