The chemistry of pyrrolic compounds. XL. A new synthesis of protoporphyrins III and XIII and a ¹H N.M.R. study of the preferred pathway of electron delocalization in the porphyrin macrocycle

Abstract

The synthesis of protoporphyrins III and XIII as the dimethyl esters has been completed by an oxidative cyclization of biladiene-ac intermediates. Protoporphyrin III has been converted into deuteroporphyrin III and measurements of the proton coupling constant in the allylic unit, CH₃-C=C-H, of this latter porphyrin point to the double bond of this system having a significantly diminished bond order. This finding is in accord with the view that the π-electron delocalization pathway in the porphyrin macrocycle involves the periphery of the molecule.