S N 2 versus S N 2' Competition.

We have quantum chemically explored the competition between the S _N 2 and S _N 2' pathways for X ^- + H ₂ C═CHCH ₂ Y (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., C ^γ ═C ^β -C ^α -Y, bearing a leaving-group at the α-position, proceed either via a direct attack at the α-carbon (S _N 2) or via an attack at the γ-carbon, involving a concerted allylic rearrangement (S _N 2'), 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 _N 2 or allylic S _N 2' pathway. Our activation strain analyses expose the underlying physical factors that steer the S _N 2/S _N 2' 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.