Mechanism of Asymmetric Hydrogenation of Aromatic Ketones Catalyzed by a Combined System of Ru(蟺-CH2C(CH3)CH2)2(cod) and the Chiral sp2N/sp3

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• 作者：Hiroshi Nakatsuka ; Tomoya Yamamura ; Yoshihiro Shuto ; Shinji Tanaka ; Masahiro Yoshimura ; Masato Kitamura
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：July 1, 2015
• 年：2015
• 卷：137
• 期：25
• 页码：8138-8149
• 全文大小：812K
• ISSN：1520-5126

文摘

The combination of a Goodwin鈥揕ions-type chiral N4 ligand, (R)-Ph-BINAN-H-Py ((R)-3,3鈥?diphenyl-N²,N^{2鈥?/sup>-bis((pyridin-2-yl)methyl)-1,1鈥?binaphthyl-2,2鈥?diamine; L), with Ru(蟺-CH₂C(CH₃)CH₂)₂(cod) (A) (cod = 1,5-cyclooctadiene) catalyzes the hydrogenation of acetophenone (AP) to (R)-1-phenylethanol (PE) with a high enantiomer ratio (er). Almost no Ru complex forms, with A and L remaining intact throughout the reaction while generating PE quantitatively according to [PE] = k_obst2. An infinitesimal amount of reactive and unstable RuH₂L (B) with C₂-螞-cis-伪 stereochemistry is very slowly and irreversibly generated from A by the action of H₂ and L, which rapidly catalyzes the hydrogenation of AP via Noyori鈥檚 donor鈥揳cceptor bifunctional mechanism. A CH-蟺-stabilized Si-face selective transition state, C_Si, gives (R)-PE together with an intermediary Ru amide, D, which is inhibited predominantly by formation of the Ru enolate of AP. The rate-determining hydrogenolysis of D completes the cycle. The time-squared term relates both to the preliminary step before the cycle and to the cycle itself, with a highly unusual eight-order difference in the generation and turnover frequency of B. This mechanism is fully supported by a series of experiments including a detailed kinetic study, rate law analysis, simulation of t/[PE] curves with fitting to the experimental observations at the initial reaction stage, X-ray crystallographic analyses of B-related octahedral metal complexes, and Hammett plot analyses of electronically different substrates and ligands in their enantioselectivities.}