Crown ethers Macrocyclic polyethers that

Crown ethers are crown-shaped macrocyclic polyethers that form complexes with positive Ions. 

Crown ethers are syntheHc macrocyclic polyethers that can form selective complexes with various cations. Anl8-crown-6 ether is illustrated in Fig. 8, where 18 indicates the total number of atoms in the polyether ring and 6 the number of oxygen atoms. Chiral crown ethers have been synthesized by... 

Casnati and co-workers have devised a calixarene crown ether (7.17) that shows a preference for potassium over sodium of 22,000.85 A macrocyclic polyether with attached 8-hydroxyquinoline groups (7.18) favors barium over other alkaline earth cations by a factor of more than 10 million.86 Compound (7.19) shows a preference of magnesium over calcium of 590.87 Compound (7.20) has the highest known binding constant for Ag(I), log K 19.6.88... 

Particularly striking examples of the effect of specific solvation can be cited from the study of the crown ethers. These are macrocyclic polyethers that have the property of specifically solvating cations such as Na" and K". For example, in the presence of 18-crown-6, potassium fluoride is soluble in benzene or acetonitrile and acts as a reactive nucleophile ... 

Crown ethers are synthetic macrocyclic polyethers that can form selective complexes with various cations. Chiral crown ethers such as (diphenyl-substituted l,l -binaphthyl) crown ether or (-Fill 8-crown-6)-2,3,11,12-tetracarboxylic acid, which are bound to silica gels or coated on reversed-phase materials, were utilized for the enantioseparation of underivatized primary amino acids and their esters. On the other hand, the enantiomers of underivatized and derivatized amino acids enter into... 

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... 

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules. 

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 series of polyether macrocycles [59]—[66] (Fig. 33) that contain a coordinated reducible, redox-active 16-electron molybdenum nitrosyl (Mo(NO)(3+ group have been prepared (Al-Obaidi et al, 1986 Beer et al., 1987). Compounds [59]—[63] were synthesized from the reactions between [Mo(NO)LX2] (L = tris(3,5-dimethylpyrazolyl)hydroborate X = Cl or I ) and the appropriate amine substituted benzo-crown ether. Compounds... 

With the gift of hindsight, it is obvious that the majority of synthetic chemists would turn to macrocyclic compounds in the first instance for a ready source of molecular receptors. Indeed, this is exactly what has happened, although the most popular line (that concerned with macrocyclic polyethers) to date was to come on the scene serendipitously at just the right time. To their credit, synthetic chemists grasped the opportunity and wasted no time in developing the chemistiy of the crown ethers. 

Chirality derived from the readily accessible a-amino acids has been incorporated into the side chains of aza and diaza macrocyclic polyethers. A number of procedures suitable for peptide synthesis have proved (178) to be unsuitable for acylating the relatively unreactive secondary amine groups of aza crown ethers. Eventually, it was discovered that mixed anhydrides of diphenylphos-phinic acid and alkoxycarbonyl-L-alanine derivatives do yield amides, which can be reduced to the corresponding amines, e.g., l-172. By contrast, the corresponding bisamides of diaza-15-crown-S derivatives could not be reduced and so an alternative approach, involving the use of chiral A-chloroacetamido alcohols derived from a-amino acids, has been employed (178) in the synthesis of chiral receptors, such as ll-173 to ll-175, based on this constitution. 

As stated by Pedersen [1] at the very beginning, systematic names of polyethers are too cumbersome for repeated use. Thus, he coined an efficient system of their trivial names that is in common use. The polyethers were dubbed crown ethers or crowns . The total number of atoms in the macrocycle preceded this term whilst the number ofoxygen atoms followed it. The presence ofaromatic 44 or saturated 60 rings in the systems is marked at the beginning. In Lehn s system for bicyclic molecules called cryptands the number of OCH2CH2 units in each branch ofthe bicyclic hosts 61-63 ([1.1.1]-, [2.1.1]- and [2.2.2]cryptands... 

Although some scattered examples of binding of alkali cations (AC) were known (see [2.13,2.14]) and earlier observations had suggested that polyethers interact with them [2.15], the coordination chemistry of alkali cations developed only in the last 30 years with the discovery of several types of more or less powerful and selective cyclic or acyclic ligands. Three main classes may be distinguished 1) natural macrocycles displaying antibiotic properties such as valinomycin or the enniatins [1.21-1.23] 2) synthetic macrocyclic polyethers, the crown ethers, and their numerous derivatives [1.24,1.25, 2.16, A.l, A.13, A.21], followed by the spherands [2.9, 2.10] 3) synthetic macropolycyclic ligands, the cryptands [1.26, 1.27, 2.17, A.l, A.13], followed by other types such as the cryptospherands [2.9, 2.10]. 

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