Double ring expansion from a 5,15-diarylporphyrin to a 5,16-diaryl-10,11,21,22-tetradehydroporphyrin(184.108.40.206) occurred through a reaction sequence consisting of oxidation with PbO2to 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 tetradehydroporphyrin(220.127.116.11) 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.
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