Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst
Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst
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DOI：
10.1021/jacs.6b02173
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发表时间：
2016-04-13
影响因子：
15
通讯作者：
Gade, Lutz H.
中科院分区：
文献类型：
--
作者：
Bleith, Tim;Gade, Lutz H.
对以高活性和对映选择性铁(II) boxmi配合物为催化剂（高达>99% ee）的酮催化氢化硅烷化进行了详细的机理研究，以阐明催化剂前活化的途径和铁催化氢化硅烷化的机理。发现羧酸盐预催化剂通过将羧酸盐配体还原成相应的醇盐而被活化，然后进入铁催化的氢化硅烷化的催化循环。对映体比率温度依赖性的 Eyring 型分析建立了 ln(S/R) 和 T-1 的线性关系，表明在 -40 至 +65 摄氏度的整个温度范围内有一个单一的选择性决定步骤（Delta Delta G(sel)（双匕首），(233 K) = 9 +/- 1 kJ/mol）。速率决定步骤的速率定律和活化参数是通过哈米特分析、自由基时钟实验、动力学同位素效应 (KIE) 测量 (k(H)/k(D) = 3.0 +/- 0.2）、催化活性醇盐中间体的分离以及整个反应序列的 DFT 建模得出和补充的。所提出的反应机理的特征在于醇盐络合物与硅烷的a-键复分解反应，随后酮与氢化铁络合物的配位，以及酮插入Fe-H键以再生醇盐络合物。
A detailed mechanistic study of the catalytic hydrosilylation of ketones with the highly active and enantioselective iron(II) boxmi complexes as catalysts (up to >99% ee) was carried out to elucidate the pathways for precatalyst activation and the mechanism for the iron-catalyzed hydrosilylation. Carboxylate precatalysts were found to be activated by reduction of the carboxylate ligand to the corresponding alkoxide followed by entering the catalytic cycle for the iron-catalyzed hydrosilylation. An Eyring-type analysis of the temperature dependence of the enantiomeric ratio established a linear relationship of ln(S/R) and T-1 indicating a single selectivity-determining step over the whole temperature range from-40 to +65 degrees C (Delta Delta G(sel)(double dagger),(233 K) = 9 +/- 1 kJ/ mol). The rate law as well as activation parameters for the rate-determining step were derived and complemented by a Hammett analysis, radical clock experiments, kinetic isotope effect (KIE) measurements (k(H)/k(D) = 3.0 +/- 0.2), the isolation of the catalytically active alkoxide intermediate, and DFT-modeling of the whole reaction sequence. The proposed reaction mechanism is characterized by a rate-determining a-bond metathesis of an alkoxide complex with the silane, subsequent coordination of the ketone to the iron hydride complex, and insertion of the ketone into the Fe-H bond to regenerate the alkoxide complex.