Grignard reagents configurational stability

The extra stability of vinyl Grignard reagents is clearly shown by Knochel s preparation of 22 by exchange with i-Pr2Mg. Notice that the alkenyl group is transferred rather than the alkyl group to benzaldehyde and that both metallation and reaction with the aldehyde occur with retention of configuration.2... 

Reactions of methyllithium or methyl Grignard reagents resulted in the formation of optically active silyl anions which were characterized after hydrolysis and reaction with allyl bromide (243). The observed overall predominant retention of configuration provides evidence for the optical stability of silyl anions. Their first preparation was reported by Sommer and Mason (245) and they appeared configurationally less stable than the chiral germyl anions (31). 

The configurational stability of primary Grignard reagents can be studied by an interpretation of exchange effects in the proton resonance spectra as a function of temperature (Whitesides et al., 1963). These changes in spectra are related to the high energy barrier in heavily substituted ethanes. The molecule studied is shown below in three rotamer forms of which only two are non-equivalent.)... 

Hydrolysis or condensation with allyl bromide showed retention of configuration to occur at silicon. However, products of low optical purity were obtained. It is difficult to know whether some racemisation had occurred in the silicon-cobalt bond cleavage or whether it is due to a low configurational stability of the silyl Grignard reagent. 

Methyl methacrylate polymerizations, initiated by oiganomagnesium compounds, also yield abnormal products. Here, the active centers are unusually persistent and stabile. In addition, the a-carbon atoms of the monomers were found to assume tetrahedral configurations.This suggests that the active centers contain covalent magnesium carbon bonds. Also, gel permeation chromatography curves of the products show that more than one active center operates independently. A pseudoanionic mechanism was therefore postulated for polymerizations of acrylic and methacrylic esters by Grignard reagents. 

Many studies of the addition of nucleophiles to palladium-allyl complexes have been conducted. Hayashi has shown that the additions of stabilized anions, such as malonate anions or amine nucleophiles, to chiral, non-racemic allyl complexes occur with inversion of configuration.Addition of excess phosphine and either diethyl malonate or dimethylamine to a chiral, non-racemic allyl complex results in nucleophilic attack with nearly complete inversion. The reaction with sodium dimethylmalonate is shown at the right of Equation 11.40. In contrast, nonstabilized carbanions such as allyl or phenyl magnesium chloride react with the same Ti -allylpalladium complex with retention of configuration as shown at the left of Equation 11.40. The stereochemistry from reaction of the Grignard reagents likely results from nucleophilic attack at the metal, followed by reductive elimination. 

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