Novel artificial receptors for alkylammonium ions with remarkable selectivity and affinity

Abstract

The benzene-based tripodal tris(oxazolines) have been developed as the most selective and strong receptors toward linear alkylammonium ions reported to date. Among six tris(oxazolines) based on 2,4,6-trimethylbenzene framework, the phenylglycinol-derived receptor 4 exhibits the largest association constant toward nBuNH(3)(+) (logK(ass) = 6.65 +/- 0.02), while a similar value toward tBuNH(3)(+) (logK(ass) = 3.80 +/- 0.01) compared with others, which corresponds to the selectivity ratio of nBuNH(3)(+)/tBuNH(3)(+) as high as approximate to 700. The tris(oxazoline) 6 that has bare oxazoline ring exhibits still a large association hindered constant tBuNH(3)(+) toward sterically (logK(ass) = 5.26 +/- 0.02). Both receptors 4 and 6 extract beta-phenethylammonium ion from water into chloroform almost completely. When the benzene frame is changed from 2,4,6-trimethylbenzene to 2,4,6-triethylbenzene, dramatic changes in the affinity as well as in the selectivity are observed. The association constant observed by tris(oxazoline) 8 toward nBuNH(3)(+) approaches 10(8) M-1 and the selectivity ratio of nBuNH(3)(+)/tBuNH(3)(+) is increased to 2700. This selectivity is even more enhanced to 4000 with tris(oxazoline) 9. The enhanced binding affinity and high selectivity observed with receptors 4 and related derivatives 7-9 compared with others can be explained by an optimized steric and electronic environment provided by the phenyl substituents, which has been unambiguously demonstrated by X-ray crystallographic and H-1 NMR spectroscopic studies on the host-guest complexes. The new receptor system has several unique features such as ready availability, structural simplicity, and in particular versatility in derivatization. By virtue of these advantages, it can be readily tailored as selective receptors toward biologically important amines.