R= any alkyl; R1=Methyl or alkyl or 20 cycloalkyl
The given reaction is Corey - house synthesis. An alkyl halides is first treated with Li metal in Dry ether to form alkyl lithium. This upon reaction with Cuprous iodide yields Lithium dialkyl cuprate.
R−XLi−−−−−→dry etherRLi+LiX
2RLi+Cul⟶Li(R)2Cu+Lil (Lithium dialkyl cuprate)
Lithium dialkyl cuprate is known as Gilman reagent
R′X+R2LiCuDry−−−→etherR′−R+RCu+LiX
Foe Better yields,
R = any alkyl
R'= Methyl or Primary alkyl or secondary alkyl or secondary cycloalkyl or vinyl or aryl
In other words , R'X should not be a tertiary alkyl halide.