OXIDATION OF THIOANISOLES AND METHYL PHENYL SULFOXIDES BY OXO(PHOSPHINE)RUTHENIUM(IV) COMPLEXES

The oxo(phosphine)ruthenium(IV) complexes [Ru(bpy)2(O)PR3](ClO4)2 (bpy = 2,2'-bipyridine; PR3 = P(C6H5)3, P(p-C6H4OCH3)3, P(p-C6H4CH3)3, P(p-C6H4F)3, P(C6H5)(p-C6H4CF3)2, P(p-C6H4CF3)3) cleanly oxidize sulfides to sulfoxides and in separate experiments oxidize sulfoxides to sulfones. The substrate oxidations are second order overall, first order in substrate concentration and first order in ruthenium concentration. Through the use of para-substituted thioanisoles and para-substituted methyl phenyl sulfoxides, linear free energy relationship analyses of the kinetics of substrate oxidations by [Ru(bpy)2(O)P(C6H5)3](ClO4)2 were conducted. For the oxidation of sulfides to sulfoxides, an excellent correlation (R2 = 0.99) was obtained between log (k(X)/k(H)) and the sigma- values for the substituents of para-substituted thioanisoles (rho = -1.56). For the oxidation of sulfoxides to sulfones, a good correlation (R2 = 0.98) was obtained between log (k(X)/k(H)) and the sigma+ values for the substituents of para-substituted methyl phenyl sulfoxides (rho = -0.42). In addition, the kinetic isotope effect for the oxidation of thioanisole and methyl-d3 phenyl sulfide produced k(H)/k(D) = 1.14, and the oxidation of methyl phenyl sulfoxide and methyl-d3 phenyl sulfoxide resulted in an inverse isotope effect of k(H)/k(D) = 0.64. Finally, through the use of [Ru(bpy)2(O)PR3](ClO4)2 complexes, Hammett-type analyses of the kinetics of thioanisole oxidation and methyl phenyl sulfoxide oxidation by the [Ru(bpy)2(O)PR3](ClO4)2 complexes afford a different approach in analysis of the mechanism of sulfide and sulfoxide oxidation. Hammett correlations of log (k(X)/k(H)) versus SIGMAsigma (where sigma is the Hammett substituent constant for each of the para substituents on the triphenylphosphine ligands) also show linear relationships. The slopes of these plots gave rho = 0.49 (R2 = 0.99) for the oxidation of thioanisole and rho = 0.37 (R2 = 0.99) for the oxidation of methyl phenyl sulfoxide by the [Ru(bpy)2(O)PR3](ClO4)2 complexes. The results of all our experiments suggest that the rate-determining step in the oxidation of thioanisole by [Ru(bpy)2(O)P(C6H5)3](ClO4)2 involves primarily single-electron transfer, whereas the rate-determining step of the oxidation of methyl phenyl sulfoxide involves primarily an S(N)2 mechanism.