A compound contains both alpha,beta unsaturated carbonyl group and a normal carbonyl group. Is there any method to reduce the normal carbonyl group keeping the alpha,beta unsaturated carbonyl group intact?
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As for α,β-Unsaturated ketones, they tend to be reduced by NaBH 4 in a 1,4-sense, although mixtures are often formed. But addition of cerium chloride as an additive greatly improves the selectivity for 1,2-unsaturated ketone (luche Reduction).
Luche reduction is the selective organic reduction of α,β-unsaturated ketones to allylic alcohols with sodium borohydride (NaBH 4 ) and lanthanide chlorides, mainly cerium(III) chloride (CeCl 3 ), in methanol or ethanol.
The selectivity can be explained in terms of the HSAB theory : carbonyl groups require hard nucleophiles for 1,2-addition. The hardness of the borohydride is increased by replacing hydride groups with alkoxide groups, a reaction catalyzed by the cerium salt by increasing the electrophilicity of the carbonyl group. This is selective for ketones because they are more Lewis basic. In one application, a ketone is selectively reduced in the presence of an aldehyde. Actually, in the presence of methanol as solvent, the aldehyde forms methoxy acetal that is inactive in the reducing conditions.