Driving Forces in the Sharpless Epoxidation Reaction: A Coupled AIMD/QTAIM Study

Abstract

In order to better understand the epoxide-formation step of the Sharpless epoxidation process, a set of 263 oxygen-transfer reactions reflecting the complexity of the Sharpless epoxidation process were studied using density functional theory (DFT) and Bader's quantum theory of atoms in molecules (QTAIM). The diversity within these reactions reflects the different ligands in the coordination sphere of vanadium and also different substrates (alkene and an allylic alcohol both free and in the form of an alcoxo ligand). The transition states for 76 of these reactions were also characterized using DFT and QTAIM, allowing for an estimation of the impact of the different ligands and substrates on the activation barriers. A smaller subset of the latter was further subjected to an ab initio molecular dynamics (AIMD) simulation coupled to QTAIM analysis. The results show that the type of active catalyst plays an important role in the thermodinamic outcome of these reactions, with vanadium(V) tert-butylhydroperoxide adducts being responsible for the most exoenergetic reactions. On the other hand, the different ligands tested play only a limited role in modulating the thermodynamics and kinetics of these reactions. Moreover, no evidence was found to support a thermodynamic or kinetic preference for the epoxidation of an allylic alcohol over that of an unfunctionalized alkene. However, the results suggest that the reaction path is strongly influenced by the orientation of the substrate upon approximation to the active catalyst, confirming the well-known regioselectivity of the Sharpless epoxidation process.