Bis-porphyrin arrays. Part 2. The synthesis of asymmetrically substituted bis-porphyrins

A strategy for the synthesis of asymmetrically substituted tetraazaanthracene linked bis-porphyrins in which the two porphyrin rings contain differences in their peripheral substituents has been developed. The method is illustrated by the preparation of bis-porphyrins with a single meso-halophenyl and seven meso-3,5-di-tert-butylphenyl substituents. The bis-porphyrins were prepared by condensation of a porphyrin-alpha-dione with benzene-1,2,4,5-tetraamine to form a porphyrin diaminoquinoxaline intermediate which was subsequently condensed with a second different porphyrin-alpha-dione. The key issue in the synthesis was the separation of the desired asymmetrically substituted bis-porphyrin from the symmetric bis-porphyrin by-products of similar polarities. Enhanced separation of the bis-porphyrin products was achieved by chelation of a metal into one of the porphyrin rings, the metal being introduced at the porphyrin-alpha-dione stage. Copper was successfully used when chelated into the less polar porphyrin-alpha-dione but the use of zinc in the more polar porphyrin-alpha-dione to enhance bis-porphyrin separation was unsuccessful as the pyridinium hydrochloride produced in the reaction was found to demetallate the porphyrins.