The catalytic asymmetric hydrogenation of ketones reflect an important way to prepare valuable chiral alcohols. Understanding how transition metals promote these reactions is key to the rational design of more active, selective and sustainable catalysts. A highly unusual mechanism for the asymmetric hydrogenation of benzophenone, catalysed by an anionic IrIII hydride system with a strong counterion dependence on catalyst activity, is explored and rationalised here. The active catalyst, generated in situ from [IrCl(COD)]2 and a bidentate P,SR ligand under H2 in the presence of a strong base (M+iPrO- in isopropanol, M = Li, Na, K), is the solvated M+[Ir(H)4(P,SR)]- salt (P,SR = CpFe[1,2-C5H3(PPh2)(CH2SR)], with R = iPr, Ph, Bz and Cy). Catalyst activity increases, for all the R derivatives as the counterion is varied in the order Li < Na < K. For the most active K system, the addition of 18-crown-6 drastically reduces the activity. While the cation proves to strongly affect catalyst activity, it does not significantly influence the enantioselectivity. DFT calculations are used to explore these effects in detail, and show that the solvation model used in the calculations is critical. Only by using a hybrid implicit/explicit solvent model, including sufficient explicit solvent molecules to properly describe the first solvation shell of the cation, are the experimental observations reproduced. This model reveals the fundamental importance of the alkali-metal cations coordination sphere in understanding the counterion effects. The turnover-determining step in the catalytic cycle involves outer-sphere hydride transfer to the substrate. This step leads to coordination of the alkoxide product to the alkali-metal cation, and proceeds with significant rearrangement of the coordination sphere of M, whereas there is little change in the geometrical parameters around iridium or the alkoxide. The DFT calculations also pinpointed the major enantio-discriminating interactions, and rationalise the insensitivity of enantioselectivity to alkali metal cation placement.

中文翻译：

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了解阴离子铱 （III） 配合物的酮加氢催化：反离子和溶剂化的关键作用


酮的催化不对称氢化反映了制备有价值的手性醇的重要途径。了解过渡金属如何促进这些反应是合理设计更具活性、选择性和可持续性的催化剂的关键。本文探讨了二苯甲酮不对称氢化的一种非常不寻常的机制，该机制由阴离子 IrIII 氢化物系统催化，对催化剂活性具有很强的反离子依赖性。在强碱（M+iPrO- 异丙醇中，M = Li、Na、K）存在下，由 [IrCl（COD）]2 和双齿 P，SR 配体在 H2 下原位生成的活性催化剂是溶剂化的 M+[Ir（H）4（P，SR）]-盐（P，SR = CpFe[1,2-C5H3（PPh2）（CH2SR）]]，其中 R = iPr、Ph、Bz 和 Cy）。对于所有 R 衍生物，催化剂活性增加，因为反离子以 Li < Na < K 的顺序变化。对于最活跃的 K 系统，添加 18-冠-6 会大大降低活性。虽然阳离子被证明对催化剂活性有很大影响，但它不会显着影响对映选择性。DFT 计算用于详细探索这些影响，并表明计算中使用的溶剂化模型至关重要。只有通过使用混合隐式/显式溶剂模型，包括足够的显式溶剂分子来正确描述阳离子的第一个溶剂化壳层，才能重现实验观察结果。该模型揭示了碱金属阳离子配位球在理解反离子效应方面的根本重要性。催化循环中的周转决定步骤涉及外球氢化物转移到衬底。 此步骤导致醇盐产物与碱金属阳离子的配位，并继续进行 M 配位球的显着重排，而铱或醇盐周围的几何参数几乎没有变化。DFT 计算还确定了主要的对映体-鉴别相互作用，并合理化了对映体选择性对碱金属阳离子放置的不敏感性。