Longer guests drive the reversible assembly of hyperextended capsules
Longer guests drive the reversible assembly of hyperextended capsules
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DOI：
10.1002/anie.200702245
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发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Rebek, Julius, Jr.
中科院分区：
文献类型：
--
作者：
Ajami, Dariush;Rebek, Julius, Jr.
我们最近报道，当合适的客人在场时，圆柱形胶囊宿主1.1结合了糖脲间隔物2a， b，以形成一个扩展的组装1·24·1（图1）插入四个乙二醇分子，它们增加了空腔的长度，以容纳较长的烷烃和其他分子。使用弱碱性的二羟基乙酸乙酯2c，设计了一个弹簧加载系统[2]，使用外部酸和碱在两个扩展组件和原始组件之间可逆切换。2c的高溶解度允许获得更大比例的间隔剂与空腔，我们在这里报告了意想不到的结果。我们发现，在客人在场的情况下，新的甚至进一步扩大的胶囊会自发组装，这些胶囊太长，无法在1·24·1范围内安装。我们提出了这些包封复合物在溶液中的结构和它们涌现行为的一些基本原理。许多核磁共振波谱研究表明，包封烷烃的化学位移与其在胶囊内的位置和构象有关。图2显示了正十四烷（nC 14H30）封装在1·1和1·24·1中的前场区域。如图2a所示，该烷烃在1·1中卷曲（压缩），在1·24·1中扩展（松弛）。在后者中，光谱（图2b）反映了亚甲基信号从胶囊末端向内部手性微环境移动时的下场位移。乍一看，不对称元件离腔体的末端很远，但在组装中心的糖基脲间隔物的手性排列，通过这些壁的刚性有效地转移了八面墙包封的正构烷烃的核磁共振光谱通常显示倒数第二碳原子上的氢原子是非对映异构的。例如，在1·24·1内的十四烷烃的C2和C4（以及对称的C13和C11）上的质子都表现出这种效应。在较高的温度下，糖脲分子的对映体排列开始相互转换，客体的非对映体信号结合。在新的组装体1·24·1中可以看到C15链和一直到C18的正构烷烃的封装，并且我们报道了在。[1]中更长的分子来宾C19H40的情况令人费解，因为烷烃太长，不能在1·24·1的扩展构象中适应（表1）。然而，前场区域
We recently reported that the cylindrical capsule host 1· 1 incorporates glycoluril spacers 2a, b when suitable guests are present to give an expanded assembly 1· 24· 1 (Figure 1).[1] Four glycoluril molecules are inserted, and they increase the cavity s length to accommodate longer alkanes and other molecules. With the weakly basic glycoluril 2c, a springloaded system [2] was devised using external acids and bases to switch reversibly between the two expanded and original assemblies. The high solubility of 2c allows access to larger ratios of spacer to cavitand, and we report herein the unexpected consequences. We find spontaneous assembly of new and even further expanded capsules in the presence of guests that are too long to fit within 1· 24· 1. We propose structures for these encapsulation complexes in solution and some rationale for their emergent behavior. Many NMR spectroscopic studies have shown that the chemical shifts of encapsulated alkanes are related to their positions and conformations within the capsule 1· 1. Figure2 shows the upfield regions of n-tetradecane (nC 14H30) encapsulated in 1· 1 and in 1· 24· 1. This alkane is coiled (compressed) in 1· 1, as shown by the close spacing of the signals (Figure 2a) and extended (relaxed) in 1· 24· 1. In the latter, the spectrum (Figure 2b) reflects the downfield shifts of the methylene signals as they move away from the capsule s ends and toward the chiral microenvironment inside. At first glance, the asymmetric elements are far from the cavity s end, but the chiral arrangement of the glycoluril spacers at the center of the assembly shifts the eight walls in a manner that is transmitted effectively by the very rigidity of those walls.[3] NMR spectra of encapsulated n-alkanes often show hydrogen atoms on the penultimate carbon atoms to be diastereotopic. For example, the protons at both C2 and C4 (and, by symmetry, C13 and C11) of ntetradecane inside 1· 24· 1 show this effect. At higher temperatures, the enantiomeric arrangements of the glycoluril molecules begin to interconvert, and the diastereotopic signals of the guest coalesce. The encapsulation of the C15 chain and normal alkanes up to C18 are seen in the new assembly, 1· 24· 1, and we reported even longer molecular guests inside.[1] The case of C19H40 was puzzling, as the alkane is too long to fit within 1· 24· 1 in an extended conformation (Table 1). Yet the upfield region of