硼基酚與酚自由基之合成、電化學、與磁性質探討

Borylated Phenol and Phenoxyl Radical: Synthesis, Electrochemistry, and Magnetic Property

馮博彥(Po-Yeng Feng)

指導教授 ： 邱靜雯

國立臺灣大學/理學院/化學研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.02388

摘要

由非金屬元素所組成之自由基分子，通常是較不穩定的，所以需要特定的電子與立體穩定效應來降低其反應性。至今，將含硼官能基引入有機自由基的例子相當少。由於硼的正電性致使其具有較一般有機取代基要強的sigma-donating能力，此外，硼原子上空的p軌域，亦可離域自由基中的未成對電子，所以引入含硼的官能基預期可以幫助穩定有機自由基。為驗證此想法，我們合成了帶有雙三均甲苯硼基之酚自由基，並且去探討在氧化還原反應中結構上的改變。依據酚自由基之電子自旋共振光譜及其模擬數據得知，硼原子以及均三甲苯基上對位甲基上的氫皆有貢獻，證實了硼取代基的確可以穩定酚自由基上未配對的π電子。除此之外，我們還發現並證實雙三均甲苯硼基酚會藉由一連串的單電子還原-氫原子轉移-單電子還原來形成帶兩個負電荷的酚氫硼酸鹽。另外，電中性的雙三均甲苯硼基酚自由基還可以藉由單電子還原來形成相對應的苯昆硼烯陰離子，並發現苯昆硼烯陰離子中π電子的離域程度會受到陽離子配位能力的影響。

關鍵字

酚自由基 ； 硼烷 ； 電子轉移 ； 氫原子轉移 ； 還原

並列摘要

Free radicals that are exclusively composed of non-metal elements are relatively unstable, and generally require specific electronic and steric stabilization effects to realize the structural characterization of these open-shelled species. To date, substitution of organic free radicals with diarylboryl group has only been reported for few times. Taking into account the sigma-donating ability governed by the electropositive nature of boron, the boryl substituent should help in stabilizing the electron deficient phenoxyl radical. Furthermore, the empty p-orbital at boron could delocalize the un-paired electron and lead to stable free radicals. To evaluate the idea, we have prepared the dimesityl boryl substituted phenoxyl radical, and to investigate its structural changes under redox events. Interestingly, we have also discovered an electron reduction triggered sequential electron and hydrogen atom transfer process, which leads to transformation from a hydroxyphenyl-substituted borane to a hydridoborate phenoxide dianion. The neutral borylated phenoxyl radical could be further reduced with cobaltocene to the corresponding benzoquinone borataalkene derivative. ESR spectroscopic measurement of the free radical reveals a hyperfine-coupling constant of 3.424 G and 1.142 G to the 11B (I = 3/2) and 10B (I = 3) nuclei, respectively. This result indicated that the boryl substituent indeed stabilizes the open-shelled species through delocalization of the spin density on the phenol ring.

並列關鍵字

Phenoxyl Radical ； Borane ； Electron Transfer ； Hydrongen-atom Transfer ； Reduction

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