Abstract
| - It is textbook knowledge that nucleophilic substitution at carbon (SN2@C) proceeds via a central reactionbarrier which disappears in the corresponding nucleophilic substitution reaction at silicon (SN2@Si).Here, we address the question why the central barrier disappears from SN2@C to SN2@Si despite thefact that these processes are isostructural and isoelectronic. To this end, we have explored and analyzedthe potential energy surfaces (PES) of various Cl- + CR3Cl (R = H, CH3) and Cl- + SiR3Cl modelreactions (R = H, CH3, C2H5, and OCH3). Our results show that the nature of the SN2 reaction barrieris in essence steric, but that it can be modulated by electronic factors. Thus, simply by increasing thesteric demand of the substituents R around the silicon atom, the SN2@Si mechanism changes from itsregular single-well PES (with a stable intermediate transition complex, TC), via a triple-well PES (witha 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 SN2@C reactions.
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