Mechanistic Insight into Phenol Dearomatization by Hypervalent Iodine: Direct Detection of a Phenoxenium Cation
Mechanistic Insight into Phenol Dearomatization by Hypervalent Iodine: Direct Detection of a Phenoxenium Cation
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DOI：
10.1021/acs.joc.2c01765
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发表时间：
2022-10-10
影响因子：
3.6
通讯作者：
Frenette, Mathieu
中科院分区：
文献类型：
--
作者：
Juneau, Antoine;Lepage, Iannick;Frenette, Mathieu
苯酚脱芳化反应是由高价碘试剂引起的几种氧化反应之一。然而，在苯酚脱芳过程中是否存在游离苯氧苯中间体在文献中存在争议。在这里，我们报道了利用紫外-可见和共振拉曼光谱在富电子苯酚，2,4,6-三甲氧基苯酚和（二乙酰氧基）苯的反应中明确检测到游离苯氧基中间体。相比之下，我们主要使用紫外-可见和电子顺磁共振（EPR）光谱检测电子含量较低的酚类或烷氧基取代芳烃与（二乙酰氧基）苯反应的单电子氧化产物。我们的结论是，通常假设的游离苯氧苯中间体，虽然可能具有高度稳定的取代基，但不太可能是典型酚与高价碘试剂反应的一般机制途径。极性溶剂1,1,1,3,3,3-六氟异丙醇（HFIP）或使用氧化性更强的高价碘试剂，如[双（三氟乙酰氧基）碘]苯（PIFA）或[羟基（tosyloxy）碘]苯（HTIB），可以帮助减少自由基副产物的形成，有利于苯氧基中间体的形成。
Phenol dearomatization is one of several oxidation reactions enabled by hypervalent iodine reagents. However, the presence of a proposed free phenoxenium intermediate in phenol dearomatization is a matter of debate in the literature. Here, we report the unambiguous detection of a free phenoxenium intermediate in the reaction of an electron-rich phenol, 2,4,6-trimethoxyphenol, and (diacetoxyiodo)benzene using UV-vis and resonance Raman spectroscopies. In contrast, we predominantly detect single electron oxidation products of less electron-rich phenols or alkoxy-substituted aromatics in their reaction with (diacetoxyiodo)benzene using UV-vis and electron paramagnetic resonance (EPR) spectroscopies. We conclude that the often-postulated free phenoxenium intermediate, while possible with highly stabilizing substituents, is unlikely to be a general mechanistic pathway in the reaction of typical phenols with hypervalent iodine reagents. The polar solvent 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) or the use of more strongly oxidizing hypervalent iodine reagents, such as [bis(trifluoroacetoxy)iodo]benzene (PIFA) or [hydroxy(tosyloxy)iodo]benzene (HTIB), can help reduce the formation of radical byproducts and favors the formation of phenoxenium intermediates.