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 m ethylrhenium trioxide, CH3ReO3. The kinetics were investigated in acet onitrile-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 CH3 Re(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 substitu ents on phosphorus were made to vary the nucleophilicity of the phosph ine 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 PP h(3). The rhenium peroxides A and B show a small difference in reactiv ity. The bimolecular reactions between A and most of the phosphines ha ve 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.