Meso-oxidation of some metalloporphyrins

Treatment of porphyrins with thallium(III) trifluoroacetate in the presence of trifluoroacetic acid results in uncontrolled oxidation at the macrocyclic meso-positions, presumably via radical processes. For example, a mixture of the thallium(III) α γ-dioxoporphodimethene (4a; R = Et), the αβγ-trioxo compound (3), and octaethylxanthoporphyrinogen (6) is obtained when octaethylporphyrin (1; R = Et) is oxidised in the presence of air. More controlled oxidation is achieved when the meso-trifluoroacetoxyporphyrin complexes (8a, b) or metallo-oxophlorins (7a, b) are treated with mild bases in air, the major products being metallo-αγ-dioxoporphodimethenes (4b, c).β-Hydroxy-α-oxophlorins (16) are isolated and characterised for the first time; aspects of the chemistry of this novel oxygenated porphyrin system are reported.