Asymmetric depletion

Asymmetric depletion ((—)-NLE) is less frequently reported than (+)-NLE, perhaps because it seems less worthy of investigation. However, it can be informative with respect to the reaction mechanism, as demonstrated by Hayashi et al. in the catalyzed enantioselective 1,4-addition of arylboronic acids on enones. ... 

Keywords Asymmetric amplification. Asymmetric catalysis, Asymmetric depletion, Autocatalysis, Chiral auxiliary. Enantiomeric excess. Kinetic models. Non-linear effects. Reservoir effect... 

Asymmetric depletion may originate from several mechanisms, for example ... 

The asymmetric sulfide oxidation described in Scheme 2 has a (-)-NLE until eeaux=70% [5]. The (S)-proline catalyzed cyclization of a triketone shows a weak asymmetric depletion [5], as does allylic oxidation of cyclohexene in the presence of a catalyst prepared from Cu(OAc)2 and (S)-proline [41]. 

The largest asymmetric depletion found in the literature is shown in Scheme 7 (Curve A), it was reported by Kobayashi et al. during their studies involving BINOL/lanthanide triflate combinations [49]. It is also interesting to mention the multi-shaped curve B (Scheme 7 ) observed by Pfaltz and Zhou in the conjugated addition of a Grignard reagent to cycloheptenone, in the presence of a chiral copper catalyst [50]. 

Recently, the concept of kinetic resolution has been extended to the case where enantioimpure catalysts are used. Kagan discovered the first examples of nonlinear effects in asymmetric catalysis, where there was no proportionality between the ee of the auxiliary and the ee of product (Figure 5.27) and gave some mathematical models to discuss these effects. The nonlinear effect (NLE) originates from the formation of diastereomeric species when the chiral auxiliary is not enantiomerically pure, either inside or outside the catalytic cycle. The observed effects were classified as (+)-NLE and (-)-NLE where "asymmetric amplification" and "asymmetric depletion" respectively occured. 

Schematic energy level diagrams of a metal/polymer/metal structure before and after the layers are in contact are shown in the top two drawings of Figure 11-6. Before contact, the metals and the polymer have relative energies determined by the metal work functions and the electron affinity and ionization potential of the polymer. After contact there is a built-in electric field in the structure due to the different Schottky energy barriers of the asymmetric metal contacts. Capacitance-voltage measurements demonstrate that the metal/polymer/metal structures are fully depleted and therefore the electric field is constant throughout the bulk of the structure [31, 35]. The built-in potential, Vhh i.e. the product of the constant built-in electric field and the layer thickness may be written...

The vibrational teiaperatnre does not change appreciably until we reach spectrum 10 in the sequence shown in Figure 1, i.e., until oxygen depletion has nearly stopped the reaction. Our crude estimate of the vibrational temperature based on the centroid position and the assumption that all vibrational temperature are the same suggests that the vibrational temperature, like the rotational teia-perature, falls below the surface temperature. This cannot be the case because the emission intensity would be too low to detect in our system if the asymmetric temperature were in fact that low. Our Interpretation 1 that symmetric stretch and bending have cooled to the point where they make very little contribution to the emission,... 

Fig. 14.6 Experimental and calculated enrichments or depletions of all possible ozone iso-topomers. The labels 6, 7 and 8 represent lsO, 170, and lsO respectively. Ozone (gray bars) was produced in well scrambled oxygen mixtures at about 90 mbar and room temperature (Mauersberger et al. Adv. Atomic Mol. Opt. Phys. 50, 1 (2005)). The calculated values (vide infra) are those of Gao, Y. Q. and Marcus, R. A., J. Chem. Phys. 116, 137 (2002) setting the parameter r = 1.18 (black bars) or r = 1.0 (white bars). A typical symbol, such as 668, denotes an ozone with the isotopic composition 160160180 and consists of a mixture of symmetric (160180160) and asymmetric (160160180) isotopomers (After Gao, Y. Q. and Marcus, R. A., Science 293, 259 (2001))...

It was soon recognized that in specific cases of asymmetric synthesis the relation between the ee of a chiral auxiliary and the ee of the product can deviate from linearity [17,18,72 - 74]. These so-called nonlinear effects (NLE) in asymmetric synthesis, in which the achievable eeprod becomes higher than the eeaux> represent chiral amplification while the opposite case represents chiral depletion. A variety of NLE have been found in asymmetric syntheses involving the interaction between organometallic compounds and chiral ligands to form enantioselective catalysts [74]. NLE reflect the complexity of the reaction mechanism involved and are usually caused by the association between chiral molecules during the course of the reaction. This leads to the formation of diastereoisomeric species (e.g., homochiral and heterochiral dimers) with possibly different relative quantities due to distinct kinetics of formation and thermodynamic stabilities, and also because of different catalytic activities. 

The differences in the hydration energies of the ions and in their hydrated radii are expected to lead to d1 =d2. As indicated by simulations [28], the negative ions are less repelled, therefore they can approach the interface closer than the positive ions. The asymmetric ion depletions generate a surface potential, even in the absence of an external surface charge. The charge that generates the diffuse double layer is located a few Angstroms from the interface, and is due to the anions. 

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