The porphyrins from the ‘annulene chemist's' perspective

Abstract

The [18]annulene model of porphyrin, emphasizing the presence of an 18π main conjugation pathway in the molecule, invites the application of Hückel's 4n+2 rule to the tetrapyrrolic macrocycle. Although the usefulness of this model had already been demonstrated in the late sixties by the landmark syntheses of sapphyrins by R. B. Woodward and of corroles by A. W. Johnson, the cornucopia of novel porphyrinoid chromophores it entails became apparent only in recent years. Guided by experience in annulene chemistry, we embarked some time ago on a systematic exploration of potentially aromatic porphyrin structural variants that could be expected to generate interdisciplinary interest. As a result, the porphycenes and metalloporphycenes as well as the tetraoxa-, tetrathia- and tetraselenaporphyrin dications were discovered. In this account our latest contributions to the field - homologues of porphyrin, of porphycene and of the tetraoxaporphyrin dication, isophlorins, isocorroles and metalloisocorroles - will be surveyed. © 1993 IUPAC