Diamagnetic ring current, relationship aromaticity

Free-base corrole (e.g., 2.6), like porphyrin, contains an 18 n-electron pathway and, as such, can sustain a diamagnetic, aromatic ring current (Scheme 2.1.1). Corroles are much stronger acids (and weaker bases) than porphyrins. Thus, unlike porphyrin, corrole forms a stable anion when treated with aqueous alkali. This anion, best represented by structure 2.7, is also an aromatic 18 Tt-electron system, which may account in part for its special stability. In any event, the neutral form of the macrocycle may be regenerated on acidification. Thus, the corrole-to-porphyrin relationship in terms of acidity allows for comparison to be made between these two systems and their smaller six n-electron counterparts benzene and cyclopentadienyl anion (Figure 2.1.2). In other words, cyclopentadienyl anion may be regarded as a contracted benzene just as corrole anion may be considered as being a contracted porphyrin . 

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