Alkane synthesis using R₂CuLi

Summary

• Organolithium cuprates, R₂CuLi, react with alkyl halides forming a new C-C, giving alkanes.
• Primary alkyl iodides make the best substrates otherwise elimination can be a problem.
• The R group of the cuprate can also be aryl or vinyl.
• The R' group in the halide can also be aryl or vinyl.
• Although the mechanism looks like a S_N2, it is more complex and is not well understood.

Question:
What evidence suggests that these reactions are not just simple S_N2 reactions ?  

Related Reactions

• Conjugate addition of R₂CuLi to a,b-unsaturated carbony