Phosphorus and Arsenic-containing Macrocycles

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). 

Quite a number of cyclic phosphonium salts have been prepared over the years. We have not generally included compounds here which contain fewer than three potential binding sites in a ring unless they are mentioned as by-products in another reaction. Horner, Kunz and Walach have utilized the well-known alkylation approach to prepare cyclic phosphonium salts containing four phosphorus atoms. The formation of the cyclic tetraphosphonium salts is shown below in Eq. (6.14). 

In the reactions characterized in the general sense above, phenyl and benzyl were the organic residues bound to phosphorus. It is not clear which of these groups is removed in the LAH dealkylation step. It should also be noted that as tetraquaternary phosphonium salts, these species do not qualify as crown analogs in the strictest sense because of the absence of lone pair electrons on phosphorus. Furthermore, the quaternary phosphorus can resist oxidation much better than secondary or tertiary phospho- 

The first polyphosphino maeroeyeles designed speeifieally for use as transition metal binders were reported in 1977 in back-to-baek eommunications by Rosen and Kyba and their eoworkers. The maeroeyeles reported in these papers were quite similar in some respeets, but the synthetic approaches were markedly different. DelDonno and Rosen began with bis-phosphinate 18. Treatment of the latter with Vitride reducing agent and phosphinate 19, led to the tris-phosphine,20. Formation of the nickel (II) complex of 20 followed by double alkylation (cyclization) and then removal of Ni by treatment of the complex with cyanide, led to 21 as illustrated in Eq. (6.15). The overall yield for this sequence is about 10%. 

Kyba and eoworkers prepared the similar, but not identical compound, 26, using quite a different approach. In this synthesis, pentaphenylcyclopentaphosphine (22) is converted into benzotriphosphole (23) by reduction with potassium metal in THF, followed by treatment with o t/20-dichlorobenzene. Lithium aluminum hydride reduction of 23 affords l,2-i /s(phenylphosphino)benzene, 24. The secondary phosphine may be deprotonated with n-butyllithium and alkylated with 3-chlorobromopropane. The twoarmed bis-phosphine (25) which results may be treated with the dianion of 24 at high dilution to yield macrocycle 26. The overall yield of 26 is about 4%. The synthetic approach is illustrated in Eq. (6.16), below. 

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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. 

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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