Chirality and isomerism

The concepts of chirality and isomerism may readily be extended to pairs or larger assemblages of molecules, hence the reference to chiral and achiral environments above. 

In both encodings, stereoisomerism is discussed with an example of chirality and cisjtrans isomerism. 

The experimentally observable phenomenon of optical activity is usually considered in the context of variation of molecular chirality arising from a particular stereochemical configuration at a particular atom such that the molecule has no improper rotation S axis. Molecules with opposite chirality configurations are enantiomers and show oppositely signed optical activity. Molecules differing only in conformation are called conformers or rotational isomers. In most cases, the difference in energy between rotational isomeric states is very small, such that at ambient temperature all are populated and no optical activity results. However, if one particular conformer is stabilized, for example, by restriction of rotation about a bond, the molecule can become chiral, and thus optically active. 

When Wa = substituted aminoacyl, that is, when Wa-Xaa is a peptide, there is a strong tendency to form an oxazolone. The 2-alkyl-5(4//)-oxazolone that is formed is chirally unstable. Isomerization of the 2-alkyl-5(4//)-oxazolone generates diaste-reomeric products. When Wa = R0C=0, there is a lesser tendency to form an oxazolone. The 2-alkoxy-5(4/7)-oxazolone that is formed is chirally stable. No isomerization occurs under normal operating conditions. Finally, when Wa = R0C=0, an additional productive intermediate, the symmetrical anhydride, can and often does form. 

Acid-induced mcemization and isomerization of chiral allylic alcohols. Bimolecular nucleophilic displacements in allylic compounds are known to proceed via the four possible pathways shown in Scheme 19. 

Table 22 Arrhenius parameters for the gas-phase racemization and isomerization of chiral ions 35 and 36 Reaction...

Isomers are compounds with the same molecular formula but different structural formulas. Some organic and biochemical compounds may exist in different isomeric forms, and these different isomers have different properties. The two most common types of isomers in organic systems are cis-trans isomers and isomerism due to the presence of a chiral Ccirbon. 

The hydrovinylation reaction, the codimerization of ethene and styrene (Scheme 2), provides easy access to chiral building blocks from inexpensive hydrocarbon feedstocks, which can be used further for the preparation of fine chemicals. Key problems in this reaction include the selectivity of the reaction and the stability of the catalyst. The main side reactions are oligomerization and isomerization of the product to internal achiral alkenes. The latter reaction can be suppressed by... 

Compounds in which rotation is restricted may exhibit cis-trans isomerism.155 These compounds do not rotate the plane of polarized light (unless they also happen to be chiral), and the properties of the isomers are not identical. The two most important types are isomerism resulting from double bonds and that resulting from rings. 

The trans isomer is chiral and has been resolved, while the cis isomer is a meso form. Another example of cis-trans isomerism resulting from restricted rotation about single bonds367 is found in 1,8-di-o-tolylnapthalene368 (see also p. 128). 

Optical isomerism is said to occur when a molecule and its mirror image are non-superimposable, that is when there is no rotation or translation of the mirror image which brings it into coincidence with the original. Molecules that can exist in optically isomeric forms are said to be chiral and... 

The discovery and development of highly efficient Rh-catalysts with chiral diphosphites and phosphine-phosphites have dramatically improved the enantioselectivity of asymmetric hydro-formylation to >90% ee in the first half of the 1990s. It appears that the success of the chiral Rh-catalyzed process has replaced the chiral Pt-catalyzed process used extensively in 1980s, which often suffer from side reactions, such as hydrogenation and isomerization, as well as low selectivity to branched aldehydes. It can be said that the enantioselectivity of asymmetric hydroformylation has reached the equivalent level to that of asymmetric hydrogenation and... 

When a molecule is chiral, then it will have two isomeric forms called enantiomers, each of which is the nonsuperimposable mirror image of the other. Enantiomers are distinct stereoisomers because they are compounds that have die same molecular formula and sequence of bonded elements but which differ in tile spatial arrangement of groups in the molecule. If a molecule is chiral, and thus has two enantiomers, it usually (but not always) contains at least one chiral center. In organic compounds a chiral center usually corresponds to an asymmetric tetrahedral carbon atom. 

Substrate Chiral ligand Metal- to-ligand molar ratio Reaction conditions Con- versionh Yield Hydroformylation product and isomeric composition1 Chiral reaction product Ref. 

Con- ver- sion % Yield1 % Hydroformylation products and isomeric composition Chiral reaction product o.p. abs. % conf. Ref. 

Propellers of the type A13ZX may be analyzed in an analogous manner.9) The four classes of rotational mechanisms are retained, and an additional non-rotational pathway is added inversion at Z along the Z—X bond. Such inversion changes the configuration only with respect to the center of chirality, and has no effect on the other elements of isomerism. Enantiomerization pathways for an AraZX molecule of type 3 thus must include an odd number of inversions. 

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