Iron(II)-Catalyzed Hydrogenation of Acetophenone with a Chiral, Pyridine-Based PNP Pincer Ligand: Support for an Outer-Sphere Mechanism,Organometallics

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Iron(II)-Catalyzed Hydrogenation of Acetophenone with a Chiral, Pyridine-Based PNP Pincer Ligand: Support for an Outer-Sphere Mechanism
Organometallics ( IF 2.5 ) Pub Date : 2018-01-24 00:00:00 , DOI: 10.1021/acs.organomet.7b00816
Raffael Huber ₁ , Alessandro Passera ₁ , Antonio Mezzetti ₁

Affiliation

We report here the tridentate, P-stereogenic, C₂-symmetric PNP pincer ligand (S_P,S_P)-2,6-bis((cyclohexyl(methyl)phosphanyl)methyl)pyridine (1a) and its iron(II) complexes [FeBr₂(CO)(1a)] (2a), [FeHBr(CO)(1a)] (3a), and [FeH₂(CO)(1a)] (4a). In the presence of base, bromocarbonylhydride 3a catalyzes the hydrogenation of acetophenone to (S)-1-phenylethanol with 48% ee. The transition states of the enantiodetermining transfer of hydride from 3a to the carbonyl group of acetophenone were studied by density functional theory (DFT) with a full conformational analysis of the PNP ligand for the three different mechanistic models recently proposed for a related achiral catalyst. The DFT calculations show that the outer-sphere monohydride mechanism originally proposed by Milstein reproduces the experimentally observed sense of induction (S) and enantioselectivity, whereas the dihydride and inner-sphere pathways predict the formation of the R enantiomer.

中文翻译：

铁（II）催化的手性，吡啶基PNP钳配体苯乙酮的加氢：支持外层机制

我们在这里报告的三齿，P-stereogenic，C ₂对称PNP钳配体（S _P，S _P）-2,6-双（（环己基（甲基）膦基）甲基）吡啶（1a）及其铁（II）配合物[FeBr ₂（CO）（1a）]（2a），[FeHBr（CO）（1a）]（3a）和[FeH ₂（CO）（1a）]（4a）。在碱的存在下，溴羰基氢化物3a催化苯乙酮氢化为ee为48％的（S）-1-苯基乙醇。对映体确定氢化物从中转移的过渡态通过密度泛函理论（DFT）研究了苯乙酮的3a羰基，并针对最近提出的有关非手性催化剂的三种不同机理模型对PNP配体进行了完整的构象分析。DFT计算表明，Milstein最初提出的外球一氢化物机理再现了实验观察到的诱导感（S）和对映体选择性，而二氢化物和内球途径预测了R对映体的形成。

更新日期：2018-01-24

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