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 cata lysts in aqueous enantioselective hydrogenation reactions. A series of cata lysts in which the organometallic species was incorporated during gel synth esis generally gave good conversions but low ee values which were usually l ess than 25%. Preformed silicas with a narrow range of pore size (26 and 37 Angstrom) and an amorphous silica (average pore size 68 Angstrom) were mod ified by external surface deactivation and/or derivatization of the interna l surface for potential tethering of the catalytic species. In general, exc ellent conversions (80-100%) but modest enantioselectivities (40-50%) were obtained when using 37 and 68 Angstrom silicas, similar to those values obt ained using unmodified silicas. Only low conversions were obtained using th e modified 26 Angstrom silicas. Pore volume measurements suggest that this is due to very restricted access to the pores when the tether is present. T he results indicate that for aqueous hydrogenation the van der Waals intera ctions of the catalyst with the porous matrix are sufficiently strong to re tain the catalyst, thus allowing for its reuse. No major advantage then app ears to be gained by the use of potential internal tethers. (C) 2001 Elsevi er Science B.V. All rights reserved.