Preparation of Organomagnesium Compounds Grignard Reagents

Grignard reagents are prepared from organic halides by reaction with magnesium, a Group II metal. 

Anhydrous diethyl ether is the customary solvent used when preparing organomagnesium compounds. Sometimes the reaction does not begin readily, bnt once started, it is exothermic and maintains the temperatnre of the reaction mixtnre at the boiling point of diethyl ether (35°C). 

The order of halide reactivity is I Br Cl F, and alkyl halides are more reactive than aryl and vinyl halides. Indeed, aryl and vinyl chlorides do not form Grignard reagents in diethyl ether. When more vigorons reaction conditions are reqnired, tetrahy-drofnran (THF) is used as the solvent. 

THF forms a more stable complex with the Grignard reagent and, with a boiling point of 66°C, allows the reaction to be carried out at a higher temperature. 

Write the structure of the Grignard reagent formed from each of the following compounds on reaction with magnesium in diethyl ether  

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Metal-halogen exchange between Grignard reagents and organic halides is an alternative useful method for the preparation of organomagnesium compounds (Scheme 3.30). We will discuss this method in detail in Section 3.4. 

There are a number of pertinent reviews and books [1] for the preparation of organomagnesium compounds and their reactions. Thus, this chapter summarizes the representative methods for the preparations of organomagnesium (Grignard) reagents. 

Oddo reported that the organomagnesium derivatives of p3Trole, indole, skatole, and carbazole could be prepared in a single operation by mixing the parent heterocyclic compound with an alkyl halide and magnesium in anhydrous ether.The product formed was reported to be the same as that obtained by the more conventional procedure. However, this approach to the synthesis of the indole Grignard reagents does not seem to have been exploited in subsequent work. 

Extensive studies have been made of the metallation of 1-alkynes by organomagnesium compounds [D], For preparative purposes, metallation by Grignard reagents (commonly ethylmagnesium bromide) in diethyl ether or THF is usually straightforward and convenient. Magnesiation of acetylene itself has special features and is described separately. 

Many straight-chain and branched secondary alcohols have been made by the action of organomagnesium compounds on higher aldehydes. The method is popular for the preparation of arylalkylcarbinols from either the aromatic aldehyde or the aromatic Grignard reagent. ... 

A striking exaple of the influence of the Schlenk equilibrium on the progress of reactions of organomagnesium compounds was observed in, what was named, a ligand-assisted nucleophilic addition reaction (LANA), in which the substrate was a lithium quinol alkoxide [29]. In a typical LANA reaction (Scheme 14) the lithium quinol alkoxide is first prepared in situ, after which follows the sequential addition of l,3-dimethyl-3,4,5,6-tetrahydro-2(l//)-pyrimidone (DPMU) and a Grignard reagent. 

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