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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- 作者：Tim Bleith ; Lutz H. Gade
- 刊名：Journal of the American Chemical Society
- 出版年：2016
- 出版时间：April 13, 2016
- 年：2016
- 卷：138
- 期：14
- 页码：4972-4983
- 全文大小：766K
- 年卷期：0
- ISSN：1520-5126
文摘
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.