Synthesis
Cucurbiturils are aminals and synthesized from urea 1 and a dialdehyde (e.g., glyoxal 2) via a nucleophilic addition to give the intermediate glycoluril 3. This intermediate is condensed with formaldehyde to give hexamer cucurbituril above 110 °C. Ordinarily, multifunctional monomers such as 3 would undergo a step-growth polymerization that would give a distribution of products, but due to favorable strain and an abundance of hydrogen bonding, the hexamer is the only reaction product isolated after precipitation.
Decreasing the temperature of the reaction to between 75 and 90 °C can be used to access other sizes of cucurbiturils including CB, CB, CB, CB, and CB. CB is still the major product; the other ring sizes are formed in smaller yields. The isolation of sizes other than CB requires fractional crystallization and dissolution. CB, CB, CB, and CB 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.
Cucurbituril is particularly difficult to isolate. It was first discovered by Day and coworkers in 2002 as an inclusion complex containing CB by fractional crystallization of the cucurbituril reaction mixture. The CB•CB 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 CB within the CB cavity mimics the independent rotation of a flywheel within the frame of a gyroscope.
Isolation of pure CB could not be accomplished by direct separation methods since the compound has such a high affinity for CB. The strong binding affinity for the CB can be understood since it has a complementary size and shape to the cavity of the CB. Pure CB was isolated by Isaacs and coworkers in 2005 by introducing a more strongly binding melamine diamine guest that is capable of displacing the CB. The melamine diamine guest was then separated from the CB 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 CB has an unusually large cavity (870 Å3) that's free and capable of binding extraordinarily large guests including a cationic calixarene.
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