Spherands classes

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. 

Although some scattered examples of binding of alkali cations (AC) were known (see [2.13,2.14]) and earlier observations had suggested that polyethers interact with them [2.15], the coordination chemistry of alkali cations developed only in the last 30 years with the discovery of several types of more or less powerful and selective cyclic or acyclic ligands. Three main classes may be distinguished 1) natural macrocycles displaying antibiotic properties such as valinomycin or the enniatins [1.21-1.23] 2) synthetic macrocyclic polyethers, the crown ethers, and their numerous derivatives [1.24,1.25, 2.16, A.l, A.13, A.21], followed by the spherands [2.9, 2.10] 3) synthetic macropolycyclic ligands, the cryptands [1.26, 1.27, 2.17, A.l, A.13], followed by other types such as the cryptospherands [2.9, 2.10]. 

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. 

In general, there are two types of macrocycles or cryptands containing the ferrocene unit. The first is that in which the ferrocene is appended to either a macrocycle — represented schematically by Fig. 6-1, or to a cryptand, spherand or cavitand (Fig. 6-2). The second class of compounds has ferrocene incorporated... 

The facile condensation reaction between formaldehyde and phenols or their derivatives provides a major route into rigid macrocycles used in supramolecular chemistry. Calixarenes, the best-known class of phenol-derived macrocycles, are prepared this way, as are spherands and their relatives. Cyclotriveratrylene, however, is an excellent exemplar of the molecular basket type of ligand and has been known for the best part of a century. The basic cyclotriveratrylene synthesis is shown in Figure 3.1. The original procedure by Mrs. Gertrude Maud Robinson [1] has since been refined by others and many variations are known [2,3]. 

The preceding molecular baskets belong broadly to two classic classes of macrocycles namely the calixarenes and cycloveratrylenes. There are several other macrocycles with the potential to bind guests in an aromatic-rich cavity that are worthy of discussion. Of the vast amount of work that emanated from the Cram group, the rigid spherands and carcerands stand out. In addition there are the cyclophanes and Baeyer s early contribution to macrocyclic chemistry, resorcinar-enes and calixpyrroles. 

Another class of rr-spherands with a belt-shaped structure originates from the connection of 1,4-cyclohexanediylidene units to macrocycles of type 24. However, in contrast to the beltenes the rr-bonds are in the plane of the ring and not perpendicular to it. The most simple representative of this class (n = 2) is tricyclo[4.2.2.2 ]dodeca-l,5-diene (25) synthesized in 1981 by Wiberg et al. [23], but due to its small size it cannot be called a molecular belt. 

The ion-bearer coneept applies to cyclic and bicyclic molecules as well as to the open-chained counterparts deseribed above. Complexing agents such as crown ethers eomprise such an enormous class, however, that they are usually referred to by the more speeific eognomen. This tradition applies to lariat ethers, spherands. eryptands, carcerands, calixarenes, sepulehrands, and numerous other compounds that are dealt with separately in this Encyclopedia. Struetures exemplifying these various families are shown in Fig. 6. 

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. 

Spherand A class of cyclophanes that position heteroatoms—generally O-atoms—in a pseudospherical arrangement around a cationic (generally alkali metal)... 

The cation-binding potential of ligands can be due to the relief of electron-electron repulsion upon complexation. The class names Spherands have been suggested for ligands of this type, e.g. (11) and metallospherium salts for their complexes with metal salts. Ideal hosts of this new type describe an oxygen shell whose cavity can be occupied only by spherical entities, such as single atoms or monoatomic ions, and not by parts of the hosts or by solvents. ... 

Since the discovery of crown ethers by Pedersen there have been many attempts to design and synthesize hosts whose ion affinities and selectivities surpass those of the original cyclic polyethers. Particularly important examples are the cryptands and spherands. These polycyclic receptors form stronger complexes and are generally more selective than crown ethers, however their complexes equilibrate more slowly as a consequence of their more rigid, encapsulating structures. This report is focused on the development of a new class of macrocyclic hosts, the torands, whose rigid toroidal structures should permit rapid equilibration of complexes. 

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