Use of Crown Ether as Phase-Transfer Catalyst

Crown ether is the other important class of phase-transfer catalysts which critically differs from the onium salt, in that the whole inorganic salt is transferred into the organic phase. The reaction modes described above can generally be accommodated in such crown ether-catalyzed reactions, simply by replacing the onium cation by a metal cation complex of crown ether. 

2 Makosza s pioneering contributions are representative, see (a) Makosza, M. (1966) Tetrahedron Lett., 4621 (b) Makosza, M. (1966) Tetrahedron Lett., 5489 (c) Makosza, M. (1969) Tetrahedron Lett., 673  

3 (a) Dehmlow, E.V. and Dehmlow, S.S. (1993) Phase Transfer Catalysis, 3rd edn. VCH, Weinheim (b) Starks, C.M., Liotta, C.L. and Halpem, M.E. (1994) Phase-Transfer Catalysis, Chapman Hall, 

New York (c) Sasson, Y. and Neumann, R. (eds), (1997) Handbook of Phase-Transfer Catalysis, Blackie Academic Professional, London (d) Halpern, M.E. (ed.), (1997) Phase-Transfer Catalysis, ACS 

Symposium Series 659, American Chemical Society, Washington, DC. 

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The unique ability of crown ethers to form stable complexes with various cations has been used to advantage in such diverse processes as isotope separations (Jepson and De Witt, 1976), the transport of ions through artificial and natural membranes (Tosteson, 1968) and the construction of ion-selective electrodes (Ryba and Petranek, 1973). On account of their lipophilic exterior, crown ether complexes are often soluble even in apolar solvents. This property has been successfully exploited in liquid-liquid and solid-liquid phase-transfer reactions. Extensive reviews deal with the synthetic aspects of the use of crown ethers as phase-transfer catalysts (Gokel and Dupont Durst, 1976 Liotta, 1978 Weber and Gokel, 1977 Starks and Liotta, 1978). Several studies have been devoted to the identification of the factors affecting the formation and stability of crown-ether complexes, and many aspects of this subject have been discussed in reviews (Christensen et al., 1971, 1974 Pedersen and Frensdorf, 1972 Izatt et al., 1973 Kappenstein, 1974). 

One final note regarding the use of crown ethers as phase transfer catalysts there is little literature which directly compares quaternary ammonium catalysts with crown ethers in liquid-liquid processes (see Sect. 1.10) [48]. There are examples where both have been tried and are effective. In general, however, it appears that for solid-liquid phase transfer processes, the crowns are far better catalysts than are the quaternary ammonium ions. In order for a solid-liquid phase transfer process to succeed, the catalyst must remove an ion pair from a solid matrix. The quaternary catalysts have no chelating heteroatoms with available lone pairs which would favor such a process. The combination of a quaternary catalyst and some simple coordinating amine or ether would probably succeed [28, 32, 34]. It seems likely, as mentioned above, that it is the combination of diamine and quaternary catalyst generated in situ which accounts for the success of Normanf s catalysts [28]. It is interesting to speculate on the possibility of using a quaternary ammonium compound and a drop of water as a catalytic system. 

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