Organocuprates mechanism

The stereoselective 1,4-addition of lithium diorganocuprates (R2CuLi) to unsaturated carbonyl acceptors is a valuable synthetic tool for creating a new C—C bond.181 As early as in 1972, House and Umen noted that the reactivity of diorganocuprates directly correlates with the reduction potentials of a series of a,/ -unsaturated carbonyl compounds.182 Moreover, the ESR detection of 9-fluorenone anion radical in the reaction with Me2CuLi, coupled with the observation of pinacols as byproducts in equation (40) provides the experimental evidence for an electron-transfer mechanism of the reaction between carbonyl acceptors and organocuprates.183... 

Scheme 4.4. Proposed mechanism for the 1,6-addition of organocuprates to acceptor-substituted enynes.

E. -i. Nakamura, S. Mori, Wherefore Art Thou Copper Structures and Reaction Mechanisms of Organocuprate Clusters in Organic Chemistry, Angew. Chem. Int. Ed. Engl. 2000, 39, 3750-3771. 

R. A. J. Smith and A. S. Vellekoop, 1,4-Addition Reactions of Organocuprates with ,/3-Unsat-urated Ketones, in Advances in Detailed Reaction Mechanisms (J. M. Coxon, Ed.), Vol. 3, Jai Press, Greenwich, CT, 1994. 

Organocuprates and a few other carbon nucleophiles sometimes react with allylic halides (or acetates) to give the product of what looks like an SN2 reaction. However, the mechanism is completely different— preliminary coordination by the copper or other transition metal to the C=C n bond is the first step, and the coordination of the copper changes from rj-2 to rj-1 or rj-3 before a reductive elimination step establishes the C—C bond.394 Each of these steps affects the overall regiochemistry (and the stereochemistry discussed in the next chapter), which may look like an SN2 or an SN2 reaction, while mechanistically being neither. 

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