Grignard Schlenk equilibrium

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

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 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 . 

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... 

SCHEME 7. Schlenk equilibrium which describes the composition of a wide range of Grignard... 

In spite of various theoretical studies of Grignard reactions shown in the previous section, their mechanisms seem to be as yet unsettled. In particular, the connection between the R MgX dimer in the Schlenk equilibrium and its reactivity toward carbonyl groups is still unclear. In this section, a systematic computational study of several Grignard reactions is presented in order to reveal their unclear points. The polar vs. SET problem will be explained in a forthcoming new mechanism. 

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... 

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. 

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