The World of Cucurbiturils — From Peculiarity to Commodity

Israel Journal of ChemistryVolume 51, Issue 5-6 p. 492-494 Guest EditorialFree Access The World of Cucurbiturils — From Peculiarity to Commodity First published: 24 May 2011 https://doi.org/10.1002/ijch.201100052Citations: 28AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat More than a century ago, while Robert Behrend (Figure 1) and his coworkers were investigating acid-catalyzed condensation reactions between glycoluril and formaldehyde,1 a mysterious white material was the result. The German chemist had unknowingly synthesized the parent cucurbituril, yet recognized that this as-yet-unnamed material could be dissolved in water in the presence of either protons or alkali ions, and easily formed complexes with metal salts and organic dyes.2 This peculiarity has now matured into an internationally well-recognized area of supramolecular chemistry that is intensively being pursued. While the first synthesis of cucurbituril is, unarguably but in retrospect, Behrend’s biggest achievement, he was known during his lifetime for the first synthesis of uric acid,3 for carrying out the first potentiometric titration,4 and even for an organic name reaction, the Behrend rearrangement.5 Figure 1Open in figure viewerPowerPoint Potrait picture of the Geheime Regierungsrat Prof. Dr. phil., Dr.-Ing. h. c. Anton Friedrich Robert Behrend (1856–1926). For a biography in German see ref. 6; for a partial translation into English see ref. 7. After 75 years of dormancy, investigations by Mock and coworkers in the 1980 s led to the structural elucidation of the white solid as being a discrete macrocyclic pumpkin-shaped hexamer8 as opposed to an open-chain ladder polymer. They coined the name “cucurbituril” in reflection of the botanical name for pumpkin. Recognition of the structure immediately led to comprehensive investigations of its host–guest inclusion behavior, which also revealed a particularly high affinity towards organic ammonium cations.9 At present, unique applications of cucurbiturils in diverse areas are rapidly unfolding that will soon render these macrocycles a commodity. Homologues of different sizes, ranging from cucurbit[5]uril to cucurbit[10]uril (Figure 2), as well as equatorially alkylated and hydroxylated derivatives, were isolated and characterized,10–12 showing marked differences in water solubility, guest affinity as well as guest size selectivity, and binding stoichiometry. In particular, the possibility to study these fascinating macrocycles in neutral aqueous solution has attracted much attention since applications in many areas, including biology, are now in sight. Moreover, it has become possible to contrast the inclusion properties and binding strengths of this new class of synthetic host molecules with established macrocyclic receptors, such as cyclodextrins and water-soluble calixarenes. In fact, cucurbiturils are competitive in several respects, such as their low toxicity,13, 14 and far superior in many others, particularly their high chemical stability, large complexation-induced pKa shifts, as well as their tight, selective binding (see articles in this special issue). Figure 2Open in figure viewerPowerPoint Chemical structures of isolated cucurbituril homologues. Initial investigations on cucurbit[6]uril focused on its binding preferences and the determination of the binding constants of its host–guest complexes.15–17 These studies have continued to attract attention also for the larger homologues, driven, among other things, by the exceptional kinetics and thermodynamics of binding. It was Mock and coworkers who also laid the foundation for many lines of applications of cucurbiturils, which are currently intensively pursued. These include pH-responsive fluorescent switches18 and their use in catalysis.19–21 Examples of rotaxane architectures and polymeric assemblies followed.22–27 Very recently, applications in biology,28, 29 in drug binding and delivery,30, 31 for sensors,32–34 and on the surfaces of materials35, 36 have emerged. The mechanistic aspects of their binding have retained their fascination,37–39 while the search for new derivatives and homologues remains a great challenge.40, 41 The most recent advances in the world of cucurbiturils are reviewed in this special issue, which has been compiled in the context of ICCB 2011, the 2nd International Conference on Cucurbiturils. The initial international meeting on cucurbiturils was held in the form of a workshop at the University of Maryland, USA, in 2007, organized by L. Isaacs and A. E. Kaifer. Following lively and very open discussions on the progress in the field, with a large participation from young scientists, it was decided to hold biannual meetings on cucurbiturils. The workshop was followed in 2009 by the 1st International Conference on Cucurbiturils (ICCB 2009), which took place in Pohang, Republic of Korea, and was organized by K. Kim. ICCB 2009 showed that the field was gaining much momentum, including in particular advanced applications in biological disciplines and nanoscience. ICCB 2011, held at the University of Cambridge, UK, and organized by the two guest editors of this special issue, highlights the full spectrum of mechanistic insights, synthetic possibilities, and applications that these remarkable host molecules have enabled. Our most recent research has revealed that Behrend was actually of Jewish parentage,42 a hidden fact that is not apparent from any of his professional biographies.6 He would certainly have taken great interest in this (his) special issue on cucurbiturils in the Israel Journal of Chemistry. 1 1 Werner M. Nau Oren A. Scherman Guest Editor Guest Editor References 1R. Behrend, E. Meyer, F. Rusche, Justus Liebigs Ann. Chem. 1905, 339, 1– 37. Wiley Online LibraryWeb of Science®Google Scholar 2E. Meyer, 1904, Inaugural-Dissertation, Heidelberg, Germany. Google Scholar 3R. Behrend, O. 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