Phases with crown ethers

Complexation with crown ethers increases the notoriously low solubilities of diazonium salts in most solvents (with the obvious exception of water). Therefore, it is possible to carry out phase-transfer reactions with complexed diazonium ions (review Gokel et al., 1985). Useful examples can be found in a paper from Gokel s group (Beadle et al., 1984a) on the Gomberg-Bachmann and Pschorr reactions (see Sec. 10.10). 

The reaction can be carried out efficiently using aryl diazonium tetrafluoroborates with crown ethers, polyethers, or phase transfer catalysts.103 In solvents that can act as halogen atom donors, the radicals react to give aryl halides. Bromotrichloromethane gives aryl bromides, whereas methyl iodide and diiodomethane give iodides.104 The diazonium ions can also be generated by in situ methods. Under these conditions bromoform and bromotrichloromethane have been used as bromine donors and carbon tetrachloride is the best chlorine donor.105 This method was used successfully for a challenging chlorodeamination in the vancomycin system. 

Ha is the more and HB is the less complexed host enantiomer in the aqueous phase. In the second type of experiment excess of racemic valine and optically pure (S)-[269] were distributed between two immiscible phases. In this experiment a 1 1 complex is formed in the non-aqueous phase in which L-valine dominates by an amount of 12.5% (CRFchc1j 1.28 and EDC 1.50). In terms of differences in free energy between the two diastereoisomeric complexes this means a difference in A(AG°) of 0.23 kcal mol-1 in favour of the (l)-S-[269] complex. Similar experiments have been carried out with crown ethers [270]—[280]. 

The general concept of phase transfer catalysis applies to the transfer of any species from one phase to another (not just anions as illustrated above), provided a suitable catalyst can be chosen, and provided suitable phase compositions and reaction conditions are used. Most published work using PTC deals only with the transfer of anionic reactants using either quaternary ammonium or phosphonium salts, or with crown ethers in liquid-liquid or liquid-solid systems. Examples of the transfer and reaction of other chemical species have been reported(24) but clearly some of the most innovative work in this area has been done by Alper and his co-workers, as described in Chapter 2. He illustrates that gas-liquid-liquid transfers with complex catalyst systems provide methods for catalytic hydrogenations with gaseous hydrogen. 

Several phosphines with crown ether substituents were synthetized in order to accelerate reactions catalyzed by their (water-insoluble) Rh(I) complexes by taking advantage of a built-in phase-transfer function [66,67]. Indeed, hydrogenation of Li-, Na-, K- and Cs-cinnamates in water-... 

The reaction can be carried out efficiently using aryl diazonium tetrafluoroborates with crown ethers, polyethers, or phase-transfer catalysts.95 In solvents that can act as halogen-... 

Hydroxy-bridged complexes [Pt2(,u-OH)2(PEt3)4]2+ can also be prepared. The structure consists of two square planar platinum(II) centers bridged by hydroxide ligands with an angle of 36.4° between the mean plane normals.1569 A useful method to prepare these complexes involves the use of phase-transfer catalysis with crown ethers to facilitate the reaction of KOH with platinum(II) chloro complexes.1570... 

The treatment of aqueous solutions containing Znn or Pbu with crown ethers results in the preferential complexation of Pbu, to give a complex which is readily transported across the CHC13-H20 interface, and the method appears to have potential for the separation of these two metals.1132 Polyethers have also been used for the extraction of zinc from aqueous media containing thiocyanate, and the distribution of the metal between the two phases has been shown to depend upon the solvent and any other cations which may be present.1133... 

Some synthetically useful oxidations of alkenes by permanganate can be performed under controlled conditions. For example, 1-decene could be oxidized to nonanoic acid in 91% yield by permanganate in the presence of the phase-transfer agent Aliquat 336.319 In a benzene solution with crown ether and permanganate, a-pinene is oxidized to cis-pinonic acid in 90% yield (equation 110).314... 

In summary, phthalocyanines modified with crown ethers are interesting synthetic targets as they are prone to form columnar phases. Their electron conductivity and complexation properties make them interesting candidates for the design of sensor materials or supramolecular switches. 

