Double Ring Expansion from an Aromatic [18]Porphyrin(1.1.1.1) to an Antiaromatic [20]Porphyrin(2.1.2.1)

Double ring expansion from a 5,15‐diarylporphyrin to a 5,16‐diaryl‐10,11,21,22‐tetradehydro[20]porphyrin(2.1.2.1) occurred through a reaction sequence consisting of oxidation with PbO2 to 5,15‐dioxoporphodimethene, a Corey–Fuchs reaction with tetrabromomethane in the presence of triphenylphosphine, and Fritsch–Buttenberg–Wiechell rearrangement triggered by tert‐butyllithium. The obtained tetradehydro[20]porphyrin(2.1.2.1) and its mono‐ and dihydrogenated congeners exhibited 20 π antiaromatic character, whereas overhydrogenated congeners bearing a saturated bridge were nonaromatic owing to disrupted π conjugation. Building bridges: A reaction sequence involving oxidation with PbO2, a Corey–Fuchs reaction, and Fritsch–Buttenberg–Wiechell rearrangement allowed double ring expansion from a porphyrin to a tetradehydro[20]porphyrin(2.1.2.1) (see scheme), which was reduced to a didehydro[20]porphyrin(2.1.2.1) and a [20]porphyrin(2.1.2.1). These planar porphyrins exhibited 20 π antiaromatic character.