Kinetics and mechanism of RuCl2PPh3)3-catalyzed oxidation of sulfides by N-methylmorpholine N-oxide

Oxidation of dibutylsulfide, diphenylsulfide, methylphenylsulfide and dibenzylsulfide to the corresponding sulfoxides by N-methylmorpholine N-oxide (NMO) catalyzed by RuCl₂(PPh₃)₃ in DMF solvent is reported. The reaction is first order in both catalyst and N-oxide. The order with respect to the substrate is variable, being zero at higher concentrations and fractional at lower concentrations. The order of reactivity observed for the substrates is as follows: dibutylsulfide ≈ dibenzylsulfide > methylphenylsulfide > diphenylsulfide. RuCl₂(PPh₃)₃ oxidizes olefins to form epoxides at a slower rate. The order of reactivity observed for the two types of substrates parallels their nucleophilicity (sulfides > alkenes).Spectral studies indicate 1:1 complex formation between RuCl₂(PPh₃)₃ and the sulfides. The active oxidant is the Ru(IV)oxo complex formed by the oxidation of Ru(II) by NMO.