Question

How does sodium borohydride, `Na^(+)BH_4^(-)` act as a reducing agent towards imines?

Answer 1

We can picture sodium borohydride as a hydride transfer reagent, if we write its formula as `NaH*BH_3`.

Explanation:

So the reaction of imine with sodium borohydride may be rationalized on the basis of sodium borohydride as a hydride transfer reagent:

`RC(=NR')R'' + NaBD_4 rarr [RC(D)(N^(-)R')R'']Na^+ + BH_3`.

Here, I have used the borodeuteride (`D=""^2H`) so that we can follow the hydride transfer reaction. Standard work up gives the (deuterium labelled) amine `RC(D)(NHR')R''`.

Lithium aluminum hydride transfers hydrides in precisely the same way, except that lithal is a much more potent reductant.