研究室工作進展 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.
亮點介紹
還原消除和β-H消除反應是金屬有機化學中的基元反應。具有“順式-β-H”的烷基鹵化钯中間體通常會發生β-氫消除反應。對于這一基元反應，在有機化學人名反應Heck反應中已經诠釋的非常清楚。然而，最近我們發現含有“順式-β-H”的烷基碘化钯中間體，可以高效的發生還原消除反應生成C(sp3)-I鍵(Angew. Chem. Int. Ed. 2014, 53, 14533. Org. Chem. Front. 2015, 2, 1080.)。
在研究這類還原消除反應機理的過程中，本研究室結合理論計算和實驗，發現該反應中LiOtBu在非極性條件下可以穩定烷基鹵化钯中間體，降低C(sp3)-I鍵還原消除反應勢能。而烷基鹵化钯中間體由于與LiOtBu存在相互作用，在發生β-H消除反應前需要脫除一分子LiI。但在非極性條件下，脫除一分子LiI是不利的。