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Published on: February 7, 2017
Cucurbit[n]uril formation proceeds by step-growth cyclo-oligomerization.
Wei-Hao Huang1, Peter Y Zavalij, Lyle Isaacs
1Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA.
Journal of the American Chemical Society
|June 6, 2008
Summary
Glycoluril oligomers and nor-seco-cucurbit[n]urils form with limited formaldehyde. Acyclic pentamer and hexamer oligomers bind guests, expanding cavities for larger molecules, revealing a step-growth cyclo-oligomerization mechanism in cucurbit[n]uril formation.
Area of Science:
- Supramolecular Chemistry
- Organic Synthesis
- Crystal Engineering
Background:
- Cucurbit[n]urils (CB[n]) are macrocyclic compounds typically synthesized with a 1:2 ratio of glycoluril to formaldehyde.
- Incomplete condensation reactions yield mixtures of CB[n] analogs and linear or cyclic glycoluril oligomers.
Purpose of the Study:
- To isolate and characterize acyclic glycoluril oligomers formed under sub-stoichiometric formaldehyde conditions.
- To investigate the guest-binding properties and structural features of these oligomers.
- To elucidate the mechanism of cucurbit[n]uril formation.
Main Methods:
- Chromatographic purification of glycoluril oligomers (dimer to hexamer).
- Solution-state characterization (NMR, MS).
- X-ray crystallography for structural determination.
- Guest-binding studies.
- Product resubmission experiments.
Main Results:
- Isolation and structural characterization of C-shaped glycoluril oligomers (dimer-hexamer).
- Acyclic pentamer and hexamer oligomers exhibit guest-binding capabilities similar to CB[6], with an expanded cavity accommodating larger guests.
- Product resubmission revealed preferences for specific ring sizes during CB[n] formation.
- A comprehensive mechanistic scheme for CB[n] synthesis was proposed.
Conclusions:
- Glycoluril oligomers, including acyclic pentamers and hexamers, can be isolated and characterized.
- These oligomers possess unique guest-binding properties, demonstrating cavity size adaptability.
- The formation of CB[n] and related compounds proceeds via a step-growth cyclo-oligomerization pathway.
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