High Activity and Selectivity for Catalytic Alkane–Alkene Transfer (De)hydrogenation by (tBuPPP)Ir and the Importance of Choice of a Sacrificial Hydrogen Acceptor

The triphosphorus-coordinating pincer iridium fragment (tBuPPP)Ir was recently reported to be highly active for the catalytic dehydrogenation of n-alkanes. Dehydrogenation is calculated to be highly regioselective for the terminal position of n-alkanes. The extremely high intermolecular selectivity observed in n-alkane/cycloalkane competition experiments supports the prediction of extremely high regioselectivity for dehydrogenation of n-alkanes. The use of sterically unhindered hydrogen acceptors is key to observing the high activity of the (tBuPPP)Ir fragment. 4,4-Dimethylpent-1-ene (TBP) is found to be particularly convenient for this purpose. With the commonly used hydrogen acceptor 3,3-dimethylbut-1-ene (TBE), (tBuPPP)Ir affords n-alkane dehydrogenation at a rate no different than that obtained with the well-known fragment (iPrPCP)­Ir. However, with the use of TBP as acceptor, (tBuPPP)Ir shows much greater activity for n-alkane transfer dehydrogenation than previously reported catalysts, affording appreciable rates even at 50 °C, an unprecedentedly low temperature for catalytic alkane transfer dehydrogenation. Also critical to the identification of (tBuPPP)Ir as a highly effective catalyst is the use of n-alkane substrate rather than the commonly used “model” dehydrogenation substrate, cyclooctane, with which dehydrogenation rates are much lower than those with n-alkanes.