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] ≫ [Ir(IV)]0 reveal the rate law to be -d[Ir(IV)]/dt = k[Ir(IV)](2)[CysSO2H]/[Ir(III)] with a negligible pH dependence. The value of k is (6.8 ± 0.12) × 10(3) 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 Ir(IV) and CysSO2(-) to form Ir(III) and CysSO2(•). The second step is the rate-limiting inner-sphere oxidation of CysSO2(•) by Ir(IV). Production of CysSO3H is proposed to occur through hydrolysis of an Ir(III)-bound sulfonyl chloride that is the immediate product of the inner-sphere second step.