Molecular clefts

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

The condensation reactions described above are unique in yet another sense. The conversion of an amine, a basic residue, to a neutral imide occurs with the simultaneous creation of a carboxylic acid nearby. In one synthetic event, an amine acts as the template and is converted into a structure that is the complement of an amine in size, shape and functionality. In this manner the triacid 15 shows high selectivity toward the parent triamine in binding experiments. Complementarity in binding is self-evident. Cyclodextrins for example, provide a hydrophobic inner surface complementary to structures such as benzenes, adamantanes and ferrocenes having appropriate shapes and sizes 12) (cf. 1). Complementary functionality has been harder to arrange in macrocycles the lone pairs of the oxygens of crown ethers and the 7t-surfaces of the cyclo-phanes are relatively inert13). Catalytically useful functionality such as carboxylic acids and their derivatives are available for the first time within these new molecular clefts. 

PAMAM dendrimers develop molecular clefts [10] on their surfaces which may be visualized from de Gennes Starburst dendrimer model (Fig. 44) [85]. De Gennes hypothesizes that the Starburst structures should be quite flexible in the early generations, whereas the surface branch cells should become substantially more... 

J. Rebek, B. Askew, M. Killoran, D. Nemeth, F.T. Lin, Convergent Functional Groups 3. A Molecular Cleft Recognizes Substrates of Complementary Size, Shape, and Functionality , J. Am. Chem. Soc., 109, 2426 (1987)... 

Actual RNA has been cleaved by the artificial enzyme shown in Fig. 6.14 which is a mimic of Staphylococcal nuclease. This is molecular-cleft-type artificial enzyme, where guanidinium residues connected to a rigid backbone... 

Porphyrins 14 and 15, containing four convergent hydroxy groups, form molecular clefts that bind tetramethoxy-p-benzoquinone to give 1 1 adducts [68, 90-94]. Association constants of the quinone guest with 14 and 15, determined from absorption and fluorescence titration experiments, are K/i, = 7.9 x 10 M and... 

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