Tetra-tosylurea

Figure 5.4 Representative sections of the 1H NMR-Spectra (CDCI3, 400 MHz, 25°C) of (a) tetra-tolylurea (1) (Y-C5Hnn), (b) tetra-tosylurea (4), and (c) a 1 1 mixture of 1 and 4.

On the other hand, there are examples of exclusive heterodimerization. Both tetra-aryl- and tetra-tosylureas readily form homodimers when they are dissolved alone in an apolar solvent. However, heterodimers are the only detectable species in a solution containing both compounds in a 1 1 ratio. An example of this selective heterodimerization [41] is demonstrated by the NMR-spectra in Figure 5.4. Only when one of these tetra-ureas is present in excess can signals for its homodimer be seen by NM R. This early observation by Rebek and Castellano [42] is especially important for the template syntheses described below. Other examples of exclusive formation of heterodimers will be discussed later. 

In practice it is reasonable to use the non-reactive compound in the heterodimer, the tetra-tosylurea 4, in a small excess, to exclude the undesired formation of homodimers 5-5 or 6-6. After metathesis, the dimer is decomposed by a hydrogen-bond breaking solvent (usually THF), and the crude product is usually hydrogenated directly, to avoid complications by cis-trans isomerism. The whole reaction sequence is schematically summarized in Scheme 5.12. Bis- and tetraloop calix[4]-arenes 8 and 9 were easily prepared in high yields (70-95%) [51,52] for residues Y = methyl, pentyl, and dodecyl, and loop sizes of n = 2 m + 2 = 8, 10, 14, and 20.4 Several aspects show the generality of this approach ... 

Figure S.9 Approaches to tetraloop compounds with different size and structure of the loops via heterodimerization with tetra-tosylurea 4 followed by olefin metathesis and hydrogenation.

A second calix[4]arene, the tetra-tosylurea (template II) is used to bring the reactive groups into the correct position for their connection. 

Like in heterodimers with tetra-tosylurea (4), the alkenyl groups of 5 and 6 are prearranged in these heterodimers with 8 or 9 in a way which allows only one well-defined connection by metathesis reaction. The only difference is that this now leads... 

Figure 5.19 Molecular shape of rotaxane 23 (n = 6), a hetero-dimeric assembly of a tetraloop tetra-urea 9 (stick representation) and tetra-tosylurea 4 (space filling), based on MD simulations. Ether groups (OY = OC5Hnn,) areomitted in the formula.

Therefore, we connected the tri-urea 27 with three tetra-ureas 28, via covalent linkers, to a building block A. In apolar solvents molecules A alone would form crosslinked polymers via dimeric substructures formed by the tri- and tetra-ureas. Together with the building block B, a tetra-tosylurea substituted by four phthalimido alkylether residues, this polyassociation is prevented by the formation of heterodimers between tetra-aryl- and tetra-tosylureas. A solution of A and B in the ratio 2 6 therefore contains exclusively the dendritic assembly shown in Figure 5.21, as proved by light scattering and DOSY NMR spectroscopy [69]. 

Thondorf I, Rudzevich Y, Rudzevich V, Bohmer V (2007) Reasons for the exclusive formation of heterodimeric capsules between tetra-tolyl and tetra-tosylurea calix[4]arenes. Org Biomol Chem 5 2775-2782... 

The covalent connection of two tetra-urea calix[4]arenes via their narrow rims leads to molecules which are bifunctional with respect to their ability to form dimeric capsules. This was used by Rebek and Castellano to form self-assembled polycaps , including also alternating structures based on the heterodimerization of aryl- and tosylureas [42,63]. The idea was later extended to switchable polymers [64]. We were recently able to show that the covalent connection of two tetra-urea calix[4]arenes via a rigid spacer between urea functions also leads to molecules 24, which can form polymeric assemblies (Scheme 5.22) [57,65]. 

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