College of Chemistry and Chemical Engineering, Graduate University of Chinese Academy of Science, Beijing, 100049, PR China.
Dalton Trans. 2010 Jun 21;39(23):5519-26. doi: 10.1039/c001399d. Epub 2010 May 11.
A computational study has been carried out to examine if the metal-free catalyst (1) designed for imine hydrogenation is able to hydrogenate ketones, using the cyclohexanone (3) and its derivatives (4-6) as ketone models. The catalytic cycle includes two major steps: hydrogen activation and hydrogen transfer. The concerted pathway in the hydrogen transfer step is preferred over the stepwise pathway. The two separated steps for hydrogen activation and hydrogen transfer can benefit the hydrogen addition to the substrates (e.g., ketones) which do not have strong Lewis base centres, because the substrates need not to be involved in the hydrogen activation. In general, the larger the steric effect of the substrate is, the less severe the side reactions become, and the more difficultly the desired reaction occurs. The energetic results show that the hydrogenations of 3-5 are kinetically and thermodynamically feasible under ambient conditions, but the hydrogenation of 6 is less energetically favourable. Therefore, it is important to establish a proper balance between promoting the desired reaction and meanwhile avoiding the undesired reactions. The issue of the resting state, caused by forming stable alkoxide complexes like in the ketone hydrogenation catalyzed by the metal-ligand bifunctional catalysts, is also discussed.
已经进行了一项计算研究，以检查为亚胺氢化设计的无金属催化剂（1）是否能够氢化酮，使用环己酮（3）及其衍生物（4-6）作为酮模型。催化循环包括两个主要步骤：氢活化和氢转移。氢转移步骤中的协同途径优先于分步途径。氢活化和氢转移的两个分离步骤有利于向没有强路易斯碱中心的底物（例如酮）添加氢，因为底物不需要参与氢活化。一般来说，底物的空间位阻越大，副反应越不严重，所需反应越难发生。能量学结果表明，在环境条件下，3-5 的氢化在动力学和热力学上都是可行的，但 6 的氢化在能量上不太有利。因此，在促进所需反应的同时避免不必要的反应之间建立适当的平衡非常重要。还讨论了由金属-配体双功能催化剂催化的酮氢化中形成稳定的烷氧基配合物等引起的休眠状态问题。
