Other Macrocyclic Systems

Other Macrocyclic Systems. The first synthesis of crown arsanes (58), which are air-stable, and which should prove useful in chelating transition metals, has been reported. The novel fourteen-membered rings (59) are of interest because of their potential in chelating metals. 

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Other macrocyclic systems have been prepared by crossed condensations between 1,3-diiminoisoindoline and other heterocyclic bases.120,122... 

A series of zinc(II) phthalocyanines with other sorts of solubilizing groups in exo or endo positions (carboxyalkyl, carboxyalkoxy, amino acid) has been synthesized and subjected to preliminary in vitro assays.257 Interestingly, the seryl derivative zinc(II) 2,9,16,23-tetrakis(l-car-boxy-2-hydroxyethylaminocarbonyl)phthalocyanine proved to be cytotoxic (i.e., toxic in the dark) which is not so commonly observed with macrocyclic systems. 

Since 1960, a very large number of other synthetic macrocycles has been prepared and this has resulted in a great increase in interest in all aspects of the chemistry of macrocyclic systems. From about this time there has also been enhanced interest in the role of metal ions in biological systems and many such bioinorganic studies have involved complexes of both natural and synthetic macrocycles. Thus there has been an element of cross-fertilization between these two developing areas, viz ... 

Other donor systems. Although not as common as all-nitrogen donor rings, macrocycles incorporating sulfur donor atoms have been widely reported. Structures (42)-(45) illustrate four representative examples of this type. 

Co-facial dimers consist of two simple macrocyclic systems connected by linkages such that the respective cyclic portions are constrained to stack one above the other. Such molecules define three cavities two within the macrocyclic subunits and a third between these subunits. 

In this chapter, the discussion has centred on the redox behaviour of the cyclic systems of a limited range of metal ions. Nevertheless, the examples are of sufficient breadth to illustrate that the redox behaviour of a particular system usually depends upon a fine balance between kinetic, thermodynamic and structural factors in which both the nature of the central metal ion and of the cyclic ligand are major influences. Of course, such considerations are not restricted to macrocyclic systems - however, the latter have provided convenient models for the elucidation of a variety of redox behaviour - much of which is of relevance to other areas and, for example, to many of the natural redox systems. 

Other applications of the polymer substrate technique include the synthesis of threaded macrocyclic systems (hooplanes, catenanes, knots), the retrieval of a minor component from a reaction system, and the trapping of reaction intermediates [Frechet, 1980a,b Hodge, 1988 Hodge and Sherrington, 1980 Mathur et al., 1980],... 

In the bisbenzylisoquinolines the size of the macrocyclic system affects the chemical shifts of some groupings of protons in a characteristic way. In certain conformations of the 21-membered ring in 5-7 ether linked systems the 2- and 6-substituents lie over aryl rings with a consequent shift to lower frequency. This is illustrated by the H NMR spectrum of thalictine [39] (38) which shows absorption for the 8-and 8 -protons to low frequency of the other aromatic protons. 

Hydrophobic forces are also important in the assemblies of metallo-supramolecular catenanes. One of the most interesting examples is formed when one of the unpolar bipyridine ligands of one macrocycle is included spontaneously in the other macrocycle s internal cavity [39]. Here, the benzene unit of the one macrocycle serves as a guest molecule for the other macrocycle, and the cyclization is favored by n-n interactions. In addition, the minimization of hydrophobic surfaces in polar medium constitutes the second driving force for the catenane formation. The quantitative formation of the [2]catenanes 31a and 31b based on this principle are depicted in Figure 13. Formation of catenane 31b was found to be reversible. Even at room temperature, two monomeric ring structures equilibrate quickly due to the labile nature of Pd-N bond and interlocked molecular ring system 31b is formed. 

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