Oxidation and Reduction of Porphyrins

As a result of their large aromatic π-systems, porphyrins, hydroporphyrins, and their metal complexes display a rich reversible redox chemistry that is frequently studied by electrochemical methods. In their metal complexes, the porphyrinic ligands act as non-innocent ligands when partaking in redox events. Numerous irreversible chemical reductions are known that convert porphyrins to a range of hydroporphyrinoids in which either one β,β′-double bond (chlorins), two β,β′-double bonds (bacteriochlorins, isobacteriochlorins), or the meso -position (phlorins) are reduced. Likewise, an assortment of chemical oxidation methods convert the porphyrins to a range of β-, meso -, and N -oxidized derivatives. Some of these oxidation reactions lead to cleaved β,β′-bonds (secochlorins), excised β-carbons (chlorophins), or carbon atoms being replaced by oxygen atoms (porpholactones). meso -Ring-opened products (bilins) accessible by a range of oxidation methods are another important class of porphyrin oxidation products. The key chemical porphyrin redox methodologies and products, organized by the position of the attack of the reagents (β- or meso -position) and the formal type of chemical conversion (reduction or oxidation), are reviewed.