Carboxylate Catalyzed Isomerization of ß,γ-Unsaturated N-Acetylcysteamine Thioesters.

We demonstrate herein the capacity of simple carboxylate salts - tetrametylammonium and tetramethylguanidinium pivalate - to act as catalysts in the isomerization of ß,γ-unsaturated thioesters to α,ß-unsaturated thioesters. The carboxylate catalysts gave reaction rates comparable to those obtained with DBU, but with fewer side reactions. The reaction exhibits a normal secondary kinetic isotope effect (k1H /k1D =1.065±0.026) with a ß,γ-deuterated substrate. Computational analysis of the mechanism provides a similar value (k1H /k1D =1.05) with a mechanism where γ-reprotonation of the enolate intermediate is rate determining.