Arunachalam Chellamani

Mechanism of oxidation of organic sulphides by oxo(salen)manganese(v) complexes

Abstract The kinetics of oxygen atom transfer from several cationic oxo(salen)manganese(V) [salen = N,N′-ethylenebis(salicylidineaminato)] complexes to organic sulphides have been studied spectrophotometrically in acetonitrile at 25 °C. The reaction follows an overall second-order kinetics, first-order each in sulphide and oxomanganese(V) complex. Electronic-substrate and electronicoxidant effect studies reveal that the single electron transfer from sulphide to the oxo complex is the rate-controlling step. The redox potentials of the couple MnV/MnIV have been estimated by applying Marcus theory to the experimentally observed rate constants.

Mechanism of picolinic-acid-catalysed chromium(VI) oxidation of alkyl aryl and diphenyl sulphides

The kinetics of picolinic acid (PA)-catalysed CrVI oxidation of organic sulphur compounds have been studied with several alkyl aryl sulphides and diphenyl sulphides in acetic acid–water mixtures. The PA-catalysed oxidation follows third-order kinetics, first order each in oxidant, sulphide, and catalyst, at constant [H+] and ionic strength. A good correlation exists btween log k2 and Hammett σ constants for both aryl methyl and diphenyl sulphides, and the reaction constants are negative. Steric congestion at the reaction centre, sulphur, has been revealed by studies with C6H5SR (R = Me, Et, Pr, Pri, and But). Three mechanisms are proposed to account for the observed experimental results. In aryl methyl sulphides the rate benefit (kcat–kuncat)/kuncat is the least for p-methoxyphenyl methyl sulphide ad the largest for p-nitrophenyl methyl sulphide and this is in accordance with the reactivity–selectivity principle.

Applicability and mathematical verification of thereactivity–selectivity principle in the oxidation of thioanisolesby oxo(salen)manganese(V) complexes

The second-order rate constants for the oxidation of various para-substituted thioanisoles with substituted oxo(salen)manganese(V) complexes have been measured spectrophotometrically in acetonitrile at 25 °C. Electron-withdrawing substituents in thioanisoles and electron-releasing substituents in oxo(salen)MnV complexes decrease the rate of oxidation. Correlation analyses show the presence of an inverse relationship between reactivity and selectivity in the reactions of various thioanisoles with a given oxo(salen)MnV complex and also in various oxo(salen)MnV complexes with a given thioanisole. Mathematical treatment of the results has also been carried out.

Separation of electronic and steric effects: Oxidation ofortho-substituted phenyl methyl sulphides by peroxoanions, chromium(VI) and picolinic acid catalysed chromium(VI)

The kinetics of oxidation ofortho-substituted phenyl methyl sulphides by peroxoanions, Cr(VI) and picolinic acid catalysed Cr(VI) have been investigated. Regression analyses of the rate data by using Taft and Charton equations to separate steric effect from electronic effects have been carried out. In peroxoanion oxidations, the localized electronic effect plays a major role. In the uncatalysed and picolinic acid catalysed Cr(VI) oxidations, both localized and delocalized effects are significant. Steric effect plays a minor role in all the oxidations