Open Shell Interactions, Nonbonded Attraction, and Aromaticity. Implications for Regiochemistry

Molecules and transition state complexes can be constructed, in theory, from union of component fragments. Whenever one of these fragments has an open shell electronic configuration, there is a strong one-electron bias toward cis regioselectivity. This simple theoretical analysis can be used to rationalize unexpected chemical phenomena such as the greater stability of the cis relative to the trans isomer of various olefins, the gauche effect in conformational analysis, and the cis regioselectivity of cycloadditions.