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”.