Dimeric capsule, formation

Scheme 3.13 Dimeric capsule formation in a triureidocalix[6]arene.

The internal volume of the capsule (190-200 A3) cannot be empty, and the inclusion of a suitable guest (often the solvent) is a necessary condition for the dimerization. Various organic molecules (for example, (lR)-(+)-nopinone, (lR)-(+)-cam-phor, [32b,c]) have been included in the cavity [35], and the upfield shifts of their 1 H NMR signals provide an independent proof of the formation of dimeric capsules,... 

A dimeric capsule can also be formed by two different tetra-urea molecules, and in aprotic solvents a mixture of two tetra-urea derivatives usually contains not only the two homodimers but also the heterodimer in a more or less statistical ratio. This formation of heterodimers is an additional proof of the dimerization [39], which is also valid when other indications (e.g. m-coupled doublets for aryl protons) fail. As an example, sections of the spectra of tetra-tolylurea (1), tetra-hexylurea (3), and of a mixture of the two are shown in Figure 5.3a-c [37a],... 

Formation of the dimeric capsule 5.55 based on calixarene ligand 40 (reprinted with permission... 

Molecular capsules are structurally elaborated receptors that completely surround the hosted molecule(s). Encapsulation based on covalent bonds yields permanent arrangements of molecules-within-molecules. Reversible encapsulation, on the other hand, is based on self-assembling through formation of weak supramolecular bonds and offers possibilities for a dynamic in out exchange of encapsulated molecules. Most of the dimeric capsules developed by Rebek and his group are obtained through reversible self-assembly of resorcinarene subunits. When simultaneously encapsulated in the cylindri-cally shaped inner space of these capsules, two reactant molecules are temporarily isolated from others in solution and display reactivity features different from those in bulk solution. The matter has been extensively reviewed,and will not be discussed here. 

Fig. 1 Capsule formation and early examples of Rebek s self-assembling eapsules self-replicating dimer 1-1, the tennis ball 2. and the softball 3.

Dibenzyl ketones are suitable guests for this dimeric capsule and they undergo exclusive formation of decarboxylation products in the absence of the hosting dimer, while in the presence of the capsule unusual massive formation of rearrangement products derived by recombination of radical residues and minor amounts of decarbonylation products is observed (Figure 24a). Moreover, as far as the decarbonylation products are concerned, photolysis within the capsule provided only mixed products and no homodimers, confirming that the recombination of radicals occurs in the capsule faster than the in-out exchange. 

Figure 24 (a) Hydrophobic dimeric capsule leads to selective formation of l-(4-methylphenyl)-2-phenylethanone with no decarbony-... 

Melegari M, Massera C, Ugozzoli F, Dalcanale E (2010) Tetraphosphonate cavitands interplay between metal coordination and H-bonding in the formation of dimeric capsules. CrystEngComm 12 2057-2059... 

When crystallized in the presence of alcoholic solvent which can act simultaneously as hydrogen bond donor and acceptor, Atwood and coworkers [45] prepared a unique dimeric capsule (26c)2(/-PrOH)g (Fig. 3.7a). The two subunits are not directly but indirectly connected in a rim-to-rim fashion by eight isopropanol (/-PrOH) molecules via 16 intermolecular hydrogen bonds. A similar capsule TEA" (26b)2(H20)g was reported by Aoki and coworkers [46] (Fig. 3.7b) at the same time. The capsule formation is driven by a tetraethylammonium ion (TEA ) via cation- -n interactions. Using this template synthesis strategy, more solvent mediated dimeric capsules have come out in literature with different ammonium guests [47]. 

Murayama K, Aoki K (1998) Resorcin[4]arene dimer linked by eight water molecules and incorporating a tetraethylammonium ion guest-driven capsule formation via cation-p interactions. Chem Commun 607... 

Crossley (89) described the self-assembly of a spherical cage-like structure made up of two zinc porphyrin dimers bound to a tetramine ligand. The 2 1 complex is stable at 10-6m concentrations, but addition of excess of ligand causes dissociation of the capsule with formation of a 1 1 complex. 

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