Anion-exchange reactions Lewis-acid-based

Treatment of 37 with 1 equiv of SbCh furnished the bicyclic compound 117 featuring the structural motif of a phosphastibirane. Removal of the chloride anion by a Lewis acid (e.g., AICI3) or exchange of chloride by reaction with Ag[Al OC(CF3)3 4] yielded the cage compound 118 with the antimony atom located in the apex of the square-based pyramidal structure (Scheme 43) <2006CC1375>. 

Cation- and anion-exchange resins are widely applied as catalysts, when reactions can be carried out at temperatures lower than 423 K [161-171], Cation-exchange resins in acid forms have Bronsted acid sites and when exchanged with metallic cations contain Lewis acid sites, while anion exchange resins forms base species to carry out base-catalyzed reactions [169], The principles previously described in this chapter for these sites in other catalysts, can be applied for the catalytic action of these polymers. 

Product isolation is very simple as only the solvent has to be removed after separation from the polymer. The polymer can be returned to the active chlorinating agent by reaction with carbonyl chloride. A variation of this procedure does not require polymer-bound phosphorus halide but uses Lewis acid-Lewis base complexes between anion-exchange resins, such as Amberlite IRA 93, and phosphorus pentachloride (equation 4). Again, isolation of the resultant acid chloride is simple and the exhausted polymer can be regenerated for further use by simply washing it with aqueous acidic and basic solutions. Yields range from 51 % for crotonic acid chloride to 86% for decanoic acid chloride. 

During typical surface complex formation, or ligand exchange, the surface hydroxyl group on the hydrous oxide exchanges with a similar Lewis base electron pair donor in the solution. Adsorption of either protons or hydroxide ions is interpreted in terms of an acid-base reaction at the oxide surface, i.e., the surface hydroxyl group is either protonated or deprotonated. The adsorption of ligands (anions and weak acids) on a metal-oxide surface can also be compared with complex formation reactions in solution, e.g. ... 

LA can be purified either by precipitation of metal lactates followed by a neutralizing reaction with sulfuric acid [8] or by esterification with alcohol, distillation and hydrolysis of the formed ester, or by electro-dialysis. A more recent purification process is to extract LA by liquid-liquid extraction making use of at least one organic solvent not miscible with water in the presence or not of at least one Lewis base such as a ternary amine for instance. With this process, lactic acid must be recovered in a second step with a liquid-liquid back-extraction. This step allows to re-transfer the lactic acid to water. Finally, LA in acid- and/or ammonium lactate or metal lactate-form may be purified by processing it on cationic and/or anionic ion-exchange columns. 

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