Reactions Involving Organocopper Reagents and Intermediates

Sn2 and SN2 Reactions with Halides and Sulfonates. Corey and Posner discovered that lithium dimethylcuprate can replace iodine or bromine by methyl in a wide variety of compounds, including aryl, alkenyl, and alkyl derivatives. This halogen displacement reaction is more general and gives higher yields than displacements with 

For substituted allylic systems, both a- and y-substitution can occur. Reaction conditions can influence the a- versus /-selectivity. For example, the reaction of geranyl acetate with several butylcopper reagents was explored. Essentially complete a- or y-selectivity could be achieved by modification of conditions.28 In ether both CuCN and Cul led to preferential /-substitution, whereas a-substitution was favored for all anions in THF. 

3-Acetoxy-2-methyl-l-alkenes react primarily at C(l), owing to steric factors.29 

5-Acetoxy-1,3-alkadienes give mainly e-alkylation with dialkylcopper-magnesium reagents. 

High y-selectivity has been observed for allylic diphenyl phosphate esters.26a O 

Secondary bromides and tosylates react with inversion of stereochemistry, as in the classical SN2 substitution reaction.21 Alkyl iodides, however, lead to racemized product. Aryl and alkenyl halides are reactive, even though the direct displacement mechanism is not feasible. With there halides the mechanism probably consists of two steps. The addition of halides to transition-metal species with low oxidation states is a common reaction in transition-metal chemistry and is called oxidative addition. An oxidative addition to the copper occurs in the first step of the mechanism, and the formal oxidation 

Allylic halides usually give both SN2 products and products of substitution with an allylic shift (SN2 products) although the mixed organocopper reagent RCu— BF3 is reported to give mainly the SN2 product.22 Allylic acetates undergo displacement with an allylic shift (SN2 mechanism).23 The allylic substitution process may involve initial 

The reaction shows a preference for anti stereochemistry in cyclic systems.2 

It has been suggested that the preference for the anti stereochemistry is the result of 

Suzuki, T. Suzuki, T. Kawagishi, and R. Noyori, Tetrahedron Lett. 21 1247 (1980). 

State of copper after this addition step is 3+. This step is followed by combination of two of the alkyl groups from copper. This process, which is also very common for transition-metal intermediates, is called reductive elimination. 

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