Aryl halides Grignard reagent

Alkyl and aryl magnesium halides (Grignard reagents, represented by the formula RMgX) are extremely well known on account of their uses in synthetic organic chemistry (see further reading list for this chapter). The general... 

Carbanions are classified as soft nucleophiles. It would be expected that they would be good nucleophiles in Sn2 reactions, and this is generally true. The reactions of aryl-, alkenyl-, and alkyllithium reagents with primary alkyl halides and tosylates appear to proceed by Sn2 mechanisms. Similar reactions occur between arylmag-nesium halides (Grignard reagents) and alkyl sulfates and sulfonates. Some examples of these reactions are given in Scheme 7.2. 

The formation of the organomagnesium halide (Grignard reagent) involves a heterogeneous reaction between magnesium metal and an alkyl, alkenyl, or aryl halide in ether solution. The solvent may be any one of a number of ethers, but diethyl ether and tetrahydrofuran are by far the most popular. 

Since Grignard reagents can easily be obtained from aryl halides, they are of special value in the s nthesis of many aromatic compounds, particularly as, for reasons already stated (pp. 270, 276), aromatic compounds cannot generally be prepared by means of ethyl acetoacetate and ethyl malonate. 

The order of halide reactivity is I > Br > Cl > F and alkyl halides are more reac tive than aryl and vinyl halides Indeed aryl and vinyl chlorides do not form Grignard reagents m diethyl ether When more vigorous reaction conditions are required tetrahy drofuran (THF) is used as the solvent... 

Overall the carboxylation of Grignard reagents transforms an alkyl or aryl halide to a carboxylic acid in which the carbon skeleton has been extended by one carbon atom... 

The major limitation to this procedure is that the alkyl or aryl halide must not bear substituents that are incompatible with Grignard reagents such as OH NH SH or... 

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 proceeds by formation of the Grignard reagent from o bromofluorobenzene Because the order of reactivity of magnesium with aryl halides is Arl > ArBr > ArCl > ArF the Gngnard reagent has the structure shown and forms benzyne by loss of the salt FMgBr... 

Grignard reagent (Section 14 4) An organomagnesium com pound of the type RMgX formed by the reaction of magne Slum with an alkyl or aryl halide... 

In some instances the reactions of the indole Grignard reagents resemble those of simple aryl or alkyl magnesium halides, but in many eases they appear to reaet anomalously. 

Grignard reagents are a very important class of organometallic compounds. For their preparation an alkyl halide or aryl halide 5 is reacted with magnesium metal. The formation of the organometallic species takes place at the metal surface by transfer of an electron from magnesium to a halide molecule, an alkyl or aryl radical species 6 respectively is formed. Whether the intermediate radical species stays adsorbed at the metal surface (the A-modelf, or desorbs into solution (the D-model), still is in debate ... 

---