Nucleophilic substitution at unactivated vinylic carbon.: Factors conducive to the energetic preference for the in-plane SN2 pathway

The gas-phase nucleophilic bimolecular substitutions at unactivated vinylic carbon (CH2=CHCl, 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 S(N)2 path with retention (S-N pi) but an in-plane S(N)2 path (S-N sigma) 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 (sigma*, or pi*), (ii) the proximate sigma-sigma* 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 S-N pi transition state is predominantly influenced by the proximate sigma-sigma* type interactions, whereas electrostatic interactions are the major factor conducive to the energetic preference for the S-N sigma over the S-N pi processes for Cl- and Br-. Solvent effect raises the barrier height but the mechanism and preferred path are not affected.