Condensations, using supported fluorides

Examples of the Michael-type addition of carbanions, derived from activated methylene compounds, with electron-deficient alkenes under phase-transfer catalytic conditions have been reported [e.g. 1-17] (Table 6.16). Although the basic conditions are normally provided by sodium hydroxide or potassium carbonate, fluoride and cyanide salts have also been used [e.g. 1, 12-14]. Soliddiquid two-phase systems, with or without added organic solvent [e.g. 15-18] and polymer-supported catalysts [11] have been employed, as well as normal liquiddiquid conditions. The micellar ammonium catalysts have also been used, e.g. for the condensation of p-dicarbonyl compounds with but-3-en-2-one [19], and they are reported to be superior to tetra-n-butylammonium bromide at low base concentrations. 

Ohtani et al. used polystyrene-supported ammonium fluoride as a phase transfer catalyst (triphase catalysis) for several base-catalyzed reactions, such as cyanoethylation, Knoevenage reaction, Claisen condensation and Michael addition. The catalytic activity of the polystyrene-supported ammonium fluid was comparable to that of tetrabutylammonium fluoride (TBAF). The ionic loading and the ammonium structure of the fluoride polymers hardly affected the catalytic efficiency. The reaction was fast in a non-polar solvent (e.g., octane or toluene) from which the rate-determining step of the base-catalyzed reaction is very similar to that of the nucleophilic substitution reactions. 

The first conversion on a polymer to introduce a ethyl methyl ketone moiety was performed, without dilution, in the liquid reactants vinyl methyl ketone and condensed hydrogen fluoride, which scarcely solvate the polystyrene support. Recently, Tesser and co-workers [78] reported an unsuccessful attempt to verify this type of transformation on polystyrene, supposing that their failure depended upon the particular batch of polymer used. Obviously the swelling properties of the cross-linked supports in the poorly solvating reaction liquid — condensed hydrogen fluoride - differed in these independent experiments. Therefore, to describe completely a procedure to modify an insoluble, but swelling, polymer, it is essential to specify the solvation power of the hquid mixture of reactants. 

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