Calix urea substituted

The synthesis of fourfold [2]rotaxanes by clipping requires the efficient formation of heterodimers between a tetra-urea substituted by bulky stopper groups and an octaalk-enyl urea 6. We initially hoped that the steric crowding in homodimers of a tetra-tritylphenylurea calix[4]arene would be sufficient to shift the equilibrium toward the heterodimers in a mixture with 6. However, the distribution of the dimers was close to the statistical ratio and the desired rotaxane could be obtained in only 5% yield [59]. 

Urea-functionalized calix[4]arene syntones shown in Scheme 3.4, fixed in their cone-conformation, have been synthesized in [4]. These syntones nndergo dimerization and hybridization in apolar solvents, leading to hybrid dimeric capsules with two nonequivalent urea-substituted arene fragments and their homodimeric analogs as weU. 

Calix[4]arenes 133, substituted at their wide rim by four urea-functions, are self-complementary. In aprotic, apolar solvents such as benzene or CDC13, they exist as dimers held together by hydrogen bonds between the urea functions of opposite molecules as indicated in Figure 28. A suitably sized guest (often the solvent) is included in the cavity thus created.269... 

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. 

Around the same time as this, Reinhoudt developed a calix[4]arene system with only two urea or thiourea functionalities attached on opposite faces 70-72 [167]. These less substituted systems display both inter- and intramolecular hydrogen bonding as a result of the calixarene adopting a pinched cone conformation (demonstrated by the use of NOESY NMR). In some spectra it is impossible for the connectivities to be made within a single molecule, so the only possibility left is that dimerisation occurs. As with the initial experiments of Rebek and Bohmer, the extent of hydrogen bonding was observed to be solvent dependent. Concentration dependant FTIR was also used, to observe the effects on the NH stretching vibrations, but no concentration dependence was observed. Of the three urea derivatives used, only 72 showed no evidence of dimerisation... 

Copper-mediated coupling reactions of / -iodocalixarenes with phthalimide followed by hydrazinolysis should be mentioned as an alternative and independent strategy to obtain p-aminocalixarenes . The carbazole-substituted derivatives 50 (Figure 8) were obtained similarly by Ulhnan coupling . CMPO derivatives (51c), urea compounds (51b), available also via the isocyanates (51a), may be mentioned additionally. Mono- (52) and diimides with acidic functions pointing towards the cavity, and the calix[6]arene-based acetylcholine esterase mimic (53) are more sophisticated examples. 

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