Multiple bonds between transition metals and main-group elements - Part 168. 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. (C) 1997 Elsevier Science S.A.