Functionalized Cucurbiturils and Their Applications
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View Article Online / Journal Homepage / Table of Contents for this issue TUTORIAL REVIEW www.rsc.org/csr | Chemical Society Reviews Functionalized cucurbiturils and their applications Kimoon Kim,* Narayanan Selvapalam, Young Ho Ko, Kyeng Min Park, Dongwoo Kim and Jeeyeon Kim Received 7th September 2006 First published as an Advance Article on the web 7th November 2006 DOI: 10.1039/b603088m Cucurbit[n]uril (CB[n], n = 5–10), a new family of molecular hosts comprising n glycoluril units, have gained much attention in the new millennium for their exceptional molecular recognition ability. The CB homologues have brought dynamism to CB chemistry, as witnessed by the heightened interest in the field for the last several years. Compared to the chemistry of cyclodextrins and calixarenes, however, that of CB[n] has developed slowly until recently, which may be attributed mainly to their poor solubility in common solvents, and inability to functionalize these molecules. The direct functionalization method of CB[n] propelled CB chemistry to a new height as this new method not only solved the solubility problem but also opened up the gateway to the generation of tailor-made CB[n] derivatives. The functionalization of CB[n] led us to investigate numerous applications including artificial ion channels, vesicles, stationary phases in chromatography, ISEs, polymers, nanomaterials, and many others. This tutorial review describes the recent advances and challenges in the functionalization of CBs along with the applications of functionalized CBs. 1. Introduction host–guest behavior of CB[6] were conducted by Mock and others.3–5 While the hydrophobic interior provides a potential Cucurbit[n]uril (CB[n]) is a family of macrocyclic compounds inclusion site for nonpolar molecules, the polar ureido comprising n glycoluril units, self-assembled from an acid- carbonyl groups at the portals allow CB[6] to bind ions and catalyzed condensation reaction of glycoluril and formalde- molecules through charge–dipole and hydrogen bonding hyde. Although the synthesis of the parent compound, CB[6] interactions. The unique structure and recognition properties 1 was first reported by Behrend et al. in 1905, its chemical make CB[6] attractive not only as a synthetic receptor but also nature and structure had been unknown until 1981 when full as a building block for the construction of supramolecular 2 characterization was reported by Mock and co-workers. The architectures. A wide variety of supramolecular species such as Downloaded by Dalhousie University on 03 December 2012 pumpkin-shaped molecule has a hydrophobic cavity and rotaxanes, catenanes, and molecular machines incorporating two identical carbonyl-laced portals. Extensive studies on the CB[6] have been reported.6 Other applications of CB[6] such as Published on 07 November 2006 http://pubs.rsc.org | doi:10.1039/B603088M in catalysis7 have also been demonstrated. National Creative Research Initiative Center for Smart Supramolecules In 2000, we reported the discovery of other members of the and Department of Chemistry, Pohang University of Science and CB family, CB[n](n = 5–11) and successful isolation of CB[n] Technology, San 31 Hyojadong, Pohang 790-784, Republic of Korea. 8 E-mail: [email protected]; Fax: +82-54-279-8129; (n = 5, 7 and 8), which has broadened the scope of CB Tel: +82-54-279-2113 chemistry enormously. Within a few years, their host–guest Kimoon Kim studied chemistry Narayanan Selvapalam, born in at Seoul National University India, received his MS (1990) (BS, 1976), Korea Advanced and PhD (1997) degrees from Institute of Science and Mysore University and IIT Technology (KAIST) (MS, Delhi, respectively. During 1978) and Stanford University 1997–1999, he worked at the (PhD, 1986). After postdoc- R&D center of the Cancer toral work at Northwestern Institute (WIA), Chennai. University, he joined Pohang After working at KAIST as a University of Science and postdoctoral fellow for a year Technology where he is now a he joined CSS in 2001. Since Professor of Chemistry. He is 2005, he has been a research also director of the Center for assistant professor in the group Smart Supramolecules (CSS) of Professor Kimoon Kim, Kimoon Kim supported by the Creative Narayanan Selvapalam and actively involved in the Research Initiative Program cucurbituril synthesis. of the Korean Ministry of Science and Technology. His current research focuses on developing novel supramolecular systems toward functional materials and devices. This journal is ß The Royal Society of Chemistry 2007 Chem. Soc. Rev., 2007, 36, 267–279 | 267 View Article Online chemistry as well as novel supramolecular assemblies built with formaldehyde produces a complicated mixture of with CB[n] has been extensively studied by us as well as unsubstituted and substituted CBs, separation of desired others.9,10 For example, redox control of guest binding, products from which is practically impossible or laborious. formation of a macrocycle within a macrocycle, inclusion