Adsorption geometry of modifiers as key in imparting chirality to platinum catalysts
Adsorption geometry of modifiers as key in imparting chirality to platinum catalysts
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DOI：
10.1021/ja016722n
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发表时间：
2001-11-07
影响因子：
15
通讯作者：
Zaera, F
中科院分区：
文献类型：
--
作者：
Kubota, J;Zaera, F
手性在生物体的生物化学中非常重要，因为自然界已经进化到有利于一种利手而不是另一种利手。在药物应用中使用错误的对映体可能是致命的：以沙利度胺为例，其中一种对映体的镇痛特性被另一种对映体引起的胎儿畸形所抵消。1目前合成对映体纯化合物的最佳方法是均相催化法。然而，由于这种方法通常需要昂贵且难以处理的金属络合物，其难以从产物中分离，因此更优选使用多相催化剂。最有希望的途径是通过将手性改性剂添加到常规催化剂中，3但这种方法迄今为止受到缺乏对相应表面过程的理解的限制。4手性修饰的多相催化的少数成功例子之一是在用金鸡纳生物碱修饰的负载型铂催化剂上的R-酮酯的氢化。5-8不幸的是，由于改性剂的结构6 -9或浓度8 -10、金属催化剂的粒度、溶剂的性质6-9和反应条件12的微小变化都会以不可预测的方式影响其活性和选择性，因此，为特定应用调整该方法受到阻碍。在这里，我们报告的结果从原位反射-吸收红外光谱（RAIRS）研究金鸡纳从溶液中吸附到铂表面上，其中揭示了一些光的原因，这些变化。化学吸附金鸡纳碱的吸附特性由图1中提供的振动数据来说明，图1比较了在铂表面暴露于辛可尼定在四氯化碳中的饱和（6.8 mM）溶液期间获得的原位RAIRS迹线与纯生物碱的光谱。在该图的插图中提供了辛可尼丁的结构，以突出显示在这些改性剂中已经鉴定的三个功能部分，即：（1）锚定喹啉芳族环，据信负责吸附到金属的部分，（2）叔奎宁环，具有碱性氮原子的胺基，其促进与反应物的络合，和（3）负责产物手性的C8和C9碳原子周围的立体区域。7溶剂的选择是为了简化红外光谱的分析。通过记录p/s极化比以利用所谓的表面选择规则13，实现了吸附分子与液相分子的区分，并进行了许多测试以确保记录的光谱确实对应于吸附的物质。[14]首先，在厚度增加的液膜的光谱中，由于溶剂而产生的峰增加，而那些分配给吸附物的峰根本没有变化。此外，在用纯四氯化碳冲洗系统后，吸收带仍然存在，这表明吸附具有高度的不可逆性。最后，在氧化的铂表面上没有检测到红外峰;辛可尼定的化学吸附只发生在干净的金属铂上，吸附后保留了金鸡纳改性剂的分子结构，这可以通过在1591、1568和1507 cm-1附近的主要喹啉面内变形模式的持久性来证明（尽管后两者分别移动到1574和1510 cm-1）。另一方面，很明显，表面诱导特定的吸附配置。特别是1462 cm ~（-1）峰相对于1452 cm ~（-1）峰的相对重要性的增加，说明了1462 cm ~（-1）峰的相对重要性的增加，说明了1462 cm ~（-1）峰相对于1452 cm ~（-1）峰的相对重要性的增加。
Chirality is prominent in the biochemistry of living organisms, since nature has evolved to favor one handedness over the other. The use of the wrong enantiomer in pharmaceutical applications can be deadly: witness the case of thalidomide, where the analgesic properties of one enantiomer is offset by the fetal malformations caused by the other. 1 At present, the best way to synthesize enantiomerically pure chemicals is via homogeneous catalysis. 2 However, since such processes often require expensive and hard-to-handle metal complexes which are difficult to separate from the products, the use of heterogeneous catalysts is much preferred. The most promising route for that is via the addition of chiral modifiers to regular catalysts, 3 but such approach has so far been limited by the lack of understanding of the corresponding surface processes. 4 One of the few successful examples of chiral-modified heterogeneous catalysis is the hydrogenation of R-ketoesters on supported platinum catalysts modified with cinchona alkaloids. 5-8 Unfortunately, tuning that process for specific applications is hampered by the fact that small changes in structure6-9 or concentration8-10 of the modifier, particle size of the metal catalyst, 11 nature of the solvent, 6-9 and reaction conditions12 all affect its activity and selectivity in unpredictable ways. Here we report on results from in situ reflection-absorption infrared spectroscopy (RAIRS) studies on the adsorption of cinchona from solution onto platinum surfaces, which shed some light on the reasons for those changes. The adsorption characteristics of chemisorbed cinchona are illustrated by the vibrational data provided in Figure 1, which compares the in situ RAIRS trace obtained during exposure of a platinum surface to a saturated (6.8 mM) solution of cinchonidine in carbon tetrachloride against a spectrum from the pure alkaloid. The structure of cinchonidine is provided in the inset of that figure to highlight the three functional parts that have been identified in these modifiers, namely:(1) the anchoring quinoline aromatic ring, the moiety believed to be responsible for adsorption to the metal,(2) the tertiary quinuclidine ring, an amine group with a basic nitrogen atom which facilitates complexation with the reactant, and (3) the stereogenic region around the C8 and C9 carbon atoms responsible for the chirality of the product. 7 The choice of solvent was made to simplify the analysis of the infrared spectra. Discrimination of the adsorbed molecules from those in the liquid phase was achieved by recording p/s polarization ratios to take advantage of the so-called surface selection rule, 13 and a number of tests were performed to ensure that the recorded spectra do indeed correspond to adsorbed species. 14 For one, in spectra from liquid film of increasing thickness, the peaks due to the solvent grow, while those assigned to the adsorbate do not change at all. In addition, the absorption bands persist after flushing the system with pure carbon tetrachloride, an indication of the high degree of irreversibility of the adsorption. Finally, no infrared peaks were detected on an oxidized platinum surface; the chemisorption of cinchonidine only occurs on clean metallic platinum.Retention of the molecular structure of the cinchona modifier upon adsorption is evidenced by the permanence of the main quinoline in-plane deformation modes around 1591, 1568, and 1507 cm-1 (although the latter two shift to 1574 and 1510 cm-1, respectively). On the other hand, it is clear that the surface induces a specific adsorption configuration. In particular, the increase in relative importance of the peak at 1462 cm-1 with respect to that at 1452 cm-1 in the …