Magnesium, reaction with aryl halides

Grignard Synthesis.— The most important method for synthesizing aromatic alcohols is by the Grignard reaction, with magnesium alkyl or aryl halides (p. 77). The one given as an example of a tertiary aromatic alcohol may be prepared by the action of magnesium phenyl bromide, CeHs—Mg—Br, upon acetone. 

In a similar attempt to determine the nature of the rate-determining step in the reaction of aryl halides with magnesium, the kinetics of the reaction of substituted aryl bromides with magnesium and with tri-/i-butyltin hydride in ethereal solvents were examined [81c], using competitive kinetics techniques, and were correlated with the substituent effect using Hammett cr parameters. A Hammett plot log iK/K)=f(( x) then provided the value of the Hammett constant p for the reactions of aryl halides with magnesium and with tri-u-butyltin hydride in various solvents (Fig. 4 and Table 11). 

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. 

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... 

Grignard reagents, R-MgX or Ar-MgX, are typically prepared by the reaction of an alkyl halide, R-X, or an aryl halide, Ar-X, with magnesium metal in an anhydrous ethereal solvent (Eq. 19.3) the organometallic reagent dissolves as it is formed. You may note that carbon is transformed from an electrophilic center in the starting material R-X or Ar-X into a nucleophilic center in the product R-MgX or Ar-MgX in this process. 

Formation of aryl Grignard reagents (Section 14 4) Aryl halides react with magnesium to form the corresponding arylmagnesium halide Aryl iodides are the most reac tive aryl fluorides the least A similar reaction occurs with lithium to give aryllithium reagents (Section 14 3)... 

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 ... 

Grignard reagent (Section 14.4) An organomagnesium compound of the type RMgX formed by the reaction of magnesium with an alkyl or aryl halide. 

The reaction of magnesium metal with an alkyl or aryl halide in diethyl ether is the standard method for synthesis of Grignard reagents. The order of reactivity of the halides is RI > RBr > RC1. 

Similarly to the alkyl derivatives, the most common route for arylcopper compounds is the reaction of a copper halide and aryllithium compounds (Equation (4)). Organocuprates with aryl groups are obtained by using an appropriate excess of the lithium reagent. Magnesium aryls have also been employed in transmetallation reactions with Cu(l) salts to yield both arylcopper compounds and arylcuprates (Equations (5) and (6)). 

The preparation of a Grigncird reagent begins with magnesium metal and dry ether (in most cases, either diethyl ether or THF, tetrahydrofuran). The ether cleans the surface of the metal and takes the reagent into solution for reaction. (If either the ether or the reaction vessel contains moisture, the yield is poor.) The magnesium then reacts with either an alkyl halide or an aryl halide. The ease of reactivity decreases in the order R1 > RBr > RCl. Iodides may react too rapidly, but chlorides may react too slowly. Thus bromides are usually the best. The general reaction is... 

Reaction IX. (a) Action of Magnesium Alkyl or Aryl Halide on certain Alkyl or Aryl Halides in the presence of Absolute Ether (Grignard). (C. 1906, II., 748.)—The Grignard reaction has perhaps a wider application than even the Friedel-Crafts or the diazo reaction. When an alkyl or aryl bromide or iodide is treated with magnesium powder, usually in presence of absolute ether, a magnesium alkyl or aryl halide is formed. 

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