Magnesium, reaction of with alkyl and aryl halides

McGwire, Mark, 1041 Macrolide antibiotics, 758 Magnesium, reaction of with alkyl and aryl halides, 550-551, 571 Magnetic field... 

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. 

Reactions of Activated Magnesium with Alkyl and Aryl Halides... 

Organophosphorus compounds find wide use in the chemical industry as catalysts, intermediates, complexes, and end-use products. Arylphosphines and phosphine oxides are often produced by the reaction of a preformed Grignard reagent with a halophosphine or phosphine oxide. Yields are reduced by the production of unwanted side-reaction products such as biaryls. These unwanted products are reduced when the reaction is conducted under Barbier conditions. When alkyl and aryl halides are reacted with magnesium metal, a trihalophosphine or phosphine oxide, a metal halide or amine catalyst, in THE benzene mixtures, at reflux, good yields of phosphines or phosphine oxides are obtained [74]. For example, triphenylphosphine can be prepared in a 97.2% yield from the reaction of bromobenzene, trichlorophosphine, magnesium metal, aluminum chloride, and sodium chloride in THF-benzene at 70 80 C. 

An efficient method for the reduction of alkyl and aryl halides is by reaction with magnesium and isopropanol. 1-Bromonaphthalene is reduced at the boiling point of isopropanol (82°). With a less reactive halide, for example chlorobenzene, a... 

We have examined the reactions of the activated magnesium and the Kl-activated magnesium with various alkyl and aryl halides.1-4 The results of these studies are summarized in Table 4. 

The reaction is carried out by first reacting the alkyl or aryl halide with magnesium shavings in an ether suspension and then treating with silicon tetrachloride (prepared by passing chlorine over heated silicon). With methyl chloride the following sequence of reactions occur ... 

Such cyclohexadienes are easily oxidizable to benzenes (often by atmospheric oxygen), so this reaction becomes a method of alkylating and arylating suitably substituted (usually hindered) aryl ketones. A similar reaction has been reported for aromatic nitro compounds where 1,3,5-trinitrobenzene reacts with excess methyl-magnesium halide to give 2,4,6-trinitro-l,3,5-trimethylcyclohexane. Both... 

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