Proposed Descriptors of Paired Molecular Faces

A detailed analysis of stereotopic faces in organic molecules was given in Chapter 12. The eleven subclasses of stereotopic faces, in effect, belong to five categories - achirohomotopic, chirohomotopic enantiotopic, achirodiastereotopic and chirodiastereotopic. Achirohomotopic (H/H), enantiotopic (E, H) and achirodiastereotopic (D/F) faces are located in achiral molecules chirohomotopic (H, H ), and chirodiastereotopic (D, F ) faces are foimd only in chiral molecules. The asterisks in the H, H and D, F descriptors indicate that the paired 

As in the case of ligands, there is a correspondence between stereotopicity and chirotopicity of stereotopic faces. Table C.2 indicates the relationship between the stereotopicity of molecular faces and the in-plane, 2D-clocksense (chirotopicity) of these faces. The latter clocksense is defined with respect to an off-plane observer, on either side of the molecular face. 

The specification of faces of alkenes, dienes and other systems by the proposed system requires criteria for determining the 2D-clocksense in these systems. This is done in Addendum D for a wide range of acyclic and cyclic systems. 

The proposed descriptors of homotopic/diastereotopic faces/ligands establish a common basis for the discussion of their reactivities and selectivities (vide infra). These descriptors are not intended to replace either the pro-R, pro-S, pro-r, pros descriptors for ligands, or the Re/Si/re/si descriptors for faces. The proposed specifications are short, concise, and universal they emphasize the similarities in reactivity for ligands and faces further, they reveal the stereotopic nature of the ligands (homotopic vs. enantiotopic vs. diastereotopic), as well as the chirotopicity vs. achirotopicity of the molecular environment. 

The rules we will present help one assign a simple and unambiguous stereotopicity descriptor - H, H, E, 3, D, D, or F - to each molecular face of an acyclic or monocyclic planar moiety. Paired homotopic (equivalent) faces are designated by the same descriptor H/H or H /H. For enantiotopic faces, the rules will help determine which face is E and which one is 3 for diastereotopic faces they will help tell which face is D, which one is F, which one is D and which one is F. The rules for each molecular system will be detailed below. 

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