Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst

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 °C (ΔΔG‡sel, 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 (kH/kD = 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 σ-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.以高活性且具备对映选择性的铁(II) boxmi配合物（iron(II) boxmi complexes）作为催化剂（对映体过量值最高可达>99% ee），针对酮类化合物的催化氢化硅化反应开展了详尽的机理研究，旨在阐明预催化剂活化路径与铁催化氢化硅化的反应机制。研究表明，羧酸酯类预催化剂可通过还原羧酸酯配体生成对应的烷氧基配合物，随后进入铁催化氢化硅化的催化循环。通过对映体比率的埃林型温度依赖性分析，可得到ln(S/R)与1/T的线性相关关系，证实-40 ℃至+65 ℃的全温度区间内仅存在单一选择性决速步（ΔΔG‡sel, 233 K = 9 ± 1 kJ/mol）。本研究推导了速率决速步的速率定律与活化参数，并辅以哈米特分析、自由基钟实验、动力学同位素效应（Kinetic Isotope Effect, KIE）测量（kH/kD = 3.0 ± 0.2）、催化活性烷氧基中间体的分离，以及全反应序列的密度泛函理论（Density Functional Theory, DFT）建模。所提出的反应机制具有如下特征：烷氧基配合物与硅烷发生速率决速的σ键复分解反应，随后酮类化合物配位至氢化铁配合物，再经Fe–H键的插入反应再生烷氧基配合物。