Macrocyclic modification using organometallic methodologies. Regiochemically controlled mono- and bis-homologation reactions of porphyrinogen with carbon monoxide assisted by early transition metals

Early oxophilic transition metals facilitate the homologation of a pyrrole to a pyridine ring within the porphyrinogen skeleton by assisting the insertion of CO into metal-carbon and metal-hydrogen bonds, producing eta2-formyl or eta2-acyl carbenium ion. The bonding mode of the pyrrolyl anion to the metal determines the regiochemistry of the reaction. Electronic and geometric characteristics of a tetrapyrrolic macrocycle can be modified by introducing one or two pyridines.