Modulation of catalytic activity by ligand oxides in the sulfoxidation of phenylmercaptoacetic acids by oxo(salen)chromium(V) complexes

Mechanism of sulfoxidation of eleven para-substituted phenyl mercaptoacetic acids (PMAAs) by three oxo(salen)chromium(V)+ PF6− complexes in the presence of different ligand oxides (LOs) such as triphenylphosphine oxide, pyridine N-oxide and 4-picoline N-oxide have been studied spectrophotometrically in 100% acetonitrile medium. Spectral and kinetic profiles establish the formation of adduct, OCr(V)(salen)+-LO as the reactive intermediate in the catalytic cycle. The rate of sulfoxidation is found to be enhanced significantly by the addition of LOs and introduction of substituent in PMAA and salen complex. Both electron releasing and electron withdrawing substituents in the substrate and oxidant facilitate the rate of sulfoxidation. Correlation with Hammett constants yields a non-linear concave upward curve. Based on the experimental results and substituent effects two different mechanisms, a direct oxygen atom transfer (DOT) for PMAAs with electron withdrawing substituents and a single electron transfer for PMAAs with electron donating substituents have been postulated.

Graphical AbstractThe presence of triphenylphosphine oxide, pyridine N-oxide and picoline N-oxide in the oxidation of phenylmercaptoacetic acid by [oxo(salen)chromium(V)]+ causes rate acceleration. Substituent effects and non-linear Hammett correlation paved way to propose a single electron transfer mechanism for electron donors and direct oxygen atom transfer mechanism for electron acceptors.Figure optionsDownload as PowerPoint slide