Urea-based receptors

ION PAIR COMPLEXATION BY AMIDE- AND UREA-BASED RECEPTORS... 

Fig. 6.7 Host-guest properties of heteroditopic tris-amide and tris-urea-based receptors XCU and XCA toward organic contact ion-pairs. H-bonds are indicated by dashed lines. Inset H NMR spectra (CDCI3,300 MHz, 298 K) of (a) XCA (b) after addition of EtNHa Pic , TBA- CP, TBA N03 and TBA AcO (c) after addition of TBA F ...

Amendola V, Fabbrizzi L, Mosca L. Anion recognition by hydrogen bonding urea-based receptors. Chem. Soc. Rev. 2010 39 3889-3915. 

Hughes, M. P., Smith, B. D., Enhanced carboxylate binding using urea and amide-based receptors with internal Lewis acid coordination A cooperative polarization effect. J. Org. Chem. 1997, 62, 4492—4499. 

Appropriately strapped calixarenes have also been shown to complex anions and cations simultaneously. Such a system is the ditopic, calixarene-based receptor 11 (Scheme 9.4) reported by Tumcharern and coworkers [51[. Receptor 11, in which the strap incorporates two different binding motifs (urea and amide), showed selectivity for the tetrahedral phenylphosphinate anion (PPhH02) over simple Y-shaped anions, such as acetate. After initial screening studies, quantitative analyses were carried out using NMR spectroscopic methods. Specifically, titrations were carried out in CD3CN using the TBA salts of three promising anions, namely acetate, phenylphosphinate, and diphenylphosphate. Standard analyses of the... 

Other groups have also exploited these simple motifs for the recognition of carboxylates (Fig. 14). Wilcox used ureas such as 18 to complex various anions including carboxylates [55]. Similar receptors have been utilized by Curran to alter the stereochemistry of the alkylation of a-sulfinyl radicals [56] as well as accelerate a Claisen rearrangement [57]. Moran also created a urea-based... 

Abstract This review article provides a broad overview to the area of anion coordination by synthetic organic receptors and includes examples of different functional groups used to bind anions. The first section examines neutral anion receptors containing amide-, sulfonamide-, urea- and thiourea-based receptors. Then aromatics such as pyrrole, car-bazole and indole are discussed before concluding the discussion of neutral systems with examples of hydroxy OH donors. A brief overview of charged systems is also provided. 

Prohens and co-workers have synthesized compounds 8a and 8b, simple squaramido-based receptors and investigated their ability to coordinate car-boxylate anion in competitive solvents [16]. The amide NH groups of the squaramide form a more open cleft (similar to ureas) than the isophthala-mides. Receptors 8a and 8b therefore adopt a more suitable geometry for the coordination of bidentate anions, such as carboxylate anions, through two approximately linear hydrogen bonds. Proton NMR titration experiments revealed association constants of 217 M and 1980 for the binding of acetate by 8a and 8b respectively in DMSO-de at 295 K. 

---