Aromaticity Switching Via Rearrangement of a Pyrrolyl-Substituted Phosphenium Cation

A pyrrolyl-substituted bicyclic phosphenium cation was synthesized and observed in situ to undergo a spontaneous rearrangement in which the aromaticity switched between the [C4N] and [C2N2P]+ ring. The driving force for this rearrangement is stabilization of the vacant p-orbital of the phosphenium cation and formation of a new aromatic system. Nucleus-independent chemical shift calculations showed the [C4N] and [C2N2P]+ rings to have aromatic characteristics in each isomer with the adjoining ring dearomatized. Addition of chloride ions reversed the aromatic switching in forming a chlorophosphine. Switching of aromaticity via rearrangement is a rare phenomenon (cf. 2H-isoindole) with this example showing tautomerism across both rings, driven by increased stabilization of the Lewis acidic phosphenium cation through π-donation.