Minor-image isomers

A FIGURE 24.15 The two enantioineric fbmis of 2-bromopentane. The minor-image isomers are not superimposable on each other. 

Alfred Werner conjectured as early as 1899 that octahedrally coordinated metal complexes should occur in nonidentical minor image isomers. For such objects. Lord Kelvin, in 1893, had coined the adjective chiral , a term never used by Werner. It can be proved by examination of the original sample of [Co(NC)2)2(en) Br, prepared by Edith Humphrey, a Ph.D. student of Wemer s, that crystals of optically pure samples were obtained in Werner s laboratory as early as 1899 or 1900. However, Werner did not publish die first successful resolution of an octahedral metal complex until 1911. Presently, interest in chirality in coordination compounds is booming, mainly because of the importance of coordination compounds in enantioselective homogeneous catalysis. Other interesting sq )plications are enantioselective interactions of chiral coordination species with biomolecules, and the stereoselective synthesis of multicenter systems. 

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. 

When a species cannot be superimposed on its minor image, the two forms are known as enantiomers or optical isomers. Most... 

The irons isomer and its minor image are identical. They are not isomers of each other. 

Most of the physical and chemical properties of optical isomers are identical. The properties of two optical isomers differ only if they are in a chiral environment— that is, one in which there is a sense of right- and left-handed-ness. In the presence of a chiral enzyme, for example, ihe reaction of one optical isomer might be catalyzed, whereas the other isomer would not react Consequently, one optical isomer may produce a specific physiological effect wiihin the body, whereas its minor image produces a different effect or none at all. Chiral reactions are also extremely important in the synthesis of pharmaceuticals and other industrially important chemicals. The 2001 Nobel Prize in chemistry was awarded to W. S. Knowles and K. B. Sharpless of the United States and R. Noyori of Japan for Iheir research on the catalysis of chiral reactions. 

Optical isomers are nonsuperimposable minor images. We call a pair of optical isomers enantiomers. The rotation of polarized light is measured with a polarimeter. 

The methyl substituted spiro system JO (Fig. 3) was also studied (24, 25). With this system, two isomers JJ and J2 are possible and molecular models show that they can each exist in four different conformations. Evaluation of the anomeric and the steric effects of each conformation leads to the prediction that isomer JJ exists in the conformation 11A only (0 kcal/mol relative to the other conformations) whereas isomer 12 is a mixture of a major (12A, 2.4 kcal/mol) and a minor (12B, 2.9 kcal/mol) conformer. However, since isomers JJ and J2 are interconvertible (JJ can be converted into the mirror image of J2 by opening and reclosure of the acetal function) and... 

The fluorescence of SAA is roughly a mirror image of the absorption of the cis-qulnold isomer (Table 7). Minor blue shifts have been observed if the hydrocarbon solvent is replaced by an alcoholic solvent, but the evidence is not sufficient to permit postulating an n—it fluorescent state (53). Evidence of the tt-tt character of the emitting state is the high absorption coefficient of the quinoid form of the order of e = 10. Furthermore, fluorescence from n-TT levels of ketonlc compounds is unusual. The fluorescence decay time has been measured to be xp = 20 10 ns... 

Compounds that are neither racemic nor enantiomerically pure are usually called enantio-merically enriched. Chemists have two ways of referring to the ratio of enantiomers in an enantiomerically enriched sample. The first is the simple one we have just used e.r. or enantiomeric ratio, expressed as two numbers adding to 100. More common, however, is to express this ratio as an enantiomeric excess. Enantiomeric excess (or ee) is defined as the excess of one enantiomer over the other, expressed as a percentage of the whole. So a 94 6 mixture of enantiomers consists of one enantiomer in 88% excess over the other, and we call it an enantiomerically enriched mixture with 88% ee. Why not just say that we have 94% of one enantiomer Enantiomers are not like other isomers because they are simply mirror images. The 6% of the minor enantiomer can be paired with 6% of the major isomer to form a racemic mixture amounting to 12% of the total. The mixture contains 12% racemate and 88% of one enantiomer, hence 88% ee. 

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