Water-stabilized cavitands.

Abstract

Tetrabenzimidazole cavitands 4 were prepared by condensation of ortho esters with octaamino cavitand 3 in 70-80% yield. Molecular modeling predicted that no intramolecular hydrogen bonds are possible between the imidazole fragments in the vase conformation of 4. Instead, this conformation provides four perfect binding sites for hydroxyl-containing molecules through an N-H---O-H---N pattern. Such interactions provide the means for sealing the cavitand's cavity. Accordingly, dry compounds 4 are not soluble in dry CDCl3 but readily dissolve upon addition of small amounts of alcohols or by saturation of the solution with water. 1H NMR spectroscopy revealed that in these solutions molecules 4 adopt a vase conformation while 1D GOESY experiments revealed their monomeric nature. In water-saturated CDCl3, these cavitands 4 form kinetically stable 1:1 inclusion complexes with tetramethylphosphonium bromide and triethylammonium chloride in which the cation is incorporated into the pi-basic cavity. Thus, cavitands 4 are a novel class of open-ended molecular containers capable of the formation of highly kinetically stable complexes upon assistance by hydrogen-bonding water molecules.