Unraveling Non-Linear Hammett Behavior in Aromatic Sulfinyl Acetic Acid Oxidation Catalyzed by [FeIII(salen)]Cl Complexes

Oxido-decarboxylation mechanism of phenylsulphinylacetic acid (PSAA) by means of eight iron(III)salen complexes in aqueous N,N′-dimethylformamide (DMF) was studied. The active oxidizing species is identified as [FeIII(salen)]+ ion. Complex formation between PSAA and [FeIII(salen)]+ ion was confirmed through Michaelis-Menten kinetics and spectral changes. Introducing diverse electron-withdrawing and releasing groups (except p-OMe) in PSAA slows the reaction rate, with a non-linear Hammett correlation indicating that the influence of substituents shifts in PSAA. Electron-withdrawing groups impede electron transfer (ET) reaction between PSAA and iron-complex, while electron-releasing groups enhance it but also reduce the nucleophilic attack by water. A mechanism involving simultaneous electron transfer and decarboxylation leading to sulfone formation is proposed, with the non-linear Hammett correlation attributed to the development and decay of sulfoxide radical cations influenced by means of substituents.