From Si(II) to Si(IV) and Back: Reversible Intramolecular Carbon–Carbon Bond Activation by an Acyclic Iminosilylene

Reversibility is fundamental for transition metal catalysis, but equally for main group chemistry and especially low-valent silicon compounds, the interplay between oxidative addition and reductive elimination is key for a potential catalytic cycle. Herein, we report a highly reactive acyclic iminosilylsilylene 1, which readily performs an intramolecular insertion into a CC bond of its aromatic ligand framework to give silacycloheptatriene (silepin) 2. UV–vis studies of this Si­(IV) compound indicated a facile transformation back to Si­(II) at elevated temperatures, further supported by density functional theory calculations and experimentally demonstrated by isolation of a silylene–borane adduct 3 following addition of B­(C6F5)3. This tendency to undergo reductive elimination was exploited in the investigation of silepin 2 as a synthetic equivalent of silylene in the activation of small molecules. In fact, the first monomeric, four-coordinate silicon carbonate complex 4 was isolated and fully characterized in the reaction with carbon dioxide under mild conditions. Additionally, the exposure of 2 to ethylene or molecular hydrogen gave silirane 5 and Si­(IV) dihydride 6, respectively.

可逆性是过渡金属催化（transition metal catalysis）的核心要素，对于主族化学（main group chemistry）亦至关重要；而在低价硅化合物（low-valent silicon compounds）领域，氧化加成（oxidative addition）与还原消除（reductive elimination）之间的相互作用，是构建潜在催化循环的关键所在。本文报道了一种高活性无环亚氨基硅基硅烯（acyclic iminosilylsilylene）1，该化合物可轻松对其芳香配体骨架中的C=C键发生分子内插入反应，得到硅环庚三烯（silacycloheptatriene，silepin）2。对该四价硅（Si(IV)）化合物的紫外-可见（UV–vis）光谱研究表明，其在高温下可轻易发生逆反应回到二价硅（Si(II)）状态；该结果得到了密度泛函理论（density functional theory）计算的进一步佐证，并通过以下实验得到验证：在加入三(五氟苯基)硼（B(C6F5)3）后，分离得到了硅烯-硼烷加合物（silylene–borane adduct）3。我们利用该还原消除倾向，以硅庚因2作为硅烯（silylene）的合成等价物，研究其在小分子活化中的应用。事实上，在温和条件下与二氧化碳发生反应后，我们分离得到了首个单核四配位硅基碳酸酯配合物4，并对其完成了全面表征。此外，将硅庚因2分别暴露于乙烯或氢气中，可得到硅丙环（silirane）5与四价硅二氢化物6。