Rational Synthesis of Antiaromatic 5,15-Dioxaporphyrin and Oxidation into β,β-Linked Dimers

5,15-Dioxaporphyrin was synthesized for the first time by a nucleophilic aromatic substitution reaction of a nickel bis(,-dibromodipyrrin) complex with benzaldoxime, followed by an intramolecular annulation of the -hydroxy-substituted intermediate. This unprecedented molecule is a 20-electron antiaromatic system, in terms of Huckel's rule of aromaticity, because lone pair electrons of oxygen atoms are incorporated into the 18-electron conjugated system of the porphyrin. A theoretical analysis based on the gauge-including magnetically induced current method confirmed its antiaromaticity and a dominant inner ring pathway for the ring current. The unique reactivity of 5,15-dioxaporphyrin forming a ,-linked dimer upon oxidation was also revealed.