Studies in Porphyrin Chemistry: A Synthetic Approach

Abstract

This review summarizes porphyrin synthesis in the author's laboratory, and deals mainly with the preparation of porphyrins substituted with electron-withdrawing groups. Oxidative cyclization of 1,19-dimethylbilenes-b has provided the most successful avenue to porphyrins of this class, and the procedure works best when the terminal rings of the bilene -b are substituted with electron-withdrawing groups. This methodology has yielded a wide range of porphyrins carrying this class of substituent on adjacent pyrrolic rings, and has led to the preparation of many biologically important molecules including porphyrin a and the chlorophylls c₁ and c₂. Also described are the necessary modifications which are required to adapt the general strategy to the synthesis of porphyrins carrying electron-withdrawing groups on opposite pyrrolic rings as well as for cases where only one such substituent occurs. All these procedures were designed specifically to handle the preparation of porphyrins which were unsymmetrically substituted. However, some biologically significant members of the porphyrin series have a symmetrical arrangement of substituents in part of the molecule, and an adaption of the general synthetic procedure allows advantage to be taken of this substitution pattern. A mechanistic study of the oxidative cyclization of bilenes -b, which has been the reaction central in all the above synthetic endeavours, has produced a broad understanding of the manner in which the cyclization proceeds. Very recent studies involving the synthesis of petroporphyrins, besides consolidating the structure of many of these compounds, have provided sufficient material to allow an investigation of the properties of these porphyrins to commence, and in some cases this had led to a clearer understanding of their orgin.