Dissymmetric configuration

However, these structures were in doubt for a number of years. Strong evidence for criticizing them was given in 1927 by Kuhn and Albrecht [53], They reported that optically active D- and L-2-nitrobutanes can be converted into their optically active sodium salts and then regenerated without complete racemization. This was confirmed by Shriner and Young [54] who found that 2-nitrooctane behaves similarly. Subsequently the structures la and Ha for the aci-forms and Ilia and Illb for the aci-anion appeared to be excluded as they cannot correspond to dissymmetric configurations. If they were correct, the regenerated nitro compounds should be racemic. 

Kuhn and Albrecht (67) noted that, when 4,4 -dinitrodiphenic acid is converted to its quinine (Q) salt, the latter is unexpectedly and strongly dextrorotatory they attributed this to the adoption by the biphenyl skeleton of a dissymmetric configuration induced by the optically active alkaloid, and persisting for only so long as the salt exists. This can only be inferred and not proved directly, for only an inactive acid can be liberated from the salt on acidification ... 

Stereochemical analysis can add detail to the mechanistic picture of the Sj l substitution reaction. The ionization mechanism results in foimation of a caibocation intermediate which is planar because of its hybridization. If the caibocation is sufficiently long-lived under the reaction conditions to diffirse away from the leaving group, it becomes symmetrically solvated and gives racemic product. If this condition is not met, the solvation is dissymmetric, and product with net retention or inversion of configuration may be obtained, even though an achiral caibocation is formed. The extent of inversion or retention depends upon the details of the system. Examples of this effect will be discussed in later sections of the chapter. 

Krow, G., The Determination of Absolute Configuration of Planar and Axially Dissymmetric Molecules, 5, 31. 

A general approach for the determination of the absolute configuration of a chiral carbon consists of attaching to it a labile chiral unit and a dissymmetric chromophore and measuring the optical rotation at the sodium D line. This has been successfully applied to amino acids401. 

Atomic Inversion, Pyramidal (Lambert) Axially and Planar Dissymmetric Molecules, Absolute Configuration 6 19... 

Absolute Configuration of Planar and Axially Dissymmetric Molecules... 

The application of exciton coupling between the benzoate and the s-trans-diene or -enoate chromophore in axially dissymmetric molecules derived from hydroxy-substituted adaman-tanone allowed determination of not only the absolute but also the relative configuration. All benzoates 2-5 of 4R configuration show negative exciton Cotton effects, with amplitudes lower for (2 )-adamantylidene compounds 2, 4 compared with 2Z-isomers 3, 5, in which the two interacting chromophores are at a closer distance143. 

These are extremely common and can be a problem when the synthesis of the related tris complexes is being attempted, particularly as they are of rather limited solubility. They have been known for many years.1148,1160,1161 The isomers possible for these species are profuse.1149 For the general formula [(aa)2Cr-( U-OH)2Cr(aa)2] there are three possible geometric configurations about each metal ion tram N cis O, cis N tram O and cis N cis O. In addition, each metal ion is a dissymmetric centre bringing the total number of geometric and optical isomers to 21. 

This obviously is unlikely for the given example because there is no reason for cyanide ion to have anything other than an exactly equal chance of attacking above or below the plane of the ethanal molecule, producing equal numbers of molecules of the enantiomers, 21 and 22. However, when a chiral center is created through reaction with a dissymmetric (chiral) reagent, we should not expect an exactly 1 1 mixture of the two possible isomers. For example, in an aldol-type addition (Section 18-8E) of a chiral ester to a pro-chiral ketone the two configurations at the new chiral center in the products 23 and 24 are not equally favored. That is to say, asymmetric synthesis is achieved by the influence of one chiral center (R ) on the development of the second ... 

Planar and Axially Dissymmetric Molecules, Absolute Configuration of (Krow).. 5 31... 

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