Forty years of hydride reductions

Forty years ago, it was observed that diborane, a hydride, reduces aldehydes and ketones with exceptional ease even at 0°. Since then various hydride reagents have evolved for the convenient reduction of typical organic functional groups. Electrophilic reducing agents such as borane and alane possess markedly different reducing characteristics than those of nucleophilic reducing agents such as sodium borohydride and lithium aluminum hydride. Explorations have revealed means of both enhancing and diminishing the electrophilic character of borane and alane on the one hand and both enhancing and diminishing the nucleophilic character of borohydrides and aluminohydrides on the other. Such derivatives reveal unique characteristics making possible valuable selective reductions, not practical previously. These developments have completely altered the procedures normally used in organic chemistry for the reduction of functional groups.