Relative anion stabilities and transition state energies regarding vinylic vs allylic deprotonation of cyclic vinyl ethers by organolithium reagents: An ab initio study

Abstract

The relative energies of allylic and vinylic anions of several vinyl ethers were determined by ab initio calculations at the Hartree-Fock and second-order Møller-Plesset perturbation theory (single point) levels using basis set 6–31++G(d,p) in an attempt at explaining experimental results concerning allylic vs vinylic deprotonation. A general trend with cyclic vinyl ethers has been discovered where the stability of the allyl anion over the vinyl anion in terms of ring size is 8≈7>6>5≈4. In general, optimized vinyl anions exhibit a vinyl angle compression whereas optimized allyl anions exhibit an allyl angle expansion. Additionally, transition state structures are examined that invoke a pre-equilibrium complexation of lithium to the electron rich vinyl ether oxygens prior to the formation of the deprotonation products. These transition states are suggestive of multi-center processes, precluding the formation of free ions during these deprotonation reactions. In many cases, the stabilization energy of the appropriate transition state is in agreement with the experimentally observed product.