Gas-phase organocatalysis with crown ethers
Gas-phase organocatalysis with crown ethers
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DOI：
10.1039/c2sc00796g
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发表时间：
2012-03
期刊：
影响因子：
8.4
通讯作者：
H. D. Winkler;Egor V. Dzyuba;A. Springer;Luisa Losensky;C. Schalley
中科院分区：
文献类型：
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作者：
H. D. Winkler;Egor V. Dzyuba;A. Springer;Luisa Losensky;C. Schalley
催化循环可以在气相中通过串联质谱实验以逐步的方式详细研究，该串联质谱实验允许在后续步骤之前对每个中间体进行质量选择。在本文中，我们描述了大冠醚（二苯并-24-冠-8二苯并-30-冠-10），以介导的E2-消除丙烯从冠醚-丙基铵复合物产生NH 4 +/冠醚复合物。氨与丙胺的反交换完成催化循环。据我们所知，这个反应循环代表了气相有机催化的第一个例子。此外，较大的冠醚显着加速H/D-交换反应的五个NH氢原子在单质子化乙二胺相比，类似的较小的冠醚配合物。这种效果，这是没有观察到的丙基铵配合物，可以合理化的质子化乙二胺离子的冠醚的空腔内通过氢键结合到铵和胺基团的有利的预组织。在这种复合体中，H/D交换可以通过有效的“中继”机制进行。不同尺寸的冠醚铵配合物的H/D-交换行为的比较揭示了一个独特的反应模式，强烈依赖于冠醚的大小。虽然18-冠-6作为非共价保护基团几乎完全抑制H/D-交换，但较小和较大的冠醚表现出较高的交换速率。
Catalytic cycles can be investigated in detail in the gas phase in a step-by-step manner by tandem mass spectrometric experiments that allow for the mass-selection of each of the intermediates prior to the subsequent step. In the present article, we describe large crown ethers (dibenzo-24-crown-8 to dibenzo-30-crown-10) to mediate the E2-elimination of propene from crown ether-propylammonium complexes giving rise to NH4+/crown ether complexes. A back exchange of the ammonia against propyl amine completes the catalytic cycle. To the best of our knowledge, this reaction cycle represents the first example of organocatalysis in the gas-phase. In addition, the larger crown ethers significantly accelerate the H/D-exchange reaction of the five NH hydrogen atoms in singly protonated ethylenediamine as compared to the analogous smaller crown ether complexes. This effect, which is not observed for the propylammonium complexes, can be rationalized by a favorable pre-organization of the protonated ethylenediamine ion within the cavity of the crown ether through hydrogen bonding to both the ammonium and the amine groups. Within this complex, the H/D-exchange can proceed through an efficient “relay” mechanism. A comparison of the H/D-exchange behavior of ammonium complexes with differently sized crown ethers reveals a distinct reactivity pattern that strongly depends on the crown ether size. While 18-crown-6 acts as a non-covalent protective group suppressing the H/D-exchange almost completely, smaller and larger crown ethers exhibit higher exchange rates.