# How can organocuprates generated with catalytic Cu(I) be competitive in 1,4-addition to an enone?

Organocopper compounds (e.g. $$\ce{LiCuR_2}$$) are often prepared from an organolithium or Grignard reagent when a soften nucleophile is needed, such as for a conjugate addition to an $$\alpha,\beta$$-unsaturated carbonyl. Clayden et al. (p.499) recommend generating it in situ catalytically via transmetallation with 1% $$\ce{CuI}$$.

My question: if at the start, there is 1% of organocopper reagent and 99% Grignard reagent, why don't those 99% do direct addition? Why is the organocopper reagent so much more reactive that conjugate addition is the main outcome of the reaction?

• My guess would be the greater nucleophilicity of the cuprate over the $\ce{R-Mg-X}$ Jan 16 '18 at 10:45
• @EashaanGodbole Thanks! But why is it more nucleophilic? Jan 16 '18 at 10:46
• The formation of -ate complexes (e.g. boronate, aluminate (as in $\ce{LiAlH4}$) results in the formation of high-energy $\sigma$-bonds. There's the extra reduction in effective electronegativity due to the negative charge too... Jan 16 '18 at 10:48
• @EashaanGodbole But isn’t the C-Mg bond higher in energy than C-Cu? Jan 16 '18 at 11:03
• Would it be higher in energy than $\ce{C-Cu^{-}}$? Jan 16 '18 at 17:03