Covalent heterogeneous catalysis, enantioselective

Obviously, the way to heterogeneous enantioselective catalysis is open when chiral ligands are introduced into the polymers. This idea was first proposed by Okamoto and Still (231). However, the first catalytic results were published by Cazaux and Caze (235). These authors use the asymmetric Mo(VI) complex (9) covalently bound to a cross-linked polystyrene resin for the enantioselective epoxidation of geraniol. Unfortunately, the enantiomeric excess obtained was low—far below that obtained with the Ti-tartrate catalyst (Section II,B). Moreover, the polymeric structure appeared to be unstable. 

Enantioselective alkylations have been achieved using modified MCM-41 materials.135 Mesoporous templated MCM-41s with covalently linked chiral ephedrine are active heterogeneous chiral auxiliaries in the enantioslective alkylation of benzaldehyde by diethylzinc. Lower rates, selectivities and enantioselectivities are obtained under heterogeneous conditions compared to homogeneous catalysis. This can be explained either by the participation of the uncovered surface to the racemic alkyl transfer or by a restricted accessibility to the catalytic sites in the heterogeneous reactions. 

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