Summary factors controlling conjugate addition

The organocopper is shown here as Me-Cu because its precise structure is not known. But there are other organocopper reagents that also undergo conjugate addition and are much better understood. The simplest result from the reaction of two equivalents of organo-lithium with one equivalent of a copper (1) salt such as CuBr in ether or THF solvent at low temperature. The lithium cuprates (R2CuLi) that are formed are not stable and must be used immediately. 

The addition of lithium cuprates to a, 3-unsaturated ketones turns out to be much better if trimethylsilyl chloride is added to the reaction— we shall explain what this does shortly, but for the moment here are two examples of lithium cuprate additions. 

The silicon works by reacting with the negatively charged intermediate in the conjugate addition reaction to give a silyl enol ether—a type of molecule we met in Chapter 20. Here is a possible mechanism for a reaction between Bu2CuLi and an a, 3-unsaturated aldehyde in the presence of MeaSiCl. The silyl enol ether simply hydrolyses to the ketone at the end of the reaction. 

At this point in the chapter it is worthwhile talking stock of the factors controlling the two modes of addition to a, 3-unsaturated carbonyl compounds. 

CHAPTER 22 CONJUGATE ADDITION AND NUCLEOPHILIC AROMATIC SUBSTITUTION 

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We end with a summary of the factors controlling the two modes of addition to a, ( -unsaturated carbonyl compounds, and by noting that conjugate addition will be back again—in Chapters 23 (where we consider electrophilic alkenes conjugated with groups other than C=0) and 29 (where the nucleophiles will be of a different class known as enolates). 

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