Organocuprates reactivity with substrates

Double substitution of geminal dibromides is best accomplished by treatment with an organocuprate, followed by the addition of an alkyl iodide to the crude reaction mixture. " The reaction has found especial application for the synthesis of dimethylcyclopropanes, ubiquitous in natural product chemistry. Excellent yields are often obtained with Me2CuLi, but with less reactive substrates higher order cyano-and thiocyano-cuprates sometimes provide better results. Spiro dialkylation of geminal dibromides has also been accomplished. Vicinal dibromides, on the other hand, afford alkenes upon treatment with dialkyl cuprates. ... 

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