What is the Luche Reduction?

It is a specific type of highly regioselective reduction that results in a 1,2 reduction of ketones, aldehydes, or enones.

What are the conditions required for this reduction, and how do they make it selective?

• NaBH4 : this is the main reducing agent in the reaction that is responsible for adding a hydrogen atom to the carbonyl group.

• A lanthanoid chloride (CeCl3, for example): the Ce3+ degrades the reducing agent from a hydrogen based reducing agent to a –OR group based reducing agent.  This hardens the reducing agent, making it more likely to add a hydrogen atom specifically to a carbonyl group.  If there were a ketone and an aldehyde present, it will be more likely to add to a ketone because a ketone is more Lewis basic than an aldehyde

• An alcohol solvent (i.e. methanol or ethanol): the solvent provides the hydrogen atom necessary to add onto the alkoxide group after the reducing agent has added a hydrogen atom to the carbonyl group.  The alcohol solvent is activated by the Ce3+ ion, making its proton very acidic, which then comes off and is added to the oxygen atom of the ketone.

What is the actual reaction scheme for the Luche reduction?

1. Ce3+ catalyzes the degradation of the reducing borohydride by the alcohol solvent.

2. Ce3+ also activates the alcohol solvent, making it easier for the oxygen of the carbonyl group to pick up that hydrogen atom.

3. The degraded reducing agent adds a hydrogen atom to the carbon atom of the carbonyl group, creating an alkoxide.

4. The alkoxide then takes a hydrogen atom away from the activated alcohol solvent, completing the reduction of a carbonyl to an alcohol.

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