Nucleophilic substitution at silicon (S_N@Si) via a central reaction barrier

It is textbook knowledge that nucleophilic substitution at carbon (S _N2@C) proceeds via a central reaction barrier which disappears in the corresponding nucleophilic substitution reaction at silicon (S _N2@Si). Here, we address the question why the central barrier disappears from S_N2@C to S_N2@Si despite the fact that these processes are isostructural and isoelectronic. To this end, we have explored and analyzed the potential energy surfaces (PES) of various Cl ^- + CR₃Cl (R = H, CH₃) and Cl^- + SiR₃Cl model reactions (R = H, CH₃, C₂H ₅, and OCH₃). Our results show that the nature of the S_N2 reaction barrier is in essence steric, but that it can be modulated by electronic factors. Thus, simply by increasing the steric demand of the substituents R around the silicon atom, the S_N2@Si mechanism changes from its regular single-well PES (with a stable intermediate transition complex, TC), via a triple-well PES (with a pre- and a post-TS before and after the central TC), to a double-well PES (with a TS; R = OCH₃), which is normally encountered for S_N2@C reactions.