Insight into the Mechanism of Carbonyl Hydrosilylation Catalyzed by Brookhart's Cationic Iridium(III) Pincer Complex
Insight into the Mechanism of Carbonyl Hydrosilylation Catalyzed by Brookhart's Cationic Iridium(III) Pincer Complex
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DOI：
10.1021/ja503254f
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发表时间：
2014-05-14
影响因子：
15
通讯作者：
Oestreich, Martin
中科院分区：
文献类型：
--
作者：
Metsaenen, Toni T.;Hrobarik, Peter;Oestreich, Martin
新的实验结果表明，部分修正了目前公认的由Brookhart介绍的铱（III）钳形配合物催化羰基硅氢化反应的机制。采用硅立体硅烷作为立体化学探针的结果是外消旋，而不是在硅原子的构型反转。外消旋化程度受硅烷/羰基化合物比的影响，硅烷过量时发生反转。独立制备硅基碳鎓离子中间体并测试其反应性有助于使这种效应合理化。立体化学分析以及这些对照实验、严格的多核磁共振分析和量子化学计算清楚地证明了另一种硅烷分子参与了氢化物转移。硅烷的活化作用是意想不到的，但事实上，它对催化循环的结束至关重要。
New experimental findings suggest partial revision of the currently accepted mechanism of the carbonyl hydrosilylation catalyzed by the iridium(III) pincer complex introduced by Brookhart. Employing silicon-stereogenic silanes as a stereochemical probe results in racemization rather than inversion of the configuration at the silicon atom. The degree of the racemization is, however, affected by the silane/carbonyl compound ratio, and inversion is seen with excess silane. Independently preparing the silylcarboxonium ion intermediate and testing its reactivity then helped to rationalize that effect. The stereochemical analysis together with these control experiments, rigorous multinuclear NMR analysis, and quantum-chemical calculations clearly prove that another silane molecule participates in the hydride transfer. The activating role of the silane is unexpected but, in fact, vital for the catalytic cycle to close.