Base catalysis using supported fluorides

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

The observed activation of allyltrihalosilanes with fluoride ion and DMF and the proposition that these agents are bound to the silicon in the stereochemistry-determining transition structures clearly suggested the use of chiral Lewis bases for asymmetric catalysis. The use of chiral Lewis bases as promoters for the asymmetric allylation and 2-butenylation of aldehydes was first demonstrated by Denmark in 1994 (Scheme 10-31) [55]. In these reactions, the use of a chiral phos-phoramide promoter 74 provides the homoallylic alcohols in high yield, albeit modest enantioselectivity. For example, the ( )-71 and benzaldehyde affords the anti homoallylic alcohol 75 (98/2 antUsyn) in 66% ee. The sense of relative stereoinduction clearly supports the intermediacy of a hexacoordinate silicon species. The stereochemical outcome at the hydroxy center is also consistent with a cyclic transition structure. 

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. 

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