Schlenk equilibrium Grignard reagents

By reaction of zinc-copper couple with diiodomethane 2 an organozinc species 4 is formed, similar to a Grignard reagent. Its structure cannot be fully described by a single structural formula. The actual structure depends on the reaction conditions—e.g. the solvent used this corresponds to the Schlenk equilibrium as it is observed with the Grignard reaction ... 

Boiling-point elevation and freezing-point depression measurements have demonstrated that at all concentrations in THF and at low concentrations in ether (up to 0.1M) Grignard reagents prepared from alkyl bromides and iodides are monomeric, that is, there are few or no molecules with two magnesium atoms. Thus, part of the Schlenk equilibrium is operating... 

However, Grignard reagents prepared from alkyl bromides or iodides in ether at higher concentrations (0.5-1 M) contain dimers, trimers, and higher polymers, and those prepared from alkyl chlorides in ether at all concentrations are dimeric, so that 24 is in solution, probably in equilibrium with RMgX and R2Mg that is, the complete Schlenk equilibrium seems to be present. 

The weight of evidence, accumulated over many years of detailed studies using a combination of physicochemical techniques, including NMR, reveals that the solution composition of Grignard reagents is best represented by extended Schlenk equilibria (Scheme 1) . However, because of the complexity of the solution behaviour, the vast majority of their NMR spectra are analysed on the basis of the basic equilibrium first proposed by Schlenk and Schlenk (Scheme 2). 

Schlenk equilibrium - The "Grignard reagent is really an equilibrium. 

Schlenk equilibrium concept sot forth above, the magnesium halide component of tertiary alkyl Grignard reagents causes halohydria formation faster than the diaJkylmagnesium component can add. Spocial conditions have, howover. been found, which permit addition of (erf-butylmagnesium chloride to ethylene oxide (Eq. 818), giving a modest yield of the desired primary alcohol.1484 1101... 

K. C. Cannon, G. R. Krow, The Composition of Grignard Reagents in Solution The Schlenk Equilibrium and Its Effect on Reactivity, in Handbook of Grignard Reagents (G. S. Silverman, P. E. Rakita, Eds.), Marcel Dekker Inc., New York, 1996, 271-290. 

As noted in Section 2.1.2, Grignard reagents in solution may be described in terms of the Schlenk equilibrium (or modifications), represented in its simplest form by... 

Analytical RISM-MP2 free energy gradient method Application to the Schlenk equilibrium of Grignard reagent ... 

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