Compact Rotaxane Superbases

Challenges for the development of efficacious new superbases include their ease of synthesis, chemical stability and high basicity, while minimizing nucleophilicity is important for reducing unwanted side reactions. Here we introduce a new family of organic superbases, compact amine-crown ether rotaxanes, which show desirable characteristics in all these respects. Metal-free active template synthesis provides access to a range of rotaxanes with as little as three atoms between the stoppering groups, locking the location of a small crown ether (21C7 and 24C8 derivatives) over the amine group of the axle. The forced proximity of the interlocked protophilic components results in pKaH+ values as high as 32.2 in acetonitrile, which is up to 20 pKaH+ units greater than the pKaH+ values of the non-interlocked components, and brings the conjugate base rotaxanes into the basicity realm of phosphazene superbases. The rotaxane superbases are generally chemically stable and, in a model reaction for superbases, eliminate HBr from a primary alkyl bromide with complete selectivity for deprotonation over alkylation. Their modest size, ease of synthesis, high basicity, low nucleophilicity and, in the best cases, rapid substrate deprotonation kinetics and excellent hydrolytic stability, make compact amine-crown ether rotaxane superbases intriguing candidates for potential applications in synthesis, supramolecular and materials chemistry.