Role of Equilibrium Associations on the Hydrogen Atom Transfer from the Triethylborane鈥揗ethanol Complex

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• 作者：Guillaume Povie ; Mattia Marzorati ; Peter Bigler ; Philippe Renaud
• 刊名：The Journal of Organic Chemistry
• 出版年：2013
• 出版时间：February 15, 2013
• 年：2013
• 卷：78
• 期：4
• 页码：1553-1558
• 全文大小：324K
• 年卷期：v.78,no.4(February 15, 2013)
• ISSN：1520-6904

文摘

The triethylborane鈥搈ethanol system used in radical deoxygenation and dehalogenation processes has been investigated. Unambiguous evidence for the formation of a complex between triethylborane and methanol is provided. It was shown that the complexation process is exothermic (螖H掳 鈮?鈭?.6 kcal mol^鈥?) while being entropically disfavored (螖S掳 鈮?鈭?4 cal mol^鈥? K^鈥?). This study demonstrates that only very small quantities of complex (1鈥?%) are present in most of the reported conditions used in dehalogenation and deoxygenation processes. Recalculating the rate constant for the hydrogen transfer to a secondary alkyl radical with this concentration suggests a value in the 10⁶ M^鈥? s^鈥? range for the complex itself, indicating a much more important activation of the O鈥揌 bond than previously thought. The importance of solvent effects is also highlighted. The formation of a larger amount of complex by the addition of methanol is accompanied by its deactivation via hydrogen bonding. These observations open new opportunitites for the future preparation of more effective hydrogen atom donors involving borane complexes.