Macrocyclic polyethers-crown ethers

Due to their pronounced selectivity in metal ion ccmplexation (6), crown ethers (macrocyclic polyethers) and related macrocyclic multidentate ligands are attractive mobile carriers for metal ion transport across liquid membranes. As summarized in recent reviews of macrocycle-facil itated transport of ions in liquid membrane systems (7,8), most studies have been conducted with macrocyclic carriers which do not possess ionizable groups. For such carriers, metal ions can only be transported down their concentration gradients unless some type of auxiliary complexing agent is present in the receiving aqueous phase. 

Artificial ionophores have been known since 1967 when C.J. Pedersen published his crown ethers , macrocyclic polyethers of any desired number of members, built according to the general principle -[—O—CH—CH— ] - ... 

Crown ether Macrocyclic polyether so called because of its appearance in molecular models. 

Polyoxometalates also play an important role in the selection of a metal ion for its complete encapsulation in the cavity of a crown ether to form an unusual supramolecular cation structure. For example, the crown ethers (macrocyclic polyethers), generally, do not readily form complexes with first-row transition metals in their low oxidation states because such metal ions provide only soft coordination (acceptor) sites and crown ethers have hard donor atoms. Naturally, only a few first-row transition metal rown ether complexes had been structurally characterized in which a direct bond formation between a transition metal and the crown ether oxygen atoms became possible rare examples of this kind are offered by the smaller ring crown ethers (e.g., 15-crown-5 and... 

Crown Ethers, Crown compounds. Macrocyclic polyethers with the repeating unit (-CH,-CH2-0-), where n is greater than 2. Crown compounds with other heteroatoms (N, S, P) are known. Prepd by C. J. Pedersen,... 

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

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. 

Crown ethers are cyclic polyethers designated [n]crown-m where n is the ring size and m the number of oxygen atoms, for instance [18] crown-6 1. They show a high affinity for cationic guest molecules, especially alkali metal cations, where the cation is commonly complexed within the cavity of macrocycle or sand-... 

Additional macrocyclic polyether groups show no co-operative effect when the polyether rings diverge. For instance, the bis(crown ethers) [58] and [59]... 

Anionic units have not only been attached to macrocyclic polyethers via flexible arms but have also been incorporated into the cycle itself. Alberts and Cram (1976, 1977) have studied the ion-binding properties of crown ethers containing / -diketone units, such as [72] and [73]. 

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... 

Complexes of alcohols like methanol, ethanol, 2-propanol and n-butanol (116-122), and ethers like Diox (47,120,123-125) and THF (126-128) have been prepared. The bonding between these ligands and the metal ions is considered to be very weak. In recent years, complexes of the lanthanides with a few macrocyclic polyethers have been reported. Cassol et al. (129) have prepared the complexes of benzo-15-crown-5 and dibenzo-18-crown-6 with lanthanide nitrates and isothiocyanates. King and Heckley (130) have also reported the complexes of these ligands with lanthanide nitrates. The heavier lanthanide nitrate complexes of dibenzo-18-crown-6... 

Crown-ethers are macrocyclic polyethers capable of forming host-guest complexes, especially with inorganic and organic cations. Modification of the crown-ether by the introduction of four carboxylic groups makes it possible to use this class of compounds as chiral selectors in CE. ... 

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. 

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... 

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. 

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