Organocopper reagents undergo conjugate addition
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص509
2025-06-11
696
The copper works by transmetallating the Grignard reagent to give an organocopper reagent— simply put, the magnesium is exchanged for copper. Organocoppers are softer than Grignard reagents, and add in a conjugate fashion to the softer C=C double bond. Once the organo copper has added, the copper salt is available to transmetallate some more Grignard, and only a catalytic amount is required.
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 (I) 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 α,β-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. Here is a possible mechanism for a reaction between Bu2CuLi and an α,β-unsaturated aldehyde in the presence of Me3SiCl. The silyl enol ether simply hydrolyses to the ketone at the end of the reaction.
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