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In host–guest chemistry, cucurbiturils are macrocycle made of glycoluril () linked by (). The oxygen atoms are located along the edges of the band and are tilted inwards, forming a partly enclosed cavity (cavitand). The name is derived from the resemblance of this molecule with a pumpkin of the family of Cucurbitaceae.
Cucurbiturils are commonly written as cucurbit nuril, where n is the number of glycoluril units. Two common abbreviations are CB n , or simply CBn.
These compounds are particularly interesting to chemists because they are suitable hosts for an array of neutral and cationic species. The binding mode is thought to occur through hydrophobic interactions, and, in the case of cationic guests, through cation-dipole interactions as well. The dimensions of cucurbiturils are generally on the ~10 Angstrom size scale. For instance, the cavity of cucurbit6uril has a height ~9.1 Å, an outer diameter ~5.8 Å, and an inner diameter ~3.9 Å. Review: The Cucurbitnuril Family Jason Lagona, Pritam Mukhopadhyay, Sriparna Chakrabarti, Lyle Isaacs Angewandte Chemie International Edition Volume 44, Issue 31, Pages 4844 - 4870 2005 Abstract
Cucurbiturils were first synthesized in 1905 by Robert Behrend, by condensing glycoluril with formaldehyde, Ueber Condensationsproducte aus Glycoluril und Formaldehyd, Robert Behrend, Eberhard Meyer, Franz Rusche, Justus Liebig's Annalen der Chemie 1905, 339, 1–37. but their structure was not elucidated until 1981. Cucurbituril W. A. Freeman, W. L. Mock, and N.-Y. Shih J. Am. Chem. Soc., 1981, 103, 7367. The field expanded as CB5, CB7, and CB8 were discovered and isolated by Kim Kimoon in the year 2000. To date cucurbiturils composed of 5, 6, 7, 8, 10, and 14 repeat units have all been isolated,Cucurbituril Homologues and Derivatives: New Opportunities in Supramolecular Chemistry Acc. Chem. Res., 36 (8), 621 -630, 2003. Cheng, Xiao-Jie, et al. "Twisted Cucurbit14uril." Angewandte Chemie International Edition 52.28 (2013): 7252–7255. Web. which have internal cavity volumes of 82, 164, 279, 479, and 870 Å3 respectively. A cucurbituril composed of 9 repeat units has yet to be isolated (as of 2009). Other common molecular capsules that share a similar molecular shape with cucurbiturils include , , and .
Decreasing the temperature of the reaction to between 75 and 90 °C can be used to access other sizes of cucurbiturils including CB5, CB7, CB8, and CB10. CB6 is still the major product; the other ring sizes are formed in smaller yields. The isolation of sizes other than CB6 requires fractional crystallization and dissolution. CB5, CB6, CB7, and CB8 are all currently commercially available. The larger sizes are a particularly active area of research since they can bind larger and more interesting guest molecules, thus expanding their potential applications.
Cucurbit10uril is particularly difficult to isolate. It was first discovered by Day and coworkers in 2002 as an inclusion complex containing CB5 by fractional crystallization of the cucurbituril reaction mixture. A Cucurbituril-Based Gyroscane: A New Supramolecular Form AnthonyI. Day, Rodney J. Blanch, Alan P. Arnold, Susan Lorenzo, Gareth R. Lewis, and Ian Dance Angew. Chem. Int. Ed.; 2002; 41(2) pp 275-277. The CB10·CB5 was unambiguously identified by single crystal X-ray structural analysis that revealed the complex resembled a molecular gyroscope. In this case, the free rotation of the CB5 within the CB10 cavity mimics the independent rotation of a flywheel within the frame of a gyroscope.
Isolation of pure CB10 could not be accomplished by direct separation methods since the compound has such a high affinity for CB5. The strong binding affinity for the CB5 can be understood since it has a complementary size and shape to the cavity of the CB10. Pure CB10 was isolated by Isaacs and coworkers in 2005 by introducing a more strongly binding melamine diamine guest that is capable of displacing the CB5. Cucurbit10uril Simin Liu, Peter Y. Zavalij, and Lyle Isaacs J. Am. Chem. Soc.; 2005; 127(48) pp 16798 - 16799; (Communication) The melamine diamine guest was then separated from the CB10 by reaction with acetic anhydride that converted the positively charged amine groups to neutrally charged amides. Cucurbiturils strongly bind cationic guests, but by removing the positive charge from the melamine diamine guest reduces the association constant to the point it can be removed by washing with methanol, DMSO, and water. The CB10 has an unusually large cavity (870 Å3) that's free and capable of binding extraordinarily large guests including a cationic [calixarene|calix[4arene]].
Host–guest interactions also significantly influence solubility behavior of cucurbiturils. Cucurbit6uril dissolves poorly in just about any solvent but solubility is greatly improved in a solution of potassium hydroxide or in an solution. The cavitand forms a positively charged inclusion compound with a potassium ion or a hydronium ion respectively which have much greater solubility than the uncomplexed neutral molecule.
CB10 is large enough to hold other molecular hosts such as a calixarene molecule. With a calixarene guest different chemical conformations (cone, 1,2-alternate, 1,3-alternate) are in rapid equilibrium. Allosteric control is provided when an adamantane molecule forces a cone conformation with a calixarene–adamantane inclusion complex within a CB10 molecule.
In another rotaxane system with a CB7 wheel, the axle is a 4,4'-bipyridinium or viologen subunit with two carboxylic acid terminated aliphatic N-substituents at both ends. In water at concentration higher than 0.5 mM complexation is quantitative without need of stoppers. At pH = 2 the carboxylic are protonated and the wheel shuttles back and forth between them as evidenced by the presence of just two aromatic viologen protons in the proton NMR spectrum. At pH = 9 the wheel is locked around the viologen center. More recently, rotaxaneV. Ramalingam and A. R. Urbach, Org. Lett., 2011, 13, 4898 with a CB8 wheel was synthesized. This rotaxane can bind neutral guest molecules.
A cucurbituril cut in half along the equator is called a hemicucurbituril.
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