Luche reduction

Luche reduction is the selective organic reduction of ketones to alcohols with lanthanoid chlorides, such as cerium(III) chloride, and sodium borohydride. The Luche reduction can be conducted chemoselectively toward ketone in the presence of aldehyde or toward α,β-unsaturated ketone in the presence of non-conjugated ketone.^[1][2][3]

An enone forms an allylic alcohol in a 1,2-addition. Competing conjugate 1,4-addition is suppressed. The solvent is an alcohol such as methanol or ethanol.

The selectivity can be explained in terms of 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 it is more Lewis basic.

In one application a ketone is selectively reduced in presence of an aldehyde. Actually in presence of methanol as solvent, the aldehyde forms methoxy acetal which is inactive in the reducing conditions.

References

1. Strategic Applications of Named Reactions in Organic Synthesis (Paperback) by Laszlo Kurti, Barbara Czako ISBN 0-12-429785-4
2. Lanthanides in organic chemistry. 1. Selective 1,2 reductions of conjugated ketones Jean Louis Luche J. Am. Chem. Soc.; 1978; 100(7); 2226–2227. doi:10.1021/ja00475a040
3. Lanthanoids in organic synthesis. 6. Reduction of .alpha.-enones by sodium borohydride in the presence of lanthanoid chlorides: synthetic and mechanistic aspects Andre L. Gemal, Jean Louis Luche J. Am. Chem. Soc.; 1981; 103(18); 5454–5459 doi:10.1021/ja00408a029