Mechanism for the Oxidation of Sulfides and Sulfoxides with Periodates: Reactivity of the Oxidizing Species

DFT computations have been performed at different levels of theory to identify the mechanism for the oxidation Of Sulfides and sulfoxides with periodates. The periodate ion (IO4-), periodic acid (HIO4) and their hydrated derivatives all oxidize sulfides to sulfoxides in one-step oxygen-transfer reactions and the relative reactivities are HIO4 >> H5IO6 > IO4- > H4IO6- >> H3IO62- . The hydration and dissociation equilibria of the periodates are shifted towards IO4- in neutral and moderately acidic solutions, and sulfides are oxidized mainly with IO4- under normal experimental conditions. The oxygen atoms of the periodates attack the sulfides perpendicularly to the plane of the C-S-C atoms and the S center dot center dot center dot O center dot center dot center dot I atoms are in a linear arrangement in the very early transition state (TS). The sulfides are the electron donors and periodates are the electron acceptors in the reactions; the geometries of the TSs are determined by the overlap of the HOMO and the LUMO of the reactants. Other mechanisms can be ruled out because the attack of the sulfur atom of the substrate on the iodine atom of the reactants increases the energy of the system continuously very steeply as the S center dot center dot center dot I distance decreases. Experimentally derived Delta G double dagger; values are in good agreement with the Delta G double dagger data computed for the oxygen-transfer mechanism. Sulfoxides are also oxidized to sulfones with IO4- in a one-step oxygen-transfer reaction, and the structures of the TSs and the Delta G double dagger data are similar to those obtained for the reactions of sulfides. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)