THE SCHLENK EQUILIBRIUM

For many years the equilibrium was formulated as in equation 7 and the monomer RMgBr was thought not to exist.  

Thermochemically, the association is not a major factor since the enthalpies of dilution of Grignard reagents are very small in diethyl ether as well as in THF An explanation 

Joel F. Liebman, Torkil Holm and Suzanne W. Slayden 

TABLE 1. Equilibrium constants (A schienk) and enthalpies of reaction for the Schlenk reaction R2Mg + MgBr2 2RMgBr 

RMgBr Solvent Schlenk (kJmoL ) Method Reference 

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

The study of the Schlenk equilibrium for organozinc compounds represents a major chapter in the understanding of these reagents in general [26]. Before elaborating the studies on zinc carbenoids, it is appropriate to briefly review the definitive investigations on organozinc halides themselves. 

Eor ethylzinc chloride, CH3CH2ZnCl, and ethylzinc bromide, CH3CH2ZnBr, there is a linear relationship between the observed chemical shift and the ratio of ethylzinc halide to diethylzinc. Extrapolation of these lines to x=l (mol fraction of CH3CH2Z11X) gives predicted values for the average chemical shift that closely match those measured for these species. This indicates that for these two organozinc halides, the Schlenk equilibrium lies heavily on the side of the ethylzinc halide in toluene. However, in the case of ethylzinc iodide, CH3CH2ZnI, there is a... 

In a nucleophilic solvent, the organomagnesium species not only exists as RMgX, but is rather described by the Schlenk equilibrium ... 

In addition dimeric species are formed, being in equilibrium with the monomeric RMgX. The Schlenk equilibrium is influenced by substrate structure, the nature of the solvent, concentration and temperature. 

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... 

Solutions of the alkyl derivatives [TpMe2MgR are generally stable at room temperature. However, heating to 80-120°C results in ligand redistribution, analogous to the Schlenk equilibrium, and the formation... 

The kinetics of the Grignard reaction was recognized to be much more complex than originally considered because of several factors the Schlenk equilibrium produces many reacting species, a metal impurity induces an ET process, and the primary product, ROMgX, contributes to the reaction . 

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. ... 

TABLE 2. Enthalpy of solvation of components of the Schlenk equilibrium in Et20 and THE (kJmol ) ... 

The only dialkylmagnesium compound whose enthalpy of formation has been measured is dineopentyl magnesium (s) —236.8 7.2 kJmol and (g) —74.3 7.6 kJmol. Unfortunately there are no enthalpies of formation for any of its isomers or homologs. We cannot even calculate the enthalpy of the Schlenk equilibrium because, although the enthalpy of formation of neopentylmagnesium bromide is for the ether solution, that for dineopentylmagnesium is not, and there is no experimental value for the enthalpy of solution. 

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