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)/5tppts/water/nitrile (tppts = sodium salt of tri(m-sulphophenyl)phosphine)) is performed by controlled deposition on silica of the catalyst in its aqueous phase (SAP catalyst). The resulting heterogeneous molecular catalysts are active for the alkylation of ethyl acetoacetate or morpholine by (E)cinnamyl ethyl carbonate. They are very selective for the monoalkylated product, and the secondary reactions due to water itself, such as formation of cinnamyl alcohol, are suppressed. Finally, the active Pd(0) entity is stable toward decomposition into metallic particles. 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 correlated to a large enhancement of the interphase surface area. The water content of the SAP catalyst intervenes in different ways. Thus, a minimum amount of water (ca. 15-20 wt %) is necessary to observe any catalytic activity, a phenomenon which is correlated with a certain mobility of the complex on the surface of silica, confirmed by P-31 MAS NMR, but also probably to a minimum concentration of the reactants in the aqueous phase. Activity increases when the water content reaches 30-50 wt %. Within these boundaries, no detectable palladium leaching is observed but the catalysts progressively loose part of their activity after several recyclings, a phenomenon which seems to be correlated with water leaching into the organic phase. P-31 MAS NMR reveals that the main 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 that of free tppts (delta -5.6 ppm), in accordance with a fast exchange between free tppts and coordinated tppts. No strong interaction between the complex and the silica surface could be evidenced; in the presence of small amounts of water, free tppts interacts with the silica surface (delta approximate to 0 ppm) via hydrogen bonding. Above 50 wt % of water, a number of problems are encountered which lead to an overall drop in activity: the solid agglomerates and palladium and water leaching both become significant. The SAP catalysts supported on a nonporous silica present the same behavior, i.e., increasing activity with increasing water content up to a content corresponding roughly to the wetting volume.