含有機官能基之介孔 SBA-15 材料在不對稱烷化及 Aldol 反應上的催化應用

Organofunctionalized Mesoporous SBA-15 Materials as the Catalysts in Asymmetric Alkylation and Aldol Reactions

范啟寬(Chikuan Fan)

指導教授 ： 鄭淑芬

國立臺灣大學/理學院/化學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.02402

摘要

本論文主要是利用 SBA-15 介孔材料具有規則孔洞及高表面積的特性，將有機觸媒固定於材料的孔壁上而得到可重覆使用之異相觸媒材料。在酸性條件下，以 P123 做為模板試劑， TEOS 當做矽源，利用直接合成法可合成出修飾有有機官能基的 SBA-15 材料。在合成溶液中若添加適當量的鋯鹽 (ZrOCl2) 可得到平板狀的形貌，而添加 1.5 M 氯化鈉則可得到纖維狀的形貌，若沒有額外添加鹽類則會得到棒狀的形貌。所得之 SBA-15 材料經由 XRD 、氮氣吸脫附、熱重分析以及紅外線光譜儀等儀器的鑑定可得知其結構的資訊。
修飾有氯丙基的 SBA-15 介孔材料經由親核性取代反應可將脯胺酸衍生物α,α-Diphenyl-L-prolinol 固定於材料上，並應用在二乙基鋅對苯甲醛的不對稱加成反應上。利用含脯胺酸衍生物平板狀 SBA-15 催化二乙基鋅對苯甲醛的不對稱加成反應，以甲苯做為溶劑雖然產物的產率較低，但是卻能避免反應物與 SBA-15 表面的矽羥基直接接觸而發生消旋的現象。催化效果最好的是具有 20% 脯胺酸衍生物含量的平板狀 SBA-15 ，推測是與其具有較多的有機觸媒含量有關。共修飾有疏水官能基的材料，由於妨礙了親核性取代反應的進行，所以催化效果並不好，但後修飾大量甲基的材料，由於較靠近活性中心，不僅增加疏水性，也減少消旋的現象產生，其中 post-Me-20% Cl-Zr-SBA-15-cata 其催化活性最佳，可達 52% 之鏡像選擇率。經由中和處理過後之觸媒，除了纖維狀材料外，棒狀及平板狀材料的催化效果都下降，推測是由於纖維狀材料的氧化矽骨架縮合較完全，在中和處理時，不易破壞骨架而讓矽羥基裸露進而造成消旋的現象。
修飾有乙烯基的 SBA-15 材料，經由自由基反應與水解、去保護以及調節酸鹼度的過程，可獲得固定有半胱胺酸的 SBA-15 觸媒。利用含半胱胺酸平板狀 SBA-15 材料催化環己酮與對硝基苯甲醛間的 aldol 縮合反應，發現反應速率會隨著反應溫度的提升而大為提高，同時也會增加 syn / anti 產物的比例。若添加質子型溶劑如乙醇，由於溶劑與介孔材料表面的矽羥基形成氫鍵，會使得反應中間物傾向 re-facial attack 的攻擊模式而得到較多的 anti 產物。改變反應物苯甲醛對位的官能基，發現不具拉電子特性的氫以及具有較弱拉電子特性的氯取代基反而比強拉電子特性的硝基轉化率來的高，推測苯甲醛對位的硝基可能會與 SBA-15 表面的矽羥基形成氫鍵使得對硝基苯甲醛無法靠近活性中心而讓反應性下降。而固定有半胱胺酸的纖維狀 SBA-15 的催化效果比棒狀及平板狀 SBA-15 的催化活性來的好，推測是由於其孔徑在三種形貌的孔洞材料中雖然居中，但可能大小較適當，使反應物更能靠近活性中心，而得到較好的催化活性。

關鍵字

平板狀 SBA-15 ； 親核性取代 ； 自由基反應 ； 不對稱烷化 ； aldol 反應

並列摘要

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.
Proline-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.
Cysteine-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.

並列關鍵字

platelet SBA-15 ； nucleophilic substitution ； free radical reaction ； asymmetric alkylation ； aldol reaction

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