Ethers polyether macrocycles

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... 

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. ... 

The macrocyclization reaction described above has been used to generate a great number of catenane (12) and rotaxane (13) architectures (Scheme 10.4) using both crown ethers [preformed macrocyclic components 14 (strategy A)] and hydroqui-none-based dumbbell-shaped polyethers [preformed acyclic components 15 ( clipping )] as templates [14b, 15]. These templates are also relatively robust with regard to the substitution of different groups into both the tetracationic cyclophane and the neutral frameworks. 

The lariat ethers comprise a snbset of the polyether macrocycles, and are identified by their pendant chains. They can be categorized as either N-pivot (19) or C-pivot (20), depending on which type of atom the chain is attached. As for their polyether parents, much of the focus on these macrocycles has been on complexation of alkali and alkaline earth metal ions. 

Polyether macrocycles (18) are the simplest of the polyoxa macrocycles. The commonly used name for these macrocycles is the crown ethers, due to their crown-like structure in the solid state. These molecules have been extensively studied... 

Crown Ethers and Related Hosts - The First Class of Artificial Hosts Crown ethers are macrocyclic polyethers with crown-like shapes. Various cations are selectively bound to the crown ether, depending on the size of the macrocyclic ring. More precise recognition can be accomphshed using modified crown ethers such as lariat ethers and cryptands. 

Aryl-ether-based dendrons were used as stoppers in a series of rotaxanes containing one to three bipyridinium units surrounded by polyether macrocycles (compounds 74-76) [150], Electrochemical investigation in acetonitrile shows that a first reversible cathodic wave is due to the bipyridinium units, which are all reduced simultaneously and independently at the same potential. Electrode adsorption problems prevented accurate measurements for a second reduction process, also due to the bipyridinium units. On oxidation, two not fully reversible processes are observed, involving both the polyether macrocycle surrounding the bipyridinium units and the dioxybenzene units of the dendritic stoppers. 

S.8.2.2. Crown Ether and Phase Transfer Catalysts Particularly striking examples of the effect of specific solvation occur with the crown ethers. These macrocyclic polyethers specifically solvate cations such as Na+ and K+. 

Lastly, a convenient fcw-crown ether derivative of triaminotriazine, prepared from a phthalic acid derivative of a polyether macrocycle, would make an excellent reagent in the preparation of dendrimers desirous of surface crown ether(s) <97BCSJ671>. 

Functionalized polyether macrocycles (e.g., 1) were prepared (<60%) by the Rh(ll)-catalyzed decomposition of methyl diazoacetoacetate in the presence of substituted or unsaturated 5- and 6-membered cyclic ethers (13ASC3161). A supramolecular organogel was prepared by mixing two different glycidyl... 

The synthesis of macrocycles is very important due to their versatile applications in medicinal and naturtil product chemistry [123]. The rhodium-catalyzed one-pot synthesis of functionalized polyether macrocycles, at high concentrations from four components, was achieved under mild conditions by Lacour et aL the synthesis of 16- to 18-membered macrocycles by the regioselective condensation of two a-ditizo-p-ketoesters, and two cyclic ethers, in the presence of RhjlOct) is depicted in Scheme 9.11 [124]. Various diazo esters have been successfully employed as substrates with up to 75% yields. 

The combination of reactions of rhodium carbenoids with polyether-macrocycle synthesis offered interesting procednres for the synthesis of this important class of compounds. One elegant example is the Rh-catalyzed four-component reaction of two a-diazo- 3-keto esters and two cyclic ethers, such as tetrahydrofuran or 1,4-dioxane, to yield functionalized 16- to 18-membered macrocycles 65 (Scheme 5.44) [42]. The process involves the generation of electrophilic rhodium carbenoid A, the addition of cyclic ether to this intermediate, as well as the formation and dimerization of the oxonium ylide intermediate B. Another example is the Rh-catalyzed macrocyclization of oxetanes with a-diazocarbonyls (Scheme 5.45) [43]. In this case, three oxetanes and one rhodium carbenoid intermediate condense in a one-step process. It is noteworthy that these macrocyclizations could proceed under high-concentration conditions (1M). 

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. 

Macrocyclic polyethers containing the 2.2-paracyclophane unit are interesting structures and several such compounds have been prepared . Despite the diverse structural possibilities, the syntheses of these molecules have generally been accomplished by straightforward Williamson ether syntheses. The only unusual aspect of the syntheses appears to be a novel approach to certain paracyclophanes developed by Helgeson (see footnote 7a in Ref. 91). The first step of Eq. (3.28) illustrates the formation of the required tetrol, which is then treated with base (KOH or KO-t-Bu) and the appropriate diol dito-sylate to afford the macrocycle. 

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. 

Markies et al. have studied the coordination of zinc to cyclic and acyclic polyether ligands. In some cases the zinc is not coordinated within the ligand cavity but only through two of the ether donors in the macrocyclic ring.360 The bis(methoxyethyl)ether complex of zinc diphenyl (41) is five-coordinate with three ether oxygens coordinating from the acyclic ligand.361... 

Further comments. The preceding discussion outlines typical syntheses for simple polyether crown rings. It needs to be noted that a considerable number of other types of crown derivatives, displaying a variety of molecular architectures, has also been synthesized. Many of these types parallel the structurally developed macrocycles (which incorporate mainly donor atom types other than ether oxygen) discussed in Chapter... 

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