Aluminum hydride

(A) Reduction of α,β-unsaturated carbonyls and esters: The conversion of α,β unsaturated ketones, aldehydes, and esters into allylic alcohols can be carried out with very good selectivity using AlH 3. However, DIBAL is a reagent of choice for this transformation but is costly. Carboxylic acids and esters are rapidly reduced by AlH3 than LiAlH4 in presence of halides and nitro group. (Diagram presented). (B) Reduction of acetals: Cyclic acetals can be reduced to the half protected diols, which has wide applications in carbohydrate chemistry. For instance, acetals (benzylidene derivative) can be selectively reduced to a monobenzylated diol. (Diagram presented). (C) Reduction of amides: During the reduction of amides to amine, there is a competition between C-O and C-N bond and the cleavage depends upon the reaction conditions. This complication can be avoided with AlH3. A quantitative yield of amine is obtained within a short reaction time. Conjugated amine can be cleanly reduced to allylic amines, whereas LiAlH4 reduces also the conjugated double bond. Reduction of β-lactams to azetidines can be accomplished with AlH3 8 while ring opening was observed with LiAlH4. (Diagram presented). (D) Reduction of nitriles: The less basic AlH3 appears to be better than LiAlH4 for reducing nitriles to amines. (Diagram presented). (E) Desulfurisation: Desulfurisation of sultones is rapid and proceeds in good yields with AlH3, while LiAlH4 affords poor yields with long reaction times. (Diagram presented). (F) Epoxide ring opening: With most epoxides, hydride attack occurs at the least sterically hindered side to give the corresponding alcohol. However due to the electrophilic nature of AlH3 compared to LiAlH4, it is possible for ring opening to occur at the more hindered side. With phenyl subtituted epoxides mechanistic studies have shown that attack at benzylic carbenium ion or 1,2-hydride shift followed by hydride attack gives products with the same regiochemistry but with different stereochemistry. The stereo-selectivity of AlH3 mediated epoxide ring opening reaction has been studied in depth. (Diagram presented). (G) SN2′ allylic arrangements: Displacement of good leaving group to give the rearranged allylic system can be carried with AlH3. This reaction appears not to be sterically demanding as a variety of displacement are possible. (Diagram presented). (H) Preparation of allenes: Preparation of allenes from propargylic system can also be accomplished. Most systems show a preference for syn elimination. This same procedure has been used to prepare fluroallenes. (Diagram presented). (I) Miscellaneous: Though alkyl halides are usually inert to AlH3, facile reduction of cyclopropyl halides to cyclopropanes and glycosyl fluorides to tetrahydropyrans is known. (Diagram presented). © Georg Thieme Verlag Stuttgart.