研究室工作进展 Feb. 25th, 2016
A DFT Study on Conversion of Aryl Iodides to Alkyl Iodides: Reductive Elimination of R–I from Alkylpalladium Iodide Complexes with Accessible β-Hydrogens
Wei Hao, Junnian Wei, Yue Chi, Patrick J. Walsh,* and Zhenfeng Xi*
Chem. Eur. J. 2016, 22, 3422-3429.
β-Hydride abstraction is a well-accepted elementary step for catalytic cycles in organometallic chemistry. It is usually anticipated that alkylpalladium halides containing syn-β-hydrogens will undergo β-hydride abstraction to afford the Heck-type products. However, our recent experimental results demonstrate that the reductive elimination of alkyliodides from alkylpalladium iodides containing syn-β-hydrogens may surpass the β-hydride abstraction or even become exclusive in certain cases (Angew. Chem. Int. Ed. 2014, 53, 14533; Org. Chem. Front. 2015, 2, 1080.)
Inspired by our recent exciting experimental observations, a DFT study was conducted herein on the reductive elimination pathway. The C(sp3)–I reductive elimination was compared to the β-hydride elimination pathway to understand the energetics that govern product formation. The reaction involves oxidative addition, alkyne insertion, C–N bond cleavage and reductive elimination. For the alkylpalladium iodide intermediate, LiOtBu stabilizes the intermediate in the non-polar solvent, thus promoting reductive elimination and preventing β-hydride elimination. The C–N bond cleavage process was explored and the computations show that PPh3 is not bound to the Pd center during this step. Experimentally, it was demonstrated that LiOtBu is not necessary for oxidative addition, alkyne insertion or C–N bond cleavage steps, lending support to the conclusions from the DFT calculations. The turnover-limiting steps were found to be C–N bond cleavage and reductive elimination, while oxidative addition, alkyne insertion and formation of the indole ring provide the driving force for the reaction.
亮点介绍
还原消除和β-H消除反应是金属有机化学中的基元反应。具有“顺式-β-H”的烷基卤化钯中间体通常会发生β-氢消除反应。对于这一基元反应，在有机化学人名反应Heck反应中已经诠释的非常清楚。然而，最近我们发现含有“顺式-β-H”的烷基碘化钯中间体，可以高效的发生还原消除反应生成C(sp3)-I键(Angew. Chem. Int. Ed. 2014, 53, 14533. Org. Chem. Front. 2015, 2, 1080.)。
在研究这类还原消除反应机理的过程中，本研究室结合理论计算和实验，发现该反应中LiOtBu在非极性条件下可以稳定烷基卤化钯中间体，降低C(sp3)-I键还原消除反应势能。而烷基卤化钯中间体由于与LiOtBu存在相互作用，在发生β-H消除反应前需要脱除一分子LiI。但在非极性条件下，脱除一分子LiI是不利的。