Phenyl Shifts in Substituted Arenes via Ipso Arenium Ions
Phenyl Shifts in Substituted Arenes via Ipso Arenium Ions
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DOI：
10.1021/jo301848g
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发表时间：
2012-11-02
影响因子：
3.6
通讯作者：
Johnson, Richard P.
中科院分区：
文献类型：
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作者：
Ajaz, Aida;McLaughlin, Erin C.;Johnson, Richard P.
取代芳烃通过自身偶氮阳离子的异构化反应是一种重要而普遍的分子重排反应，它导致结构异构体的相互转化。我们在这里表明，超强酸三氟甲磺酸（TfOH），约。1 M的二氯乙烷（DCE）溶液，为这些重排提供了可靠的催化反应条件，易于在环境温度、回流（84 ℃）或微波反应器中（用于更高温度）下应用。三联苯异构体在TfOH/DCE中在84 ℃下的相互转化得到邻位/Meta/帕拉平衡比为0：65：35，几乎与Olah先前报道的AlCl 3催化的值相同。对于三种三苯基苯，TfOH催化的平衡强烈（>95%）有利于1，3，5-三苯基异构体。平衡三种可能的四苯基苯得到61：39的1，2，3，5-和1，2，4，5-取代异构体的混合物。在探索的反应条件下，这些结构都没有发生显著的Scholl环化。包含溶剂化的DFT计算支持一个机械方案，其中所有的苯基迁移发生在一系列的ipso arenium离子。在研究的每种情况下，在平衡时优选的异构体是那些通过最大放热，因此最不可逆的1，2-H位移产生高度稳定的阳离子的异构体。
The isomerization of substituted arenes through ipso arenium ions is an important and general molecular rearrangement that leads to interconversions of constitutional isomers. We show here that the superacid trifluoromethanesulfonic acid (TfOH), ca. 1 M in dichloroethane (DCE), provides reliable catalytic reaction conditions for these rearrangements, easily applied at ambient temperature, reflux (84 degrees C), or in a microwave reactor for higher temperatures. Interconversion of terphenyl isomers in TfOH/DCE at 84 degrees C gives an ortho/meta/para equilibrium ratio of 0:65:35, nearly identical to values reported earlier by Olah with catalysis by AlCl3. For the three triphenylbenzenes, TfOH-catalyzed equilibration strongly (>95%) favors the 1,3,5-triphenyl isomer. Equilibration of the three possible tetraphenylbenzenes gives a 61:39 mixture of the 1,2,3,5- and 1,2,4,5-substituted isomers. Under the reaction conditions explored, none of these structures undergoes significant Scholl cyclization. DFT calculations with inclusion of solvation support a mechanistic scheme in which all of the phenyl migrations occur among a series of ipso arenium ions. In every case studied, the preferred isomers at equilibrium are those that yield highly stable cations by the most exothermic, hence least reversible 1,2-H shift.