Mesoporous Silica SBA-15-supported Norephedrine and Ephedrine as Heterogeneous Chiral Ligands

Abstract

Successful development of the homogeneous catalysts has been sometimes followed by attempts to attach the catalysts on an insoluble polymeric support. A suitably designed immobilized catalyst can retain the advantages of a heterogeneous catalyst in terms of simplified workup procedure and recyclization while maintaining reactivity and selectivity of a homogeneous catalyst. The immobilization of homogeneous species is of particular interest in the field of asymmetric catalysis since it is desirable to be able to regenerate and reuse expensive chiral catalysts. Polystyrene resin and silica gel have been most commonly used as insoluable supports for the immobilization of the catalysts. Recently, mesoporous silica SBA-15 with large uniform pore diameters and high specific surface areas have become of high interest as inorganic supports. Our interest in the field led to prepare SBA-15-supported ephedrine 1 and norephedrine 2. Herein, we report our investigation on the asymmetric addition of diethylzinc reagent to aldehydes and on the asymmetric transfer hydrogenation of ketones mediated by the supported ligands. As shown in Scheme 1, (−)-ephedrine was anchored on SBA-15 silica in a similar procedure as for silica gelsupported ephedrine 1. Treatment of SBA-15 with an excess of (3-chloropropyl)triethoxysilane in refluxing toluene gave chloropropylated SBA-15. The free silanol group was endcapped by treatment with hexamethyldisilazane (HMDS). Reaction of the samples are summarized in Table 1. The surface area was decreased considerably in the chloropropylation, but the pore diameter was not changed substantially during the modification steps. chloropropylated SBA-15 with 1.5 equiv. excess of (−)-ephedrine in refluxing toluene in the presence of 1 equiv of diisopropylethylamine afforded SBA-15-supported ephedrine 1. SBA-15-supported norephedrine 2 was also prepared from (−)-norephedrine by the same method. The degrees of functionalization were determined by weight gain or nitogen elemental analysis for the modified SBA-15. The characteristics of the two samples are summarized in Table 1. The surface area was decreased considerably during the modification, but the pore diameter was not changed substantially. With the supported ephedrine 1 in hand, we examined its catalytic efficiency in asymmetric addition of diethylzinc to aldehyde in hexane. Aldehydes were converted to the corresponding (R)-secondary alcohols with moderate ee's in high yields. The results were compared to the data previously obtained with amorphrous silica gel-supported ephedrine. As can be seen in Table 2, SBA-supported ephedrine 1 gave much higher reaction rate and better