Purpurins Bearing Functionality at the 6,16-meso-Positions: Synthesis From 5,15-Disubstituted meso-[β-(Methoxycarbonyl)vinyl]porphyrins

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Abstract
Porphyrins bearing p-X-phenyl substituents (X=Me, OH, OTs , NMe2, NO2) at the 5,15-meso-positions have been synthesized. meso-Formylation of the nickel derivatives produced the derivatives (4), (12), (17) and (23) which show atypical broadened Soret and Q-band absorptions, the latter at 570-750 nm. The optical and n.m.r. spectra of the corresponding free base monoformyl derivatives also indicate electronic perturbations of the porphyrin rr -system. Wittig reactions with ( methoxycarbonylmethylene ) triphenylphosphorane, followed by demetallation, resulted in the meso-acrylate porphyrins (8), (14), (19) and (25), which also exhibit unusual electronic spectra consistent with a distortion of the macrocyclic ring current. The p-dimethylaminophenyl derivative (14) also shows protonation-dependent spectra indicating a quinomethene structure. The meso-acrylate porphyrins were efficiently converted into type A purpurins under basic conditions. Type B purpurins were also formed, and the relative yields of types A and B are temperature and solvent dependent, but independent of the presence of oxygen. These results are in contrast to those for the formation of octaethylpurpurin derivatives; likewise, no evidence of a purpurin - porphyrin equilibrium was observed for these compounds. In the presence of oxygen and strong visible light the meso-formyl-2- ( methoxyoxalyl ) chlorins are formed. The electronic spectra of the purpurins are typified by strong absorptions in the red region of the spectrum; the p- dimethylaminophenyl derivative (27) also exhibits a solvent-dependent spectrum, which is explained in terms of a dimethylaminoquinomethene tautomerism. The purpurins and porphyrin derivatives have potential application in photodynamic cancer therapy.