Oxidation potentials, Grignard reagent

Electrolysis of Grignard reagents in ether produces the saturated and unsaturated hydrocarbons to be expected if the anode oxidizes a real or potential carbanion to the corresponding free radical. 

Finally, Cristau and coworkers have reported on a quite efficient preparation of triphenylphosphine oxide (Figure 2.13) by a similar addition-elimination reaction of red phosphorus with iodobenzene in the presence of a Lewis acid catalyst followed by oxidation of an intermediate tetraarylphosphonium salt.42 This approach holds the potential for the preparation of a variety of triarylphosphine oxides without proceeding through the normally used Grignard reagent. Of course, a variety of approaches is available for the efficient reduction of phosphine oxides and quaternary phosphonium salts to the parent phosphine, including the use of lithium aluminum hydride,43 meth-ylpolysiloxane,44 trichlorosilane,45 and hexachlorodisilane.46... 

Pyridine A-oxides are potential starting materials for the synthesis of a multitude of target molecules. Rapid addition of Grignard reagents to pyridine A-oxides 67 under mild conditions gave stereodefined dienal oximes 68 in good to excellent yields (Scheme 36). ... 

Unfortunately, the preparation of functionalized Grignard reagents via direct oxidative addition of magnesium metal to organic halides still suffers from severe limitations. This is mainly due to the intrinsic high reducing potential of magnesium metal. 

Sulfur-lithium exchange is easier and has much greater potential (much of it still unrealised) when the sulfur is at the sulfoxide oxidation level. It has long been known that organolithiums, like Grignard reagents, will attack a sulfoxide, displacing with inversion at sulfur the substituent best able to support an anion. The reaction has been commonly used to form sulfoxides with defined stereochemistry 152157... 

Magnesium is a greyish-white metal with a surface oxide film that protects it to some extent chemically—thus it is not attacked by water, despite the favorable potential, unless amalgamated. It is readily soluble in dilute acids and is attacked by most alkyl and aryl halides in ether solution to give Grignard reagents. 

A better knowledge of electrochemical properties of the Grignard reagents would allow a more accurate prediction of their reactivities toward one and the same ketone, since electron transfer (ET) involves the transfer of one electron. It was hoped that it would be possible to relate reaction rates to the anodic oxidation potential of RMgX. 

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). 

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