Catalysis is one of the central themes in chemical science. The promotion of physicochemical and biological processes, as demonstrated by enzymes in living cells, is an extremely important subject to be elucidated in all branches of science. Therefore, the design and application of new and efficient types of promoters for organic transformations are of fundamental importance.

A range of important classes of organic reactions can be conducted with fluoride anions as a catalyst or nucleophilic reagent.[1] Although alkali metal fluorides are a readily available source of the fluoride ion, their applications are limited because of their low solubility in organic solvents. To increase the solubility of alkali metal salts, crown ethers are frequently employed to generate a “naked” fluoride ion (Figure 1 I).[2] This simple but innovative concept of crown ethers helped lay the foundation for the important fields of ion and molecular recognition and has therefore had a profound impact on science.[3] However, their application to chemical reactions is often hampered by the strong basicity of fluoride, which causes it to induce the formation of by-products. During the course of our efforts to develop general catalyst (promoter) systems employing alkali metal salts as nucleophilic sources, it was observed that bulky protic solvents (eg tBuOH) can be suitable to generate a “flexible” fluoride ion from CsF through controlled hydrogen bonding to reduce the basicity of the nucleophile, fluoride (Figure 1 II).[4] However, owing to