Oxidation of thioanisole by peroxomolybdate ions: direct oxygen transfer from tetraperoxomolybdate ion

Addition of thioanisole (PhSMe) to Na2MoO4 in dilute hydrogen peroxide at pH 9.4 rapidly quenches the absorbance of the tetraperoxo complex, Mo(O-2)(4)(2-),which gradually regenerates as the PhSMe is oxidized to the sulfoxide. This direct oxygen transfer is followed by the decreasing absorbance of Mo(O-2)(4)(2-) at 452 nm. In dilute H2O2 the Mo(O-2)(4)(2-) becomes a steady-state intermediate, and kinetics monitored by H-1 NMR spectroscopy allow estimation of the second-order rate constant for oxidation of PhSMe by MoO(O-2)(3)(2-) and rate constants for interconversion Of MoO(O-2)(3)(2-) and MoO(O-2)(4)(2-) The steady-state approximation breaks down at higher [H2O2], and with [H2O2] = 2 m the MoO(O-2)(3)(2-) and Mo(O-2)(4)(2-) are approximately in equilibrium; based on the rate constants of oxidation of PhSMe by MoO(O-2)(3)(2-) and Mo(O-2)(4)(2-) and the related association equilibrium constant, the observed and predicted rate constants for the overall oxidation of PhSMe are similar. Copyright (C) 2001 John Wiley Sons Ltd.