Solvophobically-Driven Oligo(ethylene glycol) Helical Foldamers. Synthesis, Characterization, and Complexation with Ethane-1,2-diaminium.

Oligo(ethylene glycols) 1a-h, which are incorporated with one to eight 2,3-naphthylene units, respectively, have been synthesized and characterized. The conformational changes of the new oligomers have been investigated in chloroform-acetonitrile binary solvents by the UV-vis, 1H NMR, and fluorescent spectroscopy. It has been revealed that the naphthalene units in hexamer If, heptamer lg, and octamer 1h are driven by solvophobic interaction to stack in polar solvents. As a result, compact helical conformations are formed that give rise to a cavity similar to that of 18-crown-6. Shorter oligomers lb-e exhibit weaker folding tendency. 1H NMR studies reveal that lf-h are able to complex ammonium or ethane-l,2-diaminium 19, but not secondary ammonium compounds. The association constants of complexes lf·19, lg·19, and 1h·19 in acetonitrile are determined to be 3.5(±0.4) × 103, 1.0(±0.12) × 104, and 2.5(±0.4) × 104 M-1, respectively, with the 1H NMR titration method. For comparison, hexamer 22, which incorporates six 1,5-naphthylene units, is also prepared. The UV-vis and fluorescent investigations show that 22 is also able to fold in polar solvents, but no helical structure can be produced due to mismatch of the stacking naphthalene units and consequently there is no obvious complexation between 22 with ethane- 1,2-diaminium ion. The structures of the longest foldamer 1h and its complex with 19 have been studied with molecular mechanics calculations. This work represents a new approach to building folding conformations from flexible linear molecules. [ABSTRACT FROM AUTHOR]