Enantiomeric excess, measurement

For instance, when a-[2H]-styrene is hydroformylated with the Rh/(—)-DIOP catalytic system, the two reaction products obtained have almost identical optical purity and opposite absolute configuration (Fig. 2)51). Therefore, in this case, the enantiomeric excess measured indicates both the type (the re-re enantioface reacts preferentially) and extent of enantioface discrimination ( 15%) occurring during the reaction. 

In 1987, Hoffinann introduced a new method for investigating configurational stability [109]. It is particularly usefiil for the investigation of the optical stabihty of organometals, as it does not require the use of these enantiomerically enriched reagents. Furthermore, no enantiomeric excess measurement is necessary. It thus appears as completely different to that of the classical methods run to investigate configurational stability. 

Despite these problems and limitations, chiral derivatisation remains the most widely used NMR technique for enantiomer resolution. CDAs are usually simple, multifunctional compounds which are often commercially available and claimed to be pure enantiomers. Derivatisation frequently involves esterification or amidation under non-racemising conditions. The method is reliable, which is not always the case with CSAs or CLSRs. In addition, the sense of nonequivalence is consistent allowing assignment of absolute configuration on the basis of chemical shift to be made with some confidence. Diastereomeric anisochronicity is usually sufficient to permit enantiomeric excess measurement to within 1% even with small applied magnetic fields (< 100 MHz). 

Conversion estimated by TLC analysis or by GC relative to internal standard (tiidecane) Enantiomeric excess measured by chiral HPLC or chiral GC... 

Enantiomeric excess measured by chiral GC Data reported are the average of two trials... 

The separation of amino acids into particular enantiomers was the subject of mass spectrometric studies but also ESI MS was applied to determine by kinetic resolution the enantiomeric excess of optically active alcohols and amines in nanoscale by diastereoselective derivatization with optically active acids [38], This method has several distinctive features among others, easily available chiral acids can be used (the authors used A -benzoyl proline derivatives), no chromatographic separations are required, it is insensitive to certain impurities, it is fast and requires only small amount of substrate (10 nmol or less). The method can take an advantage when accuracy of enantiomeric excess measurement is sufficient within 10% limits. 

By simply hydrolyzing the easily accessible 2-hydroxy-2-methylalkanenitriles with concentrated acid, 2-hydroxy-2-methylalkanoic acids are obtained without measurable racemization (Table 3). The reaction sequence from the starting ketone to the carboxylic acid can be carried out in one pot without isolation of the cyanohydrin. The enantiomeric excesses of the (/ )-cyanohydrins and the (ft)-2-hydroxyalkanoic acids are determined from the ( + )-(/T)-Mosher ester derivatives and as methyl alkanoates by capillary GC, respectively. The most efficient catalysis by (R)-oxynitrilase is observed for the reaction of hydrocyanic acid with 2-alkanoncs. 3-Alkanoncs are also substrates for (ft)-oxynitrilase, to give the corresponding (/ )-cyanohydrins32. 

The submitters report obtaining the product in 99% yield. The enantiomeric excess of the Mosher ester of 3 was measured to be 98% using a Chiralcel OD column (40% 2-propanol/hexane). This optical purity measurement substantiated the optical purity assessment made by 111 NMR studies of 3 and racemic 3 prepared using a different method3. Addition of the chiral shift reagent tris[3-(heptafluoropropylhydroxymethylene)-(+)-camphorato]europium (III) resulted in clear resolution of the respective aromatic proton signals for the two enantiomers, which was demonstrated with the racemate. Under similar conditions, NMR analysis of 3 showed that within the detectable limits of the experiment (ca. <3%), there was none of the disfavored enantiomer. 

