Aldehydes Knochel cuprate addition

Alkyl zinc iodides RFG—Zn—I are poor nucleophiles. However, they are turned into good nucleophiles when they are converted into the so-called Knochel cuprates RFG— Cu(CN)ZnHal with solubilized CuCN—that is, CuCN containing LiHal. In the presence of a Lewis acid Knochel cuprates add to aldehydes, provided these are a,/f-unsaturated. With substituted oc,/3-unsaturated aldehydes a 1,2-addition can be observed, as shown in Figure 10.38. With acrolein (an unsubstituted f/./l-u nsaturated aldehyde) or a,/J-unsaturated ketones (Fig. 10.43), however, Knochel cuprates undergo 1,4-additions. 

The use of BF3-Et20 to accentuate the reactivity of otherwise sluggish cuprates toward 1,2-additions is also the subject of a recent report by Knochel. Functionalized lower order cyanocuprates incorporating Znl+ as the gegenion in place of Li+ or MgX+ readily add to aldehydes at low temperatures provided excess Lewis acid is present (Scheme 9). Isolated yields of products are very good, and the observation that ketones do not react under similar conditions adds an element of chemospecificity to this method. 

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