Anion binding through hydrogen bonding

II. BIOLOGICAL APPROACHES TO ANION BINDING A. Binding through Hydrogen Bonding... 

A number of simple amides and sulfonamides (Figure 50) have been shown by Reinhoudt and coworkers to bind anions exclusively through hydrogen bonding. For example, the naphthalenesulfonamide binds H2PO4 , HSO4 and Cl with association constants of 1.42 x 1O M, 38 M and 1600M , respectively [71]. 

Sessler et al. introduced the anion-recognition ability of calix[4]pyrrole (1.11). 1.11 has the ability to bind halide anions efficiently through hydrogen-bonding interactions between pyrrolic NHs and the anions when the macrocyclic is fixed in the cone conformation. Counter cations are encapsulated at the same time in the opposite side of the bowlshaped calix[4]pyrrole cavity because of the electron-rich pyrrole cavity (Figure 1.9). 

The crystal structme of 28 showed that the compound adopts a pinched-cone conformation with the ethylene glycol substituents properly disposed to fold in and bind a metal cation at the base of the cahxarene and that the two thiourea units are effectively oriented to provide an anion-binding site through hydrogen bonding. NMR titrations in CD3CN of 28 with the TBA... 

The preference for the reaction wherein the intermediate compound incorporates the mefa-xylylene spacer—which must arise for its preference to bind the chloride ion—was shown by the extremely efficient formation of the cyclo-phane incorporating two of these moieties that can be prepared in 88% yield in the presence of additional chloride ions. The anions are bound by the imidazolium and meM-xylyl moieties through hydrogen bonds. Competition experiments in the synthesis back up this hypothesis. In addition, careful kinetic studies on the ring closure of an intermediate component prove the template effect. The experiments showed that the rate of the ring closure of the monocationic intermediate to give the dicationic macrocycle is increased up to 10 times in the presence of 40 mM tetrabutylammonium chloride. 

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