Ajami Dariush, Rebek Julius
The Skaggs Institute for Chemical Biology and Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, USA.
J Org Chem. 2009 Sep 4;74(17):6584-91. doi: 10.1021/jo901072m.
Self-assembled, hydrogen-bonded capsules emerge from synthetic resorcinarene-derived cavitands and soluble glycolurils when appropriate guest molecules are present. The assembly consists of 2 cavitands, 4 glycolurils and guest(s), and the arrangement of glycolurils leads to a chiral structure. The capsule features a space of approximately 620 A(3) and accommodates narrow guests such as n-alkanes from C(14) to C(19), or other molecules (e.g., capsaicin) and combinations of molecules of up to approximately 22 A in length (e.g., two p-methylstyrene molecules). Positions of encapsulated nuclei can be predicted from NMR chemical shifts, with intense shielding of deltaDelta = -5 ppm near the resorcinarene ends and mild deshielding of +0.5 to 1 ppm near the glycolurils at the capsule's center. Computational methods using nucleus independent chemical shifts (NICS) were used to map the induced magnetic shielding/deshielding for the inner space of the cavity. The asymmetric arrangement of the spacers creates a chiral steric and magnetic environment in the capsule and the geminal hydrogen atoms of encapsulated alkanes show diastereotopic proton signals. The two enantiomers interconvert (racemize) through an achiral intermediate involving a slight rotation of the spacers and lengthening of the cavity. Accordingly, longer, compressed alkanes accelerate the racemization by applying pressure from the inside on the capsule's ends. Guests that place hydrogen bond donors and acceptors near the glycolurils in the middle (e.g., p-isopropylbenzyl alcohol) also accelerate the racemization by facilitating the rotation of the glycolurils. Slow tumbling of guest on the NMR time scale inside the capsule leads to social isomerism of para-disubstituted benzenes such as p-methylstyrene. Flexible guests such as hexane tumble inside the cavity with an activation barrier of DeltaG(++) =16.2 kcal/mol. The middle of the extended capsule is narrow, but still accommodates phenyl groups such as those presented by p-quaterphenyl and alkylated biphenylcarbonitriles. The aromatic units in these guests report their positions by imparting magnetic anisotropy to the capsule components. Gases such as propane, butane, isobutane, propylene, 2-methylpropene, and 1,3-butadiene even xenon are coencapsulated with other guests and their motions inside are examined.
当存在合适的客体分子时，由合成的间苯二酚杯芳烃衍生的穴状配体和可溶性甘脲自组装形成氢键连接的胶囊。该组装体由2个穴状配体、4个甘脲和客体组成，甘脲的排列导致形成手性结构。该胶囊的空间约为620 ų，可容纳窄的客体，如C(14)至C(19)的正构烷烃，或其他分子（如辣椒素）以及长度达约22 Å的分子组合（如两个对甲基苯乙烯分子）。包封核的位置可通过核磁共振化学位移预测，在间苯二酚杯芳烃末端附近有强烈的屏蔽，δδ = -5 ppm，在胶囊中心的甘脲附近有+0.5至1 ppm的轻微去屏蔽。使用核独立化学位移（NICS）的计算方法来绘制腔内空间的感应磁屏蔽/去屏蔽图。间隔基的不对称排列在胶囊中创造了手性空间和磁性环境，包封烷烃的偕氢原子显示出非对映异位质子信号。两种对映体通过涉及间隔基轻微旋转和腔延长的非手性中间体相互转化（外消旋化）。因此，更长、压缩的烷烃通过从内部对胶囊末端施加压力来加速外消旋化。在中间的甘脲附近放置氢键供体和受体的客体（如对异丙基苄醇）也通过促进甘脲的旋转来加速外消旋化。客体在胶囊内核磁共振时间尺度上的缓慢翻滚导致对二取代苯（如对甲基苯乙烯）的自旋异构体。像己烷这样的柔性客体在腔内翻滚，活化能垒为ΔG‡ = 16.2 kcal/mol。伸长的胶囊中间很窄，但仍能容纳诸如对四联苯和烷基化联苯腈所呈现的苯基。这些客体中的芳香单元通过赋予胶囊组分磁各向异性来报告它们的位置。丙烷、丁烷、异丁烷、丙烯, 2-甲基丙烯和1,3-丁二烯甚至氙等气体与其他客体共包封，并研究它们在内部的运动。
