Dimeric capsule

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, 

2) A and B just characterize different phenolic units, where the difference may be due to the ether residues Y or to the urea residues R or both. 

Organic cations are especially good guests, since their inclusion is favored by cation-7t interactions with the aromatic walls. This enables the encapsulation to be studied by mass spectrometry also [36]. If cations such as tetraethylammonium or cobaltocenium are included [37], the directionality of the hydrogen-bonded belt 

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Acceleration of a Diels-AIder reaction by a self-assembled dimeric capsule... 

H NMR spectroscopy in CDC13 and C6D6 solution - the NH protons are shifted significantly downfield relative to model compounds that do not form dimeric capsules because of the electron-withdrawing presence of the carbonyl oxygen atoms hydrogen bonded to the NH protons. 

Gonzalez, J. J., Ferdani, R., Albertini, E., etal, Dimeric capsules by the self-assembly of triureidocalix[6]arenes through hydrogen bonds. Chem. Eur. J. 2000, 6, 73-80. 

Calix[4]arene provides a suitable semi-rigid concave scaffold for the construction of dimeric capsules. Both the upper and lower rim can be decorated with a wide variety of functional groups, in particular the urea moiety is known to provide a good combination of hydrogen bond donor and acceptor character implemented in the same functional group and this sequence of atoms have been extensively employed in the constructions of calix[4]arene dimeric molecular capsules (Fig. 9). Characteristic of these dimers is the presence of a cyclic seam of hydrogen bonds that seal the capsule and provides an internal molecular cavity that easily accommodates molecules as big as benzene. 

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. 19 Achiral calix[4]arenes with different substituents on the urea residues lead to diastereoisomeric chiral dimeric capsules...

Fig. 26 A Chiral memory effect applied to the construction of a chiral softball homo-dimeric capsule made of achiral units. B Racemic softball binds a chiral guest leading preferentially to one diastereoisomer. Fast exchange, compared to capsule s racemization rate, with a better achiral guest allows preparation of enantioenriched capsule...

Scheme 3.9 Hydrogen-bonded dimeric capsule (35)2 (R1 = R2 = 4-n-heptylphenyl, R3 = OH) catalyzes a Diels-Alder reaction.

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

Tetra-urea Calix[4]arenes - From Dimeric Capsules to Novel Catenaries and Rotaxanes... 

Hydrogen-bonded dimers of tetra-urea calix[4]arenes offer a unique, unprecedented preorganization [28], Before we describe how this can be used for the rational construction of molecules with novel topologies, it is reasonable to discuss the properties of such dimeric capsules and the conditions under which they exist. 

Figure 5.1 Sections of nH NMR spectra (400 MHz, 25 °C) of tetra-urea 1 (a) in DMSO-d6 (monomers) (b) in CDCI3 (dimeric capsules 1CDCI31) (c) in CDCI3 in the presence of Et4N+-BF4 (dimeric capsules [lEt4N+l]BF4 ). The solvent signal is marked by an asterisk.

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],... 

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