Silane Inversion Catalyzed by Weak Nucleophiles: Pseudorotation of 10-Si-5 Intermediates

Article abstract of DOI:10.1021/ja00308a031

The inversion 1 at silicon observed in weakly nucleophilic media is proposed to occur by a mechanism involving pseudorotation of 10-Si-5 intermediate 2 (i.e., Nu+1<*>2<*>2<*><*>1<*>+Nu, where 1<*> and 2<*> are enantiomers of 1 and 2).Supporting this mechanism are kinetic studies that indicate (a) the rate of inversion of 1 is first-order in nucleophile, (b) electron-donating groups on the phenyl ring of the nucleophilic catalyst benzaldehyde accelerate the inversion of 1, indicating that the benzaldehyde is coordinated to 1 in the transition state for inversion of 1, (c) ΔS*=-27.9 eu for the benzaldehyde-catalyzed inversion of 1, and (d) there is no apparent correlation between the inversion rate of 1 and the solvent ionizing power of the reaction medium, as measured by the E_T(30) values.The equilibrium between 1 and 2 is frozen out in the low-temperature 1H, 19F, and 29Si NMR spectra of mixtures of 1 and nucleophiles p-(dimethylamino)benzaldehyde (DMAB) or methanol.For the coordination of DMAB with 1, ΔH⁰=-12.4 kcal/mol, ΔS⁰=-52.3 eu, ΔH*=-2.1 kcal/mol, and ΔS*=-47.7 eu.Low-temperature rate studies of the inversion of 10-Si-5 siliconate 4(2, with Nu=DMAB) show it to proceed by a nondissociative intramolecular pseudorotation with ΔG*_183K=10.2 kcal/mol.The reaction is fast enough to explain the inversion of silane 1 observed at higher temperatures where NMR observation of intermediate siliconate (2) is not possible.