Locating Stereocenters

For purposes of this course, we will define a stereocenter as a carbon atom with four different groups on it. For example. 

This drawing has a carbon in the center with four different groups ethyl, methyl, bromine, and chlorine. Therefore, we have a stereocenter. Anytime you have four different groups connected to a carbon atom, there will be two ways to arrange the groups in space (always two never more and never less). These two arrangements are different configurations  

These two compounds are different from each other even though the atoms are connected in the same way. The difference between them comes from their positions in 3D space. Therefore, they are called stereoisomers ( stereo for space). More specifically, they are called enantiomers, because the two compounds are mirror images of each other and they are not superimposable. If we construct models of these two compounds, we see that they are not the same—i.e., they cannot be superimposed. 

You must learn how to recognize when an atom has four different groups attached to it. To help you with this, let s begin with examples that are not stereocenters  

The carbon atom indicated above is not a stereocenter because there are two groups that are the same (there are two ethyl groups). The same is trae in the following case  

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In molecules, the regions that can be R or S are called stereocenters (or chiral centers—chiral is Greek for hand, and we can understand the symbolism there). In this chapter, we will learn how to locate stereocenters, how to draw them properly, how to label them as R or S. and what happens wdien you have more than one stereo-center in a compound. 

When locating stereocenters, it is often helpful to draw in the hydrogens in line-angle drawings. Carbon atoms with only one or two lines extending from them, as well as sp and sp hybridized carbons, can be excluded from consideration. Once the stereocenters are identified, use dashed and solid wedges to show the bonds to substituents. 

Let s redraw jnst the stereocenter showing the location of the fonr priorities ... 

We redraw just the stereocenter showing the location of the four priorities, and then we spear the molecule with a pencil and rotate 180° to put the 4 on a dash ... 

When we learned how to name compounds, we said that we would skip over the naming of stereocenters until we learned how to determine configuration. Now that we know how to determine whether a stereocenter is R or S, we can now see how to include this in the name of a compound. It is actually quite simple. If there is only one stereocenter, then you simply place either (R) or (S) at the beginning of the name. For example, 2-butanol has one stereocenter, and can either be (I )-2-bu-tanol or (S)-2-butanol, depending on the configuration of the stereocenter. If more than one stereocenter is present, then you must also use numbers to identify the location of each stereocenter. Consider the following example ... 

Stereochemical Control by the Enolate or Enolate Equivalent. The facial selectivity of aldol addition reactions can also be controlled by stereogenic centers in the nucleophile. A stereocenter can be located at any of the adjacent positions on an enolate or enolate equivalent. The configuration of the substituent can influence the direction of approach of the aldehyde. 

Low 3delds indicate areas for improvements Convergent routes preferred, in principle If chiral, location of resolution step or integrity of induced stereocenters... 

A directing effect of a methyl group at the allylic stereocenter, located between the nitrile oxide and the alkene moieties, on the stereochemistry of cycloaddition was found with the carbon analogues (Scheme 6.46 and Table 6.14) (18,256). 

Cozzi and co-workers (243,263) studied the influence of the double-bond configuration on the stereochemical course of the intramolecular cycloaddition of chiral alkenes, where the stereocenter is located outside the isoxazoline ring (Table 6.15). On the basis of experimental results as well as theoretical calculations, two models were proposed for the reaction with (Z)- and ( )-aIkenes, in accord with the model proposed for a-X-substituted alkenes (see Section 6.2.3.1). 

Mechanistically, in the transition state leading to 316, conformer 315 bearing most of the substituents in a pseudoequatorial orientation should be operative, resulting in formation of the carbocycle with the substituent at the newly formed stereocenter located in the a-orientation. 

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