Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4.
J Am Chem Soc. 2013 Jan 16;135(2):941-7. doi: 10.1021/ja311842r. Epub 2013 Jan 4.
The perfluoro- and perprotiopentaphenylboroles 1 and 2 react with dihydrogen to effect H-H 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 H(2). The reaction is first-order in both [borole] and [H(2)] with activation parameters of ΔH(‡) = 34(8) kJ/mol and ΔS(‡) = -146(25) J mol(-1) K(-1). A minimal kinetic isotope effect of 1.10(5) was observed, suggesting an asynchronous geometry for H-H 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 H(2) 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.
全氟和全丙戊二烯基五苯硼烷 1 和 2 与氢气反应，导致 H-H 键断裂，并形成硼环戊烯产物。通过评估 2 与 H(2)之间较慢反应的动力学性质，对该反应的机理进行了实验研究。该反应在[硼烷]和[H(2)]中均为一级反应，其活化参数为 ΔH(‡) = 34(8) kJ/mol 和 ΔS(‡) = -146(25) J mol(-1) K(-1)。观察到最小动力学同位素效应为 1.10(5)，这表明在限速过渡态中 H-H 断裂具有异步几何形状。为了解释观察到的产物的立体化学，提出了开环/闭环机制，并通过分离合成了提议的中间体及其观察到的转化为产物来支持。此外，对反应机理的广泛 DFT 映射支持了这一建议的合理性。该研究说明了在没有外部碱的情况下，强主族路易斯酸对 H(2)的活化的一种新机制，该过程部分由 1 和 2 中硼烷环的反芳香性驱动。
