5,10,15,20-TETRAPHENYLSAPPHYRIN - IDENTIFICATION OF A PENTAPYRROLIC EXPANDED PORPHYRIN IN THE ROTHEMUND SYNTHESIS

The Rothemund-type condensation of pyrrole and benzaldehyde yields, ap art from 5,10,15,20-tetraphenylporphyrin (TPPH2) and inverted tetraphe nylporphyrin 2-aza-21-carba-5,10,15,20-tetraphenylporphyrin (CTPPH2), a unique pentapyrrolic macrocyclic molecule with the aromatic nucleus of sapphyrin, namely, 5,10,15,20-tetraphenylsapphyrin (TPSH3). Its uno rthodox structural skeleton with an inverted pyrrole ring lying opposi te to the bipyrrole unit accounts for the spectroscopic properties of the novel sapphyrin. The diprotonation of TPSH3 acts as a trigger for a structural transformation involving a flip of the pyrrole units, whi ch relocates the 27-NH pyrrolic nitrogen from the periphery into the c enter of the macrocycle. The formation of 5,10,15,20-tetraphenylsapphy rin proves that the pentapyrrolic product is accessible by the mechani sm of the Rothemund synthesis.