Zeijden [112] used chiral M-functionalized cyclopentadiene ligands to prepare a series of transition metal complexes. The zirconium derivative (82 in Scheme 46), as a moderate Lewis acid, catalyzed the Diels-Alder reaction between methacroleine and cyclopentadiene, with 72% de but no measurable enantiomeric excess. Nakagawa [113] reported l,T-(2,2 -bis-acylamino)binaphthalene (83 in Scheme 46) to be effective in the ytterbium-catalyzed asymmetric Diels-Alder reaction between cyclopentadiene and crotonyl-l,3-oxazolidin-2-one. The adduct was obtained with high yield and enantioselectivity (97% yield, endo/exo = 91/9, > 98% ee for the endo adduct). The addition of diisopropylethylamine was necessary to afford high enantioselectivities, since without this additive, the product was essentially... 

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 ... 

A range of L-cysteine derivatives bearing a 1,2,4-triazolyl residue on the sulfur atom has been prepared by the asymmetric Michael addition of 4,5-dialkyl-3-mercapto-l,2,4-triazoles to a nickel Schiff base complex. The enantiomeric excesses of the product aminoacids were measured and found to be greater than 98.5% in some cases <2004TA705, 2004RCB932, 2004IZV894>. 

The enantiomeric excesses of the phosphonic acids were measured using 31 P NMR after treatment with (IS, 2S)-(—)-N,A -dimethyl(diphenylethy-lene)-diamine in CDC13 and a catalytic amount of CD3OD. 

For a nonracemic mixture of enantiomers prepared by resolution or asymmetric synthesis, the composition of the mixture was given earlier as percent optical purity (equation 1), an operational term, which is determined by dividing the observed specific rotation (Mobs) of a particular sample of enantiomer with that of the pure enantiomer ( max), both of which were measured under identical conditions. Since at the present, the amount of enantiomers in a mixture is often measured by nonpolarimetric methods, use of the term percent optical purity is obsolete, and in general has been replaced by the term percent enantiomeric excess (ee) (equation 2) introduced in 197163, usually equal to the percent optical purity, [/ ] and [5] representing the relative amounts of the respective enantiomers in the sample. 

The enantioselectivity factors in Table 10 indicate that amino acid samples with different enantiomeric excess (ee) should show differences in the R = (1 + I5)/Iref term. Indeed, In Rchkai is linearly related to optical purity of the specimen and the relevant calibration curve can be established. Accordingly, it is possible to rapidly determine ee of an amino acid sample by a single measurement of Rchirai in a tandem... 

In measuring enantiomeric excess (e.e.) one must first ascertain... 

Conversions were measured on an achiral GC column using calibration curves. For the determination of the enantiomeric excess, the 2-octanol formed was derivatized into the corresponding acetate ester using acetic anhydride (60 fiL) and catalytic DMAP overnight. The reaction was quenched with tap water (300 uL), centrifuged for 2 min at... 

In simple experiments, particulate silica-supported CSPs having various cin-chonan carbamate selectors immobilized to the surface were employed in an enantioselective liquid-solid batch extraction process for the enantioselective enrichment of the weak binding enantiomer of amino acid derivatives in the liquid phase (methanol-0.1M ammonium acetate buffer pH 6) and the stronger binding enantiomer in the solid phase [64]. For example, when a CSP with the 6>-9-(tcrt-butylcarbamoyl)-6 -neopentoxy-cinchonidine selector was employed at an about 10-fold molar excess as related to the DNB-Leu selectand which was dissolved as a racemate in the liquid phase specified earlier, an enantiomeric excess of 89% could be measured in the supernatant after a single extraction step (i.e., a single equilibration step). This corresponds to an enantioselectivity factor of 17.7 (a-value in HPLC amounted to 31.7). Such a batch extraction method could serve as enrichment technique in hybrid processes such as in combination with, for example, crystallization. In the presented study, it was however used for screening of the enantiomer separation power of a series of CSPs. 

NMR spectroscopy [225]. The measurement of enantiomeric excesses of supported substrates has also been achieved by HR-MAS NMR [226]. 

As emphasized by Bakken et al. (2001), the observation of an excess of inversion of the configuration in the C-C alkylated products obtained by the reaction in Scheme 6.12 may well be the result of a single transition state that gets divided into the electron-transfer product and substitution product. In fact, the amount of enantiomeric excess in such a reaction can serve as a measure of the bonding in the electron-transfer transition state. Scheme 6.13 depicts the S 2 route. 

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