Enantioselective Formation of a Dynamic Hydrogen-Bonded Assembly Based on the Chiral Memory Concept

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Abstract
In this paper, we report the enantioselective formation of a dynamic noncovalent double rosette assembly 1a3·(CYA)6 composed of three 2-pyridylcalix[4]arene dimelamines (1a) and six butylcyanuric acid molecules (BuCYA). The six 2-pyridyl functionalities of the assembly interact stereoselectively with chiral dicarboxylic acids 3a − e via two-point hydrogen-bonding interactions. One of the two enantiomeric assemblies (P- or M-) 1a3·(CYA)6 is formed in excess as the result of the complexation of the chiral diacids, resulting in formation of optically active assemblies. The complexations with dibenzoly tartaric acids d-3a and l-3a (3 equivalent), respectively, leading to the formation of diastereomeric assemblies (P)-1a3·(BuCYA)6·(d-3a)3 and (M)-1a3·(BuCYA)6·(l-3a)3 with 90% diastereomeric excess. The diastereomeric excess in (M)-1a3·(BuCYA)6·(l-3a)3 is “memorized” when l-3a is removed by precipitation with ethlylenediamine (EDA). The assembly (M)-1a3·(BuCYA)6 is still optically active (90% enantiomeric excess), although none of its individual components are chiral. (M)-1a3·(BuCYA)6 has a high kinetic stability toward racemization (Ea = 119 kJ mol-1, half-life of (M)-1a3·(BuCYA)6 is ca. 1 week at 20 °C).