Luche reduction
{{Short description|Primary alcohol}}
{{Use dmy dates|date=March 2024}}
{{Reactionbox
| Name = Luche reduction
| Type = Organic redox reaction
| NamedAfter = Jean-Louis Luche
| Section3 = {{Reactionbox Identifiers
| OrganicChemistryNamed = luche-reduction
| RSC_ontology_id = 0000286
}}
}}
Luche reduction is the selective organic reduction of α,β-unsaturated ketones to allylic alcohols.{{cite book |last1=Kürti |first1=László |last2=Czakó |first2=Barbara |year=2005 |title=Strategic Applications of Named Reactions in Organic Synthesis: Background and Detailed Mechanisms |publisher=Elsevier Academic Press |isbn=0-12-429785-4}}{{cite journal |last=Luche |first=J.-L. |year=1978 |title=Lanthanides in Organic Chemistry. 1. Selective 1,2 Reductions of Conjugated Ketones |journal=J. Am. Chem. Soc. |volume=100 |issue=7 |pages=2226–2227 |doi=10.1021/ja00475a040}}{{cite journal |last1=Luche |first1=J.-L. |last2=Rodriguez-Hahn |first2=L. |last3=Crabbé |first3=P. |year=1978 |title=Reduction of Natural Enones in the Presence of Cerium Trichloride |journal=J. Chem. Soc., Chem. Commun. |issue=14 |pages=601–602 |doi=10.1039/C39780000601}} The active reductant is described as "cerium borohydride", which is generated in situ from NaBH4 and CeCl3(H2O)7.{{cite book |doi=10.1002/047084289X.rc041.pub3 |chapter=Cerium(III) Chloride |title=Encyclopedia of Reagents for Organic Synthesis |date=2021 |last1=Paquette |first1=Leo A. |last2=Sabitha |first2=G. |last3=Yadav |first3=J. S. |last4=Scheuermann |first4=Angelique M. |last5=Merchant |first5=Rohan R. |pages=1–15 |isbn=9780471936237 }}
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.{{cite journal |last1=Gemal |first1=A. L. |last2=Luche |first2=J.-L. |year=1981 |title=Lanthanoids in Organic Synthesis. 6. The Reduction of α-Enones by Sodium Borohydride in the Presence of Lanthanoid Chlorides: Synthetic and Mechanistic Aspects |journal=J. Am. Chem. Soc. |volume=103 |issue=18 |pages=5454–5459 |doi=10.1021/ja00408a029}}
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.