Research news, 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.