Assessment of Conjugation\nPathways in <i>N</i>‑Methylporphyrins that Are Fused\nto Acenaphthylene, Phenanthrene,\nor Pyrene: Evidence for the Presence of Alternative Aromatic Circuits

Acenaphtho-, phenanthro-, and pyrenopyrrole esters, readily\navailable\nfrom Barton-Zard reactions of ethyl isocyanoacetate with nitroarenes,\nwere reacted with methyl iodide and KOH in DMSO to give <i>N</i>-methylpyrroles and subsequent cleavage of the ester moieties was\naccomplished with KOH in ethylene glycol at 200 °C. Condensation\nwith two equiv of an acetoxymethylpyrrole in refluxing acetic acid-2-propanol\nafforded a series of annulated tripyrranes. Cleavage of the terminal <i>tert</i>-butyl ester groups with trifluoroacetic acid, followed\nby condensation with a diformylpyrrole and oxidation with FeCl<sub>3</sub>, gave <i>N</i>-methyl acenaphtho-, phenanthro-,\nand pyrenoporphyrins. The <i>N</i>-methyl substituent effectively\nfreezes the tautomeric equilibria to maximize interactions between\nthe porphyrin nucleus and the fused aromatic substructures. Analysis\nof the proton NMR spectra provides evidence of the presence of extended\naromatic circuits within these structures. Anisotropy of induced ring\ncurrent (AICD) plots clearly shows the presence of 30π electron\npathways in phenanthro- and pyrenoporphyrins that run around the exterior\nof the benzenoid fragments. These results demonstrate that <i>N</i>-alkylation can be used to relocate aromatic pathways in\nporphyrinoid systems.