The Synthesis and Protonation of 1,1‐Di(1‐pyrrolyl)alkenes

It is shown that lithiation of di(1‐pyrrolyl)methane (3) can be directed either towards the C(α) ring positions or to the central CH2 group, depending upon the solvent and the complexing agents chosen. Di(1‐pyrrolyl)‐methyllithium (6), resulting from CH2 deprotonation, is intercepted by aldehydes and converted to the title alkenes in a few straightforward steps. Protonation of 1,1‐di(1‐pyrrolyl)ethylene (10) is found to occur under kinetic control at the terminal olefinic position. In HBF4. Me2O the resulting 5‐azionafulvene‐type ion 14 can be observed by low‐temperature NMR spectroscopy. In FSO3H, however, protonation is directed under thermodynamic control to both pyrrole rings. The resulting symmetrical dication 13 persists even at room temperature.