Mechanism of the Grignard Reagent Formation Reaction

The mechanism of the Grignard reagent formation reaction has been the subject of intensive study for quite some time. 

The notion, that radicals played an important role in heterogeneous reactions of this type was mentioned at the very beginning of the development of organometallic chemistry Frankland discovered the formation of organozinc compounds when searching for, what in those days were named organic radicals (see chap. 1). 

In his book Le Magnesium en Chimie Organique, Courtot surveyed the progress in Grignard Chemistry, a quarter of a century after the discovery of the mixed organomagnesium compounds. 

In discussing some sidereactions that occur during the formation of the Grignard reagent, no mention is made of any involvement of radical species [20]. 

Inspired by earlier research, and theoretical considerations in which monovalent magnesium species were suggested as intermediate species in the forma- 

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Anteunis, M., van Schoote, J. Grignard reaction. VII. Mechanism of the Grignard reagent formation and the Wurtz side reaction in ether. 

The mechanism of the Heck reaction with Pd(OAc)2 probably involves the initial in situ reduction of the Pd " ion to Pd(0), which then inserts in the C-X bond (recall the formation of the Grignard reagent from Mg and halides). The species R-PdX that is formed adds to the double bond in the sense of R and PdX, which in an intramolecular reaction will lead to a cyclic product. The adduct eliminates H-PdX to regenerate a double bond. This proposed mechanism is illustrated with the synthesis of the benzofuran 4.36 in Scheme 4.39. 

Reaction of Bromobenzene with Magnesium Detailed Reaction Scheme 1963. Accurate analysis of the reaction mixture of bromobenzene and magnesium in diethyl ether [25], by yet another research group led to the rejection of an ionic mechanism which had been suggested earlier [26] a rather detailed reaction scheme for the Grignard reagent formation was presented which-in the authors view-was close to the mechanism proposed by Kharasch and Reinmuth. 

The mechanism of formation of the Grignard reagent involves radical-transfer reactions in which a transient alkyl radical is formed in the presence of a magnesium-centered radical (Fig. 6.7). 

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