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(SiMe3)(2)] using 1,4-cyclohexadiene (1,4-CHD) as a H source. Reaction of 1,4-CHD with AeN(2) gave benzene, NH, and the metal hydride species NAeH (or aggregates thereof), which is a catalyst for alkene hydrogenation. BaN2 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 degrees 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 BaN2, its high activity and selectivity, and the many advantages of TH make this a simple but attractive procedure for alkene hydrogenation.