Heterogeneous enantioselective carbon-carbon bond formation:: role of the inorganic support in the synthesis and activity of supported chiral auxiliaries

Heterogeneous enantioselective alkylation of benzaldehyde with diethylzinc was performed using (-)-ephedrine grafted on mesoporous micelle templated aluminosilicates or silicates, as chiral auxiliary. Supports were characterized by a regular mesoporosity and a same initial pore diameter. Immobilization of the chiral aminoalcohol was performed through covalent anchoring of 3-halogenopropyltrimethoxysilane (XPTMS) and substitution of the halogen by (-)-ephedrine. Comparison of the efficiency of the catalysts was carried out. Results were analyzed taking into account the accessibility to the catalytic sites by changing their density (decrease of XPTMS concentration, spacing of the sites by alkyl goups) and the effect of the uncovered mineral surface on activities and enantioselectivities.