Hurst Michael R, Zakharov Lev N, Cook Amanda K
Department of Chemistry and Biochemistry, University of Oregon Eugene OR 97403 USA
Chem Sci. 2021 Sep 14;12(39):13045-13060. doi: 10.1039/d1sc04419b. eCollection 2021 Oct 13.
The oxidative addition of Pd to Si-H bonds is a crucial step in a variety of catalytic applications, and many aspects of this reaction are poorly understood. One important yet underexplored aspect is the electronic effect of silane substituents on reactivity. Herein we describe a systematic investigation of the formation of silyl palladium hydride complexes as a function of silane identity, focusing on electronic influence of the silanes. Using [(μ-dcpe)Pd] (dcpe = dicyclohexyl(phosphino)ethane) and tertiary silanes, data show that equilibrium strongly favours products formed from electron-deficient silanes, and is fully dynamic with respect to both temperature and product distribution. A notable kinetic isotope effect (KIE) of 1.21 is observed with H/DSiPhMe at 233 K, and the reaction is shown to be 0.5 order in [(μ-dcpe)Pd] and 1 order in silane. Formed complexes exhibit temperature-dependent intramolecular H/Si ligand exchange on the NMR timescale, allowing determination of the energetic barrier to reversible oxidative addition. Taken together, these results give unique insight into the individual steps of oxidative addition and suggest the initial formation of a σ-complex intermediate to be rate-limiting. The insight gained from these mechanistic studies was applied to hydrosilylation of alkynes, which shows parallel trends in the effect of the silanes' substituents. Importantly, this work highlights the relevance of in-depth mechanistic studies of fundamental steps to catalysis.
钯对硅氢键的氧化加成是多种催化应用中的关键步骤，而该反应的许多方面仍未得到充分理解。一个重要但尚未充分探索的方面是硅烷取代基对反应活性的电子效应。在此，我们描述了作为硅烷特性函数的硅基钯氢化物配合物形成的系统研究，重点关注硅烷的电子影响。使用[(μ-二环己基膦基乙烷)钯]（二环己基(膦基)乙烷）和叔硅烷，数据表明平衡强烈有利于由缺电子硅烷形成的产物，并且在温度和产物分布方面都是完全动态的。在233K下，H/DSiPhMe的动力学同位素效应（KIE）为1.21，反应对[(μ-二环己基膦基乙烷)钯]为0.5级，对硅烷为1级。形成的配合物在核磁共振时间尺度上表现出温度依赖性的分子内氢/硅配体交换，从而能够确定可逆氧化加成的能垒。综合起来，这些结果为氧化加成的各个步骤提供了独特的见解，并表明σ-配合物中间体的初始形成是限速步骤。从这些机理研究中获得的见解被应用于炔烃的硅氢化反应，该反应在硅烷取代基的影响方面呈现出平行趋势。重要的是，这项工作突出了对催化基本步骤进行深入机理研究的相关性。
