14.15 : הלידים חומציים לקטונים: מגיב גילמן

Grignard reagents reduce acid halides to alcohols, whereas reduction with lithium dialkyl cuprate, also known as the Gilman reagent, yields ketones.

This reaction is carried out in an ether solution at -78°C to obtain ketones in good yields.

The mechanism proceeds in two steps. In the first step, one of the alkyl groups of the organocuprate acts as a nucleophile and attacks the carbonyl carbon, forming a tetrahedral intermediate.

Next, the carbonyl is re-formed with the departure of a halide ion, giving ketone as the final product.

However, unlike the Grignard reduction, why does this reaction stop at the ketone? The answer lies in the reactivity of the reagents.

Copper is more electronegative than magnesium, but closer to the electronegativity of carbon. So, the carbon-copper bond is less polarized than the carbon-magnesium bond.

Consequently, the alkyl carbon in the Gilman reagent is weakly nucleophilic and less reactive than the Grignard reagent, preventing further reduction of ketones to alcohols.