5,10,15,20‐Tetraphenylsapphyrin‐Identification of a Pentapyrrolic Expanded Porphyrin in the Rothemund Synthesis

Abstract

Abstract The Rothemund‐type condensation of pyrrole and benzaldehyde yields, apart from 5,10,15,20‐tetraphenylporphyrin (TPPH 2 ) and inverted tetraphenylporphyrin 2‐aza‐21‐carba‐5,10,15,20‐tetraphenylporphyrin (CTPPH 2 ), a unique pentapyrrolic macrocyclic molecule with the aromatic nucleus of sapphyrin, namely, 5,10,15,20‐tetraphenylsapphyrin (TPSH 3 ). Its unorthodox structural skeleton with an inverted pyrrole ring lying opposite to the bipyrrole unit accounts for the spectroscopic properties of the novel sapphyrin. The diprotonation of TPSH 3 acts as a trigger for a structural transformation involving a flip of the pyrrole units, which relocates the 27‐NH pyrrolic nitrogen from the periphery into the center of the macrocycle. The formation of 5,10,15,20‐tetraphenylsapphyrin proves that the pentapyrrolic product is accessible by the mechanism of the Rothemund synthesis.