Acyclic cucurbit[n]uril type receptors: secondary versus tertiary amide arms

Abstract

ABSTRACTTwo acyclic CB[n]-type hosts (1 and 2) which possess four 2° or 3° amide arms are reported. Host 2 has four 3° amide arms that exist as a mixture of E- and Z-isomers. 1H NMR was used to qualitatively investigate the binding properties of 1 and 2 which indicates they retain the essential binding features of macrocyclic CB[n] hosts. We measured the Ka values of 1 and 2 toward guests 6–14 by ITC. Neutral hosts 1 and 2 bind less tightly than tetraanionic hosts M1, ACB1, and ACB2. We attribute the lower Ka values to the absence of secondary ion-ion electrostatic interactions for host•guest complexes of 1 and 2. The secondary amide functionality on 1 decreases affinity by the formation of intramolecular NH•••O=C H-bonds. Tertiary amide host 2 binds even more weakly than 1 due to backfolding of the amide N-CH3-groups of 2 into its own cavity.