Controlled Core-Modification of a Porphyrin into an Antiaromatic Isophlorin

Partial core-modification of a porphyrin can be employed to synthesize the 20 pi antiaromatic isophlorin. Unlike the tetra-, tri-, and dipyrrole derivatives of a porphyrin, a monopyrrole porphyrin exhibits antiaromatic characteristics. It undergoes a two-electron reversible ring oxidation to yield the 18 pi aromatic dication. H-1 NMR analysis provides distinct evidence of the altered electronic characteristics through typical paratropic and diatropic ring current effects for the 4n and the (4n + 2) pi-electron systems, respectively.