Enantiomer-selective deactivation

Enantiomer-selective deactivation of racemic catalyts by a chiral deactivator affects the enantiomer-selective formation of a deactivated catalyst with low catalytic activity (Scheme 8.2). Therefore, it is crucial for a chiral deactivator to interact with one enantiomer of a racemic catalyst (Scheme 8.2a). As the chiral deactivator does not interact with the other enantiomer of racemic catalyst, the enantiomeri-cally enriched product can be obtained. Therefore, the level of enantiomeric excess (% ee) could not exceed that attained by the enantiopure catalyst. On the other hand, nonselective complexation of a chiral deactivator would equally and simultaneously deactivate both catalyst enantiomers, thereby yielding a racemic product (Scheme 8.2b). Although this strategy tends to use excess chiral poison relative to the amount of catalyst, it offers a significant advantage in reducing cost and synthetic difficulty since readily available racemic catalysts and often inexpensive chiral poisons are used. 

Faller demonstrated the enantiomer-selective deactivation of racemic BINOL-Ti complex by using DIPT-derived titanium complex as a chiral poison (vide infra) (Sch. 10) [44], The enantiomeric excess (ee) of the allylation product increased as the amount of DIPT employed was increased. 

Whilst non-racemic catalysts can generate non-racemic products with or without the NLE, racemic catalysts (0% ee) inherently produce only racemic (0% ee) products. A strategy whereby a racemic catalyst is enantiomer-selectively deactivated by a chiral molecule as a catalyst poison has recently been reported to yield non-racemic products (Fig. 7-3) [46 8]. A unique resolution of racemic CHIRAPHOS has been attained with a chiral iridium complex to give a deactiva-... 

BINAP Ru catalyst and (lR,25 )-ephedrine (Scheme 8-53). This result is similar to that obtained when catalyzed by pure (R)-BINAP. In pure (R)-BINAP complex-catalyzed hydrogenation, (S )-2-cyclohexenol can also be obtained with over 95% ee. This means that in the presence of (R)-BINAP-Ru catalyst, (R)-cyclohexenol is hydrogenated much faster than its (S )-enantiomer. When ephedrine is present, (R)-BINAP-Ru will be selectively deactivated, and the action of (S -BINAP-Ru leads to the selective hydrogenation of (S)-2-cyclohexenol, leaving the intact (R)-2-cyclohexenol in high ee. 

Combination of the asymmetric activation and asymmetric deactivation protocols as asymmetric activation/deactivation can be achieve the difference in catalytic activity between the two enantiomers of racemic catalysts can be maximized through selective activation and deactivation of enantiomeric catalyst, respectively (Scheme 8.15). 

Based on the concept mentioned above, Brown realized the asymmetric deactivation of a racemic catalyst in asymmetric hydrogenation (Scheme 9.18) [35]. One enantiomer of (+)-CHIRAPHOS 28 was selectively converted into an inactive complex 30 with a chiral iridium complex 29, whereas the remaining enantiomer of CHIRAPHOS forms a chiral rhodium complex 31 that acts as the chiral catalyst for the enantioselective hydrogenation of dehydroamino acid derivative 32 to give an enantio-enriched phenylalanine derivative... 

Additional substances (buffer additives) are often added to the buffer solution to alter selectivity and/or to improve efficiency, and the wall of the capillary may be treated to reduce adsorptive interactions with solute species. Organic solvents, surfactants, urea and chiral selectors are among the many additives that have been recommended (table 4-24). Many alter or even reverse the EOF by affecting the surface charge on the capillary wall, whilst some help to solubilize hydrophobic solutes, form ion-pairs, or minimize solute adsorption on the capillary wall. Chiral selectors enable racemic mixtures to be separated by differential interactions with the two enantiomers which affects their electrophoretic mobilities. Deactivation of the capillary wall to improve efficiency by minimizing internet ions. with solute species can be achieved by permanent chemical modification such as silylaytion or the... 

The enzyme was not deactivated by SCCO2 and was active even in the absence of water, but the optimum rate was found at a water concentration of 0.5-1 mL/L. At 25% conversion, the e.e. of the ester was 92%, in favor of the S ester (e.e. = enantiomeric excess). The same reaction was found by Overmeyer et al. (76) to be catalyzed by a lipase from Candida antarctica B, which esterified the R enantiomer, but the enantioselectivity was poor. Wu and Liang (77) found that the enantioselectivity of esterification of racemic naproxen [Eq. (2)] was a strong function of the concentration of alcohol, with the greatest selectivity being obtained at lower alcohol concentrations. 

Mikami et al. [101] devised catalytic system in which two KRs were acting simultaneously on a racemic catalyst, leading to an activation/deactivation process. One enantiopure activator selected one enantiomer of the catalyst, leading to activation, whereas another enantiopure compound selected preferentially the other enantiomer to give a deactivated catalyst. 

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