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

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