PEDERSEN Crown Ether

PEDERSEN Crown Ethers Crown ether formation and its use in substitutions, oxidations,etc... 

C. J. Pedersen Crown ethers (complexes with planar, macrocyclic ligands)... 

PEDERSEN Crown ethers 292 PERKIN Carboxylc acid(ester) synthesis 293 PERKIN Coumann rearrangement 293 PERKOW Vinyl phosphate synthesis 294... 

Another type of phase-transfer catalysts is synthetic macro-cyclic polyethers, so-called cro m ethers, and poly(ethylene glycol) derivatives. Since the discovery of crown ethers and their complex-ing capabilities toward metal and ammonium ions in 1967 by Pedersen, crown ethers and modified compounds such as cryptates > have been attracting ever increasing interest among scientists having different applications in their mind. Complexes formed between these compounds and cations correspond to Q" " shown in Fig. 1 and possess phase-thransfer capacity for anions. The nature of cations is known to greatly influence the complexation with a given crown either or cryptate. ... 

The strength of this bonding depends on the kind of ether Simple ethers form relatively weak complexes with metal ions but Charles J Pedersen of Du Pont discovered that cer tain polyethers form much more stable complexes with metal ions than do simple ethers Pedersen prepared a series of macrocyclic polyethers cyclic compounds contain mg four or more oxygens m a ring of 12 or more atoms He called these compounds crown ethers, because their molecular models resemble crowns Systematic nomencla ture of crown ethers is somewhat cumbersome and so Pedersen devised a shorthand description whereby the word crown is preceded by the total number of atoms m the ring and is followed by the number of oxygen atoms... 

In 1967, DuPont chemist Charles J. Pedersen (21) discovered a class of ligands capable of complexing alkaU metal cations, a discovery which led to the Nobel Prize in Chemistry in 1987. These compounds, known as crown ethers or cryptands, allow gready enhanced solubiUty of sodium and other alkaU metals in amines and ethers. About 50 crown ethers having between 9—60 membered oligoether rings were described (22). Two such stmctures, dibenzo-18-crown-6 (1) and benzo-9-crown-3 (2), are shown. 

Extensive and important as Pedersen s efforts were, they might have been even greater had he not prepared the first examples of crown ethers when he was beyond sixty years of age. After giving birth to a remarkable child, he was unable to nurture it because of his retirement in 1969. 

Two of the most widely used crown ethers have been dibenzo-18-crown-6 and dicyclo-hexano-18-crown-6.(In older literature, the latter is often referred to as dicyclohexyl-18-crown-6 .) A major reason for this is that Pedersen reported complete details of the preparation of both compounds in Organic Syntheses in 1972. As a result, both compounds were readily prepared and available. 

Almost as soon as Pedersen announced his discovery of the crown ethers (see Chaps. 2 and 3) it was recognized by many that these species were similar to those prepared by Busch and coworkers for binding coinage and transition metals (see Sect. 2.1). The latter compounds contained all or a predominance of nitrogen and sulfur (see also Chap. 6) in accordance with their intended use. The crown ethers and the polyazamacrocycles represented two extremes in cation binding ability and preparation of the intermediate compounds quickly ensued. In the conceptual sense, monoazacrowns are the simplest variants of the macrocyclic polyethers and these will be discussed first. 

A number of bridged crown ethers have been prepared. Although the Simmons-Park in-out bicyclic amines (see Sect. 1.3.3) are the prototype, Lehn s cryptands (see Chap. 8) are probably better known. Intermediates between the cryptands (which Pedersen referred to as lanterns ) and the simple monoazacrowns are monoazacrowns bridged by a single hydrocarbon strand. Pedersen reports the synthesis of such a structure (see 7, below) which he referred to as a clam compound for the obvious reason . Although Pedersen appears not to have explored the binding properties of his clam in any detail, he did attempt to complex Na and Cs ions. A 0.0001 molar solution of the clam compound is prepared in ethanol. The metal ions Na and Cs are added to the clam-ethanol solutions as salts. Ultraviolet spectra of these solutions indicate that a small amount of the Na is complexed by the clam compound but none of the Cs . 

