Carbaporphyrins are porphyrin analogues where one of the pyrrolic nitrogen atoms have been replaced by carbon. Although carbaporphyrins have been the focus of numerous investigations, little work has been conducted on modified structures with fused aromatic rings that extend the conjugation. In this work, the first examples of carbaporphyrins 1 with fused anthracene units have been synthesized. Condensation of anthracene-fused cyclopentene dialdehyde 2 with tripyrrane 3 in the presence of trifluoroacetic acid, followed by neutralization of the reaction mixture with triethylamine and oxidation with DDQ gave anthracarbaporphyrin 1a in 22% yield. The macrocycle proved to be fully aromatic, and the proton NMR spectrum was consistent with the presence of an 18π electron conjugation pathway. Reaction with methyl iodide and potassium carbonate in refluxing acetone selectively afforded N-methyl carbaporphyrins 1b. Metalation with palladium(II) acetate in refluxing chloroform-acetonitrile was accompanied by methyl group migration to give C-methyl palladium(II) complex 4. The electronic spectrum of 4 was greatly altered due to the presence of new conjugation pathways, including a 30π electron circuit. These results provide new insights into the aromatic characteristics of π-extended porphyrin systems.

Synthesis, metalation and aromatic characteristics of anthro[2,3-b]-21-carbaporphyrins

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