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| - Enhanced Carboxylate Binding Using Urea and Amide-BasedReceptors with Internal Lewis Acid Coordination: A CooperativePolarization Effect
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| - A structural design strategy is described that greatly improvesthe acetate binding ability of neutralurea and amide-based receptors. The enhanced binding is due to acooperative polarization effectwhich is induced by intramolecular coordination of the urea or amidecarbonyl to a Lewis acidicboronate group. A series of boronate-ureas, 3, and arelated bis(boronate-amide), 23, werepreparedin two steps from 2-(aminophenyl)boronic acid and their structureselucidated using X-raycrystallography and other spectrometric methods. The abilities ofthe receptors to associate withtetrabutylammonium acetate in dimethyl sulfoxide solution weredetermined by 1H NMR titrationexperiments. Association constants were calculated using nonlinearcurve-fitting methods. Theboronate-ureas 3 strongly bind to acetate in dimethylsulfoxide solution with association constantsas high as 6 × 104 M-1.This is more than 150 times greater than the association constantsforcontrol urea receptors that lacked an appropriate boron substituent.Thermodynamic studiesindicate that the enhanced association is due to a favorable enthalpicchange. Additional NMRstudies eliminated the possibility of proton transfer to the acetateduring complex formation.Molecular modeling indicates that the boronate-ureas exhibitimproved acetate binding becausethe intramolecular coordination (i) induces a larger host dipole momentwhich strengthens theguest/host ion−dipole interaction, and (ii) increases the positivesurface potential at the urea NHresidues which strengthens short range Coulombic interactions with theanionic acetate. Theobserved association constants correlate better with calculated hostdipole moments, suggestingthat for the boronate-ureas described here this is the more influentialfactor controlling association.
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