Urea-based calixarene dimers

Various other examples have been provided in the literature involving the formation of molecular capsules. However, guest binding by these capsules may not have been studied, or evidence of a template requirement for formation may not have been present. These include urea-based calixarene dimers [50], deep-cavity resorcinarenes [51],... 

As illustrated in Figure 5.6 for a tetra-tolylurea (as an example), the two calix[4]-arenes forming the dimer are turned by 45° around their common fourfold axis. The tolyl residues attached to the urea groups point in opposite directions. Based on M D-simulations, the distance between adjacent tolyl methyl groups in the same calix-arene is slightly larger (13.4 A) than the distance to the adjacent methyl groups in the other calixarene (12.3 A). 

The first examples of urea-containing dimeric capsules were discovered independently by Rebek and Bohmer [165,166]. These are based on calix[4]arene ethers, in which the lower rim ether units help to fix the calixarenes in a cone conformation via intramolecular interactions. Figure 60 shows the generic structure of such systems. Synthetically, the urea calixarenes are readily prepared. The calixarenes are first nitrated, followed by a reduction of these groups into amines. The urea derivatives are fixed to the upper rim by reaction of the p-amino calixarenes with isocyanates. Many systems were studied using differently substituted ureas which contain either short alkyl chains or simple phenyl derivatives. Studies also involved the changing of the lower rim ether substituents. 

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