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

A strategy for the synthesis of asymmetrically substituted tetraazaanthrace ne linked bis-porphyrins in which the two porphyrin rings contain differenc es in their peripheral substituents has been developed. The method is illus trated by the preparation of bis-porphyrins with a single meso-halophenyl a nd 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-t etraamine to form a porphyrin diaminoquinoxaline intermediate which was sub sequently condensed with a second different porphyrin-alpha-dione. The key issue in the synthesis was the separation of the desired asymmetrically sub stituted bis-porphyrin from the symmetric bis-porphyrin by-products of simi lar polarities. Enhanced separation of the bis-porphyrin products was achie ved by chelation of a metal into one of the porphyrin rings, the metal bein g introduced at the porphyrin-alpha-dione stage. Copper was successfully us ed when chelated into the less polar porphyrin-alpha-dione but the use of z inc in the more polar porphyrin-alpha-dione to enhance bis-porphyrin separa tion was unsuccessful as the pyridinium hydrochloride produced in the react ion was found to demetallate the porphyrins.