Quantifying Periselectivity Through Ion Pair Association

Work by Steckhan attempted to quantify the periselectivity induced by different sensitizers using the Weller equation (Eq. 4.1), as estimation to the degree and direction of charge transfer.  

TABLE 4.6 Effect of Sensitizer Variation on Reaction Periselecitivity Between 8a and 13a in CH2CI2 

The relative orientation of the two reaction partners in cycloadditions leads to different endo and exo diastereomers and is therefore a key event that has been studied extensively in both neutral and ETC cycloadditions. The study of the endo/exo selectivity not only is crucial for the stereochemical outcome of the reaction but also has yielded important mechanistic insight into both types of reactions. In the context of this chapter, only a few illustrative examples will be shown.  

SCHEME 4.14 Endolexo selectivity when using electron-rich and electron-poor 

In neutral Diels-Alder reactions, the endo/exo selectivity is often rationahzed through secondary orbital interaction and differences in the electron density of 

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