Schlenk equilibrium solutions

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

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. 

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 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. ... 

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. 

As was already mentioned, the constitution of reagents 5 in ethereal solutions and of the complexes formed by them with a number of Lewis acids was investigated by electrochemical methods and the nature of existing ions and electroactive species was elucidated. It was also possible to evaluate the equilibrium constant for the Schlenk equilibrium for 5b, 5e and 5i in DME solutions from polarographic measurements . 

Such equilibria are driven by thermodynamics and therefore a selective synthetic route towards one of these species and isolation of such heteroleptic zincates in pure form is often very difficult or impossible. Only if one of the species has a sufficiently enhanced thermodynamic stability compared to the others in the equilibrium is its isolation as a pure compound possible. This is often the case when the various groups bound to zinc have a sufficiently different electronegativity, for example when one of the groups is bound to zinc via a heteroatom, or when the steric requirements of the groups bound to zinc are rather different. Sometimes it is possible to isolate one of the species present in the Schlenk equilibrium as a solid material, for example when one of the species preferentially crystallizes from solution. 

Although the simple alkylzinc halides RZnX (R = Me or Et, X = Cl, Br, I) were among the first known organozinc compounds, for a long time nothing was known about the actual structures of such compounds in solution or in the solid state. The constitution of these compounds in solution seemed to depend on the particular solvent employed " . It should be noted that in ethereal solutions, the possibility of the presence of a Schlenk equilibrium should always be considered. In these solvents the organozinc halides most... 

The solution structure of halomethylzinc derivatives has been the subject of several debates. Among those, the possible Schlenk equilibrium between IZnCH2l and Zn(CH2l)2/ Znl2 has been invoked to account for the stereochemical outcome of some cyclopropanation reactions, but little has been established unequivocally. Recently, in-depth low temperature spectroscopic studies of iodomethylzinc complexes have clearly established that the equilibrium between these species lies heavily on the side of IZnCH2l (equation 11) . ... 

Fig. 10.5. Calculated structures of Grignard compounds in the gas phase (numbers to stability given in decreasing order) or experimentally proven structures of Grignard compounds in solution (shaded pink). The Schlenk equilibrium ensures that not only RMgHal (shaded grey) occurs as a monomeric organometallic compound, but some I Mg as well.

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... 

The role of Z11I2 is that an equimolar quantity of the compound drives the Schlenk equilibrium from the reagent bis(iodomethyl)zinc to (iodomethyl)zinc iodide, which is the actual cyclopropanation catalyst and has high reactivity and stereoselectivity [50c,52], The structure of the active catalyst, Zn-bis(sulfonamide) complex XXIV, was characterized by NMR analysis and X-ray study of the structure of its bipyri-dyl complex 66 (Sch. 28) [53]. The Zn-bis(sulfonamide) complex XXIV aggregates in solution and functions as a divalent Lewis acid. 

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