Pyrazinoporphyrins: Expanding a Pyrrolic Building Block in meso-Tetraphenylporphyrin by a Nitrogen Atom

Abstract

Application of a variant of our "porphyrin breaking and mending strategy" to free base and Ni^II complexes of meso-tetraarylporphyrin – dihydroxylation, oxidative diol cleavage and reaction of the resulting seco-chlorin bis(aldehyde) with ammonia – generates pyrazinoporphyrins. Thus, a nitrogen atom was inserted between the two former β-carbon atoms, overall formally replacing a pyrrolic building block by a pyrazine moiety. Treatment of the initially formed pyrazine hemiaminals with alcohols converts them into the corresponding hemiaminal ethers. This stabilizes the chromophores significantly, but the free-base pyrazinoporphyrins still remain sensitive toward (acid-induced) degradation reactions. Free-base pyrazinoporphyrins possess slightly redshifted porphyrin-type UV/Vis spectra, while the spectra of the pyrazinoporphyrin Ni^II complexes resemble more those of metallochlorins.

The stepwise modification of meso-tetraphenylporphyrin to affect the formal replacement of a pyrrolic building block by a pyrazine moiety is described. The optical properties of the resulting pyrazinoporphyrins are porphyrin-like.

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