Spherands synthesis

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. 

Figure 7.2.8. The template synthesis leading to spherand after demetallation.

The synthesis of spherands involves ring closures using aryllithiums with Fe(acac)3. Yields are increased by high dilution techniques (equation 1 1).29 63... 

The synthesis of pyridine analog to the spherands has also been reported as shown for ligand (39).119,120 Corey-Pauling-Kotun (CPK) molecular models of cyclosexipyridine molecules indicate a cavity size appropriate for Rb+ and K+ incorporation.120... 

Trueblood, K. N., Knobler, C. B., Maverick, E., Helgeson, R. C., Brown, S. B., Cram, D. J., Spherands, the 1st ligand systems fully organized during synthesis rather than during complexation. J. Am. Chem. Soc. 1981,103, 5594-5596. 

Recently, Cram and his group reported the synthesis and chromogenic properties of the azophenol spherand 37 which is a chromogenic ion-selective indicating system capable of detecting Li+ and Na+ at concentrations as low as 10 8 M in the presence of other common ions [17]. 

The following procedure for the synthesis of spherand 1 by dimerization of 1,1 3, 1"-terphenyl 5 is representative for this type of spherand ring closure reaction. 

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]. 

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). 

Syntheses of Polycyclic Systems.- Cram and co-workers have published a series of elegant syntheses of spherands- polycyclic compounds in which the rigid cavity is formed during their synthesis rather than in the course of complexation with cations. The preparation of (88) illustrates the strategy of their method. The binding units (89), (90), (91), (92) and (93) are assigned the... 

The synthesis of larger rings has also been accomplished including the fully unsaturated azolo-fused 1,3-diazepines (222) from bis(iminophosphoranes) (221) and novel cyclic carbodiimides (e.g. 223). Iminophosphoranes are also intermediates in the synthesis of macrolactams (e.g. 225) from the reaction of co-azido acid anhydrides (e.g. 224) with tributylphosphine. The synthesis of iminophosphorane-containing cryptands and a spherand-type structure have been reported. 

Yamato. T. Yasumatsu, M. Samw atari. Y. Doamekpor, L.K. Synthesis and ion selectivity of macrocyclic metacyclophanes analogous to spherand-type calixarenes. J. Inclusion Phenom. Molec. Reeognit. Chem. 1994. 79,... 

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