SILICA IMPREGNATED WITH TETRAMETHYLAMMONIUM SALTS AS SOLID SOLID LIQUID TRIPHASE CATALYSTS

Since its introduction, phase-transfer catalysis (PTC) has become a very common synthetic tool. However, the separation of soluble phase-transfer catalysts from the reaction products is usually a laborious step, where many losses occur. In the recent decade much effort has therefore been put in the research of polymer-bound catalysts (triphase catalysis), which were usually based on polystyrene resins.1–3 Although at first quite promising, this approach has encountered some difficulties,4,5 and it is rarely used in synthetic chemistry.4,6 Some of the catalysts are commercially available, but they are quite expensive.7 Also, separation by filtration and recycling seems to be far from trivial. Their low physical stability makes them prone to pulverization when reaction mixtures are vigorously agitated,3,4 and they have a tendency to swell and form gels.3,4 To improve this physical instability of the catalysts, polystyrene has been substituted by inorganic supports such as silica or alumina.3,8 But these catalysts can be loaded only to a relatively low degree,9 their preparation requires some synthetic effort, and the supports may catalyze some side reactions by themselves.10,11 In a different approach, the need of a phase-transfer catalyst is circumvented by impregnation of the inorganic reagent itself on a support, but then, the latter is required in large amounts.12,13. © 1990, American Chemical Society. All rights reserved.