Stable carbocations. 210. .sigma.-Bond bridged carbonium ions. 8. The chemistry of protoadamantane. 7. Rapidly equilibrating unsymmetrically bridged 1,3,5,7-tetramethyl- and rapidly equilibrating trivalent 1,2,3,5,7-pentamethyl-2-adamantyl cations. Addivity of carbon-13 NMR chemical shifts relating
Stable carbocations. 210. .sigma.-Bond bridged carbonium ions. 8. The chemistry of protoadamantane. 7. Rapidly equilibrating unsymmetrically bridged 1,3,5,7-tetramethyl- and rapidly equilibrating trivalent 1,2,3,5,7-pentamethyl-2-adamantyl cations. Addivity of carbon-13 NMR chemical shifts relating
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稳定的碳正离子。
DOI：
10.1021/ja00522a041
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发表时间：
1980
期刊：
影响因子：
--
通讯作者：
G. Olah
中科院分区：
文献类型：
--
作者：
P. Schleyer;D. Lenoir;P. Mišoň;G. Liang;G. Prakash;G. Olah
在sffs - schclf溶液中，通过l3C NMR研究发现，稳定的仲离子1,3,5,7 -四甲基-2-金刚烷基阳离子（3，R= H）被描述为一组快速平衡的简并不对称桥接离子（5a-d）。相反，相应的叔原子1,2,3,5,7 -五甲基-2-金刚烷基阳离子（3，R= CFG）在低温（低于80℃）下是静态的三元碳正离子。在较高温度下，3 （R= CH3）通过相应的原金刚烷基阳离子进行骨架重排的简并平衡。在较高的温度下，3 （R= FI）以类似的机制异构化为1,2,5,7 -四甲基-2-金刚烷基阳离子（6，R= FF）。二级和三级2-金刚烷基化（3，R= FI vs. CH3）的不同结构和性质通过l3C化学移位加性准则进行了分配。通过与已知的非经典离子和三价（经典）离子的比较，以及与相关硼化合物的比较，支持了这种可加性偏差是由于碳（桥接或非经典结构）上较高的配位所致的结论。碳正离子和相应的中性烃之间的总化学位移差（3C）也提供了一个粗略但有用的结构指标。经典的碳正离子表现出很大的化学位移差异，通常为350ppm或更多，而相关的非经典阳离子表现出的差异通常小于百万分之几百。这些类别之间没有明显的区别；有些阳离子在性质上是中间的（例如，它们可能是不对称桥接的）。
The stable secondary 1, 3, 5, 7-tetramethyl-2-adamantyl cation (3, R= H), studied by l3C NMR spectroscopy in SbFs-SChClF solution, was found tobe described best as a rapidly equilibrating set of degenerate unsymmetrically bridged ions (5a-d). The corresponding tertiary 1, 2, 3, 5, 7-pentamethyl-2-adamantyl cation (3, R= CFG), in contrast, is a static triva-lent carbocation at low temperatures (below—80 C). At highertemperatures, 3 (R= CH3) undergoes degenerate equilibration involving skeletal rearrangement via the corresponding protoadamantyl cations. At higher temperatures, 3 (R= FI) isomerizes to the 1, 2, 5, 7-tetramethyl-2-adamantyl cation (6, R= FF) by a similar mechanism. The differing structures and nature of the secondary and tertiary 2-adamantylcations (3, R= FI vs. CH3) are assigned by means of the l3C chemical shift additivity criterion. The conclusion that deviations from such additivity are due to a higher coordination at carbon (bridged or non-classical structures) is supported, eg, by comparisons with known nonclassical and trivalent (classical) ions and by compari-sons with related boroncompounds. The total, 3C chemical shift difference between a carbocation and the corresponding neu-tral hydrocarbon also provides a rough, but useful, structural index. Classical carbocations show large chemical-shiftdiffer-ences, typically 350 ppm or more, whereas related nonclassical cations display differences often hundreds of parts per million less. There is no sharp distinction between these categories; some cations are indicated to be intermediate in character (eg, they may be unsymmetrically bridged).