Enantioselective effects

There are no mechanistic details known from intermediates of copper, like we have seen in the studies on metathesis, where both metal alkylidene complexes and metallacyclobutanes that are active catalysts have been isolated and characterised. The copper catalyst must fulfil two roles, first it must decompose the diazo compound in the carbene and dinitrogen and secondly it must transfer the carbene fragment to an alkene. Copper carbene species, if involved, must be rather unstable, but yet in view of the enantioselective effect of the ligands on copper, clearly the carbene fragment must be coordinated to copper. It is generally believed that the copper carbene complex is rather a copper carbenoid complex, as the highly reactive species has reactivities very similar to free carbenes. It has not the character of a metal-alkylidene complex that we have encountered on the left-hand-side of the periodic table in metathesis (Chapter 16). Carbene-copper species have been observed in situ (in a neutral copper species containing an iminophosphanamide as the anion), but they are still very rare [9],... 

Various experiments conducted using a-amino acids as a ligand gave marked enantioselective effects for Cu(II) and Ni(II). Table 7.4 illustrates the results for the reaction (1) in Figure 7.8. ... 

Capillary electrophoresis offers a set of important advantages that make it a premier technique for the investigation of enantioselective effects in the affinity interactions between chiral drugs and cyclodextrins. The most important advantage of CE is the inherently high separation efficiency offered by this technique. As already known, the most important contributors to peak resolution (R) are a separation selectivity (a) and an efficiency (N). A relationship between these parameters in CE is described by the following equation (2) ... 

Catalyst stability. Under the conditions used for preparative experiments, the optical yield remains constant up to complete conversion, suggesting that the modified catalyst is rather stable [62]. However, experiments at low modifier concentration indicate that the cinchona alkaloid deteriorates slowly and its enantioselective effect is lost [33,63]. 

Gil-Av, E, and Schurig, V. (1994) Resolution of non-racemic mixtures in achiral chromatographic systems a model for the enantioselective effects observed. J. Chromatogr. A, 666, 519-525. 

In addition to the enantioselective effect, cinchona alkaloids also produce a rate acceleration, i.e. this is an example of ligand accelerated catalysis [14]. The model of a non-closepacked ordered array of cinchonidine molecules adsorbed on platinum, proposed by Wells and co-workers, was abandoned in their later study [15]. Augustine [16] deduced from the behaviour of this system at low modifier concentrations that the chiral sites are formed at the edge and comer platinum atoms, which involve the adsorbed cinchonidine and a metal adatom. The different authors agreed that the quinoline ring of the modifier is responsible for the adsorption on platinum, the quinuclidine part, through the nitrogen atom, interacts with... 

Reductive aldol reaction of l-alken-3-ones and cinnamic esters depends on generating Rh enolates and the presence of chiral ligands turns such a process enantioselective. Effective ligands of very different structural types have been identified, and they include TADDOL phosphine 71 and BOX ligand 72. ... 

The temperature, concentration, and pH of the modifying solution have strong influences on the effectiveness of the resulting modified catalyst. Increasing the time of modification and the concentration of modifier at optimal temperature and pH enhances enantioselectivity of the catalyst up to an optimal value due to corrosion of the nickel surface with formation of nickel-tartrate chelates coming into the solution. Therefore, for the best enantioselective effect the amounts of TA on the surface of Ni must reach an optimal value. 

In addition to the enantioselective effect from complex formation, also observed was the enhancement effect of pH and the evolution of hydrogen during modification of the hydrides of the IMC s. During these processess the presence of components (especially La) in the modifying solution resulted in enrichment of IMC surface Ni. Data on Table 4.11. from X-ray photoelectronic spectra show the composition of surface layers of a number of IMC s... 

The effect of Cnd is much higher than by the above mentioned amines despite its lower pKa, because its specific adsorption on the Pt surface results in simultaneously increasing the reaction rate and the enantioselective effect Such a result was explained by the "Template effect", which is a mutual adsorption of reactant and modifier molecules on the Pt catalyst surface (Sutherland et al. ). 

Oian, X. Hall, S.D. Enantioselective effects of experimental diabetes mellitus on the metabolism of ibuprofen. J. Pharmacol. Exp. Ther. 1995, 274, 1192-1198. 

Adsorption of fluorescent molecules on chirally modified surfaces and sol-gel films can also generate enantioselective effects [39]. 

The macrocyclic antibiotics were incorporated as mobile phase additives to determine the enantioselective effects on the chiral analytes. The resolution and the capacity factor of each analyte were assessed by a variation of the structure of the macrocyclic antibiotic and the mobile phase buffer pH. 

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