In contrast to the high yield formation of cucurbit[n]uril (CB[n]) from a 1:2 ratio of glycoluril to formaldehyde, the condensation of glycoluril with less than 2 equiv of formaldehyde delivers a reaction mixture that contains glycoluril oligomers (2-6) and CB[n] compounds that lack one or more methylene bridges known as nor-seco-cucurbit[n]urils (ns-CB[n]). In this paper we report the chromatographic purification of C-shaped glycoluril oligomers (dimer-hexamer), their characterization in solution, and their X-ray crystal structures. Quite interestingly, despite being acyclic glycoluril pentamer 5 and hexamer 6 retain the ability to bind to guests typical of CB[6] but are also able to expand their cavity to accommodate larger guests like cationic adamantane derivatives. We performed product resubmission experiments with glycoluril oligomers 2-6 and found preferences for the formation of specific ring sizes during CB[n] formation. A comprehensive mechanistic scheme is proposed that accounts for the observed formation of 2-6 and ns-CB[n]. Overall, the experiments establish that a step-growth cyclo-oligomerization process operates during CB[n] formation.

The condensation of glycoluril and formaldehyde in concentrated HCl at 50 degrees C delivers nor-seco-cucurbit[10]uril (ns-CB [10]). 1H NMR and X-ray crystallographic evidence indicates that the two cavities of ns-CB[10] accommodate guests that are typically bound within CB[6] or CB[7]. Several interesting types of selectivity are possible within these ternary complexes-top/center isomerism, diastereoselective complexation of chiral (but racemic) guest pairs, and guest size controlled homotropic allosterism.

No abstract

Strongly coupled and porous MoS2–CNT with leaves-and-branch structure shows a remarkably improved electrocatalytic activity towards hydrogen evolution reaction.

No abstract