Nakagawa, T., Shibukawa, A., Kaihara, A., Itamochi,T., Tananka, H. (1986) Liquid Chromatography with Crown Ether-containing Mobile Phases VI. Molecular Recognition of Amino Acids and Peptides. 

Zintl compounds to be mentioned in other chapters are compounds or phases that have anions such as Sng-, Sn2Bi2 or Pb1. The alkali metal salts can often be isolated crystalline by complexation of the cation with crown ethers or cryptands. Recent examples are Na jSn16 and Li2Ba4Si6, the latter having a SiJ0- ring.17 Saline hydrides were discussed in Chapter 2. 

Lee, M. et al. Selective sohd-phase extraction of catecholamines by chemically modified polymeric adsorbents with crown ether, J. Chromatogr. A. 2007, 1160, 340-344. 

Although the hydrolysis of alkyl halides to alcohols has been extensively investigated, an alternative two-step sequence involving substitution with carboxylate ion is more practical for the preparation of alcohols. Activation of the carboxylate anion prepared by the reaction of the acid with a base can be achieved (i) by use of a polar aprotic solvent and (ii) by use of aprotic apolar solvents under phase transfer catalysis, polymer conditions, or with crown ethers. 

Fig. 3-32. Separation of the sodium salts of various anions on silica coated with crown ether polymers. - Stationary phase dibenzo-18-crown-6 eluent water flow rate 1 mL/min detection direct conductivity solute concentrations 0.7 ppm Na2S04, 0.1 ppm NaCI 1 ppm Nal, 4 ppm NaSCN, and 8 ppm NaHC03 (taken from [37]).

In contrast to conventional cation exchangers, a reversed elution order is observed with crown ether phases, which is mainly determined by the size ratio between crown ether ring and alkali metal ion. Due to the high affinity of poly(benzo-15-crown-5) toward potassium and rubidium ions, these are more strongly retained than lithium, sodium, and cesium ions, respectively. However, the complexing properties of crown ethers also depend on the counter ion being employed. Thus, in potassium salts, for example, an increase in retention in the order KC1 < KBr < KI is observed with an increasing size of the counter ion. 

Alkaline-earth metal ions, on the other hand, elute from a crown ether phase in the normal elution order (Mg2+ < Ca2+ < Sr2+ < Ba2+). Such a separation is only of pure academic interest, since the resolution between magnesium and calcium is extraordinarily poor due to the low interactions of both ions with crown ethers. 

One example of a separation cascade is the liquid/liquid extraction system where the desired isotopic enrichment takes place in phase A. Using systems with crown ethers or cryptands, these ligands should have a good solubility in one phase, whereas the same compounds should be insoluble or difficult to dissolve in the other phase. This also applies to chemical reactions in solid/liquid systems, e.g. in the system ion... 

Macrocycles (crown ethers, cryptands) with chromogenic groups combine the natural selectivity of macrocycles with the possibility of direct spectrophotometric determination of some metals (e.g., K, Ca) in an organic phase after extraction [127-129]. 4-Picrylamlnobenzo-15-crown-5 crown ether (formula 4.41) is applied in the extraction and spectrophotometric determination of potassium. The determinations are based on extractable ion-associates of metals (e.g., Li, Na, K, Pb) with crown ethers and xanthene or sulphophthalein dyes [130]. 

The reaction employed was the substitution of the bromine of 1-bromohexane by nitrile anion. The alkyl bromide composed one phase and a concentrated aqueous solution of either KCN or NaCN made up the other phase. The crown ethers functioned as soluble PTC s while polymer 2 suspended at the interface served as an insoluble catalyst. A reaction temperature of 85° was employed to give reasonable conversion times. The reactions were followed with NMR by monitoring the integrated intensities of the hydrogens adjacent to the halide and nitrile groups. The two triplets associated with these peaks are well separated at 3.236 and 2.156 allowing direct comparison of their relative ratios in the reaction mixtures. While this method has limited accuracy, it does allow rapid initial evaluations. Figure 3 shows the relative rates of conversion of 1-bromohexane to the nitrile with KCN. 

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