Lithium aluminum hydride, reducing agent for aldehydes and ketones

For most laboratory scale reductions of aldehydes and ketones catalytic hydro genation has been replaced by methods based on metal hydride reducing agents The two most common reagents are sodium borohydride and lithium aluminum hydride... 

Lithium aluminum hydride, LiAIH4/ is another reducing agent often used for reduction of aldehydes and ketones. A grayish powder that is soluble in ether and tetrabydrofuran, LiAlH4 is much more reactive than NaBH4 but also more dangerous. It reacts violently with water and decomposes explosively when heated above 120 °C. 

Strong reducing agents like sodium borohydride and lithium aluminum hydride are capable of reducing aldehydes to primary alcohols and ketones to secondary alcohols. The general reaction is the reverse of the reactions used to form aldehydes and ketones by the oxidation of primary and secondary alcohols, respectively (to review, see the earlier section Oxidation reactions ). However, the mechanisms for reduction are different. 

Aldehydes can be reduced tojjrimary alcohols, and ketones to secondary alcohols, either by catalytic hydrogenatloiToTB jse of chemical reducing agents Ke lithium aluminum hydride, LiAlH4. Such reduction is useful for the preparation of certain alcohols that are less available than the corresponding carbonyl cc mpounds, in particular carbonyl compounds that can be obtained by the aldol condensation (Sec. 21.7). For example ... 

Ketones and aldehydes are most commonly reduced by sodium borohydride (see Sections 10-11 and 18-12). Sodium borohydride (NaBH4) reduces ketones to secondary alcohols and aldehydes to primary alcohols. Lithium aluminum hydride (LiAlH4) also accomplishes these reductions, but it is a more powerful reducing agent, and it is much more difficult to work with. Sodium borohydride is preferred for simple reductions of ketones and aldehydes. 

The carboxyl group is one of the organic functional groups that is most resistant to reduction. It is not affected by catalytic reduction (H2/M) under conditions that easily reduce aldehydes and ketones to alcohols and that reduce alkenes to alkanes. The most common reagent for the reduction of a carboxylic acid to a primary alcohol is the very powerful reducing agent lithium aluminum hydride (Section 12.10). 

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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