Concerning the mechanism of the reaction of trisubstituted Phosphines with episulfides

Triphenylphosphine and tributylphosphine react with cis-2-butene episulfide and trans-2-butene episulfide to give cis-2butene and trans-2-butene, respectively. The kinetics of the reaction of triphenylphosphine with these two episulfides and 1-butene episulfide have been measured in several solvents of different dielectric constants. The rates of the reactions are relatively insensitive to large changes in dielectric constant of the medium. These data indicate that the desulfurization reaction proceeds by a nucleophilic attack by the phosphine on the sulfur to give the phosphine sulfide and olefin in one step, This process is envisioned as a smooth displacement on sulfur with little or no charge separation in the transition state.