Combined Experimental and Theoretical Study on the Reductive Cleavage of Inert C-O Bonds with Silanes: Ruling out a Classical Ni(0)/Ni(II) Catalytic Couple and Evidence for Ni(I) Intermediates
Combined Experimental and Theoretical Study on the Reductive Cleavage of Inert C-O Bonds with Silanes: Ruling out a Classical Ni(0)/Ni(II) Catalytic Couple and Evidence for Ni(I) Intermediates
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DOI：
10.1021/ja311940s
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发表时间：
2013-02-06
影响因子：
15
通讯作者：
Martin, Ruben
中科院分区：
文献类型：
--
作者：
Cornella, Josep;Gomez-Bengoa, Enrique;Martin, Ruben
本文报道了以硅烷为还原剂，Ni（COD）（2）/PCy 3催化C-OMe键还原断裂的机理和计算研究。具体而言，我们证明了这种转化的机制并不是通过Ni（0）预催化剂氧化加成到C-OMe键中来进行的。在不存在外部还原剂的情况下，原位生成的氧化加成络合物在室温下快速经历β-氢化物消除，最终导致Ni（0）-羰基-或Ni（0）-氢化物结合的络合物。这些复合物的X-射线晶体学表征明确提出了不同的机制的情况下，硅烷存在于反应介质中。同位素标记实验、动力学同位素效应和计算研究显然加强了这种看法。此外，我们还发现，水通过形成新的Ni桥连的羟基物种使Ni催化剂失活而具有有害影响，所述新的Ni桥连的羟基物种的特征在于X射线晶体学。通过在不同浓度下绘制C-OMe键断裂的初始速率来确定每种组分中的顺序。这些数据与原位监测实验的H-1 NMR，EPR，IR光谱，和理论计算提供了一个机制的图片，涉及Ni（I）作为关键的反应中间体，这是通过comproportionation最初形成的Ni（II）物种。该研究有力地支持了经典的Ni（0）/Ni（II）对C-OMe键断裂不起作用，从而为其他相关的C-O键断裂反应开辟了新的视角。
A mechanistic and computational study on the reductive cleavage of C-OMe bonds catalyzed by Ni(COD)(2)/PCy3 with silanes as reducing agents is reported herein. Specifically, we demonstrate that the mechanism for this transformation does not proceed via oxidative addition of the Ni(0) precatalyst into the C-OMe bond. In the absence of an external reducing agent, the in-situ-generated oxidative addition complexes rapidly undergo beta-hydride elimination at room temperature, ultimately leading to either Ni(0)-carbonyl- or Ni(0)-aldehyde-bound complexes. Characterization of these complexes by X-ray crystallography unambiguously suggested a different mechanistic scenario when silanes are present in the reaction media. Isotopic-labeling experiments, kinetic isotope effects, and computational studies clearly reinforced this perception. Additionally, we also found that water has a deleterious effect by deactivating the Ni catalyst via formation of a new Ni-bridged hydroxo species that was characterized by X-ray crystallography. The order in each component was determined by plotting the initial rates of the C-OMe bond cleavage at varying concentrations. These data together with the in-situ-monitoring experiments by H-1 NMR, EPR, IR spectroscopy, and theoretical calculations provided a mechanistic picture that involves Ni(I) as the key reaction intermediates, which are generated via comproportionation of initially formed Ni(II) species. This study strongly supports that a classical Ni(0)/Ni(II) for C-OMe bond cleavage is not operating, thus opening up new perspectives to be implemented in other related C-O bond-cleavage reactions.