Asymmetric identity SN2 transition states: Nucleophilic substitution at α-substituted carbon and silicon centers

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We have quantum chemically investigated the archetypal nucleophilic substitution reactions at carbon (SN2@C) and at silicon (SN2@Si) in the model reaction systems Cl−+A(CH3)2(CH2X)Cl (A=C, Si; X=H, F, Cl, Br, I) using relativistic density functional theory (DFT) at ZORA-OLYP/TZ2P. Our purpose is twofold. We wish to understand: (i) how the α-substituent X affects SN2 reactivity; and (ii) how methyl substituents at the central electrophilic atom A exactly participate in the transition vector of the Walden inversion. Interestingly, despite the fact that our SN2 model reactions are symmetric, i.e., constitute identity reactions, they proceed via slightly asymmetric transition states. We have also explored competing E2 pathways.