Alkene Transfer Hydrogenation with Alkaline-Earth Metal Catalysts

The alkene transfer hydrogenation (TH) of a variety of alkenes has been achieved with simple AeN′′ ₂ catalysts [Ae=Ca, Sr, Ba; N′′=N(SiMe ₃ ) ₂ ] using 1,4-cyclohexadiene (1,4-CHD) as a H source. Reaction of 1,4-CHD with AeN′′ ₂ gave benzene, N′′H, and the metal hydride species N′′AeH (or aggregates thereof), which is a catalyst for alkene hydrogenation. BaN′′ ₂ is by far the most active catalyst. Hydrogenation of activated C=C bonds (e.g. styrene) proceeded at room temperature without polymer formation. Unactivated (isolated) C=C bonds (e.g. 1-hexene) needed a higher temperature (120 °C) but proceeded without double-bond isomerization. The ligands fully control the course of the catalytic reaction, which can be: 1) alkene TH, 2) 1,4-CHD dehydrogenation, or 3) alkene polymerization. DFT calculations support formation of a metal hydride species by deprotonation of 1,4-CHD followed by H transfer. Convenient access to larger quantities of BaN′′ ₂ , its high activity and selectivity, and the many advantages of TH make this a simple but attractive procedure for alkene hydrogenation.