Macrocycles spherand

Lariat ethers crown ethers with pendant chains Sepulchrates bicyclic caged macrocycles Spherands macrocycles with phenyl backbones... 

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. 

Spherands, hemispherands, and other similar macrocycles capable of inclusion complex formation [33]... 

Spherands (10) and hemispherands (11) are macrocyclic ligands which consist of arrangements of phenyl groups. 

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]. 

AC and AEC complexation is also effected efficiently by other macrocyclic ligands such as the spherands 13, cryptospherands 14 [2.9, 2.10], calixarenes [2.38, A.6, A.23], torands [2.39], etc., some of them, for instance the spherands displaying particularly high stabilities. A special case is represented by the endohedral complexes of fullerenes in which the cation (Sr2, Ba2+, lanthanides) is locked inside the closed carbon framework [2.40],... 

An additional nuance in the nomenclature of these compounds concerns their complexes. The open-chained compounds are often referred to as podands and their complexes as podates. The cyclic ethers may also be called coronands and their complexes are therefore coronates. Complexed cryptands are cryptates. The even more complicated structures known as spherands, cavitands, or carcerands are called spherates, cavitates, or carcerates, respectively, when complexed. The combination of a macrocycle (crown ether or coro-nand) and a sidechain (podand) is typically called a lariat ether. 

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). 

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. 

The three structures shown at the right of the second line represent important families of receptor molecules that have been extensively studied in their own right. The aromatic spherand grew out of crown ether chemistry while calixarene chemistry developed on a separate evolutionary pathway. Still, their similarity is apparent. The structures of calixarenes and resorcarenes (also resorcinarenes) are included here because they have often been fused to macrocycles to form complex receptor systems. A few examples of fused receptor systems are discussed below. 

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... 

Fig. 6-2. Schematic representation of a ferrocene-containing macrocycle in which the ferrocene moiety is attached to a cryptand, spherand or cavitand. Fc = 1-substituted or 1,1 -disubstituted ferrocene jc = O, S, or NR m,n,p = 1,2, or 3 y = number of ferrocene units attached, usually 1 to 4.

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. 

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). 

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. 

Although imines have generally not been included in the Chapter, there, however, needs to be one exception. Majoral and his coworkers utilized the previously unknown thioxo6 (l-methyl-hydrazino)phosphoranyl azide in the preparation of macrocycles, cryptands and spherands, such as 16. All compounds are isolated in a pure form and are colorless to yellow stable solids. 

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