Stiff, and sticky in the right places: The dramatic influence of preorganizing guest binding sites on the hydrogen bond-directed assembly of rotaxanes

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 accepters (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 accepters testers) 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 x more strongly than single. unorganized, ester groups! The structures of the rotaxanes are established unambiguously in solution by H-1 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 . . .O=C hydrogen bonds are similar to1 kcal mol(-1) weaker than the corresponding amide-amide interactions.