Formation of ameso-Tetraphenylsecochlorin and a Homoporphyrin with a Twist

The osmium tetroxide-mediated dihydroxylation of meso -tetraphenylporphyrin leads to the formation of the vic -diol meso -tetraphenyl-2,3- vic -diol-2,3-chlorin. The corresponding Ni(II) complex was converted by lead tetraacetate into ( meso -tetraphenyl-2,3-secochlorinato-2,3-dialdehyde)nickel(II). This novel pigment undergoes an almost quantitative, intramolecular double acetal formation when treated with methanol in the presence of acid to produce a porphyrinoid in which one pyrrolic unit is formally replaced by a six-membered ring. An X-ray crystal structure determination, the first for such a homoporphyrin, reveals the severely twisted conformation of its chromophore.