What are the pros and cons of using Gilman reagent over Grignard reagent to synthesize a hydrocarbon?

As far as I know, in order to synthesize a hydrocarbon $\ce{R-R'}$, Grignard reagent or Gilman reagent could be used.

$\ce{RMgX + R'X -> RR' + MgX2}$

$\ce{R2CuLi + R'X -> RR' + LiX + RCu}$

What are the pros and cons of using Gilman reagent over Grignard reagent? Gilman reagent is significantly harder to prepare and involves a reactive organolithium compound. I'm not looking for the pros and cons in terms of cost and yield, but rather specificity:

1. Are there any $\ce{RR'}$ groups that must be synthesized via Gilman reagent and not Grignard reagent?

2. Are there any $\ce{RR'}$ groups that must be synthesized via Grignard reagent and not Gilman reagent?

Someone (outside of SE) pointed out to me how pointless it is for chemists to synthesize hydrocarbons using these methods (as they are readily available to purchase). While I agree with him, I'm still interested to find out "theoretically" if Gilman/Grignard reagent is specific to certain R/R' groups.

• Not all hydrocarbons are available for purchase though, so you might want to know how to make them. – jerepierre Oct 10 '14 at 19:23

I am not sure if Gilman reagents are harder to make than Grignard but they are softer nucleophiles and so they are used to perform 1,4-(conjugate) additions on $\ce\alpha,\beta-$unsaturated electrophiles. Grignard reagents will generally go for 1,2-attack instead. See this Wikipedia article for more info:

There is a flaw in your first reaction. $\ce{RMgX + R'X}$ is in equilibrium with $\ce{R'MgX + RX}$. This is a type of halogen metal exchange.

Since the reaction is in equilibrium you will get 3 products instead of one $\ce{RR', RR, R'R'}$ and therefore sheer wastage of Grignard reagent. On the other hand, Gilmann reagent is pretty good at it.

Only symmetrical alkane could be prepared by Grignard reagent (Here I am talking only about synthesis of alkane). So, as you say, Gilmann reagent is harder to care so its better you use Grignard reagent there, but I am not sure of that. Though I am pretty much sure that preparation of unsymmetrical alkane leads to complete wastage if Grignard reagent is used.

• @t.c thanks for pointing out the flaw . I think I have NOT used congusing language by the way I was talking for purpose of making alkanes only – DSinghvi Oct 10 '14 at 18:24
• Yes, I realised, in my question I said R-R', and I did not mention about C=O – t.c Oct 10 '14 at 18:28
• @t.c By using "SPECIFIC" . Are there any special features among alkanes that one reaction which is happening with some reagent might not react with other . I think the reason usually is steric hinderance which is not the case here. – DSinghvi Oct 10 '14 at 18:29
• Are there any reasons why wouldn't PhMgBr react with CH3Br to form toluene, for example? – t.c Oct 10 '14 at 18:33
• @t.c I think the reaction you mention do occur by Sn2 mechanism – DSinghvi Oct 10 '14 at 18:46