Mechanistic Studies on the Metal-Free Activation of Dihydrogen by Antiaromatic Pentarylboroles

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• 作者：Adrian Y. Houghton ; Virve A. Karttunen ; Cheng Fan ; Warren E. Piers ; Heikki M. Tuononen
• 刊名：The Journal of the American Chemical Society
• 出版年：2013
• 出版时间：January 16, 2013
• 年：2013
• 卷：135
• 期：2
• 页码：941-947
• 全文大小：424K
• 年卷期：v.135,no.2(January 16, 2013)
• ISSN：1520-5126

文摘

The perfluoro- and perprotiopentaphenylboroles 1 and 2 react with dihydrogen to effect H鈥揌 bond cleavage and formation of boracyclopentene products. The mechanism of this reaction has been studied experimentally through evaluation of the kinetic properties of the slower reaction between 2 and H₂. The reaction is first-order in both [borole] and [H₂] with activation parameters of 螖H = 34(8) kJ/mol and 螖S = 鈭?46(25) J mol^鈥? K^鈥?. A minimal kinetic isotope effect of 1.10(5) was observed, suggesting an asynchronous geometry for H鈥揌 cleavage in the rate-limiting transition state. To explain the stereochemistry of the observed products, a ring-opening/ring-closing mechanism is proposed and supported by the separate synthesis of a proposed intermediate and its observed conversion to product. Furthermore, extensive DFT mapping of the reaction mechanism supports the plausibility of this proposal. The study illustrates a new mechanism for the activation of H₂ by a strong main group Lewis acid in the absence of an external base, a process driven in part by the antiaromaticity of the borole rings in 1 and 2.