Radical mechanisms Grignard reagent formation

The mechanism of Grignard reagent formation involves free radicals," and... 

Fig. 17.44. Mechanism of the formation of a Grignard compound ( e indicates electron migration). The reaction is initiated by an electron transfer from the metal to the substrate. The extra electron occupies the d(C—I) orbital, whereby the C—I bond is weakened and breaks. This cleavage leads to the formation of a methyl radical and an iodide ion on the metal surface. In the third step of the reaction, the valence electron septet of the methyl radical is converted into an octet by formation of a covalent bond between the methyl radical and a metal radical. The Grignard reagent is thus formed.

The halide radical anion (6) dissociates to give a carbon radical (R, where a radical is defined as a reactive intermediate with one electron in a p-orbital see Chapter 7, Section 7.4.3) and a halide anion, X. If X and 5 react with each other, a Mg-X bond is formed and the magnesium complex becomes a radical (6) rather than a radical cation. Note that the radical cation can accept electron density from the electron-rich X. If 6 subsequently reacts with the carbon radical (R ), Grignard reagent (7) is generated. This sequence is taken to be the mechanism of Grignard reagent formation from alkyl halides 1. The ether... 

The mechanism for the formation of Grignard reagents might involve radicals ... 

A concerted electron transfer mechanism, with formation of an alkyl radical and quinone radical anion, has been proposed to account for the products of reaction of benzophenone with alkyllithium or Grignard reagents 92 the ratio of addition to reduction products is dependent on the alkyl group and not on the metal. 

If the addition of a Grignard reagent to a carbonyl compound leads to the formation of such an unexpected alcohol, it is clear that this reaction cannot have proceeded according to the polar mechanism outlined in the upper row of Figure 10.27. If the respective addition does not produce anything hut the expected alcohol, the polar mechanism provides the easiest explanation. Nevertheless, the radical mechanism could also have applied—provided, though, that the isomerization H —> iso-H would have been much slower than the radical annihilation reaction H — I. 

The majority of main group organometallic compounds for which SET mechanisms were postulated involves Grignard reagents [13,69-75] in their reactions with carbonyl compounds. Even the formation of these important laboratory reagents RMgHal (which does not fall within the scope of this article) is now believed to involve electron transfer mechanisms and radical formation at the surface of metallic Mg [76,77]. In contrast to Grignard compounds with their... 

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