In Pedersen s early experiments, the relative binding of cations by crown ethers was assessed by extraction of alkali metal picrates into an organic phase. In these experiments, the crown ether served to draw into the organic phase a colored molecule which was ordinarily insoluble in this medium. An extension and elaboration of this notion has been developed by Dix and Vdgtle and Nakamura, Takagi, and Ueno In efforts by both of these groups, crown ether molecules were appended to chromophoric or colored residues. Ion-selective extraction and interaction with the crown and/or chromophore could produce changes in the absorption spectrum. Examples of molecules so constructed are illustrated below as 7 7 and 18 from refs. 32 and 131, respectively. 

Pedersen, Cram and Lehn received the Nobel Prize in Chemistry in 1987 for their work on synthetic macrocyclic compounds. In their Nobel lectures, Pedersen (1988) described the discovery of crown ethers, Cram (1988) and Lehn (1988) the further development of work on new synthetic macrocycles and their host-guest properties. 

The 1987 Nobel Prize in Chemistry was awarded to Charles J. Pedersen, Donald J. Cram, and Jean-Marie Lehn for their work on crown ethers and related compounds (host-guest chemisty). 

Since Pedersen s original work on the use of cations to template the formation of crown ethers [18-20], a large number of different templating agents for macro-cyclization reactions have been reported. While the initial work concentrated on the use of metal cations, further developments demonstrated that species with hydrogen bonding donor or acceptor properties could be equally useful to template the synthesis of macrocyclic molecules. 

The unique ability of crown ethers to form stable complexes with various cations has been used to advantage in such diverse processes as isotope separations (Jepson and De Witt, 1976), the transport of ions through artificial and natural membranes (Tosteson, 1968) and the construction of ion-selective electrodes (Ryba and Petranek, 1973). On account of their lipophilic exterior, crown ether complexes are often soluble even in apolar solvents. This property has been successfully exploited in liquid-liquid and solid-liquid phase-transfer reactions. Extensive reviews deal with the synthetic aspects of the use of crown ethers as phase-transfer catalysts (Gokel and Dupont Durst, 1976 Liotta, 1978 Weber and Gokel, 1977 Starks and Liotta, 1978). Several studies have been devoted to the identification of the factors affecting the formation and stability of crown-ether complexes, and many aspects of this subject have been discussed in reviews (Christensen et al., 1971, 1974 Pedersen and Frensdorf, 1972 Izatt et al., 1973 Kappenstein, 1974). 

Pedersen s (1967a) original model, in which the cation is captured in the cavity of the crown ether, has been fully substantiated by X-ray crystallographic studies (Truter, 1973 Dailey, 1978). On the basis of this model, the main features of cation binding can be explained surprisingly well. The size of the cavity can be estimated from CPK molecular models (Table 1). The binding of... 

The first examples of the application of phase-transfer catalysis (PTC) were described by Jarrousse in 1951 (1), but it was not until 1965 that Makosza developed many fundamental aspects of this technology (2,3). Starks characterized the mechanism and coined a name for it (4,5), whilst Brandstrom studied the use of stoichiometric amounts of quaternary ammonium salts in aprotic solvents, "ion-pair extraction" (6). In the meantime Pedersen and Lehn discovered crown-ethers (7-9) and cryptands (10,11), respectively. 

Since the accidental discovery (24) of dibenzo-18-crown-6, 3, by Pedersen (25), a large number of macrocyclic polyethers have been reported in the literature (1-23). However, the most extensively studied of the so-called crown ethers is the parent macrocycle 1,4,7,10,13,16-hexaoxacyclooctadecane or 18-crown-6, 1. It forms molecular complexes with a wide range of substrate species including... 

More than 50 macrocyclic crown ethers were synthesised by Pedersen, and many were found to solublise alkali metal salts in non-polar solvents. He isolated 1 1 complexes with metal salts (87) and also showed that if the cation was too large to fit in the central hole, complexes with ratios of 1 2 or 2 3 (metal ether) could be obtained (88). Some of the larger ethers have been shown to complex two metal atoms simultaneously (89). Stability constants in solution are affected by the nature of the anion and the solvent. Both are also important in obtaining crystalline products. 

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