Monometallic organomagnesium reagents

An important feature of the halogen/metal interconversion is that it is an equilibrium-controlled process, which is therefore shifted toward the most stable organolithium or organomagnesium reagent or, to put it in other words, toward the weakest base [18-20], 

SCHEME 28.1 Halogen/Iithium exchange using Buli and tBuLi. 

A second consequence concerns the exchange reaction between a haloarene and an arylmetal— and, as a corollary, the evolution of a halogen-substituted arylmetal over time. In those cases, the equilibrium is shifted toward the arylmetal bearing the metal closest to as many -I (inductively electron-withdrawing) and -hM (mesomerically electron-donating) substituents as possible, the -I effect being the most influential [26]. 

Interestingly, the thermodynamically favored product of such a reaction is also the kinetically favored one [27-30], thanks to the same kind of stabihzation by electron-donating or electron-withdrawing substituents of the aromatic ring in the ate complex transition state or intermediate (vide infra). 

Another important feature of the halogen/metal interconversion is that it proceeds almost exclusively on reactants RX where X is iodine or bromine. In the case of haloarenes, a few examples of chlorine/metal exchange have been reported in very particular cases—strong electron depletion in the arene [31-33] or buttressing effect [34] (vide infra)—but fluorides are unreactive. Iodides are the most reactive toward exchange and are several orders of magnitude faster than bromides. When 

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