Reduction by lithium aluminum hydride or similar compounds

The applicability of lithium aluminum hydride to reduction of C=C bonds is strongly dependent on their position in the molecule, on the reaction temperature, and on the solvent. 

When the usual method of operation is used, namely, in boiling ether, addition of hydrogen is limited to special circumstances. Isolated as well as conjugated olefinic bonds, and aromatic systems, are normally resistant ful-venes are exceptional, being converted relatively well into alkylcyclopentadienes. Nitrogen heterocycles are attacked more easily at the C=N than at the C=C bond, wherefore, for instance, pyridine and quinoline afford their 1,2-dihydro derivatives.79 

Even ethylenic bonds in x,/ -position to polar groups are not always reduced. As a general rule, applicability of the reaction is confined to the groups 

Ar—C=C—N (jS-nitrostyrenes,83 1-methylindoles84) however, bicyclic nitroalkenes are smoothly converted into the saturated amines by lithium aluminum hydride85 although their reduction by Raney nickel is incomplete salo, / -unsaturated cyanoacetic esters86 and occasional vinyl ketones87 can be reduced in good yield. 

Normally the ethereal solution of the substance is added to the solution of the hydride in ether other reducible groups are then attacked in addition to the olefinic bond. If, however, the lithium aluminum hydride is added gradually to the substance (the so-called inverse reduction ), then saturation of the C=C bond can generally be avoided. For instance, cinnamaldehyde affords 3-phenyl-l-propanol by one method and cinnamyl alcohol by the other, both 

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