Magnesium reaction with alkyl halides, mechanism

Oxidative-addition reactions of alkyl halides also occur with metal ions such as thallium(I). In this case, the HOMO is an s orbital on the metal of a symmetry. The concerted mechanism is now impossible and only a two-step reaction can occur. The same is true for reactions of metals in the free state, such as zinc or magnesium, with alkyl halides. Note that free radicals may be formed, instead of ions, in all cases discussed. Free radical reactions are usually allowed by symmetry. 

In 1966, a method for the synthesis of organotin compounds by electrolysis of alkyl halides using a tin anode and a magnesium cathode was developed.94,95 Such electrochemical reactions have been studied extensively by Tuck and coworkers.9 These studies have been reviewed and a general mechanism proposed for this class of electrochemical reactions.9 The reactions of organic halides with a metal anode resemble oxidative addition reactions, Eqs. 7.19 - 7.20 ... 

There are very few studies of the Barbier reaction mechanism with magnesium as the metallic source. However, with the discovery that the reaction will often proceed to a greater extent with other metals such as lithium, some good mechanistic studies have been conducted. The focus of most of these studies has been the reaction of an alkyl or aryl halide with a ketone. The conclusions drawn from these studies are considered to be valid for magnesium as well, and they will be referenced in this section. 

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