Experimental and theoretical study on thermodynamic stabilities of silyl cations. Substituent effect on the stability of dimethylphenylsilyl cation in the gas phase†

Abstract

Thermodynamic stabilities of aryldimethylsilyl cations were determined by measuring equilibrium constants of hydride-transfer reaction in the gas phase using a FT ion cyclotron resonance spectrometer. An application of the Yukawa–Tsuno equation to the substituent effect gave a ρ of –5.87 and an r⁺ of 0.29. Both r⁺ and ρ values are significantly smaller than those for a carbon analog, α-cumyl cation. The same trend was observed for the results of ab initio calculations. In addition, the r⁺ value of 0.3 for gaseous silyl cations agrees with that observed for the kinetics of the hydride-transfer reaction from aryldimethylsilanes to diarylcarbenium ions in dichloromethane. It is concluded that the dimethylphenylsilyl cation is characterized by no significant π-delocalization of the positive charge into the benzene ring in addition to a small ρ value compared to that of the α-cumyl cation.