of However, our recent discovery of a direct functionalization two identical or different guest molecules, encapsulation of method of CB[n]13 allowed us to synthesize a wide variety of drugs, promotion of reactions with a large rate acceleration CB derivatives easily and to study many applications. and regio- and stereochemical control, and stabilization of Ion channels, vesicles, polymers, ion selective electrodes unstable species by encapsulation have been observed. In incorporating CB[n], and CB-immobilized solid surfaces and particular, our discovery of formation of a stable charge- silica gel have been reported and numerous other applications transfer (CT) complex inside CB[8]11 led us and others to are being explored. As such, the functionalization has paved build a number of supramolecular assemblies based on this the way to applications of CBs, in the same manner as the chemistry, including polyrotaxanes, molecular necklaces, upsurge in applications of CDs after they were functionalized. dendrimers, vesicles, and molecular machines, and to recognize This review is the documentation of recent developments amino acids and small peptides carrying aromatic residues.12 in CB chemistry including the synthesis and applications of The rich supramolecular chemistry made the host family functionalized CB[n] primarily based on the results obtained join other celebrity host families such as crown ethers, from our own laboratory. cyclodextrins, and calixarenes, but their practical applications were still limited mainly because of their poor solubility in 2. Characteristic features of CB[n] and recent common solvents, and difficulty in introducing functional developments in CB[n] chemistry groups on their surfaces. In particular, unlike other host families, functionalization of these molecules was a daunting Since the chemistry of CB[n] has been recently reviewed by us9 task. For example, introduction of substituents at the ‘equator’ and more comprehensively by Isaacs et al.,10 in this section we of CBs by condensation of substituted glycoluril with form- will just briefly describe some characteristic features including aldehyde was achieved with only limited success. Reaction of a the host–guest chemistry of the CB family and the latest mixture of substituted glycoluril and unsubstituted glycoluril developments in the chemistry of unfunctionalized CB[n]. Young Ho Ko graduated from Kyeng Min Park was born in Konkuk University with a BS Daejeon, Korea in 1976. He degree in 1990, and received his studied industrial chemistry at MS and PhD degrees from Hongik University and received Pohang University of Science a BS degree in 2003. He is and Technology in NMR spec- currently a PhD student work- Downloaded by Dalhousie University on 03 December 2012 troscopy under Professor Hee ing on biological applications Published on 07 November 2006 http://pubs.rsc.org | doi:10.1039/B603088M Cheon Lee in 1992 and 1998, of cucurbituril derivatives at respectively. After three years Pohang University of Science postdoctoral work at the and Technology under the Center for Biofunctional supervision of Professor Molecules, he joined the group Kimoon Kim. of Professor Kim in 2001. His current research interests Young Ho Ko encompass NMR spectroscopy Kyeng Min Park of supramolecular systems and host–guest chemistry. Dongwoo Kim received a BS Jeeyeon Kim was born in degree in chemistry from Seoul Yongin, Korea in 1980. She National University in 2000, earned a BS degree in both and MS and PhD degrees in applied chemistry and bio-engi- supramolecular chemistry under neering from Ajou University Professor Kimoon Kim from in 2004. She is currently a Pohang University of Science PhD student under Professor and Technology in 2002 and Kimoon Kim at Pohang Univer- 2006, respectively. His doctoral sity of Science and Technology. thesis focused on the fabrication Her research focuses on bio- of supramolecular polymers on logical applications of function- surfaces and the direct synthesis alized cucurbiturils. of polymer nanocapsules and two-dimensional polymers. He Dongwoo Kim currently is a postdoctoral Jeeyeon Kim research fellow at CSS. 268 | Chem. Soc. Rev., 2007, 36, 267–279 This journal is ß The Royal Society of Chemistry 2007 View Article Online Scheme 1 2.1. Synthesis, structures, physical properties of CB[n] Reaction of glycoluril with formaldehyde in mineral acids, such as 9 M H2SO4 or conc. HCl, at 75–90 uC, yields a mixture of the CB[n] family (Scheme 1). The lower reaction tempera- ture compared to that employed in conventional CB[6] synthesis (.110 uC) is the key to the formation of significant amounts of CB homologues in addition to CB[6]. CB[n](n =5, 7 and 8) can be separated in pure form using fractional 8 14 crystallization and dissolution, or chromatography. From Fig. 1 X-ray crystal structures of CB[7] and iCB[7]. the reaction mixture, Day and co-workers also isolated CB[5]@CB[10], in which CB[5] is trapped in the cavity of y12.6 A˚ , respectively, and its cavity volume is almost two CB[10].15 Recently, Isaacs and co-workers successfully isolated times that of CB[8].