Cucurbit[n]uril formation proceeds by step-growth cyclo-oligomerization
Cucurbit[n]uril formation proceeds by step-growth cyclo-oligomerization
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DOI：
10.1021/ja8013693
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发表时间：
2008-07-02
影响因子：
15
通讯作者：
Isaacs, Lyle
中科院分区：
文献类型：
--
作者：
Huang, Wei-Hao;Zavalij, Peter Y.;Isaacs, Lyle
与糖脲与甲醛的比例为1:2的高产率形成葫芦[n]脲（CB[n]）相反，糖脲与少于2等量甲醛的缩合产生的反应混合物包含糖脲低聚物（2-6）和CB[n]化合物，这些化合物缺乏一个或多个亚甲基桥，称为非次级葫芦[n]脲（ns-CB[n]）。本文报道了c型乙二醇二聚体（二聚六聚体）的色谱纯化、溶液表征及其x射线晶体结构。非常有趣的是，尽管是无环糖基五聚体5和六聚体6保留了与CB[6]典型的客体结合的能力，但也能够扩大它们的腔以容纳更大的客体，如阳离子金刚烷衍生物。我们对二聚乙二醇低聚物2-6进行了产品再提交实验，发现了在CB[n]形成过程中形成特定环尺寸的偏好。提出了一个综合的机制方案来解释观察到的2-6和ns-CB的形成[n]。总的来说，实验表明，在CB[n]形成过程中发生了一个阶梯生长环寡聚化过程。
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.