Urea-Based Anion Recognition

Hamilton and co-workers reported that N,N -dimethylurea 23 could form 1 1 H-bonded complex with CH3COO in DMSO-dg = 45 M ), while ditopic receptor 24, in which two urea subunits are linked through a 1,4-xylyl spacer, formed a 1 1 complex with glutarate ion in DMSO-de = 6.4 x 10 M ) [62]. 

Gale and co-workers reported the synthesis of monourea 26a and thiourea 26b with attached indole groups [64]. The carbonate complex of 26a was isolated as Et4N salt with the anion surrounded by two ligands (Fig. 5.19a). The N- 0 H-bond distances ranged from 2.739 to 2.938 A. The bicarbonate analogue with thiourea 26b 

Gunnlaugsson and co-workers reported receptors 27a-c, which differed only in the position of their amide functionality relative to the urea moiety. This modification was shown to have a significant effect on the anion recognition ability and the binding stoichiometry [65]. 

Barboiu and co-workers reported heteroditopic ureido crown ether receptor 28. The coordination modes differed, depending on the anion, as illustrated by two complexes, one with NaCl and the other with NaNOa. In the NaNOa complex, the crown ether portion, containing the sodium ion, lies adjacent to a neighboring urea podand chain that binds the nitrate by two long H-bonds (N 0 = 3.063 and 3.073 A) (Fig. 5.20a). In the NaCl complex, the appended urea chains use four donor hydrogen atoms to hold the chloride ion, with N C1 distances ranging from 3.333 to 3.517 A (Fig. 5.20b) [66]. 

Loeb and co-workers reported the preparation of complex [Pt°(34)4] from 34. Due to the free rotation around the Pt -N bond, the complex could assume four basic up and down conformations of the butyl urea substituents. The crystal structure of [Pt (34)4] S04 showed that the receptor was arranged in a cone conformation, with all eight N-H groups oriented toward the cavity including the sulfate anion (Fig. 5.24a), with 10 M . In chloride complex [Pt (34)4] (Cl )2, the receptor adopted a 1,2-altemate conformation (Fig. 5.24b), while the two chloride ions were located above and below the Pt° center, with = 1.2 x 10 M and A a2 = 2.2 X 10 M respectively [72]. 

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Amide- and Urea-Based Anion Receptors, p. 51 Amino Acids Applications, p. 42 Chiral Guest Recognition, p. 236 Deoxycholic, Cholic, and Apocholic Acids, p. 441 Fluorescence Sensing of Anions, p. 566 Guanidinium-Based Anion Receptors, p. 615 Hydrogen Bonding, p. 658 lonophores, p. 760... 

Similarly, a series of urea-based vinyl monomers were synthesized for stoichiometric oxy anion recognition [22], One of these urea-based monomers,... 

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... 

A ditopic receptor consisting of two calix[4]arene units linked by two ethylene chains at the lower rim for cation recognition and urea-based hydrogen-bonding recognition sites for anions, 40, has been developed by the group of Beer in the United Kingdom." ... 

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

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