Cahn- Ingold- Prelog priority rules

Cahn-Ingold-Prelog Priority Rules (Table 5 1, p 175)... 

To avoid potential uncertainties in the application of the Cahn-Ingold-Prelog priority rules, by convention the enolate oxygen is assigned the higher priority. 

The absolute configuration system stems from the Cahn-Ingold-Prelog priority rules, which allow a precise description of a stereocenter without using any reference compound. In fact the basis is now the atomic number of the stereocenter substituents. 

We will normally use the view on the left hand side for square planar (octahedral) and tetrahedral geometries around the optically active atom. The second view for a tetrahedral geometry shows the numbering scheme according to the Cahn, Ingold, Prelog priority rules whereas the third view shows that a tetrahedron consists of two pairs of substituents that are in planes perpendicular to each other. 

Scheme 6.20 Examples of Pd-catalysed alkenylation and arylation of 45. Apparent inversions in the absolute configurations are due to the Cahn-Ingold-Prelog priority rules.

Cahn-Ingold-Prelog Priority Rules and the Steering Wheel Model... 

Further analysis of the products from cyclodimerization revealed that each of the cis- and trans- stereoisomers has two enantiomers. As can be seen in Scheme 14.10, the cis- product has two enantiomers (a) and (b), being (S, R) and (R, S) configuration, respectively, according to the Cahn-Ingold-Prelog priority rules. Both the cis- enantiomers are meso compounds and optically inactive. The trans- product also has two enantiomers, being (S, S) and (R, R), which exist as non-superimposable mirror images of each other. Enantiomers (c) and (d) should have identical physical and chemical properties in the absence of a chiral environment, while the trans and cis diastereomers will display differences in some physical and chemical properties, such as reactivity. 

FIGURE 1.9 cis trans (or Z/ ) stereochemistry. If a = a we use the cis/trans nomenclature, but if a a (which is the case for fatty adds because the terminus of the a group is the CH3 group and the terminus of the a group is carboxylate), we should more correctly use the Z/E system based on Cahn-Ingold-Prelog priority rules. In practice, it must be noted that most people still use the cis/treats system for the stereochemistry of fatty acids. 

The 5-configuration of the fluoro alcohol has opposite stereochemistry compared to the 5-isomer of a-phenethanol. This is an artifact of Cahn-Ingold-Prelog priority rules. Cahn, R. S. Ingold, C. Pielog, V. Aneew. Chem. Int. Ed. Enel. 1966.5.385. 

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