Chiral molecules halogenation

All pheniramines are chiral molecule.s. and the halogen-.sub.siilulcd derivatives have been resolved by crystallization or salts Tormed with d-larlaric acid. Aniihislaminic activity resides almost exclusively in the S siereoisomers. ... 

To see the inherent asymmetry in the two enantiomers, I drew below the molecules in Figure 7.7 the positions of the three halogens when looking at the molecules from the bottom viewpoint such that the C—H bond is away from me. Now, let me trace the direction in going from F to Cl and then to Br. As you can see, the direction in one of the molecules is clockwise, and in the other enantiomer it is anticlockwise. These clockwise/anticlockwise rotation arrows are not superimposable. This is the asymmetry that is the root cause of the handedness of the two molecules. Another way to identify chiral molecules is if there is no way to cut them into two identical halves... 

Radical halogenation of a chiral molecule containing one stereocenter will give a racemate if the reaction takes place at the stereocenter. When reaction elsewhere leads to two diastereomers, they will be formed in unequal amounts. 

Two wider ranging, more systematic investigations of conformational dependence have since been performed to establish whether the conformational sensitivity noted in the above PECD smdies may generally provide a means for identifying and distinguishing gas-phase structure of suitable chiral species. The B-spline method has been applied to the model system (l/f,2f )-l,2-dibromo-l,2-dichloro-l,2-difluoroethane [60]. Rotation around the C C bond creates three stable conformational possibilities for this molecule to adopt. The results for both core and valence shell ionizations reaffirm an earlier conclusion a and p are almost unaffected by the rotational conformation adopted, whereas the PECD varies significantly. Eor the C Ij ionization to show any sensitivity at aU to the relative disposition of the halogen atoms further reinforces the point made previously in connection with the core level PECD phenomenon. 

Chemical Family a group of elements that share similar chemical properties and share the same column in the periodic table, for example, halogens, alkali earth Chirality condition that describes the handedness of a molecule or whether a molecule exists in forms that can be superimposed on each other Chlorofluorocarbons also called CFCs, compounds consisting of chorine, fluorine, and carbon that are responsible for stratospheric ozone destruction Coagulation precipitation or separation from a dispersed state Coefficient of Thermal Expansion measure of the rate at which a substance will expand when heated... 

A similar chiral environment is given by inclusion to cyclodextrins (CDs), cyclic oligosaccharides (3). The outside of the host molecule is hydrophilic and the inside hydrophobic. The diameters of the cavities are approximately 6 (a), 7-8 (j3), and 9-10 A (7), respectively. Reduction of some prochiral ketone-j3-CD complexes with sodium boro-hydride in water gives the alcoholic products in modest ee (Scheme 2) (4). On the other hand, uncomplexed ketones are reduced with a crystalline CD complex of borane-pyridine complex dispersed in water to form the secondary alcohols in up to 90% ee, but in moderate chemical yields. Fair to excellent enantioselection has been achieved in gaseous hydrohalogenation or halogenation of a- or /3-CD complexes of crotonic or methacrylic acid. These reactions may seem attractive but currently require the use of stoichiometric amounts of the host CD molecules. 

An unusual feature of dactomelyns is the presence of two halogen-substituted chiral centers. Since few methods are so far known for selective introduction of halogen atoms ai sp -hvbridi/ed carbon, the dactomelyns pose an extraordinary synthetic challenge. Another interesting feature of these natural products is the appearance of both chlorine and bromine atoms in the same molecule. One is struck, upon closer examination of the bicyclic pyran skeleton 1. by the fact that the chlorine atom is oriented toward the steri-cally less lavorable side of the molecule (the concave side), whereas the bromine atom is directed toward the convex side. 

To differentiate between these alternative mechanisms, H. C. Brown, M. S. Kharasch, and T. H. Chao, working at the University of Chicago, carried out the photochemical halogenation of optically active S-(H-)-l-chloro-2-methylbutanc. A number of isomeric products were, of course, formed, corresponding to attack at various positions in the molecule. Problem What were these products ) They focused their attention on just one of these products i,2--dichloro-2-mcthyl-butane, resulting from substitution at the chiral center (C-2). 

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