The photo-oxidation of meso-tetraphenylporphyrins

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Abstract
The photo-oxidation of meso-tetraphenylporphyrin (TPP) dianion in methanol affords a purple benzoyl bilinone (5b) by addition of singlet oxygen, followed by ring-opening and addition of methanol; similar products (5a) and (5c) can be formed by addition of water, or ethanol, respectively. The water adduct (5a) had previously been obtained by other workers from the photo-oxidation of metallo derivatives of TPP, by cerium(IV) or thallium(III) oxidation of TPP, or by treatment of the cation radical of TPP with nitrite, but a different structure had been assigned. Our new structure(5) has been deduced from detailed NMR and mass spectral studies including decoupling and NOE measurements, as well as field desorption mass spectra at low wire currents; the presence of quaternary meso-carbon atoms in structures (5a) and (5b) has been shown by the 13C NMR spectra of the meso13C -labelled pigments. Confirmation of these assignments has been provided by photo-oxidation of the meso-tetra(4-methoxyphenyl)porphyrin and further spectral studies of the resulting bilinones (7a) and (7b), including NOE and 13C -1H shift correlated NMR spectra.