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In previous chapters we learned that a phase-transfer catalyst must have two particular chemical functions to be successful, that is, it must rapidly transfer one of the reactant species into the normal phase of the other reactant, and second it must make the transferred species available in a highly reactive form. The need for one or the other of these two functions to be strongly catalyzed often plays an important role in the selection of the best transfer catalyst for a particular reaction, as, for example, to pick a phase-transfer catalysis (PTC) catalyst that is especially useful for activating an anion, or for a catalyst that is especially good for facilitating anion transfer to the organic phase. Catalyst structure may also be of great significance when more than one product can be formed, to cause one product to be favored over another. Sometimes catalyst structure is not important at all, and almost any PTC catalyst will perform satisfactorily for some reactions, whereas for others it may be desirable to have two (or more) catalysts to facilitate each of the various steps in a reaction sequence. Some examples of the effect of catalyst structure are discussed in this chapter to illustrate these points. |
