Enantiomers optical purity

EinaHy, kinetic resolution of racemic olefins and aHenes can be achieved by hydroboration. The reaction of an olefin or aHene racemate with a deficient amount of an asymmetric hydroborating agent results in the preferential conversion of the more reactive enantiomer into the organoborane. The remaining unreacted substrate is enriched in the less reactive enantiomer. Optical purities in the range of 1—65% have been reported (471). 

Enantiomeric excess (% ee) is the percentage of the major enantiomer minus that of the min or enantiomer optical purity (% op) is the ratio, in percent, of the optical rotation of a mixture of en antiomers to that of the pure enantiomer. 

Mr = +183 for the (S) enantiomer. Optical purity 100% based on the optical purity of the starting (S)-l-phenylethyl-amine 6). 

Mr = —12.6 for the (R,R) enantiomer. Optical purity is higher than 92% which is the maximum optical yield obtained in the rhodium-catalyzed asymmetric hydrogenation with this ligand m. Md2 = —65.5 for the (R,R) enantiomer. Optical purity should be similar to that of DIOP because both ligands have the same precursor78>. 

Specific rotation data may assist in the identification of a specific enantiomer, or may be used to determine the optical purity (enantiomeric purity) of a mixture of enantiomers. Optical purity is defined as the percent excess of one enantiomer over another in a mixture and is expressed in Eq. (3) ... 

Mixtures containing equal quantities of enantiomers are called racemic mixtures Racemic mixtures are optically inactive Conversely when one enantiomer is present m excess a net rotation of the plane of polarization is observed At the limit where all the molecules are of the same handedness we say the substance is optically pure Optical purity or percent enantiomeric excess is defined as... 

Pure miantiomeric substances show rotations that are equal in magnitude but opposite in direction. Unequal mixtures of enantiomers rotate light in proportion to the composition. The relationship between optical purity and measured rotation is... 

The addition of an achiral organometallic reagent (R M) to a chiral carbonyl compound 1 (see Section 1.3.1.1.) leads to a mixture of diastercomers 2 (syn/anti) which can be either racemic, or enantiomerically enriched or pure, depending on whether the substrates are race-mates or pure enantiomers. This section incorporates only those reactions starting from optically pure a-amino aldehydes, however, optical purity of the starting material has not been demonstrated in all cases. 

An analogous stereochemical outcome was observed when the ethylation of racemic 2-phenyl-propanal was catalyzed by (-)-(/ )-l-(diisopropylamino)-3,3-dimethyl-2-butanol36. The reaction was run to 70% conversion and again the ratio of syn- to anff-diastercomers was 3 1. In this case, however, the 7 -configurated aldehyde was predominantly consumed and both enantiomers of the aldehyde were predominantly alkylated from the Re-side. The optical purity of the unreacted (S)-2-phenylpropanal was 85.7% ee. 

The same reaction utilizing chlorotriisopropoxytitanium gives a lower yield and optical purity of the (Z)-anti product ( + )-4 (yield 33% 64% ee). Utilization of tetraisopropoxytita-nium causes complete racemization16. The reaction of (Z)-l-methylbutenyltitanium with both enantiomers of 2-( er/-butyldimethylsilyloxy)propanal proceeds only very sluggishly with approximately 20% yield99. The results are best explained by the assumption of a (twist)boat transition state. 

The ee is determined by liberution of (3S)-3-hydroxy-5-nicthylhcxanoic acid by acidic hydrolysis, which is then derivalized by heating with isopropylisocyanatc and analyzed 011 a chiral capillary GC column (Chirasil-L-Val, 50 m, 190 C, carrier 90 kPa) (,V)-cnantiomcr / 9.35 min 97% (fJ)-enantiomer 9.58 min 3% 94% ee. Material of 99% optical purity is obtained by rccrystallization. 

It is well known that spontaneous resolution of a racemate may occur upon crystallization if a chiral molecule crystallizes as a conglomerate. With regard to sulphoxides, this phenomenon was observed for the first time in the case of methyl p-tolyl sulphoxide269. The optical rotation of a partially resolved sulphoxide (via /J-cyclodextrin inclusion complexes) was found to increase from [a]589 = + 11.5° (e.e. 8.1%) to [a]589 = +100.8 (e.e. 71.5%) after four fractional crystallizations from light petroleum ether. Later on, few optically active ketosulphoxides of low optical purity were converted into the pure enantiomers by fractional crystallization from ethyl ether-hexane270. This resolution by crystallization was also successful for racemic benzyl p-tolyl sulphoxide and t-butyl phenyl sulphoxide271. 

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. 

Assuming a linear relationship between [a] and concentration, which is true for most cases, the optical purity is equal to the percent excess of one enantiomer over the other ... 

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