Hosts spherands

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). 

Fig. 3. Crown compounds/cryptands and analogous inclusion hosts. (1 4) Crown macro rings bicyclic cryptands (5) [37095-49-17, (6) [31250-06-3J, (7) [31364-42-8] (8) [23978-09-8]-, (9) spherical cryptand [56698-26-1]-, (10) cylindrical cryptand [42133-16-4]-, (11) apodand [57310-75-5]-, and (12) a spherand...

Cyclophanes or 7r-spherands have played a central role in the development of supramolecular chemistry forming an important class of organic host molecules for the inclusion of metal ions or organic molecules via n-n interactions. Particular examples are provided by their applications in synthesis [80], in the development of molecular sensors [81], and the development of cavities adequate for molecular reactions with possible applications in catalysis [82]. The classical organic synthesis of cyclophanes can be quite complex [83], so that the preparation of structurally related molecules via coordination or organometallic chemistry might be an interesting alternative. 

The hemispherands, spherands, calixarenes, and related derivatives. A number of hosts for which the pre-organization criterion is half met (the hemispherands) (Cram et al., 1982) or fully met (the spherands) (Cram, Kaneda, Helgeson Lein, 1979) have been synthesized. An example of each of these is given by (251) and (252), respectively. In (251), the three methoxyl groups are conformationally constrained whereas the remaining ether donors are not fixed but can either point in or out of the ring. This system binds well to alkali metal ions such as sodium and potassium as well as to alkylammonium ions. The crystal structure of the 1 1 adduct with the f-butyl ammonium cation indicates that two linear +N-H - 0... 

Fig.9.10 Tetrahedral hosts with Td symmetry, (a) Lehn s spherand, (b) Vogtle s spheraphane, (c) Saalfrank s metal-based cage.

In addition to macrocyclic hosts discussed above, many other molecules capable of selective complexation have been synthesized. They belong to so-called macrocyclic chemistry [30] encompassing crown ethers discussed in this Chapter, cryptands 61-63 [21], spherands 70 [31], cyclic polyamines 71 [32], calixarenes 18 [5], and other cyclophane cages such as 72 [33] to name but a few. Hemicarcerand 5 [2b] discussed in Chapter 1 and Section 7.3 also belongs to this domain. Typical macrocyclic host molecules are presented in Chapter 7. 

Donald Cram spherand hosts produced to test the importance of preorganisation... 

Figure 1.12 Comparison of the effects of preorganisation and complementarity on the magnitudes of the binding constant of polyether hosts for alkali metal cations. The figure for Li+ is given for the highly preorganised spherand-6 since it is too small to accommodate K+.

By way of comparison, acyclic podand hosts analogous to compound 3.33 have been produced in order to assess the importance of the rigid preorganisation afforded by a cyclic host. Comparison of the closely related podand 3.36 (which is estimated to possess over 10 000 possible conformations, only two of which can bind cations in a convergent manner) and 3.33 (which is locked in only one conformation) shows that the spherand binds Li+ more than 1012 times more effectively. This highlights the importance of preorganisation effects in host design. 

Cross-fertilisation between the crown ethers (or, more generally, corands), cryptands, spherands and podands has produced an enormous range of hybrid hosts such as cryptaspherands and hemispherands, many exhibiting all the useful features of the parent materials (Figure 3.16). 

A comparison of the X-ray crystal structures of spherand 3.33 and its highly stable spheraplex 3.33Ti+ reveals that in both instances the host has the same conformation (Figure 3.38). Host... 

As hosts for cations, the phenolic oxygen atoms at the calixarene lower rim (Figure 3.81) have the potential to act in a similar way to the anisole residues of the spherands, either in the original hydroxyl form or as alkyl ether derivatives. This kind of behaviour has been observed for the methyl ether of the parent /j-f-butylcalix[4] arene (compound 3.121) upon reaction with a mixture of sodium benzoate, one... 

Compare and contrast the properties of the following classes of host molecule. Include in your answer information on selectivity, solubility and binding kinetics (a) naturally occurring ionophores (b) podands (c) corands (e) lariat ethers (f) cryptands (g) calixarenes and (h) spherands. You may find it helpful to present the information in the form of a comparative table. 

Carcerand 6.97 was designed by Cram s group, just as they had designed the spherands, with the aid of CPK molecular models. In his Nobel Prize address, writing just two years after the preparation of 6.97, Cram describes his interest in the host-guest chemistry of this new capsule ... 

C. B. Trueblood, K. N. Host-guest complexa-tion. 32. Spherands composed of cyclic urea and anisyl units./. Am. Chem. Soe. 1984, 106, 7150-7167. 

Since the pioneering work of Pedersen (1), Lehn (2), and Cram (3) on synthetic macrocyclic and macropolycyclic host systems such as the crown ethers, cryptands, and spherands, there has been an enormous development of the field of host-guest or supramolecular chemistry. Molecular hosts designed to bind inorganic and organic, charged and neutral guest species via cumulative, noncovalent interactions have all been reported and extensive reviews on this subject have appeared (4-8). 

Chromoionophores of crown 4 and spherand cyclophane-type 5 with 2,4-dinitrophenylazophenol groups as a chromophore were also designed [7 a]. They show marked color changes from yellow into blue on complexation with cations in ethanol and chloroform solutions. This chapter describes selective complexation and coloration of azophenol hosts or fluorescent emission of benzothiazolyl crowns with alkaline metal ions and alkyl amines. 

In order to develop hosts with a much higher level of lithium selectivity, we planned a macrocyclic ligand which is of a capsular type, has a rigid structure as well as the function of concomitant coloration on complexation. An azophenol biphenylophane 5, which has a rigid cavity to accommodate only the lithium ion in the center, was designed by considering the fact that Cram s spherand 24 [14] can reject multivalent metal ions perfectly. 

It should be noted that the intriguing stereochemical properties of these spherand calixarenes are not yet entirely understood and hence, their potential as chiral host molecules is not yet fully developed. 

D. J. Cram and G. M. Lein, Host-guest complexation. 36. Spherand and litium and sodium complexation rates equilibria, J. Am. Chem. Soc. 107 (1985), 3657. 

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