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.