Antiaromatic Sapphyrin Isomer: Transformation into Contracted Porphyrinoids with Variable Aromaticity

Sapphyrin is a pentapyrrolic expanded porphyrin with a 22 pi aromatic character. Herein, we report the synthesis of a 20 pi antiaromatic sapphyrin isomer 1 by oxidative cyclization of a pentapyrrane precursor P-5 with a terminal beta-linked pyrrole. The resulting isomer 1, containing a mis-linked bipyrrole unit in the skeleton, exhibits a reactivity for further oxidation due to the distinct antiaromatic electronic structure, affording a fused macrocycle 2, possessing a spiro-carbon-containing [5.6.5.6]-tetracyclic structure. Subsequent treatment with an acid afforded a weakly aromatic pyrrolone-appended N-confused corrole 3, and thermal fusion gave a [5.6.5.7]-tetracyclic-ring-embedded 14 pi aromatic triphyrin(2.1.1) analog 4. The cyclization at the mis-linked pyrrole moiety of P-5 played a crucial role in synthesizing the antiaromatic porphyrinoid susceptible to facile transformation to novel porphyrinoids with variable aromaticity.

Automated summary

In this study, we report the synthesis of a 20 pi antiaromatic sapphyrin isomer, which exhibits a reactivity for further oxidation due to its distinct antiaromatic electronic structure. The resulting compound possesses a unique spiro-carbon-containing [5.6.5.6]-tetracyclic structure and can be easily transformed to novel porphyrinoids with variable aromaticity. This research highlights the synthesis of porphyrinoid compounds with different aromatic properties.