Huang Wei-Hao, Zavalij Peter Y, Isaacs Lyle
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA.
J Am Chem Soc. 2008 Jul 2;130(26):8446-54. doi: 10.1021/ja8013693. Epub 2008 Jun 5.
In contrast to the high yield formation of cucurbit[n]uril (CB[n]) from a 1:2 ratio of glycoluril to formaldehyde, the condensation of glycoluril with less than 2 equiv of formaldehyde delivers a reaction mixture that contains glycoluril oligomers (2-6) and CB[n] compounds that lack one or more methylene bridges known as nor-seco-cucurbit[n]urils (ns-CB[n]). In this paper we report the chromatographic purification of C-shaped glycoluril oligomers (dimer-hexamer), their characterization in solution, and their X-ray crystal structures. Quite interestingly, despite being acyclic glycoluril pentamer 5 and hexamer 6 retain the ability to bind to guests typical of CB[6] but are also able to expand their cavity to accommodate larger guests like cationic adamantane derivatives. We performed product resubmission experiments with glycoluril oligomers 2-6 and found preferences for the formation of specific ring sizes during CB[n] formation. A comprehensive mechanistic scheme is proposed that accounts for the observed formation of 2-6 and ns-CB[n]. Overall, the experiments establish that a step-growth cyclo-oligomerization process operates during CB[n] formation.
与由甘脲与甲醛按1:2比例高产率生成葫芦[n]脲（CB[n]）不同，甘脲与少于2当量甲醛的缩合反应得到的反应混合物中含有甘脲低聚物（2 - 6）以及缺少一个或多个亚甲基桥的CB[n]化合物，即去亚甲基葫芦[n]脲（ns - CB[n]）。在本文中，我们报道了C形甘脲低聚物（二聚体 - 六聚体）的色谱纯化、它们在溶液中的表征以及它们的X射线晶体结构。非常有趣的是，尽管是非环状的甘脲五聚体5和六聚体6仍保留了与CB[6]典型客体结合的能力，但它们也能够扩大其空腔以容纳更大的客体，如阳离子金刚烷衍生物。我们用甘脲低聚物2 - 6进行了产物重新提交实验，发现在CB[n]形成过程中对特定环尺寸的形成有偏好。提出了一个全面的机理方案来解释观察到的2 - 6和ns - CB[n]的形成。总体而言，这些实验表明在CB[n]形成过程中发生了逐步增长的环低聚过程。
