Enantiomer excess

The enantiomer composition of a sample may be described by the enantiomer excess (ee), which describes the excess of one enantiomer over the other ... 

The goal of an asymmetric reaction is to obtain one enantiomer in high excess of the other. For this reason, after the reaction one has to measure the enantiomer excess. The larger the excess of one enantiomer over the other, the better the result of the asymmetric reaction or the higher efficiency of the asymmetric induction. 

Figure 41 Composition dependence of the enantiomer excess 2fP - 1 of the P helical state of polysilanes 109(P) O 109(S) o, 110 A and 111 (calculated solid curves experimental symbols).326 Reprinted with permission from Sato, T. Terao, K. Teramoto, A. Fujiki, M. Macromolecules 2002, 35, 5355-5357, 2002 American Chemical Society. 

Ultrapurification of 50 mmolL DNB-D,L-Leucine in a Cascade of Five Stages with Two Modules and Two Enantiomeric Carrier (Quinine and Quinidine Derivative with 5 vol% Polysiloxane-supported Carrier) Transmembrane Material Stream J, Enantioselectivity a, Enantiomer Excess ee, Purity, and Yield of DNB-D-Leucine... 

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS Enantiomer excess/Enantiomeric excess,... 

Recently, a variety of measures for assessing stereoselectivity has been used. For the purposes of this volume it has been agreed to measure enantioselectivity by the enantiomer excess value [ee (%)] or the optical purity [op (%)] of the product and diastereoselectivity by the ratio of diastereomeric products (d.r., normalized to 100). 

The reasons for distinguishing enantiomer excess and optical purity of a product are again practical. As will be discussed in Section A.3., optical purity and enantiomer excess are equivalent only in the case of an ideal solution, i.e., in general ee = op. Furthermore, reliable reference values of xmal are rarely available. Thus, the op value is often a fairly unreliable measurement, and this should be apparent. 

Enantioselective gas chromatography can provide three quite different kinds of information (1) the amount of each enantiomer present in a food, determined as the enantiomeric purity or the enantiomer excess, and the separation factor a for each pair of enantiomers (2) enantiospecific sensory evaluation using gas chromatography-olfactometry (GC-O) and (3) data used as part of an authenticity determination. 

Efficient asymmetric hydrogenation of alkenes other than the amino acid and dipeptide precursors described above has met with only limited success. This appears to be at least in part due to the inability of many alkenes to function as bidentate chelates. Ethyl 2-acetoxyacrylate was hydrogenated with an enantiomer excess of 89% using [Rh(cod)(R,R-DIPAMP)]+, giving the S-enantiomer (equation 53). The ligands CHIRAPHOS, PROPHOS, DIOP, BPPM and CAMP were less effective.266... 

The a-ketolactone (63) can be hydrogenated to give pantolactone (64 equation 57). Using [RhCl(cod)]2 and the ligand BPPM (Table 5) an enantiomer excess of 81% was obtained. Related ligands possessing different substituents on the nitrogen atom of BPPM were less effective.280 When a neutral chloro complex of BPPM was employed as catalyst, the optical yield was raised to 87%.281 With DIOP (49) the best result was only 40%. 

Nishiyama and Mizuno investigated the enantioselectivity by the addition of chiral sources 276-280 into the photoreaction of 255a however, enantiomer excess (ee) was very low [297] (Scheme 80, Table 6). The diastereodifferentiation of compound 281 that connects 1-cyanonaphthalene and an alkene by di-(/)-men-thyl malonate gave 282 in 14% de moreover, the addition of Lewis acid at low temperature increased the de to 17% (Table 7). 

Fig. 2 Illustration of a positive non-linear effect, where the enantiomer excess in the product exceeds that of the catalyst...

The catalyst term in the rate equation is first order as [dimer] [1-4]. In further analysis of this model, the progress in enantiomer excess over time matches well to that predicted for a dimeric catalyst. This is further augmented by a later paper from Blackmond and Buono [74], They discovered additional kinetic complexity, in that the kinetics under their chosen conditions fit better to an [R2Zn]°[aldehyde]2 [dimer] model. Even so, there is a robust correlation between the evolution of ee and the extent of reaction. Notably, the 2-methylpyrimidine was employed in all the kinetic studies described in [20,21],... 

Some N15 enrichment in individual Murchison amino acids (versus terrestrial counterparts) suggests an extraterrestrial origin for an L-enantiomer excess-Engel, M. H., and Macko S. A. Nature, 1997, 389,265. 

Der EnantiomerenuberschuB (ee, von engl. enantiomer excess) ist ein indirektes Mafi fur die Enantiomerenreinheit einer Verb indung. Er ist definiert durch die Gleichung... 

Recently, as mentioned above, stereoselective organic synthesis with high d.e. (diastereomer excess) and high e.e. (enantiomer excess) by means of the radical reactions, has been partly established under mild conditions. 

Racemic chiral secondary allylic alcohols can be subjected to a kinetic resolution by means of the Sharpless epoxidation (Figure 3.39). The reagent mixture reacts with both enantiomers of the allylic alcohol—they may be considered as a-substituted crotyl alcohols—with very different rates. The unreactive enantiomer is therefore isolated with enantiomer excesses close to ee = 100% in almost 50% yield at approximately 50% conversion. The other enantiomer is the reactive enantiomer. Its epoxidation proceeds much faster (i.e., almost quantitatively) at 50% conversion. The epoxide obtained can also be isolated and, due to its enantiomeric excess, used synthetically. 

The homogeneous catalytic asymmetric hydrogenations of 2-arylacrylic acids have been studied. Both rhodium and ruthenium catalysts have been examined. The reaction temperatures and hydrogen pressures have profound effects on the optical yields of the the products. The presence of a tertiary amine such as triethylamine also significantly increases the product enantiomer excess. Commercially feasible processes for the production of naproxen and S-ibuprofen have been developed based on these reactions. 

The 191 problems in this book cover most of the area of stereochemistry, including nomenclature, stereogenic elements (centers, axes, planes) and their descriptors, symmetry, inorganic stereochemistry, determination of enantiomer excess, conformation of acyclic and cyclic compounds, and more. The answers, in addition to providing solutions to the problems, frequently include additional explanations of the underlying principles. The problems are ordered more or less in order of increasing difficulty. (I had a hard time with some of the problems toward the end myself )... 

---