("PTC") an. Seit den grundlegenden Arbeiten E n d e der 70er J a h r e von M. MAKOZA und A. BRANDSTROM sind zu d e n ursprunglichen quaternaren Anlinonium-und Phosphoniurnsalzen inzwischen Kronenether, Chelatreagenzien und unlosliche polyniergehundene Katalysatoren (,,Dreiphasenkatalyse") hinzugekornmen. Nachteil: Sie sind alle ziemlich teuer. F u r das rasch expandierende Gebiet gibt es bereits Monographien, und thenienrelevante Tagungen sprechen viele Interessenten an. Siehe d a s vorliegende Buch. Seine behandelten Schwerpunkte lassen sich grob in A. Theorie und Mechanismen, B. Katalysatorentwicklung, C. Ariwendung in der Polynierchemie gliedern. Aufgefallen sind rnir die offenbar aussichtsreichen Versuche, in einer A r t HOFMANN-Reaktion P T C ziir Aufbrechung der Polysulfidvernetzungen in Altgummi zu nutzen, dabei die H a u p t k e t t e intakt zii lassen und die Strnktnr neiien Gummis anzustreberl (Goodrich). F u r die Carbonylierung und Carboxyalkylierung mittels organornetallischer Anionen eignet sich anionisch akt,iviertes A1,0, (irnmobilisierte Polyethylenglykole) a m besten (Texaco). Sogar freie RadikalPolytrierisationen lassen sich PT-katalysieren (K,S,O,/Cyclodextrin 1mw. Aliquat 336 = Tricapryltriniethyla~~iinoniuinchlorid) ( 3 M). Man sucht verstarkt nach PTC, die sich bei hohen Temperaturen einsetzen lassen, z.

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The development of Phase Transfer Catalysis (PTC) represents a major step forward in the employment of many organic reactions and renders them very convenient and useful processes. These reactions involve the application of nucleophiles in general, anions and bases in particular, in reactions carried out in a water‐organic solvent system. They can be performed both in the laboratory and on an industrial scale. The ease of application of PTC processes is the main reason for their increasing utilization in industry. An outstanding achievement of this technique is the employment of aqueous bases in reactions which traditionally would otherwise require a strong base in a nonaqueous medium. The classical procedures that require severe anhydrous conditions, expensive solvents and dangerous bases such as metal hydrides and organometallic reagents are now replaced by aqueous solutions of, e.g., sodium or potassium hydroxides (PTC/OH processes). In contrast to the extensive synthetic applications of PTC/OH systems, the detailed mechanisms of these processes have been the subject of a great deal of controversy and various mechanisms have been suggested. However, it would seem that our knowledge concerning the mechanistic aspects of such reactions has now reached the stage where it can be used to advantage in synthesis planning. A better understanding of the various factors which influence the reaction would undoubtedly help to optimize PTC/OH processes such as to enable higher yields in shorter reaction times at lower temperatures. The importance of, inter alia, the catalyst will be pointed out and it is highly recommended that such catalysts be always available in the laboratory, for the range of organic reactions that they can efficiently, conveniently and safely catalyze is vast indeed.

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A. IntroductionCritical to the success of phase-transfer catalytic (PTC) processes are (1) the maximization of the rate of transfer of reactant anions from the aqueous or solid phase to the organic phase, (2) the maximization of the rate of transfer of product anions from the organic phase to the aqueous or solid phase, and (3) the related equilibrium partitioning of the reactant and product anions between the organic and aqueous or solid phases. The common organic solvents employed in phasetransfer processes are usually relatively nonpolar and usually aprotic. Because anions do not have a great affinity for such solvents and prefer to reside in an aqueous environment, the desired transfer is not a particularly favorable process. The transfer of anions from an aqueous to an organic phase, however, may be achieved by choosing a phase-transfer cation that is not strongly solvated by water and that has organic-like characteristics and is thus compatible with the organic phase. For instance, the volume-to-charge ratio (as well as the organiclike nature) of quaternary ammonium and phosphonium salts can be adjusted over a wide range of values by simply changing the length of the alkyl (or aryl) substituents bonded to the quaternary heteroatom. Tetramethylammonium salts are highly soluble in aqueous media and only slightly soluble in most organic solvents, whereas tetradoecylammonium salts are soluble in most organic media but only slightly soluble in water. The former salt represents a quaternary ammonium ion with a small organic volume-to-charge ratio whereas the latter salt has a large organic volume-to-charge ratio. In a similar manner, macrocyclic multidentate ligands (crown ethers, cryptands, polyethylene oxides, etc.) may be employed to complex metal cations and carry them, along with their anions, from the aqueous or solid phase into the organic phase.The factors that affect the mass transfer and distribution of the phase-transfer 23 C. M. Starks et al., Phase-Transfer Catalysis © Chapman & Hall, Inc. 1994 r >Br->CI-CIO; > BrO; > 10; NO; > NO; CIO; > 10;The divalent anions listed in Table 2-2 have comparatively little affinity for the organic phase. An important measure of the competitive partitioning of anions between the aqueous and organic phases in the presence of a quaternary cation is the selectivity constant, KX1y set, which is defined by Eq. (2-1) and (2-2).(2-1)