Reaction Stereochemistry Stereoselectivity and Stereospecificity

The terms enantiotopic and diastereotopic describe the relationship between a pair of atoms or groups in a molecule. Sometimes it is also useful to describe the local environment of a single atom, group, or location in a molecule (even if it does not coincide with an atomic center) as chiral or not. A chirotopic atom or point in a molecule is one that resides in a chiral environment, whereas an achirotopic atom or point does not. All atoms and all points associated with a chiral molecule are chirotopic. In achiral molecules, achirotopic points are those that remain unchanged (are invariant) upon execution of an S that is a. symmetry operation of the molecule. For most situations, this means that the point either lies on a mirror plane or is coincident with the center of inversion of the molecule. Importantly, there will generally be chirotopic points even in achiral molecules. 

These terms can be clarified by looking at some specific examples. In the following ro-tamers of ) cso-l,2-dichloro-l,2-dibromoethane, the only achirotopic site in rotamer A is the point of inversion in the middle of the structure. Every atom is in a locally chiral environment, and so is chirotopic. For rotamer B, all points in the mirror plane (a plane perpendicular to the page of fhe paper) are achirotopic. All other points in these conformers are chirotopic, existing at sites of no symmefry. In other words, all other points in these conformers feel a chiral environment, even though the molecule is achiral. 

Topicity relationships and symmetry arguments provide a powerful approach to anticipating reactivity patterns. Whether by habit, intuition, or full realization, it is the topicity relationships discussed above that synthetic chemists use to develop chemical transformations that yield asymmetric induction. 

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