A number of calix[6]arenes bearing ureas at the upper rim positions of alternate rings 1, 3 and 5 were prepared and studied in detail by NMR spectroscopy and gel permeation chromatography. N-Unsubstituted ureas were shown to dimerize through a cyclic array of hydrogen bonds to give cylindrical cavities capable of encapsulating small molecules such as dichloromethane, benzene and fluorobenzene. Slow equilibria between dimer and monomer were observed in [D6]DMSO-CDCl3 mixtures. By contrast, N-substituted ureas are monomeric. All urea monomers with bulky O-substituents display a solvent-dependent, slow equilibrium between C3v and Cs cone conformations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.