SUPPORTED AQUEOUS-PHASE PALLADIUM CATALYSTS FOR THE REACTION OF ALLYLIC SUBSTITUTION - TOWARD AN UNDERSTANDING OF THE CATALYTIC-SYSTEM

The heterogeneization of the catalytic biphasic system Pd(OAc)(2)/5tpp ts/water/nitrile (tppts = sodium salt of tri(m-sulphophenyl)phosphine) ) is performed by controlled deposition on silica of the catalyst in i ts aqueous phase (SAP catalyst). The resulting heterogeneous molecular catalysts are active for the alkylation of ethyl acetoacetate or morp holine by (E)cinnamyl ethyl carbonate. They are very selective for the monoalkylated product, and the secondary reactions due to water itsel f, such as formation of cinnamyl alcohol, are suppressed. Finally, the active Pd(0) entity is stable toward decomposition into metallic part icles. This method (SAP) is most appropriate with true biphasic media. Thus, with CH3CN, the SAP catalyst is less active than the monophasic homogeneous catalyst, whatever the water content of the solid. On the contrary, with PhCN, the SAP catalyst is drastically more active than the biphasic homogeneous catalyst, a phenomenon which must be correla ted to a large enhancement of the interphase surface area. The water c ontent of the SAP catalyst intervenes in different ways. Thus, a minim um amount of water (ca. 15-20 wt %) is necessary to observe any cataly tic activity, a phenomenon which is correlated with a certain mobility of the complex on the surface of silica, confirmed by P-31 MAS NMR, b ut also probably to a minimum concentration of the reactants in the aq ueous phase. Activity increases when the water content reaches 30-50 w t %. Within these boundaries, no detectable palladium leaching is obse rved but the catalysts progressively loose part of their activity afte r several recyclings, a phenomenon which seems to be correlated with w ater leaching into the organic phase. P-31 MAS NMR reveals that the ma in surface complex is Pd(0)(tppts)(3). Its chemical shift corresponds to the weighted average of that of Pd(tppts)(3) (delta 22.6 ppm) and t hat of free tppts (delta -5.6 ppm), in accordance with a fast exchange between free tppts and coordinated tppts. No strong interaction betwe en the complex and the silica surface could be evidenced; in the prese nce of small amounts of water, free tppts interacts with the silica su rface (delta approximate to 0 ppm) via hydrogen bonding. Above 50 wt % of water, a number of problems are encountered which lead to an overa ll drop in activity: the solid agglomerates and palladium and water le aching both become significant. The SAP catalysts supported on a nonpo rous silica present the same behavior, i.e., increasing activity with increasing water content up to a content corresponding roughly to the wetting volume.