Comprehensive Synthesis of Monohydroxy-Cucurbit[n]urils (n=5, 6, 7, 8): High Purity and High Conversions
Comprehensive Synthesis of Monohydroxy-Cucurbit[n]urils (n=5, 6, 7, 8): High Purity and High Conversions
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DOI：
10.1021/jacs.5b04553
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发表时间：
2015-08-19
影响因子：
15
通讯作者：
Ouari, Olivier
中科院分区：
文献类型：
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作者：
Ayhan, Mehmet M.;Karoui, Hakim;Ouari, Olivier
我们描述了一种光化学方法，利用过氧化氢和紫外线直接在葫芦[n]尾端（n = 5,6,7,8）上引入单个醇函数，其转化率为95-100%。c[6]和c[7]的反应很容易扩大到1g。葫芦脲自由基的自旋捕获结合质谱实验，使我们对反应机理有了更深入的了解，并对CB[5]、CB[6]、CB[7]和C13[8]单功能化合物进行了表征。用O-18同位素标记水进行的实验表明，该反应机理复杂，同时存在自由基和离子中间体。DFT计算可以估算出CB系列中每种氢原子类型的键解离能（BDEs），从而解释了CB[n]化合物的“赤道”C-H位置具有较高的反应活性。这些结果还表明，对于CB[8]，在葫芦脲骨架上的直接功能化更加困难，因为其中一个亚甲基氢原子（H-b）的BDE在系列内降低，接近H-c的BDE，从而为CB[8]骨架上产生的其他类型的自由基开辟了道路，导致几种副产物。然而，CB[5]-(OH)(1)和CB[8]-(OH)(1)是CB[8]的第一个衍生物，由于本文提出的软方法，产率很高。
We describe a photochemical method to introduce a single alcohol function directly on cucurbit[n]urils (n = 5, 6, 7, 8) with conversions of the order 95-100% using hydrogen peroxide and UV light. The reaction was easily scaled up to 1 g for CB[6] and CB[7]. Spin trapping of cucurbituril radicals combined with MS experiments allowed us to get insights about the reaction mechanism and characterize CB [5], CB[6], CB[7], and C13[8] monofunctional compounds. Experiments involving O-18 isotopically labeled water indicated that the mechanism was complex and showed signs of both radical and ionic intermediates. DFT calculations allowed estimating the Bond Dissociation Energies (BDEs) of each hydrogen atom type in the CB series, providing an explanation of the higher reactivity of the "equatorial" C-H position of CB[n] compounds. These results also showed that, for CB [8], direct functionalization on the cucurbituril skeleton is more difficult because one of the methylene hydrogen atoms (H-b) has its BDE lowering within the series and coming close to that of H-c, thus opening the way to other types of free radicals generated on the CB[8] skeleton leading to several side products. Yet CB[5]-(OH)(1) and CB[8]-(OH)(1), the first CB[8] derivative, were obtained in excellent yields thanks to the soft method presented here.