Enantiomeric excess, definition

In 1968,Horner et al. [22] and Knowles and Sabacky [23] independently demonstrated that low but definite enantiomeric excesses (up to 15% ee) were produced in the rhodium-catalyzed asymmetric hydrogenation of simple alkenes using methylpropylphenylphosphine 7 as chiral ligand (Scheme 1). 

A quantitative study of the cholesteric induction and of the chiral transfer from dopant to phase requires the definition of the helical twisting power 3. This quantity expresses the ability of a chiral dopant to twist a nematic phase and can be numerically expressed in Eq. (2) where p is the cholesteric pitch, c the dopant molar fraction, and r its enantiomeric excess its sign is taken to be positive or negative for right-handed (P), or left-handed (M) cholesterics, respectively. This relation holds for molar fractions <0.01-0.05 ... 

Fig. 3.7. Definition of the enantiomeric excess ee using the Sharpless epoxidation of allylic alcohol as an example. The chiral auxiliary is tartaric acid diethyl ester (diethyl tartrate, DET).

For the definition of the asymmetry factors gx, see Appendix A.) The first tei is the Stevenson and Verdieck result [27] (for circularly polarized light, AUI = 1 The second term describes the enantiomeric excess due to the true magnetochin anisotropy. 

Enantiomeric excess (e.e.) definition, 107 footnote Enantiosdective retro-synthesis, 200-204 Enantioselective syntheses ... 

The final definition concerns formation of enantiomers, and the terms enantioselective and enantiospecific are used. If a reaction produces an unequal mixture of enantiomers it is enantioselective. If it generates only one enantiomer of two possibilities, it is enantiospecific. The baker s yeast reduction (see sec. 4.10.F) of 134 gave 135 with >99% ee (S). (Here % ee means percent of enantiomeric excess.) A 0% ee means a 50 50 mixture (racemic mixture), 50% ee means a 75 25 mixture and 90% ee means a 95 5 mixture. The predominance of the (S) enantiomer makes this reaction highly enantioselective. 

Cronin, John R., and Sandra Pizzarello. Enantiomeric Excesses in Meteoric Amino Acids, Science 275 (1997) 951-55. This is the first truly definitive report of an excess of uncommon T.-amino acids on the Murchison meteorite. 

By definition, enantioselective reactions are transformations in which a prochiral substrate is converted into a chiral product such that one of the two enantiomers is formed in significant excess. The degree of enantioselectivity is given by the enantiomeric excess (ee). Schematically, such a transformation is depicted in Scheme 1, with the triangle (substrate S) representing a prochiral unit and the two tetrahedrons (products P and ent-P) representing two enantiomeric chiral products. 

This definition refers to an enantiomeric mixture produced in an asymmetric synthesis. In some cases where a diastereomeric mixture is produced, the definition has to be altered accordingly. Percent optical purity is an operational term that depends on optical rotation measurements. It is not necessarily equal to the percent enantiomeric purity (13), which is a more meaningful term and is the extent to which one enantiomer is formed in excess over the other ... 

The optical purity is usually, but not always, equal to enantiomer excess. In order for the two to be equal, it is necessary that there be no aggregation. It is possible, for example, that a homochiral or heterochiral dimer (see Glossary, Section 1.6, for definitions) would refract the circularly polarized light differently than the monomer (or each other). In 1968 [19] Krow and Hill showed that the specific rotation of (S)-2-ethyl-2-methylsuccinic acid (85% ee) varies markedly with concentration, and even changes from levorotatory to dextrorotatory upon dilution. In 1969 [20], Horeau followed up on Krow and Hill s observation, and showed that the optical purity (at constant concentration) and enantiomer excess of (5)-2-ethyl-2-methylsuccinic acid were unequal except when enantiomerically pure or completely racemic. This deviation from linearity is known as the Horeau effect, and its possible occurence should be remembered when determining enantiomeric purity by polarimetry. 

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