S2 versus S2' Competition.

We have quantum chemically explored the competition between the S2 and S2' pathways for X + HC═CHCHY (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═C-C-Y, bearing a leaving-group at the α-position, proceed either via a direct attack at the α-carbon (S2) or via an attack at the γ-carbon, involving a concerted allylic rearrangement (S2'), 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 S2 or allylic S2' pathway. Our activation strain analyses expose the underlying physical factors that steer the S2/S2' 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.