Electrophilic Aromatic Coupling of Hexapyrrolylbenzenes. A Mechanistic Analysis

Oxidative transformation of hexapyrrolylbenzenes into azacoronenes using bromine electrophiles as alternative coupling agents is shown to occur according to a distinct mechanism, different from those proposed for typical high-potential oxidants. It is shown that consecutive cyclizations do not involve brominated intermediates and can be rationalized by assuming a relayed arenium cation attack followed by deprotonation and conjugate elimination. The final cyclization is incompatible with this mechanism and is found to involve double-electrophilic bromination followed by thermal elimination of dibromine. These findings provide insight into the reactivity of sterically congested aromatic systems and may help in designing new methods of C–C bond formation.