14.15
Acid Halides to Ketones: Gilman Reagent
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Q1: What is a Gilman reagent and how does it reduce acid halides?
A Gilman reagent, or lithium dialkyl cuprate, selectively reduces acid halides to ketones. One alkyl group acts as a nucleophile and attacks the carbonyl carbon of the acid halide, forming a tetrahedral intermediate. The carbonyl then reforms with halide departure, yielding a ketone as the final product.
Q2: Why does the Gilman reagent stop at the ketone instead of reducing it to an alcohol?
The Gilman reagent stops at the ketone because copper is more electronegative than magnesium, making the carbon-copper bond less polarized than the carbon-magnesium bond. This renders the alkyl carbon weakly nucleophilic and less reactive, preventing further reduction of the ketone to an alcohol.
Q3: What are the reaction conditions required for Gilman reagent reduction of acid halides?
The Gilman reagent reduction is carried out in an ether solution at -78°C to obtain ketones in good yields. These low-temperature conditions and the ether solvent are essential for maintaining reagent stability and achieving selective ketone formation from acid halides.
Q4: How does the Gilman reagent differ from Grignard reagents in reducing acid halides?
Grignard reagents reduce acid halides to alcohols because the carbon-magnesium bond is highly polarized, making the alkyl carbon strongly nucleophilic. In contrast, Gilman reagents produce ketones because the carbon-copper bond is less polarized, resulting in weaker nucleophilicity that stops reduction at the ketone stage.
Q5: What is the mechanism of the Gilman reagent reduction in the first step?
In the first step of the mechanism, one alkyl group of the organocuprate acts as a nucleophile and attacks the acyl carbon of the acid halide. This nucleophilic attack forms a tetrahedral intermediate, which is the key intermediate structure before carbonyl reformation occurs.
Q6: What happens in the second step of the Gilman reagent reduction mechanism?
In the second step, the carbon-oxygen double bond reforms with the loss of a halide ion as the leaving group. This reformation of the carbonyl completes the reduction, yielding the ketone as the final product from the tetrahedral intermediate structure.
Q7: How does electronegativity affect the reactivity of Gilman reagents?
Copper's electronegativity is closer to carbon's than magnesium's electronegativity is. This similarity reduces the polarity of the carbon-copper bond compared to the carbon-magnesium bond, resulting in weaker nucleophilicity of the alkyl carbon and significantly lower reactivity of the Gilman reagent.