Grignard reagents equilibria

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 usual designation of Grignard reagents as RMgX is a useful but incomplete representation of the composition of the compounds in ether solution. An equilibrium exists with magnesium bromide and the dialkylmagnesium.20... 

Grignard reagents, there is an equilibrium between an alkylmagnesium halide and a dialkylmagnesium. 

Electron-rich olefins are nucleophilic and therefore subject to thermal cleavage by various electrophilic transition metal complexes. As the formation of tetraaminoethylenes, i.e., enetetramines, is possible by different methods, various precursors to imidazolidin-2-ylidene complexes are readily available. " Dimerization of nonstable NHCs such as imidazolidin-2-ylidenes is one of the routes used to obtain these electron-rich olefins [Eq. (29)]. The existence of an equilibrium between free NHC monomers and the olefinic dimer was proven only recently for benzimidazolin-2-ylidenes. In addition to the previously mentioned methods it is possible to deprotonate imidazolidinium salts with Grignard reagents in order to prepare tetraaminoethylenes. " The isolation of stable imidazolidin-2-ylidenes was achieved by deprotonation of the imidazolidinium salt with potassium hydride in THF. ... 

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). 

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