Mechanism of the Pummerer reaction: a computational study.

DFT calculations are used to investigate the mechanism of the Pummerer reaction between a chiral sulfoxide and acetic anhydride under classical and stereoselective reaction conditions (without and with additives, respectively). The first step involving acetylation of the sulfoxide with release of acetate is found to be rate-determining in both cases. For the stereoselective Pummerer reaction in the presence of trimethylsilyl triflate (TMSOTf) and N,N-dimethylacetamide (DMAC), TMSOTf- and DMAC-assisted transition states as well as ion exchange reactions are considered to account for the role of TMSOTf and DMAC.