Some representative macrocyclic systems

Based on donor atom type, macrocyclic ligands can be considered to span two extreme types. First there are those systems which chiefly contain nitrogen, sulfur, phosphorus, and/or arsenic donors. These macrocycles tend to have considerable affinity for transition and other heavy metal ions they usually show much less tendency to form stable complexes with ions of the alkali and alkaline earth metals. The present discussion will be restricted to a consideration of a selection of such ligands and their complexes. 

The second ligand type consists of a large group of cyclic compounds incorporating numbers of ether functions as donors. Structure (22) illustrates a typical example. Such crown polyethers usually show strong complexing ability towards alkali and alkaline earth ions but their tendency to coordinate to transition metal ions is less than for the above 

As background to the material described in the chapters which follow, it is appropriate at this point to introduce a few further examples of particular (monocyclic) ring types. When taken together with the macrocycles described so far, these examples enable some perspective to be gained of the structural diversity for simple rings which now exists. Nevertheless, it is emphasized that collectively these systems represent only a very minor selection from the large number of such rings now reported. 

All N-donor systems. The N3-donor macrocycle (23) has ten atoms in its macrocyclic ring and is too small to completely encircle a metal ion 

What is different about macrocyclic ligand complexes  

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Some representative systems. A further type of binuclear species incorporates two metal ions bridged by one or two small groups within the macrocyclic ring. Such complexes differ from the previous type in that the bridges between the metal ions do not involve donor groups which are directly attached to the macrocycle - see (122) and (123). A variety of... 

Some representative examples. Many crown macrocycles incorporating other heteroatom types besides ether oxygens have been synthesized. In an early preparation of this type, Lehn et al. reported a synthesis for the 04N2-system (175, diaza-18-crown-6) usingthe procedure outlined by [4.5] (Dietrich, Lehn Sauvage, 1969). 

Table 4 lists some representative examples of macrocyclization via the intramolecular SNAr reaction. In addition to the 14- and 17-membered cycloisodityrosines shown in the table, a variety of mono-, bi-, and tricyclic systems from the vancomycin family of natural products including the orienticin C/40 vancomycin J44-47 and teicoplanin[48 aglycons have been prepared by this method.This versatility, coupled with the high yields obtained, makes the intramolecular SNAr reaction currently the most widely applicable method for cycloisodi-tyrosine formation. 

A class of macrocyclic system that bears some structural similarity to the still-hypothetical [18]annulene triimino species 2.339 has recently been described by Black, et al. The first representative of this new family, the so-called calix [3]indole 2.376, was prepared in 12% yield via the POCl3-catalyzed condensation between indole 2.372 and benzaldehyde (2.154) (Scheme 2.3.24). The related symmetrical meso-ary systems 2.311-2.319 were prepared in analogous fashion in yields of 83%, 10%, and 81%, respectively. 

The field has been the subject of several reviews of the properties, structures, and stability of numerous ligands and their complexes, including both mixed donor open-chain and macrocyclic polydentates. " Detailed reports of coordination chemistry that focus on the metal include many examples of mixed donor ligand systems. The nature of this article requires that only limited references are cited herein as examples, but these include some key reviews readers may access original work through these reviews or else by structure searching from compounds represented in line drawings that appear here. 

Coordination number six appears, from the limited available data, the most common (Table 7). Lower coordination numbers are known but appear to be fairly rare three is represented by some silylamides (Section 41.3.3.1) and in a dehydrated zeolite (Section 41.3.5.2.i) four-planar is known only for macrocyclic [N,] systems and as [MnOJ in a few unusual crystalline compounds and four-tetrahedral is well characterized but still apparently less common than for Zn, Co and possibly even Ni. Coordination number five is also known, but few examples are yet characterized. On the other hand, higher coordination numbers seven and eight are less unusual here—hardly surprising in view of the larger size of Mn". Examples include seven coordinate [Mn(edta)H20] (Section 41.3.9), [Mn(py3tren)] (Section 41.3.3.6) and an [Mn07] species (Section 41.3.5.3.iii), and eight coordinate [Mn(04)2] species (Section 41.3.5.4). 

The preparation of (alk-l-ynyl)thiophenes 154 is possible via palladium-catalyzed Sonogashira cross-coupling reactions between iodo- or bromothiophenes 152 and terminal alkynes 153 (Scheme 60, Table 38) [309, 360, 363, 364], Alkynylated thiophenes represent an important structural motif found in many ir-electronic systems such as molecular rods or conjugated macrocycles [163, 362, 365, 366], In the case of bromoiodo-substituted thiophenes, the more reactive iodine atom is replaced selectively [367]. Some coupling methods require stoichiometric amotmts of paUadium(0) [360, 368], whereas other methods proceed successfully tmder photochemical conditions without the need of a catalyst [369]. Recent... 

In addition to these pyridine-containing helical macrocycles, there are numerous reports of polyaza-macrocyclic species that are chiral due to the adaptation of a non-planar conformation. Some current examples included the ruffled benzimidazole-based ligands and the distorted phthalocyanines recently prepared by the groups of Chan [40] and Kobayshi [41], respectively. These systems do not fall within the context of this Chapter due to their resemblance to porphyrinic systems but they nevertheless represent an interesting class of chiral macrocycles based upon the incorporation of heterocyclic rings. 

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