Reduction of compounds containing double bonds

The most common type of reduction reaction over solid supports is hydrogenation [98, 99]. However, as this class of reaction involves hydrogen in the gas phase, it does not fall within the bounds of this chapter. In liquid phase processes the most widely investigated type of reaction is the reduction of compounds containing carbon-oxygen double bonds. Ketones and aldehydes are readily reduced by a number of methods using solid catalysts or supported reagents [100]. 

Lithium borohydride supported on zeolite A or X will bring about the selective reduction of aldehydes without reducing any ketones that may be present [102]. Lithium aluminium hydride in the presence of silica gel reduces ketones rapidly and in high yield without affecting cyano or nitro groups (e.g. equation 4.19) [103]. The same reagent also selectively reduces carboxylic esters when other reducible groups are present and facilitates the formation of hydroxyesters from ketoesters [104]. 

Alumina can also be used as a support for reducing agents. Sodium dispersed on the surface of alumina reduces ketones, esters and oximes [105]. Although the selectivities and yields in these reactions do not differ greatly from those obtained using unsupported sodium, the alumina-supported reagent has a long shelf life and is easier to handle. 

Zinc borohydride supported on alumina reduces ketones without causing hydrolysis of an enol acetate group (e.g. equation 4.20) [106]. The reagent also reduces P-nitrostyrenes selectively [107]. 

Carbonyl compounds can be reduced efficiently in hydrosilylation reactions with an inorganic solid acid or base catalyst present [108, 109]. Iron montmorillonite catalyses hydrosilylation reactions most effectively (e.g. equation 4.21) [108], while sodium montmorillonite is completely inactive. 

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