Mechanistic study on the oxidation of (phenylthio)acetic acids by oxo(salen)manganese(V) complexes and the reactivity–selectivity principle

A systematic study on the kinetics and mechanism of oxidation of several (para-substituted phenylthio)acetic acids with various substituted oxo(salen)manganese(V) complexes in acetonitrile at 20 °C is presented. The kinetic data indicate that the reaction is second-order overall, first-order each in (phenylthio)acetic acid and oxo(salen)manganese(V) complex. Rate studies with substituted (phenylthio)acetic acids give an excellent Hammett correlation with σ+/σ− constants and the ρ-values are in the range of −0.82 to −1.17 for different oxo(salen)manganese(V) complexes. The log k2 values observed in the oxidation of each (phenylthio)acetic acid by substituted oxo(salen)manganese(V) complexes correlate with 2σ, giving ρ-values from 0.26 to 0.56. A mechanism involving single electron transfer from the sulfur center of the substrate to the oxo complex in the rate-controlling step is envisaged. Kinetic data were obtained over a temperature range of 15–35 °C and the activation parameters evaluated. Correlation analyses show the presence of an inverse relationship between reactivity and selectivity in the reactions of various (phenylthio)acetic acids with a given oxo(salen)manganese(V) complex and also in various oxo(salen)manganese(V) complexes with a given (phenylthio)acetic acid. Mathematical treatment of the results shows the operation of a valid reactivity–selectivity principle in this redox system.

Graphical abstractThe oxygenation of several (para-substituted phenylthio)acetic acids with various substituted oxo(salen)manganese(V) complexes follows an overall second-order kinetics, first-order each in the oxidant and substrate. Substituent effect studies reveal the operation of a SET mechanism. This redox system falls under strong reactivity–selectivity principle.Figure optionsDownload full-size imageDownload as PowerPoint slide