Gas-phase acidities of aryldimethylsilanes

Abstract

Gas-phase acidities of aryldimethylsilanes have been determined by measuring equilibrium constants of proton-transfer reactions using a FT ion cyclotron resonance mass spectrometer. The acidity of dimethylphenylsilane was found to be 370.5 kcal mol⁻¹, i.e., it is a stronger acid than dimethylsilane by 2.7 kcal mol⁻¹. The acid-enhancing effect of the phenyl group is significantly smaller than in the carbon analog. In addition, the acidities of aryldimethylsilanes were linearly correlated with the standard substituent constant (σ°), giving a ρ of 4.6, indicating that the thermodynamic stabilities of silyl anions are less sensitive to the polar effects of substituents than benzoate, phenoxide, anilide, and benzyl anions. More importantly, it was found that the p-nitro group, which has a large π-electron-accepting ability, also conforms to the σ° correlation, suggesting that π-delocalization does not play a role in the stabilization of silyl anions; this is in contrast to the π-effects observed in carbon analogs. This is supported by theoretical stabilization energies and optimized geometrical features based on ab initio calculations.