Fusion of aromatic rings to porphyrins and their derivatives has been shown to cause variable bathochromic shifts that can give insights into how extended conjugation effects the aromaticity of the porphyrin ring. Fusion of phenanthrene has been studied in the past in an attempt to induce bathochromic shifts in the UV-vis spectra, and to potentially expand the normal porphyrin 18 pi-electron pathway through the phenanthrene ring to give a 30 pi-electron circuit. These effects were not found to be as strong as predicted, possibly due to the favored tautomer 1a providing a conjugation pathway that avoids the fused aromatic unit. Addition of an N-methyl group to this system allows an alternative tautomer 1b to be locked into place, thereby enabling a 30 pi-electron pathway. A MacDonald-type “3+1” condensation was used to react N-methyl phenanthrotripyrrane 2 with a pyrrole dialdehyde 3 to afford N-methyl phenanthroporphyrin 4. Furan, thiophene, selenophene, and indene dialdehydes were similarly condensed to afford a series of N-methyl heteroporphyrins with a fused phenanthrene units. A palladium(II) complex of N-methylphenanthrobenzocarbaporphyrin 5 was also synthesized and spectroscopic evidence was obtained that demonstrated that methyl group migration across the porphyrinoid macrocycle to give 6 had occurred. These compounds all showed bathochromic shifts towards the red region and downfield shifts in the 1H NMR spectra when compared to N-unsubstituted porphyrinoids.

Synthesis of N-methylphenanthroporphyrinoids with extended conjugation pathways

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