Calculations enantiomeric excess

The catalyst + aliphatic n-alkene system, for all of the olefins from propene to 1-decene, give almost 40,000 possible conformations. The most stable conformations for each of the olefins were selected (around 1,700) and their transition states were optimized at IMOMM level. The calculated enantiomeric excesses are shown in Fig. 13. Calculations are able to reproduce the observed increase in ee for short chains, and the presence of a ceiling value after which the increase in enantioselectivity is much smaller, in excellent agreement with experiment. 

In contrast to the aforementioned fullerenes, C76 is a chiral molecule containing 30 different types of carbon-carbon bond. In this molecule five different pyracylene-type carbon-carbon bonds repeat to form chrysene-shaped units. Kinetic resolution of this fullerene has been achieved via asymmetric osmylation in the presence of a cinchona based chiral ligand (see Section 4.4.4.1.1., ligand 1 d/2 d, Table 5). The calculated enantiomeric excess of the recovered material (after 95% conversion) is >97%, whereas the regenerated C76, formed by tin(II) chloride reduction of the osmylated material (after 33 % conversion), is enriched in the opposite enantiomer. Analysis of the local curvature of the C76 molecule indicates that Os04 should selectively add to two of the 30 types of bonds86. 

TABLE 2. Actual and Calculated Enantiomeric Excess of Solution of 10 mM Atenolol in (S) chi a I N Ionic Liquid. 

Traditionally and even today, polarimetry is used in many laboratories for control of optical purity. However, this method suffers from some well-known specific drawbacks. Furthermore, often calculation of the enantiomeric excess from optical rotation is impossible, because the specific rotation of the pure enantiomer is not known precisely, or calculated enantiomeric excess values may be wrong owing to impurities. For these reasons direct chromatographic analytical procedures are preferred. 

Table 3 Actual and calculated enantiomeric excess (ee%) of solution of 10 propranolol in 5 -CHTA+ TfiN- IL. Data from [127]...

For the determination of the enantiomeric excess. 23 mg of the mi-hydroxy ester (containing 2 5% //-product) and 9 mg of tris[3-(heptafluoropropylhydroxymethylene)-a-camphorato]europium are dissolved in 0.5 mL of CDC13. The ec is calculated from the peak heights and areas of the resolved doublets, due to the methyl groups attached to C-2. of the minor (f) 1.74) and major (<5 1.98) enantiomers. 

However, considering practical limitations, that is, the availability of optically pure enantiomers, E values are more commonly determined on racemates by evaluating the enantiomeric excess values as a function of the extent of conversion in batch reactions. For irreversible reactions, the E value can be calculated from Equation 1 (when the enantiomeric excess ofthe product is known) or from Equation 2 (when the enantiomeric excess ofthe substrate is knovm) [la]. For reversible reactions, which may be the case in enzymatic resolution carried out in organic solvents (especially at extents of conversion higher than 40%), Equations 3 or 4, in which the reaction equilibrium constant has been introduced, should be used [lb]. 

The HPLC analyses of the reaction mixtures of the 2-benzyl-1-benzosuberone were carried out on a Chiracel OJ column (0.46x25 cm). The column contains silica-gel as packing material coated with a cellulose derivative. The eluent was hexan/2-propanol 90 10 v/v, the flow rate was 1.0 ml/min and the column pressure was 50 kg/cm2. The UV-absorbance was measured at 249 nm Enantiomeric excesses were calculated according to the following equation ... 

The pathway with the lowest overall barrier is C on the Pro- 5 manifold, meaning that for this system, we would predict the S enantiomer to be formed, in accord with experiment. Our calculated energy difference between the two manifolds is almost 6.5 kcal/mol, which is more than large enough to account for the observed enantiomeric excess of> 99% [78],... 

Progress of the reaction was monitored using a GC equipped with a FID on an achiral CP 1301 capillary column (30 m x 0.25 mm x 0.25 m film) and N2 as carrier gas. Enantiomeric purity of 2-octanol was analysed after derivatization with acetic anhydride (see below) using a CP-Chirasil Dex-CB column (25 m x 0.32 mm x 0.25 pm film, column B) and H2 as carrier gas. Enantioselectivities (expressed as the enantiomeric ratio E) were calculated from enantiomeric excess of the product and conversion as previously reported. Retention times and methods are listed in Table 3.1. 

The diastereomeric excess (de) and enantiomeric excess (ee) were determined by first converting the methyl ester to the diastereomeric acetal by acid-catalysed reaction with (2R,3 )-2,3-butanediol. The acetals were then analysed on a capillary GC HP5 column (30 m X 0.32 mm x 0.25 pm) injector 250 °C 320 °C column 130 °C isothermal. The de was calculated to be 82 % and the ee >95 %. ... 

