Organofunctionalized Mesoporous SBA-15 Materials as the Catalysts in Asymmetric Alkylation and Aldol Reactions
Date Issued
2008
Date
2008
Author(s)
Fan, Chikuan
Abstract
Mesoporous SBA-15 materials of high surface area, uniform pore size, and high hydrothermal stability were synthesized in acidic medium using triblock coploymer EO20PO70EO20 (P123) as template and TEOS (tetraethoxysilane) as silica source. To avoid pore-blocking by anchored functional groups or by the reaction molecules and solvents during catalytic reactions, our laboratory has developed a method to prepare SBA-15 with short channeling pores in 150 ~ 350 nm by adding proper amount of Zr(IV) ions in the synthesis solution. This method was applied to prepare chloropropyl and vinyl-functionalized platelet SBA-15 materials named x%Cl-Zr-SBA-15 or x%Vin-Zr-SBA-15 (x = 5 ~ 20) with short mesochannels in this thesis. roline-derivative was anchored on SBA-15 materials by nuceophilic substitution of the x%Cl-Zr-SBA-15 materials with α,α-Diphenyl-L-prolinol in 4 mL toluene. The resultant x%Cl-Zr-SBA-15-cata were applied as the catalysts in asymmetric addition of diethylzinc to benzaldehyde. Catalyst 20%Cl-Zr-SBA-15-cata gave the highest enantioselectivity, probably due to its highest incorporating amount of α,α-Diphenyl-L-prolinol. In comparison to the catalytic reaction in hexane, the reaction proceeded in toluene gave milder conversion and selectivity but higher enantioselectivity. The relatively higher polarity of toluene probably avoids direct contact of the products with the silanol groups, which might cause racemization of the products. Post-decorating a large amount of methyl group can increase catalytic efficiency to 52% e.e., attributing to its hydrophobicity and end-capped silanol groups. On the other hand, the platelet SBA-15 catalysts gave higher conversion and enantioselectivity than those with long meso-channels. It is attributed to their short meso-channel that might facilitate reactants to approach the active centers. ysteine-functionalized SBA-15 materials were prepared by anchoring N-Boc cysteine ethyl ester onto x%Vin-Zr-SBA-15 materials via free radical addition, followed by hydrolysis and deprotection procedures. The materials were applied in catalytic Aldol reaction between cyclohexanone and p-nitrobenzaldehyde. Higher temperature was found to accelerate the reaction and to increase the syn / anti ratio of the diastereomers. Cysteine-functionalized SBA-15 of fiber-like morphology gave the highest catalytic activity than those of rod and platelet-like morphologies, probability due to the more suitable pore size toward diffusion of reactants and products. The p-substituted groups, of either electron withdrawing or donating character, on benzaldehyde led to lower conversions than that of benzaldehyde as the reactant. It is attributed to that surface silanol groups probably bind with the p-substituents through H-bond and obstruct the approach of benzaldehide molecules to the active sites.
Subjects
platelet SBA-15
nucleophilic substitution
free radical reaction
asymmetric alkylation
aldol reaction
Type
thesis
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