Nucleophilic Influences and Origin of the S_N 2 Allylic Effect

The potential energy surfaces for the S_N 2 reactions of allyl and propyl chlorides with 21 anionic and neutral nucleophiles was studied by using ωB97X-D/6-311++G(3df,2pd) computations. The "allylic effect" on S_N 2 barriers was observed for all reactions, and compared with propyl substrates, the energy barriers differed by -0.2 to -4.5 kcal mol^-1 in the gas phase. Strong correlations of the S_N 2 net activation barriers with cation affinities, proton affinities, and electrostatic potentials at nuclei demonstrated the powerful influence of electrostatic interactions on these reactions. For the reactions of anionic (but not neutral) nucleophiles with allyl chloride, some of the incoming negative charge (0.2-18 %) migrated into the carbon chains, which would provide secondary stabilization of the S_N 2 transition states. Activation strain analysis provided additional insight into the allylic effect by showing that the energy of geometric distortion for the reactants to reach the S_N 2 transition state was smaller for each allylic reaction than for its propyl analogue. In many cases, the interaction energies between the substrate and nucleophile in this analysis were more favorable for propyl chloride reactions, but this compensation did not overcome the predominant strain energy effect.