Rhodium-catalyzed disulfide exchange reaction

A system of RhH(PPh₃)₄, trifluoromethanesulfonic acid, and (p-tol)₃P catalyzes the disulfide exchange reaction. Treatment of two symmetrical dialkyl disulfides with the catalyst provides an equilibrium mixture of three disulfides within 15 min in refluxing acetone. The catalyst is active after reaching the equilibrium, and addition of a disulfide to the mixture changes the ratio of the products. The use of 4 mol equiv excess of one of the disulfides provides the unsymmetrical disulfide in a yield exceeding 80%. Disulfide-containing peptides also undergo an exchange reaction. The reactions of diaryl disulfides and dialkyl disulfides are even faster, and reach equilibrium within 5 min at room temperature in the presence of the rhodium complex and 1,2-bis(diphenylphosphino)ethane (dppe). This exchange reaction is considerably affected by the substituents on the disulfides. Treatment of diphenyl disulfide, di(p-tolyl) disulfide, and bis(sec-butyl) disulfide yields phenyl p-tolyl disulfide at room temperature with unchanged bis(sec-butyl) disulfide; random disproportionation occurs at reflux. The rhodium catalysis can be used for the exchange reaction of disulfides and diselenides giving selenosulfides as well as disulfides and ditellurides giving tellurinosulfides.