Borohydride intermediates pave the way for magnesium-catalysed enantioselective ketone reduction.

A magnesium precatalyst for the highly enantioselective hydro-boration of C[double bond, length as m-dash]O bonds is reported. The mechanistic basis of the unprecedented selectivity of this transformation has been investi-gated experimentally by isolation of catalytic intermediates and theoretically by DFT calculations. The facile formation of a magnesium borohydride species is critical in overcoming competing pathways in the selectivity-determining insertion step.