Spontaneous nucleophilic addition of hydroxide ions to the meso-position of high-valent antimony-oxo porphyrin complexes.

Bimolecular reactions of the antimony(V) porphyrin complexes SbO(tpp)OH, 1 and SbO(oep)OH, 2 with tetra-n-butylammonium hydroxide were investigated at 298 K in acetonitrile solution (tpp, dianion of 5,10,15,20-tetraphenylporphyrin and oep, dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin). Spontaneous nucleophilic addition of hydroxide ions to the non-oxidized porphyrin macrocycle leads to novel hydroxyphlorin derivatives, which contain a saturated meso-carbon bridge. While this process is a reversible equilibrium reaction for the TPP derivative, the OEP complex undergoes subsequent demetallation and oxidative ring cleavage in the presence of dioxygen. Possible implications for the competitive inhibition of heme-oxygenase by high-valent metalloporphyrin therapeutics are discussed.