Determination of the Enantiomeric Excess ee

We will describe three significant methods based on the measurement of optical rotation, nuclear magnetic resonance and chromatography. However, other methods are possible (infrared, Raman, etc.), while some such as calorimetry are hardly ever used for metalfic complexes. 

The specific rotation of a compound is, as we have seen (Section 2.4.1.1), defined by [ Ispedfic = [ Imeasured/ where is the observed rotation expressed in °, / is the 

This method appears simple, yet several precautions are necessary. Apart from the fact of knowing the specific rotation of the pure enantiomer, which is itself not always straightforward, the measurement of the optical rotation must be made under the same conditions of solvent, concentration, temperature and wavelength as were used to obtain the specific rotation of the pure enantiomer. Moreover, the measured rotation is often weak, which increases the error in the measurement. It also happens not infrequently that the complexes being studied are strongly coloured, and as a result the measurement is not possible with a standard polarimeter. The presence of impurities with a large optical rotation may also invalidate the reading. 

This is a technique widely used for the determination of the ee of a mixture of diamagnetic enantiomers. On its own, NMR cannot distinguish between enantiomers, rather it is an indirect method which requires that the enantiomer mixture be converted into a mixture of diastereomers. This can be achieved by  

In each case it is a matter of obtaining a diastereomeric environment around the nuclei whose resonances can then be distinguished. The ee is then calculated by integration of the peaks corresponding to each of the diastereomers. In reality, it is a diastereomeric excess that is calculated, but this can be converted into an ee if the chiral auxiliary agent used is itself enantiomerically pure. In order for the precision of the measurement to reach 5%, each peak area must be obtained with an accuracy of 1.25%. 

---

A large number of investigations have been reported on the use of lipases in KR of racemic alcohols, amines and esters. Accurate determination of the enantiomeric excess (ee) of both substrate and product wiU be crucial for reliability of the KR data obtained, and this in turn is dependent upon the development of a suitable analytical procedure for the baseline separation of substrate and product in a single chromatography run [9]. 

Figure 2.24, Determination of the enantiomeric excess of 1-phenylethanol [30, 0.1 mmol in 0.3 ml CDCI3, 25 °C] by addition of the chiral praseodymium chelate 29b (0.1 mmol), (a, b) H NMR spectra (400 MHz), (a) without and (b) with the shift reagent 29b. (c, d) C NMR spectra (100 MHz), (c) without and (d) with the shift reagent 29b. In the C NMR spectrum (d) only the C-a atoms of enantiomers 30R and 30S are resolved. The H and C signals of the phenyl residues are not shifted these are not shown for reasons of space. The evaluation of the integrals gives 73 % R and 27 % S, i.e. an enantiomeric excess (ee) of 46 %...

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,...

Some 1- and 2-hydroxyalkanephosphonates have been successfully resolved by a CALB (Candida antarctica lipase B)-catalysed acylation process to give both (R)- and (S)-isomers with high enantiomeric excess (in most cases with 95% ee). (S)-Naproxen and (S)-Ibuprofen chloride are convenient chemical derivatizing agents for the determination of the enantiomeric excess of hydroxy and aminophosphonates by PMR. . New phosphorylating agents, 3-phosphoro-2(-N-cyanoimino)-thiazolidine derivatives (3-phosphoro-NCTS) (316), can be used as a stable alternative to phosphorochloridates. Phosphoryla... 

Although separation factor a has the greatest impact on peak resolution, an accurate determination of high enantiomeric excess (ee-values) as shown in Fig. 6.23 ]11] is only possible, in the case of highly resolved enantiomer separation. 

The enantiomeric excess (ee) of the hydrogenated products was determined either by polarimetry, GLC equipped with a chiral column or H-NMR with a chiral shift reagent. Methyl lactate and methyl 3-hydroxybutanoate, obtained from 1 and 2, respectively, were analized polarimetry using a Perkin-Elmer 243B instrument. The reference values of [a]o(neat) were +8.4° for (R)-methyl pyruvate and -22.95° for methyl 3-hydroxybutcinoate. Before GLC analysis, i-butyl 5-hydroxyhexanoate, methyl 5-hydroxyhexanoate, and n-butyl 5-hydroxyhexanoate, obtained from 1, 5, and 6, respectively, were converted to the pentanoyl esters, methyl 3-hydroxybutanoate was converted to the acetyl ester, and methyl 4-methyl-3-hydroxybutanoate obtained from 2 was converted the ester of (+)-a-methyl-a-(trifluoromethyl)phenyl acetic acid (MTPA). 

Another phosphorus containing CDA (87) has also been described for the estimation of the enantiomeric excess of chiral amines80. The phosphorus CDA 87 is formed quantitatively and instantaneously in situ in an NMR tube by reaction of phosphorus trichloride (PCI3) with the chiral diamine 88. Addition to the NMR tube of the chiral amine (IC-NIIo) for which the ee is to be determined gives the diastereomeric phosphorous derivatives ... 

MIP assays can also be utilized in synthetic organic applications. For example, MIP-based assays have been used to measure the chiral purity of samples in organic solvents. An L-phenylalanine anilide (l-PAA) imprinted polymer was utilized as a recognition element to measure the enantiomeric excess (ee) of PAA samples (Chen and Shimizu 2002). The MIP displays greater capacity for l-PAA versus d-PAA samples of similar concentration, and this difference was used to estimate enantiomeric excess. The enantiomeric excess of an unknown solution was determined by comparing the UV absorbance of the PAA remaining in solution after equilibration against a calibration curve. This MIP assay was demonstrated to be rapid and accurate with a standard error of +5% ee. 

Enantiomeric purity, measured as the enantiomeric excess (ee) of an isomer, is determined by the formula (% major isomer)—(% minor isomer). Thus, if a chiral drag is said to be or 50% ee, the composite mixture contains 75% of one enantiomer and 25% of the other. Enantioselectivity refers to the greater activity of one enantiomer over its minor image. Enantiospecificily is rarely observed and implies that one enantiomer possesses 100% of the observed activity in most cases it is more accurate to use the term highly enantioselective. The pharmacologically more active enantiomer is termed the eulomei and the less active enantiomer is referred to as tire distomer. 

The enantiomeric excess (% ee) of these compounds was determined by the submitters as follows. The ester and acids were first reduced to the corresponding alcohols with DIBAL and LAH, respectively. The alcohols were then allowed to react with 100% excess of (S)-(+)-o-methoxy-o-trifluoromethylphenylacetyl chloride (Mosher s reagent) in (1 1) pyridine-carbon tetrachloride for 18 hr. The diastereomeric ratio of these derivatives was finally determined by isothermal gas chromatography on a capillary OV-17 column at 160°C. 

The enantiomeric excess (ee) of carbinol formation was determined for the three /9-keto esters 1-3. Their inhibition constants for ADH are given in Table 3.2. 

Standard reaction conditions. Catalysed reactions were conducted at 293 K in a Baskerville stainless steel reactor of volume 150 ml. 7.2 ml pyruvate ester (65 mmol) and alkaloid (0.17 mmol cinchonidine (50 mg) or 0.17 mmol quinuclidine (19 mg) or 0.17 mmol of each) were dissolved in 12.5 ml dichloromethane and added to the catalyst (250 mg) in the reactor. The reactor was operated at 30 bar in a constant pressure mode. Product analysis was by chiral gas chromatography to determine the enantiomeric excess (ee) and by GCMS to determine the masses of higher molecular weight products. 

---