Unravelling the underlying mechanism of the reduction of aldehydes/ketones with metal borohydride in an aprotic solvent.

The reduction mechanism of aldehyde/ketones with M(BH 4 ) n is not fully understood, even though the hydroboration mechanism of weak Lewis base borane complexes is known to involve a four-membered ring transition state. Herein, the reduction mechanism of M(BH 4 ) n in aprotic solvents has been elucidated for a six-membered ring, in which hydride transfer to the C atom from the B atom, formation of an L·BH 3 adduct, and disproportionation of (BH 3 (OR) - ) borane are involved. The metal cations and solvents participate in and significantly influence the reaction procedure. We believe that this mechanistic study would provide a further reference for the application of M(BH 4 ) n in organic reactions.