[darkdevil]

Hi, I found out many literatures to couple thiophene units using n-BuLi and then transforming it into a grignard (as shown in the top scheme). Afterwards by adding an aryl halide to finish the coupling. I do not understand why can't we just add aryl halide right after the lithiation? without transforming it into a grignard. Can someone explain why? Thanks

[darkdevil]

WHY??? Can anyone give some comments?

[orgopete]

I think this question is asking what role must magnesium play in the coupling. Since you know the thienyllithium doesn't couple, this should give some clue about the reaction. For the moment, forget that magnesium bromide is the coupling reagent and ask if you wanted to do this coupling with any other methods, what other methods would work? I am going to presume you will find that a different mechanism prevails. If so, then magnesium must react in a different manner than lithium, even though we may more commonly see lithium and Grignard reagents reacting similarly.