Covalent Linkage of Melamine and Cyanurate Improves the Thermodynamic Stability of Hydrogen‐Bonded Double Rosettes in Polar Solvents

Abstract

AbstractThis paper describes studies on the synthesis, self‐assembly behavior, and thermodynamic stabilities of eight different calix[4]arene di(melamine‐cyanurate) or di(melamine‐barbiturate) conjugates. The successful synthetic strategy comprises the preparation of amino‐n‐alkyl‐functionalized calix[4]arene dimelamines coupled with a carboxyl‐functionalized cyanurate or barbiturate by an amide bond‐forming reaction. 1H NMR experiments show that three of the eight conjugates form well‐defined double rosette assemblies. DMSO titration experiments illustrate that the covalent linkage between the cyanurate and dimelamine moieties produces a significant increase in the thermodynamic stabilities of these conjugates. The relative stability of the assemblies seems to be primarily governed by the structure of the component connecting the melamine and cyanurate units. The highest stability (χDMSO = 70%) was observed for the di(melamine‐cyanurate) assembly in which the components are connected through an n‐heptylamidomethyl linker. Increasing the linker size from n‐heptylamidomethyl to n‐decylamidomethyl slightly reduced the thermodynamic stability (ΔχDMSO ≈︁ 10%) of the corresponding double rosette assembly. The thermodynamic stability of the double rosette structure decreases drastically, however, on introduction of substituents at the position α to the methylcarbonyl group. With benzyl substituents (based on D‐phenylalanine), neither the n‐heptyl‐ nor the n‐decylamidomethyl‐linked compounds any longer form the double rosette structure in solution. For the conjugates with a methyl substituent at this position (based on L‐alanine), only the heptyl‐linked compound still forms the double rosette structure, the χDMSO value being reduced in this case by 15%. Neither of the di(melamine‐barbiturate) conjugates form double rosette structures at all. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)