Luche reduction
Template:Short description Template:Use dmy dates Template:Reactionbox Luche reduction is the selective organic reduction of α,β-unsaturated ketones to allylic alcohols.[1][2][3] The active reductant is described as "cerium borohydride", which is generated in situ from NaBH4 and CeCl3(H2O)7.[4]
The Luche reduction can be conducted chemoselectively toward ketone in the presence of aldehydes or towards α,β-unsaturated ketones in the presence of a non-conjugated ketone.[5]
An enone forms an allylic alcohol in a 1,2-addition, and the competing conjugate 1,4-addition is suppressed.
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 a methoxy acetal that is inactive in the reducing conditions.
References
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