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 functi onal method, we explored the three most reasonable mechanisms for H2O2 epox idation of propenol catalyzed by methyltrioxorhenium (MTO), namely: (i) coo rdination of propenol as lone pair donor to rhenium mono- and bis-peroxo co mplexes followed by intramolecular epoxidation, (ii) formation of a metal a lcoholate, derived from addition of propenol to the Re complex with the for mation of a metal-OR bond, followed by intramolecular epoxidation, (iii) in termolecular 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 ov er the other two and, in particular, they provide the first unambiguous and compelling evidence that alcoholate-metal complexes, mechanism (ii), do no t appreciably contribute to product formation. In keeping with experimental findings, theoretical data predict that the monoperoxo Re complex should b e considerably less reactive than its bis(peroxo) counterpart and suggest t hat the hydrated form of the latter complex should be the actual active epo xidant species. All transition structures exhibit a distorted spiro-like st ructure, while the most stable ones feature hydrogen bonding to the attacki ng peroxo fragment with the olefinic OH group either in an "outside" (OC1C2 C3 approximate to 128 degrees) or "inside" (OC1C2C3 approximate to 14 degre es) conformation. Previous qualitative models for transition structures of Re-catalyzed epoxidation of allylic alcohols are discussed in the light of our computational data.