Nucleophilic Substitution at Silicon (SN2@Si) via a Central Reaction Barrier
- 15 February 2007
- journal article
- research article
- Published by American Chemical Society (ACS) in The Journal of Organic Chemistry
- Vol. 72 (6), 2201-2207
- https://doi.org/10.1021/jo070076e
Abstract
It is textbook knowledge that nucleophilic substitution at carbon (SN[email protected]) proceeds via a central reaction barrier which disappears in the corresponding nucleophilic substitution reaction at silicon (SN[email protected]). Here, we address the question why the central barrier disappears from SN[email protected] to SN[email protected] 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, C2H5, and OCH3). Our results show that the nature of the SN2 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 SN[email protected] 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 SN[email protected] reactions.Keywords
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