Photo-decarboxylation of iron(III) porphyrin–amino acid complexes in aqueous solution

Irradiation (λ > 390 nm) of acidic anaerobic aqueous solutions of iron(III) tetrakis(2-N-methylpyridyl)porphyrin, in the presence of mono- or di-basic amino acids or their N-acylated derivatives, generates the iron(II) porphyrin and the corresponding acyloxyl radical; subsequent decarboxylation of the latter gives the corresponding ammonioalkyl or amidoalkyl radical. The rate and course of each reaction have been monitored by UV–VIS spectroscopy and EPR spin-trapping techniques. The large differences in the observed rates of iron(II) porphyrin formation are controlled by two factors: the binding affinity of the carboxy group for the iron(III) porphyrin to form the photoactive complex and the competitive reactions of the acyloxyl radical decarboxylation and regeneration of the iron(III) porphyrin complex, following photolysis. With cationic carboxylate ligands, such as glycine or L-alanine, charge repulsion with the cationic porphyrin results in the former effect predominating, whereas with neutral ligands, for example N-acetylglycine, it is the latter that determines the overall rate of reaction.In aqueous base, the amino acids ligate to the iron(III) porphyrin via the amino rather than the carboxy group. Subsequent irradiation brings about an electron transfer from the ligand to give the iron(II) porphyrin and the amino acid cation–radical which reacts further to give an α-amino radical either by decarboxylation or proton loss.