Enantiomer differentiation

In principle, mass spectrometry is not suitable to differentiate enantiomers. However, mass spectrometry is able to distinguish between diastereomers and has been applied to stereochemical problems in different areas of chemistry. In the field of chiral cluster chemistry, mass spectrometry, sometimes in combination with chiral chromatography, has been extensively applied to studies of proton- and metal-bound clusters, self-recognition processes, cyclodextrin and crown ethers inclusion complexes, carbohydrate complexes, and others. Several excellent reviews on this topic are nowadays available. A survey of the most relevant examples will be given in this section. Most of the studies was based on ion abundance analysis, often coupled with MIKE and CID ion fragmentation on MS " and FT-ICR mass spectrometric instruments, using Cl, MALDI, FAB, and ESI, and atmospheric pressure ionization (API) methods. 

The single most important physical property that differentiates enantiomers is their ability to rotate the plane of plane polarized light. This property is called optical activity and is displayed only by chiral molecules. Thus, stereoisomers which are also chiral are known as optical isomers. Chiral molecules that rotate polarized light in a clockwise fashion are termed dextrorotatory (d) while those that rotate the beam counterclockwise are levorotatory (/). Enantiomers have optical rotations of die same magnitude but of different signs (d or /). 

The BINAP-Ru complex effectively differentiates enantiomers of 1-hydroxy-l-phenyl-2-propanone. Hydrogenation of the racemic compounds catalyzed by an (R)-BINAP-Ru complex gives the corresponding IS,2R diol with a 92% optical purity and unreacted R substrate with 92% ee at 50.5% conversion (Scheme 1.45) [lc]. The relative hydrogenation rate of the enantiomers, k /kR, is calculated to be 64 1. 

Just as solvents can serve as CSRs, it is possible to use enantiomerically pure solvents to provide the asymmetric environment necessary to differentiate enantiomers. This then allows us to determine the relative amount of two enantiomers in a mixture of them, something that is otherwise difficult to accomplish without resorting to polarimetry. Enantiomerically pure alcohol (R)-10-11 has been used as a solvent to differentiate the enantiomers of aminoester 10-1412 ... 

The knowledge of such a mechanism, close to SN2, makes it possible to differentiate enantiomers, as well as to assay them by the use of an ad hoc chiral reagent gas... 

Our next tasks are to learn how to differentiate enantiomers in words (we do have to be able to talk to each other ) and to see how these stereoisomers differ physically. Which physical properties do enantiomers share and which are different This topic leads us to a brief discussion of how the physical differences arise. Next, we explore how enantiomers differ chemically. Which chemical properties are shared by enantiomers and which are different Next, we need to explore the circumstances under which chirality will appear. What structural features will suffice to ensure chirality Will, for example, the phenomenon we see in Figure 4.5 of four different groups surrounding a carbon be a sufficient condition to ensure chirahty Will it be a necessary condition This chapter discusses such questions. 

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