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 ((PPP)-P-tBu)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 ((PPP)-P-tBu)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), ((PPP)-P-tBu)Ir affords n-alkane dehydrogenation at a rate no different than that obtained with the well-known fragment ((PCP)-P-iPr)Ir. However, with the use of TBP as acceptor, ((PPP)-P-tBu)Ir shows much greater activity for n-alkane transfer dehydrogenation than previously reported catalysts, affording appreciable rates even at 50 degrees C, an unprecedentedly low temperature for catalytic alkane transfer dehydrogenation. Also critical to the identification of ((PPP)-P-tBu)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.

Automated summary

The triphosphorus-coordinating pincer iridium fragment ((PPP)-P-tBu)Ir has been found to exhibit high activity for the dehydrogenation of n-alkanes. The use of sterically unhindered hydrogen acceptors can enhance its activity, allowing for appreciable rates even at low temperatures.