New Entries to Water‐Compatible Lewis Acids

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Abstract
Lewis acid catalysis has attracted much attention in organic synthesis as it often affords access to unique reactivity and selectivity under mild conditions. Although various kinds of Lewis acids have been developed and applied in industry, these Lewis acids must be generally used under strictly anhydrous conditions, as the presence of even a small amount of water interferes with the reactions due to preferential reaction of the Lewis acids with water rather than the substrates. In contrast to this, rare earth and other metal complexes have been found to be water‐compatible. Furthermore, Bi(OTf)3‐ and Ga(OTf)3‐basic ligand complexes have also been found to be stable in water, and have been used as water‐compatible Lewis acids. This application is particularly significant, as Bi(OTf)3 and Ga(OTf)3 themselves are unstable in the presence of water, but are stabilized by the basic ligands. This observation has led to the development of a new approach to Lewis acid catalysis in which Lewis acids that are generally unstable in the presence of water are rendered amenable to aqueous systems when combined with basic ligands. In particular, the use of chiral basic ligands leading to new types of water‐compatible chiral Lewis acids may enable a wide range of asymmetric catalysis in aqueous media.

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