Dioxanate, Schlenk equilibrium

A suitable laboratory method is to shift the Schlenk equilibrium in a Grignard solution (p. 132) by adding dioxan to precipitate the... 

The second route (Scheme 1) is a redistribution reaction, in fact the Schlenk equilibrium. This route may be used in the reverse direction for the preparation of pure diorganomagnesium compounds from organomagnesium halides. Addition of a ligand, usually dioxane, that forms an insoluble complex with magnesium dihalide, shifts the Schlenk equilibrium completely to the left side and allows isolation of pure diorganomagnesium compounds from the remaining solution. ... 

Halogen-free diethylmagnesium reacted with 1-hexyne three times as fast as ethylmagnesium bromide [85]. By (partial) removal of magnesium bromide from such a solution, through the addition of various amounts of 1,4-dioxane and measuring the relative rate of reaction of that solution with 1-hexyne, it was expected that information could be obtained about the position of the Schlenk equilibrium. Dioxane itself had no influence on the rate of reaction of diethylmagnesium in diethyl ether. The results were rather inconclusive. 

The Schlenk equilibrium, as represented by Scheme 13 can be shifted to the right by the addition of 1,4-dioxane to an ethereal solution of a Grignard reagent. A precipitate is formed of a complex of the dioxane with the magnesium halide. When enough dioxane is added most, if not all, of the halide is removed from the solution. 

One of the key factors for yield improvement is to precipitate or remove MgX2 from solution. This can be accomplished by going to less polar solvents, such as heptane, or by forming insoluble complexes, such as MgX2 dioxane. Potential explanations for this effect are removal of a Lewis acid, shift in the Schlenk equilibrium, or the change in solvent polarity that affects the nucleophilicity of the Grignard reagent. 

These were discarded very soon, however, ami me now only of historic interest. Finally, the investigations of V. Sclilenk and VV. Schlenk led a crucial step further. They determined that the halogen ol certain Grigttard compounds can he completely removed as MgX, by precipitation with dioxane 40. Due to this information and new association data 39. they postulated the equilibrium ... 

More target-oriented approaches to shift the Schlenk-ifpe. equilibrium comparable to the well-established protocols known for the magnesium derivatives stiU have to be developed. While the addition of 1,4-dioxane to Grignard solutions leads... 

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