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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?

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  • $\begingroup$ My guess would be the greater nucleophilicity of the cuprate over the $\ce{R-Mg-X}$ $\endgroup$ – Eashaan Godbole Jan 16 '18 at 10:45
  • $\begingroup$ @EashaanGodbole Thanks! But why is it more nucleophilic? $\endgroup$ – GingerBadger Jan 16 '18 at 10:46
  • $\begingroup$ 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... $\endgroup$ – Eashaan Godbole Jan 16 '18 at 10:48
  • $\begingroup$ @EashaanGodbole But isn’t the C-Mg bond higher in energy than C-Cu? $\endgroup$ – GingerBadger Jan 16 '18 at 11:03
  • $\begingroup$ Would it be higher in energy than $\ce{C-Cu^{-}}$? $\endgroup$ – Eashaan Godbole Jan 16 '18 at 17:03

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