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Weinreb Amide, Ketone and Amine as Potential and Competitive Secondary Molecular Stations for Dibenzo-24-Crown-8 in [2]Rotaxane Molecular Shuttles.

This paper reports the synthesis and study of new pH-sensitive DB24C8-based [2]rotaxane molecular shuttles that contain within their axle the following four potential sites of interaction for the DB24C8: ammonium, amine, Weinreb amide and ketone. At the protonated state, the DB24C8 lied around the best ammonium site. After either deprotonation or deprotonation-then-carbamoylation of the ammonium, different localizations of the DB24C8 were noticed, depending on both the number and nature of the secondary stations and steric restriction. Unexpectedly, the results indicated that Weinreb amide was not a proper secondary molecular station for the DB24C8. Nevertheless, via its methoxy side-chain, it slowed down the shuttling of the macrocycle along the threaded axle, therefore partitioning the [2]rotaxane into two translational isomers on the NMR time scale. Regarding the ketone, it was successfully utilized as a secondary molecular station and its weak affinity for the DB24C8 was similar to that of a secondary amine.