Inversion versus Retention of Configuration for Nucleophilic Substitution at Vinylic Carbon

A high-level computational study using CCSD, CCSD(T), and G2(+) levels of theory has shown that unactivated vinyl substrates such as vinyl chloride would afford gas phase, single-step halide exchange by a pure in-plane σ-approach of the nucleophile to the backside of the C−Cl σ bond. Geometry optimization by CCSD/6-31+G* and CCSD(T)/6-31+G* confirms the earlier findings of Glukhovtsev, Pross, and Radom that the SN2 reaction of Cl- with unactivated vinyl chloride in the gas phase occurs by a σ attack. Complexation of vinyl chloride with Na+ does not alter this in-plane σ preference. However, moderately activated dihaloethylenes such as 1-chloro-1-fluoroethylene undergo gas-phase SN2 attack by the accepted π-route where the nucleophile approaches perpendicular to the plane of the CC. In the latter case a single-step π pathway is preferred for the Cl- + H2CCFCl reaction. This is the first definitive example at a high level of theory where a single-step π nucleophilic vinylic substitution is preferred over a multistep mechanism in the gas phase. The activation barriers for these gas-phase single-step σ- and π-processes involving both naked anions and Na+ complexes are, however, prohibitively high. Solvation and the presence of a counterion must play a dominant role in nucleophilic vinylic substitution reactions that proceed so readily in the condensed phase. In solution, nucleophilic vinylic substitution reactions involving electron-withdrawing groups on the carbon−carbon double bond (e.g., −CN, −CHO, and −NO2) would almost certainly proceed via a free discrete carbanionic intermediate in accord with experiment.