Schlenk equilibrium compounds

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. 

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. 

TABLE 3. Schlenk equilibrium constants K) for selected Grignard compounds... 

Although the interconversion of the ally lie isomers remains rapid at temperatures as low as —80 °C, the Schlenk equilibrium is slowed sufficiently to enable signals due to both the allyl Grignard and bis(allyl)magnesium compounds to be observed at such temperatures in their H NMR spectra. The NMR parameters for the bis(allyl)magnesium compounds are not very different from those of the Grignards, suggesting that they possess essentially... 

So far, only organozincates have been considered in which the three or four organic groups bound to zinc are identical (homoleptic zincates). It is well established that heteroleptic organozinc compounds undergo the so-called Schlenk equilibrium (equation 12). This is also true for heteroleptic zincates (equation 13). In this case, the equilibria are even more complicated, due to the possible formation of a variety of homo- and heteroleptic species. 

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 role of titanium salt is to activate the carbonyl compounds as Lewis acid. As described above, bis(iodozincio)methane (3) is nucleophilic enough to attack the carbonyl group of aldehydes or ce-alkoxyketones. In the reaction with simple ketones or esters, however, the addition of titanium salt is necessary to facilitate the nucleophilic attack. Instead of this Lewis acid activator, simple heating may induce the nucleophilic attack. Treatment of 2-dodecanone with 3 without titanium salt at higher temperature, however, does not improve the yield of alkene (Scheme 13). The reason for the low reactivity of 3 at higher temperature comes from the structural change of 3 into the polymeric methylene zinc 4 through the Schlenk equilibrium shown in equation 740. 

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.

Although quite broad, Mg resonances of organomagnesium compounds have been reported. The shift is sensitive to solvation and ligand electronegativity, and resonances for the Schlenk equilibrium species have been reported. Beryllium-9 NMR spectra are more informative than those of In... 

Schlenk equilibrium [13] the simpler diorganomagnesium compounds are often easier to study [14,78,79] because of the absence of complications from additional halide ligands. As the organolithium compounds, organomagnesium species often react rapidly at ambient temperatures with substrates so that the charge transfer intermediates are not observed or reported. 

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. 

In simple terms, solutions of the Grignard reagent follow the Schlenk equilibrium [Eq. (1) 1]. The chemistry is dependent on many factors (e.g., solvent used). The compounds present in solution consist of the compounds depicted in Eq. (1) as well as compounds formed from the aggregation of these basic organomagnesium monomers into their dimers, trimers, and so on. 

---