Stable carbocations. 211. 1-Phenylallyl cations and their rearrangement to indanyl cations in superacidic media
Stable carbocations. 211. 1-Phenylallyl cations and their rearrangement to indanyl cations in superacidic media
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DOI：
10.1021/jo00402a006
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发表时间：
1978-08
影响因子：
3.6
通讯作者：
G. Olah;G. Asensio;H. Mayr
中科院分区：
文献类型：
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作者：
G. Olah;G. Asensio;H. Mayr
“相对分配是不确定的。通过将样品重新冷却到-120℃，证明了异构化是不可逆的。根据~(13)C核磁共振谱，在-80℃时，可能是由于4的电环闭合而产生的苯离子8作为中间体被消除了。这一光谱显示了三个乙烯基单重态和四个乙烯基二重态，而8的光谱预计将在同一区域显示两个单重态和五个二重态。因此，10和/或12可能是重排序列4-14中可观察到的中间体。为了区分这两种离子，用FSOsH-SBFS处理了2-(间甲苯基)-3-戊烯-2-醇(6)。即使在-120℃时，也只能观察到环状中间体光谱中的一小部分。在-60℃发生重排反应，中间离子11和13的~(13)C核磁共振谱显示四个烯烃单线和三个双烯线。根据核磁共振谱，13是可能的。由于13的H(6‘)是唯一连接在庚三烯基主链上偶数碳上的质子，它应该是屏蔽最强的烯烃质子。同样的原因，H(4‘)应该是11中屏蔽最强的乙烯基氢。图1b清楚地表明，高场下的烯烃共振是单重态，与13的H(6’)一致。相反，11的H(4‘)应该表现为双重态。类似地，图1a的高场吸收可以指定为12的H(6‘)，由H(5’j)分裂的二重态，而10的H(4‘)应该表现为三重态。正如以前用其他苯离子观察到的那样，12和13的亚甲基都以宽的单重态被吸收。只检测到微量的16和12，这表明在烯丙基阳离子中，7C(3)选择性地攻击C(2‘)位，生成相应的Ar离子9。如方案I所示，建议将4和7至8和9的电环闭合作为初始步骤。通过一系列两个1，2-氢化物移位的进一步重排，分别得到12和13。去质子化-再质子化序列不能解释12和13的形成，因为这将意味着Aro-马季奇环上的凹陷的质子化。然而，这一假说与从苯烯丙醇和FSO3D制备吲哚阳离子时，它们的六元环中没有显示出氚掺入的观察结果相反。观察到7圈比4圈快这一现象可以用以下事实来解释
“Relative assignment is uncertain. isomerizations were irreversible, as demonstrated by recooling the samples to—120 C. The benzenium ion 8, which might result from the electrocyclic ring closure of 4, was eliminated as the intermediate observed at-80 C on the basis of the 13C NMR spectrum. This spectrum shows three vinylic singlets and four vinylic doublets, while the spectrum of 8 is expected to display two singlets and five doublets in the same region. Therefore, 10 and/or 12 are suggested as possible observable intermediates in the rearrangement sequence 4— 14. In orderto differentiate between these two arenium ions, 2-(m-tolyl)-3-penten-2-ol (6) was treated with FSOsH-SbFs. Even at-120 C 7 could only be observed as a minor component in the spectrum of the cyclic intermediate. Rearrange-ment to the indanyl cation 15 occurred at—60 C. As expected for both 11 and 13, the 13C NMR spectrum of the intermediate ion showed four olefinic singlets and three olefinic dou-blets.Based on the NMR spectrum, the identification of 13 was possible. Since H (6') of 13 is the only proton attached to an even numbered carbon in the heptatrienyl backbone, it should be the most shielded olefinic proton. For the same reason, H (4') should be the most shielded vinylic hydrogen in 11. Figure lb clearly shows that the olefinic resonance at highest field is a singlet, in accord with H (6') of 13. On the contrary, H (4') of 11 should display a doublet. Analogously, the high-field absorption of Figure la can be assigned to H (6') of 12, a doublet split by H (5'j, whereas H (4') of 10 should show a triplet. As observed previously with other benzenium ions, 5 the methylene group of both 12 and 13 absorbed as a broad singlet. Only trace amounts of 16 could be detected toge ther with 12, indicating that in the allyl cation 7 C (3) attacks selectively at the C (2') position to yield the corresponding arenium ion 9. Mechanism. As depicted in Scheme I, the electrocyclic ring closure of 4 and 7 to 8 and 9 is proposed to be the initial step. Further rearrangements by a series of two 1, 2-hydride shifts yield 12 and 13, respectively. A deprotonation-reprotonation sequence cannot be accountable for the formation of 12 and 13 since this would imply protonation of indenes on the aro-matic ring. This hypothesis, however, is in contrast to the observation that indanyl cations do not show deuterium incorporation in their six-membered rings when they are pre-pared from phenylallyl alcohols and FSO3D. 3 The observation that 7 cyclizes fasterthan 4 can be explained by the fact that