Multiple bonds between transition metals and main-group elements part 168. ¹ methyltrioxorhenium/Lewisbase catalysts in olefin epoxidation

Methyltrioxorhenium(VII) (MTO) forms with H₂O₂ and Lewis bases, e.g., pyridine, a powerful, highly selective oxidant for olefins, both under mono-phase (alcohols as solvents) and biphasic conditions (CH₂Cl₂/H₂O). Large excess of pyridine is required to maintain the catalytic activity due to rapid oxidation of pyridine to pyridine N-oxide, which reaction is catalyzed by MTO, too. The selectivity to epoxides, however, remains high throughout the course of the catalysis, since pyridine N-oxide also forms a highly selective but much less active co-catalyst with MTO/H₂O₂. Under two-phase conditions, the presence of the aqueous phase ensures high epoxidation activities due to rapid, efficient removal of the pyridine N-oxide.