trans-Substituted porphyrins (ABAB) are commonly synthesized through the reaction of an aldehyde with a dipyrromethane. However, in this acid-catalyzed condensation there is a constant risk of fragmentation followed by recombination. Therefore, this procedure can lead to the rearrangement of meso-substituents, ultimately yielding a mixture of porphyrins.
In this work, different approaches were evaluated to obtain 5,15-bis[4-(N,N-diphenyl)aminophenyl]-10,20-bis(pentafluorophenyl)porphyrin and 5,15-bis[4-(9-carbazolyl)phenyl]-10,20-bis(pentafluorophenyl)porphyrin. In particular, these specific substituent patterns are crucial building blocks in obtaining different materials. First, the reaction of 5-pentafluorophenyldypyrromethane with the corresponding benzaldehyde catalyzed by boron trifluoride diethyl etherate in dichloromethane led to a high level of scrambling that produces a mixture of porphyrins. These products involve ABAB (3%), A3B (15%) and A4 (4%) symmetries, where A represents a pentafluorophenyl group. These porphyrins have similar polarities and they are very difficult to separate by column chromatography. Therefore, the reagents were changed to pentafluorobenzaldehyde and dipyrromethane. When 1:1 molar ratio was used, the products were a complex mixture. However, the total yield of porphyrins increased to 20% and 25% with a molar ratio of 0.7:1 and 0.5:1, respectively. Furthermore, in the last case, A4 porphyrin was not obtained and the main products were ABAB (19%) and A3B (6%). Therefore, condensation of a dipyrromethane with pentafluorobenzaldehyde provides a general method for the rational synthesis of ABAB-porphyrins in good yield with lower scrambling.
