Mechanisms dialkylcuprate

A key step in the reaction mechanism appears to be nucleophilic attack on the alkyl halide by the negatively charged copper atom but the details of the mechanism are not well understood Indeed there is probably more than one mechanism by which cuprates react with organic halogen compounds Vinyl halides and aryl halides are known to be very unreactive toward nucleophilic attack yet react with lithium dialkylcuprates... 

The mechanism is very similar to the stoichiometric one, except that the key dialkylcuprate reagent is made from transmetallation of the Grignard reagent. 

When the isomeric acetates A and B react with dialkylcuprates, both give a give a very similar product mixture containing mainly C with small amounts of D. Only trace amounts of the corresponding Z-isomers are found. Suggest a mechanism to account for the formation of essentially the same product mixture from both reactants. 

The mechanism of conjugate addition of lithium dialkylcuprates to enones has been explored by the determination of 13C kinetic isotope effects by an NMR method reductive elimination from Cu is implicated as the rate-determining step.109... 

The coupling takes place as if a carbanion (R ) were present and the carbanion attacked the alkyl halide to displace the halide ion. This is probably not the actual mechanism, however, because dialkylcuprates also couple with vinyl halides and aryl halides, which are incapable of undergoing SN2 substitutions. 

Nucleophilic ring-opening of oxiranes by lithium dialkylcuprates occurs at the sterically less-hindered side and the corresponding alcohol is formed.The reaction can be carried out under mOd conditions and the side-reactions observed with other organometallic compounds can be avoided. The mechanism of the transformations has been treated in detail. With MejCuLi the reaction proceeds stereoselectively and the new C-C bond is formed on the side opposite the C-0 bond of the oxirane (Eq. 

Trialkylboranes are able to transfer an alkyl group onto l-acyl-2-vinylcyclopropanes with simultaneous opening of the ring. Since this reaction requires oxygen, a radical mechanism has been postulated. The yields of these reactions were too low for practical purposes however, this difficulty was overcome by the use of lithium dialkylcuprates rather than trialkylboranes as alkylating agents or by the use of copper(I) methyltrialkylborates, The reaction... 

The mechanism probably involves displacement of a tert-butyl radical from the borane by dioxygen, followed by addition of the tert-butyl radical to the vinyl group and ring cleavage. Much better yields of the same products were obtained when lithium dialkylcuprates replaced the trialkylboranes the nature of the intermediates is unknown in this case. 

However, the fact that vinylic and aryl hahdes undergo similar reactions suggests that Sn2 cannot be the only possible mechanism. An alternative, and more likely, mechanism begins by regarding the dialkylcuprate as equivalent to the product of the reaction between Cu and two methyl anions. 

Conjugate Addition Reactions.—The use of polymer-bound dialkylcuprates gives comparable or higher than normal yields in the conjugate addition to enones the major advantage lies in the ease of work-up. The mechanism of conjugate addition of cuprates and the mechanism and stereochemistry of the addition of lithium dimethylcuprate to jS-cyclopropyl-substituted enones have been reported. 

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