Crown ether cryptand, and polyethylene glycol

It was a result of demand from industry in the mid-1960s for an alternative to be found for the expensive traditional synthetic procedures that led to the evolution of phase-transfer catalysis in which hydrophilic anions could be transferred into an organic medium. Several phase-transfer catalysts are available quaternary ammonium, phosphonium and arsonium salts, crown ethers, cryptands and polyethylene glycols. Of these, the quaternary ammonium salts are the most versatile and, compared with the crown ethers, which have many applications, they have the advantage of being relatively cheap, stable and non-toxic [1, 2]. Additionally, comparisons of the efficiencies of the various catalysts have shown that the ammonium salts are superior to the crown ethers and polyethylene glycols and comparable with the cryptands [e.g. 3, 4], which have fewer proven applications and require higher... 

The advantages of PTC reactions are moderate reaction conditions, practically no formation of by-products, a simple work-up procedure (the organic product is exclusively found in the organic phase), and the use of inexpensive solvents without a need for anhydrous reaction conditions. PTC reactions have been widely adopted, including in industrial processes, for substitution, displacement, condensation, oxidation and reduction, as well as polymerization reactions. The application of chiral ammonium salts such as A-(9-anthracenylmethyl)cinchonium and -cinchonidinium salts as PT catalysts even allows enantioselective alkylation reactions with ee values up to 80-90% see reference [883] for a review. Crown ethers, cryptands, and polyethylene glycol (PEG) dialkyl ethers have also been used as PT catalysts, particularly for solid-liquid PTC reactions cf. Eqs. (5-127) to (5-130) in Section 5.5.4. 

Polymer-supported crown ethers, cryptands, and polyethylene glycols in organic synthesis 87MI43. 

PT catalysts commonly used are quaternary onium salts (ammonium and phospho-nium), crown ethers, cryptands, and polyethylene glycols. The essential characteristics of a PT catalyst are that the catalyst must have the ability to transfer the reactive anion into the organic phase to conduct the nucleophilic attack on the organic substrate, and effect a cation-anion bonding loose enough to allow a high reaction rate in the organic phase. 

Quaternary salts, crown ethers, cryptands, and polyethylene glycol (PEG) are the most common agents used for LLPTC. Over the last few decades, the two reaction mechanisms used to describe the phenomenon of a two-phase PTC reaction were the Starks extraction mechanism and M kosza interfacial mechanism. 

Crown ether, cryptand, and poly(ethylene glycol) catalysts are more stable in base than the quaternary ammonium and phosphonium ions. Only the polyethylene glycols) are likely to meet industrial requirements for low cost, although a number of more efficient, lower cost crown ether syntheses have appeared recently, such as those of sila-crowns 64 bound to silica1B9). 

Transition metal cations can be made organically soluble by complexation with a crown ether, a polyethylene glycol) or its dimethyl ether (an open crown) or tris(3,6-dioxaheptyl)amine (TDA-1, an open cryptand, 1). TDA-1 is very hydrophilic and is most useful for the solubilization of solid salts. On the other hand, it also forms complexes with some metal carbonyls. Alternatively, a very lipophilic anion (for instance stearate) can make a salt organic. Finally, some other special ligand (e.g., a bipyridine-N,N -dioxide derivative) can be used. In all these cases positively charged species are brought into the organic phase for reaction. 

Quaternary ammonium (3) and phosphonium ions (61), crown ethers such as (62), cryptands such as (63) and poly(ethylene glycol) ethers (64) bound to PS are catalysts for reactions of water insoluble organic compounds with organic insoluble inorganic salts. " Silica gel, alumina, polystyrene-polypropylene composite fibers, nylon capsule membranes, and polyethylene (Mn 1000-3000) have also been used as supports. The reactions are called phase-transfer-catalyzed because one or both of the reactants are transported from the normal liquid or solid phase into a polymer phase, where the reaction proceeds. 

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