Osmium (VIII) catalyzed kinetics and mechanism of indoles oxidation with Aryl-N-haloamines in alkaline medium

The kinetics of oxidation of the title substrates by sodium N-haloarylsulfonamides (or ary-N-haloamines), chloramine-T (CAT), bromamine-T (BAT), chloramine-B (CAB), and bromamine-B (BAB), catalyzed by osmium(VIII) in alkaline medium has been studied at 30°C. The corresponding oxindoles and arylsulfonamides have been characterized as reaction products. The reaction rate shows a first-order dependence each on |indole|₀ and |oxidant|₀, a fractional-order on |Os(VIII)|, and an inverse first-order on |OH⁻|. Addition of arylsulfon-amide, chloride and bromide, and variation of ionic strength of the medium have no effect on the reaction rate. There is a negative effect of dielectric constant of the solvent. Activation parameters have been calculated from the Arrhenius and Eyring plots. Hammett correlation of substituent effects indicates an LFE relationship with ρ = −1.0, showing the formation of an electron deficient transition state. From enthalpy-entropy relationships and Exner correlations, the isokinetic temperatures (333 K and 326 K) have been determined for the reactions of CAT and BAT, respectively. Proton inventory studies in H₂O-D₂O mixtures have shown the involvement of a single exchangeable proton of OH⁻ ion in the transition state. A mechanism consistent with the observed kinetics has been proposed. © 1996 John Wiley & Sons, Inc.