Larger Substituents on Amide Cavitands Induce Bigger Cavities.

A series of quinoxaline cavitands bearing pendant amide groups with various substituent sizes (Et, ⁱPr, ^tBu) were synthesized, and their cavity size/structure were investigated by X-ray and NMR analyses. In the case of the Et or ⁱPr amide cavitand, the conformation of the molecule was in the vase form, while the bulky ^tBu amide cavitand gave the kite conformation at room temperature. X-ray crystal structures of Et and ⁱPr cavitands clearly showed the intramolecular H-bondings to influence the conformation and the cavity sizes dependent on the bulkiness of functional groups. The ¹H NMR spectrum revealed that the Et cavitand can encapsulate an adamantane guest compound with slow exchange.