Crown ethers dissolving metals

Phase transfer catalysis succeeds for two reasons First it provides a mechanism for introducing an anion into the medium that contains the reactive substrate More important the anion is introduced m a weakly solvated highly reactive state You ve already seen phase transfer catalysis m another form m Section 16 4 where the metal complexmg properties of crown ethers were described Crown ethers permit metal salts to dissolve m nonpolar solvents by surrounding the cation with a lipophilic cloak leav mg the anion free to react without the encumbrance of strong solvation forces... 

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... 

Ethers form Lewis acid Lewis base complexes with metal ions Certain cyclic polyethers called crown ethers, are particularly effective m coor dinatmg with Na" and K" and salts of these cations can be dissolved m nonpolar solvents when crown ethers are present Under these conditions the rates of many reactions that involve anions are accelerated... 

Phase-tiansfei catalysis (PTC) is a technique by which leactions between substances located in diffeient phases aie biought about oi accelerated. Typically, one OI more of the reactants are organic Hquids or soHds dissolved in a nonpolar organic solvent and the coreactants are salts or alkah metal hydroxides in aqueous solution. Without a catalyst such reactions are often slow or do not occur at ah the phase-transfer catalyst, however, makes such conversions fast and efficient. Catalysts used most extensively are quaternary ammonium or phosphonium salts, and crown ethers and cryptates. Although isolated examples of PTC can be found in the early Hterature, it is only since the middle of the 1960s that the method has developed extensively. 

This cluster is prepared from the reaction of an alloy of Pb with an alkali metal, dissolved in liquid ammonia with the crown ether, crypt, dissolved in ethylenediamine. 

Knochel et al. (1977a) have studied the effect of the structure of the ligand on its ability to catalyse the reaction between solid metal acetates and benzyl chloride dissolved in acetonitrile. Approximate half-lives for the reactions are given in Table 29. For crown ethers, the reactivity sequence decreases in the... 

A different concept of chiral recognition was used by Lehn et al. (1978) for the differentiation between pairs of enantiomeric anions. Following the terminology used for metallo-enzymes, the chiral crown ether [309] acts as an apo-receptor, complexing a metal cation and thus becoming a chiral metal receptor that may discriminate between enantiomeric anions (cascade-type complexation). Extraction experiments with racemic mandelic acid dissolved in... 

Known as crown ethers because of their crown-like shape, these ethers contain cavities that are ideal for forming complexes with metal ions. It is this property that allows ordinary salts to dissolve in organic solvents. For example, potassium permanganate is usually insoluble in benzene, but readily dissolves in benzene if [18]-crown-6 ether is added. This solution is useful because it allows oxidation with potassium permanganate to be carried out in organic solvents. The potassium ion (shown in green) is just the right size to fit into the cavity in the crown ether. 

Soluble Zintl ions can also be obtained by electrochemical methods using the respective element as cathode material [34, 60, 61], or through the reaction of the various modifications of the tetrel (Sn and Pb) and pentel (P, As, Sb) elements with dissolved or finely dispersed alkali or alkaline-eatth metals in solution [62] as well as in molten crown-ethers [63]. 

This general trait of crown ethers and cryptands (to be discussed later) to stabilize alkali metal salts has been extended to even more improbable compounds, the al-kalides and electrides, which exist as complexed alkali metal cations and alkalide or electride anions. For example, we saw jn Chapter 10 that alkali metals dissolve in liquid ammonia (and some amines and ethers) to give solutions of alkali electrides 10 M M+ f e" (12.38)... 

As early as 1969, Pedersen was intrigued by the intense blue colour observed upon dissolution of small quantities of sodium or potassium metal in coordinating organic solvents in the presence of crown ethers. Indeed, the history of alkali metal (as opposed to metal cation) solution chemistry may be traced back to an 1808 entry in the notebook of Sir Humphry Davy, concerning the blue or bronze colour of potassium-liquid ammonia solutions. This blue colour is attributed to the presence of a solvated form of free electrons. It is also observed upon dissolution of sodium metal in liquid ammonia, and is a useful reagent for dissolving metal reductions , such as the selective reduction of arenes to 1,4-dienes (Birch reduction). Alkali metal solutions in the presence of crown ethers and cryptands in etheric solvents are now used extensively in this context. The full characterisation of these intriguing materials had to wait until 1983, however, when the first X-ray crystal structure of an electride salt (a cation with an electron as the counter anion) was obtained by James L. Dye and... 

KI and KSCN are dissolved by 5.12 but without complexation of the boron atom. The stabilisation of the K+ ion by the crown ether moiety is apparently sufficient in these cases.12 Similarly, the 18-membered phenolic crown ether analogue can be metalated (= exchange of a proton for a metal ion in this case the OH proton) with trimethylaluminium to give 5.13 which forms a ditopic complex with LiCl in solution and in the solid state. 

Inorganic salts, which contain metal ions, do not dissolve in organic solvents such as benzene. However, by surrounding the metal ion with a chelate called a crown ether the desired solution can be made. The mining industry uses cyanide ions to dissolve gold out of the quartz rocks in which it is often found. The cyanide ligands are removed in subsequent chemical steps. 

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