What happens for a nonracemic mixture of enantiomers Is it possible to calculate the values of the chiral properties of the solution from knowledge of the properties of the enantiopure compound In principle, yes, on the condition that there is no autoassociation or aggregation in solution. Then, the observed properties will be simply the weighted combination of the properties of two enantiomers. A nice example of where this normal law may be broken was discovered by Horeau in 1967 it is the nonequivalence between enantiomeric excess (ee) and optical purity (op, with op = [a]exi/[ ]max) for 2,2-methylethyl-succinic acid. In chloroform op is inferior to ee, while in methanol op = ee. This was explained by the formation of diastereomeric aggregates in chloroform, while the solvation by methanol suppresses the autoassociation. 

Chiral compounds are of wide interest as auxiliaries in asymmetric synthesis. The efficiency of the auxiliary in the process is usually expressed as the ee of the product (eCpfod)- What happens if the chiral auxiliary is not enantiomerically pure One expects a lower ee for the product. From this value one can calculate the maximum ee of the product (eCj ax) by taking into account the enantiomeric excess of the auxiliary, assuming a proportionality between eCprod and eeaux ... 

For the calculations we used a simplified model system in which all substituents were replaced by methyl groups (Scheme 4). Experimentally, the methyl substituted catalyst and methanol as nucleophile are active, but the enantiomeric excesses obtained fall below those obtained with the tert-XmcinQ amide-derived catalyst in combination with allyl alcohol (Scheme 3). 

Figure 12. NMR determination of the enantiomeric excess of glycerol acetonide without separation of the unreacted ester. Approximately 1.35 mmol of a mixture of glycerol acetonide and the acetate ester was dissolved in a 10 mm NMR tube. To the solution was added 0.45 mmol of PCI,. The tube was shaken and quickly uncapped to allow the escape of the HCl gas formed during the reaction. After the recapped tube stood for 30 min at room temp, 0.5 mL of CDCI3 was added, and the NMR spectrum was recorded on a Varian 200 MHz instrument. The ee was calculated from Korean s formula (ee = (K - 1)/(K + 1), Vigneron, J.P. Dhaenens, M. Horeau, A. Tetrahedron. 1973,...

The ratio of the enantiomeric benzyl amide products was determined by analyzing a diluted aliquot of the quenched reaction mixture by HPLC using a chiral stationary phase column (Chiralcel OD, Daicel Chemical Co.). Since racemization is a pseudo-first-order kinetic process, these data (along with the time zero value) are sufficient for determination of the intrinsic rate of racemization kR. The half-life for racemization lRU2 can be directly calculated from the l/d ratio (or % enantiomeric excess, %ee) where t was the time of benzylamine addition (the delay time) ... 

The enantiomeric excess of the product in a batch reactor now can easily be calculated because f ... 

Usually the enantiomeric excess is calculated for a standard conversion process a single irreversible batch reaction in a homogeneous solution starting from racemic or prochiral substrate. However, if the assumptions that were used for the derivation of Eqns. (10.14), (10.15) and (10.17) do not hold, different equations apply, and the enantiomeric excess may be higher or lower. Table 10.3 shows an overview of some modifications, including some potential improvements (substrate racemization) and problems (equilibration) that were treated in Chapter 2. Clearly, many modifications will lead to a decrease rather than to an increase of the enantiomeric excess. 

Finally, when working in the field of asymmetric synthesis, the organic chemist needs to quote both the chemical yield and the optical yield. The percentage optical yield or optical purity [enantiomeric excess (ee) %], is calculated thus ... 

The enantiomeric excess of substrate and product shifts during the course of a kinetic resolution, making the determination of the efficiency of the reaction dependent on the reaction time. Hence, two kinetic resolutions can only be compared at the same extent of conversion [45, 62]. For a better comparison of two reactions, equations for the calculation of the enantioselectivity were established [63, 64]. In most cases, the enantiomeric excess of both substrate and product is determined for greater accuracy, but the determination of one and the conversion can also be used for the calculation of the E value (enantioselectivity) ... 

The experimental results let us to conclude that TA does not form complexes with any of the model compounds but DBTA gives crystalline host-guest complexes with all the four model compounds. [16] The results are summarised in Table 5 where the efficiency of the resolutions is characterised by the S value calculated as the production of the yield and the enantiomeric excess. [11] Application of DBTA monohydrate in hexane suspension provided more efficient separation of 26 and 28 enantiomers than the use of anhydrous DBTA under the same conditions. Data in Table 5 refer to the resolutions made with DBTA monohydrate in all cases. 

To evaluate the enantioselectivity of the reaction, usually, enantiomeric excess, ee, of products is used. However, when a racemic substrate is reacted, ee of the product as well as that for the substrate changes depending on the conversion as shown in Figure 5(a). To evaluate the enantioselectivity of this type of reaction, the ratio of the specificity constants of the enantiomers, E-value, was introduced (Figure 5(b)).5 The methods of the calculation of E for the reaction of racemates is shown in Figure 5(c). 

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