Alkynes reduction with lithium aluminium

Anti addition across an alkyne can be accomplished with lithium aluminium hydride. This is particularly popular using propargylic alcohols and occurs with very high stereoselectivity. If the reduction is effected in the presence of sodium methoxide and the crude reduction product is treated with iodine, then the final product is exclusively the 3-iodoallylic alcohol (2.62). In contrast, reduction with lithivun aluminium hydride and alvuninium chloride, followed by iodination, gives the 2-iodoallylic alcohol (2.63). 

In a comprehensive survey, main group hydrides of aluminium and possibly tin were regarded as reagents which could add H to an alkyne . ( CuH is treated as an electron donor in equation 82.) Thus, RjAlH , RjBH , RsGeH , LiAlH -Ni[(MeCO)aCHo], , etc., which are or appear to be non-nucleophilic, are barred. Tin hydrides, which often engage in radical processes, may, however, deliver H to an alkyne - - . Obviously, conventional catalytic reductions and the curious reductions of CaHo by dispersed Fe or by powdered metals (Cr, Fe, Zn) (equation 88) do not appear to be appropriate here . Essentially, this leaves us with lithium... 

Reduction of the halides with a metal hydride such as lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane) gives the corresponding organotin hydrides These have an important place in organic synthesis for the reduction of halides to hydrides (hydrostannolysis) and the addition to alkenes and alkynes (hydrostannation), by radical chain reactions. Further reactions may intervene between the pairs of reactions shown in Equations (1.1.3) and (1.1.4), and (1.1.4) and (1.1.5), and these reactions are particularly useful for inducing ring-closure reactions. 

Useful reduction processes include the partial reduction of alkynes ( H2, Lindlar catalyst), reduction of alkenes with tetradeuterohydrazine, and reduction of carbonyl compounds with sodium borodeuteride or lithium aluminium deuteride as in the following examples ... 

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