14.15: Acid Halides to Ketones: Gilman Reagent
Lithium dialkyl cuprate, also known as Gilman reagents, selectively reduces acid halides to ketones. The acid chloride is treated with Gilman reagent at −78 °C in the presence of ether solution to produce a ketone in good yield.
As shown below, the mechanism proceeds in two steps. First, one of the alkyl groups of the reagent acts as a nucleophile and attacks the acyl carbon of the acid chloride to form a tetrahedral intermediate. This is followed by the reformation of the carbon–oxygen double bond with the loss of a halide ion as the leaving group to give a ketone as the final product.
The electronegativity of copper is closer to the electronegativity of carbon as compared to magnesium. Therefore, the carbon–copper bond in the Gilman reagent is less polarized, which makes the alkyl carbon weakly nucleophilic and less reactive. Consequently, the reaction stops at the ketone intermediate and prevents further reduction of a ketone to an alcohol.