Schlenk equilibria

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... 

Schlenk equilibrium 93 s-cis 7, 9, 26, 31, 35 sdyl-substituted 16 Simmons-Smith reaction 87 SnClj 309 SnCU 309 solid-phase 198 square bipyramidal 255 rr-stacking 8 stannyl-substituted 16 s-trans 7, 26 - acrolein 307 succinimide 227 sulfonamides 122 synchronicity 306... 

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... 

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 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 . 

Scheme 7 Schlenk equilibrium of reactive and spectator ligands of the homo- and heteroleptic forms complexes of calcium (a) and barium (b) [53-55]...

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