Spherands preorganization

From the kinetic point of view the facts are different and the order is reverse, ie, the rigid highly preorganized spherands are slow, as contrasted with the flexible barely preorganized podands that are fast both in formation and decomposition of the receptor—substrate (host—guest) complex (20,21). 

This preorganization principle not only affects the thermodynamic stability of the complex but also has a pronounced effect on the kinetic stability of the alkali complexes of the spherands. The anisyl groups in the spherands provide the preorganized ligating donor sites, and they shield the cavity from solvent molecules. As a result, the rates of complexation by the spherands are decreased and the rates of decomplexation are decreased even more when compared with related flexible crown ethers. 

In addition to the spherands, Cram synthesized compounds in which at least half of the binding sites may be considered to be preorganized, the so-called hemispherands.6 In the parent molecule 2, the molecular cavity is composed of a rigid m-teranisyl moiety in which the oxygen binding sites are conformationally organized prior to complexation, and a flexible polyether... 

Fig. 1,1, Preorganization does matter. A comparison of 18-crown-6 and the spherand on the right with respect to alkali metal ion binding reveals that the spherand has an up to 10 orders of magnitude higher binding constant.

Recently, creativity in chromogenic macrocycle synthesis has expanded. New spherand species have been synthesized that act as highly preorganized chromogenic-specific indicators for Li+ and Na (Cram et al., 1985), and an azophenol dye has been prepared with perfect selectivity for Li (Kaneda et al., 1985). Many of these chromogenic macrocycles and more complicated species such as the hemispherands and cryptohemispherands have found commercial use for Na" and K assays in body fluids (see 41) (Helgeson et al., 1989 Czech et al., 1990). 

A particularly good example of the power of preorganization combined with conformational adaptation is seen in the calix[4]spherands (142a), compounds in which a hemispherand moiety is affixed to the lower rim of a calix[4]arene. Although the hemispherand part of the molecule is... 

In order to build better hosts which have preorganized binding sites. Cram and co-workers have prepared hemispherands [50] and spherands [51],... 

Preorganization and Complementarity, p. 1158 Protein Supramolecular Chemistn. p. 1161 Spherands, p. 1344 X-Ray Crystallography p. 1586... 

Molecular Squares, Boxes, arid Cubes, p. 909 71-71 Stacking Theory and Scope, p. 1076 Preorganization and Complementarity, p. 1158 Pyrrole- and Polypyrrole-Based Anion Receptor-s. p. 1176 Rotaxanes arid Pseudorotaxanes, p. 1194 Self-Assernhling Catenanes, p. 1240 Siderophores, p. 1278 Spherands, p. 1344... 

The classic example of preorganization is the contrast between podand 3 and spherand 4 (Fig. The... 

Soon after crown ethers came on the scene as the first synthetic host molecules capable of binding guests, cryptands followed, and soon thereafter, the spherands. The preorganization of these three classes of molecules follows the order of their invention, as does overall binding affinity, i.e., crownsNobel Laureate, was the creator of the family of hosts that he named spherands. The spherand story began shortly after the genesis of supramolecular chemistry, and it demonstrates how quickly the field matured, as complex syntheses and methods of characterization enhanced the rapid sophistication of host-guest chemistry. 

Similar to the hemispherands are the cryptahemispherands. Instead of a corand part, there is a cryptand part the rest of the host again is a highly preorganized spherand unit. Cryptahemi spherand 13 binds Na" as a seven-... 

Donald Cram Spherand hosts produced to test the importance of preorganization... 

---