Regiospecific beta-functionalization of free-base porphyrins by pseudohalogens

Pseudohalogens based on iodine ('I-X') can be used to regiospecifically introduce chlorine atoms or acetoxy groups onto the beta-positions of meso-tetraphenylporphyrins (TPPs). TPPs and a quinoxaline derivative were reacted with iodine monochloride to give mono- or di-chlorinated porphyrins, such that when two chlorine atoms were added they were placed antipodally on the porphyrin ring. Reaction of the porphyrins with a mixture of iodine and silver acetate gave the corresponding mono- and di-acetoxylated porphyrins. The acetoxylated porphyrins could be simply transformed to the corresponding hydroxyporphyrins with subsequent oxidation with the Dess-Martin periodinane, giving a simple new route to chlorin-alpha-diones and bacteriochlorin-tetraones. From the products of the reactions and a UV-visible spectroscopic study, it is proposed that the reactions proceed via a single electron transfer mechanism through a porphyrin cation radical intermediate.