Stiff, and Sticky in the Right Places: The Dramatic Influence of Preorganizing Guest Binding Sites on the Hydrogen Bond-Directed Assembly of Rotaxanes

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Abstract
Structural rigidity and the preorganization of thread binding sites are shown to have a major influence on template efficiency in the synthesis of hydrogen bond-assembled rotaxanes. Preorganization is so effective, in fact, that with good hydrogen bond acceptors (amides) a “world record” yield of 97% for a [2]rotaxane is obtained. The truly remarkable feature of this efficient template, however, is that it allows even poor hydrogen bond acceptors (esters) to be used to prepare hydrogen bond-assembled rotaxanes, despite the presence of competing hydrogen bonding groups (anions) which bind the key intermediates at least 10000× more strongly than single, unorganized, ester groups! The structures of the rotaxanes are established unambiguously in solution by 1H NMR spectroscopy and in the solid state by X-ray crystallography. As a series they provide unique experimental information regarding the nature of amide−ester hydrogen bonding interactions; in particular they suggest that in CDCl3, amide−ester NH···OC hydrogen bonds are ∼1 kcal mol-1 weaker than the corresponding amide−amide interactions.

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