High affinity carboxylate binding using neutral urea-based receptors with internal Lewis acid coordination

Journal of Organic Chemistry

Volumn 61, Issue 14, 1996, Pages 4510-4511

  Hughes, Martin Patrick ^a   Shang, Maoyu ^a   Smith, Bradley D ^a  

^a UNIVERSITY OF NOTRE DAME   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0000497807     PISSN: 00223263     EISSN: None     Source Type: Journal    
DOI: 10.1021/jo9608026     Document Type: Article

References (38)

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30. The crystal structures will be described in a subsequent paper.
31. By the end of a typical titration experiment with tetrabutylammonium acetate in "dry" DMSO, 50% of 7 and 15% of 9 had hydrolyzed to produce 5. With the octyl derivatives, <10% of 8 and <5%of 10 had hydrolyzed. In the case of 8, the products from hydrolysis, namely 6 and pinacol, were found to have no ability to bind acetate or 8 in DMSO. Thus, they were treated as minor, inert impurities. In the absence of acetate or in less polar solvents, such as chloroform, the amount of hydrolysis was considerably less.
32. The reason for the increased stability seen with the octyl derivatives is not totally clear. One possibility is that the mechanism for hydrolysis of 7-10 involves initial attack of water at the electrophilic uronium carbon with subsequent cleavage of the uronium C-O bond. The resulting intermediate then recyclizes with loss of the 2X groups to generate the highly stable 5 or 6. If the initial attack by water is rate determining, then a bulky octyl could be expected to sterically hinder this step and make the compound kinetically more stable than the corresponding methyl derivative.
33. 1H NMR titration isotherms were generated for as many host signals as possible. A binding constant was derived by averaging the values obtained from the different isotherms, which were always within ±10% of each other. The values listed in Table 1 are the averages of three independent titration experiments. Control experiments showed no evidence for host self-association in DMSO.
34. Conners, K. A. Binding Constants, The Measurement of Molecular Complex Stability; Wiley: New York, 1987.
35. The curves were fitted using the simplex algorithm. Cooper, J. W. Introduction to Pascal for Scientists; Wiley: New York, 1981.
36. 2 is -0.01. Chu, K. C. In Burger's Medicinal Chemistry, Part I. The Basis of Medicinal Chemistry, 4th ed.; Wolff, M. E., Ed.; Wiley: New York, 1979; p 401.
37. 6 As expected, the ortho boron residue dramatically increases the positive electric potential surrounding the urea NH residues, as well as the magnitude of the molecular dipole. For hosts 11, 7, and 9, the positive electrostatic potentials at the NH residues were calculated to be 65.8, 81.0, and 85.7 kcal/mol; the calculated molecular dipoles were 2.4, 6.1, and 7.6 D.
38. max values in Table 1), which is indicative of stronger hydrogen bonding in the host-guest complex.