Luche reduction

Schematic of the Luche reduction. Reagents: alpha-beta-unsaturated ketone, CeCl3, Sodium borohydride (NaBH4), alcohol. Product: allylic alcohol.

The Luche reduction is an organic reaction used to convert an α,β-unsaturated ketone to an allylic alcohol using cerium trichloride, sodium borohydride, and an alcohol solvent. The role of the cerium or other lanthanide metals is to coordinate to the alcohol, making its proton more acidic which allows for it to be abstracted by the carbonyl oxygen of the ketone starting material. The NaBH4 starting regent also reacts with the cerium activated alcohol to form a series of alkoxyborohydrides and these "hard reagents" result in the 1,2 hydride attack on the protonated carbonyl to give the final allylic alcohol product.[1][2]

Mechanism

Mechanism of the Luche reduction. Coordination of the alcohol to the Cesium cation makes its proton more acidic. BH4, borohydride Soft reagent favours 1,4 attack. Same mechanism leads to the other alkoxyborohydrides, alkoxyborohydrides Hard reagents favour 1,2 attack these 3 borohydrates lead to product. B(OR)4, not a hydride source.

References:

1.
Luche, J. L. J. Am. Chem. Soc. 1978, 100, 2226–2227.
2.
Gemal, A. L.; Luche, J. L. J. Am. Chem. Soc. 1981, 103, 5454–5459.