Dimeric hydrogen-bonded capsules

The future prospects for the capsule project emerge from these considerations. Further increasing the size of the capsule and building chemical functionalities into the inner cavity would allow a closer emulation the functions of enzymes, especially those that require cofactors in order to catalyze chemical transformations. Another important aspect is to design capsules that can combine stereospecificity and catalysis - that is accelerate stereoselective transformations. Capsules that reversibly dimerize in water would probably contribute a lot more to our understanding of non-covalent forces and solvent effects in this most biorelevant medium. So far, water solubility and assembly have not been achieved with hydrogen-bonded capsules. 

Hydrogen bonded capsule consisting of calixarene dimers will be briefly presented in Section 8.3.2. 

Figure 4.4 Supramolecular hydrogen bonded capsule developed by Rebek et ai (left) a dimeric structure and (right) various tetrameric structures.

Threefold- and tetrafold-symmelric ligand syntones 123-128 have been used in [21] for the design of Dja- and D a-symmelric hydrogen-bonded capsules and their analogs. Dimerization and hybridization of these syntones by Schemes 3.20 and 3.21 gave homo- and heterodi-meric ligands 323-329, respectively, which form 1 1 cage complexes with varions encapsulated mono- and dications shown in these Schemes. 

Hydrogen-bonded capsules 307-309, which accelerate Diels-Alder reactions between dienes and dienofiles, have been prepared in [29-31] using dimerization of the ligand syntones 115, 116, and 138 by Schemes 3.28 and 3.29 kinetic and thermodynamic studies of these reactions within the above capsules were performed in [31]. Two solvent molecules occupying their cavities form the corresponding 1 2 cage complexes. 

The dimerization of nonsymmetric ligand syntone 139 by Scheme 3.30 has been used [32] for the preparation of diastereomeric hydrogen-bonded capsules 349 and 350. 

The use of RTF allowed encapsulation a guest organic molecule by dimeric para-phosphonic acid-containing calix[5]arene capsule 382 [50]. This hydrogen-bonded capsule binds carboplatin... 

Abstract Concave molecules with a periphery of complementary hydrogen-bond donors and acceptors dimerize to form non-covalently bound capsules. These host structures feature the ability to bind appropriate guest molecules... 

Figure 3. Capsules based on other hydrogen-bonding atoms omitted) Tetraurea calixarene dimer 16-16,...

An interesting example of less rigid cages is also provided by molecular self-assembling capsules obtained in the Rebek group [56]. 81 was found to form dimers held together by hydrogen bonds between the donors at its ends and the acceptors in the middle. 

Coordination interactions are not the only way to use self-assembly to produce closed, capsular systems capable of binding guest species in solution. Work by Julius Rebek Jr (Scripps, USA)43 has shown that multiple hydrogen-bonding interactions, because of their relatively weak, but directional, nature, are ideal for the strict self-assembly of closed spherical molecules and capsules. For example, component 10.49 consists of two intrinsically curved diphenylglycoluril units linked by a durene-based (1,2,4,5-tetramethyl benzene-based) spacer. In both solution and in the solid state, 10.49 self-assembles spontaneously to produce the tennis ball-shaped dimer (10.49)2 shown in Figure 10.43. The formation of the dimer has been observed by ... 

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...

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