Magnesium with electrophiles

Table 12 Representative Reactions of (2-Butene-l,4-diyl)magnesiums with Electrophiles...

Table 4.4 Stepwise reactions of (2,3-dimetyl-2-butene-l,4-diyl)magnesium with electrophiles.

Organometallic reagents and alkali metal amides can react via a cyclic transition state (Section II, B, 5) beginning with electrophilic attack at the most basic ring-nitrogen. As a result, sodamide (in dimethylaniline, 145°, 2 hr) yields the 4-amino derivatives (40% yield S)) methyl- or phenyl-magnesium iodides give the 4-adduct quantitatively.s ... 

The Grignard reagents prepared from the activated magnesium appear to react normally with electrophiles. Thus reactions with proton donors, ketones, and carbon dioxide afford hydrocarbons, alcohols, and carboxylic acids, respectively. The reductive coupling of ketones to pinacols had also been accomplished with the activated magnesium. ... 

B. Reactions of Magnesium Ketone Enoiates with Electrophiles. 472... 

D. Reactions of Magnesium Dicarbonyl Enoiates with Electrophiles. 489... 

E. Reactions of Magnesium Amide and Lactam Enoiates with Electrophiles. 499... 

C. Reactions of Magnesium Ester Enolates and Magnesium Lactone Enolates with Electrophiles... 

Lithium dibutylisopropylmagnesate proved to be quite efficient for the bromine-magnesium exchange of 5-bromo-2-picoline at —10 °C (Table 14) . The resulting picolyl-magnesium reagent reacted with electrophiles including thiuram disulfide (last entry). 

Magnesium alkyUdene carbenoids were found to be reactive with different nucleophiles to give new alkenyhnagnesium compounds which could be trapped with electrophiles. As a whole, novel methods for the synthesis of tri- or tetrasubstimted olefins from the 1-chlorovinyl p-tolyl sulfoxides in a one-pot reaction are realized. 

Similarly, nitroarenes can also be lithiated (Scheme 5.43), but reactions of meta-lated nitroarenes with electrophiles only proceed cleanly if the metalation is performed in the presence of the electrophile [403], Otherwise, the metalated arene can reduce nitro groups to nitroso groups, which quickly react with additional organo-metallic reagent to yield hydroxylamines [404, 405], Nitroarylmagnesium halides can be prepared by iodine-magnesium exchange at -80 °C to -40 °C, and react with electrophiles in the expected way [6] (Scheme 5.43). 

Treatment of indole (pKa - 17) with strong bases such as butyl lithium, Grignard reagents, or metal hydrides produces the corresponding indolyl anion, which reacts with electrophiles either on nitrogen or at the C3 position. With lithium, sodium, or potassium as counterion the indolyl anion tends to react on nitrogen, as in the preparation of 7.35. However, with magnesium as the counterion the intermediate has an essentially covalent rather than ionic structure, and reaction tends to occur at the C3 position, as in the preparation of 7.36. 

One of the benefits of the Barbier reaction over the Grignard reaction is generally the production of products with little or no R-R dimer formation. However, because of the radical nature of the reaction Wurtz-type dimer formation can be the primary reaction in the absence of an electrophilic coreagent. Some Russian chemists have reported the dimerization of halogenated alkenes and alkanes using iodine-activated magnesium with silver bromide [Eqs. (27) and (28) 64]. 

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