Synthesis of spirofluorenyl-embedded 5,10- and 5,15-porphodimethenes <i>via</i> scrambling side reactions

In this study, we investigate the synthesis of spirofluorenyl-fused porphodimethenes through the reaction of the corresponding dipyrromethane with cycloheptyl-fused dipyrromethane and benzaldehyde under acidic conditions and subsequent oxidation. This yields a mixture of the expected 5,15-porphodimethenes, along with 5,10-porphodimethenes and tetraphenylporphyrin. These unexpected products are derived from the scrambling side reactions of the 9-fluorenyl-fused dipyrromethane, whose reproducibility was also demonstrated by the reaction of 9-fluorenyl-fused dipyrromethane and benzaldehyde. The mixture of 5,10-porphodimethenes and tetraphenylporphyrin could not be separated owing to their insufficient solubility in common solvents used for the chromatography and similar mobility on silica gel. The structural determination and final elucidation of each by-product were performed using X-ray crystal structure analysis by selecting single crystals from the crude crystallization systems. Such a procedure to synthesize 5,10-porphodimethenes can function as a protocol that can also be used to synthesize cyclic congeners of tripyrrins based on their fully conjugated tripyrrolic partial structures.