Alkene Transfer Hydrogenation with Alkaline Earth Metal Catalysts

The challenging 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, N''H and the metal hydride species N''AeH (or aggregates thereof) which is a catalyst for alkene hydrogenation. Complex BaN''2 is by far the most active catalyst. Hydrogenation of activated C=C bonds (e.g. styrene) proceeded already 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 choice of the catalyst is critical to successful TH. The ligands fully control the course of the catalytic reaction which can be: (i) alkene TH, (ii) 1,4‐CHD dehydrogenation or (iii) 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.

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