Acid-catalysed oxidative ring-opening of epoxide by DMSO. Theoretical investigation of the effect of acid catalysts and substituents

摘要

DFT studies have been carried out in order to investigate the reaction mechanism of a series of acid-catalysed oxidative ring-opening of epoxides in DMSO. The role of the acid catalyst during the oxidation of ethylene oxide has been investigated by employing three acids, namely H3O+, Li+, and Mg2+. Effects of substituents on the epoxide have been investigated by including butene oxide and cyclopentene oxide as the reacting species. Stationary points have been obtained at the B3LYP/6 - 31++ G(d, p) level of theory and the reaction barriers have been evaluated through free-energy calculations. This is the first mechanistic elucidation for such a process, involving oxidation of the epoxide ring by DMSO. The mechanism proceeds in two steps, namely a ring-opening step followed by an intramolecular proton transfer which leads to an alpha-hydroxyketol. The results show that the second step is the energetically less favourable, a feature consistent with the harsh experimental conditions needed to obtain such products. The role of the acid catalyst is discussed and we show that use of H3O+ or Mg2+ as an activator leads to similar results concerning the reaction energetics.