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 H2 at 80 °C and a remarkably low H2 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 H2 (pK a ≈ 49) with a weak base M[N(SiMe3)2]2 (HN(SiMe3)2: pK a ≈ 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.
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M.A. thanks the Fund for Scientific Research–Flanders (FWO-12F4416N) for a postdoctoral fellowship and the Free University of Brussels (VUB) for financial support. F.D.P. acknowledges the Research Foundation Flanders (FWO) and Strategic Research Program funding of the VUB. He also acknowledges the Francqui foundation for a position as ‘Francqui research professor’. S.H. acknowledges the Deutsche Forschungsgemeinschaft for financial support (HA 3218/7-1).
The authors declare no competing financial interests.
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Bauer, H., Alonso, M., Färber, C. et al. Imine hydrogenation with simple alkaline earth metal catalysts. Nat Catal 1, 40–47 (2018). https://doi.org/10.1038/s41929-017-0006-0
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