Allylic alcohol epoxidation by methyltrioxorhenium: A density functional study on the mechanism and the role of hydrogen bonding

By locating all relevant transition structures with a hybrid density functional method, we explored the three most reasonable mechanisms for H₂O₂ epoxidation of propenol catalyzed by methyltrioxorhenium (MTO), namely: (i) coordination of propenol as lone pair donor to rhenium mono- and bis-peroxo complexes followed by intramolecular epoxidation, (ii) formation of a metal alcoholate, derived from addition of propenol to the Re complex with the formation of a metal - OR bond, followed by intramolecular epoxidation, (iii) intermolecular oxygen transfer assisted by hydrogen bonding where the rhenium complex acts as hydrogen bond acceptor and HOR as hydrogen bond donor. The computational results demonstrate that the last route is highly favored over the other two and, in particular, they provide the first unambiguous and compelling evidence that alcoholate - metal complexes, mechanism (ii), do not appreciably contribute to product formation. In keeping with experimental findings, theoretical data predict that the monoperoxo Re complex should be considerably less reactive than its bis(peroxo) counterpart and suggest that the hydrated form of the latter complex should be the actual active epoxidant species. All transition structures exhibit a distorted spiro-like structure, while the most stable ones feature hydrogen bonding to the attacking peroxo fragment with the olefinic OH group either in an "outside" (OC₁C₂C₃ ≈ 128°) or "inside" (OC₁C₂C₃ ≈ 14°) conformation. Previous qualitative models for transition structures of Re-catalyzed epoxidation of allylic alcohols are discussed in the light of our computational data.