Imine hydrogenation with simple alkaline earth metal catalysts

Hydrogenation of unsaturated bonds is dominated by transition metal catalysis. Compared with transition metals, the use of other metals is less explored, especially so for the s-block elements despite their ready availability and low cost. Here, we show that group 2 metal amides (M[N(SiMe3)(2)](2), M = Mg, Ca, Sr, Ba) unexpectedly catalyse the hydrogenation of aldimines with H-2 at 80 degrees C and a remarkably low H-2 pressure of 1-6 bar. Conversion rates increase with metal size: Mg < Ca < Sr < Ba (for Ba, quantitative conversion is reached within 15 min). The key to this catalysis is the unanticipated formation of metal hydride species by deprotonation of H-2 (pK(a) approximate to 49) with a weak base M[N(SiMe3)(2)](2) (HN(SiMe3)(2): pK(a) approximate to 25.8). Density functional theory calculations suggest that the most favourable pathway indeed involves metal hydride intermediates. The efficient alkaline earth metal-catalysed hydrogenation of imines with molecular hydrogen at remarkably low pressure provides an attractive alternative to transition metal catalysis.