Chiral borohydrides, asymmetric reducing agents

The asymmetric reducing agent (78) has been used to prepare naturally occurring y-lactones and enantiomerically pure secondary propargylic alcohols,whereas the chiral alkoxy(acyloxy)borohydride(79) gives 35-50% enantiomeric excess in ketone reductions. 

Although the catalytic asymmetric borane reductions mentioned above are a powerful tool to obtain highly enantio-enriched alcohols, these require the use of a rather expensive and potentially dangerous borane complex. Sodium borohydride and its solution are safe to handle and inexpensive compared to borane complexes. Thus sodium borohydride is one of the most common industrial reducing agents. However its use in catalytic enantioselective reductions has been limited. One of the most simple asymmetric catalysts is an enantiopure quaternary armnonium salt that acts as phase-transfer catalyst. For instance, in the presence of the chiral salt 81 (Fig. 9), sodium borohydride reduction of acetophenone gave the secondary alcohol in 39% ee [124]. The polymer-supported chiral phase-transfer catalyst 82 (Fig. 10) was developed for the same reduction to give the alcohol in 56% ee [125]. 

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