Influence of the Electronic and Steric Demand of the Nitrogen Bases on the Oxidation of Phenylmercaptoacetic Acids in the Presence [Oxo(Salen)Chromium(V)] ⁺ Ion

ABSTRACT The sulfoxidation of eleven phenylmercaptoacetic acids (PMAA) catalyzed by three oxo(salen)chromium(V) complexes has been performed spectrophotometrically in 100% acetonitrile medium. Four nitrogen bases (NB), pyridine (Py), imidazole (ImH), 1‐methylimidazole (MeIm), and picolinic acid (PA) were used as axial ligand. It has been found that the rate of sulfoxidation is not only tuned by the substituents in the PMAA and salen but also varied by the addition of a nitrogen base. The observed order of rate enhancement of nitrogen bases ImH > MeIm > Py > PA seems to be directly related to the binding constant values, pKa values, and π‐donating abilities. An adduct formed between [oxo(salen)chromium(V)] + ion and base has been proposed as the sole active oxidizing species. The Hammett correlation between rate constants and substituent constants shows a nonlinear concave upward curve, which is explained by the existence of two different mechanisms: electrophilic attack of the oxygen atom of the oxo(salen)chromium(V)‐base adduct on electron‐rich PMAA for electron‐withdrawing substituents and single electron transfer from electron‐rich sulfur atom to the oxo functionality of the complex for electron‐donating substituents.