After the manner in which coenzymes often participate in the binding of substrates in the active sites of enzymes, pillararene, a macrocycle containing five hydroquinone rings linked through their para positions by methylene bridges, modifies the binding properties of cucurbituril, such that the latter templates azide-alkyne cycloadditions that do not occur in the presence of only the cucurbituril, a macrocycle composed of six glycoluril residues doubly linked through their nitrogen atoms to each other by methylene groups. Here, we describe how a combination of pillararene and cucurbituril interacts cooperatively with bipyridinium dications substituted on their nitrogen atoms with 2-azidoethyl- to 5-azidopentyl moieties to afford, as a result of orthogonal templation, two rotaxanes and one rotaxane in >90% yields inside 2 h at 55 C in acetonitrile. Since the hydroxyl groups on pillararene and the carbonyl groups on cucurbituril form hydrogen bonds readily, these two macrocycles work together in a cooperative fashion to the extent that the four conformational isomers of pillararene can be trapped on the dumbbell components of the rotaxanes. In the case of the rotaxane, it is possible to isolate a compound containing two pillararene rings with local C5 symmetries. In addition to fixing the stereochemistries of the pillararene rings, the regiochemistries associated with the 1,3-dipolar cycloadditions have been extended in their constitutional scope. Under mild conditions, orthogonal recognition motifs have been shown to lead to templation with positive cooperativity that is fast and all but quantitative, as well as being green and efficient.
ASJC Scopus subject areas
- Colloid and Surface Chemistry