Arsenic-containing macrocycles

Template procedures have not been employed for synthesis of arsenic-containing macrocycles until recently. A number of compounds of this type, including L1237-L1239, were obtained by cyclisation reactions between arsenic-containing compounds and dihalogen derivatives as shown in Scheme 5-7 [22, 23]. 

There are classes of natural products, such as erythromycins (14-membered ring), which contain macrocycles as building blocks and yet which still preserve conformational restrictions. Thus, it is possible to assign configurations using the arsenal of NMR methods. An example is the determination of the stereochemical position of a hydroxy group at C-9 in macrocycle 2 by a comparison of <5( H) and 3/HH values64. ... 

As for phosphine macrocycles, arsine macrocycles involve very difficult synthetic chemistry. The reasons for the difficulties are essentially threefold (i) the volatility and well-known toxicity of the arsenic-containing precursors (ii) the instability of coordinated As—II functions, which renders template cyclizations of limited use for arsenic macrocycles (iii) the high energy barrier to inversion at arsenic (167.4kJmol 1),41 which means that high-dilution cyclizations of polydentate arsine macrocycles leads to mixtures of stereoisomers which are not at all trivial to separate. On the other hand, the macrocyclic (tertiary) arsines are less susceptible to oxidation than their phosphine analogs. 

Although most of the macrocycles that contain phosphorus or arsenic which have thus far been prepared, are primarily transition metals binders, two compounds have been prepared which are essentially crown ethers containing phosphorus. Kudrya, Shtepanek and Kirsanovhave prepared two compounds which are essentially polyoxygen macrocycles but which contain one or two methylphosphonic acid esters as part of the ring. These two macrocycles are shown below as 7d and 17 and are both prepared by the reaction of 2,2 [oxybis(ethyleneoxy)] bisphenolate with methylphosphonic dichloride in a mixture of acetonitrile and benzene. The crystalline monomer 16) and dimer 17) were isolated in 17% and 11% yields respectively as indicated in Eq. (6.13). 

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. 

As well as sulfur, macrocycles containing other large donors such as tertiary phosphorus or arsenic atoms are also known, although the metal-ion chemistry of such ligands has been somewhat less explored. In part, this reflects the synthetic difficulties often encountered in the preparation of ligands containing these heteroatoms structures (49) (Horner, Walach Kunz, 1978), (50) (Kauffmann Ennen, 1981), and (51) (Mealli etal., 1985) provide three representative examples of such macrocycles. 

Roesky and co-workers unexpectedly isolated the unique 14 membered macrocycle 38 containing arsenic, carbon, oxygen, and nitrogen atoms in the skeletal framework from reaction of arsenic(III) cyanide and hexafluoroacetone (Fig. 6.4). The structure of 38 was confirmed by single-crystal X-ray. 

D. AZA-CROWN MACROCYCLES CONTAINING ARSENIC AND SILICON ATOMS IN THE MACRORING... 

Tables listing the known aza-crown macrocycles containing sulfur, phosphorus, arsenic, and silicon atoms in the macroring are presented hereafter. Insofar as possible, tables are ordered by the number of heteroatoms and complexity of the substituents.

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