College of Chemistry and Chemical Engineering, Graduate University of Chinese Academy of Sciences, Beijing, 100049, PR China.
Dalton Trans. 2011 Mar 7;40(9):1929-37. doi: 10.1039/c0dt01297a. Epub 2011 Jan 24.
This study extends our previous work of using π-FLP strategy to develop metal-free hydrogenation catalysts. Using small MeN=CMe(2) imine (im1) as a model, we previously designed cat1 and cat2 catalysts. But it is unclear whether they are capable of catalyzing the hydrogenations of bulky imines. Using tBuN=C(H)Ph (im2) as a representative of large imines, we assessed the energetics of the cat1- and cat2-catalyzed im2 hydrogenations. The predicted energetics indicates that they can still catalyze large imine hydrogenations with experimentally accessible kinetic barriers, although the energetics becomes less favorable. To improve the catalysis, we proposed new catalysts (cat3 and cat4) by tailoring cat1 and cat2. The study indicates that cat3 and cat4 could have better performance for the hydrogenation of the bulky im2 than cat1 and cat2. Remarkably, cat3 and cat4 are also found suitable for small imine (im1) hydrogenation. Examining the hydrogen transfer substeps in the eight hydrogenations involved in this study, we observed that the mechanism for the hydrogen transfer step in the catalytic cycles depends on the steric effect between catalyst and substrate. The mechanism can be switched from stepwise one in the case of large steric effect (e.g.im2/cat2) to the concerted one in the case of small steric effect (e.g.im1/cat3). The new catalysts could be better targets for experimental realization because of their simpler constructions.
这项研究扩展了我们之前使用 π-FLP 策略开发无金属氢化催化剂的工作。使用小分子 MeN=CMe(2)亚胺（im1）作为模型，我们之前设计了 cat1 和 cat2 催化剂。但不清楚它们是否能够催化体积庞大的亚胺的氢化反应。使用 tBuN=C(H)Ph（im2）作为大亚胺的代表，我们评估了 cat1 和 cat2 催化的 im2 氢化的能量学。预测的能量学表明，尽管能量变得不利，但它们仍然可以用实验上可获得的动力学障碍催化大亚胺的氢化。为了提高催化性能，我们通过修饰 cat1 和 cat2 提出了新的催化剂（cat3 和 cat4）。研究表明，cat3 和 cat4 可能比 cat1 和 cat2 更适合氢化大体积的 im2。值得注意的是，cat3 和 cat4 也适用于小亚胺（im1）的氢化。在研究中涉及的八种氢化反应中，考察了氢转移的子步骤，我们观察到催化循环中氢转移步骤的机制取决于催化剂和底物之间的空间效应。在大空间效应的情况下（例如 im2/cat2），机制可以从逐步转移转变为协同转移，而在小空间效应的情况下（例如 im1/cat3）。由于新催化剂的结构更简单，因此它们可能是更好的实验实现目标。
