Crown ethers anion activation using

The influence of the length of the spacer on the activity of supported crown ethers and cryptands used as catalysts in anion promoted phase transfer reactions has been studied by Montanari. There is an increase in activity approaching that of the small analogue catalysts when they are bonded with a long chain to the polymeric support. 

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use. 

The proportion of the /rans-O-alkylated product [101] increases in the order no ligand < 18-crown-6 < [2.2.2]-cryptand. This difference was attributed to the fact that the enolate anion in a crown-ether complex is still capable of interacting with the cation, which stabilizes conformation [96]. For the cryptate, however, cation-anion interactions are less likely and electrostatic repulsion will force the anion to adopt conformation [99], which is the same as that of the free anion in DMSO. This explanation was substantiated by the fact that the anion was found to have structure [96] in the solid state of the potassium acetoacetate complex of 18-crown-6 (Cambillau et al., 1978). Using 23Na NMR, Cornelis et al. (1978) have recently concluded that the active nucleophilic species is the ion pair formed between 18-crown-6 and sodium ethyl acetoacetate, in which Na+ is co-ordinated to both the anion and the ligand. 

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. 

Both quaternary onium salts and cation complexes of lipophilic multidentate ligands (crown-ethers and cryptands) have been used as catalysts in two-phase systems in the presence of base (OH, F, etc.). However, under these conditions, the lack of chemical stability of quaternary salts and the very low complexation constants of multidentate ligands (especially crown-ethers) make all these systems barely effective in the activation of such anions. 

For adequate reaction rates, a high concentration of iodide anion is necessary. The cation portion of the salt appears to have little or no effect on catalytic activity or reaction selectivity. Inorganic iodides (such as potassium iodide) are the obvious first choice based on availability and cost. Unfortunately these catalysts have very poor solubility in the reaction mixture without added solubilizers or polar, aprotic solvents. These solubilizers (e.g., crown ethers) and solvents are not compatible with the desired catalyst recovery system using an alkane solvent. Quaternary onium iodides however combine the best properties of solubility and reactivity. 

The solvents used in these rhodium-catalyzed reactions may also act as complexing agents for counterions of the anionic rhodium complexes. For example, tetraglyme is known to coordinate alkali metal cations. Such solvation decreases the possibility of the cation interacting with the anionic rhodium catalyst and lowering its activity or solubility. The crown ethers, such as [18]-crown-6... 

Competition between mono- and di-valent cations has an important role in biological processes. Furthermore, the lipophilicity of a ligand and its complex plays an important role in deciding whether a species is soluble in organic media of low polarity. This has important consequences in areas such as phase-transfer catalysis, the use of crown ethers as anion activators, and in cation transport through lipid membranes. Many crown ethers have now been synthesized with incorporation of long alkyl side chains and enhanced lipophilicity and used successfully in the above areas. 

Since the discovery in 1964 that the antibiotic valinomydn exhibited alkali cation specificity in rat liver mitochondria, a new area of research has developed, based not only on biological systems but also on model systems such as crown ethers.484 The ability of neutral compounds to form lipid-soluble alkali and alkaline earth complexes was observed in 1951. The structure of the corresponding ligand, the anion of the antibiotic nigericin (78), was characterized as its silver salt in 1968.488 486 Silver was used as a heavy atom crystaUographically, since the Ag+ cation had a radius between that of Na+ and K+, which were the two alkali cations with which nigericin was most active. 

Chemical applications. The formation of AC and AEC crown ether complexes and cryptates promotes the solubilization of salts in organic media and has three major effects, decreased cation/anion interaction, cation protection and anion activation. These are usually more pronounced for the cryptates and have numerous uses in pure and applied chemistry. 

The versatile solubility properties of the crown ethers and cryptands are important in two of their major applications, phase transfer catalysis and anion activation. Phase transfer catalysis involves the transport of guest species from one phase to another. The two phases in question are usually two immiscible liquids (liquid-liquid phase transport). In practice, this usually means the use of a... 

The use of crown ethers and cryptands in phase transfer catalysis and anion activation are illustrated in the following case studies. 

---