Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, 10691 Stockholm, Sweden.
Chemistry. 2011 Sep 26;17(40):11216-22. doi: 10.1002/chem.201100827. Epub 2011 Aug 31.
The synthesis of a new series of cyclopentadienylruthenium catalysts with varying electronic properties and their application in racemization of secondary alcohols are described. These racemizations involve two key steps: 1) β-hydride elimination (dehydrogenation) and 2) re-addition of the hydride to the intermediate ketone. The results obtained confirm our previous theory that the electronic properties of the substrate determine which of these two steps is rate determining. For an electron-deficient alcohol the rate-determining step is the β-hydride elimination (dehydrogenation), whereas for an electron-rich alcohol the re-addition of the hydride becomes the rate-determining step. By matching the electronic properties of the catalyst with the electronic properties of the alcohol, we have now shown that a dramatic increase in racemization rate can be obtained. For example, electron-deficient alcohol 15 racemized 30 times faster with electron-deficient catalyst 6 than with the unmodified standard catalyst 4. The application of these protocols will extend the scope of cyclopentadienylruthenium catalysts in racemization and dynamic kinetic resolution.
描述了一系列具有不同电子性质的茂钌催化剂的合成及其在仲醇外消旋化中的应用。这些外消旋化涉及两个关键步骤：1）β-氢化物消除（脱氢）和 2）氢化物重新添加到中间酮。所得结果证实了我们之前的理论，即底物的电子性质决定了这两个步骤中的哪一个是速率决定步骤。对于缺电子醇，速率决定步骤是β-氢化物消除（脱氢），而对于富电子醇，氢化物的重新添加成为速率决定步骤。通过使催化剂的电子性质与醇的电子性质相匹配，我们现在已经表明，可以显著提高外消旋化速率。例如，用缺电子催化剂 6 进行外消旋化时，缺电子醇 15 的反应速率比未修饰的标准催化剂 4 快 30 倍。这些方案的应用将扩展茂钌催化剂在外消旋化和动态动力学拆分中的应用范围。
