Linear Extension of Carbaporphyrin Chromophores: Synthesis, Protonation, and Metalation of Anthro[2,3-b]carbaporphyrins: Evidence for 30π-Electron Aromatic Circuits in a Palladium(II) Complex.

Acid-catalyzed condensation of a naphtho[2,3-f]indane dialdehyde with a tripyrrane, followed by an oxidation step, afforded an anthro[2,3-b]-21-carbaporphyrin. The presence of a fused anthracene unit induced minor bathochromic shifts and did not significantly affect the aromatic characteristics of the carbaporphyrin core. Protonation led to the formation of a monocation with similar diatropic properties, but the dication generated in the presence of a large excess of trifluoroacetic acid had a weakened Soret band absorption and a broad absorption at 754 nm. Nucleus-independent chemical shift (NICS) calculations indicate that the dication is only weakly aromatic and possesses a 32-atom 30π electron delocalization pathway. Alkylation with methyl iodide and potassium carbonate gave a 22-methyl derivative that reacted with palladium(II) acetate to afford an aromatic palladium(II) complex. Upon heating, the methyl group migrated from the nitrogen to the internal carbon atom and the resulting complex exhibited diminished aromatic character. A comparison with related carbaporphyrin complexes without ring fusion or with benzo- or naphtho-fused units demonstrated that the diatropic character decreased with increasing conjugation. NICS calculations and anisotropy of induced current density (AICD) plots confirmed this trend and showed that the remaining aromatic properties of the anthrocarbaporphyrin complex were due to a 30π electron circuit that extends around the entire anthracene unit.

Synthesis of Chrysoporphyrins and a Related Benzopyrene-Fused System.

Reaction of 6-nitrochrysene with ethyl isocyanoacetate in the presence of a non-nucleophilic base gave a c-annulated pyrrole ethyl ester that was used to prepare chrysene-fused tripyrranes and a chrysopyrrole dialdehyde. Chrysene-fused tripyrranes were reacted with a pyrrole dialdehyde, but poor yields of chrysoporphyrins were obtained. However, condensation of the chrysopyrrole dialdehyde with a series of tripyrranes afforded excellent yields of chrysoporphyrins and an acenaphtho-chrysoporphyrin. Iron(III) chloride mediated oxidative cyclization of a dihexylchrysoporphyrin afforded a benzopyrene-fused porphyrin that exhibited a strongly red-shifted electronic absorption spectrum. DFT calculations show that both chrysoporphyrins and the benzopyrene-fused porphyrin have tautomers that possess 34π electron delocalization pathways that pass through the porphyrin nucleus and the fused polycyclic aromatic hydrocarbon (PAH) units. Protonation gave dications that favor 36-atom 34π electron circuits. c-Annulated pyrrole dialdehydes were also condensed with a carbatripyrrin to generate PAH-fused carbaporphyrins that retained fully aromatic properties.