Enolate ions aggregation

The order of enolate reactivity also depends on the metal cation which is present. The general order is BrMg < Li < Na < K. This order, too, is in the order of greater dissociation of the enolate-cation ion pairs and ion aggregates. Carbon-13 chemical shift data provide an indication of electron density at the nucleophilic caibon in enolates. These shifts have been found to be both cation-dependent and solvent-dependent. Apparent electron density increases in the order > Na > Li and THF/HMPA > DME > THF >ether. There is a good correlation with observed reactivity under the corresponding conditions. 

Aggregation of enolate ion pairs and their equilibrium dynamics 629... 

The reaction order with respect to active chain-ends concentration is 0.5, indicating that chelation of the lithium cation does not effectively perturb the aggregation state of the enolate ion pair (Figure 8). 

N,N-diethylacrylamide (DEAAm) in polar and nonpolar solvents using alkyllithium initiators is complicated due to the presence of slow aggregation dynamics of the propagating amido enolate ion pairs similar to ester enolate ion pairs in alkyl (meth) acrylate polymerization. Attempts were made to use different initiators in combination with coordinating ligands to control the polymerization, and only minimum control on molecular weight, MWD, and the stereostructure of the polymers was obtained."""-"""... 

THF and DME are slightly polar solvents which are moderately good cation solvators. Coordination to the metal cation involves the oxygen lone pairs. These solvents, because of their lower dielectric constants, are less effective at separating ion pairs and higher aggregates than are the polar aprotic solvents. The crystal structures of the lithium and potassium enolates of methyl /-butyl ketone have been determined by X-ray crystal-... 

The diastereoselectivity of protonation of enolate anions has been studied by H/D exchange.156 /i-Substituted ethyl butanoates were chosen as substrates, with conditions that rigorously excluded ion-pairing and aggregation effects. Stereoelectronic effects were found typically to produce higher stereoselection than purely steric effects, hi the specific case of H/D exchange in 3-ethoxybutanoate in ethanol-4 protonation of the enolate of 3-fluorobutanoate was chosen as a computational model.157 Similar... 

The structure of mixed aggregates involving ester enolates is also of major interest to macromolecular chemists, since ionic additives are often introduced in the polymerization medium. The more stable arrangement between lithium 2-methoxyethoxide and MIB lithium enolate was thus calculated (at the DFT level) to be a 5 1 hexagonal complex with similar O—Li lateral coordinations212. The same team has recently extended this study to complexes formed between the same enolate in THF and a-ligands such as TMEDA, DME, 12-crown-4 and cryptand-2,1,1213. Only in the case of the latter ligand could a separate ion pair [(MIB-Li-MIB),2 THF]-, Li(2,l,l)+ be found as stable, still at the DFT level, as the THF solvated dimer [(MIB-Li)2,4 THF]. 

Ion-pair acidity is an important parameter to probe highly diluted solutions, as demonstrated by the sophisticated visible or near-UV absorption methods developed mainly by Streitwieser and coworkers. This analytical approach afforded the pXa values and aggregation states of a large set of cesium and lithium enolates. Its principle relies... 

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