Novel and Improved Syntheses of 5,15-Diphenylporphyrin and its Dipyrrolic Precursors

Optimized syntheses of 5,15-diphenylporphyrin (DPP, 1) and its dipyrrolic precursors are described. A novel procedure for the synthesis of dipyrromethane (2), prepared by hydrodesulfurization of the corresponding di-2-pyrrolylthione (8), is presented, as well as an improved method to isolate large quantities of 5-phenyldipyrromethane (4), prepared by the acid-catalysed condensation of pyrrole with benzaldehyde. These dipyrromethanes are key reagents in two high-yield (2+2)-type syntheses of DPP. 5-Phenyldipyrromethane was formylated to provide 1-formyl- (11) and 1,9-diformyl-5-phenyldipyrromethane (12), and reduction of 11 provided the corresponding hydroxymethyl compound 13. These compounds (11-13), however, were much less efficient precursors to DPP. Two polypyrrolic compounds, 1,1,2,2-dipyrrolylethane (9) and 5,10-diphenyltripyrrane (10), potentially useful for the synthesis of porphyrinic macrocycles, were isolated as side-products during the dipyrromethane and 5-phenyldipyrromethane syntheses.