Attractive Nonbonded Interactions Help Stabilize the Z Form of Alkenyl Anions

Alkenyl anions (⁽⁻⁾H₂C–CHCH–Y; Y = aliphatic, aromatic, amine, ether) are often thermodynamically stable in the sterically congested Z form. This preference for Z structures is used to control regiochemistry in organometallic and Grignard reactions, allyl amine rearrangements to enamines, and allyl ether rearrangements to enol ethers. Explanations for Z stability in alkenes (Y = CH₂R) typically invoke through-space attraction (Coulomb or charge transfer) between the formally anionic carbon C1 and the Y = CH₂R hydrogens. However, this explanation is difficult to generalize to amines and ethers. We suggest that the orbital-driven so-called “attractive nonbonded interactions” suggested to stabilize the Z forms of 1,2-difluoroethylene and 1-substituted propenes also help stabilize Z alkenyl anions. We present electronic structure calculations and surveys of the experimental literature to show these effects’ relevance for alkenyl anions. Our results suggest new approaches for regiocontrol in reactions with alkenyl anion intermediates and motivate revisiting older orbital-based theories of “attractive nonbonded interactions”.