Formation of Organomagnesium Compounds

The mechanisms of the more traditional formations in solution have also been considered in a valuable series of papers. The rates of reaction, in general, of RX (R=alkyl) with Mg were proportional to both [RX] and the magnesium surface area. For RI, and most RBr, transport to the magnesium was overall rate limiting the rates of reaction of RCl are slower and are not transport limited. Either (0 initial electron transfer from Mg to RX or (//) collision of RX with Mg, followed by halogen extraction, are possibilities for the critical steps. 

Specifically in the reaction of cyclopentyl bromide (R Br) in EtgO it was established that (/) -d[R Bi]jdt is proportional not only to [R Br] and the surface area of magnesium, but also to the rate of stirring of the solution and inversely proportional to the viscosity and (i7) the activation energy is small, ca. 2—3 kcal mol. There appears to be an upper limit to the rate of reaction of alkyl halides with magnesium. For less reactive halides, e.g., MeaCCHaBr, the rates are also dependent on the dielectric constant. 

Hammett p values are similar for both the ArBr-Mg and ArBr-BusSnH reactions in EtjO, but not however in THF. The r.d.s. in the ArBr-Mg reaction is one of the following (i) electron transfer to ArBr, (ii) formation of Ar by bromine abstraction from ArBr by Mg, or, less probably, (iii) insertion of Mg into C-Br. The reactions of ArBr with Mg in THF and more polar solvents (as well as those between Arl and Mg in EtgO) are mass transport limited. 

A considerable increase in the yields of RMgX, as well as coupled products, occurs on irradiation with light (RX=2-bromofluorene, 2-bromothiophene, PhCl, and PhaCHCl). 

The exchange reaction [e.g., equation (1)], catalysed by cpaTiCl2, provides 

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Reaction with deuterium oxide, O-deuterated alcohols or acids is commonly used to provide evidence for the formation of organomagnesium compounds (e.g. see Ref. [3]). Although the presence of water normally inhibits the reaction of organic halides with magnesium, it is noteworthy that under appropriate conditions a Barbier procedure was successful [4] ... 

The following specific features of formation of organomagnesium clusters in the films obtained by codeposition of magnesium and organic compound vapors can be formulated ... 

The addition of organomagnesium compounds to the carbonyl group of aldehydes and ketones has a long history, and remains one of the most important reactions for carbon-carbon bond formation. While the overall reaction is simple, it is susceptible to a number of side-reactions, and its... 

Similar considerations apply to reactions of organomagnesium compounds with formate esters (leading to aldehydes), chloroformates or carbonates (leading to esters) or carbamoyl chlorides (leading to amides) ... 

Reactions of organomagnesium compounds with nitroso or nitro compounds are often complicated, and not useful in synthesis, or only represented by isolated examples. One exception to this generalization involves conjugate addition to a,[3-unsaturated and aromatic nitro compounds (see Section 4.2), and two others lead to the formation of carbon-nitrogen bonds. 

The formation of tetraalkylborates by reactions of organomagnesium compounds with trialkylboranes,... 

Table 1 The Influence of Solvent on Radical Formation by Organomagnesium Compounds with Ketones [36]...

The influence of the solvent on radical formation by organomagnesium compounds with ketones was made evident a few years later when dibenzylmagnesium was reacted with fluorenone [36] (Table 1). 

Grignard, in his pioneer paper on reactions of organomagnesium compounds [17], reported the formation of a small amount of benzyl alcohol (beside 56% of the expected main product, the secondary alcohol) in the reaction of benzaldehyde with isoamylmagnesium bromide (Scheme 7) ... 

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