Fixing the Conformation of Calix[4]arenes: When Are Three Carbons Not Enough?

Calix[4]arenes are unique macrocycles that through judicious functionalisation at the lower rim can be either fixed in one of four conformations or remain conformationally flexible. Introduction of propynyl or propenyl groups unexpectedly provides a new possibility; a unidirectional conformational switch, with the 1,3‐alternate and 1,2‐alternate conformers switching to the partial cone conformation, whilst the cone conformation is unchanged, under standard experimental conditions. Using 1H NMR kinetic studies, rates of switching have been shown to be dependent on the starting conformation, upper‐rim substituent, where reduction in bulk enables faster switching, solvent and temperature with 1,2‐alternate conformations switching fastest. Ab initio calculations (DFT) confirmed the relative stabilities of the conformations and point towards the partial cone conformer being the most stable of the four. The potential impact on synthesis through the “click” reaction has been investigated and found not to be significant. Selective switching: Calix[4]arenes functionalised with propynyl groups at the lower rim undergo unique uni‐directional conformational switching. 1H NMR and kinetic studies reveal that 1,2‐alternate structures switch to the partial cone rapidly in solution and in the solid state whereas 1,3‐alternate structures switch only in solvent. Ab initio calculations confirm that the partial cone conformation is the global thermodynamic minimum.