Tetraurea calix arene

Fig. 11 Heterodimeric diastereoisomeric capsules arising from dimerization of achiral sulfonyl tetraurea and chiral tetraurea calix[4]arenes...

Rudzevich V, Rudzevich Y, Bohmer V (2009) Dimerization and self-sorting of tetraurea calix[4]arenes. Synlett 1887-1904... 

L. Frish, M. O. Vysotsky, S. E. Matthews, V. Bdhmer, Y. Cohen, Tropylium cation capsule of hydrogen-bonded tetraurea calix[4]arene dimers, J. Chem. Soc., Perkin Trans. 2, 2002, 88-93. 

Figure 14 illustrates the presence of exchange crosspeaks in the NOESY spectra of a self-assembled tetraurea calix[4]arene capsule 5. For this capsule, the correlation time is long and NOE cross-peaks are of the same... 

Frish L, Matthews SE, Bohmer V, Cohen Y. A pulsed gradient spin echo NMR study of guest encapsulation by hydrogen-bonded tetraurea calix[4]arene dimers. J Chem Soc Perkin Trans 1999 2 669-72. 

Just 1 year later, in 1996, Bohmer and co-workers demonstrated, using H NMR spectroscopy, that combining two different tetraurea ca]ix[4]arenes (2 and 3 from Fig. 32.2) in benzene-dg provided a mixture of homodimers 2 2 and 3 3 in addition to the heterodimeric capsular assembly 2 3. The ratio of the three self-assembled containers was very close to the statistical distribution expected for three isoenergetic dimers (25 25 50 for 2 2,3 3, and 2 3 respectively) [15]. Immediately, following this report, Bohmer and co-workers described the first X-ray structure of a tetraurea cahx[4]arene homodimeric cOTitainer. This finding was perceived by the community of Supramolecular Chemists as the definitive proof of the self-assembly of tetraurea calix[4]arenes in dimeric capsular assemblies held together by hydrogen bonds [16]. 

Covalent connection of two tetraurea calix[4]arene units through their lower rims with a p-disubstituted benzene spacer held the tetraurea functions of both calixarene apart (molecules 6 and 7, Fig. 32.3). Consequently, in this case the formation of a unimolecular capsule was not allowed. Alternatively, supramolecular polymerization took place under adequate conditions forming functional polymeric capsules, the so-called polycaps [20, 21]. Gel permeation chromatography was used for the physical characterization (i.e. aggregate stability and molecular weight) of these polymeric capsules. Moreover, encapsulation studies of adequate guests (e.g. p-difluorobenzene) were performed to study further the self-assembly behavior of polycaps [22]. 

Limitations on the application of hydrogen bonded capsular assemblies come from the poor stability of these assemblies in polar media compared to those held by covalent bonds. In an attempt to overcome these Umitations, Bohmer described the synthesis of a mechanically locked chiral bis-catenane based on calixarene capsules [55]. Inspired by this pioneering work, our group produced a mechanically locked hybrid tetraurea calix[4]arene-calix[4]pyrrole capsule 30 (Fig. 32.14) [56]. This catenated capsule displayed reversible encapsulation (i.e. reversible assembly of the urea belt). The two hemispheres of the capsule never dissociate completely due to the bis-catenated topology. 

Mogck O, Paulus EF, Bohmer V, Thondorf I, Vogt W (1996) Hydrogen-bonded dimers of tetraurea calix[4]arenes unambiguous proof by single crystal X-ray analysis. Chem Commun 2533-2534... 

Fig. 3.6 Dimeric capsule from tetraurea calix[4]arene [44]...

Fig. 9 A Calix[4]arene tetraurea dimeric capsules with a particular head-to-tail hydrogen bond network. B Achiral tetraurea units form achiral homodimeric capsules, while C two different achiral units forms chiral heterodimeric capsules...

Fig. 10 Chiral Calix[4]arene tetraurea unit bearing stereogenic centers on the urea residues...

This dimerization reduces the symmetry of the single calix[4]arene from a time-averaged C4v symmetry to C4 symmetry stable on the NMR timescale. This can be deduced for instance from a pair of /n-coupled doublets for the aromatic protons of the calixarene skeleton, while these protons appear as a singlet in DMSO, where the tetraurea exists in the form of single molecules (see Figure 29). 

The stereochemical properties of tetraurea dimers are intriguing and challenging as well. As already mentioned for the simplest case, the directionality of the C=0 groups makes each single calix[4]arene in the dimer chiral (C4-symmetry), while the dimer as a whole is an achiral meso form (S8-symmetry) composed of a pair of enantiomers. 

Fig. 2 Left calix[4]arene tetraurea dimer,s 4. with the X-ray structure Right cylindrical capsule 5 and its computer-generated molecular model.

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