Spontaneous elimination of halides from -halo carbanions upon geometry optimizations: Computational predictions of the outcomes from elimination reactions proceeding via an E1cb-Like mechanism

The geometries of many beta-halocarbanions are not stationary points on the potential energy surface. Computational optimization of these geometries can lead to spontaneous elimination of the halogen as a halide ion and formation of alkene products. Computational optimizations might, therefore, be used to predict the experimental behavior of beta-halocarbanions according to an E1cb-like elimination reaction mechanism. We compare the optimization behavior of a variety of beta-halocarbanions according to DFT (omega B97X-D and B3LYP), MP2, and semiempirical PM3 models. For most optimizations, there was agreement about the general order of leaving group preference (Br- > Cl- > F-). Also observed was a general preference for loss of the leaving group from the least substituted carbon atom when options were presented.