Tweezers and Clefts

Guanidium-Based Anion Receptors, p. 615 Hydrogen Bonding, p. 658 Hydrophobic Effects, p. 673 Molecular Clefts and Tweezers, p. 887 Porphyrin Derivatives, Functional, p. 1139 Steroid-Based Anion Complexation Agents, p. 1365 Troeger s Base Derivatives, p. 15 6 Zwitterion Receptors, p. 1639... 

Molecular Clefts and Tweezers, p. 887 Organometallic Anion Receptors, p. 1006 Phase-Transfer Catalysis in Environmentally Benign Reaction Media, p. 1042 Siderophores, p. 1278 Swfactants, Part 1 Fundamentals, p. 1458 Surfactants, Part 11 Applications, p. 1470 The Template Effect, p. 1493... 

Liquid Clathrates, p. 804 The Lock and Key Principle, p. 809 Molecular Clefts and Tweezers, p. 887 Molecular Modeling and Related Computational Techniques, p. 901... 

A variety of molecules with a functional group located in a cleft have been synthesized (a) Zimmerman SC, Zeng Z, Wu W, Reichert DE (1991) J Am Chem Soc 113 183 ( Molecular tweezers ) (b) Rebek Jr J, (1990) Angew Chem 102 261 Int Ed Engl 29 245 (Oefts based on derivatives of Kemp s acid ). In both classes, a functional group is located in a slot. But the shielding is only from the top and from the bottom, not from the sides. 

The component analysis described above shows that the aromatic cleft in 34 contributes 4.2 kcal mol-1 toward the free energy of complexation of 9-propyladenine (50). An alternative approach to determining the contribution made by the aromatic cleft would be to measure the free energy obtained in a complex between 50 and a molecular tweezer containing an aromatic cleft, but no carboxylic acid group. This analysis, which has been carried out using molecular tweezer 11,... 

Fig. 4 Exemplary cases of enforced cavity hosts and their supramolecular compounds (guests symbolized in broken rings) (a) Cleft-type (b) tweezer-type (c) spherand (d) cyclophane (monocyclic) (e) cyclophane ibicyclic) (f) cavitand (g) carcerand (h) calixarene (calix[4]arene) (i) resorcarene (j) cryptophane (k) cyclodextrin (P-cyclodextrin) and (1) cucurbituril.

While this seminal work set the stage for further developments in the area, one problem was that the linker, despite its rigidity, still afforded the possibility of conformational isomerism such that the caffeine moieties were free to adopt orientations in which they were not syn and did not create a cleft into which binding could occur. The entropic demand associated with producing con-formers with a syn orientation of binding elements certainly lowered the efficiency of molecular tweezers of this type. 

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