There is great interest in developing synthetic mimics of the confined environments present in enzymes to increase the efficiency and specificity of organic reactions.  For practical applications these synthetic reactors should be readily constructed and adaptable to the specific reactants required.  Our goal is to use the self-assembly of simple macrocyclic ureas to develop readily tunable porous materials for use as confined environments for reactions and for host/guest chemistry.  We have demonstrated that simple bis-urea macrocycles predictably self-assemble, as 1 assembles as designed into porous crystals that can reversibly absorb guests (Figure 1).  These crystals display gas adsorption isotherms consistent with microporous materials.  The crystals can be used as a container to promote a highly selective [2+2] photocycloaddition of matched guests, such as 2-cyclohexenone or 3-methyl-2-cyclopentenone in high yield.  The product can be easily removed from the crystals by extraction and the empty crystals recovered by filtration and reused.  Alternatively, the crystals can be dissolved in DMSO, releasing particularly large guests that might otherwise be trapped.  The crystals can simply be reformed by recrystallization.  We will also report on the elaboration of this structural building block to create porous crystals from macrocyclic ureas 2 and 3 that contain channels of different and predefined size and shape.  

Figure 1 Schematic representation of the self-assembly of macrocycles into crystals with columnar structures.  The porous crystals can reversibly adsorb a variety of guests including 2-cyclohexenone.  Irradiation of the guest in the confined environment yields a highly selective photodimer in high conversion.  The guests can be readily extracted from the crystals and the crystals reused.

This research is funded by the National Science Foundation (CHE-0316424) and by the American Chemical Society Petroleum Research Fund (44682)