Polypodands

Quaternary onium salts were the first phase-transfer catalysts used subsequently, a number of compounds (linear polyethers, polypodands, crown-ethers, cryptands, cage-compounds, etc.) were found effective for the anion activation in two-phase systems. These structurally different systems must satisfy at least two fundamental conditions in order to behave as phase-transfer catalysts i) solubility in the organic phase ii) steric hindrance around the cationic center leading to a good cation-anion separation within the ion-pair. 

The interaction of oxygen-containing acyclic ligands with alkali and alkaline earth metal cations has provided a burgeoning area of interest. In historic terms, this was preceded by the advent of crown ethers and the accompanying almost retrospective look at their acyclic precedents. This section is sub-divided into five parts simple chelates, metal complexes as ligands, podands, polypodands and sugars. 

Many polypodands having terminal donor groups are also available, e.g. (39)-(41).33 These also exhibit cation complexation and phase transfer properties. KMn04 and aqueous alkali metal picrates are much more readily taken into organic phases in the presence of these podands than with dibenzo-18-crown-6.239 The KSCN complex of (39) exhibits a novel coordination geometry as all 10 donor atoms participate in coordination of the metal cation, and in order to do this the three arms wrap around the cation in a propeller-like fashion.240... 

Cyclophosphazenic polypodands. Obtained as a mixture from the commercial Brij 30, these cheap and efficient catalysts are comparable to most of the common phase-transfer catalysts. 

Metal complexes as ligands Podands Polypodands Carbohydrates... 

In this case, in fact, the nucleophilicity sequence is reversed on switching from the complexes of crown ether 5 to the corresponding cryptates. Interestingly, the same trend of crown ether 5 is found with the corresponding complexes of cyclophosphazenic polypodand 2c. As shown in Table 3. cyclophosphazenic polypodands like 2c have a reactivity only slightly lower (three to four times) than that of the corresponding crown ethers and, hence, can be considered a valid alternative to the more sophisticated cyclic ligands. 

Lipophilic polyether ligands, like crown ethers, cryptands, polypodands. and quaternary onium salts, proved to be efficient activators of anions ( naked anions ) in low-polarity media. In these solvents, they give rise to ion pairs particularly reactive due to the low stabilization by the solvent and the low interaction with the bulky cation. The reactivity of these ion pairs is even higher than that... 

In 1977, it was found that the attachment of additional ligand atoms to linear polyethers resulted in a large enhancement of complex stabilities These polypodands exhibit a cryptand-like behavior also in their phase-transfer properties. 

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