Heteroconjugated anions

Okada, T., Non-aqueous capillary electrophoretic separation of Bronsted acids as heteroconjugated anions, /. Chromatogr. A, 771, 275-284,1997. 

Further examples of gas-phase measurements which reveal the influence of stepwise solvation of homo- and heteroconjugated anionic nucleophiles, A (AH)n and A (BH)n, on the kinetics of Sn2 discplacement reactions with halomethanes can be found in referenees [475, 486], In all cases, solvation of the reaetant anion with up to three solvent moleeules leads to a decrease in the rate constants by at least three orders of magnitude. 

The finding that in a heteroconjugated anion embedded in a polar environment phenols transfer the proton to the carboxylates is rationalized schematically in... 

Figure 14.24 Schematic representation of solvation effects on the hydrogen bond geometries of heteroconjugated anions.

Isobe and coworkers407 found an interesting diastereoselective heteroconjugate addition of methyllithium to 314. The stereochemical control was considered to be determined at the stage of the intermediate 315. Since methyllithium is considered to be coordinated strongly with the methoxyethoxymethoxyl (OMEM) group, the methyl anion would attack the -carbon of the olefin only from one side, as shown below. 

Formation constant (Kf) of the heteroconjugated complex (1 1 complexation of analyte-anion) is given by... 

Like Brjinsted acids, Lewis acids are known to homoconjugate and to heteroconjugate with anions derived from them. This point, completely neglected by researchers in the field of cationic polymerisation, is particularly relevant to direct initiation mechanisms and will be amply discussed in Chap. IV, where the evidence for the occurence of such interactions is reviewed and our views on their role in provoking the premature stoppage of a polymerisation are presented. 

The homo- and heteroconjugation of halide anions with hydrogen halides is well known and virtually complete in chlorinated hydrocarbons and nitromethane as shown by NMR spectroscopy. Thus, the presence of halide ions in a system containing one of these acids in such solvents, and most probably in less polar ones, produces the sequestration of an equivalent amount of acid. Higher homoconjugated aggregates have recently been reported in the case of HCl in propylene carbonate ... 

Many other metal fluorides can homo- and heteroconjugate with related anions. Thus, boron fluoride gives readily the complex anions B2F7 and and... 

Conductance measurements on unbuffered solutions are unreliable, but useful information can be obtained from conductometric titra-tions. ii ii i Solubility - 2.Fii8 nd spectrophoto-metric measurements give more precise results. Kolthoff and his collaborators - have made a detailed spectrophotometric study of acid-base indicators in acetonitrile. Indicators with cationic acid forms can give trouble because of formation of ion pairs with anions. The most satisfactory indicators are ones like 2,6-dinitrophenol, 2,6-di-t-butyl-4-nitrophenol, and picric acid which have little tendency to form homo- or heteroconjugate ions because of steric hindrance. ... 

In this case, then, the difference in energy between the contributing p electrons in the heteroconjugated system and that of those in the AH anion is not Scti but 2S(Xi. The difference in total n energy is given as before by... 

The difference AE in 7r-bond energy between the AH anion and the heteroconjugated system is then... 

Solvent and H/D isotope effects on the proton transfer pathways in heteroconjugated hydrogen-bonded phenol-carboxylic acid anions observed by combined UV-Vis and NMR spectroscopy. / Am. Chem. Soc.,... 

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