Nucleophilic Substitution at Unactivated Vinylic Carbon. Factors Conducive to the Energetic Preference for the In-Plane SN2 Pathway

Abstract

The gas-phase nucleophilic bimolecular substitutions at unactivated vinylic carbon (CH2CHCl, 1) by four nucleophiles (OH-, SH-, Cl-, and Br-) are investigated theoretically at the G2(+)(MP2) level. The results show that the stronger nucleophiles (OH- and SH-) substitute by an out-of-plane SN2 path with retention (SN π) but an in-plane SN2 path (SN σ) with inversion of configuration is preferred for the substitution by the weaker bases, Cl- and Br-. However, the elimination pathway is much more facile for OH- than any substitution process. We have considerd three factors, (i) the LUMO symmetries (σ*, or π*), (ii) the proximate σ−σ* charge-transfer interactions, and (iii) the electrostatic interactions in the transition state, as possible causes for preferred pathway for each nucleophile. The stability of the SN π transition state is predominantly influenced by the proximate σ−σ* type interactions, whereas electrostatic interactions are the major factor conducive to the energetic preference for the SN σ ove...