A kinetic and mechanistic study of nucleophilic substitution at a pentacoordinated silicon atom

Bassindale, Alan R.; Parker, David J.; Taylor, Peter G. and Turtle, Robert (2009). A kinetic and mechanistic study of nucleophilic substitution at a pentacoordinated silicon atom. Zeitschrift für Anorganische und Allgemeine Chemie, 635(9-10) pp. 1288–1294.

DOI: https://doi.org/10.1002/zaac.200900127

Abstract

A model system for the exploration of nucleophilic substitution at a pentacoordinate silicon centre is reported. The degenerate nucleophilic exchange of free N-methylimidazole (NMI) with coordinated NMI in a chiral pentacoordinate silicon compound, 4, has been studied by dynamic NMR spectroscopy. In 4 the axial ligands are N-methylimidazole and an intramolecularly-coordinated and chelating oxygen atom of an amido group. The equatorial ligands are N-CH₂, Ph and Me. Two competing mechanisms for the nucleophilic substitution at silicon have been distinguished from the data. One mechanism takes place with retention of configuration at silicon and is zeroth order in added nucleophile. This is interpreted as involving rate limiting dissociation of the N-methylimidazole followed by rapid re-association of another NMI molecule. The other mechanism takes place with both inversion and retention of configuration at silicon but is also of zeroth order in added nucleophile. A mechanism for this reaction in which the O-Si coordination is lost in the rate-determining step is consistent with the data. In this particular example there is no evidence for a mechanism in which pentacoordinate silicon is attacked to give a hexacoordinate intermediate or transition state.

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