Oxidation of Cysteinesulfinic Acid by Hexachloroiridate(IV)

We report the results of an experimental study of the oxidation of cysteinesulfinic acid (CysSO2H) by [IrCl6]2– in aqueous media at 25 °C in order to gain insight into the mechanisms of oxidation of alkylsulfinic acids by simple one-electron oxidants. When the reaction is performed with exclusion of O2 between pH 3 and 5, it is complete in several seconds. The products are [IrCl6]3– and CysSO3H. Kinetic data obtained by stopped-flow UV–vis methods with [CysSO2H] ≫ [IrIV]0 reveal the rate law to be −d­[IrIV]/dt = k[IrIV]2[CysSO2H]/[IrIII] with a negligible pH dependence. The value of k is (6.8 ± 0.12) × 103 M–1 s–1 at μ = 0.1 M (NaClO4). A mechanism is inferred in which the first step is a rapid and reversible electron-transfer equilibrium between IrIV and CysSO2 – to form IrIII and CysSO2 •. The second step is the rate-limiting inner-sphere oxidation of CysSO2 • by IrIV. Production of CysSO3H is proposed to occur through hydrolysis of an IrIII-bound sulfonyl chloride that is the immediate product of the inner-sphere second step.