OXIDATIONS OF ER(3) (E=P, AS, OR SB) BY HYDROGEN-PEROXIDE - METHYLRHENIUM TRIOXIDE AS CATALYST

Catalytic and noncatalytic conversions of tertiary phosphines to their oxides by hydrogen peroxide have been investigated. The catalyst is methylrhenium trioxide, CH3ReO3. The kinetics were investigated in acetonitrile-water (1:1 by volume) at 25 degrees C. Stepwise interactions between CH3ReO3 and H2O2 form CH3Re(eta(2)-O-2)(O)(2)(OH2), A, and CH3Re(eta(2)-O-2)(2)(O)(OH2), B. In CH3CN-H2O ( 1:1 v/v) the equilibrium constants are K-1 = 13 +/- 2 L mol(-1) and K-2 = 136 +/- 28 L mol(-1) at pH 1.0 and 25 degrees C. The forward and reverse rate constants for the formation of A in this medium are k(1) = 32.5 +/- 0.3 L mol(-1) s(-1) and k(-1) = 3.0 +/- 0.2 s(-1). Systematic changes in the substituents on phosphorus were made to vary the nucleophilicity of the phosphine and its cone angles; the kinetic effects are discernible, although they Lie in a narrow range. Triphenylarsine and triphenylstibine were also studied, and their rates are within a factor of 2 of that for PPh(3). The rhenium peroxides A and B show a small difference in reactivity. The bimolecular reactions between A and most of the phosphines have rate constants of the order 10(5) L mol(-1) s(-1). The kinetic data support a mechanism that allows nucleophilic attack of the substrate at the rhenium peroxides.