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德国科学家解决了非经典2-降基碳正离子的结构问题（补充了C&EN）
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德国科学家解决了非经典2-降基碳正离子（2-norbornyl carbocation）的结构问题
诸平
2013年7月5日出版的最新一期《科学》（Science）杂志发表了德国多家大学的研究人员合作完成的研究成果，解决了非经典2-降基碳正离子（2-norbornyl carbocation）结构问题。
非经典碳正离子或非经典离子是有机化学中的一种碳正离子，含有三中心两电子键，σ键的电荷不只定域在一个原子上[1]，“非经典离子”一词一开始是由J. D. Roberts在1951年用来描述环丁烷正离子[2][3]，但此在1949年化学家Saul Winstein就用这种离子解释降冰片基化合物的反应性[4]。上述提及的化合物是exo-降冰片基和对甲苯磺酰基的化合物（下图1中标示1）及其endo异构物（下图1中标示3），而其反应是在乙酸中和乙酸钾进行酰化反应。重要的是在亲核取代反应中二种异构物都产生了相同的生成物（下图1中标示2），而exo构型反应的反应速率是endo构型反应或是环己烷类似反应的350倍。
图1 exo构型反应和endo构型反应
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Fig. 2 Proposed 2-norbornyl cation structures. (A) Nonclassical Cs structure of the 2-norbornyl cation, depicted in 3c-2e and "pi" complex formulations. (B) Brown’s rapidly equilibrating C1 classical norbornyl cation enantiomers. Credit: (c) Science 5 July 2013: Vol. 341 no. 6141 pp. 62-64 DOI: 10.1126/science.1238849 |
图2是提出的2-降基阳离子结构。其中A是2-降基碳正离子的非经典Cs结构，表示的是三中心二电子键π型复合物的结构；而（B）表示C1经典2-降基碳正离子的对映体之间的布朗快速均衡。这是德国化学家对于数十年争论不休，悬而未决难题——非经典2-降基碳正离子的晶体结构问题，通过X-射线结晶学终于得到解决。详见：F. Scholz, D. Himmel, F. W. Heinemann, P. v. R. Schleyer, K. Meyer, I. Krossing. Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation, Science 5 July 2013: Vol. 341 no. 6141 pp. 62-64 DOI: 10.1126/science.1238849.
对于经典和非经典2-降基碳正离子的性质争论已经持续了64年，1949年化学家Saul Winstein就用这种离子解释降冰片基（取代降莰烷）化合物的反应性。而其他化学家如赫伯特·布朗（Herbert Brown）等人对此反应消极，认为如果承认其合理性就意味着接受了碳原子可以与4个以上的其他原子结合成键。因此布朗提出了一种快速平衡的共振结构式，这样不仅可解释化学家一直观察到的一些现象,而且可以与经典的碳4价学说保持一致。
多年来, 化学家对于此争论不休，不过大多数还是最终倾向于非经典型的2-降基碳正离子结构,但是现在看来,化学家在这方面的努力终于使问题真相大白于天下，争论应该休止了。事实证明,只要严格控制实验条件，完全可以通过实验结果使争论得到令人满意的答复。这种突破性的研究已经在德国弗莱堡大学（University of Freiburg）校园里的一个实验室完成。该研究团队使用柔性溴铝酸阴离子（bromoaluminate anions）在固态稳定碳正离子。允许进行有规律的2-降基阳离子盐晶体的制备，然后,大量的工作是围绕着如何使晶体降温反应展开，经过反复实验研究人员发现，将晶体样品冷却到40 K, 然后对其慢慢加热，然后再冷却，反复五或六次，这样做的目的就在于通过加热降温过程来消除晶体结构自身可能存在的裂缝，最终终于等到晶体的真正的非经典结构。实验所得的3种晶体结构与理论计算结果吻合程度良好。更多信息请浏览原文。
C&EN 2013年7月8日出版的第91卷，第27期，第5页对此也有报道，摘引如下：
Solving An Old Bonding Debate
Chemists have used a deep-freeze crystallization method to solve the structure for the 2-norbornyl cation, likely closing an acrimonious 50-year debate on the nature of a fundamental bonding concept. Instead of two electrons shared between two carbon atoms, as in conventional single-bond structures, this cation structure shows nonclassical bonding, with two electrons delocalized over three carbon atoms (Science 2013, DOI: 10.1126/science.1238849).
The structural analysis of the cation, a bridged cyclic hydrocarbon, was led by Ingo Krossing of the University of Freiburg and Karsten Meyer of the University of Erlangen-Nuremberg, both in Germany.
The debate centered on different interpretations of thermodynamic and spectroscopic data for the cation. Last year, University of Richmond chemist and historian Jeffrey I. Seeman characterized the interactions between the two sides as “crude and rude” (C&EN, Nov. 19, 2012, page 44). A crystal structure of the cation likely would have resolved the arguments, but that experimental evidence was elusive. Many other researchers have obtained 2-norbornyl cation crystals, but their molecular disorder stumped scientists, Krossing says.
Meyer, Krossing, and colleagues managed to obtain the structure by stabilizing the 2-norbornyl cation with a bromoaluminate counterion, crystallizing the product for several days at a chilly 245 K. They developed an annealing process of cooling the crystals even further to 40 K, then repeatedly warming and cooling the crystals to freeze out molecular disorder, allowing them to collect good X-ray data.
The structure reveals the two-electron, three-carbon system as having two C–C bonds of 1.80 Å, longer than a standard 1.54-Å C–C single bond. The third bond in the system is 1.39 Å long, similar to the delocalized C–C bonds of benzene.
The work is “a triumph of creative crystallography in taming an uncooperative cation,” says theoretical organic chemist Dean J. Tantillo of the University of California, Davis. Although the existing evidence had largely convinced chemists that the nonclassical structure existed in the gas phase and in solution, the new structure demonstrates that it exists in the solid state as well, Tantillo says.
https://blog.sciencenet.cn/blog-212210-705794.html
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