Expansion of the porphyrin π-system: stepwise annelation of porphyrin β,β′-pyrrolic faces leading to trisquinoxalinoporphyrin

Abstract

Lateral extension of the porphyrin π-electron system by ring annelation of β,β′-pyrrolic faces is reported. Three separate syntheses of trisquinoxalinoporphyrin 1, differing in the sequence in which pyrrolic rings are elaborated, are reported. The most efficient pathway converts readily accessible zinc(II) corner bisquinoxalinoporphyrin 7 into 1 in five steps in 47% yield overall. Routes to 1 are also established from linear bisquinoxalinoporphyrin 8 and quinoxalinoporphyrin 10. The ring annelation process involves a nitration, reduction, photooxidation sequence to establish α-dione functionality that is reacted with 1,2-diaminobenzene. The ¹H NMR spectrum of 1 reveals that it exists predominantly as a single trans-tautomer in which the two inner hydrogens reside on the nitrogens of the unsubstituted pyrrolic ring and its opposite quinoxalino-fused pyrrolic ring. Each of the synthetic routes established in this study could have use in elaboration of more complex oligoporphyrin arrays.