Amplified chiral transformation through helical assembly
Amplified chiral transformation through helical assembly
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DOI：
10.1002/anie.200701668
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发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Aida, Takuzo
中科院分区：
文献类型：
--
作者：
Toyofuku, Kunihiko;Alam, Md. Akhtarul;Aida, Takuzo
“手性放大”是这样一种现象，其中形成的产物的对映体纯度高于所应用的手性助剂的对映体纯度。[1-6]这是不对称合成中具有挑战性的课题之一，鉴于手性对称性破缺在自然界中的应用，这也是有趣的。迄今为止报道的成功例子包括：1）催化剂本身或与配体的同/异手性组装，或2）手性产物或中间体参与反应过渡态的形成，导致某些基本步骤的立体选择性加速或减速。[1，2]在此，我们报告了一种概念上的新策略，通过将底物预先转化为螺旋聚合物前体，在不对称转化中实现“手性扩增”。已知几种能够动态螺旋反转的共价聚合物在共价或非共价地容纳手性助剂时形成单手螺旋构象。[3-6]它们中的一些能够实现单手性，甚至当附加的手性助剂不是对映体纯的。这种现象被称为“多数规则”，其中主要对映体决定螺旋手性。[3]我们期望这将适用于不对称合成中的“手性放大”，当选择适当的反应时，由这种螺旋环境介导。作为实现这一概念的第一步，我们尝试了PtII诱导的D2-对称的，具有四个吡啶基的完全取代的卟啉（12 H）的超分子聚合[7]。由于相邻取代基之间的空间排斥，完全取代的卟啉呈非平面的鞍形形状，因此具有点群D2的12 H等卟啉是手性的。[8]然而，由于大环反转的低能量势垒，这些非平面卟啉经历两种对映体形式之间的快速相互转化。在1997年，我们报道了鞍形卟啉（SP）的这种动态手性在与手性酸如（R）-或（S）-扁桃酸相互作用时冻结成两种非对映异构体形式中的任一种（MA;图式1）。[9]有趣的是，尽管SP在倒入AcOH中时失去了相互作用的MA，但由于与AcOH的再生氢键（手性记忆），它保留了从MA转移的绝对构型。[9-11]最近，我们还报道，在SP的环状二聚体中，两个面对的SP单元在构型上彼此互锁，使得环状二聚体
“Chiral amplification” is a phenomenon in which products with a higher enantiomeric purity than those of applied chiral auxiliaries are formed.[1–6] This is one of the challenging subjects in asymmetric synthesis and is also interesting in view of the use of chiral symmetry breaking in nature. Successful examples so far reported include: 1) homo/heterochiral assembly of catalysts themselves or with ligands or 2) the participation of chiral products or intermediates in the formation of transition states of the reaction, resulting in stereoselective acceleration or deceleration of certain elementary steps.[1, 2] Herein we report a conceptually new strategy toward “chiral amplification” in asymmetric transformations by prior conversion of a substrate into a helical polymeric precursor. Several covalent polymers, which are capable of dynamic helical inversion, are known to form a onehanded helical conformation when they accommodate chiral auxiliaries covalently or noncovalently.[3–6] Some of them are able to achieve one handedness even when the appended chiral auxiliaries are not enantiomerically pure. This phenomenon has been referred to as the “majority rule”, in which a major enantiomer determines the helical handedness.[3] We expected this to be applicable to “chiral amplification” in asymmetric synthesis when appropriate reactions that are mediated by such helical environments are chosen.For the first step in the realization of this concept, we attempted PtII-induced supramolecular polymerization [7] of a D2-symmetric, fully substituted porphyrin (12H) with four pyridyl groups. Because of a steric repulsion among the neighboring substituents, fully substituted porphyrins adopt a nonplanar, saddle shape and therefore porphyrins such as 12H with a point group D2 are chiral.[8] However, owing to a low energetic barrier for the macrocyclic inversion, these nonplanar porphyrins undergo a rapid interconversion between the two enantiomeric forms. In 1997, we reported that such a dynamic chirality of saddle-shaped porphyrins (SP) becomes frozen into either of the two diastereoisomeric forms when they interact with a chiral acid such as (R)-or (S)-mandelic acid (MA; Scheme1).[9] Interestingly, although SP loses interacting MA when poured into AcOH, it preserves the absolute configuration, transferred from MA, thanks to the regenerated hydrogen bonds with AcOH (chirality memory).[9–11] More recently, we have also reported that, in a cyclic dimer of SP, the two facing SP units are configurationally interlocked with one another, so that the cyclic dimer