Amphoteric Homotropic Allosteric Association between a Hexakis‐Urea Receptor and Dihydrogen Phosphate

Conformationally flexible hexakis‐urea 1 was synthesized efficiently by condensing hexakis(aminomethyl)benzene with 4‐nitrophenyl‐(3,5‐di‐ tert ‐butylphenyl)carbamate. The hexakis‐urea 1 is unexpectedly soluble in organic solvents of low polarity due to intramolecular hydrogen bonding. The hexakis‐urea 1 recognizes chloride, bromide, and acetate in a 1:2 host‐guest ratio and in a positive allosteric manner in CDCl 3 . The ability of 1 to recognize dihydrogen phosphate is a unique outcome, and the structure of the associated complex, which contains four dihydrogen phosphate ions, was clarified by single‐crystal X‐ray structural analysis. However, in solution, a complex with three dihydrogen phosphate ions was identified. The dihydrogen phosphate association in CDCl 3 proceeds in an amphoteric allosteric manner; in a positive allosteric manner ( K 1 < K 2 ) in the first step and a negative allosteric manner ( K 2 > K 3 ) in the subsequent step.