Luche reduction
The Luche reduction is a chemical reaction that serves the selective reduction of ketones in addition to aldehydes . Their principle is based on the activation of the keto function by Lewis acids . It is named after its developer Jean-Louis Luche .
If aldehyde and keto functions are present next to one another in a molecule , hard nucleophiles preferentially reduce the aldehyde function. The reason for this is that the aldehydic carbon has a stronger positive partial charge than the carbonyl carbon atom of the ketone and is therefore preferred for attack by hard nucleophiles. In a Luche reduction, the hardness of the ketonic carbon is increased by complexing its oxygen with cerium (III) chloride , thus activating this position for reduction.
mechanism
In the first step, cerium (III) chloride coordinates to the ketonic oxygen. The sodium borohydride used as reducing agent now preferably hydrogenates this position. The alcohol formed is finally released by aqueous work-up.
The preferred reduction in the keto function is reinforced by a second effect. Usually simple alcohols are used as solvents , which react with sodium borohydride to form various alkoxyborohydrides. This increases the hardness of the reducing agent and the selectivity is shifted further in favor of the reduction of the activated ketone.
More reactions
The Luche conditions are also suitable for the reduction of α, β-unsaturated ketones. In these cases, due to the conjugation to the C = C double bond, the ketonic carbon is not hard enough to be attacked by hard nucleophiles. Activation with cerium (III) chloride increases its hardness and thus enables reduction.
Under Luche conditions, other hard nucleophiles, such as Grignard reagents or organolithium compounds , can also be used. These react with the activated keto function without enolization occurring.
literature
- R. Brückner: reaction mechanisms. 3rd edition, Spektrum Verlag, Heidelberg 2004, ISBN 3-8274-1579-9 .
Individual evidence
- ↑ Andre L. Gemal, Jean Louis Luche: lanthanide in organic synthesis. 6. Reduction of α-enones by sodium borohydride in the presence of lanthanoid chlorides: synthetic and mechanistic aspects . In: Journal of the American Chemical Society . tape 103 , no. September 18 , 1981, pp. 5454-5459 , doi : 10.1021 / ja00408a029 .
- ^ Carl R. Johnson, Bradley D. Tait: A cerium (III) modification of the Peterson reaction: methylenation of readily enolizable carbonyl compounds . In: The Journal of Organic Chemistry . tape 52 , no. 2 , 1987, pp. 281-283 , doi : 10.1021 / jo00378a024 .