Mechanistically, NaBH4 works through a hydride transfer, in which a hydride is transferred from
an electropositive boron to an electropositive α-carbon in a carbonyl. (Scheme 2) The hydride
can attack from either the front or back side of a ketone (a back side attack is shown in Scheme
2), creating a new chiral center. As the hydride attacks the carbonyl’s carbon, the π-electrons in
the carbonyl move onto the oxygen. This in turn causes a lone pair from the oxygen to bond to
the electropositive boron. As a final step, this intermediate is hydrated by a protic solvent,
creating the newly formed chiral alcohol and an oxyborohydride byproduct. This oxyborohydride
then has the ability to reduce more carbonyls with its remaining hydrides.