Reduction quaternary ammonium aluminium hydride

Solid lithium aluminium hydride can be solublized in non-polar organic solvents with benzyltriethylammonium chloride. Initially, the catalytic effect of the lithium cation in the reduction of carbonyl compounds was emphasized [l-3], but this has since been refuted. A more recent evaluation of the use of quaternary ammonium aluminium hydrides shows that the purity of the lithium aluminium hydride and the dryness of the solvent are critical, but it has also been noted that trace amounts of water in the solid liquid system are beneficial to the reaction [4]. The quaternary ammonium aluminium hydrides have greater hydrolytic stability than the lithium salt the tetramethylammonium aluminium hydride is hydrolysed slowly in dilute aqueous acid and more lipophilic ammonium salts are more stable [4, 5]. 

In general, the rates of reduction by the ammonium salts are slower than those attained under normal conditions with the lithium salts, but the use of a non-ethereal solvent can be an advantage. Quaternary ammonium aluminium hydrides reduce ketones and amides effectively to alcohols and amines. Nitriles are also reduced to amines, whereas haloalkanes and arenes are reductively dehalogenated to give hydrocarbons in high yield [3]. 

General reduction procedure using quaternary ammonium aluminium hydrides... 

Most alkynones are reduced to the corresponding / -alkynols with high (62— 82%) enantiomeric excess using a complex formed from lithium aluminium hydride and the amino-alcohol (10). The asymmetric reduction of ketones by borohydride in the presence of quaternary ammonium derivatives of (25,35)-l,4-diaminobutane-2,3-diol has been described. When lithium aluminium... 

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