Asymmetric hydrogenation using Wilkinson-type rhodium complexes immobilized onto mesoporous silica

Abstract

Heterogeneous chiral catalysts based on the DIOP-Rh system were prepared by covalent immobilization onto SBA type silicas using two different strategies. In the first route, a chiral analogue of Wilkinson's catalyst, [Rh(diop)(PPh₂(CH₂)₂Si(OCH₂CH₃)₃)Cl], 2, was immobilized onto a SBA-15 surface via a classical post-grafting procedure affording [2]/SBA-15. The covalent link to the surface was introduced through the monodentate phosphine ligand while keeping the structure of the chiral directing group intact. The second approach involved a two step synthesis: achiral RhCl{PPh₂(CH₂)₂Si(OCH₂CH₃)₃}₃, 3, was first integrated within the walls of the silica framework during the synthesis of the oxide followed by incorporation of the chiral auxiliary in the pores of the material by post-synthetic grafting of a siloxane-containing DIOP ligand, 4. This led initially to the formation of [3][4]@PMOs in which stereogenic and reactive centers are not directly linked together but are in close proximity within a confined space. For characterisation, both molecular and solid state techniques were used to determine the structural and textural properties of the hybrid materials. The catalytic performances of the materials was evaluated using the hydrogenation of methyl (Z)-2-N-acetylaminocinnamate. Generally, the [2]/SBA-15catalyst leads to full, selective conversion with an ee of 57% in favor of the (R)-enantiomer. These data compare favorably to those reported for homogeneous counterparts. Rate of hydrogenation was further improved by varying the reaction parameters (temperature and pressure) or by generating in situ cationic active site just prior to the catalysis. In the case of [3][4]@PMOs, lower conversions were achieved (15 to 41%), which is attributed to restricted accessibility to the rhodium centre. Enantioinduction up to 26% was observed for these systems. Catalyst recycling was studied for [2]/SBA-15 and it was shown that the catalyst could be re-used several times without loss of enantioselectivity or activity.