Crown ethers group 16 compounds

Related to the crown ethers are compounds, such as hexamethyl-[14]-4,ll-diene (6), which differ by the replacement of one or more of the oxygen atoms by other kinds of donor atoms, particularly N or S. MacrocycHc amine and thioether compounds have been synthesized. Compounds having more than one kind of heteroatom in the ring are called mixed-donor macrocycles. The naturally occurring metaboUtes nonactin [6833-84-7] and monactin [7182-54-9] have both ether and ester groups incorporated in the macrocyclic stmcture. 

However, the chemical reduction reaction does not work efficiently for the removal of two Bingel addends, whereas the electrochemical reduction process works very efficiently. One of the most interesting examples that illustrates the difference between the chemical and electrochemical methodologies involves the cyclophane type trans-1 ( -37) conjugate [76], The chemical reductive protocol fails completely with this compound, presumably due to ion complexation by the crown ether group and consequent stabilization. When the electroreductive method was used, a clean retrocyclopropanation reaction was observed (Figure 25). 

Replacement of cyano by hydrogen in a photosubstitution reaction is not as frequently encountered as the reverse process, but the dicyanopyrazine (84), and a related compound with a crown ether group attached to the phenyl ring, undergoes an efficient photoreaction in which first one cyano group is replaced and... 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

The unique chemical behavior of KO2 is a result of its dual character as a radical anion and a strong oxidizing agent (68). The reactivity and solubiHty of KO2 is gready enhanced by a crown ether (69). Its usefiilness in furnishing oxygen anions is demonstrated by its appHcations in SN2-type reactions to displace methanesulfonate and bromine groups (70,71), the oxidation of benzyHc methylene compounds to ketones (72), and the syntheses of a-hydroxyketones from ketones (73). 

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. 

Chelation itself is sometimes useful in directing the course of synthesis. This is called the template effect (37). The presence of a suitable metal ion facihtates the preparation of the crown ethers, porphyrins, and similar heteroatom macrocycHc compounds. Coordination of the heteroatoms about the metal orients the end groups of the reactants for ring closure. The product is the chelate from which the metal may be removed by a suitable method. In other catalytic effects, reactive centers may be brought into close proximity, charge or bond strain effects may be created, or electron transfers may be made possible. 

Sodium acetate reacts with /p-nitrophenyl benzoates to give mixed anhydrides if the reaction is conducted in a polar aprotic solvent in the presence of a crown ether. The reaction is strongly accelerated by quartemary nitrogen groups substituted at the orthc position. Explain the basis for the enhanced reactivity of these compounds. 

Both of these structures are open-chained compounds corresponding to crown ethers in function if not exactly in structure (see Chap. 7). They have repeating ethyleneoxy side-chains generally terminated in a methyl group. Montanari and co-workers introduced the polypodes 22 as phase transfer catalysts . These compounds were based on the triazine nucleus as illustrated below. The first octopus molecule (23) was prepared by Vogtle and Weber and is shown below. The implication of the name is that the compound is multiarmed and not specifically that it has eight such side-chains. Related molecules have recently been prepared by Hyatt and the name octopus adopted. For further information on this group of compounds and for examples of structures, refer to the discussion and tables in Chap. 7. 

The name lariat ethers has recently been suggested by Gokel and coworkers to describe a class of compounds which are based on crown ethers and have secondary donor groups bound to the macrormg by flexible arms (see Sect. 1.3 5). The name lariat is suggested by the analogy to the lassoes used to rope and tie animals since it is hoped that the more successful molecules of this class will be able to do likewise with sundry cationic species. 

So many different crown ether systems have been prepared over the recent decade that it sometimes seems that any of them could be placed in a miscellaneous category. On the other hand, each has its interesting features and probably merits a separate section for adequate discussion. Because both of these criteria cannot be met simultaneously, we have placed a number of compounds in this section which are fully deserving of detailed discussion, but not enough examples are yet available to group them separately. 

Okahara and his coworkers have made a number of contributions to the synthesis of crown ethers using a one-pot method (see Sect. 3.13). These methods have been applied largely to the preparation of simple aliphatic crown ether systems. In addition, this group has prepared macrocyclic ester compounds using a one-pot procedure. Although... 

Active methylene compounds also displace the nitro group at the furazan ring. Thus, 3,4-dinitrofurazan reacted with the sodium salt of ethyl (3-oxo propionate or related compounds in the presence of a crown ether to give the corresponding ester, which was readily hydrolyzed and decarboxylated (92UP1) (Scheme 146). 

Closely related to the crown ether adducts are the two intramolecular arenediazonium ion-crown ether compounds 11.6 and 11.7 which were synthesized by Gokel s group (Beadle et al., 1984b). Infrared and lH NMR spectra are consistent with the insertion of the diazonio group into the 21-crown-7 cavity. The complex 11.6 can therefore be described not in an anthropomorphic, but in a zoomorphic way, as an ostrich complex reflecting the common belief that an ostrich hides its head in a hole when endangered. For the complex 11.7 the spectra correspond to... 

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