Reductions with metal alkoxyaluminium hydrides are chemical reactions that involve either the net hydrogenation of an unsaturated compound or the replacement of a reducible functional group with hydrogen by metal alkoxyaluminium hydride reagents. Introduction Sodium borohydride and lithium aluminium hydride are commonly used for the reduction of organic compounds. These two reagents are on the extremes of reactivity—whereas lithium aluminium hydride reacts with nearly all reducible functional groups, sodium borohydride reacts with a much more limited range of functional groups. Diminished or enhanced reactivity may be realized by the replacement of one or more of the hydrogens in these reagents with alkoxy groups. Additionally, substitution of hydrogen for chiral alkoxy groups in these reagents enables asymmetric reductions. Although methods involving stoichiometric amounts of chiral metal hydrides have been supplanted in modern times by enantioselective catalytic reductions, they are of historical interest as early examples of stereoselective reactions. The table below summarizes the reductions that may be carried out with a variety of metal aluminium hydrides and borohydrides. The symbol "+" indicates that reduction does occur, "-" indicates that reduction does not occur, "±" indicates that reduction depends on the structure of the substrate, and "0" indicates a lack of literature information. Mechanism and stereochemistry Prevailing mechanism Reduction by alkoxyaluminium hydrides is thought in most cases to proceed by a polar mechanism. Hydride transfer to the organic substrate generates an organic anion, which is neutralized either by protic solvent or upon acidic workup. Reductions of α,β-unsaturated carbonyl compounds may occur in a 1,2 sense (direct addition) or a 1,4 sense (conjugate addition). The tendency to add in a 1,4 sense is correlated with the softness of the hydride reagent according to Pearson's hard-soft acid-base theory. Experimental results agree with the theory—softer hydride reagents afford higher yields of the conjugate reduction product. A few substrates, including diaryl ketones, diarylalkenes, and anthracene, are known to undergo reduction by single-electron transfer pathways with lithium aluminium hydride. Metal alkoxylaluminium hydride reagents are well characterized in a limited number of cases. Precise characterization is complicated in some cases by disproportionation, which converts alkyoxyaluminium hydrides into alkoxyaluminates and metal aluminium hydride: Stereochemistry The origin of diastereoselectivity in reductions of chiral ketones has been extensively analyzed and modeled. According to a model advanced by Felkin, diastereoselectivity is controlled by the relative energy of the three transition states I, II, and III. Transition state I is favored in the