Halides, alkyl reaction with aluminum hydride reagents

Primary and secondary alkyl bromides, iodides, and sulfonates can be reduced to the corresponding alkanes with LiBHEt3 (superhydride) or with lithium aluminum hydride (LiAlH4, other names lithium tetrahydridoaluminate or lithium alanate). If such a reaction occurs at a stereocenter, the reaction proceeds with substantial or often even complete stereoselectivity via backside attack by the hydride transfer reagent. The reduction of alkyl chlorides to alkanes is much easier with superhydride than with LiAlH4. The same is true for sterically hindered halides and sulfonates ... 

Hydride reagents (lithium aluminum hydride, LiAlHt, and sodium borohy-dride, NaBHt) are a source of the nucleophilic hydride species Hr. Reaction of hydride with a suitable carbon electrophile results in a reduction of that carbon (by increasing the number of C-H bonds). Reactions of carbonyl compounds with lithium aluminum hydride (LAH) generally give an alcohol product (after workup), with the exception of amides, which give amine products. Sodium borohydride is less reactive than LAH. It does not react with esters, amides, or carboxylic acids, so it is described as being selective for aldehydes and ketones. Sodium borohydride also fails to reduce nitroalkanes or alkyl halides, so LAH must be used in those reactions. 

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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