RuCl3/CeCl3/NaIO4: A new bimetallic oxidation system for the mild and efficient dihydroxylation of unreactive olefins

摘要

The catalytic dihydroxylation of olefins represents a unique synthetic tool for the generation of two C,O-bonds with defined relative configuration. Whereas OsO4 has been established as a very general dihydroxylation catalyst within the past 30 years, the less expensive and toxic isoelectronic RuO4 has found only limited use for this type of oxygen-transfer reaction. High catalyst loading and undesired side reactions were severe drawbacks in RuO4-catalyzed oxidations of C,C-double bonds. Recently, we were able to improve the RuO4-catalyzed dihydroxylation by addition of Bronsted acids to the reaction mixture. This protocol proved to be of general applicability, however, certain limitations were observed. To address these problematic functional groups a new Lewis acid accelerated oxidation was developed. The use of only 10 mol % of CeCl3 allowed a further decrease in the catalyst concentration down to 0.25 mol % while broadening the scope of the reaction. Silyl ethers and nitrogen containing functional groups are now tolerated in this optimized protocol. Furthermore, competing scission reactions are supressed in the presence of Lewis acid allowing longer reaction times and the successful oxidation of electron-deficient tetrasubstituted double bonds that cannot be oxidized using known dihydroxylation protocols.