Bromobenzene with magnesium

Table 2.27. Yields of products from Barbier reactions of bromobenzene with magnesium and diethyl carbonate in various solvents [53]...

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. 

Indeed the formation of more than 20% benzyl alcohol was reported in the reaction of bromo- or iodoethane with magnesium and benzaldehyde (Table 2.23) and 30% in the uncontrolled reaction of bromobenzene with magnesium and benzaldehyde (Table 2.24). 

Cyclopropylidenedispiro [2.0.2.1]heptane (56) was initially prepared by reductive cyclization of the dibromide 72 under the action of phenyllithium [66, 67] (in this case 56 was obtained as a mixture with bromobenzene) or magnesium [68] (Scheme 12). 

C for a further 45 minutes or until all the bromine has disappeared (no red vapours visible) and the evolution of hydrogen bromide has almost ceased. Transfer the dark-coloured reaction product to a separatory funnel and shake successively with water, with sufficient 5-10 per cent sodium hydroxide solution to ensure that the washings are alkaline to litmus, and finally with water. Dry with magnesium sulphate or anhydrous calcium chloride. Filter through a fluted filter paper into a small distilling flask and distil slowly. Collect the crude bromobenzene at 150—170 °C pour the residue while still hot into a small porcelain basin. Redistil the liquid of b.p. 150-170 °C (2) and collect the bromobenzene at 154-157 °C the yield is about 60 g (60%). 

Inspection of Table 11 shows that p for the reaction of aryl halides with tri-u-butyltin hydride was not solvent-dependent, whereas p for the reaction with magnesium was. The small values of p for the reaction of aryl bromide with magnesium in a polar solvent were again interpreted [81c] in terms of a mass transport-limited process. Thus reactions of aryl bromides would be transport-limited in THF and more polar solvents, but not in diethyl ether and less polar solvent. On the other hand, the rate of reaction of aryl iodides with magnesium seemed to be transport-limited, even in diethyl ether, whereas the rate of reaction of chlorobenzene with magnesium, which was lO slower than that of bromobenzene, was not. 

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. 

For the preparation of this type of compounds, the corresponding substituted bromobenzene is reacted with magnesium to convert it to the substituted phenylmagnesium bromide. The Grignard reagent obtained is then treated with trifluoroacetic acid ... 

In Equation (a) bromobenzene reacts with magnesium in the presence of ether to 3held phenyl magnesium bromide i.e., the Grignard Reagent. 

In 1895, four years before Barbier s report [6] on the one-step synthesis with magnesium, sodium was used in a one-pot reaction with bromobenzene and ethyl oxalate, the unforeseen product being triphenylmethanol [7]. 

Sonication was ejffectively used [37] for synthesis of N-substituted benzamides in one-step reactions of bromobenzene or 2-methylbromobenzene, tert-huiyl isocyanate and a metal in diethyl ether or THF. With magnesium yields were almost quantitative. However, chlorobenzene proved to be unreac-tive under the same reaction conditions. 

In the presence of a catalytic amount of iodine, tetraalkoxysilanes react directly with magnesium and an alkyl or aryl halide. A mixture of 2 parts magnesium, 1 part tetraethoxysilane and 2 parts bromobenzene in toluene yields, after 6 h at 100 °C, the following organosilyl ethers triethoxyphenylsilane (18-20 % b.p. 234°C), diethoxy-diphenylsilane (40-42% b.p. 302-304°C) and ethoxytripkenylsilane (5-6% m.p. 63-64°C) [275]. At the molar ratio 4 1 4, ethoxytriphenylsilane is the favoured product (60-65%). [276]. 

Phenyl magnesium bromide can be made from bromobenzene, upon treatment with magnesium. 

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