Calixpyrroles having extended aromatic cavities have been functionalized with 4 ureas in the para position of their meso phenyl substituents. This elaboration of the upper rim was completed in 2 synthetic steps starting from the α,α,α,α-tetranitro isomer of the calixpyrrole obtained in the acid catalyzed condensation of p-nitrophenyl methyl ketone and pyrrole. In dichloromethane solution and in the presence of 4,4′-bipyridine N-N′-dioxide the tetraurea calixpyrrole dimerizes reversibly forming a cyclic array of 16 hydrogen bonds and encapsulating 1 molecule of bis-N-oxide. The encapsulated guest is bound in the cavity by hydrogen bonding to the 2 endohedral calixpyrrole centers. Further evidence for dimerization of the tetraurea calixpyrroles is provided by 1H-NMR experiments and by the formation of mixed capsules.
Self-assembly of dimeric tetraurea calixpyrrole capsules
Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 10455-10459.