Oxidation potentials, Grignard reagent correlations

A plot of kj of the reactions of methylmagnesium bromide, /i-butylmagnesium bromide, and ethylmagnesium bromide versus the standard oxidation potentials of these Grignard reagents gave a relatively straight line (correlation coefficient r = 0.998) [see also Ref. 46], which supports the reliability of the measurements, as well as the correctness of the proposed mechanism. 

A refinement of the mechanism was searched for in the more recent literature when a differentiation was made between what was called inner-sphere ET and outer-sphere ET. It was assumed that, in the reaction of a Grignard reagent with a ketone (i.e., benzophenone), the electron transfer was rate-limiting [44] furthermore, for a series of Grignard reagents, a correlation had been found between the reaction rates and their oxidation potentials [21], according to the Marcus theory for outer-sphere ET [55]. Nevertheless, it seemed questionable [56] whether the electron transfer was an independent step (steps l->2->3-+4 in Scheme 19), or whether it was concerted with the transfer of the magnesium atom (steps l->3->4). 

The reaction of alkylntagnesium reagents with di-r-bulyl peroxide leads to alkyl /-butyl ether, /-buiyl alcohol, and disproportionation products of the alkyl [96). Mechanistic studies showed that the initial step is ET. This conclusion was i.a. based on a linear correlation of log rale with the oxidation potentials of the Grignard reagents. Reactions of peroxides are extremely sensitive to metal catalysis, but take place even when high-purity magnesium is used. 

The rates of reaction for the different Grignard reagents were largely linearly correlated to the anodic oxidation potentials of the reagents. 5-Hcxenylmagnesium bromide produced I -addition product of which 509 was cycli/cd. 

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