Helicity inversion in responsive foldamers induced by achiral halide ion guests
Helicity inversion in responsive foldamers induced by achiral halide ion guests
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DOI：
10.1002/anie.200800796
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发表时间：
2008-01-01
影响因子：
16.6
通讯作者：
Hecht, Stefan
中科院分区：
文献类型：
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作者：
Meudtner, Robert M.;Hecht, Stefan
在过去的十年中，对折叠体[1]和螺旋折叠聚合物[2]的兴趣大大增加，反映了这一新兴领域的巨大潜力。[3]一个特别有吸引力的方面与骨架的明确但动态的构象相关，沿着其对外部刺激的响应性，其可用于实现靶向致动、[4，5]递送、[5]和感测中的应用的各种功能。[6]在后一种情况下，螺旋骨架的固有手性使得可以通过圆二色性（CD）光谱法方便地监测构象变化。理想情况下，各种客体的结合导致形成过量的一种螺旋形式，这与外部诱导的CD效应有关。[7，8]手性转移到通常的非手性骨架是由与手性客体的相互作用引起的，[7，9]因此，（过量）螺旋扭曲方向的反转[10]通常是由相同客体但另一种对映体形式的结合引起的。[11]进一步深入了解手性转移和放大的基本原理，作为生物大分子中同手性的一种潜在机制[12]，将能够设计用于手性识别，传感和分离以及催化的新材料。[13]为此目的，期望容易获得响应性折叠体平台，其允许方便地调节性质（甚至在非生物条件下）。在这里，我们提出了我们的工作，一个新的家庭的响应折叠分子，所谓的“clickamers”，并描述了他们的有效合成和调查，他们的折叠行为在各种条件下。重要的是，我们报告了一个前所未有的螺旋反转现象[11]，观察到响应非手性客体分子（卤素离子）。我们的折叠体设计受到Lehn及其同事开发的“螺旋密码子”方法的启发，该方法利用了联吡啶和各种氮杂芳烃的强烈偏好，以采用反构象来产生螺旋结构。[14]我们利用高效的Cu催化的芳基叠氮化物和芳基乙炔的1，3-偶极环加成反应，通常称为点击反应[15]，以生成由基于交替的三唑和吡啶部分的嵌入式螺旋基序组成的主链。所得的2，6-双（1，2，3-三唑-4-基）吡啶结构具有强烈的反、反构象偏好，[16 a]并且预组织的扭结结构单元通过间亚苯基铰链连接。围绕最小数量的连接键（在间亚苯基铰链处）的旋转导致折叠成螺旋构象，该螺旋构象通过所产生的π-π堆积相互作用而在旋光上稳定（图1）。在两亲性体系中，疏溶剂效应也可以诱导进一步的稳定化。[17]带有17个（杂）芳族部分的低聚物模型说明了我们的clickamers的另一个有趣的结构特征，
Interest in foldamers [1] and helically folding polymers [2] has increased considerably over the past decade, reflecting the vast potential of this emerging field.[3] One particularly attractive aspect is associated with the well-defined yet dynamic conformation of the backbone along with its responsiveness towards external stimuli that can be used to implement various functions for applications in targeting actuation,[4, 5] delivery,[5] and sensing.[6] In the latter case, the inherent chirality of the helical backbone makes it possible to conveniently monitor conformational changes by circular dichroism (CD) spectroscopy. Ideally the binding of various guests leads to the formation of an excess of one helical form, which is associated with an externally induced CD effect.[7, 8] The transfer of chirality to the usually achiral backbone is caused by interaction with a chiral guest,[7, 9] and as a result, the inversion of the (excess) helical twist sense [10] is typically caused by the binding of the same guest but the other enantiomeric form.[11] Further insights into the fundamentals of chirality transfer and amplification as one potential mechanism responsible for the homochirality in biomacromolecules [12] will enable the design of new materials for chiral recognition, sensing, and separation as well as catalysis.[13] For this purpose, the facile access to a responsive foldamer platform, which allows for convenient tuning of properties (even under abiotic conditions), is desirable. Here, we present our work on a novel family of responsive foldamers, so-called “clickamers”, and describe their efficient synthesis and investigations of their folding behavior under a variety of conditions. Importantly, we report on an unprecedented phenomenon of helix inversion [11] observed in response to achiral guest molecules (halide ions). Our foldamer design is inspired by the “helicity codon” approach developed by Lehn and co-workers that exploits the strong preference of bipyridine and various aza-arenes for adopting an anti conformation to generate helical structures.[14] We utilize the highly efficient Cu-catalyzed 1, 3-dipolar cycloaddition reaction of aryl azides and aryl acetylenes, commonly referred to as the click reaction,[15] to generate a backbone consisting of embedded helicogenic motifs based on alternating triazole and pyridine moieties. The resulting 2, 6-bis (1, 2, 3-triazol-4-yl) pyridine structure has a strong preference for the anti, anti conformation,[16a] and the preorganized kinked building blocks are connected by metaphenylene hinges. Rotation about a minimal number of connecting bonds (at the meta-phenylene hinges) leads to folding into a helical conformation that is enthalpically stabilized by the resulting π–π stacking interactions (Figure1). In an amphiphilic system, further stabilization can be induced by the solvophobic effect as well.[17] Models of an oligomer bearing 17 (hetero) aromatic moieties illustrate another interesting structural feature of our clickamers as