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”.

烯基阴离子（(−)H2C–CH≡CH–Y；Y = 烷基、芳基、胺基、醚基）在空间位阻较大的Z构型中通常具有热力学稳定性。这种对Z构型的偏好被用于控制有机金属和Grignard反应中的区域化学，以及烯丙胺的重新排列生成腈和烯丙醚的重新排列。对于烯烃（Y = CH2R）中Z稳定性的解释通常涉及通过空间吸引（库仑力或电荷转移）在形式上带负电的碳C1与Y = CH2R氢之间的相互作用。然而，这种解释难以推广到胺和醚。我们提出，通过轨道驱动的所谓“吸引非键相互作用”，这种相互作用被认为有助于稳定1,2-二氟乙烯和1-取代丙烯的Z构型，同样也有助于稳定Z烯基阴离子。我们通过电子结构计算和实验文献的调查展示了这些效应对烯基阴离子的相关性。我们的结果表明，在涉及烯基阴离子中间体的反应中进行区域控制的新的方法，并激励重新审视基于轨道的“吸引非键相互作用”的旧理论。