Pillar[5]arene as a Co-Factor in Templating Rotaxane Formation

Abstract

After the manner in which coenzymes often participate in the binding of substrates in the active sites of enzymes, pillar[5]arene, a macrocycle containing five hydroquinone rings linked through their para positions by methylene bridges, modifies the binding properties of cucurbit[6]uril, such that the latter templates azide-alkyne cycloadditions that do not occur in the presence of only the cucurbit[6]uril, 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 pillar[5]arene and cucurbit[6]uril 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 [4]rotaxanes and one [5]rotaxane in >90% yields inside 2 h at 55 °C in acetonitrile. Since the hydroxyl groups on pillar[5]arene and the carbonyl groups on cucurbit[6]uril form hydrogen bonds readily, these two macrocycles work together in a cooperative fashion to the extent that the four conformational isomers of pillar[5]arene can be trapped on the dumbbell components of the [4]rotaxanes. In the case of the [5]rotaxane, it is possible to isolate a compound containing two pillar[5]arene rings with local C5 symmetries. In addition to fixing the stereochemistries of the pillar[5]arene 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.

Pillar-Shaped Macrocyclic Hosts Pillar[n]arenes: New Key Players for Supramolecular Chemistry

“…264 The per-hydroxylated pillar [n]arenes have hydrogen donor OH groups at both rims. CB [6] has CO moieties as hydrogen acceptors.…”

Cucurbit[n]urils‐related Multitopic Supramolecular Components: Design, Properties, and Perspectives

“…The importance of these lateral interactions can be demonstrated on an example of molecular capsules. [38] Considering two adjacent macrocyles within one complex, the lateral interactions can be either attractive or repulsive depending on the nature of the macrocycle portals. Indeed, capsules usually encage a large space; therefore, some guests are present to imprint the final shape and/or fill in the internal voids.…”

Pillar[n]arenes and Other Cavitands: Aspects of Complex Thermodynamics

“…The electrostatic potential maps show that the peripheral hydroxyl groups of β-CD and PA [5] have weak positive charges, which act as hydrogen bond donors. [20,28] In contrast, the carbonyl oxygen atoms of CB [6] are centers of high electron density. Most notably, two dissimilar cycloarenes, namely PA [5] and CA [5], exhibited discrete differences in shape and electrostatic potential profiles ( Figure 1).…”

“…. [28] (c) Chemical structure, X-ray crystal structures and calculated electron potential profiles (DFT calculations, B3LYP/6-31G) of CA [5]. [20a] As shown in Table 1, the dimensions of PA [5] and PA [6] are similar to those of α-CD and β-CD.…”