Activation of Carbonyl and Related Compounds in Aqueous Media

14 Activation of Carbonyl and Related Compounds in Aqueous Media 

Lewis acid activation of C=0 and C=N groups has been of great interest in organic synthesis [1], While various kinds of Lewis acid-promoted reactions have been developed and many have been applied in industry, these reactions must be carried out under strictly anhydrous conditions. The presence of even a small amount of water stops the reaction, because most Lewis acids immediately react with water rather than the substrates and decompose or deactivate, and this has restricted the use of Lewis acids in organic synthesis. 

On the other hand, in the course of our investigations to develop new synthetic methods, we have found that rare earth metal triflates [Sc(OTf)3, Yb(OT03, ] [2] and some other metal salts can be used as water-stable Lewis acids for activation of C=0 and C=N groups in water-containing solvents. 

In this chapter, our research on use of the Lewis acid catalysts in carbon-carbon bond-forming reactions in aqueous solvents is overviewed. 

1 Lanthanide triflate-catalyzed aldol reactions in water-containing solvents 

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Although Lewis add-catalyzed carbon-carbon bond-forming reactions are now of great interest in organic synthesis, these reactions must be conducted under strictly anhydrous conditions, because most Lewis adds react immediately with water rather than the substrates, and are decomposed or deactivated. Sc(OTf)3, however, was found to be stable in water, and effectively activated carbonyl and related compounds as a Lewis add in water. Although it had already been found that lanthanide triflates (Ln(OTf)3 Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and yttrium triflate (Y(OTf)3) are stable in water and can act as Lewis acid catalysts in aqueous media [3], Sc(OTf)3 occasionally has even better properties even than Ln(OTf)3. Sc(OTf)3, moreover, worked well as a Lewis acid catalyst in several organic solvents, and chiral scandium triflates have also been developed. 

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