Edge Migration of Aromatic Rings by Radical Reactions─Kinetics and Directionality

A series of five- and seven-ring migration reactions of polyaromatic radicals were examined theoretically. Computed reaction energetics and rate constants demonstrate the facileness of such migrations. The results show, however, that the direction of migration is not identical for homologous reactions. It is determined by strain-induced changes in molecular geometry, which is confirmed by the harmonic oscillator model of aromaticity analysis. One of the notable outcomes of the present study is that fused five-membered rings can form, not split, thereby challenging the isolated pentagon rule.