Modified silica-heterogenised catalysts for use in aqueous enantioselective hydrogenations

Modified mesoporous silicas have been prepared, homogeneous organometallic catalysts incorporated and the resulting heterogeneous systems used as catalysts in aqueous enantioselective hydrogenation reactions. A series of catalysts in which the organometallic species was incorporated during gel synthesis generally gave good conversions but low ee values which were usually less than 25%. Preformed silicas with a narrow range of pore size (26 and 37 Å) and an amorphous silica (average pore size 68 Å) were modified by external surface deactivation and/or derivatization of the internal surface for potential tethering of the catalytic species. In general, excellent conversions (80–100%) but modest enantioselectivities (40–50%) were obtained when using 37 and 68 Å silicas, similar to those values obtained using unmodified silicas. Only low conversions were obtained using the modified 26 Å silicas. Pore volume measurements suggest that this is due to very restricted access to the pores when the tether is present. The results indicate that for aqueous hydrogenation the van der Waals interactions of the catalyst with the porous matrix are sufficiently strong to retain the catalyst, thus allowing for its reuse. No major advantage then appears to be gained by the use of potential internal tethers.