Reactivity Probes of n-o Interactions

In this section we shall examine in detail the role of n—a interactions in carbanion chemistry. Carbanion stability as well as relative acidities of organic molecules are important probes of n-a interactions. 

It has been known for many years that hydrogens adjacent to a heteroatom are more acidic when the heteroatom is sulfur than when oxygen A striking example of this phenomenon is the behavior of dithiane, A, compared to its oxygen analog, 

when treated with strong base. The sulfur compound forms a carbanion while its oxygen counterpart fails to react  

The lone pair in the dithiane carbanion C can assume either an equatorial or axial position in the ring as shown below  

It is readily recognized that the conformation in which the lone pair is equatorial corresponds to an anft -anangement of the carbon lone pair in relation to the adjacent S-C bond (darkened for the sake of clarity) and the axial case a syn orientation. Based on our approach, we would predict that the carbanion with the lone pair equatorial will be the most stable conformation. This is indeed observed experimen tally  

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