S_N2 versus S_N2′ Competition

We have quantum chemically explored the competition between the S_N2 and S_N2′ pathways for X^-+ H₂CaCHCH₂Y (X, Y = F, Cl, Br, I) using a combined relativistic density functional theory and coupled-cluster theory approach. Bimolecular nucleophilic substitution reactions at allylic systems, i.e., CaC^β-C^α-Y, bearing a leaving-group at the α-position, proceed either via a direct attack at the α-carbon (S_N2) or via an attack at the γ-carbon, involving a concerted allylic rearrangement (S_N2′), in both cases leading to the expulsion of the leaving-group. Herein, we provide a physically sound model to rationalize under which circumstances a nucleophile will follow either the aliphatic S_N2 or allylic S_N2′ pathway. Our activation strain analyses expose the underlying physical factors that steer the S_N2/S_N2′ competition and, again, demonstrate that the concepts of a reaction's "characteristic distortivity" and "transition state acidity" provide explanations and design tools for understanding and predicting reactivity trends in organic synthesis.