MTO/H₂O₂/Pyrazole-Mediated N-Oxidation of <i>meso</i>-Tetraarylporphyrins and -chlorins, and S-Oxidation of a <i>meso</i>-Tetraaryldithiaporphyrin and -chlorin

The methyltrioxorhenium (MTO)/pyrazole-mediated H2O2 oxidation of octaethylporphyrin and a number of meso-tetraarylporphyrins offers simple and good yielding access to the corresponding N-oxides, only few of which were prepared before. The crystal structure of a free base tetraarylporphyrin N-oxide demonstrates the degree to which the oxygenated pyrrole moiety is slanted with respect to the rest of the otherwise nearly planar macrocycle. The method is also suitable to the preparation of hitherto unknown chlorin N-oxides. Oxidation of meso-tetraphenyldithiaporphyrin and meso-tetraphenyldithiachlorin furnishes the corresponding novel S-oxides. The optical properties of the novel chromophores are described and rationalized.