Porphyrins Bearing Stable <i>m</i><i>eso</i>-Alkylidenyl Double Bonds. A New Family of Nonplanar Porphyrinoids

Unique core-modified porphyrinoids, such as oxabenziporphyrins, oxapyriporphyrins, and thiapyriporphyrins, bearing exocyclic C−C double bonds at meso-positions, have been synthesized and characterized. The synthesis was accomplished by utilizing typical “3+1”-type condensation. Two different stable tautomeric forms were isolated, and the two tautomeric forms can be interconvertible upon treatment with base. In contrast, only the structure bearing an exocyclic double bond was isolated in the case of oxapyriporphyrin and oxabenziporphyrin.