Recognition-directed assembly of salt-binding [2]rotaxanes

This paper describes the synthesis of four isomeric [2]rotaxanes and includes the first example of a salt binding [2]rotaxane with the following key observations. In polar solvents the [2]rotaxane undergoes a relatively slow co-conformational exchange that produces broad signals in the 1H NMR spectrum. Upon complexation with K+, the NMR signals sharpen apparently because the rotaxane adopts a single co-conformation. In contrast, the binding of Cl− has no effect on the rotaxaneʹs dynamic behavior. Although the binding of K+ induces a preorganization of the [2]rotaxane it does not change the rotaxane/Cl− association constant. The paper finishes with a discussion of the future directions of this work.