Reduction with lithium aminoborohydrides

In the last entry of Table I, it is noted that the reduction had to be done with lithium aminoborohydride (LiH3BNH2). Myers further expanded the utility of this reagent with additional examples of the reduction of alkylated pseudoephedrine amides to chiral alcohols of high ee (Table II), and the reduction of iV,iV-disubstituted dodecanecarboxamides and 1-adamantanecarbox-amides to the corresponding alcohols, respectively (Table III) (5). 

Table III. Reduction of Amides with Lithium Aminoborohydride...

Lithium aminoborohydrides, which are readily obtained by reaction of n-BuLi with amine-boranes327,328, are excellent reagents for the reduction of imines to their corresponding secondary amines (equation 80)329. If the substrate possesses chirality, then there is considerable diastereoselectivity in this reaction (up to 90%). 

A review describing the major advances in the field of asymmetric reduction of achiral ketones using borohydrides, exemplified by oxazaborolidines and /9-chlorodiisopino- camphenylborane, has appeared. Use of sodium borohydride in combination with chiral Lewis acids has been discussed.298 The usefulness of sodium triacetoxyboro-hydride in the reductive amination of aldehydes and ketones has been reviewed. The wide scope of the reagent, its diverse and numerous applications, and high tolerance for many functional groups have been discussed.299 The preparation, properties, and synthetic application of lithium aminoborohydrides (LABs) have been reviewed. 

Lithium aminoborohydrides. The reagent 1 is a typical member of a number of lithium aminoborohydrides, prepared by reaction of BuLi with amine boranes in quantitative yields. These reagents can be stored at 25 under N2 for at least six months they are not pyrophoric. They are comparable to LiAlFL as reductants. Thus 1 reduces carbonyl compounds (including esters) in high yield. Lactones and anhydrides are reduced, but carboxylic acids are not reduced. In addition 1 reduces amides, epoxides, oximes, nitriles, and even halides. 

Several years ago, we reported the synthesis and synthetic utility of lithium aminoborohydrides (LABs) a new class of powerful, safe, and highly selective reducing agents (2, 3). These reagents performed many of the transformations for which lithium aluminum hydride is usually used. Thus, the following reduction reactions were carried out with LABs aldehydes and ketones to alcohols, esters to alcohols, oc,P>unsaturated ketones to allylic alcohols, a,P-unsaturated esters to allylic alcohols, alkyl halides to hydro-carbons, azides to amines, and epoxides to alcohols. These reduction reactions are summarized in Figure 3. 

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