We are interested in controlled formation of homogeneous porous materials with nanotubular channels for use in host-guest chemistry and as confined reaction environments. Our approach is innovative because we can tailor the size, structure and functionality of the cavity by altering the structure of a small molecule, a macrocyclic bis-urea. The bis-urea macrocycles predictably self-assemble into tubular structures through non-covalent interactions, primarily urea-urea hydrogen bonds and aryl stacking interactions. We report on two derivatives: a phenylether macrocycle and a benzophenone macrocycle that assemble as designed into columnar structures, each forming porous crystals with guest-accessible channels. Both of these materials show gas adsorption isotherms consistent with microporous materials with pores < 6.5 Å in diameter. In fact, these porous crystals can be used as a container for reactions, promoting highly stereoselective [2+2] cycloaddition of enones of matched size and shape to yield the exo heat-to-tail dimers in high conversion. The product can be removed from the crystals by extraction and the empty crystals recovered by filtration and reused. Yet unlike zeolites, the self-assembled structure can be dissolved in warm DMSO, releasing particularly large guests that might otherwise be trapped.

This work was supported in part by the NSF (CHE-071817) by the Petroleum Research Fund (44682), and by a grant from the University of South Carolina, Office of Research and Health Sciences Research Funding Program.