Stable Lewis Acids in Aqueous Media [Synthetic Reagents]

　The Lewis acid-catalyzed C-C bond forming reactions have been of great interest in organic synthesis. While various kinds of Lewis acid-promoted reactions have been developed, these reactions must be carried out under strict anhydrous conditions. The presence of even a small amount of water stops the reaction, because most Lewis acid immediately react with water rather than the substrates and decompose or deactivate.
　Recently, Kobayashi and co-worker have found that certain metal triflates are stable Lewis acids in aqueous media. The reaction of silyl enol ethers with commercial formaldehyde solution smoothly proceeds in aqueous media by the certain metal triflates, especially Yb(OTf)₃ to give the corresponding aldol adducts in high yields. Moreover, the triflates can also be used in organic solvents in many reactions. In all cases, the reaction is completed by only a catalytic amount of the triflate, and the catalyst can be easily recovered after the reaction was completed and can be reused. On the other hand, in asymmetric synthesis, chiral Lewis acid which was prepared by Sc(OTf)₃, chiral binaphthol and an amine affords chiral Diels-Alder adduct in the reactions of dienophiles and dienes. Then, the certain metal triflates, especially Yb(OTf)₃, Sc(OTf)₃, etc. are expected to be useful as new types of Lewis acid catalysts in many organic reactions.
　From the view points of green chemistry, these are also remarkably effective Lewis Acids as non-toxic and nonflammable water can be used as reaction solvent.